Reactive Powder Concrete Introduction Reactive Powder Concrete (RPC) is a developing composite material that will allow the concrete industry to optimize material use, generate economic benefits, and build structures that are strong, durable, and sensitive to environment. A comparison of the physical, mechanical, and durability properties of RPC and HPC (High Performance Concrete) shows that RPC possesses better strength (both compressive and flexural) and lower permeability compared to HPC. This page reviews the available literature on RPC, and also presents the results of laboratory investigations investigations comparing RPC with HPC. Specific benefits and potential applications of RPC have also been described. High-Performance High-Performance Concrete (HPC) is not just a simple mixture of cement, water, and aggregates. It contains mineral components components and chemical admixtures having very specific characteristics, which give specific properties to the concrete. The development of HPC results from the materialization of a new science of concrete, a new science of admixtures and the use of advanced scientific equipments to monitor concrete microstructure. HPC has achieved the maximum compressive compressive strength in its existing form of microstructure. 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. This philosophy has been employed in Reactive 1 Powder Concrete (RPC) . Reactive Powder Concrete (RPC) was developed in France in the early 1990s and the world’s first Reactive Powder Concrete structure, the Sherbrooke Bridge in Canada, was erected in July 1997. Reactive Powder Concrete (RPC) is an ultra high-strength and high ductility cementitious composite with advanced mechanical and physical properties. It consists of a special concrete where the microstructure is optimized by precise gradation of all particles in the mix to yield maximum density. It uses extensively the pozzolanic properties of highly refined silica fume and optimization of the Portland cement chemistry to produce the highest 1 strength hydrates . The concept of reactive powder concrete was first developed by P. Richard and M. Cheyrezy and RPC was first produced in the early 1990s 2 by researchers at Bouygues’ laboratory in France . A field application of RPC was done on the Pedestrian/Bikeway Bridge in the city of 3 Sherbrooke, Quebec, Canada . 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 4 due to its excellent impermeability . The requirements for HPC used for the nuclear waste containment structures of Indian Nuclear Power Plants are normal compressive strength, moderate E value, uniform density, good workability, and high 5 durability . There is a need to evaluate RPC regarding its strength and durability to suggest its use for nuclear waste containment structures in Indian context. Composition of Reactive Powder Concrete RPC is composed of very fine powders (cement, sand, quartz powder and silica fume), steel fibres (optional) and superplasticizer. The superplasticizer, used at its optimal dosage, decreases the water to cement ratio (w/c) while improving the workability of the concrete. A very dense matrix is achieved by optimizing the granular packing of the dry fine powders. This compactness gives RPC ultra-high strength and 6 durability . Reactive Powder Concretes have compressive strengths ranging from 200 MPa to 800 MPa. 1
Richard and Cheyrezy indicate the following principles for developing RPC: 1. 2. 3. 4. 5. 6. 7.
Elimination of coarse aggregates for enhancement of homogeneity Utilization of the pozzolanic properties of silica fume Optimization of the granular mixture for the enhancement of compacted density The optimal usage of superplasticizer to reduce w/c and improve workability Application of pressure (before and during setting) to improve compaction Post-set heat-treatment for the enhancement of the microstructure Addition of small-sized steel fibres to improve ductility
Table 1 lists salient properties of RPC, along with suggestions on how to achieve them. Table 2 describes the different ingredients of RPC and their selection parameters. The mixture design of RPC primarily involves the creation of a dense granular skeleton. Optimization of the granular 7 mixture can be achieved either by the use of packing models or by 8 particle size distribution software, such as LISA [developed by Elkem ASA Materials]. For RPC mixture design an experimental method has been preferred thus far. Table 3 presents various mixture proportions for 1,3,9,10 RPC obtained from available literature .
Table 1: Properties of RPC enhancing its homogeneity and strength Property of Recommended Description RPC Values Coarse aggregates are replaced by fine sand, Reduction with a in reduction in aggregate the size of size the coarsest aggregate by a factor of about 50. Improved mechanical Enhanced properties of mechanical the paste by properties the addition of silica fume
Types of failure eliminated
Maximum size Mechanical, of fine sand is Chemical & 600 µm Thermomechanical
Young’s Disturbance modulus values of the in 50 GPa – 75 mechanical Gpa range stress field.
