Discussion on anti-crack technology of mass concrete in water conservancy construction?
1 Causes of cracks in mass concrete
1.1 Temperature difference cracks
The temperature difference between the surface layer of concrete formed by hydration heat of cement and the inside of concrete is too great. Generally speaking, mass concrete structures are required to be poured in one go. After pouring, it is difficult for the hydration heat of mass cement to be concentrated in the inside, so its internal temperature will rise obviously, and its surface layer will dissipate heat faster, resulting in a great temperature difference. At this time, the tensile strength of concrete is low and the age of concrete is short. Once the surface tensile stress formed by temperature difference is greater than the ultimate tensile strength of concrete, cracks will appear on the surface of concrete.
1.2 Shrinkage cracks
In the process of slowly hardening and heat dissipation, the shrinkage stress of concrete is relatively large, especially for mass concrete structures. If the stress is greater than the ultimate tensile strength of concrete at that time, shrinkage cracks will occur in concrete. In mass concrete, even if the water-cement ratio is large and the shrinkage value is not too large, the stress will increase when the shrinkage is superimposed at the same temperature. Therefore, in the process of mass concrete construction of large hydraulic structures, Chinese medicine should first measure and consider its own shrinkage factors.
1.3 stability cracks
The reason why cracks appear is mainly attributed to unqualified cement stability, which is the main manifestation of stability cracks.
2 control measures of cracks
2.1 prevention and treatment of shrinkage cracks
when preparing concrete, we should attach great importance to the control of cement dosage, species, water-cement ratio, admixture and aggregate selection, etc.
2.1.1 Selection and use of cement
When selecting dam concrete, special mud with low alkali content, high strength, long initial setting period, low calorific value and good plasticity should be preferred. The chemical composition ratio of ordinary portland cement clinker is shown in Table 1.
From the above table, it can be seen that C2S has slow splashing speed, small hydration heat, small volume shrinkage and high strength in the later stage, so it can be known that C2S is a suitable mineral, and its content should be properly controlled because C3A has the largest hydration heat. Considering the comprehensive factors, it is best to control the proportion of chemical composition of portland cement clinker within the range of C2S ≥ 8%, C3A≤5% and C4AF≤15%. It is best not to choose too many kinds of cement, and the quality inspection should be strictly carried out when the cement is transported to the construction site, and the relevant tests should be carried out.
2.1.2 dosage of cement and its water-cement ratio
In order to keep the fluidity, cohesiveness and water retention of cement, and to avoid unqualified concrete, it can't be poured. Therefore, when preparing concrete, it is necessary to reduce the amount of cement and the water consumption per unit of cement, that is, the water-cement ratio. The maximum water-cement ratio and the minimum cement consumption have certain regulations, as shown in Table 2.
2.1.3 sand aggregate
in mass concrete, the proportion of aggregate is usually 8% ~ 83% of the absolute volume of the concrete, so the aggregate with small volume expansion coefficient, low elastic modulus and uniform gradation should be selected. In addition to the size of sand particles to meet the requirements of aggregate, sand should be properly added with more stone powder or fine powder, because stone powder or fine powder can improve the compactness, working performance, durability and crack resistance of concrete. Research shows that the proportion of stone powder generally accounts for 15% ~ 18% of sand.
2.1.4 admixture
before selecting cement admixture, it must be tested and determined, and then the active materials that meet the requirements are selected from the test, because this is to avoid mixing unqualified materials and prevent affecting the strength and life of concrete. Fly ash can be mixed into concrete, which can improve the impermeability and durability of concrete, improve the tensile strength of concrete, reduce the shrinkage and bleeding of concrete, reduce its hydration heat and inhibit the reaction between alkali and aggregate, all of which are helpful to improve the crack resistance of concrete. However, it is necessary to ensure that the fineness of fly ash is equivalent to that of cement particles, and its sulfur content and alkali content are low, and the water demand ratio is small. In addition to fly ash, cement with admixture has good comprehensive performance, such as slag portland cement with slag and pozzolanic portland cement with pozzolanic ash.
2.1.5 admixture
in order to make concrete develop towards high performance, admixture is an indispensable and important component and should be actively used. Such as superplasticizer and air entraining agent in admixture, mixing them can reduce the unit water consumption of mass concrete and the amount of cementing material, which not only reduces the workload required for mixing concrete, but also improves the mechanical and thermal properties of hardened concrete and its durability.
2.2 temperature difference cracks
temperature difference cracks are caused by the excessive difference between the internal temperature and the surface temperature of concrete caused by the hydration heat of concrete. Therefore, in order to avoid temperature cracks, it is necessary to reduce the hydration heat. The factors that lead to large hydration heat are water consumption, cement consumption, water-cement ratio, admixture and admixture when preparing concrete, but there are also control problems in construction. Therefore, in order to better prevent temperature cracks, we must improve construction technology and strengthen construction management.
2.2.1 pouring method
The concrete pouring of mass structure can be divided into three pouring methods, namely, overall layering, sectional layering and inclined layering. If the concrete pouring strength is required to be high, the comprehensive layered pouring method is adopted; If the required pouring strength is small, the inclined plane layering method is adopted. The casting method can be selected according to the shape and size of the structure, the tamping method and the supply capacity of concrete. At present, the inclined plane layering method is commonly used.
2.2.2 tamping of concrete
The tamping process of concrete is the process of vibrating the concrete poured into the mould, which can improve the compactness of concrete. Because vibrating can discharge bubbles, the materials in concrete can be mixed more evenly, and the maximum compactness can be obtained. For larger concrete, in order to better resist cracking, it is best to carry out twice vibrating technology.
2.2.3 comprehensive measures to control the initial temperature of concrete
① limit the temperature of the discharge port. When the outside temperature is high, artificial cooling measures must be taken. Such as spraying aggregate with cold water and cooling aggregate with cool air.
② cold water pipes are embedded in dam concrete for cooling.
2.2.4 Formwork removal
The concrete shall be removed as late as possible, and the surface temperature of concrete shall not drop more than 15℃ after formwork removal, and the strength of field test block shall not be lower than C5.
2.3 stability
the volume stability of concrete refers to the performance of whether the volume changes evenly during the hardening process of cement. Poor stability will cause quality accidents such as expansive cracks or warping deformation of components. Excessive free calcium oxide or magnesium oxide in clinker or excessive gypsum content can make the stability poor. Therefore, the ingredients must be strictly checked.
3 concrete reinforcement treatment
3.1 grouting treatment
grouting is an effective method to treat concrete cracks. Be sure to drill holes and grout on the concrete that needs reinforcement grouting.
3.2 structural reinforcement
There are many methods for structural treatment, one is the steel bar anchor bolt used for small fullness reinforcement. The other is to dig a groove about 1m wide along the crack surface after the dam body is stabilized, and then backfill high-quality concrete. Although the construction is complicated, this method may be the best effective measure at present.
3.3 backfill after excavation
In fact, pouring accidents, cracks and insufficient concrete strength are everywhere, and their influences are very complicated. At this time, it is usually very difficult for cement grouting to achieve the expected effect, and even some structural reinforcement measures are useless. For this reason, we can try to dig it out and backfill it again according to the actual situation. Only by strictly controlling the temperature stress and construction quality in the backfilling process can the new and old concrete be combined well.
4 Conclusion
From the demonstration and explanation in the previous article, it is not difficult to see that the crack problem of mass concrete can be prevented by slow control in the whole process of concrete configuration, material selection and construction. Therefore, in practice, we must strictly control each process and carry out standardized treatment, so as to ensure the quality and safety of mass concrete hydraulic structures.
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