A Literature Review of Use of Recycled Concrete Aggregates.pdf

CHAPTER 2 – LITERATURE REVIEW

2.1

INTRODUCTION

The literature review provides the necessary background information on concrete technology in general, along with materials used for concrete manufacturing with a strong focus on concrete aggregate. The fine and coarse concrete aggregates are reviewed in terms of their properties, and the testing techniques used in the characterisation of concrete aggregate are are also reviewed. In addition, background information on the basic engineering properties of conventional concrete is presented including its acoustic acoustic characteristics. characteristics.

With reference reference to coarse coarse aggregate and

conventional concrete, porosity has been identified as one of the most decisive  properties affecting the physical, mechanical, and acoustic characteristics of concrete, subsequently, literature on porosity of coarse aggregate and concrete is reviewed. The literature review presents the current state of knowledge and examples of successful uses of alternative materials in concrete technology, and in particular the use of Recycled Concrete (RC) Aggregate as a coarse aggregate fraction in non-structural concrete. It also presents presents a review of of available literature on RC Aggregate Aggregate properties including particle size distribution, density and water absorption, and identifies the need to investigate porosity and possible chemical contamination of the aggregate. A comparison between conventionally used aggregate in concrete technology and RC Aggregate is made made based on basic engineering engineering properties. Furthermore, accounts accounts of data, opinions and experience gained from successful applications of RC Aggregate as coarse aggregate in concrete production are presented, and characteristics of RA Concrete are are compared compared with those those of concrete made made from natural aggregate. aggregate. An analysis of differences between NA Concrete and RA Concrete is presented in a range of physical, mechanical and acoustic properties. A review of conventional research techniques used in the examination of concrete aggregate and and concrete is presented. This is followed by a review of non-destructive methods to investigate the microstructure of bulk materials which includes the Small Angle Neutron Scattering (SANS) technique. technique.

A comparison comparison based based on typical typical 2-1

measurements derived by different methods of pore size distribution, of the total volume of pores and of the pore surface area of concrete is presented. The literature review also presents background information on road traffic noise, on the use of concrete as a material for road sound barriers and on the acoustic performance of commercially available concrete sound barriers. The background information reviewed in this chapter formed the basis for a formulation of the hypothesis and objectives of this research project. The aim of the research project is aimed at the characterisation of locally produced selected RC Aggregate and its differentiation from natural basalt aggregate, as well as at developing a concrete product that best utilises the inherent properties of RC Aggregate.

2.2

CONCRETE CONSTITUENT MATERIALS

Modern concrete is a sophisticated composite material which is constantly undergoing improvements and modifications. However, the basic constituents of conventional, ordinary Portland cement (OPC) concrete such as fine and coarse aggregate, cement, and water, remain the same. There are other materials such as chemical admixtures including superplasticisers, water reducers, and air entrainers that can be used to modify the characteristics of OPC OPC concrete. There is also an an increase in the use use of pozzolanic materials including fly ash, granulated blast-furnace slag and silica fume (Neville, 1999). Over the last few decades, decades, the uses of various alternative fine and coarse aggregates in the production of concrete have been investigated, including the use of RC Aggregate. Hydraulic cements produced in Australia fall into two broad categories; General Purpose (GP) which includes ordinary Portland cements and blended cements (GB); and special purpose purpose cements (CCAA&AS, 2004). In standard concrete where there is no need for special characteristics such as resistance to sulphates, development of high early strength, or reduction in heat of cement hydration, the GP cement is used.

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In Australia, the most commonly used fine and coarse aggregates in concrete technology are natural gravels, gravels, sands, and crushed crushed rock. In the Melbourne metropolitan area, crushed basalt as coarse aggregate, and natural quarry sand as fine aggregate are readily available and most commonly used (Day, 1999). Although the fine and coarse aggregate in concrete matrix provide inert filler, the aggregates’ petrographical, physical and mechanical properties can significantly affect concrete plastic and and hardened characteristics. characteristics. Nawy (1997) defines defines the most important  properties of aggregate for ordinary concrete being the particle size distribution, aggregate shape, porosity and possible reactivity with cement. Nawy (1997) also states that surface texture has significant influence on concrete strength, since cubically shaped crushed stones with a rough surface appear to produce higher strength concrete than smoother faced uncrushed gravel, as bonding between aggregate and cement paste is increased. Other properties that characterise characterise concrete aggregate aggregate include: strength and and rigidity expressed as a crushing value, soundness which defines aggregate resistance to normal weathering conditions, abrasion resistance, dimensional stability, alkali reactivity, density, and water absorption. Fine aggregate occupies approximately 30% of the total volume of conventional concrete, and the quality of fine aggregate affects the properties of concrete (CCAA&AS 2004). The recommended recommended amount of fine aggregate aggregate in workable concrete depends on the grading of the aggregate, cement content, particle shape and grading of the coarse aggregate and intended use of concrete. Ryan (1992) reports that river, pit, and quarry sands are most commonly used for fine aggregate in metropolitan metropolitan Melbourne. Fine aggregate from those those sources consists of of a high proportion of silica in various forms, which is advantageous for the bonding  between aggregate and cement, consequently leading to more durable concrete. Day (1999) identifies seven features affecting suitability of fine aggregate as concrete aggregate. These include particle size distribution, particle shape and surface texture, clay, silt and dust content, chemical impurities, presence of mechanically weak  particles, water absorption and mica content. Grading is singled out as the most important property, followed by particle shape and presence of impurities that determines acceptance of fine aggregate as suitable for concrete manufacture. 2-3

According to Day (1999), local fine aggregate is of an acceptable quality with the exception of You Yang granitic sand, which is highly absorptive. The particle size distribution of fine aggregate can be and often are represented by the fineness modulus (FM). The FM is calculated from the sum of cumulative percentages retained on standard sieves ranging from 4.75mm to 150 μm (Neville, 1999). Mindess (1981) states that although the FM is a crude depiction of aggregate grading it can be used to check uniformity of grading if small changes are expected. It is possible that aggregate of very different particle size distribution can have the same fineness modulus. The FM of fine aggregate is used in mix proportioning as a convenient  parameter describing aggregate grading which has a significant effect on the workability of concrete. Mindess (1981), states that the fineness modulus should be in a range between 2.3 and 3.2, where lower numbers represent a fine grading and higher numbers are representative of coarse grading of concrete sands. Nawy (1997) argues that fine aggregate with a fineness modulus of 2.5 and lower produces concrete with a sticky consistency, less workability and lower compressive strength, and that fine aggregate with the fineness modulus of 2.75 to 3.2 produces concrete of higher compressive strength and durability. Use of sub-standard fine aggregate in concrete can retard settings, increase bleeding and results in poor workability and increased water demand. Consequently it produces  porous, highly permeable and less durable concrete (Neville, 1999).

2.2.1 Coarse Aggregate In Australia, the properties of coarse aggregate should comply with the requirements of the Australian Standard AS2758.1 - 1998 ‘Aggregates and rock for engineering  purposes, Part1: Concrete aggregates’ (SAA, 1998). The standard identifies basalt, diorite and granite as the most commonly used coarse aggregate in Australia (CCAA&AS 2002). In metropolitan Melbourne Ryan (1992) identifies crushed basalt as the most commonly used coarse aggregate for concrete production followed by 2-4

hornfels, toscanite and to a lesser extent, river gravel as other sources for concrete aggregate. In the Sydney district, Pienmunne (2001) states that igneous rocks such as  basalt, dolerite and granite, and metamorphic rocks such as hornfel and quartzite, are used in concrete production. There is an increase in the use of river gravel as an aggregate for ready-mixed concrete. The round shape and a smooth surface texture of river gravel allow the pumping of concrete to be relatively easy. The use of river gravel is dictated by a reduced availability of other aggregate. Crushed igneous rocks are preferred as coarse aggregate for concrete, as they have higher strength and are less reactive than metamorphic or sedimentary rocks. However, the production of aggregate from igneous rocks has declined from 4.8 million tonnes per annum (tpa) (65% of the total aggregate market) in the 1970s, to 2.7 millions tpa (35%) in 2000. As the deposits of suitable igneous rock close to major metropolitan cities in Australia are becoming scarce, especially in the Sydney region, there is an increase in the production of river gravel and lower quality sedimentary rocks. There is also a sharp increase in coarse aggregate produced from concrete waste, from practically nothing in the 1970s to 1.2 million tpa in 2000 in Sydney, and 0.7 millions tpa in Melbourne, which accounts for approximately 10% of the total aggregate market in Australia. Some of the RC Aggregate is used in the production of concrete (Pienmunne, 2001). The coarse aggregate for concrete can be characterised by its shape, surface texture, grading, particle and bulk density, water absorption, and content of impurities and other  potentially harmful materials such as silt, clay, or organic matter. Mindess (1981) states that to proportion workable, of adequate strength and durable concrete, at least the following properties of coarse aggregate must be known: shape, texture, grading, moisture content, specific gravity, and bulk density. The Australian Standard AS 2758.1-1998 ‘Concrete aggregates’ expands on those requirements and identifies the following aggregate properties to be known to the concrete technologists to design suitable concrete mixes: particle density, bulk density, water absorption, particle size distribution, alkali aggregate reactivity, and soluble salts content (SAA, 1998).

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Sagoe-Crentsil (1999) confirms that the most common coarse aggregate used for concrete in Melbourne is crushed basalt.

He defines the basic properties of this

aggregate including water absorption of 1.0%, crushing value of 15%, and particle density of 2,890kg/m3. No foreign material content in locally produced basalt was reported.

Neville (1999), states that the crushing strength of basalt is approximately

200MPa, the crushing value is about 12%, and specific gravity is 2.85 on average. As a response to the growing demand for coarse concrete aggregate and the growing volume of quality recycled aggregate, the Australian Standard AS 2758.1-1998 ‘Concrete aggregates’, since its last update in 1998, also allows the use of crushed concrete as coarse concrete aggregate. However, the use of such aggregate should be authorised after additional testing is conducted, or previous experience justifies its use as coarse concrete aggregate (SAA, 1998).

2.3

ALTERNATIVE CONSTITUENT MATERIALS in CONCRETE

Although the basic concepts governing concrete technology remain unchallenged, concrete has undergone many changes. Cement and aggregate manufacturers constantly strive for higher quality products leading to a better, more economic concrete, so a wide range of chemical admixtures have been developed in order to alter concrete characteristics. Concrete technologists have also observed many advantages which result from the use of industrial by-products such as fly ash, and materials from alternative sources such as reclaimed and recycled aggregate (Tabone, 1999). An excellent example of concrete made from alternative constituent materials is the concrete used in two projects in Melbourne, the 60L office building in Carlton, and the City of Melbourne new Council House (CH2) in Collins Street. The concrete used in those two projects was made from alternative constituent materials such as reclaimed and recycled aggregate, recycled water, and supplementary cementitious materials. The total replacement materials in the 25MPa concrete accounted for 94%, and in the 32MPa and 40MPa concrete accounted for 35% of total weight of materials (Bowie, 2004).

