Question: We have an unusual case of an apparently expanding concrete that baffles local experts. The contractor removed all the old concrete of a deteriorating sidewalk and placed a new one in December 1981. Due to cold weather, the contractor ordered 4000-psi concrete and built a tent around the entire job. Salamanders were placed in the tent but no records were kept of the actual temperatures inside. The contractor did not cover the new sidewalk so the concrete was apparently subject to drying at an unknown temperature in its early stages. The base material was well compacted sand and gravel. The sidewalk, located on a hill, was placed in 16-foot-long sections separated by ½-inch joints that were subsequently sealed.

Later in 1982, it was noticed that the individual sections were apparently enlarging and each had expanded downhill and in many cases the higher section had overridden the lower one, squeezing out the joint sealant. Visually the whole thing is a mess. Since this is a show sidewalk in the center of town, it will be necessary to either replace it or make repairs if we can find a satisfactory method.

We have established conclusively that the concrete is not simply sliding down the hill, but each section is apparently expanding. Since concrete made with quality aggregate is not supposed to expand beyond its original dimensions, we don't know what to make of this situation. Do you have any ideas?

Answer: The expansion is much larger than could be accounted for by changes in moisture or temperature. We checked with William Hime, of Erlin Hime Associates, Northbrook, Illinois, about whether there has ever been trouble with alkali-silica reactivity with any aggregates in your area. He replied that such reactivity has become more likely than heretofore because of the recent trend toward higher alkali contents of cements.Most areas of the country have been free of this trouble. But in recent years, new dust-control requirements by the Environmental Protection Agency have required changes in the manufacture of portland cement. These in turn have tended to increase the alkali content of some cements. Now some aggregates that had formerly undergone no noticeable alkali-silica reaction (because they had always been used with low-alkali cements) are beginning to reveal their reactivity when used with higher-alkali cements.You might check this out by testing the cements and aggregates you use by the mortar bar test of ASTM C 227.