Mechanical properties of reactive powder concrete: A review

To promote engineering application of reactive powder concrete (RPC), based on the domestic and foreign research achievements, the preparation, mix proportion, mechanical properties of RPC were discussed. The results showed that different admixtures have different influence on mechanical properties of reactive powder concrete. High temperature curing can excitant material’s activity and improve the compressive strength of RPC. The axial tensile strength of RPC was usually in the range of 4∼12 MPa, and the splitting tensile strength and flexural strength of RPC were close and higher than that of axial tensile strength. The tensile properties of RPC can be greatly improved by increasing the fiber volume fraction.


Introduction
Reactive Powder Concrete (RPC) was a new type of ultra-high performance cement matrix composite, also known as ultra high performance fiber reinforced concrete (UHPFRC). Compared with normal strength concrete, RPC has higher strength, toughness and durability, and has a broad application prospect [1]. In the past 20 years, many researchers and engineers have extensively studied the preparation and properties of reactive powder concrete. Based on the domestic and foreign research results, the preparation technology and mechanical properties of RPC were summarized, and the problems in the development of RPC and the future development direction were analyzed.

Materials and mix proportions of RPC
Pierre et al [2] introduced the manufacture principle and production process of RPC for the first time, and put forward the mix proportions of RPC200 and RPC800. The RPC200 of compressive strength 170~230 MPa was prepared by using quartz sand as aggregate and heat curing at 90 o C. The RPC of 490~650 MPa was successfully prepared by 50 MPa compression molding and autoclaved curing at 250~400 o C. By adding steel aggregate with particle size less than 0.8 mm, combined with compression molding and autoclaved curing, the RPC800 of compressive strength 650~810 MPa was prepared successfully.
The basic mechanical properties of fly ash and granulated blast furnace slag RPC under different curing conditions were studied by Yazcl et al [3,4]. The results showed that autoclaved pressure and steam curing can improve the hydration process and compressive strength of RPC, but autoclaved pressure and steam curing decreased flexural strength and toughness of RPC. Moreover, increasing the content of slag and fly ash can reduce shrinkage and deformation of RPC and improve compressive strength and toughness. The development of ultra-high performance concrete with high efficiency cement and mineral admixture was studied by Yu et al [5]. The influence of fly ash, blast furnace slag and limestone powder instead of cement on the performance of UHPC was analyzed. The effect of the three materials on the hydration reaction of UHPC in the first five days was similar. Compared with the fly ash or limestone concrete of 28 and 91 days curing, the mechanical properties of ultra-high performance concrete of blast furnace slag were better than those of the concrete. When the water-binder ratio increased from 0.165 to 0.18, compressive strength and flexural strength of UHPC increased. When the water-binder ratio increased from 0.18 to 0.2, the mechanical properties of UHPC decreased. The effects of sand gradation ratio, cement content and silica fume content on the fluidity and mechanical properties of RPC were studied by Ahmad [6]. The compressive strength and elastic modulus of RPC were significantly affected by water-binder ratio, and the fracture modulus was influenced by cement content. With the increase of cement content, the compressive strength of RPC increased, and the fracture modulus and elastic modulus decreased. The fracture modulus of RPC was three times higher than that of normal strength concrete, and the elastic modulus of RPC was 20% higher than that of other high strength concrete. A regression equation was fitted according to the content of main admixtures and the mechanical properties of RPC to optimize the mix proportion.
He et al [7] used P.O 62.5 cement, took standard sand as aggregate, removed mold after standard curing 24 h, and adopt three curing systems (20 o C water curing 28 days, 90 o C hot water curing 48 h and hot water curing 200 o C dry heat curing 8 h). When the mix proportions was cement: sand: silica fume: quartz powder: DFS water reducer = 1: 1: 1: 0.35: 0.32.5, the ratio of water-binder was 0.2, and the volume fraction of steel fiber was 3% (the diameter was 0.175 mm, the length was 13 mm), the compressive strength of RPC was 178.8 MPa and 298.6 MPa, respectively.
Liu [8,9] used natural fine aggregates and admixtures to replace quartz powder and silica fume in RPC, respectively. Under the condition of hot water or autoclaved curing, RPC with compressive strength > 200 MPa, flexural strength > 50 MPa and pull-compression ratio > 0.27 was successfully prepared. The cement, silica fume, ultrafine fly ash and ultrafine slag powder quaternary composite system were used to give full play to the superposition and composition complementary effect among all kinds of mixed materials. A optimum mix proportion of 420 kg/m 3 cement, 105 kg/m 3 silica fume, 262 kg/m 3 ultrafine fly ash, 262 kg/m 3 superfine slag powder, 1258 kg/m 3 natural sand, 21 kg/m 3 water reducer, 147 kg/m 3 water were obtained. Standard curing 24 h after test specimen forming, mold removal, and hot water curing at 90 o C for 48 h. The compressive strength of the cement sand specimen was 192.7 MPa, and the compressive strength of the cement sand specimen was 208.5 MPa if the mould was cured in 1.7 MPa autoclaves for 8 h.
Zhang et al [10] studied the development of high aluminum RPC. By adding high aluminum material, under the general curing mode, the early strength RPC material with 28 days compressive strength up to 184 MPa and elastic modulus up to 66 GPa was obtained. The material also had the advantages of low water absorption (0.94%) and good carbonation resistance.
Zhu et al [11] studied the mechanical properties and durability of RPC contained rice husk ash, and compared the effects of quartz sand and natural sand on the properties of RPC. Test results showed that flexural strength, compressive strength and fluidity of RPC changed little when natural sand was substituted for quartz sand. The optimum water-binder ratio of RPC mixed with rice husk ash was 0.20~0.22. With the increase of the substitution rate of rice husk ash, the shrinkage of rice husk ash decreased and the change of rice husk ash decreased with the increase of age, and the resistance to chloride ion permeation decreased. It was recommended to select RPC that appropriate replacement rate of rice husk ash according to different performance requirements. 3 standard curing and high temperature steam curing. Test results showed that the compressive strength of RPC can be improved slightly by increasing the steel fiber content. When the volume content of steel fiber was less than 2.0%, the enhancement effect of high temperature curing on RPC strength was significant, and when the content of steel fiber was more than 3.5%, the effect of high temperature curing on improving the compressive strength of RPC decreased. The compressive strength of standard curing and high temperature curing RPC was higher than that of natural curing RPC, and the increase was 11.5% and 34.2% respectively. But a high amount of steel fiber significantly increases the costs of RPC.

