BENDABLE CONCRETE.pdf

BENDABLE CONCRETE

CHAPTER 1

INTRODUCTION Conventional concretes are almost unbendable and have a strain capacity of only 0.1% making them highly brittle and rigid. This lack of bendability is a major cause of failure under strain and has been a pushing factor in the development of an elegant material namely, bendable concrete also known as Engineered Cementitious Composites (ECC.) This material is capable to exhibit considerably enhanced flexibility. A bendable concrete is reinforced with micromechanically designed polymer fibres. ECC is made from the same basic ingredients as conventional concrete but with the addition of High-Range Water Reducing (HRWR) agent as it is required to impart good workability. However, coarse aggregates are not used in ECCs (hence it is a mortar rather than concrete). The powder content of ECC is relatively high. Cementitious materials, such as fly ash, silica fume, blast furnace slag, etc., may be used in addition to cement for increasing the paste content. Typically, ECC uses 2% by volume of short, discontinuous fibres. ECC incorporates super fine silica sand and tiny Polyvinyl Alcoholfibres covered with a very thin (nanometer thick), slick coating. This surface coating allows the fibre to begin slipping when they are over loaded so they are not fracturing.

Fig. 1.1: Response of ECC under Flexural Loading.

DEPT. OF CIVIL ENGG., M.TECH, NCET.

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BENDABLE CONCRETE ECC prevents the fibre from rupturing which would lead to large cracking. Thus an ECC deforms much more than a normal concrete but without fracturing. Fig. 1.1 shows the behaviour of ECC under flexural loading and it can be seen that the beam can deform well without direct failure. The different ingredients of ECC work together to share the applied load. ECC has proved to be 50 times more flexible than traditional concrete, and 40 times lighter, which could even influence design choices in skyscrapers (Vol. 4, No. 1, February 2015 © 2015 IJSCER.) Additionally, the excellent energy absorbing properties of ECC make it especially suitable for critical elements in seismic zones.


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BENDABLE CONCRETE

CHAPTER 2

SCOPE AND BACKGROUND Since ECC are more flexible than traditional concrete, it acts more like metal than glass. Traditional concrete is considered to be a ceramic, brittle and rigid. It can suffer catastrophic failure when strained in an earthquake or by routine overuse. It is studded with specially-coated reinforcing fibres that hold it together. ECC remains intact and safe to use at tensile strains up to 5%. Traditional concrete fractures and can’t carry a load at 0.01 % tensile strain. In recent times, builders reinforce concrete structures with steel bars in order to keep cracks as small as possible. But they’re not small enough to heal. So, water and deicing salts can penetrate till steel, causing corrosion that further weakens the structure. Li’s self-healing concrete needs no steel reinforcement to keep crack width tight, thereby eliminating corrosion.


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BENDABLE CONCRETE

CHAPTER 3

INGREDIENTS OF BENDABLE CONCRETE Engineered cementitious composite is a composition of cement, sand, fly ash, water, small amount of admixtures and an optimal amount of fibres. In the mix, coarse aggregates are deliberately not used because property of ECC Concrete is formation of micro cracks with large deflection. Coarse aggregates increase crack width, which is contradictory to the property of ECC Concrete.

3.1. CEMENT Cement used is Ordinary Portland cement. Numerous organic compounds which are used for adhering or fastening materials are called cements. But, these are classified as adhesives, and the term cement alone means a construction material. Blastfurnace slag may also be used in some cements and the cement is called Portland slag cement (PSC). The colour of the cement is due mainly to iron oxide. In the absence of impurities, the colour would be white. Ordinary Portland cement (OPC) – 53 grade (Ultratech Cement) is normally used.

3.2. SAND [FINEAGGREGATE] Fine aggregate / natural sand is an accumulation of grains of mineral matter derived from the disintegration of rocks. It is distinguished from gravel only by the size of the grains or particles. But it is distinct from clays which contain organic materials. Sands that have been sorted out and separated from the organic material by the action of currents of water or by winds across arid lands are generally quite uniform in size of grains. Usually commercial sand is obtained from river beds or from sand dunes originally formed by the action of winds. The most commercially used are silica sands, often above 98% pure. Beach sands usually have smooth, spherical to ovoid particles from the abrasive action of waves and tides and are free of organic matter. The white beach sands are largely silica but may also be of zircon, monazite, garnet, and other minerals.


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BENDABLE CONCRETE Sand is used for making mortar and concrete and for also used for polishing and sandblasting. Sands containing a little clay are used for making molds in foundries. Clear sands are employed for filtering water. The weight varies from 1,538 to 1,842 kg/m3, depending on the composition and size of grain. The fine aggregate passing through 4.75 mm sieve with a specific gravity of 2.68 is normally used. The grading zone of fine aggregate will be zone III as per Indian Standard specifications.

