Concrete Floor Finishes: Don’t Polish ‘Til It’s Dry!
Concrete floor finishing tastes are surprisingly subjective. However, strength and performance characteristics are beyond question when concrete slabs are properly installed. Professionals choose designer finishes to dazzle with color and light reflection, while warehouse tenants choose concrete for its resilience to forklifts and work boots. Individuals may opt for the beautiful, buff-polished beauty of bare concrete flooring.
Hold those horses. That buff comes with maintenance for those who want a glossy shine. Installers who polish before the floor is dry are haunted by the shine of persistent moisture content problems. End results reflect the integrity of the process that created them.
Burnishing and Polishing
Definition: the process of finishing concrete to provide a hard-wearing, durable finish with a surface luster.
The goal of burnishing is to produce smooth, impervious and durable concrete surfaces free of residual trowel marks. The concrete grinding process utilizes a diamond grit set in a metallic matrix to remove pits, blemishes, stains or light coatings from the concrete surface. Depending on the coarseness of the concrete, “grinding” occurs in three or four stages. The purpose is to take the rough out of the slab. This is the first stage of a polished floor.
Next, concrete polishing sets finer diamond abrasive in plastic or resin polishing disks. Installers set those disks in motion, gradually moving to finer and finer abrasives to bring more polish to the floor. The process of “lapping” continues until the concrete floor shines as desired.
During polishing, installers apply an impregnating sealer which densifies and protects the concrete. The impregnating sealer may negate the need for a topical coating, but contractors sometimes apply a commercial polishing compound to bring more sheen to the fore.
From first to last, burnishing seeks to refine the rougher surface, which is typical to new concrete slabs. Troweling smooths the surface, grinding removes unwanted dirt and materials, and polishing seals the job.
It is vital to distinguish between a concrete surface and the chemical membranes that seal it. Sealers are designed to protect concrete slabs from corrosion by blocking surface pores which absorb moisture content and salts. Moisture content permeation has several possible sources: chemical intrusion, freeze/thaw, and corrosion of reinforcements. Once a sealer is applied, it forms a thin layer between concrete and its surrounding air.
Moisture content (MC) is part of concrete from the moment that Portland cement, water and aggregates are mixed. During the drying process, this moisture migrates to the surface of the slab to evaporate away in an ongoing process until the slab is sealed or has reached the same level of relative humidity as the ambient air.
Concrete problems related to moisture content ensue when installers have not allowed sufficient time for MC to leave the slab. Stable moisture content is essential not only to the drying process, but to long-term concrete performance. Excess moisture content creates residual problems which can jeopardize concrete flooring, no matter how it is sealed or finished.
Why It’s so Important
The burnishing process can exacerbate moisture content problems if slabs have not dried properly. Put simply, the sealers whose purpose was to make concrete floors resilient and shiny end up trapping water vapor inside slabs. Left without a means of escape, concrete moisture content sits. Burnishing sealers may fail, along with flooring adhesives and chemical colorants. Excess moisture content shouldn’t be there, but it is, and builders and customers face expensive concrete remediation.
The best course, of course, is to be sure the slab has completely dried to the necessary moisture content level. Surface-based testing, like calcium chloride testing or using hand-held concrete moisture meters, can only identify the need for quantitative measure.
Relative humidity (RH) testing, on the other hand, provides a complete picture of a slab’s RH level with the use of in situ probes placed within the concrete at 40% of the slab depth. The Wagner Meters Rapid RH® measures RH inside concrete slabs for a complete picture of the slab’s readiness to begin the polishing process. The Rapid RH® in situ relative humidity test system provides crucial information on when installers can safely proceed.
A concrete moisture meter brings shine to slab integrity and performance, as polishing does to concrete surfaces.