考虑闭合筋的新旧混凝土界面抗剪试验及仿真分析

Shear Resistance Test and Simulation Analysis on New-to-old Concrete Interface with Closed Steel Bars

预制结构与现浇部分形成的新旧混凝土界面的抗剪性能对结构安全至关重要。为了准确分析界面各分项剪力及应力状态,采用ABAQUS软件建立了三维非线性有限元模型。考虑新旧混凝土界面的黏结滑移,并基于剪切-摩擦抗剪机理,采用内聚力-弹簧复合模型模拟了界面受力。基于前期设计制作的4组8个Z型直剪试件开展静载试验,从界面荷载-相对位移关系与破坏形态2个方面对有限元模型进行了验证,数值计算与试验结果吻合良好。研究了3组界面钢筋试件的界面正应力和剪应力沿界面高度的分布规律。分析了混凝土强度、界面配筋率、钢筋屈服强度对开裂荷载及抗剪承载力的影响。结果表明:加载过程中,各试件界面正应力沿高度非线性分布,界面中部应力较小;除两端应力集中区域,各阶段界面剪应力绝对值均大于正应力;峰值荷载下,剪应力均值为正应力的3.7~8.7倍,界面钢筋直径越大,对界面应力分布影响越大;增加混凝土强度可在一定范围内显著提高开裂荷载,建议新混凝土较旧混凝土提高2~3个等级;增加配筋率可带来更高的界面抗剪承载力,但对开裂荷载的影响不如混凝土强度大;界面钢筋的屈服强度对试件的抗剪承载力影响有限,使用高强钢筋较难提供更高的界面抗剪承载力。

The shear resistance performance of the new-to-old concrete interface,formed with the prefabricated structure and cast-in-place part,is crucial to the structure safety.In order to analyze the shear and stress state of each sub-item of the interface accurately,the 3D nonlinear finite element model is established with ABAQUS software.Considering the bond-slip at the new-to-old concrete interface and based on the shear-friction shear resistance mechanism,the interface stress is simulated by adopting cohesion-spring composite model.The static load test is carried out based on the preliminary designed and produced 4 groups of 8 Z-shaped direct shear specimens,the finite element model is verified from the relationship between interface load and relative displacement as well as the failure modes,and the numerical analysis is in good accordance with the test result.The distribution rules of the interface normal stress and shear stress along the interface height of the 3 groups of interface steel bar specimens are studied.The influences of concrete strength,interfacial reinforcement ratio and steel yield strength on cracking load and shear resistance bearing capacity are analyzed.The result shows that(1)during the loading process,the normal stress at the interface of each specimen distributes nonlinearly along the height,and the stress in the middle of the interface is small;(2)except for the stress concentration areas at both ends,the absolute values of interface shear stress at all stages are greater than the normal stresses;(3)under peak load,the average value of shear stress is 3.7-8.7 times of the normal stress,and the larger the diameter of the interface steel bar,the greater the influence on the interface stress distribution;(4)the cracking load can be increased significantly within a certain range by increasing the concrete strength,it is suggested to improve the strength of new concrete by 2-3 grades compared with the old concrete;(5)higher interface shear resistance bearing capacity can be got by increasing the reinforcement ratio,but the influence on cracking load is less than that of concrete strength;(6)the yield strength of the interface steel bar has limited influence on the shear resistance bearing capacity of the specimens,and it is difficult to provide higher interface shear resistance bearing capacity by using high-strength steel bars.