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Carrasquillo [1989] observed a slight increase in creep for concretes with varyingpercentages of Class C fly ash replacement, and a slight decrease in creep for concreteswith Class F fly ash. The difference was attributed to the effect of the varying CaOcontents between the two classes of fly ash.Several models have been proposed for predicting creep as a function of time for agiven concrete under a given set of load and exposure conditions. The most commonmethod in use in the United States is probably the ACI 209 [1992] method, which isbased on the work of Branson et al. [1971]. This method allows for the determination ofan ultimate creep coefficient, C cu , either by experimental tests or by assuming andmodifying a base value of 2.35. This base value was determined by analyzing the resultsof 120 specimens from several experimental studies conducted prior to 1970 on normalweight,lightweight, and sand-lightweight concretes [Branson, D.E., 1971]. The assumedbase value is meant to represent a general average for many concretes, and should bemodified using a set of multipliers developed for various factors related to concretecomposition. These factors include slump, fine aggregate percentage, cement content, andair content. The multipliers corresponding to these factors may be found in the ACI 209Committee Report [1992].The corrected (assumed) or experimentally determined ultimate creep coefficient isthen modified using multipliers corresponding to loading, curing, and environmentalconditions. The multiplier for ambient relative humidity is given as:γ = 1.27 − (0.0067 ⋅H) for H ≥ 40 ..............Equation 2.4rhwhere H is the relative humidity in percent. The multiplier γ rh defaults to 1.0 for a“standard” relative humidity of 40 percent. For relative humidity less than 40 percent, noequation is given, but γ rh must be taken as greater than 1.0.A multiplier for size and shape effects may be applied in one of two forms, onerelated to average member thickness and another related to volume-to-surface ratio. Themultiplier corresponding to average member thickness is given as:7