Fig. 5.1—Appropriate locations for joints
Fig. 5.1—Appropriate locations for joints

The most recent news from the American Concrete Institute (ACI) is that ACI 360 “Design of Slabs-on-Ground” is now revised and available for purchase through ACI—updating the previous 1992 version of the document. What is considered to be most important about the new document depends on your viewpoint. Contractors, engineers, and owners each have their biases about what is significant. To help you decipher how the updated document might help you, here's a brief list of the significant changes and additions:

  • Intent. The preface to the document more clearly states that the best slab designs and installations still crack and curl, this is to be considered a natural part of what concrete does. The intent is to define characteristics of the material to relieve contractors' and designers' problems they have no control over. In the 1992 guideline, this shrinkage and curling is regarded as a problem to solve. Committee member Joe Neuber, Neuber Concrete, Kimberton, Pa., thinks this is a very important distinction because contractors are relieved of responsibility they shouldn't be accountable for.
  • Construction joints. The old document recommends that joints be placed every 24 to 36 times the thickness of the slab. The new document recommends that joint spacing not exceed 15 feet. The assumption is that there is little test information available for most local concrete materials around the country so mixes should be considered “high shrinkage concrete mixes” and joint spacing shouldn't exceed 15 feet ( In ACI 360 see 5.2—Load-transfer mechanisms, Figure 5.6)
  • Load Transfer. There is now a complete section in Chapter 5 of the new guidelines about load transfer and load transfer devices. This is barely mentioned in the 1992 version. It stems from the recognition that joints are often the first place where maintenance is required and that aggregate interlock doesn't provide enough load transfer across joints, especially in the case of industrial floors. Changing lift truck designs aggravate this problem as they concentrate loads more and more and lift heavier weights.
  • Vapor Barriers. The use or nonuse of vapor barrier materials and where they should be placed is more fully discussed in the new guideline.
  • Light reinforcement through contraction joints. Past guidelines advise designers and contractors to stop reinforcement before construction joints in order to allow for maximum movement in the joint and to discourage midpanel cracking. More recent information is changing this theory in the new guidelines.

The new ACI 360 guidelines are in closer agreement with ACI 302 guidelines. If you are an owner, an engineer, a designer, or a contractor you need to keep up with the changes. These new guidelines represent state-of-the-art thinking.