3.3. SUPER PLASTICIZER Super plasticizer used is Melamine Formaldehyde Sulphonate. This is used to control rheological properties of fresh concrete. Super plasticizers are additives to fresh concrete which help in dispersing the cement uniformly in the mix. This is achieved by their deflocculating action on cement agglomerates by which water entrapped in the groups of cement grains is released and it is available for workability. Typically super plasticizer increase slump from say 5 cm to about 18-20 cm without addition of water. When used to achieve reduction in mixing water they can reduce water up to 15-20% and hence decrease W/C ratio by same amount. This results in increase in strength and other properties like density, water tightness. Where thin sections are to be cast super plasticizer can increase workability to pumpable level and almost no compaction is required. This helps in avoiding honeycombing. The permeability of concrete is a guide to its durability. Gross porosity is usually due to continuous passage in the concrete due to poor compaction or cracks which can be minimized by the use of super plasticizer, the incorporation of which provides increased workability maintaining low w/c ratio. It is reported that coefficient of permeability of cement paste reduces considerably with the reduction in w/c ratio. Thus super plasticizer can be used effectively to improve the properties of concrete and avoid defect. Melamine based Super plasticizer are used to assess their effectiveness in improving durability. Melamine based super plasticizer are reported to be the best and hence chosen for the research work.

3.4. FLY ASH Fly ash used is pozzocrete dirk 60. And specifications provided by suppliers are given in Table 1. In RCC construction, use of fly ash has been successful in reducing heat generation without loss of strength, increasing ultimate strength beyond 180 days, and DEPT. OF CIVIL ENGG., M.TECH, NCET.

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BENDABLE CONCRETE providing additional fines for compaction. Replacement levels of primary class fly ash have ranged from 30-75% by solid volume of cementitious material. In proportioning mixes, for minimum paste volumes, one of principal function of a fly ash is to occupy void space which would otherwise be occupied by cement or water. Occupation of void space with water would obviously result in reduction in concrete strength. The fact is that even a small amount of free lime liberated from cement is sufficient to react with large volume of fly ash. The huge amount of fly ash is produced in the thermal power stations. Class F fly ash is utilized so the acquisition cost may be reduced.

3.5. PVA FIBRES PVA fibre has suitable characteristics as reinforcing materials for cementitious composites. High modulus of elasticity, durability, tensile strength and bonding strength with concrete matrix are some of its desirable properties. PVA fibre has high strength and modulus of elasticity (25 to 40GPa) compared to other general organic fibre which are widely used for cement reinforcing. Fibre elongation is about 6-10%. The tensile strength of fibre is between 880MPa to 1600MPa. One of the remarkable characteristics of PVA fibre is strong bonding with cement matrix. The layer of Ca(OH)2 called ITZ(Interfacial transition zone) around PVA fibre is formed as white part. And in case of PP, this layer is not observed. It is known that PVA is easy to make complex cluster with metal hydroxide. It is assumed that Ca+ and OH- ions in cement slurry are attracted by PVA and makes Ca(OH)2 layer. It seems reasonable to think that Ca(OH)2 layer plays DEPT. OF CIVIL ENGG., M.TECH, NCET.

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BENDABLE CONCRETE important role for bonding strength. Fig. 2 shows images of surface for coarse PVA fibre after single fibre pull-out test. This image implies that surface of PVA fibre is peeled by Ca(OH)2 layer and this phenomena is related to strong bonding between PVA fibre and cement matrix.

3.6. WATER Water fit for drinking is generally considered fit for making concrete. Water should be free from acids, oils, alkalis, vegetables or other organic Impurities. Soft waters also produce weaker concrete. Water has two functions in a concrete mix. Firstly, it reacts chemically with the cement to form a cement paste in which the inert aggregates are held in suspension until the cement paste has hardened. Secondly, it serves as a vehicle or lubricant in the mixture of fine aggregates and cement.


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BENDABLE CONCRETE

CHAPTER 4

MIX DESIGN The mix design for ECC Concrete is basically based on Micromechanics design basis. Micromechanics is a branch of mechanics applied at the material constituent level that captures the mechanical interactions among the fibre, mortar matrix, and fibre–matrix interface. Typically, fibres are of the order of millimeters in length and tens of microns in diameter, and they may have a surface coating on the nanometer scale. Matrix heterogeneities in ECC, including defects, sand particles, cement grains, and mineral admixture particles, have size ranges from nano to millimimeter scale. However the micromechanics based mix design requires pull test to be carried on the PVA fibres, which is not possible in the laboratory. Hence the ideal mix proportion given in the literature of ECC Concrete is used as the guidelines to determine the proportion of various constituents in the concrete. The ideal Mix proportion which can be taken as reference is given in the following sections.

