Experimental Study On Early-Age Bond Behavior Between Steel Bars And Concrete Under Low Temperature

The reinforced concrete structures often encounter temperature plummeting in the process of construction.Load at early age may cause excessive defection and concrete cracking without corresponding curing method when the weather condition doesn't meet the winter construction standards.And the strength of structure would be much lower than expected,causing engineering accident.For reinforced concrete structure,reliable bond of reinforcing bars in concrete is a key factor affecting the bearing capacity of the structure,and early age is an important period for the development of concrete strength.Thus,it is of important significance for practical engineering to study the bond behavior between deformed bars and early-age concrete at critical temperature in winter construction.In this paper,the bond stress-slip behaviors between deformed bars with different diameters and early-age concrete cured at 0?and 5? is studied by experimental.The main work and conclusions are as follows:(1)A total of 271 pull-out specimens with three different diameter deformed bars were tested to study the bond behavior between deformed bars and early-age concrete cured at low temperature(0?,5?).The test age of D12,D16 specimens was 1d,3d,7d,14d,28d,the test age of D22 specimens cured at 0? was 1d,3d,7d,14d,28d and the test age of D22 specimens cured at 5? was 1d,2d,3d,4d,5d,6d,7d,14d,21d,28d.The influence of unilateral pressure was considered for 22mm diameter deformed bar specimens cured at 0?.The bond strength(?u),the slip corresponding to the ultimate bond strength(s0)and the residual bond strength(?r)at different curing temperature were obtained by experimental.On this basis,through the analysis of the experimental results,the effect of low temperature curing on the ?u,S0,?r and corresponding fitting formula were obtained.The results show that the failure mode is highly influenced by the ratio of the concrete cover thickness to the steel bar diameter(c/D),while the uniaxial pressure,the temperature and the curing age have little effect on failure modes.Low temperature curing can inhibit the development of bond strength under both of the no lateral pressure condition and the uniaxial lateral pressure condition.The lower the curing temperature or the shorter the curing time,the effect is more significant.At the age of 28 days,the concrete strength of specimens cured at 0? is close to the that of the specimens cured at 20?,while the bond strength of specimens cured at 0? is still much lower than that of the specimens cured at 20?.The results show that the effect of low-temperature curing on the bond strength is more significant than that on the concrete strength when the temperature drops to 0?.The slip corresponding to the ultimate bond strength increase with the decrease of curing temperature at the same curing age.(2)The bond stress-slip curves of deformed bars and early-age concrete cured at different temperature were obtained by pull-out test.The results show that the shape parameters can be highly influenced by the failure mode,while the temperature and concrete strength have little influence on the shape parameters of the bond slip curve.Therefore,the shape parameters formula of the bond stress-slip models were divided into two failure modes,pull-out failure and splitting failure,and prediction models of stress-slip relationship suitable for low temperature curing environment were proposed,and the results show an agreement with experimental results.