Concrete is a heterogeneous material with high brittleness, high elastic modulus and low tensile strength. External factors make the causes of cracks in cast-in-place concrete more complicated. Summarizing many engineering practices, cracks mostly occur during the initial setting to the final setting of concrete mixture, and their surface symptoms are also different. The causes of cracks in different structural members are also different.

First, the main causes of concrete cracks are:

1, the temperature changes greatly, resulting in tensile stress and temperature expansion and contraction cracks in concrete. 2, uneven humidity caused by surface drying shrinkage, open into cracks.

3. The alkali aggregate in concrete reacts with admixture to produce internal stress expansion and cracking. 4. Plastic shrinkage of concrete surface leads to cracks.

5. In the pouring process, insufficient or excessive vibration causes concrete to separate and secrete water, and a cement slurry layer with more cement content is formed on the surface, and water evaporates, which is easy to form shrinkage cracks.

6. The mixture ratio of concrete is poor, the binder-aggregate ratio and sand ratio are too large, and the concrete itself lacks the skeleton function of resisting temperature and shrinkage deformation, so it is easy to crack.

7. Improper maintenance is the main cause of cracks in cast-in-place concrete. Early curing will affect the cementation ability of concrete. Excessive curing will lead to rapid evaporation of free water on the concrete surface, lack of necessary hydration water, and sharp volume shrinkage and cracks.

8. Dismantle the form prematurely when the concrete fails to reach the specified strength during construction; Concrete is loaded before the final setting time, the reinforcement is not protected during construction, the reinforcement is displaced, and the post-pouring zone is not constructed according to the design specification, which may cause concrete cracks.

Second, the crack prevention measures

From the above analysis, it can be seen that many factors will lead to cracks in concrete to varying degrees, so various measures must be taken to control them.

(1) Concrete production

1, material selection;

Slag cement and fly ash cement with low hydration heat should be used for mass concrete.

(2) Adding appropriate additives, such as alkali-water anti-cracking agent, can improve the consistency of cement slurry and increase the tensile strength of concrete.

(3) The selection of fine aggregate adopts appropriate sand ratio.

2, mixing technical measures:

(1) All kinds of materials are weighed accurately.

(2) Strictly control the water-cement ratio and fully mix the concrete.

3, concrete transportation:

(1) The concrete transportation mode adopted shall strictly control the duration of concrete delivery from the mixer to the completion of pouring, and shall not exceed the specification requirements and the initial setting time of trial matching.

(2) Commercial concrete should be selected from the nearest mixing station, and specific technical requirements should be put forward to the mixing station, including construction site, strength grade, slump and allowable deviation, early strength and retarding requirements, initial setting time, pouring speed, etc.

4, concrete pouring

For different concrete members, the corresponding pouring technology should be adopted, and the detailed pouring construction scheme should be made before pouring mass concrete. Take practical measures.

(1) Reasonable joints and blocks, and reasonable pouring sequence.

(2) When pouring concrete in hot weather, it should be poured in layers to reduce the pouring thickness and use the pouring layer to dissipate heat.

(3) Bury cooling water pipes in concrete and introduce cold water to cool down.

(4) Arrange the pouring sequence reasonably to avoid excessive height difference and long-term side exposure.

(5) Pay attention to control the position of reinforcement to prevent the concrete protective layer from cracking due to the displacement of reinforcement.

When casting thin members, control measures are particularly important:

(1) It is forbidden to add water to concrete during pouring.

(2) Try to avoid construction in medium and high temperature weather.

(3) When pouring, move the concrete distribution pipe in time to prevent the concrete at the discharge port from being too concentrated.

(4) Use appropriate vibrating equipment. Before pouring, the base and formwork should be fully wetted to avoid excessive or insufficient vibration.

(5) After the concrete is vibrated, it shall be leveled with wood first. At the initial setting, wipe it with a wooden trowel for the first time, requiring great strength to make the surface fully dense. Before the final set of concrete, the second plastering should be carried out, and the plastering pressure should be greater than the first plastering pressure, so that the concrete surface layer can be fully compacted again.

5, concrete curing measures

(1) Cure in time, and cover with moisture for curing within 12 hours after concrete pouring. According to the different types of cement used, the curing time is determined, and it is advisable to spray water to keep the concrete in a wet state.

(2) The curing of membrane cloth should be closely covered with plastic film immediately after the second plastering by vibrating, so as to ensure that the concrete can be fully cured without losing water, supplemented by water spraying.

(3) water storage and maintenance, which can be used for large-area floor slabs.

(4) After pouring concrete in winter, appropriate and feasible curing methods should be adopted, but the ambient temperature of concrete should be guaranteed. For example, cushion storage method, covering maintenance, greenhouse maintenance, electric heating maintenance and so on.

Third, the engineering example

The Wenzhou Mansion project undertaken by our unit has a construction area of 53,600 ㎡, box foundation, frame-shear structure, raft thickness of 1.2m, waterproof board thickness of 0.4m, floor thickness of 1.20-200mm, and board spacing of 5-9.5m. Due to strict concrete crack control measures, cracks rarely occur. In particular, the raft and waterproof board have effective control measures and no cracks appear. 200㎜ thick long-span cast-in-place slab is comprehensively cured with plastic film mulching and watering, without cracks.

Four. Concluding remarks

The above analysis of the causes of cracks in cast-in-place concrete construction and the adoption of preventive measures can solve this common quality problem well. The key lies in how to implement it in construction and whether there is a reliable and sound system to regulate it.

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