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 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.