Causes and prevention of cracks in the hottest cas

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Causes and prevention of cracks in cast-in-place reinforced concrete floor slab

cracks in cast-in-place floor slab is a difficult problem that has plagued construction enterprises for a long time, and it is also a common problem of residents' housing quality complaints. Although the theory believes that the cracks in the cast-in-place floor slab are inevitable, and these cracks are generally considered not to be harmful to the use, it is still necessary to take effective measures to control them in the actual construction, especially to avoid the generation of harmful cracks. Zhidu keeps in touch with us. This paper analyzes the causes of the cracks in the roof slab of the building that reduce its weight due to cast-in-place, and puts forward prevention measures according to the construction practice

I. causes of cracks

1 The water cement ratio and collapse of concrete are too large, or excessive silt is used

the strength value of concrete is very sensitive to the change of water cement ratio, which is basically the superposition of the influence of water and cement measurement changes on strength. Therefore, the measurement deviation of water, cement, admixture and admixture solution will directly affect the strength of concrete. However, the concrete prepared with silt with large silt content has large shrinkage and low tensile strength, which is easy to produce cracks due to plastic shrinkage. In order to meet the pumping conditions of pumped concrete: large slump and good fluidity, it is easy to produce the phenomenon of less local coarse aggregate and more mortar. At this time, when the concrete is dehydrated and dried, it will produce surface cracks

2. During concrete construction, the formwork and cushion are too dry.

after concrete pouring and vibration, the coarse aggregate sinks and extrudes water and air, and the surface shows bleeding, resulting in vertical volume reduction and subsidence, resulting in the surface mortar layer, which has greater dry shrinkage than the lower concrete, and is easy to form shrinkage cracks after water evaporation. However, in recent years, the formwork and cushion have not been watered enough between pouring concrete, and if they are too dry, the formwork will absorb a large amount of water, causing plastic shrinkage of concrete and cracks

3. During the construction of cast-in-place concrete, excessive vibration, formwork and cushion are too dry

excessive troweling and calendering will make the fine aggregate of concrete float to the surface too much, forming a cement slurry layer with a large water content. Calcium hydroxide in the cement slurry reacts with carbon dioxide in the air to form calcium carbonate, causing the hydration and shrinkage of surface volume carbon, resulting in the cracking of the surface of concrete slab

4. Cracks on the slab surface caused by careless treatment of the post cast strip

in order to solve the shrinkage deformation and temperature stress of reinforced concrete, the post cast strip can be set according to the specification requirements, but some post cast strips are not constructed completely according to the design requirements, such as the construction does not leave tongue and groove joints, the post cast strip of the slab does not support the formwork, resulting in slope tenon, and the loose concrete is not completely chiseled, which may cause cracks on the slab surface

5. Influence of reinforcement engineering construction

due to the intellectualization of modern houses and the improvement of consumer requirements, the concealed burial of pipelines is more common. However, due to too many pipelines, the bond between reinforcement and concrete is reduced, resulting in uneven stress of cast-in-place floor after concrete forming, showing some small irregular cracks

6. Impact of formwork construction

some construction units unilaterally pursue high profits and reduce costs, and the insufficient number of sets of formwork equipped causes premature formwork removal, resulting in the concrete strength failing to meet the requirements of formwork removal. The experimental instruments are divided into 8 categories according to the action object and function, or the floor is deflected due to the influence of floor load due to the weak formwork support system, which may also cause full-length cracks in the slab

7. Curing works are not in place

during the curing period, the concrete strength does not reach 12MPa, that is, the construction of the next process is carried out; In particular, the impact of heavy objects is easy to cause irregular cracks on the plate surface. Improper maintenance is also the main cause of cracks in cast-in-place concrete slabs

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