Cracking and Control in Mass Concrete
Mass concrete is characterized by thick structure, large size, large pouring volume, long construction time, and high construction process requirements. The construction of mass concrete is relatively difficult, and cracking is easy to occur in processes.
Common types of cracking in mass concrete
Drying shrinkage cracking
An essential cause of concrete drying shrinkage cracking is the evaporation of water, a process that develops gradually from the surface to the inside, so the humidity and the shrinkage deformation inside and outside the concrete are uneven. The main reason for drying shrinkage is that a large number of fine pores are generated by cement hydration, producing a large amount of heat. With dry conditions, the free water in the colloid gradually evaporates, and capillary action occurs. Under compression, the volume of the colloid shrinks continuously as the water evaporates, thereby the volume of the concrete also shrinking.
Cracking results from the effects of shrinkage, creep, elastic properties and tensile strength. It occurs when the surface shrinkage deformation is restricted by the internal concrete, and the shrinkage stress reaches the tensile strength of concrete.
Plastic shrinkage cracking
Plastic shrinkage is mainly due to two aspects. One is that after the concrete is compacted, due to the differences in the density, quality, and shape of the concrete raw materials, settlement and bleeding occur. For concrete with a large water-cement ratio or obvious bleeding, after the moisture on the upper surface evaporates, the volume of the concrete is reduced. And the other is that the rate of moisture loss on the concrete surface is too fast, forming a concave surface, generating negative capillary pressure. Before the concrete hardens, the modulus of elasticity is very low. If the tensile strength of the concrete surface is lower than the tensile stress caused by the limiting shrinkage, plastic shrinkage begins.
Subsidence cracking is caused by uneven and soft soil quality of the structural foundation, or uneven settlement caused by improper backfill or water immersion, or because of insufficient stiffness of formwork, excessively large formwork support spacing or loose support bottom, etc.
Thermal cracking mostly occurs on the surface of mass concrete or concrete structures in areas where the temperature difference changes greatly. After the concrete is poured, in the hardening process, the heat of hydration is generated. Due to the large volume of concrete, a large amount of heat accumulates inside the concrete and is not easy to dissipate, causing the internal temperature to rise sharply, while the surface of the concrete dissipates heat faster. This creates a large temperature difference, and causes different degrees of thermal expansion and contraction between the inside and outside of the concrete. Certain tensile stress on the surface of the concrete is generated and cracks will occur when it exceeds the ultimate tensile strength of concrete. This cracking happens mostly in the middle and late stages of concrete construction.
Material control for mass concrete cracking
Selection of cement
Cement with small shrinkage or slight expansion is preferred. Because this kind of cement can produce certain pre-compression stress during the hydration expansion period (1-5 days), and the pre-compression stress partly offsets the temperature creep stress in the late hydration period, reduces the tensile stress in the concrete, and improves the concrete’s crack resistance ability.
Selection of aggregate
Choose aggregates with a low coefficient of linear thermal expansion, good gradation, clean surface, so that smaller porosity and surface area can be obtained, thereby reducing the cement amount, the hydration heat, and the drying shrinkage. The sand should be coarse or medium, with mud content equal to or less than 3%. The gravel or pebbles with a particle size of 0.5-3.2mm can be used.
Minimizing the amount of water
Water has a dual effect on concrete. The hydration reaction is inseparable from water, but the excess water stored in the concrete will not only affect the structure of the cement paste, and the interfacial transition zone between the aggregate and the cement paste, but also cause shrinkage in cement paste. If the internal stress generated by the shrinkage exceeds the resistance of the interfacial transition zone, microcracks may occur in this interface zone, reducing the ability of the concrete to resist tensile stress.
Control of concrete setting and hardening process
Under constrained conditions, shrinkage and deformation of hardened concrete will produce elastic tensile stress. The approximate value of tensile stress can be initially assumed to be the product of Young’s modulus and deformation. When the induced tensile stress exceeds the tensile strength of concrete, the concrete will crack. But because concrete is a heterogeneous material composed of complex phases with both viscosity and ductility, some of the stress is released by relaxation, whether the concrete cracks is determined by the residual stress after the stress relaxation.
The concrete should be covered immediately after vibrating. It is best to seal and maintain with a plastic sheet to prevent dehydration and cracking of the concrete. Covering insulation material can effectively control thermal cracking. The withdrawal time of the insulation material should be based on the temperature difference between the inside and the surface of the concrete, and the temperature difference between the surface and the atmosphere, which is less than 25°C. Generally, after the concrete is poured, the third and fourth days are the peaks of temperature rise, and then the temperature is gradually reduced. It is advisable to remove the insulation materials for more than 10 days, and the cooling rate should not be too fast to prevent cracks caused by thermal stress.
During construction, temperature measurement is very important. The temperature difference dynamics of concrete should be monitored at all times. Copper thermal sensors can be embedded in different parts of the concrete for temperature measurement. And it is also used with glass thermometers for verification. When the temperature difference is found to exceed 25°C, the insulation should be strengthened in time or the removal of insulation materials should be postponed, so as to prevent cracks caused by thermal stress.
Control of admixture
Adding fly ash to concrete can improve the impermeability and durability of concrete, reduce shrinkage, the heat of hydration of the cementitious materials, increase the tensile strength of concrete, inhibit the alkali-aggregate reaction, reduce bleeding in fresh concrete and so on. These benefits will help improve the crack resistance of concrete. But it will significantly reduce the early strength of concrete, which is disadvantageous in preventing cracking. Tests have shown that when the replacement rate of fly ash exceeds 20%, it has a greater impact on the early strength of concrete and is particularly unfavorable for crack resistance.
- Frost resistance: After 300-500 rapid freeze-thaw cycles, the relative elastic modulus of silica fume concrete is lowered by 10-20%, while that of conventional concrete is lowered by 30-73% after 25-50 cycles.
- Early strength: Silica fume reduces the induction period and helps in obtaining high early strength.
- Erosion resistance and cavitation erosion resistance: Compared with conventional concrete, the erosion resistance of silica fume concrete is increased by 50%-250%, and the cavitation erosion resistance is increased by 300-1600%.
The retarding superplasticizer can increase the tensile strength of concrete, and is extremely important for reducing the unit water consumption and cement consumption of concrete, improving the workability of fresh concrete, as well as the mechanical, thermal, and deformation properties of hardened concrete.
In addition to significantly improving the ability of concrete to resist freeze-thaw cycles and corrosion environments, air-entraining agents can significantly reduce the bleeding of fresh concrete, improve the workability of concrete, reduce the elastic modulus of concrete, and optimize the microstructure of the concrete.