Following the recommendations of ACI 302.1 will result in perfect floors.
Ductilcrete Following the recommendations of ACI 302.1 will result in perfect floors.

Pity the concrete floor — it gets no respect. Seemingly ubiquitous but nearly invisible to most of those strolling across its surface, the concrete floor performs its function admirably in most instances while being subjected to every kind of abuse imaginable. As long as there’s not a problem, it goes unnoticed and unappreciated.

And yet, there are hundreds of things that can go wrong during the design, construction, repair, and maintenance of concrete floors. With poor understanding of the role of the soil-support system, the wrong concrete mix ingredients and proportions, or improper finishing techniques, a concrete floor can fail to perform as expected either early in its life or over the long run.

There are many resources available for designers, contractors, and owners that can help them avoid the mistakes made on concrete floors, but perhaps the most comprehensive document covering slab construction is ACI 302.1, Guide to Concrete Floor and Slab Construction. The first thing to note about this document is that it is a guide. The American Concrete Institute’s definition of “Guide” states that it is not intended to be written into a specification — it “merely” provides the industry’s state of the art knowledge on a topic. But while it may not be in the contract documents directly, if there’s a dispute, an ACI guide can, and often is, cited as the industry authority.

The second important thing to understand about ACI 302.1 is that it is intended as a how-to guide for contractors and to help them understand the design provisions that are laid out in ACI 360R-10, Design of Slabs on Ground. ACI 360R-10 and ACI 302.1R-15 are a complementary set.

A new edition of ACI 302.1 has just been published — the first updated version since 2004. There have been some new developments (new understanding, in some cases) over the past 11 years but the changes are not dramatic. “We have updated the methodology of some of the processes,” says ACI Committee 302 chairman Joe Neuber, owner of Neuber Concrete, Kimberton, Pa. “The vapor retarder flow chart has been updated and we have worked out design items with Committee 360, as well as reworded the lightweight section of the guide. The chapters were rearranged for better flow of the information. We also made many minor changes in language that will better-suit the readers’ understanding of the information in the guide. Any contractor doing concrete flat work should have and understand this guide.”

This article is sort of guide to the guide — some notes on those sections to which a contractor should pay particular attention. We will also indicate where changes have been made to the new edition (ACI 302.1R-15), which is now available from ACI.

Classes of floors

Now in Chapter 4, the table listing the eight classes of floors based on intended use is very similar to the 2004 version, although the classes have been renamed. This is a big improvement, since the old names were not very descriptive. For example, a Class 4 floor was named Single Course but is now Institutional/Commercial. A Class 6 was also named Single Course in the 2004 edition but is now Heavy Industrial.

Design considerations

Chapter 5 starts with an important list of items that should be addressed in the construction documents the contractor receives (Section 5.2.1). The 2015 guide flatly states that if any of the 15 items listed — such as, concrete thickness, joint locations, and curing — are not provided, the contractor should request it from the designer. This chapter has been reduced in size, though, since Committee 302 felt that design criteria belongs in ACI 360. Committee member Pat Harrison, Structural Services Inc., Kansas City, Mo., says some committee members wanted to get rid of all design information in 302, but other members resisted. “We tried to get rid of all design recommendations but some are still there,” he says. “There’s still some redundancy with ACI 360.”

The changes to this chapter were debated at length but in the end, design criteria for joints and load transfer (dowels) was moved to ACI 360 and made clear that it is the designer’s responsibility. Section 5.32.9 states: “When the joint layout and joint details are not provided before project bid, the designer should provide a detailed joint layout along with the joint details before the slab preconstruction meeting or commencing construction.”

Another point that was strengthened in the new guide is support of reinforcement, especially welded wire reinforcement. An alternative is fiber reinforcement (steel or synthetic) which is now covered in a new Chapter 7 (more later).