Minimize Shrinkage, Minimize Cracking

by Kim Basham, PhD PE FACI
Created: March 13, 2010

Drying shrinkage is the root cause of random cracking in unreinforced concrete floors.

This out-of-joint floor crack was caused by excessive concrete shrinkage and joint spacing. Because of the wide crack width and curling, load transfer across this crack through aggregate interlock has...
This out-of-joint floor crack was caused by excessive concrete shrinkage and joint spacing. Because of the wide crack width and curling, load transfer across this crack through aggregate interlock has been lost.
Concrete shrinkage and curling typically increase joint widths resulting in failed joint filler, edge spalling and general deterioration of joints.
Concrete shrinkage and curling typically increase joint widths resulting in failed joint filler, edge spalling and general deterioration of joints.
Figure 1. A curled slab due to differential shrinkage between the top and bottom of the slab.
Figure 1. A curled slab due to differential shrinkage between the top and bottom of the slab.
Figure 2. Drying shrinkage increases with increasing water content (Ref. 1).
Figure 2. Drying shrinkage increases with increasing water content (Ref. 1).
Figure 3. Recommended joint spacing for unreinforced slabs-on-ground. Specimens with a measured shrinkage of less than 520 millionths are considered low shrinkage concrete; specimens with values...
Figure 3. Recommended joint spacing for unreinforced slabs-on-ground. Specimens with a measured shrinkage of less than 520 millionths are considered low shrinkage concrete; specimens with values greater than 780 millionths are considered high shrinkage; typical shrinkages range between 520 and 780 millionths (Ref 8).

Aggregate Size and Gradation. Use the largest practical-size coarse aggregate (ideally, 1½ or 1 inch) to reduce the water-demand of a mixture but not greater than one-third the slab thickness. Increasing the maximum size of the coarse aggregates reduces the surface area of the aggregates that must be coated with cement paste. Thus, both water and cement demands are reduced.

Gradation refers to the particle size distribution of the aggregates. A well graded aggregate contains a wide range of particle sizes and creates an aggregate matrix within the concrete with minimal void spaces because incrementally smaller particles fill the void spaces between the particles. By aggregate packing the concrete, the amount of cement paste required to fill the voids is reduced.

Popular methods of blending coarse and fine aggregates to produce a uniform gradation to minimize voids between particles include: 1) Percentage of the combined aggregate retained on each of the standard sieves, and 2) Coarseness Factor Chart (Ref. 3 and 4).

Cement Type and Content. Both cement chemistry and fineness can affect the dry shrinkage of the cement paste. Finer ground cements such as Type III (High Early Strength) typically increase dry shrinkage as compared to coarser ground cements. By increasing the maximum aggregate size, cementitious material contents can be reduced. Consequently, water and paste contents are reduced. Recommended cementitious materials content for ¾-inch maximum size aggregates is 540 to 680 pounds per cubic yard and 470 to 560 pounds per cubic yard for 1½-inch aggregates.

Chemical Admixtures. Chloride-based admixtures increase dry shrinkage. Also, some water-reducing and high-range water-reducing admixtures may increase shrinkage when used to increase slump or reduce the water content of concrete mixtures. ASTM C 494 allows concrete produced with admixtures to have up to 35 percent greater shrinkage than the same concrete without admixtures (Ref. 6). Thus, reducing the water content with a water-reducing admixture may not decrease concrete shrinkage.

Slump. Theoretically, adding water on site to increase the concrete slump increases shrinkage. However, adding small amounts (less than 2 gallons) of water does not significantly increase dry shrinkage (Ref. 5).

Curing. Researchers report that extended periods of moist curing can reduce the amount of drying shrinkage by as much as 10 to 20 percent. However, shrinkage reductions depend on the water to cement ratios and length of moist curing (Ref. 7). For curing periods greater than four to eight days and less than 35 to 50 days, shrinkage may increase. Others report that extended curing methods only delay shrinkage and curling (Ref. 8).

Shrinkage potential and maximum joint spacing

On your next job, consider the shrinkage potential of the concrete and slab restraints, especially the subbase friction, before selecting joint spacings. Ask the concrete producer if concrete shrinkage data exists. If not, evaluate the concrete's shrink potential by considering water content, maximum aggregate size, aggregate volume and gradation, cement type and content, and admixtures. Then select joint spacings as recommended for low, typical and high shrinkage concrete as shown in Figure 3. Do not exceed 15-foot joint spacings unless load transfer devices such as steel dowels or plates are used to ensure load transfer across contraction joints.

References:

Steven H. Kosmatka, Beatrix Kerhoff, and William C Panarese. "Design & Control of Concrete Mixtures, 14th Edition," PCA.
ASTM C157 / M157-08. "Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete"
Harrison, Patrick J. "For the Ideal Slab-on-Ground Mixture," Concrete International, March 2004.
ACI 302.1R-04, "Guide for Concrete Floor and Slab Construction"
Suprenant, Bruce and Malisch, Ward. "A New Look at Water, Slump and Shrinkage." Concrete Construction April 2000.
ASTM C 494-08 "Standard Specifications for Chemical Admixtures for Concrete"
ACI 209.1R, "Factors Affecting Shrinkage and Creep of Hardened Concrete"
ACI 360R-06, "Design of Slabs-on-Ground"