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Recycled water, usually in the form of water-cement slurry, is a by-product of reclaiming of plastic state concrete waste. An over-specified or rejected plastic state concrete is reclaimed at concrete batching plants where water-cement slurry and reclaimed aggregate are separated, and then used for immediate application, or stored for further use. It is now common practice that any other standard concrete produced at  batching plants that have reclaim facilities, consists of approximately 4% of watercement slurry as a water replacement. Slurry is also extensively use in special purpose ‘green’ concrete (Bowie, 2004). Blended cements are another example of the use of alternative materials in concrete manufacturing in Australia. The GB cements are defined by Australian Standard AS3972-1997 ‘Portland and blended cements’, as consisting of Portland cement and more than 5% of mineral additions (SAA, 1997). The ground granulated blast furnace (GGBF) slag, and pozzolanic materials, such as fly ash, and silica fume, are the most common blending minerals in Type GB cements. Those silicous, or silicous and alumminous materials, are by-products generated during iron production or coal  burning. Cement replacement materials, also known as supplementary cementitious materials (SCMs) which include GGBF slag, fly ash, and silica fume, are now being promoted for use in concrete as they can improve the characteristics of concrete, reduce cost, and are an example of environmentally responsible practice in the concrete industry. They aim to reduce the negative impact on the natural environment caused by the production of Portland cement. Pozzolanas and GGBF slag have either none, or have very few cementing properties. However, the silica in those minerals reacts with the calcium hydroxide Ca(OH)2, or hydraulic properties in the case of GGBF slag, being activated  by Ca(OH)2, produced during hydration of Portland cement to form calcium silicate hydrates (C-S-H). The beneficial effects of using SCMs in concrete include lower heat of hydration, lower thermal shrinkage and reduced permeability, however, these materials tend to alter setting time and rate of strength gain. Australian government agencies and professional associations, including the Concrete Institute of Australia, promote and support the use of SCMs in concrete. A prominent example is the Vicroads Specification for Structural Concrete Section 610, which 2-7

 provides scope for utilisation of SCMs as replacement for Portland cement (Vicroads, 1997). It is now at the client’s and concrete manufacturer’s discretion to specify SCMs in ordinary concrete and it is mandatory to use SCMs for concrete in marine or sulphate aggressive environments (Vicroads, 1997). The use of alternative aggregate in concrete has been initiated not only because there is an increased amount of concrete waste that can be converted into concrete aggregate but also because the quality of the natural aggregate deposits, the size of those deposits, and access and distance over which transport is economical, make the availability of high quality concrete aggregate often unobtainable or uneconomic in many parts of the world, including Australia. In Melbourne, the remote locations of aggregate sources have prompted warnings that an over-reliance on existing business practices, and overreliance on natural aggregate as the only source of aggregate for concrete production, are now considered uneconomical practices and are considered unsustainable and uneconomical uses of natural resources (Day, 1999). Apart from the increased transportation cost of concrete aggregate from distant locations, and the impact of waste, another reason to consider alternative aggregate, as Day (1999) claims, is that it is more and more difficult to obtain aggregate, both fine and coarse, conforming to typical specifications, which tend to specify ideal properties of aggregate. Day (1999) suggests that alternative concrete mix design procedures, and approaches to satisfy concrete purchaser requirements, need to be devised. The use of what would typically be defined as substandard aggregate should be at the discretion of the concrete manufacturer as long as the final product satisfies purchaser specifications (Day, 1999). Those opinions have been confirmed and expressed by many other concrete practitioners in Melbourne (Brand, 1999 – 2004; Tabone, 1999 -2004). A positive step towards economic and ecological sustainability is the provision in the current standards for the use of alternative materials, such as crushed concrete waste in concrete products, as long as the alternative aggregate satisfies requirements set for natural aggregate (SAA, 1998). However, there is a need to set technical standards for selected recycled aggregate products against target applications. These specifications could define product characteristics that must be met for specific construction application. 2-8

Pienmunne (2001) states that although the supply of recycled concrete aggregate can be erratic, as it is linked with intensity of activities in demolition and construction, the RC Aggregate is a significant source of alternative aggregate. In Victoria, a positive step in promotion of RC Aggregate for concrete manufacture was the publication of the ‘Guide for Specification of Recycled Concrete Aggregate (RCA) for Concrete Production’ in September 1998, by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Building, Construction and Engineering Division (CSIRO, 1998).

2.4

CONCRETE WASTE and CONCRETE RECYCLING

Concrete waste, which falls into the construction and demolition (C&D) waste category, is generated when creation of new, or modifications to existing urban infrastructure such as transport systems, communication networks and buildings are made. With the increased urbanisation of the world’s growing population there is also an increase in C&D waste generation. This prompts a realisation that built-in urban infrastructure along with C&D waste (unless dumped at the landfill) contains a large stock of materials, and that efficient management of concrete, steel, bricks, or their waste, is necessary to sustain the future growth and increased demand for construction materials (Lahner, 1994). In developed countries there is an increased societal demand on government agencies and industries to search for alternative materials and reduce waste to achieve ecologically sustainable development. A report prepared by the US Department of Transportation on ‘Recycled Materials in European Highway Environments’ in 2000, concludes that in most European Union countries (especially Denmark, The  Netherlands, and Germany) recycling and reuse of C&D waste is very well established (FHWA, 2000). A notable example presented in this report is of recycling and the use of recycled products in The Netherlands. It is interesting to note that 1.2 million tonnes of recycled asphalt rubble is used as concrete aggregate and that hundreds of tonnes of  bottom ash are used as lightweight aggregate in the production of concrete blocks. It is an impressive achievement that 100% of municipal waste incinerator fly ash generated 2-9

 by the municipal solid waste to energy conversion, as well as GGBF slag and electric coal fly ash, are used in cement production or used in concrete as supplementary cementitiuos materials. Further to that, almost 100% of building and demolition waste is also recycled and used. The report states that 2 million tonnes (about 20% of all concrete waste) of crushed concrete is used as concrete aggregate (FHWA, 2000). Lauritzen (1994) presents numerous examples of reuse, recycling and successful use of recycled C&D waste, especially concrete waste recycling products, in new infrastructure. The American Concrete Pavement Association states that approximately 2.6 million tonnes of concrete pavement alone is recycled each year (Nash, 2003). In Victoria, since 1986, there has been a constant increase in public and business awareness of the negative social and environmental impacts of C&D waste. Consequently, the recycling of concrete waste has increased. Currently more than 50% of concrete waste is recycled (Ecorecycle, 2002). Nolan (1998) reports that in Victoria, 0.7 million tonnes of concrete waste was recycled in 1997/98. The majority of RC Aggregate is used in road construction as a substitute for natural aggregate, mainly in the sub-base layer. However, higher value utilisation of selected RC Aggregate has  been postulated by local aggregate manufacturers. This view is supported by findings  published by CSIRO in 1998, and personal communication with Recycling Industries Pty Ltd (CSIRO, 1998).

2.4.1 Alternative Sources of Coarse Aggregate In Australia over the last decade, generation of C&D waste has steadily increased. This necessitated changes in the concrete waste stream and resulted in a change of attitude towards waste within the demolition and construction industries (Ecorecycle, 2004). The environmentally responsible approach of the government and industry to C&D waste has resulted in an increased rate of recycling, and reuse of concrete waste. It seems that there is a common understanding and consensus that depletion of natural resources is a real threat, landfill space is becoming scarce, and that waste disposal causes significant environmental and social impact. There is also a general consensus

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that recycled C&D waste including RC Aggregate can be used for construction  purposes. The main source of raw material for recycling of concrete waste comes from demolition of concrete structures. The quality and purity of the raw material affect the quality of recycling products and ultimately commercial acceptance of concrete recycling  products.

The process of manufacturing concrete recycling products is relatively

simple. To produce high quality concrete recycling products that satisfy commercial and technical specifications it is crucial to segregate concrete waste at source eliminating any low and high density and friable contaminants (Bell, 1998). Recycling  process and plant setup depends on desired grading and quality of the final product. In situations when crushed concrete waste is to be used as fill material, the use of a mobile crusher is usually sufficient. However, when crushed concrete waste is used to produce RC Aggregate for road sub-base or as a concrete aggregate, a proper plant with at least two crushers, vibrating screens, magnets and conveyor belts has to be established. Once concrete rubble has been deposited at a recycling plant it is then broken by a pulveriser mounted on an excavator. Pieces of concrete waste broken to a suitable size are then crushed in a primary jaw crusher and then passed via conveyor belts into a cone crusher. The crushed material is passed through a set of vibrating screens and sieved on the way to a stockpile.

After each crusher, the rotating magnets remove remains of steel

reinforcement whereas pickers manually remove other contaminants. Manufacturers of recycled concrete products claim that any desired grading of recycled concrete aggregate can be achieved with appropriate modifications to the plant (Curmi, 1998). Currently in the Melbourne metropolitan area, there are a number of companies recycling concrete waste. These include Delta Demolition Pty Ltd, Boral Resources (Vic) Pty Ltd, although Recycling Industries Pty Ltd at Laverton North produces the largest quantities of selected RC Aggregate of adequate quality that can be used as concrete aggregate. Figure 2.1 shows the concrete crushing plant at Laverton North.

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Figure 2.1 Concrete recycling plant setup (courtesy Recycling Industries Pty. Ltd.)

Over the past 20 years, concrete recycling and the use of its products in infrastructure  projects has increased significantly in Victoria. The range of applications for concrete recycling products has expanded from the use of aggregate in the unbound sub-base layer in road construction to the use of coarse RC Aggregate in new concrete. Concrete recycling in Victoria was initiated in 1986 by a local recycling company, Recycling Industries Pty Ltd, of the Alex Fraser Group of Companies. Initially the company produced relatively low value materials in order to get market acceptance, as well as to gain necessary experience and expertise in crushing concrete waste. Even though at the beginning, recycled concrete products were treated with suspicion, they gradually gained the attention of the construction industry, local government, and the Victorian road authority, Vicroads (Bell, 1998). Joint efforts among the Alex Fraser Group, CSIRO, Division of Building, Construction and Engineering, and Vicroads resulted in the development of the first specification for crushed concrete in Australia known as the 820 Specification for Crushed Concrete for Pavement Sub-base (Vicroads, 1992).

In 1995 the specification was revised by Vicroads.

Currently the 820Q

specification encompasses Class3 and Class4 of various nominal sizes of Crushed Concrete for Pavement Sub-base (Vicroads, 1997). Another step in the promotion of the use of alternative construction materials and in the development of specifications for crushed concrete was the introduction in July 1997 of Vicroads 821 Specification for 20 mm nominal size Class3 Cement Treated Crushed 2-12

Concrete for Pavement Sub-base (Vicroads, 1997). In the meantime the Alex Fraser Group of Companies adopted a quality management system and introduced its own commercial specifications for 20 mm Class2 Crushed Concrete, and 14/10 mm Class1 Recycled Concrete Aggregate - RCA (Bell, 1998).

2.4.2 Current Applications for Recycled Concrete Products In recent years, companies such as the Delta Group (Concrete Recycling Division), Boral Resources Pty Ltd and others, have also made a significant impact in the minimisation and recycling of concrete waste. Some of the metropolitan Melbourne councils and the Melbourne Water Authority have contributed to the increased use of recycled concrete waste by requesting and specifying the use of RC Aggregate in their  projects. There are already many examples of successful applications of this material in some of the major projects in Victoria. One such project was the Western Ring Road in Melbourne, where 75,000 tonnes of recycled concrete was used as a sub-base material in road pavements. Another successful application of recycled concrete products was a sub-base for the New Formula 1 Grand Prix racetrack at Albert Park in Melbourne, where a total of 100,000 tonnes of RC Aggregate was used for its construction (Bell, 1998). A survey conducted by Richardson (1994) revealed a growing interest in concrete recycling, and reported that 60% of municipal councils in Melbourne were engaged and committed to concrete recycling. The survey also revealed that 13% of concrete recycling products were used as a fill material, 16% as a sub-base material in footpath construction, 19% as trench base material, 43% as sub-base in road construction, and 8% as aggregate for concrete production. The survey forecasts an annual estimated demand for concrete recycling products used by municipal councils in Australia of about 750,000 tonnes per year.

2.4.3 Under-utilisation of Recycled Concrete Aggregate Mindess (1981) indicates that the escalating problem of solid waste disposal has  prompted consideration of waste as a source of aggregate for concrete. Mindess (1981) 2-13

indicates that using solid waste as concrete aggregate provides ‘the only real potential’, however, he identifies three factors to be taken into account when waste products are under consideration for use as concrete aggregate: economy (mainly the amount of transportation required), compatibility with other materials, and required properties of concrete. Mindess (1981) points out that the shape and reactivity of recycled solid waste aggregate (glass waste in particular), might affect concrete properties, and anticipation and assessment of potential problems must be carried out. In the US, in 1980, a total of 25 million tonnes of building waste such as bricks, concrete, and reinforcing steel from demolition, was considered as aggregate for concrete (Mindess, 1981). In Japan, Kasai (1994) reports that 25.4 million tonnes of concrete waste was generated in 1990 with a recycling rate of 48%. He also states that since 1991, government  policies promoting concrete recycling are in place together with an ambitious plan to recycle and reuse 100% of concrete waste. In Japan the utilisation of recycled concrete aggregate as a road base material began in 1978. The technical guidelines were first  published in 1992, and the draft guideline for utilization of RC Aggregate in concrete  production was presented in 1994 (Kasai, 1994). In Germany, Schulz (1994) reports that 23 million tonnes of C&D waste were generated and used in 1989/1990, to meet the increased demand for concrete aggregate. In Victoria, there has been a growing acceptance of the use of RC Aggregate as a sub base material in road construction. This is also due to the improved quality of the concrete recycling products produced to satisfy the requirements of the 820 Vicroads specification. It therefore seemed logical to continue to investigate the use of selected aggregate in the production of new concrete.

The CSIRO (1998) reports that

commercially available selected RC Aggregate has properties which make it a suitable substitute for natural coarse aggregate in concrete of compressive strength of 25MPa (N25). Curmi (1999) points out that in Australia the manufacturing process of RC Aggregate is now well understood, and aggregate of various grading can be produced, and that the crushing process is easily adaptable.

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The high quality recycled aggregate that is suitable for use in new concrete is the selected 14/10mm Class1 RC Aggregate. It consists of natural aggregate, with cement  paste residue adhered to it, and less than 2% of impurities of various nature. The minimum particle density of the aggregate exceeds 2,100kg/m 3  and the grading complies with industry specifications. The manufacturer of 14/10mm RC Aggregate advises that although the density of the new aggregate is lower than commonly used natural aggregates which have significant impact on the yield and the unit mass of concrete, the aggregate is suitable for concrete (Bell 1998; Brand 1998).

