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Concrete Shrinkage Issues

By Robert G. Wilkin, P.E.

When concrete is mixed, there is much more water added into the mix for workability than is needed to hydrate the cement. 
This is done so that it easily combines with the cementitious materials in the mix.  This is called excess or free water.  Eventually, this water evaporates from the concrete leaving microscopic voids in the concrete matrix.  The evaporation causes the concrete to lose mass and shrink.  Cracks can occur in framed floor slabs and slabs on grade as a result of the concrete dragging (friction) on the support material as the concrete volume reduces.  Curling can also occur when the top (exposed) surface of a slab changes volume at a rate different than that of the slab surface on the (bottom) supporting material. 

Designers usually, or should, design control joints in the slabs so the cracks are controlled and form in reduced sections of the control joints.  An example would be the lines in sidewalks. 
Joints in normal concrete on-grade floor slabs are usually no more than 20’ – 30’ on center.  These can be tooled into the fresh concrete, as in sidewalks, or saw cut in the freshly set concrete.  Cutting the joints should be done the day after placement, at the latest, since shrinkage begins as soon as the concrete starts to cure and dry out.

Shrinkage is problematic not only due to the cracking of the concrete that can occur, but also from curling of the slab.  When the top of the slab dries faster than the bottom, the top surface shrinks more and can lift the slab at the edges near the control joints.  This can be a real concern if loads, such as forklifts, travel over the joints displacing the curled edges which can result in cracking and sub grade (pumping) displacement. 

There are a number of ways to reduce the detrimental effects of shrinkage in floor slabs.  Polymer fibers are now used in slabs on grade and permitted on metal decking to replace temperature steel in the concrete.  The temperature steel, usually welded wire fabric, is typically found at the bottom of the slab, not in the middle where it would do some good.  The fibers help to bind the fresh concrete together and allow the concrete to gain strength during the early stages of curing.  At the same time, the fibers tend to distribute the volume changes that are going to occur in concrete regardless. 

There is hope however…please read on. 

There are shrinkage reducing add mixtures that can be incorporated into the concrete as well as water reducing admixtures.  The lower the water content in the mix, the less shrinkage is likely.  Also, increasing the size of the aggregate reduces the amount of cement required for strength.  This results in less water needed for hydration which also in reduces shrinkage.

Applying a curing compound or wet curing the new concrete slab also reduces the rate of the drying of the concrete until it has gained strength to counteract shrinkage tensile forces occurring as the concrete dries out. 

Curing compounds can interfere with the application of floor finishes and need to be carefully chosen.  An effective method of keeping the floor wet is using a felt backed plastic sheet or plastic over burlap to keep water on the surface continuously.  Concrete doesn’t like to be wet and dry cycled during curing which can cause the surface to chalk and alligator, so keep it moist. 

The addition of materials to the concrete and wet curing add cost but can significantly reduce shrinkage so that joint spacing can be significantly increased and cracks can be reduced.  This will save placement and maintenance costs, as well as decrease visual defects in exposed concrete.  That old saw “concrete will crack” is slowly giving way to improved placement and curing techniques and the addition of “magic elixirs.”