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