Concrete Expansion Joints

Summary: Concrete expansion joints account for volume changes in concrete at predetermined intervals. Accounting for these volume changes can prevent the concentration of forces that cause cracking.

Concrete is going to crack - that's simple fact.

However, properly-made expansion joints can eliminate random surface cracks by distributing stress forces that result from changes in moisture levels and temperature.

To be effective, concrete expansion joints must be both located and made correctly.

Whenever proper curing is interrupted, concrete dries out and the loss of moisture can cause the concrete to contract at approximately 0.6 inches per 100 lineal feet.

That may not sound like much, but it is enough of a change to cause cracking in concrete that does not have expansion joints. This rate of contraction is about the same as that which occurs when concrete undergoes a decrease of 100 degrees Fahrenheit in temperature.

The concentration of tensile stresses is the result of shrinkage restraint rather than shrinkage proper and that is when cracks develop.

Concrete that isn't restrained won't crack. The problem is, it's almost impossible to support a concrete member without restraint, whether intentional or unintentional.

Some unintentional restraints include bonds to existing footer members or walls and differences in elevations of subgrade.

Without concrete expansion joints, any restraint that concentrates drying stress forces can produce cracks in a restrained member.

There are a number of factors in the composition of the concrete itself that contribute to the drying shrinkage coefficient, but drying shrinkage can be controlled by composition adjustments too.

These changes in composition of the concrete include:

• Selection of clean, specification quality aggregate
• Minimal use of fine size aggregate in quantities that produce good workability and finishing, and no more.
• The lowest water to cement ratio possible
• Using low slump to place the concrete, which is another way of saying "use the lowest water to cement ratio"
• Possible use of functional water reducing agents in addition to low water to cement ratio
• Proper curing of concrete.

Materials and Tools Needed

• Grooving trowel
• Concrete saw with diamond blade (if you plan to dry-cut grooves)

Expansion joints should be placed in concrete, measured in feet, no more than two or three times the slab thickness in inches.

In other words, a concrete slab 4 inches thick should have joints spaced 8 to 12 feet apart.

Expansion joints should be cut to a depth of 25% of the slab depth, and they should always be at least 1/4 inch wide.

There are many ways of cutting concrete expansion joints.

Grooving tools can be used in fresh concrete. Grooving tools look like regular trowels with a v-shaped projection underneath. They may be attached to long handles when long grooves need to be made.

In recent years, contraction joints have been made using gasoline or electric power saws with shatterproof diamond-rimmed blades.

If you plan to saw your concrete expansion joints, the joints should be cut as soon as the concrete is hard enough that the saw blade won't chip the edges abutting the cut.

These dry-cut lightweight saws will cut an expansion joint from 1 to 3 inches deep.

If the weather is hot, the joints should be cut within 6 to 12 hours after the concrete has been finished.

If possible, cut joints under where walls will be placed, or under carpeted areas. Expansion joints should be planned to eliminate so-called "re-entrant" corners.

In other words, if there is a rectangular cut-out in part of a concrete slab, the inside corners of the cut-out should be the starting place for expansion joints even if it means that some of the expansion joints end up not spaced uniformly.

Without those joints, inside corners will be prone to develop cracks.

Since most concrete plans don't have these expansion joints marked on them, plan for them beforehand and if you hire a pro, tell the contractor where the inside corner expansion joints should go.

Some concrete projects may use pre-molded concrete expansion joints that are made of sponge rubber, fiber, cork composite, or plastic.

These must be chosen so that they will be able to hold up to weather conditions (i.e., non-brittle in cold weather and non-extruding in hot weather).

Many industrial or outdoor projects should have concrete expansion joints that are sealed properly with concrete sealer. I know that was a lot to absorb, but expansion joints are very important.

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