Concrete contains more water than is needed for hydration in order to facilitate placement and finishing. This extra water causes aggregate separation; aggregates sink and fine materials and water move to the top. Holland adds that macro fibers reduce aggregate and water movement, therefore reducing shrinkage and curling. He also believes the micro-cracking process enables concrete to move a little bit under load, resulting in less curling.
Installing high fiber slabs
As a concrete contractor, Bryan Birdwell, owner of Birdwell Associates, Lakeland, Fla., says he likes to know everything he can about the concrete mixes he works with and wants to control all components. He has a five-year history using macro fibers for floors, super-flat floors and traffic pavement. He both installs his own work and consults with other contractors about the installation of super-flat and high dosage macro fiber floors. His company’s specialty is super-flat floor construction, and he says they use macro fibers for that work too. “Working with 7-1/2 pounds of macro fibers per cubic yard of concrete is our magic number,” he says. “That’s where floor properties change significantly, allowing one to maximize joint spacing.”
Birdwell’s largest project using macro fibers to date is a 21,000 square foot skating rink placed in 2011. The specifications called out no joints and no cracks. After completing his work he measured the floor for flatness and elevation changes and the numbers were very small. The original measured floor flatness was FF 123 and the floor levelness was FL 123 — numbers that haven’t changed in the past two years.
To achieve the best results using macro fibers requires a systems approach to floor construction. The process starts with the careful fine grading of base materials and proper, even compaction. Uneven subbase preparation can prevent floor panels from moving without restriction. He strongly recommends placing a good vapor retarding membrane on top of the subgrade and in contact with the concrete. This also enables the unrestricted movement of floor panels to reduce the potential for cracking.
Birdwell says he prefers a placing slump of 5-1/2 to 6 inches, achieved by using polycarboxylate superplastizers. Depending on the mix design, dosage rates typically range from 4 to 8 ounces per 100 pounds of cementitious material. He recommends testing for the amount of air entrainment in the first few loads just to be sure the polycarboxylate and macro fiber product being used isn’t increasing the air entrainment level beyond acceptable levels. He adds that some contractors are also using shrinkage reducing admixtures.
Ideally, the owners of floors want perfect slabs that don’t require any maintenance. Construction joints and control joints are usually the locations where trouble begins in terms of repairs and curling issues. So increasing the spacing between joints equates to fewer issues over time. Including high dosage levels of macro fibers comes as close as anything on the market to providing owners with what they want.
The concrete industry is also beginning to see products such as Ductilcrete (a contractor franchise network) and the FAST Floor System offered by Birdwell & Associates, which use macro fibers in their proprietary process to provide maximum joint spacing, no-curl floors and long life-cycle pavements. They also provide warranties and prices that approach conventional concrete floors and pavement.