Leaderboard

It seems that ACI is clear in its provision and usage of 0.6% rather than 0.2% reinforcing ratio for temperature & shrinkage crack/stress control could not be justified for general structures and is only applicable in cases where crack width is a design consideration. As ACI -350 table 7.12.2.1 recommends only for "structures subjected to environment or required to be liquid tight" a reinforcing ratio range from .003 to .006 for Grade 40 & .003 to .005 for Grade 60 steel as a function of length between movement joints from 20' to 40'.(see ACI 350 7.12.2.1). Thus the extreme ratios ".005" & ".006" represents the maximum temp & shrinkage reinf ratio for "structures subjected to environment or required to be liquid tight" where movement joint is not provided.This maximum ratio is mentioned in ACI 224 and is supposed to be followed when crack width is a design consideration for eg in structures as defined above.(note here structures subjected to environment could simply refer to outdoor structures in accordance with CIRIA technical note 107 section 2) On the other hand as long as general structures is concerned (say building structures) crack width is not a design consideration in accordance with same code (ACI 318) and here provision of minimum reinf (0.18-0.2%) intends to limit shrinkage & temperature stresses rather than crack widths within tolerable limits (see ACI 318-11 7.12.1) which is claimed to be observed for many years satisfactorily controlled with these ratios. As long as design consideration of shrinkage & temperature effects is concerned in general structures, we include the effects of shrinkage & temp cracks indirectly in serviceability limit states as 1, In the form of additional vertical deflection due to creep & shrinkage of structural members (see ACI 435R-95 table 2.1). 2, Effective stiffness (see ACI 318-11 10.10.4) to be used in the analysis of lateral deflection accommodates the stiffness reduction due to shrinkage & temp cracking. 3, ACI 318 also recommends to analyze for the severity of stresses arising from temp & shrinkage effects (see ACI 318-11 8.2.4 & 9.2.3) Hence, usage of 0.18-0.2% reinf and 0.3-0.6% reinf is justified for general and special structures respectively.