KSC Komponent® — REPORTS AND NEWS
Shrinkage-Compensating Concrete: An Introduction
The problem with shrinking…
All concrete shrinks. It is a well-known fact that as a hardened slab of concrete dries out over weeks, months, and years, its volume will decrease. If the concrete slab is left unrestrained, this is not a problem since the slab will simply contract to a smaller volume.
However, this is rarely the case in the real world. In general there is always some sort of external restraint acting on the slab, whether from friction with the subgrade or adjacent concrete, or by tying in steel from an adjacent slab. Since the edges of the slab are not free to move, something must eventually give. When the internal stresses due to shrinkage are greater than the tensile strength of the concrete, cracking will develop.
One of the ways this cracking is controlled is to place steel rebar below the surface of the concrete. However, even with this internal reinforcement, the concrete will still shrink, resulting in many fine cracks on the surface. Another method is to use control joints. Unfortunately, saw-cutting or forming control joints can be costly and time consuming. Additionally, the joints are susceptible to damage from forklift wheels and other repeated heavy loading.
Shrinkage-compensating concrete to the rescue…
Shrinkage-compensating concrete can solve these problems by providing a “buffer” for the inevitable shrinkage the concrete will exhibit. Shrinkage-compensating concrete contains an expansive cement or expansive component. As the cement paste hydrates and begins to harden, the concrete will actually expand a very small amount (about 0.05% or so). This expansion effectively applies a tensile strain to the internal steel reinforcement. As Newton’s Third Law states, “For every action, there is an equal and opposite reaction.” The steel – now in tension – pulls back on the concrete, effectively putting the concrete into a slight compression. This result is very similar to prestressing.
Remember that cracking results when the internal stresses are greater than the tensile strength of the concrete. In shrinkage-compensating concrete, the stresses due to shrinkage are compensated by the induced compression of the expansive cement. As the concrete begins to shrink, the compressive stresses will decrease. As long as there is some residual compression remaining, the slab will not crack due to drying shrinkage.
Fewer joints, no cracking!
So what does this mean for concrete slab construction? The most obvious benefit is that the number of joints in the slab can be reduced significantly. Instead of spacing joints every 15’ or 20’, the slab can be joint-free for areas 150’x150’ or more. That amounts to about a 90% reduction of total joint length when using shrinkage-compensating concrete. Not only is there no saw cutting involved, but the costs of filling and maintaining the joints is also significantly reduced.
With shrinkage-compensating concrete, many will notice a lower life cycle cost than that of an ordinary concrete slab. This translates not only to slabs-on-grade, but also to bridge decks, parking structures, containment structures, concrete water tanks, and other applications as well.
Applications For Shrinkage Compensating Concrete
Commercial and Industrial Floors: With shrinkage-compensating concrete, your floor will have significantly fewer joints and significantly fewer cracks. Many floors are constructed with slab placements as large as 150’x150’ or more. Slab thickness can also be reduced, resulting in a lower cost of construction. A rapidly growing home improvement chain in the Midwest specifies a 4” shrinkage-compensating concrete floor in all of its new stores.
Shrinkage-compensating concrete has been successfully used in bridge decks since the 1960’s. These decks exhibit very little, and often no cracking even after a decade of service. This helps prevent corrosion of the internal steel reinforcement and deterioration of the deck. The Ohio Turnpike Commission uses shrinkage-compensating concrete exclusively in all of its new bridge decks. Michigan DOT uses CTS Type-K Cement in many of their new bridge deck construction projects. Winters in Michigan are harsh, with severe freezing and thawing conditions, compounded by the heavy use of dicing salts. Cracks in a bridge deck provide a path for the deicing salts to penetrate the deck to the enclosed steel reinforcement. The salt corrodes the steel leading to early failure of the deck. Bridge decks with CTS Type-K Cement have also been constructed in Pennsylvania, New York, Wisconsin, New Hampshire, California, and elsewhere.
Elevated slabs constructed with shrinkage-compensating concrete will exhibit significantly fewer cracks, even on slabs 22,000 square feet in size. This means fewer leaks, less crack-sealing, and lower maintenance costs. One parking structure in Santa Monica, California was constructed with shrinkage-compensating concrete over 30 years ago, and it is still performing beautifully.
Liquid Containment Structures:
Using shrinkage-compensating concrete for liquid containment means fewer joints and cracks for potential leaks. Additionally, tanks have been successfully constructed without costly waterstops or sealants in the joints. The “checkerboard” placement pattern is not necessary for tank walls, simplifying construction. Past successful projects include chemical containment, wastewater treatment plants, and aeration tanks.
Shrinkage-compensating concrete pavement requires significantly fewer joints. This means much less time and money spent on sealing, maintaining and repairing the joints. The SCC pavement can also be constructed thinner than a conventional pavement, resulting in material cost savings.