某砌体房屋部分混凝土板开裂原因分析

Cause Analysis to the Part of Concrete Slab Cracking in a Masonry Building

建筑工程交付使用前或使用一段时间后,用户常常会发现钢筋混凝土楼板板底粉刷层表面出现类似"塑性铰线"裂缝的现象,但调查发现荷载一般小于设计荷载。对于此类裂缝的产生有不同解释,本文提出一种新的解释,即混凝土终凝后,新钢筋混凝土会与模板支撑体系共同受力,模板与支撑体系刚度并非无穷大,过早施加施工荷载后,新混凝土会随着模板支撑系统共同变形,当混凝土中产生的拉应力(或应变)大于新混凝土抗拉强度(或峰值拉应变)时,即会产生此类裂缝。减少此类裂缝的方法包括增加新混凝土与模板支撑系统的刚度、限制施加施工荷载的时间等。

A similar＂plastic hinge line＂crack phenomenon was often found in the surface of finishes of bottoms of RC slabs after a building is in service for a short time or even before the turn key when the service load was less than the design service load. There were different explanations for the cause of such cracking. A new explanation was shown in this paper. After the final setting the new reinforced concrete works with formwork and supporting system as composite structure. In reality,the formwork and supporting system was deformable. So if the construction load was imposed in the new RC slab the early age concrete will deform with the formwork and supporting system. The new concrete will crack if the tensile stress（strain） was greater than the early age tensile strength（strain limit）. To reduce the probability of such kind of cracking increasing the rigidity of the composite structure and postponing the construction load imposing time were proposed.