Joints are Placed in Concrete Slabs
For such a thick and heavy surface, concrete always seems to be on the move. As newly poured concrete slabs dry, the loss of moisture causes the material to contract. Once dry, concrete will expand on hot humid days and contract on cold dry ones. For example, drying concrete will contract 2/3 of an inch per 100 linear feet. All this movement causes cracks. Actually, the problem isn’t the movement per se. The real culprit is the restraint of concrete’s movement by the underlying sub-grade, footings, walls and adjoining slabs. The solution is to place control relief joints in the concrete to ‘”pre-crack” it at pleasing, controlled intervals or at the interfaces with other structures. When done properly, the relief joints will suppress random cracking, which tends to look unsightly.
Certain steps can be taken to help minimize the amount that concrete shrinks as it dries, including:
- Minimal use of water in relation to the cement content of the concrete and the use of water-reducing agents
- Using clean, coarser aggregates
- Low slump and proper consolidation
- Adequate and continuous curing
Most of the time, control joints are introduced after new concrete has dried sufficiently, although they can also be cut with a saw in newly dry concrete — but if you don’t do this promptly, cracks will form. These saws are equipped with abrasive or diamond rimmed blades.
In this article, we’ll look at three types of control relief joints: contraction, expansion and construction.
Contraction joints break up large flat expanses of poured concrete. The joints reduce the thickness of the concrete at specified intervals to allow for shrinkage and contraction. Thinner joints that are relatively closely spaced are more effective than thicker joints lying further apart. As a rule of thumb, contraction joints are placed at five to six foot intervals. For wide areas like driveways, contraction joint can also be placed longitudinally every 20 feet or so. If the concrete is reinforced with steel bars, the intervals run 40 to 80 feet. The rebar is placed such that only one-half spans the joint.
The joint can be made by putting a groove in the wet concrete (when its reaches a plastic consistency) with a V-shaped jointing or grooving tool after the concrete has been edged but before the surface is float-finished. The tool is typically 6 inches long, 3-4 inches wide and 3/16 to 1 inch deep. The most effective joints are 3/4 to 1 inch deep, but no less than 1/4 the thickness of the slab. Corners are usually rounded or jointed.
Alternatively, metal or sealed wood forming-strips can be embedded in wet concrete and then removed after the concrete dries. A popular option for industrial flooring is to use premolded tongue and groove joints.
To provide for volume expansion in hot conditions, expansion joints are formed using prepared resilient or elastic material (cork, plastic, sponge rubber or fiber) that is 1/4 to 1/2 inch thick and as wide as the concrete’s thickness. The material is positioned before pouring the concrete. Expansion joints are a must when a slab joins or abuts another structure, such a driveway, building, curb, columns, staircases, walls, footings, etc. Often, these joints are sealed for outdoor, commercial and industrial settings.
Construction joints transfer load between two adjacent concrete slabs that are too dry for consolidation via vibration. In slabs, these joints are vertical and often called “cold joints.” Low-slump concrete is used and care is taken to prevent aggregate from sticking out. Half the length of a reinforcing steel bar can span the construction joint. Alternatively, a load transfer slip dowel device can be used across transverse joints, where one dowel end is free to move within a sleeve and the other end is embedded in the concrete. A third method to transfer load is to use permanent or removable keyways. Removable templates of beveled wood or metal strips are carefully extracted to avoid spalling.