This is the first of an every-other-month column on slabs, based on the book Designing Floor Slabs on Grade by Boyd C. Ringo and Robert B. Anderson. Bob Anderson is in the process of updating this book and a third edition is due out later this year.

Mary Hurd's introduction to the book starts with seven questions:

  • How thick should the slab be?
  • How strong should the concrete be?
  • Is reinforcement needed?
  • Where should the joints be placed?
  • Can adding fibers enhance the slab's performance?
  • When is post-tensioning appropriate?
  • What can be done to control cracking?

Although generally these may be considered decisions made by the designer, contractors should understand the basics of slab design in order to be an involved partner in a project. “Design” includes all of the decisions, specifications, and details made and documented before construction can begin. It is based on both the subgrade support and the concrete material. The authors regard design as a two-step procedure: thickness selection is done by one of the recognized design methods, then other features such as joint location and treatment and construction tolerances are determined.

This month, we will start at the bottom and look at what information about the supporting soil is needed.

Introduction. A slab on grade cannot be designed without numerical values that come directly from knowing what supports the slab. At the very least, a value is needed for the modulus of subgrade reaction, commonly referred to as k; however, the grade support system is more complicated than is indicated by a single value. In addition to k, it is necessary to know the properties of the underlying soil and available fill material. In other words, to design and construct a quality slab on grade, one needs to know as much as possible about the grade system that supports that slab. The flow chart summarizes an orderly approach to obtaining this information.

Working with a soils specialist. The first consideration of any slab on grade design should be that of securing adequate geotechnical information. This should put the person responsible for the floor design into the process at the very beginning of any planning, which must include site considerations. When alternative sites are being evaluated for a project, soils conditions are often a significant economic factor. The floor designer should be able to advise the owner as to what soils information will be needed. He should do this along with the geotechnical engineer in order to provide an optimum geotechnical report. Too often the team effort of floor designer and geotechnical engineer is missing. This can lead either to costly overspending in obtaining soils information or to unexpected construction overruns due to omissions or errors in initial information. It must be emphasized that the slab on ground designer should be engaged either before or at the same time as the geotechnical firm.

Limit risk with insufficient information. The authors have found that in much routine slab on grade design no soils information is available other than the floor designer's experience. This experience is occasionally in the jobsite area, but frequently is not within that geographical area. This situation often leads to relying on what previous experience dictated, such as “6 inches has always worked” or “the soil is good.” If forced into this situation, the designer must protect himself by stating on the construction drawings what assumptions were made in the design process. The designer should also limit his liability by noting in writing the risks and possible consequences of inadequate soil information. Such steps not only protect the floor designer and inform the client but often result in the client's favorable reconsideration in providing geotechnical backup.