Volume of the Reduction paste is at least in 20% greater By any Limitation of aggregate than the voids external sand content to matrix index of non- source (e.g., ratio compacted formwork). sand. Table 2: Selection Parameters for RPC components Selection Particle Function Types Parameters Size Good hardness 150 µm Give Natural, Sand Readily to strength, Crushed available and 600 µm Aggregate low cost. C3 S : 60%; Binding C2S : 22%; material, 1 µm OPC, Cement C3A : 3.8%; Production to Medium C4AF: 7.4%. of primary 100 µm fineness (optimum) hydrates Quartz Powder fineness Max. 5 µm Crystalline Components
reactivity to during 25 µm heattreating Filling the voids, Procured Enhance from Very less 0.1 µm rheology, Ferrosilicon Silica fume quantity of to Production industry impurities 1 µm of (highly secondary refined) hydrates L : 13 – 25 mm Good aspect Improve Steel fibres Ø: Straight ratio ductility 0.15 – 0.2 mm Less Reduce Polyacrylate Superplasticizer retarding _ w/c based characteristic Table 3: RPC mixture designs from literature P. Richard and M. S. A. V. S. 1 3 9 10 Cheyrezy Bouygues Matte Staquet [1995] [1997] [1999] [2000] Non 12 mm 25 mm Fibred Fibred fibred fibres fibres Portland 1 1 1 1 Cement Silica fume 0.25 0.23 0.25 0.23 Sand 1.1 1.1 1.1 1.1 Quartz Powder -- 0.39 -- 0.39 Superplasticizer0.016 0.019 0.016 0.019 Steel fibre --- 0.175 0.175 Water 0.15 0.17 0.17 0.19 Compacting ----pressure Heat treatment 20ºC 90ºC 20ºC 90ºC temperature
1
1
1
0.324 1.423 0.296 0.027 0.268 0.282
0.325 1.43 0.3 0.018 0.275 0.2
0.324 1.43 0.3 0.021 0.218 0.23
--
--
--
90ºC
90ºC
90ºC
The major parameter that decides the quality of the mixture is its water
demand (quantity of water for minimum flow of concrete). In fact, the voids index of the mixture is related to the sum of water demand and entrapped air. After selecting a mixture design according to minimum water demand, optimum water content is analyzed using the parameter relative density (d 0/dS). Here d0 and d S represent the density of the concrete and the compacted density of the mixture (no water or air) respectively. Relative density indicates the level of packing of the concrete and its maximum value is one. For RPC, the mixture design should be such that the packing density is maximized. Microstructure enhancement of RPC is done by heat curing. Heat curing is performed by simply heating (normally at 90°C) the concrete at normal pressure after it has set properly. This considerably accelerates the pozzolanic reaction, while modifying the microstructure of the 1 hydrates that have formed . Pre-setting pressurization has also been 1 suggested as a means of achieving high strength . The high strength of RPC makes it highly brittle. Steel fibres are generally added to RPC to enhance its ductility. Straight steel fibres used typically are about 13 mm long, with a diameter of 0.15 mm. The fibres are introduced into the mixture at a ratio of between 1.5 and 3% by 1 volume . The cost-effective optimal dosage is equivalent to a ratio of 2% 3 by volume, or about 155 kg/m . Mechanical Performance and Durability of RPC The RPC family includes two types of concrete, designated RPC 200 and RPC 800, which offer interesting implicational possibilities in different areas. Mechanical properties for the two types of RPC are given in Table 4. The high flexural strength of RPC is due to the addition of steel fibres. Table 5 shows typical mechanical properties of RPC compared to a 11 conventional HPC of compressive strength 80 MPa . As fracture toughness, which is a measure of energy absorbed per unit volume of material to fracture, is higher for RPC, it exhibits high ductility. Apart from their exceptional mechanical properties, RPCs have an ultra-dense microstructure, giving advantageous waterproofing and durability characteristics. These materials can therefore be used for industrial and 1 nuclear waste storage facilities . RPC has ultra-high durability characteristics resulting from its extremely low porosity, low permeability, limited shrinkage and increased corrosion resistance. In comparison to HPC, there is no 4 penetration of liquid and/or gas through RPC . The characteristics of RPC given in Table 6, enable its use in chemically aggressive environments and where physical wear greatly limits the life of other
concretes . Table 4: Comparison of RPC 200 and RPC 800
Pre-setting pressurization Heat-treating Compressive strength (using quartz sand) Compressive strength (using steel aggregate) Flexural strength
RPC 200 None
RPC 800 50 MPa 250 to 20 to 90°C 400°C 170 to 230 490 to 680 MPa MPa 650 to 810 -MPa 30 to 60 45 to 141 MPa MPa
Table 5: Comparison of HPC (80 MPa) and RPC 200 Property
HPC (80 MPa)
