10.7
Drying Shrinkage
When hardened concrete is exposed to air with a relative humidity of less than 100 percent, it begins to lose the free water within its capillaries. A…
When hardened concrete is exposed to unsaturated air, the free water present in its capillaries evaporates out of the pores.
This causes the movement of water adsorbed on the large surface area of calcium silicate hydrates present in the cement paste into the dried-out capillaries. This water eventually evaporates out of the concrete.
As a result, the volume of the cement paste reduces with time, causing contraction in concrete, known as drying shrinkage.
However, some drying shrinkage is reversible if the concrete is exposed to external moisture. The cement paste absorbs the moisture and undergoes wetting expansion.
The irreversible portion of drying shrinkage results from the rearrangement of particles as gel pores collapse and the formation of Si-O-Si bonds within the cement gel upon removing adsorbed water.
If the concrete member subjected to drying shrinkage is free to move, it will experience an increase in strain with time while the stress will remain zero.
Alternatively, if the member is restrained during drying shrinkage, the strain will be zero, and tensile stresses will develop in the member over time.
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Q1: What causes drying shrinkage in concrete?
Drying shrinkage occurs when hardened concrete is exposed to unsaturated air and free water in capillaries evaporates. As capillaries dry, adsorbed water on calcium silicate hydrates migrates into these empty spaces and eventually evaporates as well. This water loss reduces the cement paste volume, causing the concrete to contract over time.
Q2: Is drying shrinkage in concrete reversible?
Drying shrinkage is partially reversible. When concrete is exposed to external moisture, it reabsorbs water and undergoes wetting expansion, reversing some shrinkage. However, the irreversible portion results from permanent rearrangement of particles as gel pores collapse and new Si-O-Si bonds form within the cement gel when adsorbed water is removed.
Q3: How does concrete restraint affect drying shrinkage behavior?
Unrestrained concrete experiencing drying shrinkage develops increased strain over time while stress remains zero, allowing free deformation. Conversely, restrained concrete cannot move during drying shrinkage, so strain remains zero but tensile stresses develop progressively within the member, potentially causing cracking and contributing to types of non-structural cracks in concrete.
Q4: What factors influence the extent of drying shrinkage?
Drying shrinkage extent is influenced by the duration of wetting and drying periods, relative humidity of the air, concrete composition, and the degree of hydration when drying begins. Lower relative humidity accelerates water evaporation, while higher hydration degrees and specific cement paste compositions can increase shrinkage magnitude and rate.
Q5: How does relative humidity affect water movement in drying concrete?
When relative humidity drops below 100 percent, free water in capillaries evaporates first. As capillaries empty, adsorbed water on calcium silicate hydrates migrates toward these dried spaces. Lower humidity accelerates this evaporation process, increasing the rate and extent of volume reduction in the cement paste.
Q6: What physical changes occur in cement paste during irreversible drying shrinkage?
Irreversible drying shrinkage involves gel pore collapse and formation of new Si-O-Si bonds within the cement gel when adsorbed water is removed. These permanent structural rearrangements of hydrate particles cannot be reversed by rewetting, distinguishing irreversible shrinkage from the reversible portion caused by capillary water loss.
Q7: Why does concrete volume decrease during drying shrinkage?
Concrete volume decreases because water loss from both capillaries and adsorbed layers on calcium silicate hydrates reduces the total mass and occupies less space than the original water-filled pores. As water evaporates progressively, the cement paste contracts, causing measurable volume reduction in the hardened concrete over time.