Mitigating early-age cracks in concrete structures is critical for sufficient durability, avoiding possible strength reduction, and reducing maintenance costs.
Not only do early-age cracks diminish the aesthetic appearance of a building, but also make occupants uncomfortable and affect the contractor's reputation.
Therefore, suitable measures should be implemented to reduce or eliminate early-age crack development. The mitigation strategies begin from the design phase of the building to the construction phase, including material selection, mix proportioning, curing conditions, and environmental conditions.
How to Mitigate Early-Age Cracking in Concrete Structures?
1. Design Process
During the design process, restraints in various concrete elements should be followed as they restrict concrete movements due to thermal expansion and autogenous shrinkage. Cracks usually develop when the ratio of stress/strength exceeds 60%.
Restraints that should be accounted for during the design process are associated with concrete cover, non-uniform concrete depth, and reinforcement. Reduction in concrete cover increases cracking possibilities, and its influence is far greater than reinforcement size.
Dimensions and geometry of the structural member are critical for temperature gradient, which is a reason for early-age crack development.
2. Material Selections and Mix Parameters
Concrete mixture is commonly produced from cement, aggregate, water, and additional materials, including supplementary cementitious materials and admixtures. The influence of each material on early-age crack development is presented below:
2.1 Aggregates
Size, roughness, and coefficient of thermal expansion of coarse aggregate affect early-age cracking in concrete.
Large aggregate sizes increase the transition zone in concrete, reducing tensile strength and young’s modulus of hardened concrete. Lower tensile strength means concrete experiences cracking under lower tensile stresses.
Fully saturated aggregates increase the chance of early-age crack development. So, using dry and small-sized aggregate can improve concrete strength. Similarly, using crushed aggregate with rough surfaces improves the concrete tensile strength.
Aggregate with a low coefficient of thermal expansion can reduce thermal expansion in concrete. For example, quartzite aggregate generates higher tensile stresses than limestone by 50% due to its high coefficient of thermal expansion. Therefore, the possibility of cracking is higher in concrete made with quartzite aggregate.
2.2 Cement Types
Cement type and content significantly affect generated heat of hydration, and they can be optimized to improve concrete performance and decrease the possibilities of early-age crack development.
Portland cement , that is not too fine, along with low alkali and high sulfate content, lower the cracking temperature. So, the increase of sulfate content in cement reduces cracking temperature if the cement has low alkali content.
For water-cement ratios ranging from 0.4 to 0.7, reduction of cement content decreases drying shrinkage. However, if the water-cement ratio exceeds 0.7, the tensile strength of concrete decreases, resulting in high chances of cracking.
Using a different type of cement in high-strength concrete does not affect early-age crack development, provided that an equal water-cement ratio is used.
2.3 Supplementary Cementitious Materials and Admixtures
Admixtures and supplementary cementitious materials like fly ash, slag, water reducing admixture, shrinkage compensation cement, self-compacting admixture, and shrinkage reducing admixture can be employed to improve concrete performance.
Concrete produced by blending 50% Portland cement, 30% slag, and 20% fly ash performs better than concrete made using only ordinary Portland cement. Shrinkage reducing admixture and self-compacting admixture reduce shrinkage with the provision of sufficient moist curing. They also reduce the heat of hydration and subsequently the thermal shrinkage of concrete.
The diffusion of the heat of hydration also affects early-age cracking, especially in massive concrete structures. Compared with normal strength concrete, cracks due to heat diffusion are more frequent in high strength concrete due to a smaller extent of diffusion of the heat of hydration.
3. Construction Procedure
Concrete crack development is highly sensitive to the temperature of fresh concrete. When fresh concrete's temperature is decreased, concrete's tensile strength increases. This is because the bond created at a low temperature between calcium silicate hydrate gel is stronger than the bond formed at a high temperature.
The concrete placement procedure should be carefully planned because concrete placement sequence, formwork, insulation, cooling, and neighboring structure influence temperature reduction during the cooling phase and create temperature differences in concrete. This can lead to tensile stresses and possible early-age cracks.
An increase in formwork conductivity reduces the risk of early-age cracking during concrete casting, so choose the formwork material carefully.
Increasing curing temperature in high-strength concrete will increase autogenous shrinkage and ultimately increase the chance of early-age cracking. The optimum curing temperature for high-strength concrete to mitigate early-age cracking is 20 oC.
4. Environmental Conditions
Surrounding temperature, initial temperature, humidity, wind, and solar radiation affect the evaporation of water.
Dry air around the construction site increases the initial temperature of concrete elements, which is further increased by the heat of hydration. This can lead to improper concrete strength development due to incomplete hydration, increasing the likelihood of early-age cracking.
Rainfall during the first day of concrete placement can increase the water-cement ratio, reducing strength and increasing the chance of early-age cracking.
5. External Loading Conditions
The movement of heavy vehicles affects high slump concrete (>175 mm). Therefore, heavy traffic should be restricted during the first day of concrete casting.
FAQs
Mitigating early-age cracks in concrete structures is critical for satisfactory durability, avoiding possible strength reduction, and maintenance cost reduction. Not only do early-age cracks diminish the aesthetic appearance of a building, but they also make occupants uncomfortable and affect the contractor's reputation.
The mitigation strategies begin from the design phase of the building to the construction phase, including material selection, mix proportioning, curing conditions, and environmental conditions.
Yes, the type of aggregate is an influential factor; for example, quartzite aggregate generates higher tensile stresses than limestone by 50% due to its high coefficient of thermal expansion. So, the possibility of cracking is higher in concrete made with quartzite aggregate.
Concrete crack development is highly sensitive to the temperature of fresh concrete. When fresh concrete's temperature is decreased, concrete's tensile strength is increased. This is because the bond created at a low temperature between calcium silicate hydrate gel is stronger than the bond formed at a high temperature.
For water-cement ratios ranging between 0.4 to 0.7, cement content reduction leads to decreased drying shrinkage. The reduction of drying shrinkage means concrete is less susceptible to early-age cracking.
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