Content area
Full Text
QAs a designer of industrial concrete floors, I've always placed a priority on reducing the potential for slab cracking. One of the approaches I have used is to minimize restraint to drying shrinkage by lowering friction on the underside of the slab. For example, I might specify a layer of sand on top of a crushed stone base to choke off the surface of the stone and create a smooth interface between the base and slab. Recently, however, some contractors and owners have expressed concerns regarding "dominant joints." What are dominant joints and how can they be avoided?
AAfter placement, loss of heat and moisture result in temperature contraction and drying shrinkage in a slab. Any restraint to the resulting change in volume causes tensile stresses to develop, and these can result in cracking. Joints are cut into slabs to create thinner crosssections and stress risers that encourage the concrete to crack in a predetermined pattern rather than randomly. After a crack occurs at a joint, continued shrinkage will cause the sawcut to widen.
ACI 302.1R-151 recommends minimizing the shrinkage potential of the concrete. While that report provides recommendations for optimizing concrete mixtures for minimal shrinkage, it should be noted that shrinkage testing is needed to determine how a mixture will actually perform.
ACI 360R-102 includes a chart (Fig. 6.6) to help the designer establish the required joint spacing to minimize the potential for random cracking between joints based on the shrinkage potential of the concrete (the traditional recommendation of spacing joints at 36 times the slab thickness is only advisable when low-shrinkage concrete is used-ultimate shrinkage strain of about 0.05%). Perimeter wall connections, truck docks, columns, bollards, plumbing penetrations, and slab base friction can restrain shrinkage, and the combination of these influences determines which...