2.5

COMPARISON between NATURAL and RECYCLED CONCRETE AGGREGATE

The basic engineering properties of coarse and fine aggregate, besides many other factors, determine the quality of concrete. Most rocks and stones can be used as concrete aggregate as long as they are sound, durable and resistant to volume changes. The suitability of the coarse aggregate for concrete manufacturing is dependent also on its shape, surface texture, grading, particle, and bulk density, water absorption, and content of impurities and potentially harmful materials such as silt, clay or organic matter. Mindess (1981) states that to design workable concrete, of adequate strength and durability, a range of properties of coarse aggregate must be known, such as shape and texture, grading, moisture content, specific gravity and bulk density. Raw materials for production of the natural aggregate and RC Aggregate contribute to some differences and variations of aggregate properties. The igneous, metamorphic or sedimentary rocks used in the production of natural coarse concrete aggregate are relatively homogenous. This results in considerable consistency of natural aggregate coming from a particular rock source. The concrete waste which often consists of waste material other than concrete debris, such as timber waste, steel reinforcement, bricks,  plastic, etc., can result in an aggregate containing some impurities. As RC Aggregate is  produced from composite material, its particles vary in composition and irregular distribution of cement paste residue and rock material. Recycled concrete aggregate consists of natural aggregate coated with cement paste residue, pieces of natural aggregate, or just cement paste and some impurities. Relative 2-15

amounts of those components, as well as grading, affect aggregate properties, and classify the aggregate as suitable for production of concrete. There is a general consensus that the amount of cement paste residue has a significant influence on the quality, and the physical, mechanical and chemical properties of the aggregate, and as such has potential influence on the properties of RA Concrete. Table 2.1 presents a comparison between natural aggregate and RC Aggregate (Gomez-Soberon, 2003). Table 2.1 Comparison between natural and RC Aggregate Property

Unit

Dry specific density Specific density (surface dry) Water absorption Total porosity

kg/m kg/m3 % %

3

N Aggregate

RC Aggregate

2,570 – 2,640 2,590 – 2,670 0.88 – 1.13 2.70 – 2.82

2,260 – 2,280 2,410 – 2,420 5.83 – 6.81 13.42 – 14.86

2.5.1 Shape and Surface Texture In particular, the shape of the coarse aggregate is an important characteristic that can affect the mechanical properties of concrete. The shape and surface texture of the coarse aggregate influence the strength of concrete by providing an adequate surface area for bonding with the paste or creating unfavourable high internal stresses (Mindess, 1981). The Australian Standard AS 2758.1 - 1998 ‘Concrete Aggregate’, classifies shapes of aggregate into two categories, desirable and less desirable (SAA, 1998). The desirable shapes include rounded, irregular and angular, whereas less desirable shapes include flaky, elongated, and flaky and elongated. Neville (1999), states that a cubeshaped aggregate (as long as it is ideally graded) interlocks much better than an aggregate of elongated or flaky shape, consequently leading to stronger concrete. Concrete made from elongated or flaky aggregate is less workable and prone to develop higher amounts of entrapped air pockets. Further, the shape of an aggregate can be classified by an index known as the angularity number, which defines the amount of voids in aggregate after compaction.

For

example, the highest amount of voids created by aggregate of approximately 45% has the angularity number 12 (SAA, 1995). The coarse aggregate used in the majority of concrete manufactured in the Melbourne metropolitan area is mostly angular, (Bowie, 2002). The difference in water requirements for concrete using angular, and rounded 2-16

coarse aggregate, is approximately 10 liters per cubic meter of additional water for concrete made from angular aggregate (CCAA&AS, 2004). The surface texture of aggregate contributes significantly to the development of a  physical bond between aggregate and cement paste. It also affects the water/cement (w/c) ratio, workability, and strength. The surface texture of aggregate is classified as glassy, smooth, granular, rough, crystalline, and honeycombed (SAA, 1998). Local  basalt used as concrete aggregate has a rough surface (Curmi, 1998).

For best

workability, a smooth surface is most desirable, however, for the best bond between aggregate and cement paste, and also for optimum strength, the rough-textured particles are preferred (CCAA&AS, 2004). Tasong (1998) identifies the rough surface texture of aggregate as contributing to a better bonding between aggregate and cement paste in concrete. The optimum workability of concrete can be achieved with the use of rounded particles (CCAA&AS, 2004). Mindess (1981) states that well rounded and compact aggregate  particles close to spherical shape with a relatively smooth surface, are the ideal aggregate for concrete. Despite the lack of formal study on the surface texture and shape of RC Aggregate  produced in Melbourne, there is a universal consensus that locally manufactured aggregate has a rough texture and round shape (Curmi 1999; Bell 1999; Brand 1999).

2.5.2 Particle Size Distribution Mindess (1981) states that particle size distribution determines the cement paste requirement in concrete, and that it is more economic to use well graded aggregate as it requires less cement paste. In general, aggregate can be well (continuously) graded or gap graded. Continuously-graded aggregate is predominantly specified in concrete technology although less common gap-graded aggregate is also being used. However, above average care has to be exercised.

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Particle size distribution has a direct influence on the water demand of concrete, workability, and durability of concrete. Combined, continuous grading of fine and coarse aggregate produces cohesive, workable, and durable concrete with fewer voids  between aggregate particles (CCAA&AS, 2004). Combined grading which is coarser (deficient in fine aggregate) than stipulated by the Australian Standard AS2758.1-1998 ‘Concrete aggregate’, however, produces harsh, difficult to place and finish concrete, with insufficient amount of cement paste to fill voids between coarse aggregate (SAA, 1998).

A combined grading with excess of fine aggregate, or with excessively fine

sand, produces uneconomical concrete as the water demand of the concrete is high, hence requiring more cement (CCAA&AS, 2004). Although the continuously graded aggregate as specified by the Australian Standard AS 2758.1-1998 ‘Concrete aggregate’, leads to ideal packing of different size fractions in concrete matrix, Neville (1999), claims that from an economic view point, the use of gap-graded aggregate for concrete is an increasingly more common practice. The gapgraded aggregate is the aggregate where one or more intermediate size fractions are omitted. Table 2.2 presents the particle size distribution of gap-graded (single-size) and continuously graded aggregate. Table 2.2 Particle size distribution of coarse aggregate – AS2758.1-1998 Sieve aperture [mm]

19

13.5

9.5

% passing; nominal size 14mm continuously graded % passing; single-size 14mm % passing; single-size 10mm

100

85-100

100

85-100 100 85-100

6.7

4.75

2.36

0.75

25 -55

0-10

0-2

0-20

0-5 0-5

0-2 0-2

0-20

The combined grading of fine and coarse aggregate affects the water and cement paste amount per cubic meter of concrete. It also affects the packing of aggregate in concrete which can further be related to the amount of air voids in no-fines concrete. Figure 2.2  presents a schematic demonstration of different combined grading of aggregate in concrete matrix.

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Figure 2.2 Schematic representation of aggregate grading in an assembly of aggregate particles: (a) uniform size, (b) continuous grading, (c) replacement of small sizes by large sizes, (d) gap-graded aggregate, (e) no-fines grading (Mindess, 1981)

Figure 2.2a shows that one-sized aggregate creates a higher volume of voids between aggregate particles consequently requiring a higher amount of cement paste to fill completely the space between aggregate. When continuously graded aggregate is used (see Figure 2.2b) the smaller particles pack between larger particles reducing the amount of voids between aggregate particles consequently requiring less cement paste. Sagoe-Crentsil (1999) and Sautner (1999), report on the use of RC Aggregate in concrete of compressive strength of 25MPa. The grading of the aggregate used in their studies is shown in Table 2.3. Table 2.3 Particle size distribution of coarse RC Aggregate (Sagoe-Crentsil 1999; Sautner 1999) Sieve aperture [mm]

19

13.5

9.5

6.7

4.75

2.36

0.15

% passing; Sagoe-Crentsil % passing; Sautner

100 98

91.4 60

28.7 31

7.6 31

5.4 10

4.2 4

0.5 0.1

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It is worth noting that although there were significant differences in grading reported by Sautner (1999), and Sagoe-Crentsil (1999), the difference in reduction in the compressive strength of RA Concrete of 9% and 10% respectively, in comparison with the control concrete, was relatively small. Although there was reduction in compressive strength, the plastic state properties of concrete did not differ from those of natural aggregate. Table 2.4 presents grading of both the RC Aggregate, and locally produced basalt used  by Sagoe-Crentsil (1999) for RA25 Concrete and controlled N25 Concrete. Table 2.4 Comparison of particle size distribution of the 14mm RC Aggregate and locally manufactured basaltic aggregate Sieve aperture [mm]

19

13.5

9.5

6.7

4.75

2.36

0.15

% passing; RC Aggregate % passing; Basalt

100 100

91.4 84

28.7 43.7

7.6 5.6

5.4 2.1

4.2 1.0

0.5 0.2

Grading variations of concrete aggregate are controlled by concrete technologists and taken into account in the concrete mix design process. The difference in grading  between natural and RC Aggregate will be analysed in terms of a combined grading of coarse and fine aggregate (Tabone, 2000).

2.5.3 Water Absorption An aggregate for concrete can be in various moisture states such as oven-dry, air-dry, saturated-surface-dry, and wet. Water absorption of the aggregate is related to its  porosity and is the amount of moisture absorbed by the minute pores present in the aggregate from its air-dry state to its saturated-surface-dry state. Knowledge of this  property is necessary for determining the amount of water per cubic meter of concrete or for maintaining a desirable water/cement ratio. Any deviations in water absorption of aggregate can significantly alter fresh concrete characteristics. In the case of highly absorptive aggregate, additional water might be required to provide adequate workability. This as a consequence, introduces free water in concrete mix and results in concrete bleeding, increase drying shrinkage and affects creep characteristics (Neville, 1999). Ramachandran (2001) indicates that because water absorption of the aggregate 2-20

has the potential to increase drying shrinkage of concrete, aggregate absorbing more than 3% of water should not be used as coarse concrete aggregate. Tasong (1998) reports on an investigation on the influence of aggregate properties, and the aggregate-cement paste interface on concrete properties and, amongst various  properties identifies water absorption and aggregate surface roughness as those contributing to the development of a strong bond between paste and aggregate. Mindess (1981), states that normal weight coarse aggregate used for concrete has water absorption between 1% and 2%, and that water absorption higher than that indicates higher porosity of the aggregate.

In Melbourne, local crushed basalt has water

absorption of 1.0% (CSIRO, 1998) whereas Tasong (1998) reports water absorption of  basalt of 1.4%, Etxeberria (2004) reports on water absorption of coarse recycled aggregate of 4.3%. The CSIRO (1998), reports that the average water absorption of RC Aggregate is 5% which is much higher than commonly used natural aggregates in concrete technology. The variability of water absorption of RC Aggregate can be overcome by pre-wetting of the aggregate. This is followed by consequent adjustment of water content per cubic meter of concrete when designing mix proportions.

2.5.4 Particle Density and Bulk Density One of the basic properties used to classify aggregate, particle or bulk density is closely related to mineral composition and porosity. The Australian Standard AS 2578.1-1998 ‘Concrete aggregates’ defines particle density as the mass of a quantity of oven dried  particles divided by their saturated surface dried volume, and the bulk density, as the mass of a unit volume of oven dried aggregate. Bulk density is determined either as compacted or un-compacted. Particle density of aggregate is directly related to porosity which indirectly influences the strength of the aggregate. Aggregate density is used mainly in concrete mix proportioning as aggregates of different density influence the yield and mass per volume of concrete (CCAA&SA, 2004).

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The particle density of natural aggregate ranges between 2,100kg/m 3 and 2,700kg/m3. The particle density of locally manufactured RC Aggregate exceeds 2,100kg/m 3 (CSIRO, 1998). Ravindrarajah (1985) and Hansen (1992) report on the particle density of coarse recycled concrete aggregate of 2,200kg/m 3. The bulk density, besides depending on mineral composition and porosity of aggregate  particles, also depends on particle size distribution.

Mindess (1981) states that

maximum bulk density can be achieved when the fine aggregate content in combined aggregate is between 35% and 40%, and such aggregate is most economical as minimum cement paste is required.

The bulk density of normal density coarse

aggregate ranges between 1,450kg/m 3 to 1,750kg/m3 (Mindess, 1981).

