Freeze-Thaw Cycles and Wichita Concrete

How often it freezes

The National Weather Service office in Wichita publishes 1991–2020 climate normals for the city's airport station. They list an average of about 102.7 days a year with a low of 32 degrees or colder, but only about 16 days when the high stays at or below freezing, and less than one day a year on average at zero or below (NWS Wichita, annual climate normals for Wichita, Salina and Chanute; NOAA NCEI 1991–2020 annual normals, Wichita Eisenhower airport station). Put another way, most freezing nights in Wichita are followed by days that climb above freezing. That swing is a freeze-thaw cycle.

NOAA's monthly normals for the same station show how those freezing nights spread across the year (NOAA NCEI 1991–2020 monthly normals, Wichita Eisenhower airport station):

In December, January and February, most nights freeze, yet only a handful of days stay frozen. For wet concrete, that pattern is harder than a steady deep freeze, because the water inside keeps freezing and thawing over and over.

What freezing water does to concrete

Concrete is porous. Rain, snowmelt and slush soak into the surface. When saturated concrete reaches freezing, the National Ready Mixed Concrete Association explains, water expands as it turns to ice, causing stresses inside the concrete, and the potential for scaling rises as the number of cycles increases (NRMCA, CIP 2: Scaling Concrete Surfaces). Scaling is the flaking and peeling of the finished surface. It starts in small patches and can merge to expose large areas of stone.

De-icing chemicals make it worse. NRMCA notes that de-icers increase the saturation of concrete at the surface and the number of freeze-thaw cycles the surface experiences (NRMCA, CIP 2: Scaling Concrete Surfaces). Melting snow at noon only for it to refreeze at night adds a cycle that would not otherwise happen.

The ingredient that matters most: entrained air

Air-entrained concrete contains millions of microscopic bubbles that give expanding ice somewhere to go and keep stress from building up. NRMCA lists non-air-entrained concrete, or too little entrained air near the surface, first among the causes of scaling, and recommends a total air content of about 6 percent for typical exterior slabs with three-quarter-inch or one-inch aggregate (NRMCA, CIP 2: Scaling Concrete Surfaces). For slabs that stay moist, freeze and see de-icers, it also recommends a specified strength of 4,000 psi.

You cannot see air content once the slab hardens. That is why it belongs on a written estimate for any outdoor pour around Wichita: driveways, walks, steps, patios and approaches.

Finishing and curing make or break the surface

A great mix can still scale if the top layer is weak. NRMCA warns against finishing while bleed water is on the surface, adding excess water at the site, and overworking the surface, all of which create a weak layer on top. It also points out that placing and finishing can reduce entrained air at the surface (NRMCA, CIP 2: Scaling Concrete Surfaces). Then comes curing: keeping the new slab moist and warm enough to gain strength. A slab that dries out too fast never develops its full surface strength.

The first winter is the riskiest

New concrete is saturated and still gaining strength. NRMCA's cold-weather guidance says fresh concrete freezes if its temperature falls below about 25 degrees, that freezing can cut its potential strength by more than half, and that new concrete should be protected from freezing until it reaches about 500 psi, roughly two days after placement. It adds that concrete exposed to freeze-thaw cycles should be protected until it reaches at least 3,500 psi (NRMCA, CIP 27: Cold Weather Concreting). For homeowners, the practical version is simple: be cautious about late-fall pours, and skip de-icing chemicals the first winter. NRMCA recommends using clean sand for traction instead (NRMCA, CIP 2: Scaling Concrete Surfaces).

NWS Wichita places the average first fall freeze around October 25 and the average last spring freeze around April 9 (NWS Wichita, annual climate normals for Wichita, Salina and Chanute). A slab poured in mid-October has days, not weeks, before its first freezing night. See when to pour concrete in Kansas.

Snow is not the main problem; meltwater is

Wichita averages about 12.7 inches of snow a year at the airport (NOAA NCEI 1991–2020 annual normals, Wichita Eisenhower airport station). The snow itself does little harm. What matters is where the meltwater goes. Water ponding on a low spot of a driveway, pooling against a step, or running across a walk from a downspout keeps that spot saturated, and saturated spots scale first. Look at where water sits after a melt and fix the drainage. See drainage around driveways and patios.

A winter routine for existing concrete

• Shovel early, before snow is packed into ice by feet and tires.
• Use sand for grip when possible; when a melter is needed, choose carefully. See ice melt and concrete scaling.
• Keep downspouts discharging away from slabs.
• Rinse salt residue off in a thaw if heavy salting was needed.
• Note where scaling appears; those spots point to drainage or salt problems.

Sealing before winter

NRMCA recommends considering a breathable silane or siloxane sealer for exterior slabs facing harsh winters, applied when the concrete is reasonably dry, with late summer as an ideal time (NRMCA, CIP 2: Scaling Concrete Surfaces). A sealer slows water and salt from soaking into the surface. It will not fix a weak mix, but it helps a good slab stay good. See concrete sealing and the sealing schedule guide.

If the surface is already flaking, see repair and resurfacing and what shapes a repair scope.

Sources: NWS Wichita, annual climate normals for Wichita, Salina and Chanute; NOAA NCEI 1991–2020 annual normals, Wichita Eisenhower airport station; NOAA NCEI 1991–2020 monthly normals, Wichita Eisenhower airport station; NRMCA, CIP 2: Scaling Concrete Surfaces; NRMCA, CIP 27: Cold Weather Concreting.