Alan Face theorizes that floor panels curl spherically; the panel edges lift up, losing contact with the subbase, pushing the center of the panel down as it carries most of the weight of the panel.
Photo credit: Credit: DuctilCrete
Most floor maintenance issues occur at joints, also the place where curling presents itself. So if fewer control joints can be engineered into a system then there is less opportunity for problems downstream.
Photo credit: Credit: DuctilCrete
Shown here is a two-list concrete placement. The first lift is placed and laser-screeded (shown at the bottom right). The top lift, concrete with synthetic macro fibers and admixtures, is placed within minutes afterwards.
Photo credit: Photo Credit: SVM Digital Agency
If you are the owner of a large warehouse or manufacturing facility, one of your concerns is how well your floors will maintain over time. Robert Guarnaccio, president of Stout Development Services, Libertyville, Ill., a company that develops and sells industrial space, says most owners and developers of commercial space are concerned about performance over the long haul and understand the importance of curling issues more than anyone — more than most contractors. They want floors with fewer joints because curling occurs at joints and that’s where maintenance costs are the greatest.
If you are a concrete contractor who installs floors, curling probably isn’t an important issue for you. Your problem is to install flat, level floors that meet the specified requirements at the time of construction. But if you want continued work with owners and developers you will need to produce work that performs well over time, work that doesn’t curl and has fewer joints.
We’ve known about curling for a long time but little has occurred to address the issue — the American Concrete Institute (ACI) has few guidelines and no protocols for measuring curl. But change is needed, as contractors and manufacturers develop systems for installing no-curl floors with greater joint spacing, marketing this feature to owners. ACI does have guidelines for evaluating flatness (FF) and levelness (FL) shortly after sawed contraction joints are installed and away from initial curling zones.
What is curling?
Many think curling occurs only at the edges of a panel. But Allen Face, owner of the Allen Face Companies of Wilmington, N.C., theorizes that slab panels form a spherical shape when they curl, with the centroid of the panel at the bottom of the sphere and the edges up in the air at the top. “Curling results from drying shrinkage in the surface region of a floor panel, so this is where preventative efforts should be focused. There is very little shrinkage at the bottom portion of a slab because water is retained there for a much longer period of time,” he says.
As differential shrinkage develops, panel edges lift off the ground, requiring the center portion of the panel to carry the panel weight, compressing the base and soil under it. So the centroid of a panel loses elevation as the edges of the panel gain elevation. When floor panels curl, “panel rocking” occurs as forklift traffic passes over them, eventually causing cracks to develop parallel to panel perimeters (where the panel leaves the ground) as the flexural strength of the slab is exceeded.
How to measure curl
When slabs are evaluated for FF and FL to assure that contractors placing and finishing skills produce the specified floor, ASTM 1155 requires measurements to start two feet away from panel perimeters, measured in both the X and Y axis. This measurement avoids areas where curling activity is the greatest. There is currently no ASTM or ACI approved method for measuring curl, so Face has developed his own method. Using the same instrument for measuring FF and FL— one capable of recording 0.005 of an inch elevation changes, he takes measurements along diagonal lines across the corners of panels defined by sawed contraction joints. “As with FF and FL measurements, profiles are taken just after finishing and sawing operations are complete to profile the original surface of the slab and this information is compared with later measurements,” he adds.Reducing curl
Owners push for no-curl floors but construction companies and product manufacturers are the ones bringing change by introducing products that reduce both shrinkage and curling in floor slabs. Today any designer can specify high dosage levels of steel or synthetic macro fibers or shrinkage reducing admixtures (SRAs) to improve the long term performance of floors, or they can specify proprietary products requiring licensed installers. DuctilCrete, based in Gilberts, Ill., is an example of a proprietary system that works through an alliance of contractors in the U.S. to install floors guaranteed not to curl. Since the inception of the concept, the company has successfully placed over 30 million square feet of floors, toppings, and paving. They employ a systems approach, bringing many elements together to produce the desired results.
Their system includes: