Crack prevention in the construction of the hottes

Crack prevention in mass concrete construction

mass concrete construction is mainly characterized by large volume, and the minimum size of the general entity is greater than or equal to 1 meter. Its surface coefficient is small, the cement water Zui is large, the test force is 50kN, the chemical heat release is relatively concentrated, and the internal temperature rise is fast. When the temperature difference between inside and outside the concrete is large, the concrete will produce temperature cracks, which will affect the safety and normal use of the structure. The construction technology requirements of mass concrete are high, especially in the construction, it is not easy to choose the experimental joint of plastic materials to prevent the temperature stress crack caused by the temperature difference caused by the hydration heat of cement. Therefore, we need to make full preparations from the relevant links such as material selection and technical measures. Taking a mass concrete project as an example, this paper discusses the causes and prevention of cracks

first, the main causes of cracks

1. The heat of cement hydration. Cement needs to release a certain amount of heat during hydration. However, the section of mass concrete structure is thick and the surface coefficient is relatively small, so the heat generated by cement is concentrated in the structure and is not easy to dissipate. In this way, the hydration heat inside the concrete cannot be emitted in time, so that it accumulates higher and higher, increasing the temperature difference between inside and outside. The hydration heat of cement released by concrete per unit time is related to the amount and variety of cement per unit volume of concrete, and increases with the age of concrete. Since the surface of the concrete structure can dissipate heat naturally, in fact, most of the maximum internal temperature occurs in the first 3-5 days after pouring

2. Change of external temperature during the construction stage, the pouring temperature of mass concrete changes with the change of external temperature. In particular, the sudden drop in temperature will greatly increase the temperature difference between the inner and outer concrete. Temperature control measures shall be taken to prevent temperature stress caused by temperature difference inside and outside the concrete

3. Shrinkage of concrete the main cause of concrete shrinkage is the shrinkage of concrete caused by internal water evaporation. The alternation of dry and wet will cause the alternation of concrete volume, which is very disadvantageous to concrete

II. The preparation work before pouring

affects the shrinkage of concrete, mainly including the variety of cement, concrete mix proportion, the variety of admixtures and admixtures, as well as the construction technology (especially the curing conditions)

1. Material selection the project adopts commercial concrete pouring. The requirements for main materials are as follows:

(1) cement: considering that the hydration heat of ordinary cement is high, especially when applied to mass concrete, a large amount of hydration heat of cement is not easy to dissipate. 7. The tensile machine is protected twice a month on average. The temperature is too high, resulting in a large temperature difference with the surface of concrete, so that compressive stress is generated in the concrete, and tensile stress is generated on the surface, while glass is only 0.5-1.5kj/m2; "Ford" uses transparent plastic headlights to reduce the number of parts. Therefore, slag Portland cement with low hydration heat, grade 525, is determined to be used to improve the performance of concrete and its impermeability by adding appropriate additives

(2) coarse aggregate: crushed stone with particle size of 5 ~ 25 mm and silt content of no more than 1% The concrete prepared with large particle size and well graded stones has good workability and high compressive strength. At the same time, it can reduce the water consumption and cement consumption, so as to reduce the hydration heat of cement and reduce the temperature rise of concrete

(3) fine aggregate: medium sand, with an average particle size of more than 0.5mm and a silt content of no more than 5% Compared with the concrete mixed with fine sand, the concrete mixed with medium and coarse sand with larger average particle size can reduce the water consumption by about 10%. At the same time, the amount of cement can be reduced accordingly, so as to reduce the hydration heat of cement, reduce the temperature rise of concrete, and reduce the shrinkage of concrete

(4) fly ash: since the pouring method of concrete is pumping, in order to improve the workability of concrete and facilitate pumping, it is considered to add an appropriate amount of fly ash. The amount of fly ash shall be controlled within 10

(5) admixture: 2kg water reducing agent per cubic meter of concrete can reduce the peak value of hydration heat, compensate the shrinkage of concrete, and improve the crack resistance of concrete

2. Concrete mix ratio

(1) commercial concrete supplied by the mixing plant is used for concrete, and the concrete mixing plant is required to make concrete trial mixing in advance

(2) the concrete mix proportion should be determined through trial mix. The design shall be carried out in accordance with the relevant technical requirements in the current national code for construction and acceptance of concrete structures, code for design of mix proportion of ordinary concrete and technical code for application of fly ash concrete

(3) when fly ash is added, only the amount of sand of the same volume is deducted from the sand. In addition, the supply of cement should be considered to meet the construction requirements