Diagnosing scaled concrete flatwork

Exterior concrete flatwork in a freezing climate experiences the most severe freeze-thaw concrete exposure class, F3. It undergoes cycles of freezing and thawing in the presence of water and deicing chemicals. The most common type of failure we see is scaling of the surface. Normally if this is going to happen, you start seeing the signs after the first winter. You’ll want to know what’s happening, what caused it, who’s at fault, and what to do about it. Here’s how we go about evaluating scaled concrete flatwork.

Specifying durable concrete

air-void system in hardened concrete
Proper air entrainment results in a well-dispersed system of air voids throughout the cement paste. When water in the concrete freezes, the air voids give it a place to expand. Then it doesn’t disrupt the cement matrix. The scale on the right is in mm. Photo: Mark Lukkarila.

ACI 318, Building Code Requirements for Structural Concrete, provides the basics for durability under F3 exposure conditions. You need a water-cementitious materials ratio of 0.45 or less and a good air-void system. Some water is always present in concrete, whether from the original mixing water or infiltrated from the environment. Water expands on freezing. The air voids provide a place for it to go so it doesn’t generate enough tension to cause cracking. ASTM C457, Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete, specifies two procedures for quantifying the air-void system parameters. Generally we like to see an air-void spacing factor of 0.008 in. or less.

ACI 318 also specifies maximum limits for supplementary cementitious materials in the concrete. These materials usually delay the setting of the concrete, leaving more time for bleeding to occur. It’s important to note that ACI 318 pertains to structural concrete. That is, it doesn’t apply to concrete flatwork. The engineer may want higher proportions of supplementary cementitious materials to meet the performance requirements. For example, control of expansions due to alkali-silica reaction may necessitate more than 25% Class C fly ash.

Proper finishing

We’ve discussed the importance of finishing air-entrained concrete from the standpoint of not overworking the surface. Hard troweling and air entrainment aren’t compatible because air entrainment slows bleeding of the concrete. Hard troweling densifies the surface, making it harder for bleed water to come to the top. Instead, it gets trapped below the surface, making a weak zone that can initiate delamination later on. In addition, hard troweling elongates the air voids parallel to the surface, weakening the layer just below the surface.

It’s crucial to wait until the bleeding stops before finishing the concrete. Do just enough finishing to get a smooth surface, and no more. And don’t add water to the surface; that just raises the water-cement ratio in the worst possible place. That is, just where you want to prevent water and salts from migrating into the concrete, you’ve increased the permeability.

Sampling scaled concrete flatwork

When examining scaled concrete flatwork, we need to determine whether the problem is in the concrete itself or the finishing. If the concrete is durable below the surface, once the surface is gone the flatwork will be fine. Remedial actions would involve grinding off the remaining surface and possibly sealing or adding an overlay. If the concrete itself is not durable, it will continue to deteriorate. In that case, the only remedy is to remove and replace the flatwork.

The key to sorting out these questions is to sample the concrete in the right place. To distinguish between finishing problems and materials problems, you need to examine the concrete where failure is about to occur. If you core where the surface has already detached, you’re missing the information about the finishing and curing. If the coring itself causes the surface to detach, the surface will abrade against the bulk concrete, grinding away the evidence.

So if you’re evaluating scaled concrete flatwork, look just outside where the concrete has scaled. Tap what appears to be the intact surface with a hammer. If it sounds hollow, or if the surface breaks loose, go a little farther outside the scaled area and try again. You’re looking for incipient failure, so handle the core carefully.

Petrography of scaled concrete flatwork

In the laboratory, you need to examine both the near-surface portion and the bulk concrete. Near the surface, look for signs of premature finishing, over-finishing, addition of water, premature drying, and excessive  carbonation.

For the bulk concrete, use either of the two procedures specified in ASTM C457 to quantify the air-void system parameters. If the spacing factor is less than or equal to 0.008 in., you can consider the air-void system adequate.

Evaluating the water-cement ratio is more difficult and less certain. Ideally, you’ll have concrete thin sections in your lab of known water-cement ratio for comparison. The more closely your samples match your test specimen, the more confident you can be in your estimate of the water-cement ratio. For the concrete to be durable under F3 exposure, it must have a water-cementitious materials ratio of 0.45 or lower.

In one case I was involved in, our lab had completed a research project for the cement company that supplied the cement for the concrete we were examining. In that market, many people used a standard ternary mixture, adding fly ash to the calcined clay blended cement. With their permission, we used thin sections from the cement company’s concrete mixtures to compare with the test specimen. We could estimate the water-cementitious materials ratio with confidence because we had the same proportions of the same three cementitious materials for comparison.