| Over the last few decades,numerous postearthquake investigations have confirmed that the destroy of beam-column joints was one of the major reasons for the collapse of reinforced concrete structures,which can be mainly attributed to the premature failure of the bond between reinforcing bars and surrounding concrete.An insufficient bond will result in dramatic losses of the bearing capacity,lateral stiffness and peak-post ductility in those critical regions.A wide literature review has shown that the bond performance was closely associated to the inherent properties of concrete,especially the tensile strength and energy dissipation capacity,which however can be comprehensively enhanced by adding hybrid fibers.Given this,with the support of the Chinese National Natural Science Foundation(Grant No.51278388),the bond performance of rebar embedded in steel-polypropylene hybrid fiber reinforced concrete(HFRC)was studied upon the experimental investigation,theoretic analysis and numerical computation.The main work and results are as follows:(1)The experimental study on the bond performance.Considering the influences of fiber characteristics(volume fraction and aspect ratio),concrete strength,stirrup confinement and loading method,a total of 102 specimens were investigated by means of pull-out tests,including 51 monotonic and 51 cyclic loading cases.The results showed that compared with plain concrete(PC)specimen,the bond performance of HFRC specimen exhibit an obvious improvement,in terms of bond strength,peak slip and energy dissipation capacity,because the hybrid fibers could effectively bridge the micro-and macrocracks,therefore inhibiting their opening,and reducing the amount.For example,a respective increase of 18.2%,75.6%and 119.5%in bond strength,peak slip and cumulative dissipated energy could be obtained for the HFRC specimen having steel fiber(SF)of 1.5%in volume fraction and 60 in aspect ratio,and polypropylene fiber(PF)of 0.15%in volume fraction and 167 in aspect ratio.(2)The overall analysis on the bond response.According to the degree of crack propagation and fiber bridging effect in concrete matrix,the overall bond response of the HRFC specimens could be mainly divided into four stages,i.e.the cracks initiation stage,cracks propagation stage(initial propagation,stable propagation and unstable propagation),cracks closure stage,and friction stage(static friction and kinetic friction).For the reason that basically the fiber bridging effect is a passive actuation which requires a minimum crack width in advance,the most notable improvement in bond performance therefore was found in the cracks propagation stage.(3)The preliminary discussion on the bond mechanism.Analogous to the PC specimen,the bond stress of HFRC specimens also mainly consist of three components,i.e.the chemical adhesion,friction resistance and mechanical interlocking.However,the preferable bond performance of HFRC should be additionally attributed to the benefits from hybrid fibers,which,more specifically,were reflected in three aspects:1)for the micro-structure,they could optimize the pore structure,enhance the transition zone,and build the micro-reinforcement network;2)for the macro-response,they could inhibit the crack initiation and propagation;and 3)for the synergetic effect,they could work cooperatively,and exploit their advantages to the full,leading to a comprehensive improvement in bond performance as a consequence.(4)The propose of calculation methods for the bond strength.Based on the analyses of the experimental study,an empirical formula,in line with the theory of composite materials,was firstly established for calculating the bond strength of HFRC specimens,which was derived from the Mode Code and modified by fiber factors.Furthermore,according to the thick-walled cylinder theory,a theoretical formula upon the Tepfers model was also deduced,in which the contributions provided by stirrups and hybrid fibers were taken into account.Both the formulae were then verified by independent test results,which could offer a reference for the practical engineering design.(5)The propose of the empirical bond-slip constitutive model.According to the test results,a phenomenological model,inspired by the traditional spring model,was proposed to predict the bond behavior of HFRC specimens,in which the benefits of hybrid fibers were taken into account.Moreover,a stochastic damage variable was also introduced into the model,and the evolution of interfacial damage was assumed to conform to two classical distribution function,i.e.the Weibull distribution and lognormal distribution,respectively.The verifications between model predictions and test results clearly demonstrated that the empirical model can reasonably reflect the bond improvement caused by the hybrid fibers and the reduction in bond stress when the HFRC specimens were subjected to either monotonic or cyclic loading.(6)The propose of the theoretical bond-slip constitutive model.With the purpose of reflecting the basic characteristics of interfacial damage and plastic slip accurately,a damage-plasticity model within the thermodynamic framework was proposed,which mainly consist of two parts,that is,the evolution of bond damage and the law of hardening plasticity.Besides,in order to implement the theoretical model numerically,an implicit algorithm was adopted.The fairly good agreement between numerical predictions and test results firmly substantiated the capability of the proposed model in prediction of the bond responses of different materials(HFRC,FRC or PC),reinforcement types(deformed or plain bar),and loading histories(monotonic,repeated or cyclic loading).Finally,based on the summaries of this study,some recommendations for further research are proposed. |
PDF Full Text Request