360R-06 Design of Slabs-on-Ground
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- 8.3—Reinforcement 8.3.1
| 8.2—Thickness determination
For a slab-on-ground cast with shrinkage-compensating concrete, the determination of the slab thickness required by imposed loading is similar to that used for other slab designs. The PCA, WRI, and COE methods are all appropriate. They are discussed in Chapter 5 and illustrated in Appendixes l , 2 , and 3 . Appendix 5 illustrates other design considerations specific to the use of the shrinkage-compensating concrete. 8.3—Reinforcement 8.3.1 Restraint—An elastic type of restraint, such as that provided by internal reinforcement, should be provided to develop shrinkage compensation. Other types of restraint, such as adjacent structural elements, subgrade friction, and integral abutments, are largely indeterminate, and may provide either too much or too little restraint. Subgrade frictional coefficients in the range of one to two have been found satisfactory. High restraint will induce a high compressive stress in the concrete but provide little shrinkage compensation. Wherever possible, the design should specify the reinforcement recommended in ACI 223. 8.3.2 Minimum reinforcement—A minimum ratio of reinforcement area to gross concrete area of 0.0015 should be used in each direction that shrinkage compensation is desired. This minimum ratio does not depend on the yield strength of the reinforcement. When procedures outlined in ACI 223 are followed, however, a reinforcement ratio less than the aforementioned minimum may be used. 8.3.3 Effect of reinforcement location—The location of the steel is critical to both slab behavior and internal concrete stress. ACI 223 recommends that reinforcement be posi- tioned 1/3 of the depth from the top. The function of the top reinforcement is to balance the restraint provided by the subgrade, in addition to providing elastic restraint against expansion. Caution is advised when using smaller percentages of reinforcement because lighter gage material may be more difficult to position and maintain in the top portion of the Fig. 8.1—Typical length change characteristics of shrink- age-compensating and portland-cement concretes (ACI Committee 223 1970). Fig. 8.2—Effect of reinforcement on shrinkage and expansion at an age of 250 days (American Concrete Institute 1980). |