Finishing air-entrained concrete requires a more patient and deliberate approach compared to conventional concrete to ensure durability and a high-quality surface. The key is to delay each finishing step, keep the surface open for as long as possible, and apply a light final trowel that leaves a slight "fuzz."
Understanding Air-Entrained Concrete
Air-entrained concrete contains microscopic air bubbles intentionally introduced during mixing. These bubbles improve the concrete's resistance to freeze-thaw cycles, making it ideal for outdoor applications in cold climates. However, these air bubbles also make the concrete more sensitive to over-finishing, which can push the air out and compromise its performance.
Key Principles for Finishing Air-Entrained Concrete
Successful finishing hinges on three crucial principles: patience, minimal manipulation, and precise timing.
1. Delaying Finishing Operations
- Patience is Paramount: One of the most important rules for air-entrained concrete is to delay each finishing step as long as possible. This allows the bleed water (the water that rises to the surface as the concrete settles) to evaporate completely before the surface is worked.
- Why it Matters: Working the surface too early, while bleed water is present, can trap water beneath the finished surface, leading to scaling or delamination. Air-entrained concrete typically bleeds less and more slowly, so patience is even more critical.
- Practical Application:
- Wait until the concrete has stiffened sufficiently to support the weight of a finisher without leaving deep indentations.
- Resist the urge to start floating or troweling prematurely, even if the surface appears ready.
2. Keeping the Surface Open
- Allow for Evaporation: It is vital to keep the surface as "open" as feasible and finally close the surface as late as possible. An open surface allows trapped air and any remaining bleed water to escape naturally.
- Methods to Ensure Openness:
- Minimal Initial Floating: Use a bull float or darby to level the surface, but avoid excessive passes.
- Resist Compacting: Do not overwork the surface with floats or power trowels, as this can break the air bubbles near the surface, reducing freeze-thaw resistance.
- Environmental Considerations: Protect the surface from rapid drying (e.g., wind, direct sun) which can cause plastic shrinkage cracking, but still allow for natural evaporation.
3. The Final Trowel Pass
- "Leave Some Fuzz": When lightly troweling the surface at the end of finishing, “leave some fuzz on the surface,” as the finishers say. This means avoiding an extremely smooth, burnished finish.
- Achieving the Right Finish:
- Use a steel trowel for the final pass, but apply light pressure.
- The goal is to create a dense, durable surface without over-compacting or creating a slick, sealed layer.
- A slightly textured or "fuzzed" finish indicates that the air voids near the surface remain intact, preserving the concrete's air-entrainment benefits.
- Benefits of "Fuzz": This slight texture provides better slip resistance and ensures the microscopic air bubbles at the surface are not destroyed, maintaining the concrete's frost resistance.
Practical Tips for Success
| Finishing Step | Action for Air-Entrained Concrete | Purpose |
|---|---|---|
| Screeding | Level the concrete to grade. | Establishes uniform thickness. |
| Bullfloating | Use a wide float with minimal passes. | Levels irregularities, embed aggregates, open surface. |
| Waiting | Wait for bleed water to disappear completely and the concrete to stiffen. | Crucial for preventing surface defects. |
| Floating | Use a wood or magnesium float. Minimal passes, do not overwork. | Densifies surface, removes minor imperfections. |
| Troweling | Light, minimal passes with a steel trowel. Avoid burnishing. Leave "fuzz." | Final smoothing, leaves appropriate texture. |
| Curing | Begin curing immediately after finishing. | Prevents moisture loss, ensures strength development. |
Why These Practices Matter
Adhering to these finishing techniques for air-entrained concrete is critical for several reasons:
- Durability: Preserves the concrete's enhanced resistance to freeze-thaw cycles, preventing scaling, spalling, and cracking in cold weather.
- Surface Integrity: Minimizes the risk of delamination or blistering, which can occur if the surface is closed off too early, trapping water or air.
- Aesthetics: Ensures a uniform, durable, and appealing surface that stands the test of time.
- Safety: A slightly textured surface (from the "fuzz") provides better traction, reducing slip hazards.
By being patient and following these guidelines, you can achieve a durable, high-performing finish on air-entrained concrete that maximizes its long-term benefits.