2.5.5 Impurities and Foreign Materials in RC Aggregate The quality of an aggregate and its suitability as concrete aggregate is also determined  by the presence of reactive minerals and impurities (foreign materials), which include organic matter, sugar, silt, clay and dust. Excessive amounts of fine particles of silt, clay, and dust increase the demand for water in concrete. This results in a loss of strength, and increases concrete permeability. It may also form a coating on the aggregate decreasing the bond between the aggregate and the cement paste (CCAA&AS, 2002). The amount of impurities in any aggregate can be expressed as  both weak particle, and low-density particle content. The content of weak particles should be less than 0.5% and that of the low density particles should not exceed 1% (SAA, 1998). The low density particles, which mainly include wood and other organic matter, tend to rise to the surface of a plastic concrete which consequently produces  pop-outs and staining of finishes in the hardened concrete. Natural aggregate which is  produced from a raw material occurring in large outcrops of relatively homogeneous igneous or metamorphic rock has very few foreign materials. In contrast, raw material for production of RC aggregate is prone to impurities and foreign materials. Depending on its origin, impurities in RC Aggregate can include low-density materials such as plastic, wood or organic matter, rubber, plaster and friable materials. It can also include asphalt, clay, as well as steel reinforcement. Sautner 2-22

(1999) reports that the total amount of foreign material in RC Aggregate used for concrete for footpath construction was less than 0.1%, Sagoe-Crentsil (1999) reports a total amount of impurities of 1.2%.

2.5.6 Aggregate Porosity  Neville (1999), states that the porosity of aggregate not only affects other properties of the aggregate, such as water absorption and density, but also has significant effects on concrete properties, especially permeability and durability. Commonly used in concrete manufacturing coarse aggregates have a porosity of up to 5%. Etxeberria (2004) reports on coarse natural aggregate of average porosity of 2.3%. Table 2.5 shows the porosity of common rocks used in concrete technology (Neville, 1999). Table 2.5 Porosity of some common rock (Neville, 1999) Rock group

Porosity [%]

Basalt Quartzite Limestone Granite

0.5 –1.5 1.9 -15.1 0.0 – 37.6 0.4 – 3.8

The porosity of RC Aggregate is more complex than that of natural aggregate, and is a function of porosity of the natural aggregate used in manufacturing of original concrete,  porosity of cement paste of the original concrete, and of the in-service conditions of concrete used to produce RC Aggregate. The porosity of RC Aggregate also depends on the amount of cement paste residue present in the aggregate. A comparison between total porosity of natural and recycled concrete aggregate, is presented in Table 2.6 (Gomez-Soberon 2003; Etxeberria 2004). Table 2.6 Porosity comparison between natural and RC Aggregate Property

Unit

Natural Aggregate

RC Aggregate

Total porosity (Gomez-Soberon 2003) Total porosity (Etxeberria 2004)

%

2.70 – 2.82

13.42 – 14.86

%

1.86 – 2.81

9.13 – 10.94

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2.5.7 Other Properties The crushing value of concrete aggregate specified by the Australian Standard AS 2758.1-1998 ‘Concrete aggregate’ for conditions of the most severe exposure is limited to 30% (SAA, 1998). RC Aggregate used in Australia satisfy the specified limit. As an example, the RC Aggregate used by Sautner (1999) had an average crushing value of 19%, while aggregate used by Sagoe-Crentsil (1999) had 23% on average. Otsuki (2003) argues that the quantity and quality of cement paste residue adhered to natural aggregate in RC Aggregate affect the strength of recycled aggregate concrete especially RA Concrete with low water/cement ratio. The effect is less significant with higher w/c ratios, which could indicate that cpr has similar properties to highly porous cement pastes. The content of fines in RC Aggregate influences its suitability as concrete aggregate. Snyder (1995) reports on various studies on densely graded crushed concrete in the State of Minnesota and states that the presence of significant amount of fines in crushed concrete appears to re-cement the RC Aggregate. This is due to the presence of some content of cementitious particles in the fines. He suggests removal of cementitious fines and substitution of inert fines to reduce the ability of the RC Aggregate to re-cement.

2.6

NORMAL DENSITY and NO-FINES CONCRETE

Most of the concrete produced nowadays is made from well-graded aggregate, however, for special purposes such as reduced density concrete, the gap-graded aggregate is often used. Neville (1999) suggests three main means of reducing density of concrete. The most common is to use a lightweight aggregate which results in a lightweight aggregate concrete. The other ways include increasing air content in cement paste, which results in cellular concrete, and creating air voids between coarse aggregate particles as in the case of no-fines concrete.

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The no-fines concrete is a special concrete which is made from the gap-graded aggregate, where aggregate smaller than 4.75mm is omitted. The omission of fine aggregate in concrete mix creates voids between coarse aggregate particles.  Neville (1999) defines no-fines concrete as an agglomeration of coarse aggregate  particles, each surrounded by a coating of cement paste of up to 1.3mm thick. The density of no-fines concrete depends primarily on the size and grading of the aggregate. Well-graded aggregate should be avoided as it compacts to higher density no-fines concrete. Single sized aggregate between 10mm and 20mm is preferred with 5% oversize and 10% undersize particles allowed (Neville, 1999). However, CCAA (1999) reports that blended aggregate, a combination of 10mm and 7mm, as well as 20mm and 14mm has been found to perform satisfactorily. Neville (1999) states that no particles smaller than 5mm should be present, and that flaky, elongated or sharp edged crushed aggregate should be avoided. A pre-wetting of the aggregate in order to facilitate uniform coating by the cement paste is suggested. The workability of no-fines concrete is difficult to specify and measure; consequently there is no workability test except a visual inspection to check the consistency of concrete batches, and to assess uniformity of the cement paste coating the aggregate. It is recommended that the no-fines concrete should be placed relatively very rapidly  because the thin layer of cement paste can dry out, which consequently results in a reduced strength (Brook, 1982). No-fines concrete compacts very little, therefore compaction of no-fines concrete is not recommended except for rodding in the corners of a formwork or vibrating for a very short time to prevent the cement paste from running off. The thin layer of cement paste makes curing very important, and so moist curing is recommended. Steel reinforcing of no-fines concrete is not recommended unless it is covered with a protective layer of cement paste. Mix proportions by volume are usually specified, of the cement/aggregate ratio and water/cement ratio. Typically w/c ratios are between 0.38 and 0.52 (Malhotra, 1976). Although in Australia CCAA (1999) suggests a relatively low w/c ratio, in the range of  between 0.4 and 0.45.

The cement/aggregate ratio, which typically controls the

compressive strength of no-fines concrete, varies between 1:6 and 1:10. CCAA (1999) reports on a no-fines concrete of a w/c ratio of 0.4, cement/aggregate ratio of 1:8 2-25

resulting in concrete of compressive strength of 7.5MPa and density of 1,850kg/m 3. It has been reported by McIntosh (1956) that the aggregate/cement ratio of 1:6 and the w/c ratio of 0.38 produce no-fines concrete of the compressive strength at 28 days of 14MPa and a density of 2,020kg/ m3.  Neville, (1999) reports that the density of no-fines concrete using normal weight aggregate ranges from 1,600kg/m3  to 2,000kg/m3  with corresponding compressive strengths of between 1.5MPa and 14MPa. Malhotra (1976) reports that the flexural strength is typically 30% of the compressive strength which is relatively higher than for ordinary, normal density concrete and that shrinkage is significantly lower, ranging from 120x10-6 to 200x10-6. However, CCAA (1999) reports that the drying shrinkage of no-fines concrete can be up to 300 microstrain and that it has higher permeability than standard concrete. The density and compressive strength of no-fines concrete is dependent on the cement content, aggregate/cement ratio by volume and water/cement ratio by mass. The water/cement ratio also depends on water absorption of the aggregate, however, if the w/c ratio is higher than optimum the cement paste is not adhesive.  Neville, (1999) states that because no-fines concrete has large pores and that it is subject to limited capillary suction, the capillary pores are not fully saturated, which makes this type of concrete frost resistant. However, even limited absorption of water makes nofines concrete unsuitable for use in foundations and in situations where it may become saturated with water. Neville (1999), claims that maximum absorption can be as high as 12.5%, but under normal conditions the absorbed water does not exceed 5% by mass. To reduce air permeability Neville (1999) suggests that external walls would need to be rendered on both sides. The open texture of no-fines concrete makes it very suitable for rendering. Rendering and painting on both sides increases sound transmission loss but at the same time reduces sound absorption. In situations where acoustic properties are considered to be of paramount importance, one side of the wall should not be rendered. Although Neville (1999) suggests that the main application for no-fines concrete is as the pre-cast in-fill panels in framed structures, Meininger (1988) reports on the use of no-fines concrete in domestic car parks overlaying a permeable sub-grade, and as  pavement around trees which allows easy drainage. In Australia, CCAA (1999) reports 2-26

on the use of no-fines concrete in external and internal walls of low-rise and multistorey units, in free-draining pavements for light traffic car parks, as well as in tennis courts, drainage layers and levelling courses.

2.7

COMPARISON between STANDARD and RECYCLED AGGREGATE (RA) CONCRETE

Concrete is a two-phase composite material.

For the first few hours, after the

constituent materials are mixed together in a production process, concrete remains in a  plastic state. In this state concrete is transported, placed, compacted and finished. Proper placing techniques and adequate compaction of plastic state concrete are necessary to expel trapped air from the concrete matrix, which can adversely influence the properties of hardened concrete. Concrete starts to harden after several hours when the chemical reaction between water and mineral compounds in cement start to accelerate. It is absolutely crucial to cure concrete during the hydration process i.e. maintain the required amount of water necessary to hydrate cement to obtain the desired concrete microstructure. All the steps and processes involved in proportioning of concrete constituent materials, mixing them, placing, compacting, finishing and curing of concrete have the potential to impact on the microstructural development of concrete and its physical and mechanical properties. Regardless of the type of aggregate used in concrete, the same care has to be exercised to both concrete made from natural aggregate, and that made from recycled concrete aggregate.

2.7.1 Physical and Mechanical Properties According to Australian Standard AS1379-1997 ‘Specification and supply of concrete’ there are basic plastic state and hardened properties that have to be specified such as slump (measure of concrete rheology), compressive strength (at 7 and 28 days), and durability and shrinkage (SAA, 1997). The rheological properties of concrete describe the flow behaviour of fresh concrete. The flow behaviour (plastic flow) is characterised by two parameters, yield stress and  plastic viscosity, however, a slump of fresh concrete is the most practical measure and the most widely accepted approximation of the rheological behaviours of concrete. The 2-27

slump of plastic state concrete is not only indicative of flow behaviour, which includes the workability and finishability of fresh concrete, but also is used to control consistency of concrete mixes. Variations in slump of concrete can be caused by factors related to concrete mix design, especially to the amount of water per cubic metre of concrete and by factors related to the aggregate, especially to combined grading, shape, and water absorption. CSIRO (1998) reports on an 80mm slump of 25MPa RA Concrete and a 70mm slump of control NA Concrete of identical mix proportion, with the same w/c ratio of 0.45. The ‘Guide for Specification of Recycled Concrete Aggregate for Concrete Production’ identifies a set of properties that are equally achievable in both the RA Concrete and  NA Concrete of the same amount of cementitious binder (CSIRO, 1998). Mix  proportioning, batching, placing and finishing of concrete made from RC Aggregate require similar procedures and equipment as for conventional concrete. The optimal ratio of the coarse to fine aggregate in RA Concrete required for desired cohesiveness is the same as in NA Concrete, and cohesiveness and workability of fresh concrete are comparable providing the grading and shape of aggregates are similar. However, additional pre-wetting is suggested to control water demand in RA Concrete. Otsuki, (2003) proposes a double-mixing method for improving strength, chloride penetration, and carbonation resistance of RA Concrete. Although Lauritzen (1994) reports on a reduction in compressive strength in RA Concrete by up to 20%, the CSIRO Guide shows that it is possible for RA Concrete to achieve the same compressive strength as the (N25 grade) NA Concrete, providing commercially available RC Aggregate has adequate quality. There is a presumption that the presence of significant volumes of the cement paste residue in the aggregate might have some effect on elastic properties such as drying shrinkage, and creep. However, the measured drying shrinkage of RA Concrete with a compressive strength of 25MPa did not exceed the specified 700 microstrains (CSIRO, 1998).

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Trial mixes are recommended to optimise fresh and hardened properties of RA Concrete, and the use of fly ash is desirable to minimise or eliminate possible alkalisilica reactivity of the aggregate (Bowie 1999; Brand 1999). Sagoe-Crentsil (1999), states that the predominant mode of failure, when RA Concrete is subjected to compressive force, is either due to the aggregate or the cement paste and depends on the target compressive strength. RA Concrete of a compressive strength of 25MPa and below predominantly fails in the cement paste due to the lower binder content, whereas in RA Concrete of compressive strength higher than 25MPA, failure occurs in the aggregate. This might suggest that the compressive strength of 25MPa at 28 days is the optimum for RA Concrete. Compressive Strength

The compressive strength of concrete is affected by both the aggregate properties, and the characteristics of the new cement paste that is developed during the maturing of concrete. The potential strength of concrete is partially a function of aspects related to mix proportioning such as cement content, water/cement ratio and choice of suitable aggregate but also a function of proper curing when chemical bonding develops. The w/c ratio, proper compaction and adequate curing, affect the development of concrete microstructure, and also affect the amount, distribution and size of pores. Mindess (1981) states that porosity is the primary factor governing the strength of concrete. The bond that is developed when concrete hardens is the aggregate-paste bond, which is  both physical and chemical.

