Mass concrete structures generate hydration heat during the cement hydration process, which causes temperature variations. When the shrinkage of concrete itself is constrained by external factors, thermal stress and shrinkage stress will form inside the concrete. Once the stress exceeds the ultimate tensile strength of concrete, cracks will emerge consequently. Such cracks often cause varying degrees of damage to engineering projects. Therefore, controlling the development of thermal stress and temperature-induced deformation cracks is a major challenge in the construction of mass concrete structures.
I. Causes of Cracks in Mass Concrete Construction
Concrete is a heterogeneous material composed of multiple ingredients, and its tensile strength is far lower than its compressive strength. Cracks will occur and even lead to structural failure when tensile stress surpasses the tensile strength of concrete. Cracks in mass concrete structures are categorized into surface cracks and through cracks, both of which pose certain hazards. Comparatively speaking, through cracks impair the integrity, durability and normal service performance of structures, and even endanger structural safety. The main causes of crack formation are as follows:
Cracks caused by external loads
This type of crack is the most common, induced by primary stress calculated in accordance with conventional methods.
Cracks caused by secondary structural stress
Such cracks arise from the discrepancy between the actual working state of the structure and the computational assumption model.
Cracks caused by deformation and variation
These cracks are triggered by deformation resulting from temperature change, shrinkage, expansion, uneven settlement and other factors.
Inside concrete structures, mutual influence and mutual restriction always exist between structural components. For concrete structures with large cross-sectional dimensions, the uneven distribution of internal temperature and humidity restricts the deformation of different internal parts of the structure. Meanwhile, the deformation of concrete structures is also affected by external structures. Mass concrete features a high cement dosage; the hydration heat released by cement hydration leads to significant temperature changes and shrinkage effects, and such thermal stress is the primary cause of concrete cracking.
II. Major Measures to Prevent Crack Formation
(I) Control Concrete Temperature Rise
Select low hydration heat cement
Hydration heat refers to the heat released during the hydration of cement clinker. To reduce concrete temperature rise, under the premise of meeting design strength requirements, cement dosage can be reduced, and medium-low hydration heat cement is preferred. Slag cement or fly ash cement is generally applicable to conventional engineering projects.
Utilize the later strength of concrete
Test data shows that for each cubic meter of concrete, every 10 kg increase or decrease in cement dosage leads to a corresponding 1℃ rise or fall in concrete temperature affected by hydration heat. Therefore, based on actual structural conditions, after recalculating structural stiffness and strength and obtaining approval from design and quality inspection departments, the design strength of concrete can adopt f45, f60 or f90 instead of f28. This can reduce cement consumption by 40~70 kg per cubic meter of concrete, and correspondingly cut down the temperature rise caused by hydration heat by 4~7℃.
The utilization of concrete later strength mainly starts from mix proportion design. Tests shall verify that concrete strength continues to grow after 28 days and can reach or exceed the design strength within the scheduled time.
Admix water reducers and micro-expansion agents
Adding an appropriate amount of water reducer or retarder can reduce cement dosage, improve workability and delay the peak period of hydration heat. Mixing proper amount of micro-expansion agent or expansive cement can also reduce the thermal stress of concrete.
Add fly ash admixture
Replacing part of cement with a small amount of finely ground fly ash in concrete can not only lower hydration heat but also improve the plasticity of concrete.
Reasonable selection of aggregates
Concrete prepared with continuously graded coarse aggregates boasts good workability, lower water and cement consumption, and higher compressive strength. In addition, the silt content of sand and stone shall be strictly controlled: the silt content of sand shall be less than 2%, and that of stone shall be less than 1%.
Reduce concrete discharge temperature and pouring temperature
First, lower the mixing temperature of concrete. The most effective way to reduce concrete discharge temperature is to cool the stone aggregates. In high-temperature weather, aggregates shall be kept away from direct sunlight; when necessary, aggregates can be sprayed with water mist or rinsed with cold water before use.
In addition, procedures including loading, unloading, transportation and pouring all affect concrete temperature. Hence, in hot summer, the time from the mixing plant to formwork placement shall be minimized as much as possible.
(II) Adopt Thermal Insulation or Moisture Retention Curing to Slow Down Concrete Cooling Rate
According to different construction seasons, moisture retention curing is mainly adopted in summer and thermal insulation curing in winter to reduce the internal and external temperature difference of concrete after pouring. After the final setting of mass concrete structures, retaining a certain depth of water on the surface provides a thermal insulation effect, narrows the internal and external temperature difference of concrete, and thus restrains crack propagation. After formwork removal of mass concrete for foundation works, backfilling shall be conducted as soon as possible to avoid adverse impacts from sudden temperature changes, slow down the cooling rate and prevent crack formation.
(III) Optimize Construction Technology to Improve Concrete Crack Resistance
Adopt layered and segmented pouring method
This facilitates the dissipation of hydration heat in concrete and reduces the internal and external temperature difference.
Optimize reinforcement arrangement
Avoid stress concentration and enhance the resistance to thermal stress. Stress concentration easily occurs around holes, at variable cross-section corners and turning points. To solve this problem, diagonal steel bars and steel mesh sheets shall be additionally arranged around holes; local treatment shall be carried out at variable cross-sections to achieve gradual transition, and anti-crack reinforcement shall be added to prevent cracks. It is worth noting that small-diameter and small-spacing steel bars should be adopted as far as possible with symmetrical full-section arrangement.
Set post-pouring belts
For mass concrete with oversized plane dimensions, post-pouring belts shall be installed to reduce external constraint force and thermal stress, facilitate heat dissipation and lower the internal temperature of concrete.
Implement temperature monitoring
Timely monitor temperature differences to guide curing work dynamically, and control the internal and external temperature difference of concrete within 25℃.
Cracking is a common problem in concrete structural engineering. It not only affects the appearance of buildings, but also undermines the waterproof and impermeability requirements of structures, reduces building durability, and even impairs structural bearing capacity. The occurrence and development of cracks result from multiple factors. While improving the professional quality of construction personnel and adopting more reasonable construction techniques, we shall strictly implement the principles of pre-control, in-process control and post-control, strengthen supervision by functional departments, and leave no hidden dangers for engineering projects.
