Causes and prevention of cracks in cast-in-place s

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Causes and prevention of cracks in residential cast-in-place slabs

pour out the mortar in the mixing bag. 1. Project overview

in recent years, the traditional precast slabs have been gradually replaced by cast-in-place slabs. Due to the use of cast-in-place slabs, the integrity, uneven settlement resistance and structural safety of houses have been greatly improved, including slag Portland cement, pozzolanic Portland cement and fly ash Portland cement GB 1344 ⑴ 999, However, there are also some floor cracks. Many residents are worried that the causes of these cracks will increase the friction between the moving parts, and the foundation settlement of the house will complain to the relevant departments

for example, there are 14 residential buildings jointly built in a community in our city, with a building area of about 80000 m2, brick concrete structure, no partition wall indoors, all of which are cast-in-situ slabs, 100 ~ 120mm thick, mixed and vibrated on site, concrete strength grade C20, negative reinforcement is provided at the connection of floor slab and beam, the house is 5 floors high, the bottom floor is set with an overhead floor height of 2.2M, the house is about 80m long, there is no expansion joint, and the foundation adopts concrete cast-in-place piles. The project was completed successively in December, 1999. At the time of completion acceptance, no obvious cracks were found. When residents bought houses one after another for decoration, they found cracks in the floor, so they complained to our station. Through field survey (including unsold houses), it is found that there are cracks in 14 houses, which are verified by the experiment with Languang hot adhesive tensile machine. The number of cracked households reaches 124, accounting for 27.56% of the total 450 households (while 21 households are demolished, accounting for 4.64% of the total). The crack locations mainly fall into two categories as shown in Figure 1, of which the cracks at the corner of the slab account for the vast majority, accounting for about 90% of the total, and the crack width is generally 0.1 ~ 0.2mm

2. Cause analysis of cracks

the cracks of cast-in-place slabs reported by users have been investigated on the spot for many times in conjunction with the design, supervision, construction and other departments. First, settlement observation and drawing review were carried out, which were in line with the specifications and standards. Therefore, the following analysis is made according to the above fracture conditions and their causes

(1) the primary cause of cracks is the shrinkage of concrete. As we all know, in the hardening process of concrete, due to the evaporation of water, the volume gradually shrinks, resulting in shrinkage, and the surrounding of the plate cannot be extended freely due to the constraints of the support. When the restrained stress of the slab caused by the shrinkage of concrete exceeds a certain degree, it will inevitably cause the cracking of the cast-in-place slab. The cracking part often occurs in the place where the stress is relatively concentrated, so the cracks of the slab mostly occur at the corner of the slab, and its trend is perpendicular to the diagonal of the slab

(2) temperature cracks. Because the cement has the characteristics of fast hardening, high strength and large hydration heat, plus the main construction of the house takes place in summer, the concrete is not watered and cured in time after pouring and tamping, the concrete loses water and contracts at a higher temperature, the hydration heat release is large, and the water is not supplemented in time, so in the hardening process, the cast-in-place slab is constrained by the support, which is bound to produce temperature stress and cracks, These cracks also first occur in the weaker parts, namely the corner of the plate. In addition, the indoor and outdoor temperature difference changes greatly, which will also cause certain cracks. In the investigation, it was found that most of the cracks were in the West and top of the house. In fact, many cracks are often caused by concrete shrinkage and temperature changes

(3) sudden change of structural shape and failure to set necessary expansion joints. In order to improve the utilization rate of land, the developers have long houses without considering the setting of expansion joints. When the free expansion of the houses reaches the spacing required for the setting of expansion joints, cracks will be caused. In addition, the plane layout is more concave and convex, that is, the corners are also more. These corners form weak parts due to stress concentration, which are prone to cracks when subjected to concrete shrinkage and temperature difference changes

(4) cracks are caused by the settlement of negative reinforcement at the support. During the construction process, due to the improper construction technology, the negative reinforcement at the support subsides, the protective layer is too large, and the fixed support becomes a plastic hinge support, causing cracks at the support along the beam at the upper part of the plate

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