| Reactive Powder Concrete (RPC) is characterized by the ultra-high strength,high toughness, high durability and excellent volume stability, so it has widelyapplication prospects in the fields of oil, nuclear power, municipal works, marineengineering, mechanical engineering, aviation and military facilities. At present,RPC has become the hot research spot in international engineering materials, andthere are many studies focused on its preparation technology and mechanicalproperties at room temperature, but there are few studies on its properties afterelevated temperatures. In this paper, based on the study of high temperatureperformance of high-strength concrete, the experimental studies were performed onthe high-temperature explosive spalling of RPC, effect of fiber on RPC spallingprevention, mechanical properties and microstructure of RPC with different fibersafter elevated temperatures. The main work of this study is as follows:To find out the explosive spalling law, the effects of moisture content, heatingrate and specimen size on spalling of RPC were studied. The prevention of steelfiber, polypropylene fiber and hybrid fiber on spalling of RPC were comparativelyanalyzed. Then, based on the experimental results, the mechanism and suppressionmeasures for high-temperature spalling of RPC were discussed. The results showthat the impact of moisture content on RPC spalling is greater than heating rate andspecimen size, and the critical moisture content is0.80%~0.85%. Mixing with steelfiber volume dosage of2%or polypropylene fiber volume dosage of0.3%caneffectively prevent RPC spalling, especially, blending with steel fiber andpolypropylene fiber together can prevent RPC from spalling more significantly. RPChigh-temperature spalling mechanism can be attributed to the combined effects ofvapor pressure, thermal stress and random cracks.Through high temperature test, variation of RPC specimen appearance andmass loss with temperature was studied. With the temperature increases, theappearance characteristics and mass loss of steel fiber-reinforced RPC, RPC withpolypropylene fiber and hybrid fiber-reinforced RPC have the same vary trend. Thespecimens appear to change in color with elevated temperatures as: slate-grayâ†'grayish brownâ†'darkish brownâ†'yellow-white. After exposure to600℃, smallcracks and flaking come into being on the specimen surface. After exposure to800~900℃, a number of meshy cracks appear and the flaking and porosity becomeserious. Meanwhile the steel fibers can be easily broken off and the RPC sintering.The mass loss increases gradually with the increasing temperature, and the mass lossis most serious in200~400℃. For the same heat treatment, the higher polypropylene fiber content implies the higher mass loss.The mechanical strength tests were carried out on steel fiber-reinforced RPC,RPC with polypropylene fiber and hybrid fiber-reinforced RPC after undergoingelevated temperatures. The cubic compressive strength, flexural strength and tensilestrength corresponding three kinds of RPC were obtained. The effects oftemperature, fiber type and fiber content on RPC mechanical strength wereinvestigated. The results indicate that steel fiber can effectively improve themechanical strength of RPC after high temperature, but polypropylene fiber hasadverse effect on RPC strength. For the same heating treatment, the strength of RPC(compressive strength, flexural strength and tensile strength) increases with theincreasing steel fiber content, but decreases with the increasing polypropylene fibercontent. For steel fiber-reinforced RPC and RPC with polypropylene fiber, themechanical strength increases first and then decreases with the increasingtemperature. For hybrid fiber-reinforced RPC, with the temperature increases, thecompressive strength increases first and then decreases, while the flexural strengthand tensile strength reduce basically as linear law. Through regression analysis,equations to express the relationships between strength of different fiber-reinforcedRPC and heating temperature are established, and the theoretical curves are in goodagreement with the test data.In order to study the axial compressive stress-strain relationship of steelfiber-reinforced RPC and hybrid fiber-reinforced RPC, by the aid of the ordinarycompression testing machine with attached rigid auxiliary frame, the uniaxialcompression experiments were conducted by165specimens with the size of70.7mm×70.7mm×228mm. The stress-strain curves of steel fiber-reinforced RPCand hybrid fiber-reinforced RPC after different high temperatures were measured.The effects of fiber content and exposure temperature on the mechanical propertiesof RPC were analyzed, including the axial compressive strength, elastic modulus,peak strain and ultimate strain. The results indicate that the stress-strain curves ofsteel fiber-reinforced RPC and hybrid fiber-reinforced RPC become flatter with thetemperature increases, and the peak points of stress-strain curves move right anddownwards. The axial compressive strength and elastic modulus increase first andthen decrease with the temperature increasing. The peak strain and ultimate strainreach peaks at600℃and700℃, respectively, and they increase rapidly before thepeak points but decrease linearly after the peak points. Through regression analysis,the equations to express the relationship of the compressive strength, elasticmodulus, peak strain and ultimate strain with the exposure temperature are proposedto fit the test results. With the same heating treatment, the area under thestress-strain curves increases with the steel fiber volume dosage increases, whichmeans the greater the steel fiber content is, the better the RPC ductility and toughness are. When the temperature is lower than300℃, polypropylene fibercontent has less impact on the stress-strain curve; but when the temperature is higherthan300℃, with the polypropylene fiber content increases, the area under the RPCstress-strain curve increases. Quintic polynomial and rational fraction are used to fitthe ascending and descending of stress-strain curves, and the curves proposed byequation fit the test data well.By scanning electron microscope (SEM) technique, the microstructure andphase composition of RPC after different temperatures were studied. The resultsshow that after exposure to the temperature not higher than400℃, the cementhydration and the pozzolanic reaction promotes each other, which increases theamount of C-S-H gel, consumes part of the Ca(OH)2and improves themicrostructure of RPC. After exposure to400~800℃, with the temperatureincreases, the C-S-H gel shifts from continuous block-like form to dispersed phase,and the cracks at the bonding interface of steel fiber and matrix come into being andexpand gradually, meanwhile the melting channels of polypropylene fibersexacerbate the internal defects of RPC, so the microstructure of RPC deterioratesconstantly. The microstructure changes of RPC are the root causes of itsmacroscopic mechanical properties degradation.In this paper, the high-temperature explosive spalling, mechanical propertiesand microstructure of RPC were studied systematically. The stage achievementshave been achieved, which enriches the RPC high temperature resistance research,and provides basis for the fire resistance research and theoretical analysis of RPCcomponents and structures. This study has important significance for RPCapplication and promoting. |
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