Effects of Shrinkage In Construction

Shrinkage

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.

See More  Different Grades of Concrete

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.

Aggregate Shrinkage

Generally,the aggregate will shrink less than the cement paste, so increasing the aggregate content will reduce shrinkage.

Read More

Cracking in Concrete and Types

Factors Affecting Concrete Strength

Causes and Prevention of Segregation