How to prevent concrete cracking?
Prevention of massive concrete cracking, from the following aspects:
1. Design measures
(1) designed concrete. In ensuring good workability of the concrete situation, and minimize concrete unit water consumption, the use of "three low (low sand rates, low slump, low water-cement ratio) Second-doped (doped superplasticizer and high performance air-entraining agent) a high (high fly ash content), "the design criteria to produce" high-strength, high toughness, shot, low and high tensile value "crack concrete.
(2) was equipped with structural reinforcement, improve crack resistance. Here should be using a small diameter, closely spaced reinforcing way, the whole cross-section of the reinforcement ratio should be 0.3% to 0.5%.
(3) to avoid structural mutations produce stress concentration. Stress concentration weak link in easy to take measures to strengthen.
(4) Set the dark edge of the beam in breakable parts, improve the parts of the reinforcement ratio, improve the ultimate tensile strength of concrete.
(5) in the structural design should take full account of climatic characteristics during the construction, after setting a reasonable poured seam; under normal working conditions, after pouring seam spacing is 20 ~ 30m, the retention time is generally less than 60d can not be predicted, such as construction. Specific conditions can also be designed according to specific circumstances temporary changes.
2. Raw material control measures
(1) try to use low heat or in the heat cement (such as slag cement, fly ash cement), or take advantage of the late strength of concrete (90 ~ 180d), in order to reduce the amount of cement to reduce the heat of hydration (10kg cement because each addition and subtraction, the temperature will increase or decrease accordingly 1 ℃, and the amount is proportional to the heat of hydration of cement) under conditions permit, preference should be given small contraction or expansion of a micro cement. Because of this cement hydration expansion period (1 ~ 5d) may have some pre-stress, and in the late pre-stress can be partially offset by hydration temperature creep stress, reduce the tensile stress in the concrete, improve the crack concrete ability.
(2) appropriate fly ash. After the concrete mixed with fly ash can improve concrete impermeability, durability, reduce shrinkage, lower heat of hydration of cementitious materials, improve the tensile strength of concrete, inhibition of alkali aggregate reaction, reducing the fresh concrete The bleeding water.
(3) Select the well-graded aggregate. When aggregate mass concrete proportion is generally 80% to 83% of the absolute volume of concrete, so the choice of aggregate, should choose a low coefficient of linear expansion, low elastic modulus of rock, surface cleaning parcels no weak layer level with good aggregate. Generally, you can use 4 ~ 40mm diameter coarse aggregate, sand as far as possible; strict control of sand, gravel, clay content (less than one percent stones, sand in less than 2%), control water-cement ratio of 0.6 less; it can also mixed in the concrete retarder, slowing casting speed, in order to facilitate heat dissipation. It also can consider adding solid no cracks in mass concrete, rinse, specifications for large stone of 150 ~ 300mm. Stone doped increase not only reduces the total amount of concrete, reduces the heat of hydration, but the stones themselves also absorbed the heat, so to further reduce the heat of hydration, there are certain advantages to control the crack.
(4) The appropriate selection of superplasticizer and air-entraining agents, which reduce the massive concrete and cementitious material consumption per unit of water, improving the work of the fresh concrete, hardened concrete to improve the mechanical, thermal, deformation, durability, etc. Performance plays an extremely important role.
3. The method of construction of mass concrete construction control measures should be appropriate to set aside some internal channels.
Inside pass circulating cold water or cold air cooling, the cooling rate should not exceed 0.5 ℃ ~ 1.0 ℃ / h. For large equipment base block can be layered pouring (each interval of 5 ~ 7d), block thickness of 1.0 to 1.5 m, in order to facilitate the dissemination and reduce heat of hydration restraints. When pouring the concrete foundation on a rock or Heavy concrete cushion, on the rocks or concrete pad foundation laying anti-skid insulating layer (second degree poured asphalt glue, sprinkle sand or shop shop two 5mm thick felt three oil), the floor mutations at the ups and downs and a sectional variations made gradually to eliminate or reduce the binding effect. Furthermore, it should strengthen the water vibrated concrete, improve the density. As late form removal, concrete surface after form removal temperature should not drop more than 15 ℃. Try using two vibrating technology to improve concrete strength, improving crack resistance. Also based on specific project characteristics, UEA shrinkage-compensating concrete technology.
4. Temperature Control Measures
Concrete temperature and temperature on concrete cracks are extremely sensitive. When the concrete temperature decreases from zero stress to concrete cracking temperature, concrete concrete ultimate tensile stress exceeds the tensile stress at this time. Therefore, by lowering the temperature inside the concrete hydration heat and concrete initial temperature, reduce and avoid the risk of fracture.
Measures manual control concrete temperature cracks due to heat causes early action is not obvious. Insulation materials such as surface protection can reduce the temperature difference between inside and outside, but inevitably lead to a high body temperature of the concrete, from bound through the cracks resulting from the perspective of a potential deterioration in the conditions of crack, because sooner or later the body heat is dissipated to the. In addition, the artificial control of concrete temperature needs to be taken to prevent over-speed cooling and ultra-cold, too fast cooling not only make concrete temperature gradient is too big, too fast and early ultracold affect cement - the degree of hydration and the early strength of the colloidal system, more likely to have an early heat cracks. Ultracold make concrete temperature is too large, causing the temperature cracks. Thus, pouring time should try to arrange at night to minimize the initial setting temperature of the concrete. During the day the construction, requires sand, stone yard erection simple shading devices, or covered with wet sacks, to aggregate spray cold water if necessary. When concrete pumping, straw bags can be stamped in the horizontal and vertical tubing and sprayed with cold water.
Source: China mortar Network