? Paper Keywords: concrete crack analysis and control

? Abstract: Raft foundation is widely used in building engineering, and there are cracks in the concrete of raft foundation. This paper analyzes the surface cracks and shrinkage cracks of raft foundation, and puts forward preventive measures for concrete cracks of raft foundation, which has played a simple guiding role for future engineering practice.

? Today, with the rapid development of urban construction, high-rise building projects are constantly emerging in large and medium-sized cities. Raft foundation is more and more widely used. However, the raft foundation concrete has a large volume, and the hydration heat released in the process of pouring and hardening of concrete will produce greater temperature stress and shrinkage stress, which will lead to cracks in concrete, which will not only destroy the appearance image, but also cause the exposure and corrosion of steel bars, reduce the bearing capacity of building structures, reduce the integrity and stiffness of building structures, and become a structural hidden danger.

Mechanism analysis of temperature cracks in raft foundation concrete

(a) Temperature shrinkage caused by cement hydration

The hydration heat of cement is the main reason for cracks in mass concrete of raft foundation. When cement is hydrated, it will generate a lot of heat, but the cross section of mass concrete structure is generally thick, so it is not easy to dissipate heat when heat is gathered inside the structure, and concrete will expand greatly due to heating. In the subsequent cooling stage, the volume of concrete will gradually shrink due to the continuous decrease of its own temperature. At this time, the raft is constrained by the foundation or other structures, which will produce great temperature shrinkage stress in the concrete raft. Once the temperature shrinkage stress in the concrete raft exceeds the tensile stress intensity of the concrete at this age, the concrete will produce cracks running through the whole section, which will seriously reduce the impermeability, integrity and durability of the structure and bring serious consequences. In addition, the temperature of raft foundation concrete will be higher because of slow internal heat dissipation, and the temperature of surface part will be lower because of fast heat dissipation, which makes the difference between shrinkage value of concrete interior and surface too large, resulting in excessive surface tensile stress, thus causing cracks on concrete surface.

(B) the impact of external temperature changes

The higher the outside temperature, the higher the pouring temperature of concrete, and the lower the outside temperature, which increases the cooling range of concrete, especially the sudden drop of temperature, will greatly increase the temperature gradient between the outer concrete and the inner concrete, which is extremely unfavorable to the mass concrete of raft foundation.

The internal temperature of concrete is adiabatic temperature of hydration heat. The pouring temperature and the cooling of the structure lead to the superposition of various temperatures, and the temperature stress is caused by the temperature deformation caused by the temperature difference. The greater the temperature difference, the greater the temperature stress. At the same time, under high temperature conditions, mass concrete is not easy to dissipate heat. In this case, it is more important to study reasonable temperature control measures to prevent temperature stress caused by temperature difference between inside and outside of concrete.

(3) Internal and external constraints

Constraints can generally be classified into two categories: external constraints and internal constraints. External constraints refer to the boundary conditions of the structure, generally referring to the constraints of bearings or other external factors on the deformation of the structure. Internal constraint refers to the mutual constraint caused by the uneven distribution of temperature and shrinkage and the uneven deformation of each particle in a large cross-section structure. For the structure with large cross section, its deformation may be restricted by other objects because of its large cross section size.

Based on the above analysis, it is not difficult to see that cracks in raft concrete are common and the most harmful in construction projects. In the construction of raft concrete, it is necessary to prevent both surface cracks and shrinkage cracks of concrete. Therefore, based on these two reasons, we can draw control and prevention methods from the following mechanical analysis.

Mechanical analysis of crack control on concrete surface of double raft foundation

At the initial stage of concrete pouring, cement hydration produces a lot of hydration heat, which makes the temperature of raft concrete rise rapidly. However, due to the good heat dissipation condition on the concrete surface, the heat can be dissipated into the atmosphere, so the temperature on the concrete surface rises less; However, due to poor heat dissipation conditions and less heat dissipation, its internal temperature rises more. As a result, temperature gradient is formed inside and outside the raft concrete, which leads to compressive stress inside the raft concrete and tensile stress on the surface layer. When the tensile stress exceeds the tensile strength of concrete, cracks will occur on the surface of raft concrete.

The temperature stress that causes cracks consists of two main parts: the temperature stress caused by the constraint of relative deformation caused by the difference between the central temperature of concrete and the surface temperature of concrete during heating; In the cooling stage, when concrete is cooled from high temperature to ambient temperature, the temperature stress is caused by the external constraint of shrinkage deformation. Therefore, one of the key factors to control the cracks in raft concrete is to control the surface temperature difference of raft concrete within a certain range, so that the resulting temperature stress is less than the tensile strength of concrete at the same age. According to the current engineering practice and theoretical research, China's "Technical Specification for the Application of Concrete Admixtures" sets the temperature difference limit at 25℃.

Mechanical analysis of shrinkage crack control of concrete in three-raft foundation

The shrinkage crack of raft foundation concrete occurs in the concrete cooling stage, that is, when the concrete cools down, it shrinks due to gradual heat dissipation. In addition, during the hardening process of concrete, it will shrink due to hydration and evaporation of the mixture inside concrete and gelation of colloid. These two kinds of shrinkage will produce great shrinkage stress (tensile stress) due to the constraints of the base or the structure itself. If the shrinkage stress exceeds the ultimate tensile strength of concrete at this age, concrete will produce shrinkage cracks that run through the whole section and become structural cracks, which will bring serious harm to residential projects.

The thickness (height) of the raft foundation in building engineering is much smaller than the dimensions in the other two directions. When the ratio of the thickness to the length of the bottom plate is less than or equal to 0.2, under the action of temperature shrinkage and deformation, the entire cross section of the bottom plate leaves the end area and approaches the middle part. Therefore, for the cracks caused by this reason, we have obtained some empirical formulas to control the temperature shrinkage stress in engineering practice and theoretical research, and the control temperature shrinkage stress is less than the tensile strength of concrete at this age. The specific empirical formula should be determined according to the specific project scale and specific age, and various documents have expounded the results, thus achieving good results.

Four conclusions

In the construction of raft foundation concrete in building engineering, the development law of temperature and temperature stress is very important to the control of concrete cracks. The calculation of temperature stress should fully consider the influence of construction conditions, environmental temperature, concrete elastic modulus, creep, drying shrinkage and stress relaxation.

There are many factors that affect the temperature stress of raft foundation concrete structure, among which concrete mix proportion, pouring environment and boundary heat dissipation conditions are the main factors. Therefore, based on the above analysis and demonstration, in the future engineering practice, temperature cracks should be controlled from the following aspects.

(1) Improve the mix proportion of concrete, and add mixed materials (such as water reducer and fly ash, etc.) to reduce the hydration heat of cement and the dosage of cement per unit volume.

(2) Adding a certain amount of expansion agent into concrete, and using the shrinkage compensation principle of concrete, the crack resistance of concrete is improved. This method of combining "resistance" with "release" can solve the crack control problem of raft foundation concrete well.

(3) Reducing the pouring temperature of concrete can reduce the maximum temperature of concrete, thus reducing the foundation temperature and the temperature difference between inside and outside. To control the pouring temperature, we should try to avoid construction in high temperature season or adopt methods such as aggregate precooling to reduce the mold temperature.

(4) Improve the boundary heat dissipation conditions and constraint conditions, and adopt the maintenance measures of heat preservation and moisture preservation, so as not to make the surface concrete heat dissipation too fast, keep the concrete surface at a high temperature, and reduce the temperature difference between the inside and outside of the concrete.