Mechanical properties of RPC
Kazemi and Lubell [13] studied the effect of specimen size and fiber content on mechanical properties of UHPFRC. The dimensions of the two cube test specimens were 50 mm×50 mm×50 mm and 100 mm×100 mm×100 mm, respectively. The diameters of the three kinds of cylinder specimens were 50 mm, 70 mm and 100 mm, respectively. The volume fractions of 5 kinds of steel fibers were 0%, 2%, 3%, 4% and 5%, respectively. The compressive strength test results of ultra-high performance fiber reinforced concrete decreased with the increase of the size of the specimen. When the side length of the cube was equal to the diameter of the cylinder, the compressive strength of the cube was greater than that of the cylinder. The ratio of compressive strength of cube specimen with 50 mm length to that of cube specimen with side length 100 mm, cylinder specimen with diameter 100 mm and cylinder test specimen with diameter 50 mm were 1.09 and 1.14, respectively. The compressive strength of super high performance fiber reinforced concrete increases significantly with the increase of fiber volume fraction. Compared with ultra-high performance concrete without steel fiber, when the volume fraction of steel fiber was increased to 5%, the compressive strength of the cube specimens with 50 mm and 100 mm increased by 25% and 26%, respectively. And the compressive strength of cylinder specimens with diameter 100mm was increased by 15%.
Graybeal and Davis [14] studied the compressive strength of RPC by using three cylinder specimens of three sizes and three cubes of different sizes. The RPC compressive strength ranged from 80 MPa to 200 MPa. The diameters of the three kinds of cylinder specimens were 51 mm, 76 mm and 102 mm, respectively. The side lengths of the three cubes were 51 mm, 70.7 mm and 100 mm. respectively. The conversion coefficient of compressive strength of different sizes was shown in table 1. The results showed that when the confining pressure was less than or equal to 60 MPa, the conventional triaxial compression failure mode of RPC was mainly split failure, and when the confining pressure was 65 MPa, the failure showed the characteristics of squeeze flow. Under different confining pressures, the stress-strain curves of RPC specimens were similar in shape, and all of them undergo four stages: compaction, elasticity, stress softening and steady load reduction. The triaxial compressive strength was about 3~4 times of the uniaxial compressive strength, and the maximum compressive strength appears when the ratio of the second principal stress to the third principal stress was 0.25. The peak strain in the triaxial direction was 5~10 times of the peak strain in uniaxial compression. With the increase of the second principal stress, the peak strain in the direction of the principal pressure increased gradually. But a mathematical expression for calculating compressive strength under different confining pressures wasn't proposed. . Test results showed that the elastic modulus of RPC200 increased with the increase of compressive strength. According to the experimental results, the relationship between the elastic modulus of RPC and the compressive strength of the prism was obtained by fitting, it was = 840√ c + 34900. Lv et al [19] collected 70.7 mm×70.7 mm×210 mm, 100 mm×100 mm×300 mm prisms and measured their elastic modulus. The relation between the elastic modulus and prism compressive strength was obtained with fitting tests data, and it was = 3027√ c + 9533, as shown in figure 1.

