Industrial slabs need contraction joints to minimize random cracking. The most effective and efficient method of producing contraction joints is by sawcutting. Understanding the why, where, when, and how to sawcut joints for industrial slabs will help produce the best slab.
WHY SAWCUT JOINTS
Sawed joints are widely used in industrial slabs for efficient and cost-effective crack control. Sawcut joints minimize random cracking that result from drying shrinkage and temperature changes. A joint creates weakened sections where cracks form. Joint activation (cracking) occurs beneath the sawed slot when shrinkage and temperature stress exceed the tensile strength of the concrete. The benefits of sawcut joints include:
- Consistent joint dimensions (same width and depth at each cut)
- Straight, clean cuts
- Good joint filler performance
- Cost-effective concrete placements of any size
WHERE TO SAWCUT
Contraction or control joints should be sawcut on or at the center of column lines. Intermediate joints may be needed between column lines to keep the maximum distance between joints at 24 to 36 times the slab thickness. The resulting panels should be as square as practical, dividing a large floor area into relatively small panels. Never make the long side of a panel more than 11/2 times the short side. Avoid placing concrete around re-entrant corners. However, when necessary, corner reinforcement can be used to control crack widths.
In general, joint spacing to control drying and thermal shrinkage in industrial slabs should range from 12 to 20 feet in unreinforced and lightly reinforced concrete floors. Joint spacing may vary, depending upon local conditions such as concrete materials and mixes, climate, construction practices, and subgrade or subbase restraint. Also, you should decrease joint spacing when using concrete suspected of having high shrinkage characteristics.
Make sawcuts continuous, not staggered or offset. Locate the joints according to the plans. If not shown on the plans, request that the engineer provide joint locations. In concrete floors with steel reinforcement, don't continue the bars across the joint unless required by the engineer. If the bars are continuous, the sawcut will be relatively ineffective in controlling crack location unless the slab is only lightly reinforced.
Joint spacing design is based on a number of considerations; the most important may be the forklift load transfer at the joint. If aggregate interlock is considered to transfer load at the joint, joint spacing is usually shorter--10 to 15 feet. The load transfer at sawcut joints is typically negligible if, due to drying shrinkage and temperature shortening, the joint opens up wider than 0.04 inch through the depth of the slab. While it is common for joints to widen more than 0.04 inch at the surface, the width often narrows quickly with depth so that effective aggregate interlock load transfer occurs. Sawcut timing and base elevation control are also important to avoid dominant joints where greater widening occurs. If dowels or other load transfer devices are incorporated, the joint spacing is typically greater--from 15 to 20 feet. The dowels or other load transfer devices can maintain load transfer even when the joint opens up to about 1/4 inch.
Another consideration is the cost based on the number of joints. A few random cracks due to wide joint spacing may be preferable to the cost of maintaining a large number of joints.
Typically, the more joints a floor contains, the more it costs to maintain them. The joints are also more likely to spall under forklift traffic.