Portland cement concrete is perhaps the most versatile and economical structural material available. It is relatively inexpensive, strong, and it can be placed in nearly any shape. One of its greatest drawbacks, however, is that it shrinks appreciably as it dries, as a quick glance at the cracks in almost every concrete sidewalk will demonstrate. In contrast, since the 1960s, shrinkage-compensating concrete has been used on thousands of projects in a variety of applications. Projects have included industrial and commercial slabs on grade, bridge decks, wastewater treatment plants, and other liquid containment structures. By significantly reducing the need for joints, construction and maintenance costs can be reduced. More important, using shrinkage-compensating concrete significantly reduces cracking, which increases service life and decreases repair costs.

Understanding why it shrinks
The fundamental cause of drying shrinkage is the evaporation of excess water as the concrete dries. Water is necessary to hydrate the portland cement, but for a complete reaction, only about 1 pound of water is required for every 4 pounds of portland cement (or a water-cement ratio of 0.25). In practice, however, about double this amount is used (PCA, 1994). This additional water is necessary to allow proper mixing and sufficient fluidity for placement. Therefore, about half the water added to the concrete constituents is chemically unnecessary and will evaporate. This amounts to about 2 cubic feet of water per cubic yard of concrete, or nearly 8 percent of its total volume, which causes shrinkage as it leaves the concrete.

Further, it has been noted that using low-modulus coarse aggregate can result in a 120- to 150-percent increase in shrinkage. Shrinkage has been measured to be more than 0.10 percent for many concrete mixes containing this type of aggregate (Tarr, 2008).

Shrinkage in concrete slabs is usually handled by cutting control joints into the slab at an early age. The control joints are planes of weakness that allow the development of shrinkage cracks to occur at predetermined locations. However, even with good joint design, some drying shrinkage cracking can occur between the joints, resulting in maintenance issues and repair costs. Reducing drying shrinkage, and thus the need for joints in a concrete slab, can reduce the overall maintenance cost and improve structural performance.

Reducing shrinkage
In some applications, drying shrinkage can be minimized by reducing the amount of water mixed into the concrete. However, by reducing the water content, the concrete mix becomes stiffer and more difficult to place. A stiffer mix can cause voids to form along the formwork and around the reinforcing steel. The voids within the concrete can weaken the structure. Therefore, care must be taken to vibrate the concrete and the forms sufficiently to allow for complete consolidation. Even with these precautions, voids may still be present and may only be found when the forms are removed.

Various additives have been developed over the years to combat concrete shrinkage. Water-reducing admixtures can be added to the concrete to increase mix fluidity without additional water. These admixtures are used in most concrete placed today. However, some studies have shown that despite the reduced water content, shrinkage may remain the same or even increase versus concrete without water-reducing admixtures (Flood, 2005).

Shrinkage-reducing admixtures (SRA) are another type of admixture used to combat drying shrinkage. SRAs reduce the surface tension of the water in the concrete pores, thereby reducing the internal forces that cause concrete to shrink. While these admixtures do reduce shrinkage in the first several weeks, the effect decreases in the subsequent months and years, according to the manufacturer’s test data. The ultimate total drying shrinkage may be nearly the same over the service life of the concrete whether or not an SRA is used.

A third method for combating drying shrinkage is the use of shrinkage-compensating mineral admixtures. This type of admixture forms expansive crystals in the cement matrix that compensate for the drying shrinkage. The magnitude of the total expansion is typically small: only a fraction of an inch per 100 linear feet of concrete. When restrained by reinforcing steel, subgrade friction or other restraint, the expansion places the concrete under a slight compression of about 100 pounds per square inch (psi). This compression and small expansion is ultimately reduced by the long-term drying shrinkage. Ideally, when the drying shrinkage is completed, a small residual compression remains in the concrete and the concrete member is the same size as originally cast. As long as some compressive force remains in the concrete, shrinkage cracking cannot occur; see Figure 1.

Figure 1: Shrinkage-compensating mineral admixtures counteract shrinkage and place the concrete into compression, resulting in zero net length change of the concrete member.

