Tensile Properties of Hybrid Fiber-Reinforced Reactive Powder Concrete After Exposure to Elevated Temperatures
The paper presents a research project on the tensile properties of RPC mixed with both steel and polypropylene fibers after exposure to 20–900 °C. The direct and the indirect tensile strength (in bending) were measured through tensile experiment on dog-bone specimens and bending experiment on 40 × 40 × 160 mm prisms. RPC microstructure was analyzed using scanning electron microscope. The results indicate that, steel fibers can significantly improve the tensile performance of hybrid fiber-reinforced RPC, whereas polypropylene fibers have no obvious effect on the tensile performance. With increasing temperature, the flexural and axial tensile strength of hybrid fiber-reinforced RPC substantially decrease linearly, which attributes to the deteriorating microstructure. Based on the experimental results, equations are established to express the decay of the flexural and tensile strength with increasing temperature.
Keywordsreactive powder concrete (RPC) tensile properties elevated temperatures steel fiber polypropylene fiber scanning electron microscope (SEM)
The tensile properties of concrete after high temperature are very important for the evaluation of the residual behavior in tension of concrete structures after exposure to high temperatures (EN 1992-1-2 2004). Up to now, a lot of research on the residual mechanical properties of normal strength concrete (NSC) and high strength concrete (HSC) has been performed. It was found that the flexural strength and tensile strength of both NSC and HSC substantially decrease linearly with increasing temperature (Husem 2006; Khalig and Kodur 2011; Chang et al. 2006). Adding steel fibers to the concrete can effectively improve the tensile properties after exposure to high temperature (Song and Wang 2004), whereas the incorporation of polypropylene fibers has no obvious effect on the tensile properties (Xiao and Falkner 2006). The variations exhibited by the flexural strength for HSC with or without steel fibers and polypropylene fibers were explained by Pliya et al. (2011) Chen and Liu (2004) studied the effect of steel fibers, polypropylene fibers and hybrid fibers on the residual splitting tensile strength of HSC. Compared with plain concrete, the residual splitting tensile strength of HSC mixed with steel and polypropylene fibers improved significantly.
The difference with NSC is that HSC is more prone to spalling when subjected to high temperature. Kodur (2000) considered that the low tensile strength and low porosity led to HSC spalling under the high temperature. Kalifa et al. (2000) found that the spalling probability of HSC improved under high temperature owing to its pore pressure being much higher than the NSC. The incorporation of steel fibers and polypropylene fibers can effectively reduce the occurrence of spalling. Han et al. (2005) discussed the influence of polypropylene fibers, metal fibers, carbon fibers and glass fibers on the spalling performance of HSC.
Reactive Powder Concrete (RPC) is an ultra high strength cement-based composite material made of ultra-fine reactive powder, cement, fine aggregate, high-strength steel fibers and other components (Richard and Cheyrezy 1995). It is a very promising building material in the field of civil engineering. Currently, many studies have been completed on the mechanical properties of RPC at room temperature, and the studies show that the mechanical properties of RPC are better than NSC and HSC (Yazıcı et al. 2010; Bayard and Plé 2003; Rashad et al. 2013). The steel fibers contained in RPC can greatly improve its tensile strength and toughness (Kang et al. 2010). Few studies have been performed on the mechanical properties of RPC after exposure to high temperature, especially on the tensile properties. Tai et al. (2011) showed that the mechanical properties of steel fiber-reinforced RPC increased firstly and then decreased with increasing temperature. Due to the elimination of the coarse aggregate, RPC has a denser internal structure than HSC (Cheyrezy et al. 1995; Li et al. 2012; Vance et al. 2014). Therefore, RPC is more prone to spalling than HSC under heating. The same with HSC, the incorporation of steel fibers and polypropylene fibers can also inhibit the spalling of RPC.
From above analysis it is found that the incorporation of steel fibers can obviously improve the mechanical properties of RPC, and the incorporation of polypropylene fibers have little effect on the mechanical properties, but can greatly reduce the damage extent of high-temperature to the specimen. When RPC mixed with both steel fibers and polypropylene fibers, on the one hand, the mechanical properties can be improved evidently, on the other hand, the high temperature spalling can be effectively suppressed.