Ryan (1992) suggests that to maintain the most

advantageous aggregate-paste bond some chemical activity between some reactive elements of the aggregate and hydration products in cement paste has to develop. The  presumption is that RC Aggregate might develop an even stronger chemical bond with cement paste, as the chemical composition of the aggregate is different from those of commonly used natural aggregates and the re-bonding of some elements in cement paste residue can take place (Brand, 1999; Tabone, 2001). The most important parameters of the aggregate affecting compressive strength are its shape, texture and maximum size.

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Although the strength of the coarse aggregate is one of the dominant factors in classification of concrete aggregate, Mindess (1981) states that to some extent it is of less importance as most of the aggregates are stronger than cement paste. Despite the  presumption of a probable stronger bond between fresh cement paste and RC Aggregate in low grade concrete, Sagoe-Crentsil (1999) reports on a reduction of compressive strength gain at 28 days of 10% in comparison to control (N25) natural aggregate concrete.

Sautner (1999) indicates that the reduction in compressive strength

development in N20 concrete at 3 days was 29%, at 7 days was 20%, and at 28 days was 11.5%.

Durability

As the compressive strength of concrete remains the most recognisable and desirable  property, an understanding of various transport mechanisms and deterioration processes in concrete matrix becomes equally important. As Andrews-Phaedonos (2001) states, durability is now regarded as an integral part in the design of concrete structures, and that durability enhancing parameters are explicitly built into design specifications. Durability of concrete is a function of many design and production aspects which include the choice of an optimum w/c ratio at the mix design phase, as well as proper compaction and curing.

Further, the durability of concrete can be enhanced by

inclusion of pozzolana such as fly ash, GGBF slag and silica fume. A selection of low w/c ratios of approximately 0.4 reduces bleeding of concrete and does not result in an excess of free water in concrete, and can consequently contribute to ultimate microstructure development of relatively impermeable concrete, as the amount of capillary voids is reduced and the voids are disconnected. Whereas the selection of higher w/c ratios results in a higher volume of interconnected capillary pores which contribute to high permeability and reduced durability of the concrete. Compaction of concrete also has a significant effect on concrete durability. Lack of, or inadequate compaction contributes to low durability of concrete, as the trapped air in fresh concrete is not expelled from the material. Although the entrapped air voids are not interconnected they can also cause durability problems (Neville, 1999).

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The use of the optimum curing method and its duration of properly proportioned and compacted concrete results in durable concrete. The maintenance of continuous moist curing in particular, contributes to a higher degree of hydration which reduces the amount of capillary pores as un-hydrated cement contributes to capillary porosity (Andrews-Phaedonos, 2001). Apart from factors related to the quality of cement paste (microstructure development, capillary porosity) and its effects on durability of concrete, there is a potential for aggregate to influence durability of concrete. Aggregate containing certain siliceous minerals might undergo a reaction with soluble alkalis in concrete known as the alkalisilica reaction (ASR). Reactive forms of silica in aggregate react with alkalies such as  potassium and the sodium hydroxides present in cement to produce alkali-silica gel. Moisture transporting through the concrete matrix, then alkali-silica gel swells inducing  pressure, expansion, and cracking. It has been widely accepted that inclusion of silica fume or fly ash reduces suspected aggregate alkali-silica reactivity (Neville, 1999; Day 1999; Nawy, 1997). There are numerous indirect tests to assess durability of concrete. Those indirect tests measure either permeability or absorption of concrete and include: •

Water Permeability (this test takes one to several weeks to complete)

•

Gas Permeability – nitrogen adsorption (which is relatively quick test)

•

Rapid Chloride Permeability Test (RCPT) (which takes only up to four days to complete although a 90-day chloride pounding is necessary to properly correlate the data)

•

Initial Surface Absorption Test (ISAT)

•

Porosity Tests – Mercury Intrusion Porosimetry (MIP)

•

Sorptivity Tests – (which requires a 21day preparation and conditioning period)

Andrews-Phaedonos (2001) argues that those tests have to be undertaken by experts in specialised laboratories and that there is a need for more user friendly and simpler tests to assess permeability or absorption characteristics of concrete relatively quickly. Currently in Victoria, the Apparent Volume of Permeable Voids (VPV) test in

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accordance with the Australian Standard AS 1012.21-1999 is most commonly used (SAA, 1999). CSIRO (1998) suggests that long term durability of RA Concrete has to be closely investigated as besides adequate compressive strength, the resistance to deteriorating mechanisms is of paramount importance in concrete structures. Several areas of concern are identified: possibility of chemical contamination including chloride and sulphur based deposits affecting rheology, setting characteristics and durability, and  porosity of the cement paste residue component of RC Aggregate affecting concrete  permeability.

2.7.2 Acoustic Properties As concrete undoubtedly has become the most dominant material in noisy urban environments, its acoustic properties have become very important design and  performance criteria. In standard construction and buildings, concrete is recognised for its good sound insulating properties as it contributes to the provision of acoustically comfortable living environments expected by modern society (CCAA, 1999). There are two parameters describing acoustic properties of concrete, the sound transmission loss or class (STC) and sound absorbency. Sound absorption depends on the porosity of concrete whereas sound transmission loss depends on density of materials per unit area. The high-density concrete has higher STC and does not absorb sound energy whereas low density concrete is a good sound insulator and can have some sound absorption capacities. To increase the STC of concrete, structural and nonstructural elements such as partitions or external walls and sandwich panels are often used (Mindess, 1981). Lightweight concrete which is less dense and more porous absorbs sound energy better than normal density concrete.

However, the total porosity of concrete not only

contributes to sound absorption, but to a greater connectivity of pores. For example, concrete made from lightweight aggregate which has irregular interconnected pores,

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absorbs sound energy better than more porous foamed or aerated concrete which has discrete, unconnected air entrained bubbles (Mindess, 1981). In terms of sound absorption there are two commonly used testing methods to examine acoustic properties of acoustic materials, these are the reverberation chamber method (SAA, 1988) and the impedance tube method (SAA, 1999).

2.7.3 Porosity and Fractal Dimensions Porosity

Porosity of concrete is a function of the combined porosities of aggregates and hardened cement paste (HCP). Section 2.5.6 of this document reviews the porosity of coarse aggregate whereas this section reviews porosity of cement paste and the combined  porosity of concrete. Primarily, the porosity in HCP can be in the form of gel pores, capillary pores, or entrapped air voids. In cement pastes, the intrinsic gel porosity results from a chemical reaction between elements and compounds present in the cementitious binder and water. This type of porosity depends on the degree of hydration and maturity of the paste. The formation of gel pores is complete as long as there is sufficient water to hydrate cement, and w/c ratio is above 0.4 in concrete without water-reducing admixtures. Another type of porosity, capillary porosity, is caused by the movement of free water in hardening cement paste, which usually results from excessive (free) water if the w/c ratio is above 0.4. The presence of entrapped air voids in concrete is mainly due to insufficient compaction of concrete and also to the shape and grading of aggregate (Mindess, 1981). Although, fundamentally there are mainly three types of pores in concrete: gel, capillary, and entrapped air, different authors tend to expand the basic classifications and in some instances do not agree on the pore size range (Mindess 1981; Ramachandran 2001; Mehta 1986; Nawy 1997).

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Ramachandran (2001) classifies pores in hardened cement paste into three main types: interlayer space in C-S-H (previously termed as gel pores), capillary pores, and air voids, and states that the size of the capillary pores is dependent on w/c ratio and degree of hydration. Well hydrated, low w/c ratio cement pastes have smaller pores, in ranges  between 100Å and 500Å, whereas cement pastes of higher w/c ratio can have capillary voids up to 5,000Å. Mindess (1981), states that gel pores mainly affect shrinkage and creep of the cement  paste whereas large capillary pores in pore sizes ranging between 50nm and 10 μm affect the strength and permeability of cement paste; and medium capillaries, in a pore size range between 10nm and 50nm also affect shrinkage at high humidity. Nawy (1997) alternatively classifies pores in cement pastes by the size of pores into three groups: micropores, mesopores, and macropores. Table 2.7 presents an alternative classification of pores in concrete and the size range of each type of pores. Table 2.7 Porosity classification (Nawy 1997; Ramachandran 2001) Pore Type

Micropores Mesopores Macropores

(Gel pores) - Interparticle spacing between C-S-H Small capillaries Medium capillaries Large capillaries Entrapped air bubbles or voids

Size Range [nm]

< 2.5 2.5 - 10 10 - 50 50 – 10,000 1,000 – 1x106

Porosity is a physical property that influences mechanical properties of concrete such as its strength, durability, shrinkage, creep, permeability, and ionic diffusion. Total  porosity of properly proportioned, placed and cured hardened concrete depends on pores developed in a cement paste, entrained or entrapped air voids, and voids in the pieces of aggregate particles. Porosity in the form of continuous channels, or micro-cracks can also develop as a result of curing and environment conditions (Mehta, 1986). The total  porosity of poor quality concrete can be as high as 15%. This is derived on the assumption that the average quality hardened cement paste contains approximately 50% of air or water filled voids, and that natural aggregate commonly used in concrete technology has porosity of up to 5%. Parameters defining porosity include pore size distribution, pore size (commonly expressed as pore diameter) pore surface area, and volume of pores. Any of the 2-34

 parameters or any combination of them influence in different ways various properties of hardened concrete. Frias (2000) argues that the pore size distribution of pore system in concrete, rather than the total porosity is the critical factor affecting the performance and durability of concrete. Bagel (1997) presents results of porosity measured by the Mercury Intrusion Porosimetry (MIP) method of cement pastes of different water/cement ratios of 0.4, 0.5, 0.6, and 0.7 hydrated for 28 days. Table 2.8 summarises the results. Table 2.8 Porosity of cement paste (Bagel, 1997) Parameter

Pore volume Pore radius Surface area Total porosity

Unit 3

mm /g nm m2/g %

Water/cement ratio 0.4 0.5 0.6

0.7

43.9 71.1 3.27 12.1

66.8 78.1 2.77 14.6

48.7 68.1 3.54 11.4

75.9 74.4 3.45 16.5

 Neville (1999) further relates the quantity and characteristics of various pores and voids to concrete strength, elasticity, shrinkage, and permeability. In relation to concrete strength, the total volume of pores not their size or continuity, has a dominant influence. Drying shrinkage of concrete is influenced heavily by the total surface of the pore system, whereas permeability is reported to be affected by the volume as well as size and continuity of the pores. Eighmy (2003) also reports on the examination of a 15-year-in-service concrete  pavement (mean compressive strength of 35MPa) and prediction of future behaviour of concrete made from recycled materials of the same composition, and mix proportions of concrete compressive strength of 40MPa. Table 2.9 presents total porosity and surface area of pores in RA Concrete. Table 2.9 Porosity a 15-year-in-service concrete (Eighmy, 2003) Property

Unit

40MPa laboratory made concrete

35MPa field-aged concrete

Porosity Effective surface area

% m2/g

8.2 – 10.2 2.3 – 11.5

10.0 – 13.4 1.4 – 2.9

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Hansen (1987) investigated porosity of cement pastes of 0.4, 0.6 and 0.75 water/cement ratios. Surface area and pore volume of various neat cement pastes are presented in Table 2.10. Table 2.10 Porosity of neat cement paste (Hansen, 1987) Parameter

Unit

Total surface area Capillary pores (2.6nm - 70nm) surface area Cumulative pore volume (at p/po =0.965 corresponding to a pore diameter ~70nm) Cumulative capillary pore volume Pore volume of pore diameters <4nm Total pore volume Bulk density

m2/g m2/g cm3/g cm3/g cm3/g cm3/g g /cm3

Water/cement ratio 0.4 0.6 0.75

-

84 84.7 0.158

96 92 0.188

0. 052 0.127 1.99

0.153 0.035 0.263 1.83

0.193 0.038 0.325 1.76

Kriechbaum (1994) reports that the specific surface area of various Portland cement  pastes measured by the BET method ranges from 40m2/g to 70m2/g.  Nawy (1997) indicates that the development of the physical microstructure of a cement  paste is related to the hydration process of cement influenced also by total water content  per cubic meter of concrete which sometimes results in concrete bleeding. Excessive water causing concrete bleeding, contributes to a higher capillary porosity, the connectivity of which can be expressed as the apparent volume of permeable voids (VPV) in the cement paste of hardened concrete. Table 2.11 presents total porosity on cement pastes of various w/c ratios.