4.1. PROPORTIONING OF CONCRETE Initially, mix proportion was 1:0.8004:1.1996, PVA fibre 1% and super plasticizer dose was 1040.47 ml/bag and water to cementitious material ratio was 0.274. But by using this proportion workability was not achieved. Hence for second trial, the mix proportion was changed to 1 : 0.9 : 1.1 and PVA fibre percentage increased to 1.2% by keeping same dose of super plasticizer and increasing water to cementitious material ratio to 0.3048. Third trial mix proportion was 1:1:1 and PVA fibre 1.2%, super plasticizer dosage was reduced to 600ml/bag and water to cementitious material ratio was 0.33. Forth trial mix proportion was 1:0.9:1.1, PVA fibre percentage 1.2%, super plasticizer dosage 600ml/bag along with water to cementitious material ratio was 0.3118. To achieve workability various trials were taken. In fourth proportion super plasticizer dose was reduced to obtain workability. For each trial mix, 3 cubes were casted and cured using the accelerated curing tank and were tested to obtain desired strength requirement. After testing cubes for each trial, the trail mix no. 3 was considered as most suitable and hence the final mix proportion. However in order to increase the workability of concrete the


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BENDABLE CONCRETE water to cementitious ratio was increased to 0.35. (Volume 2, Issue 4, April 2013, IJAIEM)

4.2. CASTING PROCEDURE OF ECC- CONCRETE The performance of the ECC Concrete is influenced by the mixing. This means that a proper and good practice of mixing can lead to better performance and quality of the ECC Concrete. The quality of the concrete is also influenced by the homogeneity of the mix material, Flexural Test on Slab during the mixing and after the placement of fresh concrete. A proper mix of concrete is encouraged to the strength of concrete and better bonding of cement with the PVA fibres. Once the concrete mix design is finalized, the mixing is carried out. The mixing of ECC Concrete is carried out by using hand mixing. The procedure of hand mixing is as follows:- Add sand, cement, 50% of fly ash and 50% water and super plasticizer. Add slowly remaining quantity of fly ash, water and super plasticizer. Once the homogenous mixture is formed, add the PVA fibres slowly. Mix all the constituents till the fibres are homogenously mixed in the matrix.

4.3.

PLACING,

COMPACTION

AND

CASTING

OF

CONCRETE SPECIMENS. Before placing of concrete, the concrete mould must be oiled for the ease of concrete specimens stripping. The oil used is a mixture of diesel and kerosene. Special care is taken during the oiling of the moulds, so that there are no concrete stains left on the moulds. Once the workability test of ECC Concrete is done, the fresh concrete must be placed into the concrete moulds for hardened properties tests. During the placing of fresh concrete into the moulds, tamping is done using Tamping rod in order to reduce the honeycombing. After placing the concrete into the moulds, vibrations are done using a table vibrator. The vibration of concrete allows full compaction of the fresh concrete to release any entrained air voids contained in the concrete. If the concrete is not compacted to a proper manner, the maximum strength of the concrete cannot be achieved. After vibration operation, the levelling of concrete is done on the surface of the concrete. Levelling is the initial operation carried out after the concrete has been placed and compacted. After the levelling of the fresh concrete is done, the concrete in the mould is left overnight to allow the fresh concrete to set. DEPT. OF CIVIL ENGG., M.TECH, NCET.

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BENDABLE CONCRETE

4.4. CURING OF CONCRETE SPECIMEN After leaving the fresh concrete in the moulds to set overnight, the concrete specimens in the moulds were stripping. The identification of concrete specimens was done. After 24 hours, all the concrete specimens were placed into the curing tank with a controlled temperature of 250C in further for 28 days for the hardened properties test of concrete. Curing is an important process to prevent the concrete specimens from losing of moisture while it is gaining its required strength. Lack of curing will lead to improper gain in the strength. After 28 days of curing, the concrete specimens are removed from the curing tank to conduct hardened properties test of ECC Concrete.


