Similar to ACI 302, ACI 360 also provides recommendations for reducing the effects of slab shrinkage and curling. Note that there is no recommendation to limit placement size:
"Appropriate design and specification provisions can reduce shrinkage cracking and curling. Such provisions should include:
- Relative shrinkage of various concrete mixtures;
- Type and location of reinforcement;
- Subgrade friction;
- Concrete planarity;
- Slab thickness;
- Shrinkage restraints;
- Location of sawcut contraction joints; and
- Properly designed vapor retarder/barrier and aggregate blotter systems."
ACI 301-10 Specifications for Structural Concrete
ACI 301-10 “Specifications for Structural Concrete” added a new section on Industrial Floor Slabs. This section includes minimum default specification provisions for ground-supported industrial floor slabs. While provisions on concrete shrinkage, maximum joint spacing, and details on isolation, construction, and contraction joints are included, the specification does not limit the placement size. The specification only requires a submittal showing the extent of each placement, placement sequence and schedule for each placement.
Avoid specification limits on placement size
Large block placements are the most efficient method of slab placement, and they minimize the number of construction joints, improving overall floor flatness. If the specifications include placement size limits, exclude them from your bid. As an alternate in your bid, price the specified small placement sizes and include the impact on schedule. The Owner will then clearly see an increased cost and an extended schedule as a result of this specification clause. And with no compensating benefits.
Placing Sequence Recommendations
ACI 302.1R-04 "Guide for Concrete Floor and Slab Construction" offers the following text on placing sequence.
18.104.22.168 Placing sequence—In many cases, the most efficient way to place concrete in large areas is in long alternating strips, as illustrated in Fig. 8.1. Strip placements allow superior access to the sections being placed. Intermediate contraction joints are installed at specified intervals transverse to the length of the strips. Wide strip placements can require installation of longitudinal contraction joints.
Large block placements with interior contraction joints are an acceptable alternative to strip placements if the contraction joints are installed at specified intervals in a timely manner. The use of shrinkage-compensating concrete (because of the decrease in jointing requirements) or laser screeds (because they provide accurate strikeoff without the use of edge forms) is compatible with large block placements.
A checkerboard sequence of placement with side dimensions of 50 feet or less, as shown in Fig. 8.1, has been used in the past in an effort to permit earlier placements to shrink and to obtain minimum joint width. Experience has shown, however, that shrinkage of the earlier placements occurs too slowly for this method to be effective. Access is more difficult and expensive, and joints may not be as smooth. The committee recommends that the checkerboard sequence of placement not be used.