Q.: Our company placed about 300 cubic yards of concrete in curbs, sidewalks and aprons in a business district whose roads are serviced by the state. The concrete was certified to be 3500 psi and air-entrained. Welded wire fabric, size 6 x 6 - W1.4 x W1.4, was used. A cure-seal spray was applied at completion and the concrete was also covered with polyethylene sheet for one day.
Concrete was only 4 months old when spalling occurred in various sections totaling about 20 cubic yards. Salt from cars crossing over 6-inch aprons at the parking lot entrance caused spalling of new concrete. Could this problem have been prevented, and if so, how? Who should be responsible for this problem?
A.: Possibly the problem is scaling. This is similar to spalling except that the concrete tends to come off in thin layers. Scaling is often the result of the action of deicers during periods of freezing and thawing. The responsibility could lie with the specifier, the concrete supplier or the contractor, depending on what caused the trouble. Based on what you've reported it does not seem as though the concrete should have spalled or scaled, so there must be some condition that is either unknown or incompletely considered.
You said that the concrete was certified to be air-entrained, which we take to mean that the air content was not actually tested at the job site. You didn't indicate whether a definite air content had been specified. For a severe climate like yours, the average air content specified should have been 5.5 percent if the maximum size of the coarse aggregate was 1 1/2 inches, 6.0 percent if the maximum size was 1 inch or 3/4 inch, 7.0 percent if it was 1/2 inch, and 7.5 percent if it was 3/8 inch. If the concrete contained less air than required for the maximum aggregate size, it could have contributed to scaling or spalling.
Sometimes the job concrete contains less air than specified because either the air-entraining agent was accidentally left out while batching or the air was partially lost at some later time. It's possible for some of the initial air content to be lost during transport and delivery if the concrete contains fly ash, particularly if the fly ash has a high carbon content. Other possible causes of loss of entrained air are adding water to the concrete in the mixer to increase the slump, finishing the concrete while there is bleed water on the surface, or spraying water onto the surface of the concrete during finishing. One of these troubles could be the cause of the spalling or scaling. Sometimes one or more of these causes is unknown to the contractor and does not show up in the job records.
Although good curing requires retaining moisture in the concrete, there is a potential difficulty if the concrete is placed late in the fall. If the curing keeps the moisture content of the concrete high during the winter season, it may contribute to scaling after salt is applied. The recommendation is often given that liquid membrane curing compounds should not be applied to concrete placed late in the fall, since these compounds may retain a high moisture content for an undesirably long period during the freezing and thawing season, thereby contributing to scaling if the concrete is low in air content or high in water content. For this reason it is also suggested that concrete be allowed to cure and then dry at least 28 days before any salt is applied. Others recommend that applications of deicer salt be prohibited during the first winter.
By testing a hardened sample taken from the pavement a petrographer could ascertain the air content of the concrete and also determine whether the water-cement ratio of the cement paste in the surface layer is higher than in the rest of the concrete.
An air test with a pressure meter can be run in a few minutes. To prevent surface scaling of flatwork, air content of the concrete must be carefully controlled.