For the abovementioned reasons, it is necessary to study the mechanical properties of RPC after exposure to elevated temperatures. A previous research (Zheng et al. 2012b) has discussed the compressive behaviour of hybrid fiber-reinforced RPC after exposure to high temperature. In this paper, in order to study the tensile properties of hybrid fiber-reinforced RPC after exposure to 20, 120, 200, 300, 400, 500, 600, 700, 800 and 900 °C, bending and direct tensile tests were carried out on 40 × 40 × 160 mm prisms and dog-bone specimens. The effects of fiber type, fiber content and temperature on the flexural strength and direct tensile strength are studied. The microstructure of RPC after different temperatures is studied using SEM test. Equations to express the decay of the flexural and direct tensile strength with temperature are proposed.
2 Experimental Program
2.1 Raw Materials and Mix Proportion
RPC was prepared on the basis of the following ingredients: ordinary Portland cement with Grade of 42.5 (Chinese cement grading system); silica fume with specific surface area of 20780 m2/kg and SiO2 mass fraction of 94.5 %; slag with the 28 days activity index of 95 % and specific surface area of 475 m2/kg; quartz sand with SiO2 mass fraction of higher than 99.6 %, and diameter range of 600–360 and 360–180 μm; concentrated naphthalene water reducer with form of brown powder; high-strength steel fiber with diameter of 0.22 mm and length of 13 mm; polypropylene fiber (PPF) with melting point of 165 °C and length of 18–20 mm.
Mix proportions of HRPC.
Binding materials (kg/m3)
Quartz sand (kg/m3)
Water reducer (kg/m3)
Steel fiber (%)
2.2 Specimen Design and Fabrication
The RPC preparation has to follow certain requirements. Firstly, the pre-weighed quartz sand, cement, slag, silica fume and water reducer were poured into concrete mixer and mixed for 3 min, then the pre-weighed water was poured into mixer and mixed for 5 min, next, the polypropylene fibers and steel fibers were poured into mixer and mixed for 5 min, finally, the mixture was poured into molds and vibrated on a high-frequency vibration table. After being stored for 1 day in the standard conditions, the specimens were demoulded and cured for 3 days at 90 °C in the concrete accelerated curing box. Next, the specimens were moved into a standard curing room and cured for 60 days. Before heating treatment, the specimens were taken out of standard curing room and exposed to air for 2 months.
2.3 High Temperature Tests
2.4 Bending and Tensile Tests Regime
2.5 Scanning Electron Microscope (SEM) Test
The microstructure of concrete after exposure to high temperature determines its macroscopic mechanical properties, so it is important to study the morphology and composition of RPC after elevated temperatures. In this paper, the samples used for SEM tests were taken from the specimens tested in direct tension, and exposed to 20, 200, 400, 600 and 800 °C. Zhou (2000) detailed description of the SEM test method. Firstly, small pieces of samples about 5 mm were prepared, then the small pieces were dried, vacuum pumped and sprayed-gold successively. Next, the microstructure of RPC matrix, bonding interface between steel fiber and matrix, PPF and PPF melting channel were photographed and observed using the Quanta200 scanning electron microscope.
3 Results and Discussions
3.1 Failure Modes of Specimen
3.1.1 Flexural Failure Mode
3.1.2 Tensile Failure Mode
As can be seen from the previous discussion, the ductility and toughness of RPC gradually improves with increasing steel fiber content. The interlocked distribution of the steel fibers is the main reason for RPC failure modes shifting from brittle to ductile.
3.2 Flexural Strength
Figure 6b also shows the curves of the relative flexural strength for RPC with only steel fibers (SRPC) (Zheng et al. 2012a with steel fiber volume dosage of 1–3 %), RPC with only PPF (PRPC) (Zheng et al. 2012c with PPF volume dosage of 0.2–0.3 %) and ordinary concrete without any fiber (NSC and HSC) (Xiao et al. 2006 with concrete strength grade of C40–C100) after exposure to elevated temperatures. The corresponding equations are shown in Eqs. (2)–(4) as follows.
