沪通长江大桥水下钢管柱自密实混凝土收缩与抗裂性能研究

Research on Shrinkage and Crack-resistance Performance of Self-compacting Concrete for Underwater Steel Pipe Columns of Shanghai-Nantong Yangtze River Bridge

沪通长江大桥主航道桥钢管柱选用C40自密实混凝土,钢管柱轴向对称等分为4个隔仓,每个隔舱需要一次浇注约770 m3的混凝土。为了满足自密实混凝土的施工和防开裂要求,在优化配合比的基础上,通过自由收缩试验、限制收缩试验、约束圆环收缩试验3种不同的方法,系统地研究了自密实混凝土的收缩性能,并结合绝热温升测定结果与有限元建模,分析了混凝土早期开裂风险。研究结果表明:通过试验优选的自密实混凝土,自由收缩应变和限制收缩应变随龄期的增长逐渐增大,前7 d增长较快,后期逐渐减缓,自由收缩应变、限制收缩应变14 d时分别为407×10-6,137×10-6,60 d时分别为623×10-6,235×10-6,与相似配合比混凝土相比14 d时自由收缩应变和限制收缩应变分别降低了53.2%,45.8%;约束圆环收缩试验开裂龄期为34 d,最大约束收缩应变为115×10-6,而相似配合比混凝土开裂龄期为15 d,且最大收缩应变为194×10-6。有限元分析结果表明:隔仓混凝土径向表面与芯部最大温差只有13℃,远低于内外开裂温差限值25℃;不同龄期混凝土温度变形与体积收缩共同作用引起的拉应力最大值出现在钢管混凝土外表面,最大拉应力均低于容许拉应力,早期开裂风险低。

The self-compacting concrete with a grade of C40 is selected for the concrete-f illed steel pipe columns of the main channel bridge of Shanghai-Nantong Yangtze River Bridge.The steel pipe column is divided into four compartments symmetrically in the axial direction,each compartment requires about 770 m^3 of concrete to be poured at one time.In order to meet the construction and crack-resistance requirements of self-compacting concrete,the shrinkage performance of self-compacting concrete was systematically studied through free shrinkage test,restricted shrinkage test and restrained ring shrinkage test on the basis of optimizing the mix proportion.And the early cracking risk of concrete was analyzed through the adiabatic temperature rise results combining f inite element modeling.The research results show that the strains of free shrinkage and limited shrinkage of the optimized self-compacting concrete gradually increase with the age,they increase rapidly in the f irst seven days and slow down gradually in the later period.The strains of free shrinkage and limited shrinkage are 407×10^-6 and 137×10^-6 respectively at fourteen days.And the strains of free shrinkage and limited shrinkage at fourteen days are 53.2%and 45.8%lower than those of the concrete with similar mix proportion.The cracking age is thirty-four days according to restrained ring shrinkage test,and the maximum restrained shrinkage strain is 115×10^-6,while the cracking age of concrete with similar mix proportion is f ifteen days,and the maximum shrinkage strain is 194×10^-6.The results of f inite element analysis show that the maximum temperature difference between the radial surface and the core of concrete is only 13℃,which is far below the temperature difference limit of internal and external cracking by 25℃.The maximum tensile stress caused by the combined action of temperature deformation and volume shrinkage of concrete at different ages appears on the outer surface of concrete-f illed steel pipe,which is lower than the allowable tensile stress,and the risk of early cracking is low.