REACTIVE POWDER CONCRETE
Atri Agrawal and Nirav B. Thakkar
Reactive powder concrete is a developing composite material that will allow the concrete industry to optimize material use, generate economic benefits, build structure that are strong, durable and sensitive to environment. A comparison of the physical, mechanical and durability properties of RPC and high performance concrete shows that RPC possesses better strength and lower permeability compared to HPC. Introduction
RPC is not just a simple mixture of cement, water and aggregates. Quite often, it contains mineral components and chemical admixtures having very specific characteristics, which impart specific properties to the concrete. HPC leads the way to the achievement of the maximum compressive strength of the order 120-150 MPa. However at such a level of strength, the coarse aggregate becomes the weakest link in concrete. In order to increase the compressive strength of concrete even further, the only way is to remove the coarse aggregate. aggregate. This philosophy has been employed in what is today known as Reactive Powder Concrete . RPC is an ultra high strength and high ductility cementitious composite with .
advanced mechanical and chemical properties. It consists of special concrete where the micro structure is optimized by precise gradation of all particles in the mix to yield maximum density. It extensively uses the pozzolanic properties of highly refined silica fume and optimization of Portland cement chemistry to produce the highest strength hydrates The concept of RPC was first developed by P. Richard and M. Cheyrezy and RPC was first produced in the early 1990’s by researchers at Bouygues laboratory in France and the world’s first RPC structure, the Sherbrooke Bridge in Canada was constructed in July 1997. RPC was nominated for the 1999 Nova awards from the Construction Innovation Forum. RPC has been used successfully for isolation and containment of nuclear wastes in Europe due to its excellent impermeability Reactive powder concrete can lead to new horizon for the civil engineering field in India. Booming infrastructure development in India demands new technologies to be implemented to make structures stronger and highly durable. Reactive powder concrete provides ultra high strength and high durability which meets the present as well as future demand for the infrastructure development. .
COMPOSITION OF RPC Materials
a) Cements Cements were selected for their wide compositional range. Their C3A content, determined by quantitative XRD (internal standard), varies from 1% up to 8%.Their Blaine fineness ranges from 320 m2 /kg to 430 m2 /kg . Their soluble alkali content is very low and is comprised between 0.16% and 0.38%. b) Silica fumes Their pozzolanic potential is measured through the quantity of reacted CaO during a Chapelle test. Their specific surfaces are in range 11to12 m2 /g. The ions in solution given by silica fume particles were measured by inductively coupled plasma (ICP) with water to solid ratio of one. c) Sand Sand should be of good hardness, readily available and low cost. Its particle size ranges from 0.15mm to 0.6 mm. The type of sand generally used is natural and crushed. d) Quartz powder It is maximum reactive during heat treatment. Its particle size ranges from 0.005mm to 0.025mm. It should be crystalline in nature.
e) Steel Fibers It should have good aspect ratio and should be able to improve ductility. Its length ranges from 13mm 25mm. It should be straight. f) Water It should be clean from all the organic impurities as well as other dust particles. It should not be saline in nature. g) Super plasticizers A copolymer of acrylic ester (CAE), a polynaphtalene Sulfonate (PNS) and a polymelamine sulfonate (PMS) are normally employed for the purpose. These admixtures are synthetic polymers. CAE contains carboxylic (COOÿ) instead of sulfonic (SO3ÿ) anionic groups present in PNS and PMS polymers. The molar ratio of anionic groups per organic monomer unit: acrylic group (CAE), naphthalene (PNS), melamine (PMS) is 1 for PNS and PMS, and is much lower than 1 for CAE copolymer. To obtain a good workability and minimal secondary effects, the SP dosage were optimized with a rheometer, which is specially designed for RPC.The dosage is expressed as the mass percentage of the solids of the SP related to the mass of cement used.
