How to Minimize Shrinkage

“How to Minimize Shrinkage”

Originally published in Concrete Contractor

Written by: Kim Basham, PhD PE

Drying shrinkage is the root cause of random cracking in unreinforced concrete floors. Drying shrinkage is caused by a loss of moisture from the concrete. It occurs after concrete hardens and continues months and even years depending on the slab thickness and ambient conditions. This type of shrinkage is largely responsible for random cracking, wide joint and crack widths, and curling (or warping) of concrete floors.

Drying shrinkage causes slabs to contract, or shorten. If slabs are free to contract, tensile stresses and cracking do not occur. However, floors are restrained against contracting by subbase friction, foundations, column footings, etc. Concrete is a brittle material with little stretch or tensile strength. Unless jointed properly, drying shrinkage combined with restraints classically produces random slab cracks or wide joints, dissatisfied owners and costly repairs.

Curling is caused by differential shrinkage associated with different moisture contents in the top and bottom portions of interior floors. Because drying or moisture loss is primarily from the top of the slab, a moisture gradient is created between the top and bottom of the slab, especially when the subbase is moist. The reduced moisture content in the top portion causes the top of the slab to shrink more than the bottom. Thus, free edges of joints and cracks warp up as shown in Figure 1. The level of curling is directly related to the moisture gradient and the shrinkage potential of the concrete.

Shrinkage is expressed as dimensionless strains (length change over a given length) or percent in inch/inch. Thus, a shrinkage of 400 millionths is equal to 0.0004 inches per 1 inch or 0.04 percent. Typical concrete shrinkages for small, unrestrained laboratory concrete specimens range between 520 and 780 millionths. Due to subbase restraints, concrete shrinkage for floors in the field is less than unrestrained, laboratory specimens.

If properly jointed, slab shortening should occur in the saw-cut contraction joints and not as random or out-of -joint cracks. Proper jointing practices include locations, saw-cut timing and spacings. If joints are spaced too far apart, either out-of-joint cracking or wide joint widths will occur.

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Kim Basham is president of KB Engineering LLC, which provides engineering and scientific services to the concrete industry. Basham also teaches seminars and workshops dealing with all aspects of concrete technology, construction and troubleshooting.

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