There are many reasons for cracks in cast-in-place reinforced concrete floors. This paper analyzes the causes of cracks, expounds the measures to prevent cracks from design and construction, and puts forward the concrete measures to control cracks in cast-in-place reinforced concrete floor.

Keywords: reinforced concrete floor; Cracks; Preventive and control measures

There are many reasons for cracks in reinforced concrete floor. This paper analyzes the causes of cracks, and puts forward two sets of measures to prevent cracks in design and construction, and puts forward concrete measures to control cracks in cast-in-place reinforced concrete floor.

Keywords: reinforced concrete floor; Cracks; preventive measure

1 preface

With the rapid development of China's real estate industry, a large number of new houses have been completed, and some quality problems in the construction process have also attracted everyone's attention, such as cracks in the reinforced concrete floor of the house. Although the existence of these cracks has little effect on the overall quality of the structure, these problems are often complained by the owners. Therefore, it is of great practical significance to find out the causes of cracks in cast-in-place reinforced concrete floors and put forward preventive measures for cracks.

2 Causes of cracks in cast-in-place reinforced concrete floor slab of residence

2. 1 Improper cement dosage, slump and water-cement ratio of concrete.

The greater the amount of cement in cast-in-place concrete, the higher the hydration heat and the greater the total calorific value. The temperature of cast-in-place concrete increases with the increase of cement dosage, and the hydration heat increases the internal temperature of concrete, forming a large internal and external temperature difference, and the stress caused by the temperature difference will cause cracks in concrete. The hardening process of concrete is the combination of water, sand, stone and cement. When the water-cement ratio is high, the shrinkage of concrete increases when it hardens, resulting in cracks. The slump of commercial concrete is large, so it is easy to produce less coarse aggregate and more mortar when pouring. At this time, when the concrete is dehydrated and dried, surface cracks will occur. If prepared with sand with large silt content, it will cause large shrinkage and low strength of concrete, and it is easy to produce cracks due to plastic shrinkage.

2.2 After the concrete is cast, it shall be excessively dried and calendered.

After the concrete floor is cast and compacted, excessive plastering and calendering will make fine aggregate float to the surface of the floor too much, forming a cement slurry layer with large water content on the concrete surface, in which calcium hydroxide reacts with carbon dioxide to generate calcium carbonate, which will shrink the surface volume and cause cracks on the surface of the concrete floor.

2.3 improper concrete pouring and maintenance

Improper maintenance will also cause cracks in cast-in-place concrete floors. When the concrete floor is exposed to the atmosphere, the water on the floor surface evaporates too fast, and there is not enough hydration heat when the concrete hardens, which will cause the volume shrinkage of the concrete, and the concrete cannot resist this stress and produce cracks.

2.4 The construction vibration of cast-in-place concrete floor is too large, and the formwork and cushion are too dry.

After concrete pouring, if it is vibrated excessively, the coarse aggregate of concrete will sink, and at the same time, water and air will be squeezed out, resulting in the formation of mortar layer on the surface, which has greater drying shrinkage performance than the lower concrete, and it is easy to form shrinkage cracks after water evaporation. If the formwork and cushion are not watered enough between pouring concrete, it will cause a large amount of water absorption by the formwork, causing plastic shrinkage of concrete, thus causing cracks.

2.5 Early loading of concrete floor slab and formwork sinking

Under the action of floor load, the strength of concrete floor slab can not meet the requirements, and the formwork is removed prematurely, which exceeds the stress that floor slab concrete can bear, resulting in cracks; If the formwork support system is not firm, the floor will bend downward, which is easy to produce long cracks.

2.6 Negative reinforcement configuration at the top of the slab, such as improper spacing and protective layer thickness.

Failure to pay attention to the protection of negative reinforcement (generally thin) on the upper floor during construction will pull the concrete on the upper surface of the floor in the negative bending moment area of the bearing and produce cracks. Excessive spacing of negative reinforcement and thick protective layer will also cause cracking of concrete at the top of slab.

2.7 Cracks in embedded pipe

Cast-in-place reinforced concrete floors usually need to embed electrical conduits, which will weaken the concrete section and produce stress concentration, especially when the diameter of embedded conduits is large, these floors are prone to cracks. Therefore, short steel mesh should be used to strengthen the distribution of thicker conduits and multiple conduits.

3 Prevention and control measures

In the design, the cast-in-place reinforced concrete floor should not only consider the strength, but also check the cracks. At the same time, the thickness of the floor can be appropriately increased, the stiffness of the floor can be enhanced, and the deflection of the floor can be reduced. Under the condition of meeting the strength requirements, small-diameter and high-density steel bars should be used in the concrete floor, which can reduce the stress influence caused by temperature and shrinkage, and can also appropriately increase the reinforcement ratio, thus improving the ultimate tensile strain capacity of concrete. In addition, the design strength of concrete grade should not be too high (high grade concrete has large shrinkage deformation).

In terms of construction, metering and feeding shall be carried out in strict accordance with the mixture ratio, and at the same time, the mixing time and water cement ratio shall be controlled to keep the strength and slump of concrete consistent. Strictly control the thickness of negative reinforcement protective layer on the board. In order to control the thickness of the protective layer of negative reinforcement, there must be enough reinforced horse benches (about 800 mm apart) to fix the position of negative reinforcement, ensure that the negative reinforcement does not sink when pouring, control the thickness of the protective layer, and prevent cracks caused by the thick protective layer of negative reinforcement. The load should not be applied to the cast-in-place slab too early, and the formwork and support are not allowed to be trampled or installed until the concrete strength reaches 1.2 kg/mm2; Within the specified curing time, concrete should be reasonably cured to prevent cracks from drying and shrinkage deformation. Cast-in-place concrete floor slab must be vibrated with a flat vibrator, so that it is horizontal and vertical, and the vibrating width 1/3 is overlapped at each vibrating time, leaving no construction joints.

4 conclusion

Cracks in cast-in-place reinforced concrete floor slab are common quality problems in building engineering. Only when the factors causing cracks are comprehensively considered in the design and construction process, the design and construction specifications are strictly observed, and correct treatment measures are formulated, floor cracks can be prevented.

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