Compliance, relaxation and creep recovery of normal strength concrete.

Abstract

Calculating the long term serviceability of reinforced concrete structures requires an appropriate description of the mechanical properties of concrete. Among the long duration properties of concrete are shrinkage and creep of concrete. At relative humidities less than 96--99% concrete shrinks. For concrete, creep is the deformation under load after having subtracted the shrinkage and elastic deformation. For specimens loaded in a drying environment there is creep due to drying in addition to creep measured without moisture movement. The movement of moisture is a major complication in developing creep prediction equations. In addition, to estimate shrinkage and creep, the structural engineer is required to calculate the loss of prestress and strain/deflection under loads which vary in time. Superposition can be used to calculate recovery, relaxation, ageing and redistribution---the validity of superposition for concretes with moisture movement is discussed. In November 1999 ACI 209 adopted guidelines that should be met by prediction methods. A revised, code type, prediction model, which meets the ACI guidelines, is presented and compared to the RILEM data bank of shrinkage and creep measurements. Providing moisture movement is considered irreversible, relaxation can be calculated by superposition. Calculated relaxations are compared to experimental results. Calculating recovery by superposition is sensitive to the assumptions made concerning moisture movement. Moisture movement, drying before loading, state of moisture on reloading or unloading are critical to the adaptability, and complication of a model. Normal strength concretes, defined as concretes with mean compressive strengths less than 82 MPa, are concretes which do not experience self desiccation. The shrinkage due self desiccation is sensitive to the early age availability of moisture.