Water shapes the way we live, and it affects how our polished concrete, too. Understanding concrete’s moisture is critical to achieving the best-looking and longest-lasting surface.
Is My Concrete Moist?
Though concrete slabs seem dry, they’re actually a liquid and constantly in a state of drying. They release moisture over time through tiny pores throughout the slab.
Moisture in concrete can be measured through two key parameters: the moisture vapour emission rate (MVER) and in situ relative humidity (RH).
The moisture levels impact various aspects such as the vibrancy and longevity of the topical colour, the penetration of densifiers, the clarity of the shine, the strength development of the concrete, and the presence of efflorescence marks and differential cure.
Challenges with Dyes
Some projects have seen the concrete dye’s colour intensity change, often due to shifts in barometric pressure and vapour drive during the building’s acclimatization process. This phenomenon is from the presence of moisture and pH levels.
When dyes are applied to the concrete surface, they penetrate the surface but don’t chemically bond with reactive elements in the concrete. Moisture and alkalinity can combine to create unfavourable conditions that affect the topically applied colour.
MVER levels can fluctuate and potentially diminish or dissolve the colour of the concrete surface. Moisture testing before applying dye or other coatings ensures they will last as long as they should.
Densifier and Moisture
Moisture affects the densifier’s penetration into the concrete slab. Also, job-site conditions, such as high humidity, low airflow, and cold temperatures, can reduce densifier absorption.
Proactively conditioning the building earlier in the construction schedule using dehumidifiers, fans, heaters, and sealing off external penetrations can create drier air and lower in situ RH, resulting in improved densifier absorption.
By controlling environmental conditions, the concrete becomes more absorbent, leading to a denser, harder, and abrasion-resistant surface that polishes with greater clarity and shine.
Moisture and Concrete Sealers
Most concrete polishers apply a stain-resistant microfilm guard or sealer, which can be affected by moisture. High MVER conditions or puddles left on the surface can cause the sealer to water-whiten or develop a haze.
Water-based sealers have a tendency to revert to their original solution when in contact with water. The amount of haze or whiteness ultimately depends on the severity of the MVER and the specific product’s chemistry.
Moisture can also lead to the development of efflorescence, which is the accumulation of whitish salts on the concrete surface. Excess-free lime or calcium hydroxide are sources of efflorescence.
According to the Portland Cement Association, three factors contribute to efflorescence:
- Soluble salts present in the concrete
- Moisture dissolving these salts
- Vapour transmission or hydrostatic pressure moves the salts to the surface.
From a structural standpoint, efflorescence is generally harmless and may appear months after the project’s completion. Once the building is conditioned, controlling relative humidity, airflow, and temperature can prevent the further development of efflorescence.
Polished Concrete Expectations
In situations where efflorescence arises, you may find reassurance in knowing the excess salts are limited, and it will eventually stop.
Efflorescence issues will resolve more quickly on a closed slab (with a vapour barrier beneath the concrete) because the only moisture source is the water used during the initial concrete pour.
Once the excess salts have surfaced and the slab’s RH has equalized with the surrounding air, efflorescence will cease. However, if the vapour barrier is compromised or absent, equilibrium may never be reached. In such cases, it becomes a waiting game until the excess salts naturally disappear.