l. The main factors of concrete shrinkage:
The water consumption per unit of concrete, the composition of stones and sand, the duration of initial wet curing, and other factors such as the particle size of stones, the mix ratio of the mixture, and the slurry content of the mixture also affect the drying shrinkage, and the total demand of the mixture The amount of water is the main factor affecting drying shrinkage.
① Most of the concrete in modern industrial and civil construction has the properties of large-volume (large-area) concrete, and attention should be paid to volume stability (heat of hydration and shrinkage). Concrete (including concrete mixed with different admixtures) generally shows a slight expansion deformation in water. In the air, it generally shrinks and deforms.
②The greater the amount of cement, the greater the water consumption, the greater the amount of cement slurry, the greater the slump, and the greater the shrinkage; avoid pouring concrete in the rain, in case of light rain. Rainproof measures should be taken and the water-cement ratio should be adjusted (reduce water consumption).
③ The larger the water-cement ratio, the greater the dry shrinkage. Generally, the water-cement ratio of high-strength concrete is relatively small, which is beneficial to reduce the later dry shrinkage. However, due to the large amount of cement slurry or cementitious materials and the effect of high-efficiency water reducers Shrinkage is larger than that of medium and low strength concrete. Too low water-cement ratio is unfavorable to early plastic shrinkage. It is easy to produce early drying shrinkage cracks due to dehydration.
④ the larger the area of the storm road, the greater the shrinkage.
⑤ The shrinkage of slag cement cement is larger than that of ordinary (P.O) cement and composite (P.C) cement, the shrinkage of fly ash cement is smaller, and the shrinkage of rapid hardening cement is larger. The heat of hydration of the composite cement is lower than that of ordinary (P .0) cement, so the cement type should be selected according to the thickness, volume and construction of the concrete.
⑥Sandstone aggregate shrinkage increased significantly. Coarse and thin. The greater the mud content in the aggregate (sand, stone), the greater the shrinkage, and the lower the tensile strength, the mud content should be strictly controlled:
⑦The earlier and longer the early curing time (7-14 days), the smaller the shrinkage. Moisturizing and curing to avoid severe drying can effectively reduce shrinkage stress. Pay attention to vibration, especially at the junction of beams and slabs (or wall panels), but do not over-vibrate to prevent segregation and massive bleeding. After the floor is poured. It will be sprayed soon. Carrying out secondary calendering near the final setting (receiving pulp and grinding surface), covering with sacks or straw, secondary calendering and strengthening wet curing are very beneficial to control cracks.
⑧The greater the ambient humidity, the smaller the shrinkage: the higher the ambient temperature. The more dry the exercise, the greater the shrinkage.
⑨The coarser the aggregate (sand, stone) particle size, the smaller the shrinkage: the smaller the aggregate particle size, the higher the sand rate (that is, more sand and less stones), and the greater the shrinkage.
⑩The higher the cement activity, the finer the particles, the larger the specific surface area, and the larger the shrinkage; the higher the ambient temperature and cement temperature. The greater the shrinkage, the factory cement with high temperature should be avoided to be used directly in concrete construction, and the cement should be cooled before use.
11) the greater the reinforcement ratio, the smaller the shrinkage, and too large (coarse) reinforcement will increase the tensile stress of the concrete: the reinforcement should be thin and dense, not thick and thin. Pay attention to strengthen the structural reinforcement in the shrinkage stress concentration area, and strengthen the non-prestressed reinforcement in the prestressed structure.
12) The greater the wind speed, the greater the shrinkage. Pay attention to the high-altitude cast-in-place concrete. Avoid direct sunlight on the freshly poured concrete surface in dry winds and hot seasons, that is, to avoid rapid loss of water in the concrete: the formwork must be packed tightly to avoid water leakage (cement slurry), and the formwork must be drenched.
13) Improper selection of admixtures and admixtures will significantly increase shrinkage. Choose the right one to reduce shrinkage, especially early-morning shrinkage.
14) The higher the environment and concrete temperature, the greater the shrinkage, and the longer the shutdown exposure time, the greater the shrinkage.
15) Shrinkage and environmental cooling occur at the same time, especially the ambient temperature is cold and hot alternately, and the wind is more unfavorable to the project.
16) Backfill the soil as soon as possible, the soil is the best curing medium for concrete. Closing down and painting the house early is good for reducing shrinkage.
17) The greater the bleeding, the higher the water content on the concrete surface. Increased early surface shrinkage: However, early dehydration of the concrete surface should be avoided to increase shrinkage, since a small amount of bleeding and enhanced wet curing are beneficial to reduce cracks in concrete.
18) Medium and low-strength grade cement with small dosage. Concrete with relatively low water-cement ratio (less mixing water is required) and small slump will take about one year to complete most of the shrinkage; the amount of cement is relatively large. The water ash is relatively large and the slump is large. The completion time of most of the shrinkage of concrete with higher strength grade is about 2 years; the final shrinkage time of concrete is about 20 years. The slow shrinkage of concrete will not produce large destructive cracks, but early cracks caused by improper construction and maintenance should be avoided (at this time, the concrete strength cannot resist the shrinkage stress). The treatment and repair of slight shrinkage cracks are not "quality accidents".
