ACI 209.2R-08 Guide for Modeling and Calculating Shrinkage and Creep in Hardened Concrete

ACI 209.2R-08 Guide for Modeling and Calculating Shrinkage and Creep in Hardened Concrete.
1.2—Scope This document was developed to address the issues related to the prediction of creep under compression and shrinkage- induced strains in hardened concrete. It may be assumed, however, that predictions apply to concrete under tension and shear. It outlines the problems and limitations in developing prediction equations, presents and compares the prediction capabilities of the ACI 209R-92 (ACI Committee 209 1992), Bažant-Baweja B3 (Bažant and Baweja 1995, 2000), CEB MC90-99 (Muller and Hillsdorf 1990; CEB 1991, 1993, 1999), and GL2000 (Gardner and Lockman 2001) models, and gives an extensive list of references. The models presented are valid for hardened concrete moist cured for at least 1 day and loaded at the end of 1 day of curing or later. The models apply to concretes with mean compressive cylindrical strengths at 28 days within a range of at least 20 to 70 MPa (3000 to 10,000 psi). The prediction models were calibrated with typical composition concretes, but not with concretes containing silica fume, fly ash contents larger than 30%, or natural pozzolans. Models should be calibrated by testing such concretes. This document does not provide information on the evaluation of the effects of creep and shrinkage on the structural performance of concrete structures.
1.3.2 Linear aging model for creep—Experimental research indicates that creep may be considered approxi- mately proportional to stress (L’Hermite et al. 1958; Keeton 1965), provided that the applied stress is less than 40% of the concrete compressive strength. The strain responses to stress increments applied at different times may be added using the superposition principle (McHenry 1943) for increasing and decreasing stresses, provided strain reversals are excluded (for example, as in relaxation) and temperature and moisture content are kept constant (Le Camus 1947; Hanson 1953; Davies 1957; Ross 1958; Neville and Dilger 1970; Neville 1973; Bažant 1975; Gamble and Parrot 1978; RILEM Technical Committee TC-69 1988). Major deviations from the principle of superposition are caused by the neglect of the random scatter of the creep properties, by hygrothermal effects, including water diffusion and time evolution of the distributions of pore moisture content and temperature, and by material damage, including distributed cracking and fracture, and also frictional microslips. A comprehensive summary of the debate on the applicability of the principle of superposition when dealing with the evaluation of creep structural effects can be found in the references (Bažant 1975, 1999, 2000; CEB 1984; RILEM Technical Committee TC-107 1995; Al Manaseer et al. 1999; Jirasek and Bažant 2002; Gardner and Tsuruta 2004; Bažant 2007).ACI 209.2R pdf download.