Case studies
Parking garage — Liljestrom, one of the early developers of shrinkage-compensating concrete, reported the physical length change of shrinkage-compensated concrete slabs versus ordinary portland cement concrete slabs (Liljestrom, 1973). He examined three post-tensioned structures during a five-year period: two elevated deck parking structures containing shrinkage-compensating concrete and an elevated industrial building floor constructed with ordinary portland cement. After placement and prior to prestressing, the two shrinkage-compensated concrete decks had a slight measurable expansion, while the portland cement concrete slab had a slight measurable shortening. The prestressing caused each slab to shorten slightly. However, at the end of the five-year study, the total shortening was about four times greater in the portland cement concrete slab than in the shrinkage-compensating concrete slabs. Liljestrom concluded that using shrinkage compensating concrete greatly minimized the possibility of cracking due to drying shrinkage, and greatly reduced the magnitude of the final net shortening of the post-tensioned slabs.

More recently, shrinkage-compensating concrete was used to build an addition to a parking structure at John Wayne Airport in Santa Ana, Calif. (Chusid, 2007). The original structure built in 1993 was designed to hold 4,000 cars on two levels, and consisted of a slab on grade with a concrete deck above. The concrete was designed with ordinary portland cement and was post-tensioned about three days after placement. Six months after completion, the deck developed 75,000 linear feet of cracks requiring extensive repairs. The repairs cost approximately $500,000 and the owner allocated an additional $500,000 for future repairs. In 1998, two more decks were added to the top of the structure, but the concrete design was changed to use shrinkage-compensating concrete, with the other aspects of the design remaining essentially the same. Like the lower two levels, the new decks were post-tensioned about three days after placement. Ten years after completion, the new decks are still virtually crack-free and show very little signs of wear, in stark contrast to the lower two levels. Using shrinkage-compensating concrete reduced the maintenance cost of the new decks to almost zero since their construction.

Shrinkage-compensating concrete was used to build an addition to a parking structure at John Wayne airport in Santa ana, calif., where 10 years after completion, the new decks are still virtually crack-free and show few signs of wear.

Another parking garage was constructed at the University of Alabama, Birmingham at the Ridgecrest South Residential Community in 2008. The structure consisted of three levels of parking below and five levels of student housing above. The engineers elected to use shrinkage-compensating concrete in the lower three elevated post-tensioned slabs. The finished slabs had 300-by-300-foot plan dimensions without pour strips and were virtually crack-free. The design resulted in an estimated savings of $250,000. About one year after construction, engineers examined the shrinkage-compensated decks and found them to be in pristine condition with no significant cracking.

Concrete roof decks — Constructed in 1963, one of the oldest post-tensioned shrinkage-compensating concrete roof decks was used in the reinforced concrete home in the hills above Los Angeles, Calif. The roof — constructed with no roofing membrane or waterproofing — has been in service for more than 40 years and the owner states that it has not cracked, leaked, or required much maintenance. Furthermore, the concrete roof and walls are fireproof — an important consideration in Southern California where brush fires can run wild during the dry season (Chusid, 2006).

Airfield pavement — Shrinkage-compensating concrete was used in an innovative airfield pavement project at the Rockford Regional Airport in Rockford, Ill. In 1993, the Federal Aviation Administration (FAA) sponsored a research program to examine solutions to reduce the number of joints in airfield pavement. The FAA recognized that in certain regions of the country, frequent snow plowing of pavement leads to significant degradation of the concrete at the joints, and therefore reducing the number of joints in pavement could reduce repairs and extend service life. As part of this study, steel fiber-reinforced shrinkage-compensating concrete was placed in sections as large as 85 feet by 200 feet, and one large section was placed 85 feet by 1,200 feet and post-tensioned along its entire length.