Compressive strength Flexural strength Modulus of Elasticity Fracture Toughness
80 MPa 7 MPa 40 GPa <10³ J/m²
RPC 200 200 MPa 40 MPa 60 GPa 30*10³ J/m²
Table 6: Durability of RPC Compared to HPC Abrasive Wear Water Absorption Rate of Corrosion Chloride ions diffusion
9
10
2.5 times lower 7 times lower 8 times lower 25 times lower
Limitations of RPC In a typical RPC mixture design, the least costly components of conventional concrete are basically eliminated or replaced by more expensive elements. The fine sand used in RPC becomes equivalent to the coarse aggregate of conventional concrete, the Portland cement plays the role of the fine aggregate and the silica fume that of the cement. The mineral component optimization alone results in a substantial increase in cost over and above that of conventional concrete (5 to 10 times higher than HPC). RPC should be used in areas where substantial weight savings can be realized and where some of the remarkable characteristics of the material can be fully utilized2. Owing to its high durability, RPC can even replace steel in compression members where durability issues
are at stake (e.g. in marine condition). Since RPC is in its developing stage, the long-term properties are not known. Experimental study at IIT Madras Materials Used The materials used for the study, their IS specifications and properties have been presented in Table 7. Mixture Design of RPC and HPC
Considerable numbers of trial mixtures were prepared to obtain good RPC and HPC mixture proportions. 8 Particle size optimization software, LISA [developed by Elkem ASA Materials] was used for the preparation of RPC and HPC trial mixtures. Various mixture proportions obtained from the available literature were also studied. The selection of best mixture proportions was on the basis of good workability and ideal mixing time. Finalized mixture proportions of RPC and HPC are shown in Table 8. Table 7: Materials used in the study and their properties Sl. No. 1
2 3 4
5
6
Sample Cement, OPC, 53-grade [IS. 12269 – 1987] Micro Silica [ASTM C1240 – 97b] Quartz Powder Standard sand, grade-1 [IS. 650 – 1991] Standard sand, grade-2 [IS. 650 – 1991] Standard sand, grade-3 [IS. 650 – 1991]
Specific Gravity
Particle size range
3.15
31 µm – 7.5 µm
2.2
5.3 µm – 1.8 µm
2.7
5.3 µm – 1.3 µm
2.65
2.36 mm – 0.6 mm
2.65
0.6 mm – 0.3 mm
2.65
0.5 mm – 0.15 mm
7
8
9
10 11
Steel fibres (30 mm) [ASTM A 820 – 96] Steel fibres (36 mm) [ASTM A 820 – 96] 20 mm Aggregate [IS. 383 – 1970] 10 mm Aggregate [IS. 383 – 1970] River Sand [IS. 383 – 1970]
7.1
length: 30 mm & dia: 0.4 mm
7.1
length: 36 mm & dia: 0.5 mm
2.78
25 mm – 10 mm
2.78
12.5 mm – 4.75 mm
2.61
2.36 mm – 0.15 mm
Table 8: Mixture Proportions of RPC and HPC Materials
Mixture Proportions RPCHPCRPC HPC F* F** 1.00 1.00 1.00 1.00 0.25 0.25 0.12 0.12 0.31 0.31 1.09 1.09 0.58 0.58 2.40 2.40 1.40 1.40 1.50 1.50 0.20 0.20
Cement Silica fume Quartz powder Standard sand grade 2 Standard sand grade 3 River Sand 20 mm aggregate 10 mm aggregate 30 mm steel fibres 36 mm steel fibres Admixture (Polyacrylate 0.03 based) Water 0.25 * Fibre RPC
0.03 0.023 0.023 0.25
0.4
0.4
** Fibre HPC
Workability and density were recorded for the fresh concrete mixtures. Some RPC specimens were heat cured by heating in a water bath at 90°C after setting until the time of testing. Specimens of RPC and HPC were also cured in water at room temperature.
The performance of RPC and HPC was monitored over time with respect to the following parameters: 13 Compressive Strength (as per IS 516 on 5 cm cubes for RPC, 10 cm cubes for HPC), Flexural Strength (as per IS 516 on 4 x 4 x 16 cm prisms for RPC, 10 x 10 x 50 cm beams for HPC), Water Absorption (on 15 cm cubes for both RPC and HPC), Non destructive water permeability test using Germann Instruments (on 15 cm cubes for both RPC and HPC), Resistance to Chloride ions Penetration test (on discs of diameter 10 cm 14 and length 5 cm as per ASTM C 1202 ). Results Fresh concrete properties The workability of RPC mixtures (with and without fibres), measured 15 using the mortar flow table test as per ASTM C109 , was in the range of 120 – 140%. On the other hand, the workability of HPC mixtures (with 16 and without fibres), measured using the slump test as per ASTM C231 , was in the range of 120 – 150 mm. The density of fresh RPC and HPC 3 mixtures was found to be in the range of 2500 – 2650 kg/m . Compressive strength The compressive strength analysis throughout the study shows that RPC has higher compressive strength than HPC, as shown in Fig. 1. 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. In the context of nuclear waste containment materials, the compressive strength of RPC is higher than required.
