Effects of Shrinkage In Construction

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Shrinkage causes:

Cracking but only if the element is restrained.

Deflection,normally additional to the creep.

Autogenous Shrinkage

This is the inevitable shrinkage that results from the hydration of cement without additional water,And is typically 40 microstrain after 1 month.It is greatest for mixes with a high cement content,but Never sufficient to cause cracking.Stresses from it are rapidly relieved by creep,if no other shrinkage occurs.In wet curing,swelling occurs with similar or greater strains.

Thermal Shrinkage

Concrete is frequently relatively warm, when initial set occurs.This may be due to the heat of hydration,or other effects such as sunlight on the concrete, or the aggregate storage.When it subsequently cools,it will shrink. Typical coefficients of thermal expansion are 5–10 microstrain °C − 1.

Plastic Shrinkage

This occurs before final set,and is caused by bleeding. As water is lost from the concrete, its volume decreases. Rapid drying from the surface (i.e., faster than bleeding) will cause substantial plastic shrinkage.

Drying Shrinkage

The effect of early drying is plastic shrinkage. Drying shrinkage is a long-term phenomenon,and occurs when the pore water is lost.Typical values are 500 microstrain at 28 days at 50% RH.

Drying shrinkage is associated more with loss of water from gel pores(formed in the gel during hydration), than with capillary pores that are larger, and are initially occupied by water. Thus, the pastes, which have hydrated more and have a higher proportion of gel pores, will shrink more, for less water loss.

Continuous wet curing will cause expansion,continuous drying will cause shrinkage.Sealed curing will just cause autogenous shrinkage. Alternate wetting and drying will cause swelling and shrinkage with each cycle, with a net overall shrinkage or swelling effect depending on the mix.

Carbonation Shrinkage

Carbonation is normally of interest for durability because it causes loss of alkalinity, leading to reinforcement corrosion.It does,however,also cause some shrinkage.This shrinkage is closely related to drying shrinkage, and the combined effect of both will depend on the sequence in which they take place (i.e., carbonation during drying or after it). Typical values for mortar are 800 microstrain at 50% RH.