The rules for laying out contraction joints, also referred to as control joints, for decorative concrete are essentially the same as for any other concrete pavement. The only difference may be the decorative nature of the joints. Some think, for example, that sawed control cuts can be placed in the bottom of stamped concrete patterns, but that seldom works, especially when patterns don't run in straight lines. Some believe patterned concrete doesn't need contraction joints because the pattern lines will provide it. But pattern lines rarely go deep enough to provide the necessary weakened plane. The same holds true for handgrooved joints in fresh concrete because they extend less than the required quarter-depth of a slab.
Today, most contraction joints are placed with diamond saws within 24 hours of placement.
There are four conditions that cause concrete to crack: shrinkage, thermal contraction, subgrade moisture (including frost), and thermal gradients. Each condition causes concrete movement but shrinkage is the primary concern. You can expect that a good concrete mix with a slump of 3 to 4 inches will shrink about ¾ inches in a 100-foot-long slab during the first 28 days. Some believe that shrinkage restrained by the friction between the bottom of a slab and the subgrade material also causes cracks—flat, smooth subgrade surfaces reduce friction resulting in less cracking.
If you are installing a walkway or patio, you don't need to be too concerned about aggregate interlock, but it's important when installing pavement that heavy vehicles pass over. As the top of slabs shrink, cracks develop from the bottom of sawed joints to the bottom of a slab. These cracks are desirable because aggregates on either side interlock, providing load transfer from one side of the joint to the other. But aggregate interlock works only when crack widths don't exceed 0.035 inches. To keep shrinkage within these limits, select the right concrete mixes, keep water-cement ratios low, and install joints closer together to minimize shrinkage at each joint.
Here are some basic rules for laying out contraction joints.
- Keep panels as close to square as possible; don't exceed the length-to-width ratio more than 1½ times the width.
- To lay out joints, multiply the thickness of a slab times 2 feet. For example, if a slab is 4 inches thick, multiply 4 times 2 and lay out joints every 8 feet in both directions. Don't exceed 15 feet regardless of slab thickness.
- Avoid intersecting other joints or formlines at acute angles, especially angles less than 60 degrees.
- When “dog legs” are used to lay out joints that meet form lines properly, make them 12 to 18 inches long.
- Make joint lines bisect box-outs or drains.
- Pavement shapes that cause planes of weakness, such as inside corners, should be considered in the contraction joint layout plan.
Isolation joint is now the preferred term over expansion joint, even though you order expansion joints at supply houses. The function of these joints is to isolate your work from structures, other pavements, and objects considered nonmovable. For all practical purposes, concrete is at its most expanded state when it's placed, so expansion material isn't needed in the body of a placement and it's unsightly. Isolation joints provide space for slab movement around perimeters. The joint material should be resistant to deterioration, should be wide enough to extend to the full depth of the placement, and should be ½ to 1 inch in thickness.
Reinforcement and load transfer hardware
Don't count on steel mesh reinforcement to control cracking—it won't. What it will do, if properly placed, is limit crack width and control differential movement. Load transfer hardware shouldn't be used in 4-inch-thick slabs. Most decorative slabs don't support heavy loads so there isn't a need for transfer hardware.
For a complete guide to pavement installation, see the American Concrete Pavement Association's Web site at www.acpa.org.