Research Of The Performance Of Steel Fiber Reinforced Lightweight Concrete And Flexural Behaviors Of Superposed Beams

Structural lightweight concrete has the advantages of lightweight, high-strength,anti-seismic and good durability, it has been widely used in civil engineering. Steelfiber reinforced lightweight concrete (SFRLC) made by lightweight concrete mixingsteel fiber has not only the advantages of lightweight concrete, but also theremarkable tensile strength and cracking resistance as well as toughness, which canimprove the ductility, fatigue resistance and durability. Therefore, the experimentaland theoretical researches on performance of materials and members for SFRLC haveimportant practical value and theoretical significance. The new type superposingcomposite beam, i.e. SFRLC beam superposing with partial ordinary concrete, is thecast-in-place composite beam formed by ordinary concrete superposed on steel fiberlightweight concrete casted firstly in certain height (SFRLCB), which combines thegood performances of steel fiber lightweight concrete in lightweight and tensileresistance, and ordinary concrete in compression. This paper carried out the mixdesign of SFRLC by different method and studied the influences of differentparameters on the basic properties of SFRLC by a series of experiments, andconducted systematically experimental study and theoretical analysis on the SFRLCB.The main contents and conclusions are as follows:(1) the loose volume method was used to do mix design of the SFRLC withmachine-made sand and expanded-shale, and the steel fiber reinforced fulllightweight concrete (SFRFLC). The effects of water to cement ratio, cement dosage,sand ratio and fraction of steel fiber by volume on the properties of these two kinds ofSFRLC were systematically studied. The results show that for SFRLC withmachine-made sand and expanded-shale, the compressive strength, elastic modulus,splitting tensile strength and axial tensile strength increase with the increase of waterto cement ratio, but the flexural strength has no obvious change; the axial tensilestrength and flexural strength are affected in some extent by the sand ratio while thecement dosage is greater; when the cement dosage is lower, the axial tensile strengthand flexural strength are jointly affected by the sand ratio and cement dosage, the splitting tensile strength is not obviously affected by the sand ratio. The strengths ofSFRFLC are affected jointly by the sand ratio, water to cement ratio and cementdosage, and controlled by the strength of cement mortar and expanded-shale; theoptimum values are there for the sand ratio, water to cement ratio and cement dosage;the splitting tensile strength, the axial tensile strength and the flexural strength areobviously enhanced by the adding of steel fiber.(2) the mix design of high-strength SFRFLC was made by the direct designmethod considering the cement paste wrapping steel fibers, the properties ofhigh-strength SFRFLC were systematically experimental studied with the view ofinfluencing parameters of the fraction of steel fiber by volume, the water to cementratio, the thickness of cement paste wrapping steel fiber and the sand ratio. the resultsshow that the high-strength SFRFLC satisfies the requirements of workability of freshconcrete, dry apparent density and strength; with the increase of fraction of steel fiberby volume, the cubic compressive strength, axial compressive strength and axialtensile strength are all improved in some extent, while the splitting tensile strengthincreases obviously. After comprehensively analysis, the optimum thickness ofcement paste wrapping steel fibers is determined as1.0mm.(3) the relationship of splitting tensile strength and axial tensile strength forSFRLC on different mix proportion method was analysed. Results showed that theconversion coefficient of splitting tensile strength to axial tensile strength changewith the lightweight aggregate and characteristic value of steel fiber. It isrecommended that the testing method of axial tensile strength is suit to determine thetensile strength of lightweight concrete.(4) considering the changes of cement dosage, water to cement ratio and fractionof steel fiber by volume, experimental study was carried out on the freeze-thawresistance of SFRFLC. The results show that the freeze-thaw resistance of SFRFLC isaffected by the cement dosage and water to cement ratio, which can be obviouslyenhanced by the steel fibers. Compared with the relative dynamic elastic-modulus,the loss rate of flexural strength has more sensitivity to the freeze-thaw cycle.Therefore, it is suggested that for the member mainly subjecting to flexural actions,the loss rate of flexural strength of SFRFLC may be better as one of the evaluation indexes to the freeze-thaw resistance.(5) systematical experimental study was carried out on42SFRFLC beamssuperposed with ordinary concrete,12reinforced concrete beams and4reinforcedSFRFLC beams. The results show that the failure states of these three kinds of beamswere similar with the yield of proper reinforcement and compressive crush ofconcrete; larger influences of fraction of steel fiber by volume and depth of SFRFLCare on the ultimate resistance, which are related to the reinforcement ratio; thereinforcement ratio obviously affects to the ultimate resistance of beams; thesuperposed concrete strength has a little effect on the ultimate resistance of SFRFLCbeams superposed with ordinary concrete. Based on the theoretical analysis and withthe comparison of experimental results, the calculation model and computationmethod of bending resistance are established for the SFRFLC beams superposed withordinary concrete, the formulas for calculating the minimum and optimum depths ofSFRFLC are proposed.(6) combined with the experimental results of42SFRLC comparing with12reinforced concrete beams and4SFRFLC beams, the effects of such parameters asthe fraction of steel fiber by volume, the strength grade of ordinary concrete, thereinforcement ratio of longitudinal tensile rebar and the sectional depth of SFRFLCon the anti-cracking moment of normal section of SFRLCBs are analyzed. The resultsshow that the optimal value is there for the section depths of ordinary concrete incompressive zone and SFRFLC in tensile zone, the high-strength ordinary concreteprovides the larger anti-cracking moment of SFRLCB, the proper reinforcement ratioof longitudinal tensile rebar benefits to improve the anti-cracking moment ofSFRLCB, the enhancement of steel fiber to the anti-cracking moment of SFRLCBcoincides with the strengthening to the tensile strength of SFRFLC. Combining withthe theoretical analysis, the method for calculating the anti-cracking moment ofnormal section of SFRLCB is proposed, the formula is suggested to determine thesectional depth of SFRFLC giving full play to resisting tension.(7) based on the crack distribution of SFRFLC beams superposed with ordinaryconcrete under normal service load, the principle of classified statistical analysis forthe cracks is determined. The statistical analyses are carried out on the number of cracks, the average crack space, the average crack width and the distribution ofmaximum crack width. The calculation models and computation method are proposedfor the average crack space, average crack width and maximum crack width at thehorizontal level of longitudinal reinforcements of the SFRLCBs.