Guide for Modeling and Calculating Shrinkage and Creep in Hardened Concrete

Description

To predict the strength and serviceability of reinforced andprestressed concrete structures, the structural engineer requiresan appropriate description of the mechanical properties of thematerials, including the prediction of the time-dependantstrains of the hardened concrete. The prediction of shrinkageand creep is important to assess the risk of concrete cracking,and deflections due to stripping-reshoring.

As discussed inACI 209.1R, however, the mechanical properties of concreteare significantly affected by the temperature and availability ofwater during curing, the environmental humidity and temper-ature after curing, and the composition of the concrete,including the mechanical properties of the aggregates.Among the time-dependant properties of concrete that are ofinterest to the structural engineer are the shrinkage due tocement hydration (self-desiccation), loss of moisture to theenvironment, and the creep under sustained loads.

Dryingbefore loading significantly reduces creep, and is a majorcomplication in the prediction of creep, stress relaxation, andstrain recovery after unloading. While there is a lot of data onshrinkage and compressive creep, not much data are availablefor creep recovery, and very limited data are available forrelaxation and tensile creep.Creep under variable stresses and the stress responsesunder constant or variable imposed strains are commonlydetermined adopting the principle of superposition.

Thelimitations of this assumption are discussed in Section 1.3.Further, the experimental results of Gamble and Parrott(1978) indicate that both drying and basic creep are onlypartially, not fully, recoverable. In general, provided thatwater migration does not occur as in sealed concrete or theinterior of large concrete elements, superposition can beused to calculate both recovery and relaxation.