Creep and Shrinkage Strains in Construction Stage Analysis

During analysis of a stage, creep and shrinkage related effects are handled separately and their effects are combined and added to instantaneous deformations. Such effects are cumulative and their histories are tracked separately for each concrete member.

In the program, age of a concrete member is marked with $t_{o}$ days when it is added to a stage. It is considered that the concrete member is in a formwork for $t_{o}$ days and it is not yet subjected to any loading. When the formwork is removed at day $t_{o}$ , the member is assumed to be loaded and restrained by its supports or by adjacent members at the same time.

It is further assumed that creep related strains and stresses start to develop only after formwork is removed. Similarly, strains and stresses due to shrinkage start to develop at the same time (some shrinkage strains can be developed while still in formworks but this is ignored in the analysis).

If duration of a stage is t days and if a concrete member is

t_{o}

days old when loaded, the member is

{t + t}_{o}

days old at the end of the stage. Creep strain accumulated during the period of

(t + t_{o}, t_{o})

is expressed as

Δ ε_{c r} = ε_{c r} (t + t_{o}, t_{o})

t_{c}

marks the day when concrete member starts drying (i.e., it is the time moist curing stops), accumulated shrinkage strain at the end of the stage is

Δ ε_{s h} = ε_{s h} (t + t_{o}, t_{c}) - ε_{s h} (t_{o}, t_{c})

Note that the amount of shrinkage occurring between ages $t_{c}$ and $t_{o}$ is subtracted in the above equation due to the assumption that any shrinkage strains before formwork is removed are ignored in the analysis.