Reactive Powder Concrete.

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 comparing RPC with HPC. Specific benefits and potential applications of RPC have also been described.

High-Performance Concrete (HPC) is not just a simple mixture of cement, water, and aggregates. It contains mineral 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 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 Powder Concrete (RPC)¹.

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 strength hydrates¹.
The concept of reactive powder concrete was first developed by P. Richard and M. Cheyrezy and RPC was first produced in the early 1990s by researchers at Bouygues’ laboratory in France². A field application of RPC was done on the Pedestrian/Bikeway Bridge in the city of Sherbrooke, Quebec, Canada³. 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 impermeability⁴.
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 durability⁵. 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 durability⁶. Reactive Powder Concretes have compressive strengths ranging from 200 MPa to 800 MPa.

Richard and Cheyrezy¹ indicate the following principles for developing RPC:

1. Elimination of coarse aggregates for enhancement of homogeneity
2. Utilization of the pozzolanic properties of silica fume
3. Optimization of the granular mixture for the enhancement of compacted density
4. The optimal usage of superplasticizer to reduce w/c and improve workability
5. Application of pressure (before and during setting) to improve compaction
6. Post-set heat-treatment for the enhancement of the microstructure
7. 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 mixture can be achieved either by the use of packing models⁷ or by particle size distribution software, such as LISA⁸ [developed by Elkem ASA Materials]. For RPC mixture design an experimental method has been preferred thus far. Table 3 presents various mixture proportions for RPC obtained from available literature^1,3,9,10.

Table 1: Properties of RPC enhancing its homogeneity and strength

Property of RPC	Description	Recommended Values	Types of failure eliminated
Reduction in aggregate size	Coarse aggregates are replaced by fine sand, with a reduction in the size of the coarsest aggregate by a factor of about 50.	Maximum size of fine sand is 600 µm	Mechanical, Chemical & Thermo-mechanical
Enhanced mechanical properties	Improved mechanical properties of the paste by the addition of silica fume	Young’s modulus values in 50 GPa – 75 Gpa range	Disturbance of the mechanical stress field.
Reduction in aggregate to matrix ratio	Limitation of sand content	Volume of the paste is at least 20% greater than the voids index of non-compacted sand.	By any external source (e.g., formwork).

Table 2: Selection Parameters for RPC components

Components	Selection Parameters	Function	Particle Size	Types
Sand	Good hardness Readily available and low cost.	Give strength, Aggregate	150 µm to 600 µm	Natural, Crushed
Cement	C3 S : 60%; C2S : 22%; C3A : 3.8%; C4AF: 7.4%. (optimum)	Binding material, Production of primary hydrates	1 µm to 100 µm	OPC, Medium fineness
Quartz Powder	fineness	Max. reactivity during heat-treating	5 µm to 25 µm	Crystalline
Silica fume	Very less quantity of impurities	Filling the voids, Enhance rheology, Production of secondary hydrates	0.1 µm to 1 µm	Procured from Ferrosilicon industry (highly refined)
Steel fibres	Good aspect ratio	Improve ductility	L : 13 – 25 mm Ø : 0.15 – 0.2 mm	Straight
Superplasticizer	Less retarding characteristic	Reduce w/c	_	Polyacrylate based

Table 3: RPC mixture designs from literature

	P. Richard and M. Cheyrezy1				S. A. Bouygues3	V. Matte9	S. Staquet10
	[1995]				[1997]	[1999]	[2000]
	Non fibred		12 mm fibres		25 mm fibres	Fibred	Fibred
Portland Cement	1	1	1	1	1	1	1
Silica fume	0.25	0.23	0.25	0.23	0.324	0.325	0.324
Sand	1.1	1.1	1.1	1.1	1.423	1.43	1.43
Quartz Powder	--	0.39	--	0.39	0.296	0.3	0.3
Superplasticizer	0.016	0.019	0.016	0.019	0.027	0.018	0.021
Steel fibre	--	--	0.175	0.175	0.268	0.275	0.218
Water	0.15	0.17	0.17	0.19	0.282	0.2	0.23
Compacting pressure	--	--	--	--	--	--	--
Heat treatment temperature	20ºC	90ºC	20ºC	90ºC	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 (d₀/d_S). 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 hydrates that have formed¹. Pre-setting pressurization has also been suggested as a means of achieving high strength¹.
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 volume¹. The cost-effective optimal dosage is equivalent to a ratio of 2% by volume, or about 155 kg/m³.
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 conventional HPC of compressive strength 80 MPa¹¹. 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 nuclear waste storage facilities¹.
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 penetration of liquid and/or gas through RPC⁴. 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 concretes¹².

