- Lisa Evans
Protecting Late Season Concrete Pours
In northern climates, concrete that’s poured late in the season does not have much time to mature prior to exposure to damaging freeze/thaw cycles. That’s why surviving the first winter undamaged is often the most critical in the lifespan of newly placed concrete.
Although we like to think of concrete properties as fixed the moment it cures, it does undergo a maturation process, during which it develops its full design strength and durability. Most concrete has very high compressive strength, anywhere from 3,000 psi and beyond. Compressive strength is a measurement of how well it can withstand an external force that wants to compact it, like the weight of a car rolling over a concrete driveway.
But concrete also encounters another force that is much more damaging, and which it is far less capable of enduring. Tensile strength is a measurement of an internal force that wants to tear it apart from the inside. Inflate a balloon and then paint it. Now inflate that balloon some more, and the tensile forces imparted on the paint will cause it to crack.
Concrete cracks in much the same way during freeze/thaw cycles. Water is absorbed into the concrete where it freezes when the temperature drops. And as anyone who has left a can or bottle of pop in the freezer a bit too long knows, when water freezes it expands. Because concrete has relatively low tensile strength (about 1/10th of is compressive strength), this expansion can cause it to crack. When that crack occurs near the surface, it may become a “pop out” over an absorptive aggregate or cause the top paste to begin scaling or delaminate off the surface.
This damage can be aggravated by the use of deicing chemicals, which cause concrete to undergo additional freeze/thaw cycles as ice is melted and refrozen. Deicing chemicals can also allow concrete to become “super saturated,” and hold more water than it would naturally. And of course, more water means more expansion as it freezes.
Use Penetrating Repellants to Protect Young Concrete Concrete that has not reached full maturity is particularly prone to this kind of freeze/thaw damage. As a result, it’s crucial to provide maximum protection to late season pours. Freeze/thaw damage is best avoided by not allowing water into the concrete in the first place. Penetrating water repellants like silanes and siloxanes do a great job of reacting within the concrete to form a hydrophobic (water repellant) barrier throughout the concrete pores and capillaries. Since they work below the concrete surface, they have the added benefit of not changing its appearance or slip resistance, and last longer because they’re not subject to surface wear and abrasion.