Q.: We placed a 6-inch-thick, 16x40-foot air-entrained concrete slab during cold weather. The 12-cubic-yard load of 3000-psi concrete was made with hot water and contained an accelerator that permits placement at temperatures down to 20° F. Concrete arrived at 9:30 a.m. with a 4-inch slump, and we added no additional water at the jobsite. We finished placing it an hour and 40 minutes after the truck had left the batch plant. High air temperature during the day was 42° F and low was 24° F. Although we waited until 7 p.m., the concrete still hadn't set, so we covered it with plastic sheeting, 1 foot of straw, and another piece of plastic sheeting. We were finally able to power trowel the slab at 2 p.m. the next day. At 7 p.m. that day we covered the slab again with plastic sheeting and 18 to 24 inches of straw. When spreading the straw, workers noted that the surface still wasn't real hard.

On the second day following the pour, the weather warmed up and there was lots of fog, so we uncovered the slab. Snow fell soon afterward, and we couldn't work for 25 days. When we came back to the jobsite we removed 2 to 3 feet of snow and found that much of the slab was covered with 3 to 4 inches of ice. We used potassium chloride to melt the ice so it could be scraped off. Workers noticed that the concrete surface looked milky and seemed to be covered with a cream-colored cement film. They used squeegees to remove the rest of the deicer, but there was no water, so they couldn't wash off the surface. The weather turned cold again three days later and 14 more inches of snow fell.

When we came back to the job two weeks later and shoveled snow off the slab, most of the slab surface was badly damaged. In some places you could scrape off concrete to a depth of 3/4 inch. Was this deterioration caused by the potassium-chloride deicer?

A.: The use of a chloride deicing agent usually doesn't cause severe deterioration in such a short time. However, conditions were quite harsh because the concrete had no time to dry before being exposed to freezing and deicer application. Even air-entrained concrete may not survive the extreme conditions you describe. It's also possible that the fresh concrete froze.

To pinpoint the cause, you need to have cores taken and then examined by a petrographer for evidence that the fresh concrete froze. Chloride content, air content, and quality of the hardened concrete could also be estimated as an aid to finding out why the concrete deteriorated so rapidly.