All owners want the 3Ms: minimal joints, cracking, and curling. Concrete with a proper mix design that is properly placed, finished, and cured by a qualified contractor can achieve these results in the hardened state. The floor design, specification, and drawings must reflect current ACI specifications and standards.
First, a proper mix design is required. See the box on page 22 for the key elements. These low-shrinkage, high-fiber mixes are the proper diet for high-performance concrete floors, slabs, and toppings. A high-paste, high-shrinkage mix will not result in minimal cracking and curling. An athletic concrete mix allows us to place contraction joints at about 25 feet for 6-inch-thick slabs. When the owner wants joints only at the column lines, we reduce the required shrinkage to 0.02% at 28 days and require an fe3 (see sidebar) of 250 psi.
A pre-slab meeting with a proper agenda and accurate minutes is mandatory. The article “Successful Floors, Slabs, and Toppings,” (Concrete Construction, January 2013) clearly outlines the key requirements to be discussed and agreed upon. The meeting should take place about 40 days before the floor slab or topping work is scheduled. A successful test placement is required. Often a second test placement is required before the mix and the placing, finishing, and curing procedures are approved.
The QC Program must include the following:
- The concrete producer provides concrete in the agreed-upon slump envelope. That envelope is selected by the concrete contractor and the concrete producer. Today a contractor can have any slump or setting time he wants. “Normal” set is essential.
- All field tests are made by ACI Certified Technicians, Grades I or II.
- The key tests onsite are verification of the water content (AASHTO T318, “Standard Test Method for Water Content of Freshly Mixed Concrete Using The Microwave Oven”) and assurance that the air content of a troweled floor is less than 3%.
- Some cylinder tests are required, although compressive strength is seldom a problem.
Great floors, slabs, and toppings are being installed every day. They deliver the 3Ms in the hardened state. These results are only achievable when the properly designed system is fully executed by the concrete team. There is no “give” in these systems.
William S. Phelan is senior vice president of marketing and technical services for The Euclid Chemical Co., where he has worked for over 37 years. He is an honorary member of ACI.
Athletic Concrete
- Low Shrinkage: < 0.04% @ 28 days when tested in accordance with ASTM C157m “Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete.”
- Flexural Strength: >700 psi @ 28 days (not required for toppings).
- Water Slump: 2 to 3 inches (necessary for troweled finishes).
- Slump after addition of high-range water reducer: 5½ to 7 inches at point of deposit.
- Coarse aggregate content: 12 cubic feet of 1½-inch top-size well-graded aggregate per cubic yard for 6-inch or greater slab thickness.
- “Normal” set time: four to five hours when measured by a penetrometer.
- Macrosynthetic fibers: 4 pounds/cubic yard or higher as needed to achieve an fe3 of 200 psi when measured in accordance with ASTM C1609, “Test Method for Flexural Performance of Fiber Reinforced Concrete (Using Beam with Third Point Loading).” The term fe3 is the equivalent residual flexural strength, or the load-carrying capacity, at a deflection of 3 mm, an indication of fiber performance.
- Air Content: < 3%
- Compressive strength: f’c is generally 4000 psi at 28 days; these mixes with 530 pounds of cement/cubic yard average 5500 psi nationwide with water-cement ratio of 0.53 +/- 0.02.
* See “Athletic Concrete Revisited,” Concrete International, September, 2004.
