Study On Explosive Spalling Rules And Mechanical Properties Of Reactive Powder Concrete At Elevated Temperatures

Reactive Powder Concrete (RPC) is characterised by super-high strength, extreme durability and superior toughness. RPC has excellent durability, which can be widely used in house, office buildings and other facilities of the salt lake region and coastal areas. It is probable that the lower permeability of RPC compared to NSC and HSC prevents free water from escaping, causing considerable internal vapour pressure that often results in spalling. RPC spalling mechanism and the rule of mechanical degradation at elevated temperatures have not been reported systematically. Furthermore, spalling and mechanical properties deterioration of the concrete will result in serious damage and collapse to the building structure. Thus, mechanical properties and the mechanism for explosive spalling of RPC remain an important but unsolved problem.To study spalling and mechanical properties at elevated temperatures,957dumbbell-shaped specimen, cube specimens and prism specimens were prepared. Factors of explosive spalling, spalling prevention, effect of fibres on mechanical properties, microstructure and mass loss of RPC with different fibres were performed at elevated temperatures. The main work of this study is as follows:To study spalling law, dumbbell-shaped specimen, cube specimens and prism specimens were prepared. The results show that spalling risk increases with the moisture content. The risk of spalling increases with an increase in temperature gradient. Heating rate and specimen size are major factors which have effected on spalling of RPC. Hold time has little effect on spalling. Incorporating0.3%PP fibres (2.73kg/m3),1%steel fibres (78kg/m3) can prevent explosive spalling of RPC. Adding2%steel fibres by volume content, the critical moisture content of explosive spalling for RPC is0.85%. By smearing rationally tunnel fire resistant coating on the surface of the structure for RPC, the outside temperature of RPC can be reduced. Thus temperature gradient can be reduced to prevent spalling of RPC.To infer the maximum temperature experienced, assess fire damage and supply repair recommendations, methods are proposed according to the color change, the number and width of cracks and beating sound. Color change of RPC at different temperatures as follows: gray (20~200°C)→brown (300~400°C)→reddish gray (500°C)→dark brown (600°C)→gray (700°C)→yellow white (800°C). The appearance characteristics of RPC with PP fibres, RPC with steel fibres and RPC with hybrid fibres are similar. When the appearance of color is similar, it can infer maximum temperature of RPC experienced by beating sound. The beating sound increases gradually become dull with an increase temperature. In order to evaluate fire safety of RPC engineering, and consider degradation of the mechanical properties of RPC at elevated temperatures, the mechanical properties at elevated temperatures were carried out on RPC with PP fibres, steel fibres and hybrid fibres. To obtain cube compressive strength of RPC without fibres, RPC with PP fibres, RPC with steel fibres and RPC with hybrid fibres,270cube specimens were conducted at elevated temperatures, respectively. In order to obtain the axial compressive stress–strain relationship of RPC with steel fibres and RPC with hybrid fibres,90prism specimens were conducted at elevated temperatures, respectively. To obtain tensile strength of RPC without fibres, RPC with PP fibres, RPC with steel fibres and RPC with hybrid fibres,270dumbbell-shaped specimens were conducted at elevated temperatures, respectively. The effects of temperature, fibre content and hold time on the mechanical properties of RPC were studied about the thermal expansion, axial compressive strength, elastic modulus, peak strain and stress–strain. Mechanical properties for RPC with different fiber types and different fiber content were analyzed.Hold time is time that central temperature of specimens reaches the target temperature and maintains the target temperature a period of time. Hold time of the material properties and structure at elevated temperatures is hot problem for fire safety engineering. To solve this problem, hold time effect on mechanical properties of45prism specimens is considered at elevated temperatures. The results show that compressive strengths with hold time of1h and3h are higher than that with hold time of0h from200to400°C, are lower than that with hold time of0h between400and600°C, and higher than that with hold time of0h beyongd600°C. SO2can act with Ca(OH)2, which occurs pozzolanic reaction resulting in an increase in compressive strength at20~400°C. Decomposition of Ca(OH)2evaporation of bound water result in voids and cracks increasing, the compressive strength of RPC decreases with hold time at400~600°C. Sintering effect occurs due to sustained high temperature, the compressive strength increases with hold time at800°C. The elastic modulus and the peak strain of RPC for different hold time are more or less the same at20~600°C, but elastic modulus increases lightly and peak strain decreases rapidly at800°C. Utilising curve equation of RPC with steel fibres, stress–strain curve equation of RPC with different hold time was established at elevated temperatures.By scanning electron microscope (SEM), X-diffraction (XRD) and MIP analysis, microstructure morphology, cracks and fibres variation, the mineral composition, the phase change reaction and pore structure evolution of RPC were studied. The results show that the admixture SO2and Ca(OH)2occurs pozzolanic reaction, which leads to the increase in the number of C–S–H below400°C so that mechanical properties of RPC increase. The microcracks can be observed beyond 400°C. A large number of voids and cracks occur and microstructure becomes porous and rough at800°C, which result from decomposition of Ca(OH)2and C–S–H gel. Cracks between steel fibre and matrix increase with increasing temperature, steel fibres are completely oxidized at800°C. Bond between PP fibres and matrix at room temperature are close, and interface region is dense; holes and communication network of PP fibres melted is benefit to the escape of the water vapor beyond200°C, which resulting in reducing the occurrence of spalling. SO2content firstly reduces, and then increases with the increasing temperature. Number and peak of CaAl2Si2O8·4H2O decreases with increasing temperature. C–S–H decomposes into β-C2S and C3S at800°C, and the rapid deterioration of the RPC strength is due to CH and C–S–H decomposition. The median diameter and the porosity of RPC increase with the increasing temperature, and RPC has smaller diameter and lower porosity than ordinary strength concrete and high strength concrete. By TG–DSC analysis, Endothermic peak appears at170°C,600°C and780°C respectively, which is due to melting of PP fibres, the SO2transformation and C–S–H decomposition.Based on a large number of tests and analysis, the method for distinguishing spalling of RPC, spalling prevention, mechanical properties at elevated temperatures are summarized. Utilising these experimental results, predictive equations were developed. The data collected and the proposed RPC models were utilised to develop preventing structure fire.