A good correlation between total porosity and

apparent volume of permeable voids validates the VPV methods as a compelling indicator of total porosity. Table 2.11 Porosity and apparent VPV of various cement pastes (Nawy, 1997) Parameter

Total porosity [%] Apparent VPV [%]

0.26

Water/cement ratio 0.28 0.4 0.5 0.6

0.75

7.5 6.2

8.8 8.0

13 13.3

11.3 12.2

12.5 12.7

12.7 12.5

It is understandable that the microstructure development and porosity of concrete have direct impact on many physical and mechanical properties of concrete, such as density, which consequently might have effects on the acoustic characteristics of concrete. 2-36

However, Diamond (1999) points out that the fundamental microstructural characteristics of concrete are still not perfectly understood.

He also states that

geometrical characterisation including that of fractal geometry, has not been addressed adequately by researchers. Fractal Types and Dimensions

Furthermore, in addition to the usual characterisation of porosity expressed in terms of  pore size distribution, total pore volume, the description of the geometry of pores would complete the picture. Diamond’s (1999) study on concrete porosity using backscatter SEM revealed that pores in concrete are fractal in nature. A geometry of the capillary  pores, which are formed at the time when concrete sets and are remnants of space that existed between cement grains in the fresh concrete is far from the assumed model of regular and spherical model approximation. Diamond (1999), states that the majority of capillary pores are elongated and irregular, and highly convoluted in outline with only some of the more regular triangular or ovoid shapes. Larger pores and voids including entrapped air of typical diameter of about 50 μm are assumed to be nearly spherical. Accidental air pockets which form much larger bubbles are assumed to be of an irregular shape (Neville, 1999). The fractal nature of the concrete microstructure described by Russ (1992) is approximated to a dense object within which exists a distribution of pores, which are of a fractal nature. The boundaries between the solid and pore elements in concrete even in spherical pores, appear to be rough and highly convoluted, all of which affects  porosity measurements. In three-dimensional space a topological dimension is 2 and the fractal dimension of a boundary surface is between 2 and 3 consisting of a fractional component between 0 and 1.

Russ (1992) states that the higher the fractional

component of the fractal dimension, the greater the visual appearance of the roughness of the boundary. Mixed fractals reported by Diamond (1999) indicate the occurrence of two different classes of pore geometry where higher fractal dimension represents the more convoluted capillary pores, and lower fractal dimension describes a less rough  pore system.

2-37

Waste influence on porosity

The nature of cement and the formation of four major minerals in hardened cement  paste also affect porosity of concrete. Chandra (1997) states that some elements contained in waste have an influence not only on hydration reactivity, strength, setting time and durability, but also on microstructure of hardened cement paste. Even small amounts, as low as 0.1% of some elements on top of expected standard amounts included in cement can influence microstructure development of HCP. Increasing the content of some elements such as: addition of 0.7% of phosphorus results in a slight increase of large pores; addition of 0.5% of fluorine results in an increase of total volume of pores and of volume of larger pores; addition of chlorine results in an increase of small capillary pores; addition of 1% of chromium results in an increased amount of large capillary pores. Whereas increased amounts of manganese, and/or magnesium does not affect pore volume and pore size distribution in hardened cement  paste. Table 2.12 presents the pore structure of HCP containing various added elements that might be present in concrete waste. Table 2.12 Porosity of hardened cement paste with added various elements contained in concrete waste (Chandra, 1997) Pore size

Control porosity [%]

P 0.7%

2.8

Added element F Cl Cr Amount added 0.5% 2.0% 0.1% Pore volume [%]

Mn 0.1%

3-5nm 5-50nm 50-500nm 500nm-5μm 5-30μm

0.75 4 8.5 21.5 4.5

1.25 5 12 16 5.5

1.5 4 11 20 6

0.75 2 4.5 25.5 7

0.75 6.5 7 22.5 5.5

1 2.5 7 23.5 5

Total volume

39.25

39.75

42.5

39.75

42.25

39

USE of CONVENTIONAL and NEUTRON SCATTERING TECHNIQUES in TESTING of CONCRETE PROPERTIES

The microstractural characteristic of concrete depends on many factors including t otal  porosity, pore volume, pore size distribution, pore surface area, interfacial features, etc. An adequate understanding of these factors is crucial in devising durable concrete. 2-38

Determination of concrete properties also necessitates application of diverse techniques to examine those properties. properties. Besides gaining information on a specific specific characteristic of concrete, Day (1999) identifies three other main purposes for concrete testing: to establish whether the concrete attained sufficient maturity for stripping, stressing, etc.; to establish whether the concrete is satisfactory for the intended purpose; and to t o detect quality variations in supplied concrete to a given specification. In practice, over 90% of tests on concrete are to test its compressive strength and slump (Day, 1999). Some other tests, apart from the slump test to assess assess fresh concrete  properties include: various various workability tests other other than slump test; test; bleeding, air content, content, setting time, segregation time, unit weight, wet analysis, temperature, and heat generation. Some tests measuring hardened concrete properties include: compressive strength, tensile strength (direct tension, modulus of rupture and indirect/splitting tensile strength) density, shrinkage, creep, modulus of elasticity, absorption, permeability, freeze/thaw resistance, resistance to aggressive chemicals, resistance to abrasion, bond to reinforcement, etc. In assessment of the porosity of concrete there are numerous methods measuring various aspects and and parameters of concrete porosity. porosity. Diamond (1989) states that the following methods can be used: optical microscope, liquid displacement techniques, Mercury Intrusion Porosimetry (MIP), Brunauer-Emmett-Teller (BET) nitrogen or water vapour adsorption, helium inflow, image analysis, Low Angle X-ray Scattering and Nuclear Magnetic Resonance (NMR). It appears that there is a wide range of standard research techniques used to characterise  physical and mechanical mechanical properties properties of concrete. However, in examination examination of concrete  porosity where the range range of pore sizes typically spanning spanning six to seven orders of magnitude, from a fraction of a nanometer (~0.35nm) to a hundred microns (~350 μm) there is no single technique to measure this range and different techniques yield different values (Sereda, 1980). 1980). Commonly, Mercury Intrusion Porosimetry Porosimetry and BET gas adsorption are used to determine pore size distribution, pore volume and specific surface area of pores in concrete. concrete. In terms of non-distractive methods Bhattachrja, Bhattachrja, 2-39

(1993) suggests the Nuclear Magnetic Resonance for measuring total pore volume and surface area of cementitious materials: and Allen (1999) reports on the use of Small Angle X-ray and Neutron Scattering (SAXS and SANS) in the examination of microstructures of cementitious materials. materials. The use of non-destructive methods methods has increased as they provide supplementary data to those customarily used in concrete research (MIP and BET).

2.8.1 Conventional Techniques Conventional techniques to measure microstructure of cementitious materials include Mercury Intrusion Porosimetry, BET nitrogen (or other gases used as sorbents) adsorption, and various permeability measurement measurement techniques. Analysis of optical microscope, TEM or SEM images, can also be used in the determination of pore size distribution, and specific surface area in cementitious materials. However, all those conventional techniques require special sample preparation which also includes complete drying drying of the samples. This can damage pore pore structure in cementitious materials leading to erroneous results. The two most commonly used volumetric experimental methods of estimating pore volume and diameter are the gas adsorption method (also known as the BrenauerEmmett-Teller method BET) and the mercury porosimeter method (MIP) which measures a wide range of pore pore sizes. The range of pore size measured measured with BET is  between 0.35nm and 70nm, and measured with MIP between 3.5nm and 200 μm (Hansen, 1987) whereas Thomas (1997) reports that the range of pore size measured by BET nitrogen adsorption is from 15Å to 200Å, and the range measured by MIP is  between 100Å and 100μm. Although MIP is commonly used, Diamond (1999) argues that MIP does not provide an adequate approximation to the true size distribution of pores in hydrated cement systems, as the microstructure is damaged by the sample preparation regime, and is further damaged during examination, as the mercury is forced under significant  pressure. In the MIP method, mercury is forced into the pore system of a porous material and absorbed volume is directly measured.

Although the mercury 2-40

 porosimetery method is used used to estimate pore volume and pore pore size distribution of pores as small as 35Å in diameter, Thomas (1997) states that it is impractical to force mercury into pores of diameter lower than 100Å. Despite a widespread acceptance of the MIP method in examination of porosity of hydrated cementitious materials, Cook (1999) gives an account of the destructive nature of this technique. technique.

The pressure pressure of mercury generated generated during an experiment experiment is

approximately 300MPa, which in case of a non-continuous pore system enforces mercury penetration by by breaking through pore pore walls. MIP is said to give smaller smaller than actual porosity as mercury cannot penetrate either pores smaller than 10nm, or isolated  pores whose walls walls cannot be broken broken by high pressure. pressure. Brenauer-Emmett-Teller (BET) Method

The gas adsorption method is based on the phenomenon of gas condensation in narrow  pores at pressures lower than saturated vapour pressure of the examined material. Classically, volumes of gas progressively adsorpted by the material, and those of gas  progressively desorpted, are represented by the isotherms (plot of relative pressure p/p 0 versus pore pore volume) volume) (Gregg, 1982).

There are six principal types of adsorption

isotherms that are indicative indicative of the porosity of of tested material and pore pore nature. As an example: Type-1 isotherms are characteristic of microporous adsorbents with pore size  below 20Å; Type-2 and Type-3 isotherms are indicative of non-porous solids; Type-4 and Type-5 isotherms are characteristic of a mesoporous solid with pore size range  between 20Å and and 500Å. Figure 2.3 presents presents the six main types types of adsorption isotherms. isotherms.

2-41

Figure 2.3 Six types of adsorption isotherms (Gregg, 1982)

The non-porous solids subjected to BET nitrogen examination produce isotherms where the path of the adsorption line on a plot of relative pressure (p/p 0) versus amount of gas adsorbed is the same as the desorption line. The desorption isotherms produced by porous solids produce a hysteresis loop formed  by path difference between adsorption and desorption of gas in the pore structure of tested material.

Figure 2.4 presents the adsorption and desorption isotherms

characteristic to porous solids of microstructure of continuously graded pores.

Figure 2.4 Adsorption and desorption isotherms for a porous solid (Gregg, 1982)

2-42

When the pore size distribution of solid material microstructure has limited pore size (is not continuously graded) the adsorption and desorption isotherms tend to create a hysteresis loop above (0.0 < p/p 0 < 0.5) and below (~0.8 < p/p 0 < 1) reversible regions of the isotherms. Figure 2.5 shows the adsorption and desorption isotherms of solids with limited pore size.

Figure 2.5 Adsorption and desorption isotherms for a solid with limited pore size (Gregg, 1982)

Thomas (1997) states that the shape of the hysteresis in the adsorption-desorption isotherm is related to the pore geometry of tested porous material. The isotherms can form five types (Type A, B, C, D and E) of hysteresis loops. Figures 2.6, 2.7 and 2.8  present possible hysteresis loops and possible pore structure.

Figure 2.6 Type A hysteresis loop and possible pore structures (Thomas, 1997)

2-43

The Type A hysteresis is indicative of capillary pores of the ink-bottle shape where the  body of the pore has a radius much higher that the pore entrance. The width between the adsorption branch and the desorption branch of the isotherm indicates the difference in diameter of the neck and body of the ink-bottle-shaped capillary pores (Thomas, 1997).

Figure 2.7 Type B hysteresis loop and possible pore structures (Thomas, 1997)

Type B hysteresis is produced when in the desorption cycle, the pores are emptied at relative pressure corresponding to the width of the pore and the adsorption branch has a step at relative pressure of unity. Type B hysteresis is characteristic of pores formed by  parallel plates or ink-bottle-shaped pores with body diameter of 1,000Å.

Figure 2.8 Type C hysteresis loop and possible pore structures (Thomas, 1997)

Type C hysteresis is characteristic of spheroidal pores with circular cavity radius having various-sized entrances. 2-44

Scanning Electron Microscopy

Scanning Electron Microscopy (SEM) is one of many tools to view microscopic objects which also include the Optical Microscope (OP) and the Transmission Electron Microscope (TEM). These tools are used to investigate topographical properties of the surface of materials and to quantitatively analyse elemental composition of materials (Brinkies, 1995). Table 2.13 presents a comparison between OP, TEM and SEM. Table 2.13 Comparison between SEM and other microscopes

Specimen shape Specimen state Magnification Ultimate resolution

OM

TEM

SEM

Thin film Solid, liquid in the atmosphere x 2000 200nm

Thin film Solid in vacuum

Bulk Solid in vacuum

x 50 – 1,500,000 1Å

x 10 – 1,000,000 5Å

Some of the advantages and important features the SEM offers are the ease of changing magnifications and stereographic image display. SEM is fully computerised and images can be photographed and analysed using image analysis techniques. Specimens in sizes of 50nm to 1cm can be viewed using SEM at a range of magnifications of up to 1,000,000 times, whereas, TEM can be used to view objects in size between 1Å and 50nm (JEOL, n.d.). The SEM can operate in various modes generating various kinds of signals including Xray, secondary electrons (SEI) imaging, and backscattered electron (BEI) imaging which provide different types of information. The SEI mode is used in investigation of surface topography of bulk materials whereas in the BEI mode compositional observations of a deeper layer of specimen surface can also be achieved.

