Bond Stress-Slip Behaviors Between Deformed Steel Bars And Self-Compacting Lightweight Aggregate Concrete At Early Ages

If the structure suffers from excessive construction load at early age,there will be cracks and partial destruction.In recent years,many engineering accidents occurred in this period.Self-compacting lightweight aggregate concrete（SCLC）is a new type of concrete improved by using lightweight aggregate instead of ordinary aggregate in self-compacting concrete.It has the advantages of simple construction,low noise;good working performance,good frost resistance,good thermal insulation;light self-weight,large strength reserve and so on.Reliable bond is the basis to ensure the cooperative work between reinforcement and concrete and an important factor to determine the bearing capacity and crack control of the structure.Therefore,it is very important to study the time-varying rules of early age bonding behaviors between deformed steel bars and SCLC for the wide application of SCLC in practical engineering.In this paper,the bond stress-slip behaviors between SCLC with different strengthes and deformed bars with different diameters at early ages is studied by experiments.The main work and conclusions are as follows:（1）A total of 135 pull-out specimens with three different diameters of deformed steel bars and three different strengthes of SCLC were used to study the bonding behaviors at the early ages of 1d,3d,7d,14d and 28d,respectively.The bond failure mode between deformed steel bars and SCLC,the bond strength（τ_u）,the peak slip corresponding to ultimate bond strength（s₀）and residual strength（τ_r）at early ages were obtained by experiments.On this basis,through the statistical analysis of the experimental results,the criterion of distinguishing pull-out and splitting failure,the variation rules and fitting formula ofτ_u,s₀,τ_r with ages were obtained.The results show that the bond failure mode is related to the relative thickness of the protective layer and the measured compressive strength of concrete at different ages.When the specimen is pull-out failure,the bond strength is only related to the compressive strength of concrete;the diameter of steel bar is another factor affecting bond strength when splitting failure occurs.When the specimen is pull-out failure,s₀/D increases with the increase of the strength of concrete and relative thickness of protective layer;when splitting failure occurs,s₀/D increases with the relative thickness of protective layer but decreases with the increase of strength of concrete,and it is independent of age.（2）The bond stress-slip curves of deformed steel bars and SCLC at different ages were obtained by pull-out test.On this basis,the bond stress-slip curves were divided into two failure modes,pull-out failure and splitting failure,respectively,and the variation rules with ages were studied.The results show that the bond stress-slip curve for pull-out failure can be divided into three stages and the splitting bond stress-slip curve can be divided into two stages.The bond stress-slip constitutive relationship at early ages is independent of age and can be described by a set of bond stress-slip constitutive relations.The shape of the ascending and descending sections of the bond stress-slip curve is related to the parameters of the relative thickness of protective layer of concrete.The bond stress-slip constitutive relation between deformed steel bars and SCLC at early ages can be expressed by the mathematical model given in this paper.