Air entrainment in concrete was discovered accidentally about 70 years ago. Since then, entrained air has provided many blessings to concrete such as increased workability at lower water-cement ratios and reduced bleeding. And there is its primary function: maximizing the resistance of hardened concrete to damage from cyclic freezing and deicing chemicals.
There are also, however, some negative effects of air entrainment: there is one more concrete batch item that needs be controlled, there is a slight reduction in concrete strength, and there is the need to adjust to changes in timing and finishing techniques. In general, our industry has learned to accommodate the negatives relatively well. However, one of the difficulties of working with air entrainment has become a curse: delamination of hard-troweled concrete surfaces.
Delamination is a “plane” of separation between the surface layer and main body of concrete. Some common causes of delamination are corrosion of reinforcing steel bars aligned at a given depth, inadequate design and bonding of topping slabs, cyclic freezing, and improper finishing. This article deals only with delamination caused by hard-trowel finishing of air entrained concrete slabs.
Entrained air is essential for freeze/thaw durability and should be kept within industry guidelines (for example, 6±1½ percent with ¾- to 1-inch maximum-sized aggregate). Entrained air is unnecessary for concrete that will not be exposed to cyclic freezing unless there is a need to capitalize on its other blessings. However, there are numerous instances where machine-troweled slabs designed for use in nonfreezing environments contain entrained air where its blessings lead to the curse of delamination.
The following three case studies describe slabs that were designed to have dense, hard, flat, wear-resistant surfaces, but ended up delaminating due to entrained air and machine troweling.
A 5-inch-thick warehouse slab that incorporated a loading dock was placed on plastic sheeting. The loading dock concrete was designed to be air-entrained (5±1 percent air) because it was outside and would be subjected to cyclic freezing; it was given a broom finish. The interior slab was to be non-air-entrained because it would never be exposed to cyclic freezing; it was machine troweled. Apparently, the designer recognized that air-entrained concrete should not be machine-troweled.
Inadvertently, however, the air-entrained concrete mixture used for the loading dock was continued into the interior slab; the switch to non-air-entrained concrete was not made. Within a year after completion, large areas of the interior slab surface delaminated.
Cores were taken from both delaminated and non-delaminated areas for petrographic examinations. This revealed that the concrete was dense, well-consolidated, air-entrained, and had Portland cement contents of about 6½ bags per cubic yard and water-cement ratios estimated to be:
- 0.40 in the top ? to 3/16 inch
- 0.46 to 0.48 from 3/16 to ½ inch
- 0.44 in the rest of the concrete
The air content in the body of the concrete was determined to be 12 to 13 percent—significantly higher than the 5±1 percent specified for the loading dock concrete. This excessive air entrainment gave the paste and coarse aggregate sockets a frothy texture that significantly reduced the concrete strength.
In the top ? to 1/16 inch, the concrete contained:
- no coarse aggregate
- air contents of 1% to 2% (Figs. 1 and 2) versus 12% to 13% in the body of the concrete
- paste that is darker, harder, and has a water-cement ratio of 0.40 versus 0.44 in the body of the concrete
- paste contents higher than in the main concrete body: 38% versus 22% (Fig. 2)