Study On Bond Behaviour Of Early Age Concrete Subjected To Biaxial Lateral Pressure At Low Curing Temperatures

Nowadays increasing speed of construction in order to reduce its costs is on demanding in the super reinforced concrete projects.This issue demonstrates the importance of investigation on early-age concrete and assessing its mechanical properties.On the other hand,temperature has profound effect on the mechanical properties of early-age concrete.Unexpected temperature drops during casting or curing of concrete would degrade the concrete mechanical properties at early age,and subsequently could have irreparable damages on concrete structure and even could lead it to collapse.For instance,in an accident in an under-construction cooling tower in China sudden drop in weather temperature led the colling tower structure to collapse.Accordingly calculating and assessing mechanical properties of early age concrete is necessary.The mechanical properties that control the performance of reinforced concrete structures between concrete and the steel bar are the bond characteristics.Model codes strongly advised to control the bond characteristics including local bond strength and bond slip displacement during calculating and designing concrete structures.In addition,complex stress distribution in concrete structure members such as beam column joints would have extreme significant impacts on the bond characteristics of reinforced concrete structures.Therefore,this study aimed to investigate the simultaneous effects of early-age concrete,low curing temperature and biaxial lateral stress on bond characteristics of concrete to assess its performance on such extreme condition.One of the popular and accurate tests which could quantify the bond characteristics is pull-out test in which an embedded steel rebar in a cubic concrete specimen pulling out of concrete in order to measuring load-displacement factors.In this study 150 pull-out specimens were tested at different curing age of 1,3,7 14 and28-day age to quantify the impact of early-age concrete.Specimens were cured at 0°C and 20°C which represent low and normal curing temperature respectively to have comparison on impact of different curing temperatures.Pull-out specimens were subjected to biaxial lateral pressures varying from 0.1 f_cu to 0.4 f_cuin 5 different loading combinations to assess the effect of biaxial lateral pressure on bond behaviour between concrete and deformed steel bars.Firstly,Specimens sustained pull-out failure and mixed splitting pull-out failure mode.When the steel rebar confined adequately by concrete,the specimens sustained pull-out failure mode.However,the specimens sustained mixed splitting pull-out failure mode,inadequate confined pressure caused tangential cracks propagated in the concrete cover to reach the surface of specimens.Secondly,the bond strength for the specimens cured at 0°C decreased significantly at very early ages of 1,3 and 7 days.However,the bond strength rose steadily with the increase of the concrete age.Eventually,the specimens at the 28-day age reached almost the same bond strength at both low and normal curing temperatures.In addition,the biaxial lateral pressure has profound effect on bond strength of concrete by preventing microcracks propagating in the concrete cover so that the local bond strength increased by increasing lateral pressure.Thirdly,slip displacements corresponding to ultimate bond stress for the specimens under low curing temperature reached scatter results under different loading combinations and curing temperatures.However,the slip increased by concrete aging in most loading combinations.Finally,this study proposed predicting equations for bond stress and slip at ultimate bond stress based on the regression analysis of experimental results for both low and normal curing temperatures.The predicting equations reached reasonable regression results comparing to experimental results with agreeable correlation coefficients.Furthermore,an empirical model of bond stress-slip relationship aiming at the early age concrete under low curing temperatures was proposed.Comparing with the existing model,the proposed bond stress-slip relationship more reasonably agreed with the experimental results,validating its applicability for the early age concrete under low curing temperatures.