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一、裂缝成因 砖混楼房顶层纵墙端部普遍出现接近45°的斜裂缝(图1、图2),这已成为砖混楼房的工程质量通病。此裂缝产生的原因主要是由于外界温度使屋面和墙体产生不同的热胀变形所致。例如,在夏季混凝土屋面温度有时可达砖墙温度的两倍,而混凝土屋面的热胀变形系数又是砖墙热胀变形系数的两倍。因此,混凝土屋面的单位热胀变形就是砖墙热胀变形的四倍,这就使屋面在热胀变形过程中产生一个较大的水平推力Pw,这个水平推力Pw恰好作用在纵墙端部裂缝易发部位的墙体上方(图3),在这个水平推力作用下,裂缝易发部位的墙体内部就形成了较大的剪应力。同时,在窗洞口的下角也就出现了应力集中,当墙体的抗剪强度小于这个剪应力时,在窗洞口下角的应力集中处就出现了接近于45°的斜裂缝。随着
First, the cause of cracks Brick and concrete buildings on the top of the vertical wall of the general emergence of nearly 45 ° oblique cracks (Figure 1, Figure 2), which has become a common problem in the quality of brick and concrete buildings. The reason for this cracking is mainly due to the external thermal deformation of the roof and wall caused by different thermal expansion. For example, in summer, the concrete roof temperature can sometimes reach twice the brick wall temperature, and the thermal expansion coefficient of the concrete roof is twice that of the thermal expansion coefficient of the brick wall. Therefore, the unit thermal expansion deformation of the concrete roof is four times the thermal expansion deformation of the brick wall, which causes the roof to generate a large horizontal thrust Pw during the thermal expansion deformation. This horizontal thrust Pw just acts on the cracks at the end of the vertical wall. Above the wall of the susceptible area (Figure 3), under this horizontal thrust, a large shear stress is formed inside the wall of the crack-prone area. At the same time, stress concentration occurs at the lower corner of the window opening. When the shear strength of the wall is smaller than this shear stress, an oblique crack near 45° occurs at the stress concentration point of the lower corner of the window. along with