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BENDABLE CONCRETE

CHAPTER 5

COMPARISON WITH OTHER COMPOSITE MATERIALS

Properties

FRC

Design N.A. Methodology

Fibre

Matrix

Common HPFRCC

Use high Vf

ECC

Micromechanics based, minimize Vf for cost and processibility

Any type, Vf usually Mostly steel, Vf Tailored, polymer fibres, less than 2%; df for usually > 5%; df ~ 150 Vf usually less than 2%; steel ~ 500 micrometre df < 50 micrometre micrometre

Coarse aggregates

Fine aggregates

Controlled for matrix toughness, flaw size; fine sand

Interface

Not controlled

Not controlled

Chemical and frictional bonds controlled for bridging properties

Mechanical Properties

Strain-softening:

Strain-hardening:

Strain-hardening:

Tensile strain 0.1%

<1.5%

>3% (typical); 8% max

Crack width

Typically several Typically < hundred micrometres, micrometres unlimited beyond strain-hardening 1.5% strain

Unlimited

100 during

Note: FRC=Fibre-Reinforced Cement. HPFRCC=High-Performance Fibre Reinforced Cementitious Composites


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BENDABLE CONCRETE

CHAPTER 6

FIELD APPLICATIONS ECC have found use in a number of large-scale applications in Japan, Korea, Switzerland, Australia and the U.S. These include: 

The Mitaka Dam near Hiroshima was repaired using ECC in 2003. The surface of the then 60-year-old dam was severely damaged, showing evidence of cracks, spalling, and some water leakage. A 20 mm-thick layer of ECC was applied by spraying over the 600 m2 surface.



Also in 2003, an earth retaining wall in Gifu, Japan, was repaired using ECC. Ordinary portland cement could not be used due to the severity of the cracking in the original structure, which would have caused reflective cracking. ECC was intended to minimize this danger; after one year only microcracks of tolerable width were observed.



The 95 m (312 ft.) Glorio Roppongi high-rise apartment building in Tokyo contains a total of 54 ECC coupling beams (two per story) intended to mitigate earthquake damage. The properties of ECC (high damage tolerance, high energy absorption, and ability to deform under shear) give it superior properties in seismic resistance applications when compared to ordinary portland cement. Similar structures include the 41-story Nabeaure Yokohama Tower (four coupling beams per floor.)



The 1 km (0.62 mi) long Mihara Bridge in Hokkaido, Japan was opened to traffic in 2005. The steel-reinforced road bed contains nearly 800 m3 of ECC material. The tensile ductility and tight crack control behavior of ECC led to a 40% reduction in material used during construction.



Similarly, a 225-mm thick ECC bridge deck on interstate 94 in Michigan was completed in 2005. 30 m3 of material was used, delivered on-site in standard mixing trucks. Due to the unique mechanical properties of ECC, this deck also used less material than a proposed deck made of ordinary portland cement. Both the University of Michigan and the Michigan Department of Transportation are monitoring the bridge in an attempt to verify the theoretical superior durability of ECC; after four years of monitoring, performance remained undiminished.


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BENDABLE CONCRETE 

The first self-consolidating and high-early-strength ECC patch repair was placed on Ellsworth Road Bridge over US-23 in November 2006. The high-early-strength ECC can achieve a compressive strength of 23.59 ± 1.40 MPa (3422.16 ± 203.33 psi) in four hours and 55.59 ± 2.17 MPa (8062.90 ± 315.03 psi) in 28 days, allowing for fast repair and re-opening the session to traffic. The high-early-strength ECC repair has shown superior long-term durability in field conditions compared to typical concrete repair materials.


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BENDABLE CONCRETE

REFERENCES [1]Kong, H.J., Bike, S.G., and Li, V.C., 2002, “Constitutive rheological control to develop a self-consolidating engineered cementitious composite reinforced with hydrophilic poly(vinyl alcohol) fibres,” in press, Cement and Concrete Comp [2]Li, V. C.; Wu, H. C.; Maalej, M.; Mishra, D. K., "Tensile Behavior of Cement-based Composites with Random Distributed Steel Fibres," J. Am. Ceram. Soc., V.79, No.1, 1996, pp. 74-78. [3]ECC Technology International Network, [4]Li VC. Advances in strain hardening cement based composites. In: Engineering Foundation Conference on Advances in cement and concrete New Hampshire, July, 1994.p.24-9. [5]Li VC, Kanda T. Engineered cementitious composites for structural applications. ASCE J Mater Civil Eng 1998, [6] Li, V.C. and Hashida, T., Engineering Ductile Fracture In Brittle Matrix Composites, J. of Materials Science Letters, 8, 1993, pp. 898-901. [7]. Li, V.C., Chan, C.M. and Leung, C.K.Y., Experimental Determination Of The Tension- Softening Curve In Cementitious Composites, Cement and Concrete Research, 17(3) 1987, pp. 441-452. [8]. ACI Committee 224, Control of Cracking In Concrete Structures, ACI Manual of Concrete Practice Part 3-1991: Use of Concrete in Buildings-Design, Specifications, and Related Topics, American Concrete Institute, Detroit, 1991. [9].Li VC. From micromechanics to structural engineering––the design of cementitious composites for civil engineering applications. [10] JSCE J Struct Mech Earthquake Eng 1993;10(2):37–48.


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BENDABLE CONCRETE.pdf

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