Back to basics: The four steps to creating effective concrete

Concrete, especially high-consistency mixes, can be over-consolidated, usually from over-vibration. In these instances, a thick film of water will form on the surface. This water can negatively impact the concrete with reduced surface strength, lower permeability, and poor aesthetics. Seth Mailman, a project manager at Baker Concrete, says, “Concrete placed without proper consolidation will not perform as expected. Understanding the relationship between design and construction with respect to concrete is necessary to ensure the final product performs as expected.”

Finishing

The third phase, finishing, is the process of smoothing out the surface of the concrete to achieve a desired performance texture and appearance. The American Concrete Institute (ACI) 301-20: Specifications for Concrete Construction is a solid starting point for information on finishing to make attractive, durable concrete surfaces capable of resisting abrasion, weathering, and other physical stresses. Finishing helps with setting, prevents chipping, and is climacteric to both decorative and industrial polished concrete surfaces.

Types of finishes

• Broom finish: Brushing a broom over the concrete’s wet surface will create a noticeable textured groove. This is a long-accepted finish for exterior concrete.
• Bull float finish: A bull float is a tool used by a finisher to spread and smooth concrete before initial set is achieved. A bull full float finish is the texture left behind by the smooth paddle facing of the bull float without additional screening or trowel work.
• Sand finish: A sand finish involves removing the cream off the surface of the concrete so the sand just below is exposed to become the new surface. This can be beneficial for exterior locations in climates that deal with significant freeze-thaw swings.
• Salt finish: Rock salt can be rolled across/into the surface of fresh concrete and then washed off after final set to create a subtle texture with enhanced skid resistance.
• Stamped finish: Stamped concrete has been imprinted to resemble brick, slate, flagstone, and other material types through texturing and pattern molds that are stamped into the concrete.
• Troweled finish: By using either a hand or power trowel, a trowel finish is executed by the smoothing and compacting of the unformed surface of
  fresh concrete.

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  Polished finish: A polished finish is a higher degree of troweling in which the concrete is further refined for aesthetics and abrasion resistance. It is worth noting that polished concrete on existing slabs requires the concrete’s surface to return to a semi-plastic state through the creation of slurry to achieve maximum refinement and maintain surface integrity.

A polished concrete finish is most successfully achieved through the specification of surface micro texture (SMT) performance benchmarks in Part 3 of concrete specifications section. As an example, a hard trowel finish may be specified to meet an average 2 to 1 micrometer (µm) (80 to 40 microinches [µin]), as measured by ASME B46.1-2019, Surface Texture (Surface Roughness, Waviness, and Lay), ahead of the polished concrete contractor’s final benchmark (i.e. 0.7 to 0.2 µm [30 to 10 µin]).

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