Fig 1: Compressive strength of RPC and HPC he maximum compressive strength of RPC obtained from this study is as high as 200 MPa, while the maximum strength obtained for HPC is 75 MPa. The incorporation of fibres and use of heat curing was seen to enhance the compressive strength of RPC by 30 – 50%. The incorporation of fibres did not affect the compressive strength of HPC significantly. Flexural strength Plain RPC was found to possess marginally higher flexural strength than HPC. Table 9 clearly explains the variation in flexural strength of RPC and HPC with the addition of steel fibres. Here the increase of flexural strength of RPC with the addition of fibres is higher than that of HPC. Table 9: Flexural strength (as per IS 516) at 28 days (MPa) RPC NC* HWC** 11 12
RPC-F NC* HWC** 18 22
*Normal Curing 3
HPC NC* 8
HPC-F NC* 10
**Hot Water Curing
As per literature , RPC 200 should have an approximate flexural strength of 40 MPa. The reason for low flexural strength obtained in this study could be that the fibres used (30 mm) were long. Fibre reinforced RPC (with appropriate fibres) has the potential to be used in structures
without any additional steel reinforcement. This cost reduction in reinforcement can compensate the increase in the cost by the elimination of coarse aggregates in RPC to a little extent.
Water absorption Fig. 2 presents a comparison of water absorption of RPC and HPC. A common trend of decrease in the water absorption with age is seen here both for RPC and HPC. The percentage of water absorption of RPC, however, is very low compared to that of HPC. This quality of RPC is one among the desired properties of nuclear waste containment materials.
Fig. 2: Water absorption of RPC and HPC
The incorporation of fibres and the use of heat curing is seen to marginally increase the water absorption. The presence of fibres possibly leads to the creation of channels at the interface between the fibre and paste that promote the uptake of water. Heat curing , on the other hand, leads to the development of a more open microstructure (compared to normal curing) that could result in an increased absorption. Water permeability The non-destructive assessment of water permeability using the Germann Instruments equipment actually only measures the surface permeability, and not the bulk permeability like in conventional test methods. A comparison of the surface water permeability of RPC and HPC is shown in Fig. 3.
It can be seen from the data that water permeability decreases with age th for all mixtures. 28 day water permeability of RPC is negligible when compared to that of HPC (almost 7 times lower). As in the case of water absorption, the use of fibres increases the surface permeability of both types of concrete.
Fig. 3: Surface Water Permeability of RPC and HPC Resistance to chloride ion penetration Results of rapid chloride permeability test conducted after 28 days of curing are presented in Table 10. Data indicate that penetration of chloride increases when heat curing is done in concrete. Total charge passed for normal-cured RPC is negligible compared to the other mixtures. Even though heat-cured RPC shows a higher value than normal-cured RPC, in absolute terms, it is still extremely low or even negligible (<100 Coulombs). This property of RPC enhances its suitability for use in nuclear waste containment structures. The data also indicate that addition of steel fibres leads to an increase in the permeability, possibly due to increase in conductance of the concrete. The HPC mixtures also showed very low permeability, although higher compared to RPC. Table 10: Rapid Chloride Permeability Test (as per ASTM C 1202) RPC
RPC with fibres
HPC
NC*
HWC** NC* HWC** NC* HWC*
Cumulative 4 Charge (less than 94 140 400 250 850 passed in 10) Coulombs ASTM Very Very Very Very C1202 Negligible Negligible low low low low classification *Normal Curing
**Hot Water Curing
Summary Reactive Powder Concrete (RPC) is an emerging technology that lends a new dimension to the term ‘high performance concrete’. It has immense potential in construction due to its superior mechanical and durability properties compared to conventional high performance concrete, and could even replace steel in some applications. The development of RPC is based on the application of some basic principles to achieve enhanced homogeneity, very good workability, high compaction, improved microstructure, and high ductility. RPC has an ultra-dense microstructure, giving advantageous waterproofing and durability characteristics. It could, therefore, be a suitable choice for industrial and nuclear waste storage facilities. A laboratory investigation comparing RPC and HPC led to the following conclusions:
A maximum compressive strength of 198 MPa was obtained. This is in the RPC 200 range (175 MPa – 225 MPa). The maximum flexural strength of RPC obtained was 22 MPa, lower than the values quoted in literature (~ 40 MPa). A possible reason for this could be the higher length of fibres used in this study. A comparison of the measurements of the physical, mechanical, and durability properties of RPC and HPC shows that RPC possesses better strength (both compressive and flexural) and lower permeability compared to HPC. The extremely low levels of water and chloride ion permeability indicate the potential of RPC as a good material for storage of nuclear waste. However, RPC needs to be studied with respect to its resistance to the penetration of heavy metals and other toxic wastes emanating from nuclear plants (such as Cesium 137 ion in alkaline medium) to qualify for use in nuclear waste containment structures.
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