Evaporation retarders keep the surface from drying out so it becomes stronger and more durable. But they’re often used incorrectly, and that’s where our story starts.
Despite manufacturer warnings, contractors use evaporation retarders as finishing aids on everything from driveways to spillways. If you’re one of them, we understand you may feel you have little choice. Little is known about finishing aids, so specifications often limit surface additives to resin-based curing compounds, dry shakes, and evaporation retarders. Using an evaporation retarder instead of a finishing aid, however, compromises surface strength, durability, and aesthetics. Premature failure has caused some municipalities to prohibit the use of this extremely useful product.
A finishing aid is exactly what the name implies: a product that facilitates finishing by rehydrating the surface to a creamy state without compromising strength and resistance to physical and chemical attack.
They’re both spray-applied liquids that create a finishable surface. One, however, is meant to be worked into the surface and one is not. Furthermore, they’re different chemicals, which affects rate and type of reactivity and impact on hardened properties when mixed into cementitious paste. To make it easier for you to justify a finishing aid, we devised a research project to prove the root of the problem is the misuse of evaporation retarders to finish plastic concrete surfaces.
One big difference between two similar products
Finishing aids and evaporation retarders are necessary when placing concrete in hot, sunny, dry, or windy conditions. Interior placements can experience similar stress from HVAC units or local ambient conditions.
Evaporation retarders mitigate the loss of water at the surface and subsurface that was intended for finishing operations and cement hydration. According to ACI 302.1R-15 “Guide to Concrete Floor and Slab Construction,” they may be applied more than once during finishing to minimize plastic-shrinkage cracking when evaporation rates are high. ACI 302.1 goes on to note they should be used only according to the manufacturer’s directions and never during final troweling because they discolor the surface.
Evaporation retarders come as a concentrate the contractor formulates into a solution of one part concentrate to 10 parts water and applies after finishing with a low-pressure hand-held sprayer at a coverage rate of 200 square feet to 300 square feet per gallon. They should not be worked into the surface. When used correctly, they reduce plastic shrinkage cracking, flaking, crusting, and dusting, and create a film that allows hydration to increase surface hardness and strength.
Finishing aids are formulated specifically to rehydrate dried surfaces to a creamy state for broom finishing or hard troweling. They’re applied after screeding with a low-pressure hand-held sprayer at a coverage rate of 200 square feet to 1,000 square feet per gallon and are meant to be worked into the surface. They also can be applied or reapplied during finishing if the surface dries out.
Applying an evaporation retarder to dried concrete creates the same creamy surface, but a lot must be used to maintain workability for an extended period of time. This softens the concrete, leading to premature surface failures such as:
- Popouts, cone-shaped cavities left after a near-surface aggregate particle has expanded and fractured
- Delamination, when the top 1/8 inch to ¼ inch separates from the slab
- Lower resistance to freeze-thaw cycles and deicing salt attack and cracks
- Dusting, which can begin within days or weeks under any traffic conditions and is a result of a low-strength surface layer.
Putting our assumptions to the test
A finishing aid is not an evaporation retarder and an evaporation retarder is not a finishing aid. To prove this, we analyzed the hardened properties of three slabs finished with:
- Water (shown as REF in the bar charts) at 150 square feet per gallon
- An evaporation retarder (EVAP R) at 75 square feet per gallon
- A finishing aid (FIN AID) at 500 square feet per gallon.
The slabs were cured for 56 days in a humidity- and temperature-controlled room before being subjected to three tests.
Surface hardness was determined using ASTM C805 “Standard Test Method for Rebound Number of Hardened Concrete” and converted into surface strength. The finishing aid increased strength by more than 15% compared to water and 60% compared to evaporation retarder. As hypothesized, working an evaporation retarder into the surface significantly softened the hardened concrete.
ASTM C1583 “Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-off Method)” was used to determine near-surface strength. The results indicate finishing aids are better suited than evaporation retarders to enhancing hardened concrete properties. The surface tensile strength samples finished with an evaporation retarder had a significant amount of aggregate pullouts in the failure zone, while the slab with the finishing aid had aggregate fractures throughout the failure zone, indicating higher paste strength in the finishing aid sample. The finishing aid increased surface tensile strength by more than 30% over water and by 200% over the evaporation retarder.
A modified method of ASTM C672 “Standard Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals” was used to determine resistance to freeze-thaw cycles in the presence of deicing chemicals. Based on the number of popouts, the surface finished with an evaporation retarder was much more sensitive to the freeze-thaw environment. The finishing aid decreased popouts by more than 50% and 80%, respectively, when compared to water and evaporation retarder.
As shown in our laboratory analyses, working evaporation retarders into the surface reduces hardness, tensile strength, and resistance to freeze-thaw and deicing salt attack. We encourage you to continue using them for their intended purpose – to reduce the loss of water at the surface – and use finishing aids to make the surface creamy, dreamy, and ultimately stronger and more durable.
We thank the team at Obex Co. in Portland, Ore., for its generous support, especially President Jared Murray for his advice during sample fabrication.
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