The causes of temperature cracks are as follows:
1. Due to the large temperature difference, the cement releases a large amount of hydration heat during the hardening of the concrete structure, and the internal temperature continues to rise. , causing a large temperature difference between the surface and the inside of the concrete, and the internal expansion of the concrete is higher than the outside. At this time, the concrete surface will be subject to great tensile stress, and the early tensile strength of the concrete is very low, so cracks appear. This temperature difference is generally only large at the surface and weakens quickly away from the surface. Therefore, cracks only occur close to the surface, and the structure below the surface layer remains intact.
2. The structural temperature difference is large. When a large volume of concrete is poured on a constrained foundation (such as a pile foundation), due to external constraints, no special measures are taken to reduce, relax or cancel the constraints, or simply Without the ability to eliminate constraints, cracks can easily develop until they penetrate the entire concrete.
The process of temperature crack formation is generally divided into three periods:
The first is the initial crack, which is during the heating period of concrete pouring. Due to the heat of hydration, the cracks will appear 2-3 years after the concrete is poured. The temperature rises sharply, and internal heat and external cold cause "restraining force", which exceeds the tensile strength of concrete and causes cracks.
The second is the mid-term crack, which is the hydration heat cooling period. When the hydration heat reaches the peak, the temperature gradually decreases. When the hydration heat is dissipated, the temperature of the structure is close to the ambient temperature, and the structure temperature causes "external constraints" Force" exceeds the tensile strength of concrete and causes cracks.
The third is late-stage cracks. When the concrete is close to the surrounding environmental conditions, it remains relatively stable. However, when the environmental conditions change drastically, because concrete is a poor conductor, a temperature gradient is formed. When the temperature gradient is large, the concrete will produce cracks. crack.
The occurrence of temperature cracks is generally inevitable. What is important is how to control it within the range allowed by the specification. To carry out effective control, scientific prediction must be carried out to ensure the accuracy of control. . The control of temperature stress is generally carried out on-site. When pouring concrete, use temperature sensors and thermometers to measure the temperature from the beginning of pouring (including mold entry temperature, ambient temperature), and apply pressure (especially before initial setting) and maintain heat and moisture in a timely manner. After pouring, the thermal insulation, moisturizing and curing conditions should be adjusted in time according to the temperature control indicators. It should be noted here that the temperature influence coefficient is affected by many factors, among which temperature, humidity, heat dissipation interface (soil, air, etc.), initial setting time, wind speed, temperature difference, etc. have a greater impact. Especially when the wind speed and temperature difference are large, the temperature The influence coefficient will be greatly reduced and the temperature rise will be reduced. However, in order to prevent the temperature from cooling too fast and forming a large temperature gradient, water storage is used for maintenance in summer, and straw bags and sponges are covered in autumn and winter. If the construction site is windy and the climate is dry, wind protection and insulation measures should be taken in time after the formwork is removed, and backfilling should be carried out in time. Soil, practice has proven that these methods are very useful in changing the temperature influence coefficient, and the control is also very successful.