The

 backscatter electron image and data on elemental composition are a function of the average atomic number of the substances composing the specimen surface. In the X-ray mode of operation of SEM, an elemental analysis of specimen can be examined. When an incident electron beam irradiates a surface of a specimen, characteristic X-rays are emitted and then are detected on a signal detector. Analysis using the energy dispersive X-ray spectrometer (EDS) determines elements and their atomic weight concentration in

2-45

an examined area of specimen. To identify elements in unknown specimens, the multichannel analyser has to be calibrated using known standard specimens (JEOL, n.d.). The function of a common SEM is based on the operating principle of a cathode ray tube (CRT). In the CRT, heating of a tungsten filament generates electrons at a  potential of several thousand volts.

A generated electron beam is then focussed and

directed by electromagnetic coils, also known as condenser lenses, onto examined specimens.

A beam of 10nm diameter continually scans the surface of a specimen

where electrons are scattered and emitted from the specimen’s surface. Scattered and emitted electrons produce images of the topographical or composition features of the sample’s surface which are detected by secondary electron or backscattered electron detectors (JEOL, 1998). One of the limitations of SEM is that only very small area of the sample can be examined (Brinkies, 1995). Although the X-ray Diffraction (XRD) is considered as a more appropriate method to determine compound composition, when the objectives of the project were taken into account, the SEM was deemed as an adequate method as it contributes to SANS experimentation, and porosity and microcracks can be investigated. Apparent Volume of Permeable Voids Test

A whole range of tests to indirectly measure durability of concrete is available. Although the apparent volume of permeable voids (VPV) test, which is relatively simple and quick, is preferred in Victoria (Vicroads, 1997). The VPV test allows relatively quick measurement of the amount of space occupied by interconnected,  permeable voids, which is regarded as a compelling indication of durability of concrete. Some of the advantages of the VPV test is that simple laboratory equipment including standard drying oven, heated water bath, water tank and balance are required to carry out the test, and that it can be completed in 5 days.

2-46

Day (1999) indicates that concrete specified for major projects in Melbourne has 100 year durability requirements and adequate indication of such durability is an equivalent measure of apparent volume of permeable voids in hardened concrete of 9%.

2.8.2 Small Angle Neutron Scattering Despite the lack of a rigorous approach to data interpretation and availability of equipment the neutron scattering techniques have a potential to be very attractive research methods allowing non-destructive examination of bulk materials in their natural saturated state. The non-distractive nature of these techniques combined with the lack of special preparation or pre-treatment of tested samples, are the main advantages of neutron or x-ray scattering over conventional techniques. The lower accessible pore size with the use of SAXS is of 30nm and by SANS is of 1Å, which extends the accessibility of conventional techniques such as MIP and BET gas adsorption by an order of magnitude. Livingston (1995) states that because SANS can access porosity not accessible to those examined by BET or MIP, the microstructure characteristics including pore size distribution and inner surface area will not be commensurate. In neutron scattering, the microstructure information is in the form of a scattering  profile where the intensity of the scattering follows the scattering vector. The scattering is dominated by the interface between solid and empty, or filled with water (D 2O or H2O) pores in concrete matrix. The scattering profile is a kind of modified Fourier transform of the solid-pore structure over many length-scales ranging from nanometer to micrometer, which at high scattering angles directly follows Porod’s law. Determination of quantitative microstructural parameters of cementitious materials is only possible if the composition and density of the solid are accurate. One way of determining possible composition and density of hydrated cements and concrete is by contrast variation studies where there is a strong difference between scattering from D2O bound solid (C-S-H particles) and H 2O bound C-S-H (Allen, 1987). Radlinski and Hilde (2003) state that elemental composition measured with SEM can be used to calculate density of solid part of concrete matrix. Table 2.14 presents a comparison of

2-47

 parameters determined by conventional and neutron or X-ray scattering techniques related to microstructure of cementitious materials (Livingston, 1995). Table 2.14 Range of porosity related parameters measured by conventional and neutron scattering techniques

Conventional Methods Parameter Method Total surface area BET gas adsorption Total porosity MIP Capillary porosity MIP Gel porosity MIP -

SANS/SAXS Parameter Total surface area Porod surface area Fractally rough surface area Surface fractal dimension Total porosity Capillary porosity Gel porosity C-S-H volume Gel volume + fractal porosity Volume fractal dimension Volume fractal correlation length C-S-H globule radius

Symbol Si Sv Sf Dv Φ N ΦC-S-H ΦGEL Dv E ν Rg

It can be seen that neutron scattering techniques seem to be a very versatile tool that offer wider than conventional techniques, possibility of examining a broader range of  parameters, and as such have the ability to provide more precise characterisations of microstructure of cementitious materials including RA Concrete, and cement paste residue of RC Aggregate. Despite pioneering work done by Winslow, Allen, Hansen, Livingston and others, Sabine (1995) points out that interpretation of SANS profiles from hydrated cement is not simple and is also a function of scattered particle shape, which can be quasi-spheres, quasi-spheres plus disks, or characterised by fractal parameters.

Small Angle Neutron Scattering

Livingston (1995), states that the materials science of hydrated Portland cement  products is still not completely worked out. This is partially due to the fact that conventional methods have certain limitations and are destructive to a certain extent. Inner porosity examinations by either MIP or BET gas adsorption methods, or outer  porosity examinations with Scanning Electron Microscopy require sample preparation 2-48

that has the potential to disturb its microstructure, or the techniques themselves can be destructive. Various neutron scattering techniques including the Ultra Small Angle  Neutron Scattering (USANS), Small Angle Neutron Scattering (SANS), Small Angle X-ray Scattering (SAXS), and Small Angle Light Scattering (SALS) provide nondestructive alternatives to MIP, BET nitrogen adsorption, or SEM. They provide information on the statistical average structure over the whole sample (Allen, 1987). In each of these techniques radiation is elastically scattered by a sample and the resulting scattering pattern is analysed to provide information about the size, shape and orientation of some component of the sample as well as fractal dimensions of the concrete microstructure. Aldridge (1995) states that the gel pores (5Å – 100Å) and medium capillary pores (100Å – 500Å) can be investigated with the use of SANS. The Australian SANS instrument emits the incident flux of neutron radiation of wavelengths from 0.06nm to 10nm, which allows a range of pores from 6Å to 10nm in diameter to be probed. The incident flux of neutron radiation is collimated and directed at a sample. Some of incident radiation is transmitted by the sample, some neutrons are absorbed and some are scattered. The scattered neutrons are detected by a detector which is positioned at some distance and scattering angle. As the scattering from H2O and D2O is very different, the SANS technique, other than its non-destructive nature for examining bulk material, offers a possibility of undertaking the isotopic contrast variation experiments yielding information on the  permeation and diffusion of different types of water into porous systems (Allen, 1987). However, the hydrogen present in light water (H2O) contributes to a multiple scattering. Livingston (1995) proposes that to avoid the possibility of multiple scattering the thickness of samples of cementitious materials should not exceed 1mm if the samples are prepared with H2O. Concrete specimens prepared with D2O can be thicker. Aldridge (1995) relates multiple scattering with the wavelength of the neutron beam and sample thickness and proposes sample thickness of 2mm for wavelengths greater than 4Å.

2-49

Although it is recommended that samples for SANS examination do not require oven drying at 1050C, if the results are to be compared with those porosity data derived from conventional methods, the samples can be oven dried. Livingston (1995) also suggests that representative specimens of cement paste including aggregate should be examined, as the microstructure development of concrete greatly depends on the amount of aggregate and its characteristics. Livingston (1995) indicates that neat cement paste and concrete specimens should be  prepared, cured and stored in a CO2 free atmosphere as carbonation can rapidly take  place in such thin samples. Aldridge (1995) concludes that although the volumes of capillary pores of cement  pastes of 0.25 and 0.8 water/cement ratio should differ significantly the SANS scattering profiles obtained in his experiment were not different. He speculates that scattering was from amorphous globules of C-S-H for data obtained at the scattering wave vector (Q) of 0.0056 to 0.05Å-1and from the gel pores in the C-S-H at Q range  between 0.05 and 0.21Å-1.

He calculated the average radii of globular entities

 producing scattering profile to be 350Å and gel pores to be 50Å.

2.9

USE of CONCRETE in ACOUSTIC BARRIERS

2.9.1 Road Traffic Noise and Noise Mitigation Methods Kotzen (1999) states that an urban population in modern cities is constantly exposed to excessive noise due to an increase in large-scale transport facilities including motorways and railways. Noise levels generated in urban transport infrastructure depend mainly on type, volume, and speed of the transportation noise source. Road traffic noise is characterised by two parameters; sound pressure level (SPL) and frequency spectrum of the sound. Intensity of road traffic noise is predominantly a function of the volume of traffic as well as type and proportion of vehicle types (heavy and light vehicle) in the traffic flow. The sound pressure level generated by vehicles is mainly a function of engine and exhaust emitted noise, which is further amplified by the 2-50

noise from a tyre-surface interaction. The tyre-surface interaction noise contribution is  predominant at frequencies above 1,000Hz. The dominant noise of transportation traffic lies between frequencies of 100Hz and 1,000Hz. Figure 2.9 illustrates the noise spectra of a typical road traffic noise caused by heavy and light vehicle traffic (Nelson, 1987).

Figure 2.9 Noise spectra for heavy and light vehicles traffic (Nelson, 1987)

Average road traffic noise generated by heavy vehicles exceeds 76dB(A) (which corresponds to no-weighted SPL of 85dB) in the low frequency range (up to 200Hz) and in the mid-frequency range (between 200Hz and 500Hz). In the high frequency range  between 500Hz and 2kHz it exceeds 73dB(A) (which corresponds to absolute measure of noise of 80dB). Light vehicle road traffic noise is generally lower than heavy vehicle noise by approximately 9dB at the corresponding frequencies. Human response and effects on health and effects on day and night activities to excessive transportation noise vary, and are dependent on many social and environmental aspects. Although Nelson (1987) identifies the SPL of 65dB(A) as the absolute upper tolerated acceptable limit by the community, the noise generated by transportation infrastructure is much higher than the acceptable limit of 65dB(A). To mitigate excessive transportation noise in urban environments, the noise levels of newly constructed transportation infrastructure must now comply with the specified limits. The most stringent requirements on transportation noise reaching residents 2-51

alongside transport infrastructure are in countries such as the Netherlands and Denmark where the day noise level is limited to 55dB(A) (Kotzen, 1999). In Australia, the Victorian State Road Authority, Vicroads sets the 63dB(A) as the limit for newly constructed roads and 68dB(A) for existing roads (Vicroads, 1994). There are various measures that can be employed to mitigate road traffic noise which include increasing the distance between noise source and affected residential areas, the use of tunnels, false cuttings and earth mounds, employment of quieter road surfaces,  provision of better noise insulation of residential dwellings, and installation of acoustic  barriers. Currently acoustic barriers seem to be the most practical way of mitigating transportation noise.

Despite that, Hemond (1983) questions the effectiveness of

highway sound reflective barriers to reduce low frequency noise generated by heavy vehicles, due to diffraction (bending of waves around barriers) of long wavelength noise. Kotzen (1999) states that in urban environments the most feasible option in reducing the annoyance of transportation noise is to erect vertical, cantilevered or bio barriers. In terms of acoustic performance there are three types of barriers; reflective, dispersive, and sound absorptive. Kotzen (1999), states that in Europe there is a tendency to use absorptive barriers that acoustically soften the environment because the height of the barrier can be reduced and/or to reduce the buffer zone width to achieve required noise reduction. Day (2004)  proposes a general guide to illustrate the benefits of using absorptive barriers over reflective type barriers. For example, to reduce noise level to 60dB(A) a 3-meter high absorptive barrier or 4-meter high reflective barrier would be required. Day (2004) indicates that the buffer zone reduction of approximately 50 meters can be achieved by substituting a 4-meter high reflective barrier with an absorptive barrier of the same height. 2.9.2 Sound Absorbing Barriers In Victoria, reflective barriers made from concrete, timber or plastic are usually installed alongside busy arterial roads with lesser use of sound absorptive barriers. 2-52

 Noise attenuation of reflective barriers generally depends on their location, height and mass per unit area. This type of barrier is effective in situations when a noise source is close to a residential area and the opposite side is an open space. Materials used for reflective barriers include timber, pre-cast concrete, fibreglass reinforced concrete, metal, and plastic. The life span of reflective barriers depends on material used, and ranges from 15 years in the case of timber, to 40 years for concrete barriers (Schubert, 1988). The main difference between reflective and dispersive barriers is the added benefit of the dispersion of sound waves in any desired direction, usually upwards or downwards, which prevents the noise build-up, which is likely to occur in the case of classic vertical reflective barriers. The same type of materials as for reflective barriers can be used to manufacture dispersive barriers. The acoustic effectiveness of sound absorptive barriers depends on the internal structure of the material used for the barrier, which must allow incident sound waves to enter the matrix of the barrier. Part of the energy of the sound waves can then be dissipated and the energy is converted into a combination of mechanical vibration and heat. The remaining sound energy is reflected back into the noise source (Kotzen, 1999). Figure 2.10 presents a cross-section of an absorptive barrier.