Table 1 Composition of RPC
Cement Silica fume 730 240 All quantities are in kg/ m3
Quartz flour 220
Sand 1040
water 150
superplasticizer 15
PRINCIPLES FOR DEVELOPING RPC
a) Elimination of coarse aggregate for enhancement of homogeneity b) Utilization of pozzolanic properties of silica fume. c) Optimal usage of super plasticizer to reduce W/C and at the same time improves compaction d) Post- set heat treatment for enhancement of the microstructure e) Addition of small sized steel fibers to improve ductility.
PROPERTIES a) Fresh concrete properties
Workability of RPC mixtures measured using the mortar flow table test is in the range of 120 to 140 %. On the other hand, workability of HPC mixtures using slump test is in the range of 120 to 150 mm. Density of fresh RPC and HPC mixtures in the range of 2500 to 2650 kg/m3. b) Compressive strength
The compressive strength of RPC is usually higher than HPC. Compressive strength at early ages is also very high for RPC. Compressive strength is one of the factors linked with the durability of a material. The maximum compressive strength of RPC is in the range of 150 to 200 Mpa,
while the maximum strength for HPC is 75 Mpa. The incorporation of fibers did not affect the compressive strength of HPC significantly. c) Flexural strength
Plain RPC is found to possess marginally higher flexural strength than HPC. The increase of flexural strength of RPC with the addition of steel fibers is higher than that of HPC. RPC with compressive strength 200 Mpa should have an approximate flexural strength of 40 Mpa. The reason for low flexural strength can be due to long fibers and their relatively higher diameter. Fiber reinforced RPC has the potential to be used in structures without any additional steel reinforcement. This cost reduction in reinforcement can compensate the increase in cost by elimination of coarse aggregates in RPC to some extent.
d) Water absorption and permeability
The percentage of water absorption of RPC is very low compared to that of HPC. This quality of RPC is one among the desired properties of nuclear waste containment materials. The incorporation of fibers and use of heat curing are seen to marginally increase the water absorption. The permeability of RPC is almost 7 times lower than that of HPC
.
INFLUENCE OF FLY ASH AND SLAG CONTENT ON COMPRESSIVE STRENGTH OF STEAM CURED SPECIMENS
The hydration of the cement compounds proceeds more rapidly but the hydration reactions are basically the same. However, this higher rate of hydration will lead to a lower ultimate strength.
The reactions of pozzolans are also accelerated by the higher curing temperatures. Fig. shows the influence of FA content on compressive strength of steam cured (90 C) and standard cured specimens. It can be seen from the figure that, the increasing amount of FA content up to 60% replacement level, generally caused an increase in compressive strength at all curing conditions. After 12 days of steam curing, compressive strength of control specimens, FA20, FA40 and FA60 mixtures are 154, 152, 159 and 155MPa, respectively.
Fig. shows the influence of PS content on compressive strength of steam cured (90 C) and standard cured mortars. It can be seen from the figure that, with the increasing amount of PS content up to 60% replacement level, generally an increase in compressive strength is observed at all curing conditions. After 12 days of steam curing, compressive strength of control specimens, PS20, PS40 and PS60 mixtures are 154, 150, 156 and 145 MPa, respectively. Although, using 80% PS as a cement replacement caused large reduction in compressive strength for standard curing, this mixture showed 134Mpa.
Furthermore, using 80% FA as a cement replacement, caused disintegration during steam curing period. The compressive strength of 6 days and 12 days steam cured specimens are more than that of standard cured specimens in water during 28 days at all FA replacement levels.
FA, which has high lime content, used with SF and generally mechanical properties were improved by FA replacement under steam curing possibly due to the hydration reactions between extra lime released from FA and SF compressive strength after 12 days steam curing.
INFLUENCE OF DIFFERENT CURING CONDITIONS ON COMPRESSIVE STRENGTH
hydration process especially with combined mineral admixtures.
