What Is Drying Shrinkage Of Concrete?
What Is Drying Shrinkage Of Concrete?
Drying shrinkage is a phenomenon in which concrete contracts due to the loss of water from its capillary pores, leading to an increase in tensile stress that may cause cracking, internal warping, and external deflection before the concrete is subjected to any external load.
This type of shrinkage occurs in all Portland cement concrete as it ages and can affect various structures such as slabs, beams, columns, walls, prestressed members, tanks, and foundations.
The amount of drying shrinkage that occurs depends on various factors such as the properties and proportions of the concrete components, the mixing method, the amount of moisture during curing, the dryness of the environment, and the size of the structure.
Additionally, the shrinkage potential of a particular concrete can be influenced by the amount of mixing, the elapsed time after adding water, temperature fluctuations, slumping during placement, and the curing conditions.
The specific characteristics of the cement and aggregate used in the concrete, as well as the amounts of water and admixtures added during mixing, can also affect the drying shrinkage of the concrete. This shrinkage is primarily caused by the evaporation of water from the concrete’s capillary pores.
The extent of the shrinkage will depend on the physical properties of the concrete, including the size and location of the structure, as well as the surrounding temperature.
Factors Affecting Shrinkage Of Concrete
The relative humidity of the atmosphere where the concrete is stored is a significant factor that affects concrete shrinkage. If the concrete is placed in an environment with 100% relative humidity, it will not shrink, but rather will undergo slight swelling. The relationship between shrinkage and time for concrete stored at different relative humidities shows that shrinkage increases with both time and reduction in relative humidity.
The rate of shrinkage decreases rapidly over time, with 14-34% of the shrinkage that occurs over a 20-year period occurring within the first 2 weeks, 40-80% occurring within the first 3 months, and 66-85% occurring within the first year.
The water-cement ratio and richness of the concrete also have an impact on shrinkage, as does the type and size of the aggregate used and its modulus of elasticity. Harder, high modulus aggregates like quartz cause less shrinkage than softer aggregates like sandstone.
Concrete will shrink when exposed to drying conditions with low relative humidity and will swell when kept in 100% relative humidity or when placed in water. However, not all of the initial drying shrinkage is recovered when the concrete is stored in water for a prolonged period, indicating that the drying shrinkage process is not fully reversible.
Swelling when placed in wet conditions and shrinking when placed in drying conditions is known as moisture movement in concrete.
The water-cement ratio of the concrete, which determines the quality of the cement paste, can significantly impact the magnitude of shrinkage. Concrete with a higher water-cement ratio tends to experience greater shrinkage. Therefore, it can be concluded that, for a given aggregate content, the shrinkage of concrete is directly related to the water-cement ratio.
The type, size, and shape of the coarse aggregate used can also affect the loss of moisture and, consequently, the shrinkage of concrete. In general, using larger aggregate sizes can reduce shrinkage because they have a smaller surface area, leading to less water absorption.
Drying Shrinkage Cracks
Drying shrinkage cracks in concrete are caused by volume changes due to the loss of excess water, which leads to shrinkage. These cracks usually occur several months to three or four years after the concrete has been poured and can be more likely to form in thin members with large surface areas, such as slabs, as well as in concrete near corners and edges.
The cracks may form in any location where there is a restraint to shrinkage movement and are typically perpendicular to the direction of restraint.
Some aggregates can also be susceptible to changes in volume due to moisture fluctuations and should be used with caution. The occurrence of drying shrinkage cracks can be minimized through appropriate mix design and adequate curing.
How Do You Prevent/Reducing Drying Shrinkage Cracks?
To reduce the risk of drying shrinkage cracking in concrete, it is effective to use a mix with a lower water content or to increase the volume of aggregates in the mix, which can minimize the volume of cement paste.
Proper curing, especially for slabs, can also help the concrete to develop greater tensile strength before drying occurs.
At the design stage, incorporating movement joints and adequate crack control reinforcement can help to limit the extent of shrinkage cracking, aiming to produce a greater number of fine cracks rather than a few wider ones.
These measures can help to reduce the occurrence of drying shrinkage cracking in concrete.