斜拉桥施工阶段π型混凝土梁翼缘板裂缝成因分析与防裂措施研究

Flange Cracking Causes ofπ-Shaped Concrete Girders in Under-Construction Cable-Stayed Bridge and Prevention Measures

为了解斜拉桥施工阶段π型混凝土梁翼缘板斜向裂缝成因并制定合理的防裂措施,以某主跨120 m的π型预应力混凝土梁斜拉桥为背景,针对其悬臂施工阶段主梁翼缘板下表面出现斜向裂缝的现象进行分析。采用ANSYS软件建立该桥施工阶段杆系模型与混凝土梁实体模型,分析梁体自重、斜拉索张拉、预应力张拉、混凝土收缩、混凝土水化热作用下主梁翼缘板下表面应力情况。结果表明:梁体自重及斜拉索张拉作用下,翼缘板下表面未出现拉应力;横向及纵向预应力张拉作用下,翼缘板下表面产生一定的拉应力,但未超过混凝土抗拉强度标准值;混凝土收缩作用下,翼缘板下表面产生极小的拉应力,不会引起裂缝;混凝土养护过程中水化热作用下,翼缘板下表面混凝土与周边混凝土存在一定温差,部分测点第一主应力超过混凝土抗拉强度标准值,是开裂的主要原因。针对混凝土水化热作用影响,提出主肋布置冷却水管与翼缘板下方火炉加热并用的防裂措施,建立热流耦合模型分析防裂效果。结果表明:所采用的措施能够改善主梁养护过程中水化热温度场分布,各测点第一主应力均未超过混凝土抗拉强度标准值。

This study presents the causes of oblique cracks in flanges ofπ-shaped concrete girders in under-construction cable-stayed bridge and effective cracking preventions.A cable-stayed bridge with a main span of 120 m,which bore oblique cracks in flanges of theπ-shaped prestressed concrete girders during construction,is used as a case.Link-element model of the bridge and solid-element model of the concrete superstructure during construction were built up in ANSYS,to analyze the influence of multiple factors on stresses in the soffits of the flanges,including self-weight of girders,stay cable tensioning,prestressing tendons tensioning,shrinkage and hydration heat of concrete.It is shown that the self-weight of girders and the tensioning of stay cables do not induce tensile stresses in the soffits of flanges,while the tensioning of transverse and longitudinal prestressing tendons causes a degree of tensile stresses in the soffits of flanges.Minor tensile stresses are observed in the soffits of flanges due to the shrinkage of concrete,but not sufficient to trigger cracking.The hydration heat generated during the concrete curing process is considered the main cause of cracks,given that there exists temperature difference between the concrete in the soffits of flanges and the surrounding concrete,at some measurement points,the first principal normal stresses even exceed the tensile strength of concrete.To address the hydration heat,it is recommended to place cooling water pipes in the main ribs of theπ-shaped concrete girders and heat the flanges from beneath using burners.A thermal-fluid coupling model was developed to analyze the anti-cracking performance of the recommended measures.It is shown that the proposed measures can help improve the hydration temperature field distribution during the main girder curing process,and the first principal stresses at measurement points do not exceed tensile strength of the concrete girder.