Large volume concrete is one of the common concrete structures in water conservancy construction, and due to its large volume, the quality requirements are high. Therefore, in the process of pouring, the materials, mix ratio, water-cement ratio, strength, etc. that meet the requirements should be selected according to the engineering design requirements of the contract. However, in the actual construction, the concrete often cracks after the completion of casting, in order to solve the problem of large volume concrete cracks, it is necessary to strengthen the study of its.

Analysis of the causes of cracks in large volume concrete in water conservancy construction

　　(A) the factors of the concrete itself

　　The concrete used in water conservancy construction is usually made of cement, sand and gravel, admixtures, water and so on in accordance with a certain proportion with the mixing, but if the proportion is not reasonable, or the preparation of the mixing is not uniform will cause the quality of concrete decline. Based on the characteristics, concrete is a shrinkage body, which has a high elastic mold and low tensile strength, so it is extremely easy to produce cracks in the construction process. And there are several reasons for its cracks.

Firstly, whether the strength of sand and gravel, the type and dosage of admixture and the acidity and alkalinity of water used in the preparation of concrete meet the requirements of construction design; secondly, whether the size of shrinkage of cement meets the design standard.

Again, whether the water-cement ratio of the prepared concrete is large under the condition that the construction requirements and ease are met.

Finally, in the preparation, whether the mixing time and temperature are well controlled according to the requirements of mixing.

In addition, in the process of pounding, whether the amount of pounding is controlled within a reasonable range. The above factors will affect the quality of concrete to varying degrees, and once the quality of concrete is not up to standard will inevitably cause the rapid evaporation of water inside the concrete, which in turn will cause a huge internal shrinkage force, eventually causing cracks on the surface of concrete.

　　(B) the influence of external factors

　　1、Temperature

　　Concrete cracks caused by changes in the external temperature is the most direct and common cause of cracks in concrete. Specifically, the temperature during construction and the temperature difference after construction is too large, it is easy to produce cracks. The main reason for this is that, in the bulk concrete structure, often take a molding pouring, and such a large concrete will gather a large number of heat generated by the hydration of cement inside, can not be distributed in a timely manner, thus leading to the external temperature of concrete is much lower than its internal temperature, in the principle of thermal expansion and contraction of cement will be expansion or contraction, this period of internal and external forces will not be consistent, in The force difference between the internal and external forces will be inconsistent, and eventually lead to the production of concrete cracks.

　　2、Shrinkage

　　The preparation of concrete requires the addition of water, and there will be a hardening process when concrete is placed in a molding. In this process, the water in the concrete will continue to evaporate, which in turn takes away the concrete heat, making the concrete shrink continuously and increasing strength. Due to the difference in the quality of the concrete prepared, poor quality concrete will crack when it is subjected to a shrinkage force greater than that which it can withstand. Generally speaking, the concrete is able to withstand this shrinkage force, but in practice, this shrinkage force will overlap with the shrinkage force generated by the temperature difference, eventually causing the total shrinkage force on the concrete is greater than the concrete’s capacity, resulting in the appearance of concrete cracks.

　　3、Later maintenance

　　According to the relevant investigation, improper maintenance is also a major cause of cracks in concrete. After the concrete is poured, the concrete body is not covered in time to avoid rapid heat dissipation, coupled with the lack of timely and regular watering maintenance work, thus causing the concrete in a short period of time will evaporate its surface moisture, especially in the hot summer, in a high-temperature environment, the moisture of the concrete will also evaporate quickly, and then produce cracks.

Anti-cracking technology of large volume concrete in hydraulic construction

　　(A) Solve the influence of temperature difference in construction
First, for the problem of cracks caused by temperature differences, it should focus on the prevention of water vaporization, the main purpose is to avoid excessive temperature differences in concrete, and then make the shrinkage force is greater than the concrete bearing force, and produce cracks. The specific method is to control the temperature of concrete, optimize the amount of water used in the preparation of concrete.
Second, in the water conservancy construction of concrete pouring has a certain universality, so the construction personnel may not pay attention to the pouring work, in order to rush the schedule when pouring, the speed is too fast, reducing the quality of pouring. In this regard, it is best to choose the way of layered pouring to ensure the strength of the concrete, and at the same time also according to the actual demand for vibrating work to ensure that the concrete filled the entire mold, in order to discharge the air bubbles in the concrete, improve its compactness, in order to complete the pouring work with high quality and efficiency.
Third, after the completion of casting, under the requirements of ensuring the quality of concrete and the requirements of the schedule and other conditions, appropriately extend the demoulding time, and try to control the temperature of the concrete at about 15 ℃ when it is formed.

　　(B) to solve the impact of shrinkage in construction
First, reasonable control of the quality of cement. For concrete, the quality of the cement determines its quality. Concrete is the basis of the entire water conservancy construction, Goo reasonable control of the quality of cement is very important. The choice of cement is best in combination with the construction design requirements on the basis of the best possible choice of plasticity, high strength, and good heat dissipation of cement, in order to avoid cracks caused by large temperature differences.
Second, the reasonable choice of sand and gravel aggregate. The proportion of sand and gravel aggregate in the concrete is also relatively large, and in the selection to try to choose low modulus, small expansion coefficient and relatively clean sand and gravel aggregate. This can improve the crack resistance of concrete.

　　(C) to address the impact of post-conservation

　　After the completion of the concrete placement, but also in accordance with the construction requirements, timely and regular maintenance of it. Generally speaking, within twelve hours after the completion of the overall pouring to cover it all up, and watering and moisturizing treatment; specific watering and moisturizing time or times, depending on what kind of cement it takes, if it is ordinary silicate cement need to carry out more than seven days of maintenance, if the addition of retarder, it needs to be in more than half a month.

　　Conclusion

　　Through the above analysis, it can be seen that the causes of cracks in mass concrete are mainly due to concrete itself and external causes, and external causes include temperature, shrinkage and post-conservation factors. For these factors, to solve its crack problem, one should solve the influence of temperature difference in construction, the second should solve the influence of shrinkage in construction, and the third should solve the influence of post-conservation.