Through comparative analysis, it is found that the temperature-induced decay of HRPC and SRPC is almost the same, whereas the decay of PRPC is significantly more pronounced. This means that the flexural strength of PRPC after high temperature is the worst among the three types RPC. The curve corresponding to NSC/HSC locates at the bottom, and the curve decline rate is faster than the HSC, that is the bending performance of HRPC is better than NSC and HSC. The flexural strength curves of SRPC and PRPC exhibits a rising process, but the flexural strength decay curve of HRPC exhibits a linear decrease. The reason is that when undergoing a relatively low temperature, SRPC and PRPC are equivalent to experiencing a “high temperature curing” process, so that the cement hydration reaction is more fully, and more C–S–H gel is generated, which makes the internal structure of RPC is more compact, so the flexural strength curves of SRPC and PRPC has a rising process. For HRPC, although there is the positive effect of “high temperature curing” existence, but PP fibers and PP fiber melting channels not only increase the internal defects of RPC matrix, but also weaken the bonding properties between steel fibers and RPC matrix, and this weakening is considered to be a negative effect. Furthermore, the bonding properties of steel fiber and matrix present great effect on the flexural and tensile strength, so the flexural strength curve of HRPC decreases linearly with increasing temperature due to the negative effect, as shown in Fig. 6b.
3.3 Direct Tensile Strength
The curves of the relative tensile strength of SRPC (Zheng et al. 2012a with steel fiber volume dosage of 1–3 %), PRPC (Zheng et al. 2012c with PPF volume dosage of 0.1–0.3 %) and NSC/HSC (EN 1992-1-2: 2004) after exposure to elevated temperatures are also given in Fig. 7b. The corresponding equations are shown in Eqs. (6)–(8) as follows.
It is found that the decay of HRPC and SRPC is almost the same, whereas the decay of PRPC is significantly more pronounced. The same with the flexural strength, the tensile properties of PRPC after high temperature also is the worst among the three types RPC. The tensile strength curve of NSC/HSC locates at the bottom, and its decline rate is faster than the HSC, that is the tensile performance of HRPC is better than NSC and HSC, which is the same with the flexural strength too. After heating to 800–900 °C, the steel fibers loss strength due to the oxidizing decarbonization, and they can be broken off gently. The concrete surrounding the specimen becomes hardening, and the brittleness of RPC increases. The tensile strength curves of three different types of HRPC exhibits rebounding.
3.4 Ratio of Flexural Strength to Tensile Strength
Average values of ffT/ftT of HRPC in different temperature ranges.
Temperature Range (°C)
4 Analysis of Microstructure
At room temperature, the bonding interface of polypropylene fiber (PPF) and RPC matrix is dense, but as the temperature exceeds the PPF melting point of 165 °C, the PPF melt and leave interconnecting channels inside the RPC matrix, meanwhile the PPF melting channels also weaken the bonding properties between steel fibers and RPC matrix, which lead to the flexural and tensile strength of HRPC decreases linearly. The interconnecting PPF melting channels also provide channels for steam overflowing, that is why the incorporation of PPF can inhibit the spalling of concrete (Kalifa et al. 2001).
When the temperature is higher than 400 °C, the internal structure of RPC matrix becomes loose, and numbers of pores appear. The cracks along the bonding interface between steel fibers and matrix begin to expand, and the cracks across the melting channel of PPF begin to form. That is why the strength of RPC decreases gradually with increasing temperature. After heating to 800 °C, the internal structure resembles a honeycomb, and a number of pores appear. The interface between steel fibers and RPC matrix shows some debonding, to the detriment of the tensile properties.
With the steel fiber content increasing, the residual flexural and direct tensile strength of hybrid fiber-reinforced RPC improves significantly. With increasing temperature, the flexural and direct tensile strength substantially decreases linearly.
Steel fibers can effectively improve the direct tensile properties of RPC after high temperature. Polypropylene fibers have an adverse effect on the mechanical strength of RPC exposed to a lower temperature, but can improve the strength of RPC exposed to a higher temperature.
Based on the experimental results, equations are established to express the decay of the flexural and tensile strength with increasing temperature. Compared with normal-strength and high-strength concrete, the hybrid fiber-reinforced RPC has excellent capacity in resistance to high temperature.
With increasing temperature, the microstructure of RPC deteriorates, and the bonding interface between steel fibers and RPC matrix becomes loose gradually. The PPF melt and leave interconnecting channels inside RPC matrix. The basic reason for the degradation of mechanical properties of RPC is the deterioration microstructure.
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