After 10 years in service, engineers examined the pavement sections and found that the conventional concrete pavement with joint spacing of approximately 20 feet had deteriorated significantly, with many instances of cracking and spalling at the joints. The fiber-reinforced shrinkage-compensating concrete sections performed much better because of the reduction in joints and cracking. The post-tensioned section performed best, exhibiting almost no deterioration or cracking. The study confirmed that by using shrinkage-compensating concrete to reduce joints in airfield pavement, maintenance costs are reduced and service life is increased (Herrin, 2004).

Water treatment — Shrinkage-compensating concrete also has been successfully used in water treatment plants and liquid containment structures. For these applications, it is critical that the concrete should not crack, as this would cause leaks and possible environmental contamination. Jointing details must also be designed very carefully to avoid leaking. Using shrinkage-compensating concrete significantly reduces the chance for cracking in vertical tank walls. Shrinkage compensation also allows engineers to design structures with fewer joints. Joint spacing can be increased to 75 to 150 feet, instead of the typical 20 to 30 feet. Reducing the number of joints cuts down on costs associated with installing water stops, and reduces the number of areas where potential leaks can occur. Further, less reinforcing steel is necessary in the tank design because the tensile loads due to shrinkage can be essentially eliminated.

A number of liquid containment structures have been completed with shrinkage-compensating concrete, many with capacities well over 1 million gallons. For example, a water treatment plant in Midland, Texas, was built with below-grade clearwells 320 feet by 130 feet, and water stop locations at 110 foot intervals — a 60-percent joint reduction from normal practice, thereby reducing the construction cost. Near Eugene, Ore., the James River Paper Company built a 280-foot-diameter clarifier tank with shrinkage-compensating concrete in 1991. The tank was constructed with 100 feet between joints, and no water stops were used at the joints. Ten years after construction, an inspection showed no leaking from the joints and no cracks in the walls. In El Paso, Texas, the Bustamante Waste Water Treatment Plant and Rogers Water Treatment Plant were constructed with shrinkage-compensating concrete in 1991 and 1993, respectively. More than 10 years after construction, both structures were found to be performing very well with very little cracking (Flax, 2006).

The ridgecrest South residential community at the university of alabama, Birmingham used shrinkage-compensating concrete in the lower three elevated post-tensioned slabs. The finished slabs had 300-by-300-foot plan dimensions without pour strips and were virtually crack-free.

A proven solution
Drying shrinkage is a ubiquitous problem that may be addressed several ways. Shrinkage-compensating mineral admixtures offer a unique advantage: They do not simply reduce shrinkage, they counteract shrinkage and place the concrete into compression, resulting in zero net length change of the concrete member.

References

• Portland Cement Association, 1994, Design and Control of Concrete Mixtures, 13 ed.

• Tarr, Scott M., 2008, Concrete Cracks: A Shrinking Problem? Concrete Technology, Portland Cement Association, Feb. 14, 2008.

• Flood, Walter H., IV, 2005, Minimizing the shrinkage of concrete mixtures: a low-cost approach, Masters Thesis, University of Colorado College of Engineering.

• Liljestrom, William P., and Milos Polivka, 1973, A five-year study of the dimensional stability of shrinkage-compensating lightweight concrete used in post-tensioned slabs, Klein Symposium on Expansive Cement Concretes, American Concrete Institute, Publication SP-38.

• Chusid, Michael, 2007, A Perfect Match: Post Tensioning and Shrinkage-Compensating Concrete Form a Durable Union at John Wayne Airport, PTI Journal, July 2007.

• Chusid, Michael, 2006, All-Concrete House Built for a Legend of Post-Tensioning, PTI Journal, July 2006.

• Herrin, Stanley M., and John E. Naughton, III, 2004 10-year performance of innovative pavements: Greater Rockford Airport, Airfield Pavements: Challenges and New Technologies, American Society of Civil Engineers.

• Flax, David, 2006, A Nasty Environment: High performance concrete in Texas heat, Government Engineering, January-February 2006.

Kyle de Bruyn is a product development engineer at CTS Cement Manufacturing Corporation. Eric Bescher, Ph.D., is an adjunct associate professor in the Department of Materials Science & Engineering at University of California Los Angeles.