Compressive strain at peak load.
The experimental of the compressive properties of RPC200 without fiber added was studied by Wu et al [19]. Test results showed that the compressive strain of RPC at peak load was higher than that of normal strength concrete and high strength concrete, and it was 0.00286~0.00384. Wang [20] measured the compressive strain of basalt polypropylene hybrid fiber RPC at peak load was 0.00285~0.00335. The relationship between RPC compressive strain and axial compressive strength of hybrid fiber was obtained by fitting, it was 0 = (1093.24 + 23.19 c ) × 10 −6 (see in figure 2). The uniaxial compression test of RPC with 0.1% volume fraction of steel fiber was carried out by Jin et al [21]. The relation between the peak strain of RPC and the compression resistance of prism was 0 = (1030 + 23.10 c ) × 10 −6 .

Tensile mechanical properties of RPC
Nguyen et al [22] studied the relationship between direct tensile stress and strain of ultra-high performance fiber reinforced concrete with different sizes and geometric shapes. Test results showed that with the increase of length, cross-section area and volume, the crack resistance strength of UHPFRC decreased slightly, the strain capacity and absorption ability decreased obviously, and the crack spacing With the increase of the thickness of the specimen, the strength of the specimen increases slightly, the strain capacity and the absorptive capacity increased obviously, and the crack spacing decreased obviously. The axial tensile tests of three groups of RPC specimens with different volume content of steel fiber were carried out by Yuan [23]. The diameter of steel fiber was 0.22 mm, the length of steel fiber was 12~15 mm, the volume content was 0, 1% and 2%, respectively. The average cube compressive strength of the three groups was 98.7 MPa, 140.9 MPa and 159.1 MPa, respectively, and the corresponding tensile strength was 4.75 MPa, 5.94 MPa and 6.77 MPa, respectively. The experimental results showed that the steel fibers in the cracks were pull-out and broken, and the pullout fibers were distributed in a random direction and appeared to be interlaced.
The effect of steel-polypropylene hybrid fiber on the splitting tensile strength of RPC was studied by Smarzewski et al [24]. Test results showed that the splitting tensile strength of RPC increases with the increase of steel fiber volume content. Among them, the concrete with 1% steel fiber content has the highest splitting tensile strength, which was 55% higher than that of non-fiber concrete. Du et al [25] studied the tensile strength of RPC with different content of steel fiber. Table 2 showed splitting tensile strength, axial tensile strength and flexural strength of RPC. It can be seen from the test results that the splitting tensile strength and the flexural strength of the steel fiber RPC were close, which were significantly higher than the axial tensile strength. The axial tensile strength, splitting tensile strength and flexural strength of RPC increased with the increase of volume content of steel fiber, and the increase of splitting tensile strength was larger.

Conclusion
The above studies showed that a great deal of research results have been made on the basic mechanical properties of RPC. The basic mechanical parameters of RPC have been measured and by referring to the test method of normal strength concrete mechanical performance, including the compressive strength, the flexural strength, splitting strength and elastic modulus and so on. The RPC stress-strain curves were derived and fitted. These results can basically meet the requirements of determining the basic mechanical parameters and constitutive relations of RPC. However, there were still some problems to be solved.
• Due to there was no uniform specification and standard for the design of RPC mix proportions, the mechanical properties of RPC may differ greatly under different mix proportions. It was necessary to study the relationship between the mechanical properties and the mix proportions of RPC, and to develop an optimal design method of RPC mix proportions, which was easy to calculate, which can realize the design of RPC mixture properties based on performance.
• The tensile strength of RPC was affected by the direction of fiber distribution in the matrix. The development of a method to design fiber distribution in matrix was an effective way to further improve the tensile properties and reduce the brittleness of RPC.
• At present, the high strength and high performance of RPC mainly depend on high temperature curing, which was difficult for cast-in-place construction, so its application in practical structural engineering was greatly restricted. It was necessary to further study the mix proportions and construction technology of RPC materials, develop a RPC material that can meet the requirements of field construction, and further broaden its application scope.