What constitutes acceptable surface finishing for a concrete slab, and when should finishing occur?

• A. A rough, irregular surface with tearing; finish after hardening.
• B. A perfectly smooth surface with no laitance; finish before plastic state.
• C. A matte textured surface; finish at any time.
• D. A smooth, uniform surface with no excessive laitance or tearing; finish while plastic or as directed, avoiding over-troweling.

Answer: (D)

The key idea is that the surface of a concrete slab should be transformed into a smooth, uniform finish without a weak laitance layer or surface tearing, and the finishing must occur while the concrete is still plastic or as the project directs. Finishing while the mix is plastic allows proper consolidation and control of the surface texture, reducing the chance of creating a thin, weak laitance film that can dust or fail to bond coatings. It also helps prevent tearing and gouges that come from finishing after the concrete has stiffened. Following the project’s direction ensures any required texture (such as a broom finish or other specified surface) is achieved. Over-troweling is to be avoided because it can push paste to the surface, increasing laitance and causing a rough, susceptible finish. So the best practice is a smooth, uniform surface with no excessive laitance or tearing, finished while plastic or as directed, avoiding over-troweling.