Table 4: Comparison of RPC 200 and RPC 800

	RPC 200	RPC 800
Pre-setting pressurization	None	50 MPa
Heat-treating	20 to 90°C	250 to 400°C
Compressive strength (using quartz sand)	170 to 230 MPa	490 to 680 MPa
Compressive strength (using steel aggregate)	--	650 to 810 MPa
Flexural strength	30 to 60 MPa	45 to 141 MPa

Table 5: Comparison of HPC (80 MPa) and RPC 200⁹

Property	HPC (80 MPa)	RPC 200
Compressive strength	80 MPa	200 MPa
Flexural strength	7 MPa	40 MPa
Modulus of Elasticity	40 GPa	60 GPa
Fracture Toughness	<10³ J/m²	30*10³ J/m²

Table 6: Durability of RPC Compared to HPC¹⁰

Abrasive Wear	2.5 times lower
Water Absorption	7 times lower
Rate of Corrosion	8 times lower
Chloride ions diffusion	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.
• Particle size optimization software, LISA⁸ [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.	Sample	Specific Gravity	Particle size range
1	Cement, OPC, 53-grade [IS. 12269 – 1987]	3.15	31 µm – 7.5 µm
2	Micro Silica [ASTM C1240 – 97b]	2.2	5.3 µm – 1.8 µm
3	Quartz Powder	2.7	5.3 µm – 1.3 µm
4	Standard sand, grade-1 [IS. 650 – 1991]	2.65	2.36 mm – 0.6 mm
5	Standard sand, grade-2 [IS. 650 – 1991]	2.65	0.6 mm – 0.3 mm
6	Standard sand, grade-3 [IS. 650 – 1991]	2.65	0.5 mm – 0.15 mm
7	Steel fibres (30 mm) [ASTM A 820 – 96]	7.1	length: 30 mm & dia: 0.4 mm
8	Steel fibres (36 mm) [ASTM A 820 – 96]	7.1	length: 36 mm & dia: 0.5 mm
9	20 mm Aggregate [IS. 383 – 1970]	2.78	25 mm – 10 mm
10	10 mm Aggregate [IS. 383 – 1970]	2.78	12.5 mm – 4.75 mm
11	River Sand [IS. 383 – 1970]	2.61	2.36 mm – 0.15 mm

Table 8: Mixture Proportions of RPC and HPC

Materials	Mixture Proportions
	RPC	RPC-F*	HPC	HPC-F**
Cement	1.00	1.00	1.00	1.00
Silica fume	0.25	0.25	0.12	0.12
Quartz powder	0.31	0.31	-	-
Standard sand grade 2	1.09	1.09	-	-
Standard sand grade 3	0.58	0.58	-	-
River Sand	-	-	2.40	2.40
20 mm aggregate	-	-	1.40	1.40
10 mm aggregate	-	-	1.50	1.50
30 mm steel fibres	-	0.20	-	-
36 mm steel fibres	-	-	-	0.20
Admixture (Polyacrylate based)	0.03	0.03	0.023	0.023
Water	0.25	0.25	0.4	0.4

* Fibre RPC     ** 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:
Compressive Strength (as per IS 516¹³ 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 and length 5 cm as per ASTM C 1202¹⁴).
Results
Fresh concrete properties
The workability of RPC mixtures (with and without fibres), measured using the mortar flow table test as per ASTM C109¹⁵, was in the range of 120 – 140%. On the other hand, the workability of HPC mixtures (with and without fibres), measured using the slump test as per ASTM C231¹⁶, was in the range of 120 – 150 mm. The density of fresh RPC and HPC mixtures was found to be in the range of 2500 – 2650 kg/m³.
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		RPC-F		HPC	HPC-F
NC*	HWC**	NC*	HWC**	NC*	NC*
11	12	18	22	8	10

*Normal Curing    **Hot Water Curing

As per literature³, 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 for all mixtures. 28^th 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 Charge passed in Coulombs	4 (less than 10)	94	140	400	250	850
ASTM C1202 classification	Negligible	Negligible	Very low	Very low	Very low	Very low

*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.

References

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