APPLICATIONS
Depending on the FA content, the compressive strength of steam cured (12 days 90 1C) specimens relative to the high-pressure steam cured specimens (8 h, 2MPa, and 210 1C) is shown in Fig. In steam curing, 60% FA replacement and 80% PS replacement enabled greater compressive strength than the high pressure steam curing. Fig. shows the effectiveness of steam curing and high pressure steam curing with respect to the standard curing. It can be concluded from the figure; compressive strength at all mixtures after high-pressure steam curing (8 h) is greater than the strength of water cured specimens. Furthermore, the effectiveness of steam curing increases with increasing PS content, while this situation is not valid for increasing FA replacement. Similar trend is also observed with steam curing. In other words, 12 days of steam curing improved the compressive strength of all mixtures with respect to the standard curing. These findings findings can be explained by the superior inter-reactions of the ingredients (FA, PS, SF and cement). It is clear that steam curing provides perfect conditions for complete and accelerated hydration media for
a) It can can be be used used in in nuclea nuclearr waste waste containment structures due to lower water absorption and low permeability. b) It can can be used used for for heavi heavily ly load loaded ed structures like bridges due to its ultra high strength and durability. c) It can also also be used used for for mari marine ne structures due to high sulphate resistance. d) As no no reinf reinforc orceme ement nt is is needed needed for fibred RPC it minimizes overall cost.
ADVANTAGES
a) As frac fractur turee toughn toughness ess is high higher, er, RPC exhibits high ductility. b) Since Since RPC RPC is an an Ultra Ultra dense dense micro structure, porosity and permeability is less and therefore can be used for waste storage holding facility. c) RPC RPC has lim limit ited ed shri shrink nkage age,, increased corrosion resistance and so can be used in aggressive chemical environments.
LIMITATIONS
a) In RPC RPC mix mix desi design gn low low cos costt component of conventional concrete are replaced by more expensive elements b) Since Since RPC RPC is in its its infa infancy ncy,, a long term properties are not yet known.
REACTIVE POWDER CONCRETE USED IN THE SIDEWALK SYSTEM OF THE QINGHAI-TIBET RAILWAY BRIDGE
same time, the structure members of sidewalk system for the Qinghai-Tibet railway, including flats, brackets, and rails, have been manufactured.
1. Introduction
2. Material and mechanics function of the reactive powder concrete
The Qinghai-Tibet railway lies in Qinghai-Tibet plateau in the west area of China, at an altitude of more than 4,000 meters and in a high intensity earthquake region. The 576-km railway is being built on frozen earth. The climate conditions are harsh in that area: low air temperature, concentrated rain and snow, and heavy sandstorms. The area also has complicated geology conditions, and groundwater causes corrosion. The conditions mentioned above require the concrete used for the bridges to have superior mechanical properties and high durability. As the subsidiary facility of the railway bridge, the sidewalk system needs to support the load from the passersby and from some small machines and equipment for maintenance of the bridge. The traditional sidewalk system of the railway bridge is composed of angle steel brackets and a concrete flat. The traditional sidewalk system will be prone to be out of order due to low durability of concrete and angle steel bracket and thus needs a great deal of maintenance to keep it in service. To adapt to severe conditions on the Qinghai-Tibet plateau and to meet the requirements of railway developments, reactive powder concrete (RPC) has been developed by the common production process workflow with national material in this research. RPC has high strength and high durability, including excellent impermeability of chloride and frost-resistance. At the
2.1. Material preparation
This research concerns 42.5# Portland cement, active mineral powder, including high-quality silica fume whose ratio surface exceeds 200,000 cm3/g, low-need water superfine fly ash, super plasticizers that have good compatibility with cement, whose rate of reducing water is above 32 percent, and quartz sands whose grain size are 0.16 to 0.315 mm, 0.315 to 0.63 mm, 0.63 to 1.0 mm, and that make up the most close-grained preparation. It is detected that grain size of the two materials is very different, and they form discontinuous preparation in the course of arranging cementitious property and aggregates. Therefore, higher mineral powder whose grain size is between cementitious properties and aggregates has been produced. 2.2. Mechanics function of the reactive powder concrete In order to study the influence of the different mixture ratio on the RPC material function, the following has been studied: the influence of w/b ratio, admixture of mineral additive, compounding between different preparation aggregates and cementitious property, admixture of additive, admixture of steel fibers, curing temperature, and curing system. Through hundreds of mixture ratio experimentation, the optimal mixture ratio, whose w/b ratio is 0.16, whose curing temperature is 75°C and whose
