Pumping concrete can not only improve the construction performance of concrete, but also reduce shrinkage, prevent cracks and improve the impermeability and durability of thin-walled multi-ribbed structures. However, some projects show that the strength of pumped concrete is insufficient, and the abnormal condensation, especially cracks, affect the impermeability and durability of the structure to a certain extent, which should be paid enough attention to. This paper focuses on the analysis of the causes of temperature cracks and finds out the preventive measures.

1. Causes and characteristics of temperature cracks

Temperature cracks often appear on the surface of mass concrete or concrete structures in areas with large temperature difference. After concrete pouring, in the process of hardening, cement hydration produces a lot of hydration heat, which leads to a sharp rise in internal temperature. However, the rapid heat dissipation on the concrete surface leads to a large temperature difference between inside and outside, which leads to different degrees of thermal expansion and cold contraction inside and outside, and causes a certain tensile stress on the concrete surface. When the tensile stress exceeds the tensile strength limit of concrete, cracks will occur on the surface of concrete, which mostly occurs in the middle and late stage of concrete construction.

The trend of temperature cracks is usually irregular, and large-area structural cracks are usually criss-crossed; For the structure with long beam and slab length, the cracks are mostly parallel to the short side. The crack width is different, which is obviously affected by the temperature change, wider in winter and narrower in summer.

1.2 influencing factors and preventive measures

The internal temperature of concrete is related to the thickness of concrete, the variety and dosage of cement. The thicker the concrete, the greater the dosage of cement, the higher the hydration heat of cement, the higher its internal temperature, the greater the temperature stress and the greater the possibility of cracks.

For mass concrete, its temperature stress is related to its structural size. In a certain size range, the larger the size of concrete structure, the greater the temperature stress, and the greater the risk of cracks, which is the main reason why mass concrete is prone to temperature cracks. Therefore, the most fundamental measure to prevent cracks in mass concrete is to control the temperature difference between the inside and the surface of concrete.

1.2. 1 Selection of concrete raw materials and mixture ratio

A. cement variety selection and cement dosage control

The main cause of cracks in mass reinforced concrete is the massive accumulation of hydration heat of cement, which leads to the early temperature rise and late temperature drop of concrete, resulting in the temperature difference between inside and outside. The measure to reduce the temperature difference is to choose medium-heat portland cement or low-heat slag portland cement, and slag portland cement can also be selected when adding pumping agent or fly ash. In addition, it can make full use of the later strength of concrete and reduce the dosage of cement. Therefore, with the consent of the design unit, the compressive strength of 56 days or 90 days can be used instead of 28 days as the design strength.

B. Adding additives

A large number of experimental studies and engineering practices at home and abroad show that adding a certain amount of high-quality fly ash into concrete can not only replace part of cement, but also improve the fluidity, cohesiveness and water retention of concrete mixture due to the ball effect of fly ash particles, thus improving the pumpability.

It is particularly important that the hydration heat of cement in concrete can be reduced and the temperature rise under adiabatic conditions can be reduced by adding undisturbed or ground fly ash.

1.2.2 construction technology improvement

A. Mixing process

Using pure slurry to wrap stone or secondary feeding mortar technology can effectively prevent water from gathering on the interface between cement mortar and stone, make the interface transition layer structure compact, and increase the bonding force after hardening, thus improving the concrete strength by 65,438+00% or saving cement by 5%, and further reducing the hydration heat and cracks.

B. Vibration technology

For the poured concrete, the secondary vibration before the final setting can eliminate the gaps and moisture formed by concrete bleeding in the lower part of the stone and horizontal steel bars, improve the cohesive force and tensile strength, reduce internal cracks and pores, and improve the crack resistance.

C. Maintenance technology

In order to strictly control the internal and external temperature difference of mass concrete, ensure the quality of concrete and reduce cracks, maintenance is a very important and key process, which must be done well.

Concrete curing is mainly to maintain proper temperature and humidity conditions. Thermal insulation can reduce the thermal diffusion of concrete surface, reduce the temperature difference of concrete surface and prevent surface cracks.

2, crack treatment measures

The appearance of cracks will not only affect the integrity and stiffness of the structure, but also cause the corrosion of steel bars, accelerate the carbonation of concrete and reduce the durability, fatigue resistance and impermeability of concrete. Therefore, cracks should be treated differently and treated in time according to the nature and specific conditions to ensure the safe use of buildings.

The repair measures of concrete cracks mainly include the following methods: surface repair, caulking and structural reinforcement.

2. 1 surface repair method

Surface repair method is a simple and commonly used repair method, which is mainly suitable for the treatment of surface cracks and deep cracks that have no influence on stability and structural bearing capacity. The usual treatment measures are to smear cement slurry, epoxy mastic or anticorrosive materials such as paint and asphalt on the surface of cracks. In order to prevent concrete from cracking due to various effects, measures such as sticking glass fiber cloth on the crack surface can usually be taken.

2.2 caulking method

Sealing method is the most commonly used method in crack sealing. Usually, a groove is opened along the crack and filled with plastic or rigid water-stopping material to seal the crack. Commonly used plastic materials include PVC mastic, plastic ointment, butyl rubber, etc. The commonly used rigid waterproof material is polymer cement mortar.

2.3 Structural reinforcement methods

When cracks affect the performance of concrete structures, it is necessary to consider the reinforcement of concrete structures. There are several common methods for structural reinforcement: increasing the cross-sectional area of concrete structure, wrapping steel bars at the corners of members, prestressing reinforcement, sticking steel plates, adding fulcrum reinforcement and shotcreting reinforcement.

Cracks are common phenomena in concrete structures. Their appearance will not only reduce the impermeability of buildings, affect the use function of buildings, but also cause corrosion of steel bars and carbonation of concrete, reduce the durability of materials and affect the bearing capacity of buildings. Therefore, concrete cracks should be carefully studied, treated differently, treated with reasonable methods, and various effective preventive measures should be taken during construction to prevent the occurrence and development of cracks and ensure the safety and stability of buildings and components.