Figure 2.10 Cross-section of typical absorptive-type barrier (GRC, 1990)

2-53

A subcategory of sound absorptive barriers is a reactive barrier, which according to Kotzen (1999) incorporates a certain amount of cavities or resonators per area. The reactive barrier is designed to attenuate specific frequencies of noise. In this type of  barrier, sound waves enter the cavities through the openings in the barrier surface. A variety of materials can be used to manufacture any type of acoustic barrier with the inclusion of concrete. In general, concrete barriers are designed and used as reflective or dispersive barriers, however, Kotzen (1999) reports on the concrete reactive New Jersey ‘Laghi’ barrier, and absorptive woodfibre concrete, and granular concrete  barriers. Granular and woodfibre concrete absorptive barriers comprise ‘either wood fibres or small cementaceuos balls that are used as the aggregate’ Kotzen, (1999). Figure 2.11 presents a granular concrete barrier.

Figure 2.11 Granular concrete barrier (Kotzen, 1999)

The acoustic performance of sound absorbing barriers is represented by two parameters, sound absorption coefficient (α), and also calculated average known as noise reduction coefficient (NRC). Table 2.15 presents a summary of the acoustic performance of commercially available sound absorbing barriers used in Melbourne (Schubert 1988; Hollow Core 2000; Unicrete Concrete 2000; GRC Composites 1990)

2-54

Table2. 15 Sound absorption coefficient ( ) and NRC of commercial barriers Frequency 1  / 3 Octave Band [Hz]

Rocla

Soundtrap

Boral

Durisol

Krusscrete

Anti Sound

100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000

0.91 1.05 1.07 1.18 1.07 1.02 1.08 1.08 1.04 1.02 0.93 0.86 0.77 0.75

0.29 0.23 0.25 0.48 0.68 0.81 1.02 1.17 1.2 1.13 0.97 0.85 0.82 0.92

0.5 0.75 0.85 0.95 0.95 0.9 0.5 0.45 0.4 0.3 0.28 0.25 0.25 0.25

0.1 0.15 0.2 0.25 0.35 0.45 0.6 0.8 1 1.1 1 0.85 0.8 0.9

0.2 0.25 0.25 0.4 0.5 0.7 0.95 0.95 0.9 0.75 0.7 0.6 0.4 0.4

0.13 0.21 0.28 0.49 0.74 0.91 0.96 0.99 1 1 0.99 0.96 0.99 0.92

Unibloc5060

0.15 0.25 0.7 0.95 0.75

GRC

0.9 1.05 1.06 1.14 1.1 1.05 1.06 1.08 0.95 0.94 0.91 0.88 0.8 0.75

Noise reduction coefficient

0.96

0.94

0.48

0.76

0.64

0.91

0.66

0.96

In Victoria, the Victorian Road Authority, Vicroads sets the requirements for sound absorbency of the noise attenuation barriers in its standard specifications for road and  bridge works (Vicroads, 1997). Section DC270A.05 specifies acoustic performance and structural requirements related to sound absorptive noise barriers. It states that; •

the minimum mass density of surface area shall not be less than 15kg/m 2

•

the minimum area of wall tested for sound absorption in a reverberation room shall  be 12m2

• •

appropriate structural design standards should be used  barriers should be designed such that any panel or fragments of the panels will not fall on roadway if vehicles impact them

•

 barriers should not have holes or gaps allowing noise to pass through them

•

materials used must be corrosion and ultraviolet radiation resistant

•

the sound absorption coefficient should not be less than specified in Table DC270A.051 (please refer to Table 2.16) Table 2.16 Sound absorption coefficient ( ) – Vicroads requirements Frequency [Hz] Sound absorption coefficient

125 0.70

250 0.80

500 0.90

1000 0.90

2000 0.80

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Calculated Noise Reduction Coefficient of barriers satisfying Vicroads requirements is  NRC 0.82. There are numerous factors related to the choice of a type of sound barrier and material used for its manufacture, including engineering, environmental and cost considerations. Kotzen, (1999) identifies the following engineering and environmental considerations: 

structural strength including wind, self-weight, static and dynamic loading



impact effects from stones and vehicles



safety of vehicles on collision with a barrier



durability of barrier materials including resistance to chemical agents, heat, and ultra-violate light



maintenance requirements

Increasingly, ecological sustainability aspects are also considered when materials for  barrier design are chosen. Considerations include: the minimal energy used to transport raw materials; manufacture and erection of the barrier; the maximum use of locally available materials, preferably from renewable sources (Kotzen, 1999). Visual integration and landscape integrity are also very important factors in devising transportation infrastructure and although they are often given greater weight than cost of the barrier in a cost-benefit analysis, in most situations, cost remains a dominant factor in choosing the type and material for acoustic barriers. The cost of the barrier is influenced by: design and materials of the barrier; support structure and foundation; method of installation; maintenance requirements such as frequency inspections, repair, cleaning, and treatment. Kotzen (1999) identifies maintenance cost as a significant factor in selection of barrier and states that the cost of maintenance of concrete barriers is relatively low when compared with other barrier types and materials. Schubert (1988) specifies costs for the supply and erection of a range of different types of acoustic barriers in Victoria. Although the prices are outdated, the relativity of cost  between different types of barrier has not changed. In general, the cost of supply and installation of an absorptive barrier is on average twice that of the dispersive barrier and three times as much as for the reflective.

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Apart from noise attenuation performance, structural properties, cost and aesthetics, Schubert (1988) identifies the service life of acoustic barrier as an important criterion in selecting material for the barrier. Commonly used reflective timber barrier has a lifespan of up to 15 years whereas pre-cast concrete is up to 40 years. Limited studies on sound absorbency of no-fines concrete were undertaken by the Road Construction Authority however, as Schubert (1988) reports, it has been found that the  barrier produced of no-fines concrete made from a single size 20mm aggregate,  performed poorly in the 100Hz – 1000Hz frequency range. Table 2.16 presents sound absorption capabilities of no-fines concrete in a frequency range between 63Hz and 500Hz (Vicroads, 2001). Table 2.16 Sound absorption coefficient ( ) of no-fines concrete barrier (Vicroads, 2001) Frequency [Hz]

coefficient

63 0.17

80 0.16

100 0.15

125 0.12

160 0.1

200 0.08

250 0.07

315 400 0.09 0.1

500 0.1

Day (2004) argues that using sound absorptive barriers instead of purely reflective  barriers has two main benefits; it reduces the height of the barrier to achieve required sound pressure levels at the receiver or reduces a buffer zone between the barrier and the receiver. Table 2.17 presents the benefits of using sound absorbing barriers (Day, 2004). Table 2.17 Buffer zone width for reflective and absorptive barriers Barrier [m]

 None 3 6

height

Barrier type and buffer zone width [m] Reflective Absorptive

500 180 35

500 90 20

Reduction of the height of the barrier leads to various benefits such as a reduction of the amount of material used to manufacture a barrier, reduction of transportation and installation requirements, resulting in the overall benefit of reduced use of resources and energy.

The use of sound absorbing barrier also results in lessening of visual

obstruction, which is due to the lower height of the barrier. Alternatively, if the same 2-57

height of sound absorbing barrier remains, the buffer zone can be reduced. The application of sound absorptive barriers results in more urban land being made available for residential development, or for quieter urban environments adjacent to transportation routes.

2.10

SUMMARY

The literature review outlined the current state of knowledge on aggregate for concrete, especially on alternative coarse aggregate produced from concrete waste. The rationale for concrete recycling and the use of alternative aggregate in concrete technology is examined. It is supported with examples of the successful use of concrete made from recycled concrete aggregate. The literature review also introduces transportation noise  pollution and the means of mitigating it in urban environments. Sound reflecting and absorptive barriers have been reviewed and concrete barriers evaluated. A number of conclusions have been reached based on available literature, discussions and consultations with professionals from relevant engineering, research and scientific disciplines, and the three main issues identified. Firstly, there is growing evidence of the feasibility of substituting RC Aggregate for natural aggregate in concrete manufacturing and also an increased use of selected RC Aggregate in concrete production. Secondly, there is evidence that some properties of RC Aggregate such as porosity, shape and surface texture of the aggregate could have some positive impacts on the mechanical and acoustic properties of concrete made from such aggregate. And thirdly, the inherent and purposely introduced porosity of RA Concrete leads to increased sound absorption of acoustic barriers made from such concrete. This thesis reports upon the use of RC aggregate in a high value product, an absorptive  barrier for sound to replace reflective barriers. This new device utilises the properties of RC Aggregate and enhances the performance of the acoustic barrier. 2-58

The following chapter outlines methodology, and experimental and developmental  programs devised to increase the understanding of properties of locally produced selected RC Aggregate, differentiate the aggregate from commonly used natural aggregate, and characterises concrete and acoustic barrier made from the 14/10mm RC Aggregate.

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CHAPTER 2 – LITERATURE REVIEW ........................................................................1 2.1 INTRODUCTION ............................................................................................1 2.2 CONCRETE CONSTITUENT MATERIALS .................................................2 2.2.1 Coarse Aggregate......................................................................................4 2.3 ALTERNATIVE CONSTITUENT MATERIALS in CONCRETE ................6 2.4 CONCRETE WASTE and CONCRETE RECYCLING ..................................9 2.4.1 Alternative Sources of Coarse Aggregate...............................................10 2.4.2 Current Applications for Recycled Concrete Products ...........................13 2.4.3 Under-utilisation of Recycled Concrete Aggregate................................13 2.5 COMPARISON between NATURAL and RECYCLED CONCRETE AGGREGATE ............................................................................................................15 2.5.1 Shape and Surface Texture......................................................................16 2.5.2 Particle Size Distribution ........................................................................17 2.5.3 Water Absorption....................................................................................20 2.5.4 Particle Density and Bulk Density ..........................................................21 2.5.5 Impurities and Foreign Materials in RC Aggregate................................22 2.5.6 Aggregate Porosity..................................................................................23 2.5.7 Other Properties ......................................................................................24 2.6 NORMAL DENSITY and NO-FINES CONCRETE .....................................24 2.7 COMPARISON between STANDARD and RECYCLED AGGREGATE (RA) CONCRETE ......................................................................................................27 2.7.1 Physical and Mechanical Properties........................................................27 2.7.2 Acoustic Properties .................................................................................32 2.7.3 Porosity and Fractal Dimensions ............................................................33 2.8 USE of CONVENTIONAL and NEUTRON SCATTERING TECHNIQUES in TESTING of CONCRETE PROPERTIES .............................................................38 2.8.1 Conventional Techniques........................................................................40 2.8.2 Small Angle Neutron Scattering .............................................................47 2.9 USE of CONCRETE in ACOUSTIC BARRIERS.........................................50 2.9.1 Road Traffic Noise and Noise Mitigation Methods................................50 2.9.2 Sound Absorbing Barriers.......................................................................52 2.10 SUMMARY....................................................................................................58

Figure 2.1 Concrete recycling plant setup (courtesy Recycling Industries Pty. Ltd.) ....12 Figure 2.2 Schematic representation of aggregate grading in an assembly of aggregate  particles: (a) uniform size, (b) continuous grading, (c) replacement of small sizes  by large sizes, (d) gap-graded aggregate, (e) no-fines grading (Mindess, 1981).... 19 Table 2.9 Porosity a 15-year-in-service concrete (Eighmy, 2003) .................................35 Table 2.10 Porosity of neat cement paste (Hansen, 1987)..............................................36 Table 2.12 Porosity of hardened cement paste with added various elements contained in concrete waste (Chandra, 1997)..............................................................................38 Figure 2.3 Six types of adsorption isotherms (Gregg, 1982) ..........................................42 Figure 2.4 Adsorption and desorption isotherms for a porous solid (Gregg, 1982) .......42 Figure 2.5 Adsorption and desorption isotherms for a solid with limited pore size (Gregg, 1982) ..........................................................................................................43 Figure 2.6 Type A hysteresis loop and possible pore structures (Thomas, 1997) ..........43 2-60