mechanics function, is confirmed on the basis of routine agitating and moulding technology. It is known that concrete’s 28-day compressive strength is 168.6 MPa, bending strength is 20.6 MPa, and elastic modulus is 46.8 Gpa, 3. Durability of Reactive Powder Concrete In such severe conditions as on the Qinghai-Tibet plateau, the durability, security, and service life of structures will be distinctly reduced due to the reasons such as the initial flaw of the concrete cast in-suite in a low temperature, subjectivity of the concrete to frosting and thawing, and erosion caused by groundwater brine. Chloride impermeability is the ability of concrete to resist high-pressed liquid such as water penetrating the concrete. The wearing off of concrete, either physical or chemical, is concerned with water penetration. So, impermeability of chloride almost becomes the core problem of durability of most concrete. 3.1. Chloride impermeability There was no water leakage when hydraulic pressure varies from 0.1 to 1.6 MPa with increase of 0.1 MPa/8h. When hydraulic pressure was taken away, there was only 2.7-mm penetration. As a result, the conclusion can be reached that RPC has excellent chloride impermeability, caber-resistance, and corrosion-resistance. 3.2. Frost resistance These specimens were dropped in 20oC water for 4 days. The temperature ranged from negative 17oC to negative 13oC, while another from 4 oC to 8 oC, and every cycle lasted 4 hours. h ours. After 800 cycles of freezing and thawing, relative modulus of elasticity of RPC was still
100%, whereas the lost weight ratio was 0, and the durability ratio was 2.67. From the results, the conclusion can be reached that RPC has excellent frostresistance. Therefore, RPC is more suitable for the bridges of Qinghai-Tibet Railway than conventional concrete. 4. Study on the RPC Sidewalk System There are a lot of different d ifferent problems with conventional concrete sidewalks such as corrosion, rust of reinforcing steel bars, and breakability of concrete slab. The steel brackets rust in a conventional concrete sidewalk. As a result, a conventional concrete sidewalk system requires maintenance every year. Dead weight of the traditional sidewalk system is greater than that of the RPC sidewalk system, which makes bridges have a bad dynamic performance. To improve the conventional sidewalk system, RPC sidewalk system has been developed in Beijing Jiao tong University. An RPC sidewalk system composed of pre-cast slabs, brackets, and rails is assembled in-situ. RPC, based on optimal mixture ratio, meets the design requirements of sidewalk that the compressive strength should be greater than 120 MPa, and the split tensile strength should be greater than 12 MPa. The slump of RPC is greater than 180 mm. With an excellent workability, wo rkability, RPC can meet the requirements of construction. The sidewalk system made of RPC has such merits as small deadweight, excellent durability, low cost, and minimum maintenance. Thus, RPC is more suitable for the sidewalk system of Qinghai-Tibet Railway than conventional concrete.
CONCLUSIONS
1). Elimination of coarse aggregates combined to optimization of the granular mixture allows the obtention of an homogenous and dense cementitious matrix that exhibits high mechanical performance 2). Application of a confining pressure to the fresh concrete combined to excess water removal further improves the density. Induced micro cracking does not impede compressive strength enhancement. 3). Addition of small size steel fibers gives RPC a ductile behavior.
ACKNOWLEDGEMENT
The authors would like to thank Prof U.V.Dave for his guidance and support.
REFERENCES
1). A.S Dili and Manu Santhanam, Investigations on concrete powder concrete: A developing ultra highstrength technology, 2004. 2). Halit Yazici, The effect of curing conditions on compressive strength of ultra high strength concrete with high volume mineral admixtures, 2006. 3). Pierre Richard and Marcel Cheyrezy, Composition of Reactive powder concretes, 1995 4). Ching- Tsung Liu and Jong- Shing Huang, Highly flowable reactive powder mortar as a repair material, 2007
5). Helene Zanni, Marcel Cheyrezy, Vincent Maret, Samuel Philippot and Pedro Nieto, Investigation of hydration and pozzolanic reaction in reactive powder concrete, 1995
