Causes and prevention of concrete cracks on the ex

Reasons for the formation of concrete cracks on the outer wall of the basement and its prevention and control

(author: Feng Qingrong)

first, the concrete on the outer wall of the basement often has different degrees and different numbers of cracks in the construction stage, and most of the cracks are vertical cracks. The main reason for cracks is that concrete cracks after being restrained by reinforcement and boundary during drying shrinkage

concrete drying shrinkage refers to the volume reduction and deformation of concrete placed in saturated air due to water loss. The water in the concrete and the surrounding air are in a certain equilibrium state. If the state of the surrounding medium air changes, such as the temperature changes, the concrete will produce drying shrinkage. Drying shrinkage includes irreversible shrinkage at the beginning and volume expansion after re wetting, and reversible shrinkage during later drying

at the same time, the factors affecting concrete drying include: water cement ratio, hydration degree, curing temperature, water content, cement content, component thickness, ratio of volume to surface area, relative temperature, drying rate, drying time, etc. After the formwork is removed, the concrete of the outer wall of the basement is watered for curing, but due to the limitations of on-site conditions such as the diameter of the collet that can hold the sample, it is impossible to achieve constant temperature and wet curing, but can only be watered for curing, which will inevitably lead to the drying and shrinkage of the concrete of the outer wall of the basement. At the same time, due to the small volume and surface area of the concrete of the outer wall of the basement, the drying speed is fast and the time is short

the cracking of basement concrete usually occurs within 15 days after pouring. The cracks are mainly concentrated in the wall height of 1 and 2 places, which expand up and down to the concept of sustainable development and the demand for environmental protection. There is almost no root and top, and there is one crack every 2 ~ 3M along the wall length. Of course, the shape of the basement, the design structure, the length of the outer wall, the reinforcement and the construction conditions have a certain relationship

II. Preventive measures for concrete cracks on the outer wall of the basement

1. Design: select the appropriate thickness of the outer wall, 400 ~ 500mm thick for the basement on the 1st ~ 2nd floor, and 600 ~ 800mm thick for the lower two floors of the basement on the 3rd ~ 4th floor; Attached walls or concealed columns are set every 3 ~ 4m on the outer wall of the basement, concealed beams are set at the top of the outer wall and 1/2 of the wall height, and the horizontal construction joints are set, so as to strengthen the rigidity of the side wall, instruments and equipment worth nearly 100million yuan, which can effectively prevent cracks; The horizontal reinforcement of the outer wall is set outside the vertical reinforcement, with a spacing of ≤ 150mm; The effect of crack resistance is obvious by using cold-rolled ribbed steel welded strip or unbonded prestressed reinforced concrete technology; The concrete adopts medium strength (C30 ~ C40); The vertical post cast strip of the external wall shall be reserved

2. Raw materials and mix proportion: cement should be selected from varieties with low hydration heat; Medium and coarse sand shall be used. Fineness modulus shall not be less than 2.6; Under the condition that the stones meet the pumpability, the large particle size and continuous grading shall be selected as far as possible, and the silt content of sand and stone shall be controlled within 1.5%; Add an appropriate amount of expansion agent into the concrete to compensate for the shrinkage of the concrete; Optimize the mix proportion, and on the premise of ensuring the workability of concrete. 2. Refueling: reduce the water cement ratio and appropriately increase the sand ratio and sand ratio to reduce the number and aperture of pores

3. Construction and maintenance: the spacing, size and protective thickness of reinforcement should be strictly controlled according to the design and specification requirements. Square hoops should be used to support the inner and outer reinforcement of the outer wall, and the longitudinal and transverse reinforcement should be bound at each point; During concrete pouring, the distribution thickness shall be controlled at about 60cm, and the blanking height shall not exceed 3M. During vibration, it shall be inserted quickly and pulled out slowly, and the concealed columns and beams shall be vibrated carefully; After the final setting of the concrete, watering and curing shall be started immediately. Before formwork removal, watering and curing shall be carried out from the upper opening of the slab wall. After dismantling and grinding, fine and dense holes can be drilled on the plastic pipe or steel pipe, and the full length of the wall shall be watered and cured by using the pressure of water. The curing time shall not be less than 14 days; The construction waterproof layer and earthwork backfilling shall be done in time, so as to facilitate the later strength growth of concrete and greatly benefit the crack resistance of concrete

III. repair measures for cracks in the concrete of the outer wall of the basement

if cracks occur in the concrete of the outer wall of the basement, they should be repaired in time. The materials to be repaired mainly include epoxy resin and modified epoxy resin. The repair methods include surface treatment, grouting and filling

the surface treatment method is aimed at repairing cracks with a width of 0.3mm, a depth of deeper or through. The air compressor is used to pour the resin slurry and polymer cement slurry into the deep part of the crack, so as to restore the integrity, durability and waterproof of the structure

the filling method is applicable to the repair of large cracks and joints. First chisel the cracks into V-shaped grooves, then brush them with steel wire, carefully tamp them with 1:1 cement mortar made of cement above P.O42.5 # and finally add an additional waterproof layer with a width of 150mm on both sides of the cracks