Materials: Concrete Creep

Description

When concrete is subjected to sustained loading, deformations continue to increase with time, even if the loading is constant. This time-dependent deformation under load is known as creep. It is a permanent deformation, which is not recovered even if the load is removed. Therefore, it is a type of plasticity.

In addition to creep, concrete also shrinks over time; shrinkage may occur even in the absence of loading.

Creep and shrinkage deformations in concrete structures are frequently larger than the initial elastic deformations produced when external loads are first applied, and therefore have significant effects on service-load behaviour.

The rate and extent of both creep and shrinkage deformations are influenced greatly by ambient conditions of temperature and humidity. Other relevant factors include type and content of aggregate, the water content in the mix and the size and shape of the structural member.

Whilst underlying mechanisms are inter-related, for most practical design calculations the two phenomena are regarded as being independent and additive. Therefore the total strain, as a function of time, is calculated as:

The instantaneous strain is usually taken to be elastic and hence to depend on the stress, , and the elastic modulus, , at time of loading. That is,

The creep strain depends on the stress, , the concrete age, , at the time of loading, and the duration of loading .

The shrinkage strain, , depends on the age, , of the concrete, as measured from the time of casting or completion of curing.