In the February issue, Concrete Construction's editors requested reader feedback on the subject of rolling stones. Here are their responses:

In your February Problem Clinic there is a question about rolling stones and the problems that they cause. We have seen this problem a few times. Although rare, when it does show up, it is usually a problem for a few weeks (or just one job) and then disappears. When we get this problem, it is frustrating and we change many finishing methods to try to get the problem to go away. Sometimes the changes work, sometimes they don't. Here's what we do (among other things):

  1. Minimize the number of trips over the slab for the pans. Usually we like to pan a slab as much as six to seven times. When the rock problem occurs, we reduce the number of trips to as few as two. Of course, our F-numbers go way down when we do this.
  2. After panning, the next machines must keep their combination blades as flat as possible.
  3. On the final burn, use as light a machine as possible and keep the finish blades as flat as possible.
  4. We try to minimize the total number of trips over the slab with both the combo and finish machines. This is risky because the final appearance of the slab is at stake.
  5. We have the concrete supplier remove 100 pounds of sand and replace it with 100 pounds of course aggregate. Sometimes this can't be done because of submittal time and specifications.

There have been times when frustration sets in and no changes affect the number of rocks that show after the final burn. We then go back just before the building is turned over to the owner and patch any holes that are showing.

I would like to know more about the problem that the contractor had. The picture of the hole is very similar to what we have except the color of the rock. The article didn't say if the offending rocks were foreign material or part of the course aggregate. It would help to know if the contractor has seen this problem again? What methods has he used? Does the problem vary from load to load or mix to mix?

- Mike Poppoff Poppoff, Inc., Yakima, Wash.

From looking at the photo it appears to me that the aggregate particles were actually lifted from the concrete. This could happen when the downward pressure on the aggregate (caused by the trowel blades) was released after the trowel moved over it. If the downward movement of the aggregate was prevented, possibly by the curing process or some other reason, the stored energy in the aggregate would cause it to spring from the concrete. Try pressing down hard on an object and then moving horizontally off the object. The object will spring upward. Another possibility is to check the aggregate for iron content. Maybe they were lifted by a static charge caused by the power trowel blades.

- Harry L. Whitaker Symons Corp., Des Plaines, Ill.

With respect to your question about rolling stones, this is a problem that has been encountered locally from time to time. I have generally refereed to this defect as rocking stones, since the troweling operation appears to cause the coarse aggregate particle to rock back and forth in the top of the slab. The rocking motion forms a socket in the top of the slab, and most times the stone is left in the socket. Generally the rocking stone is a round stone, 10 to 16 mm (3/8 to 5/8 inch) in diameter. The first time I encountered this defect was with a defective dry shake hardener that contained occasional larger gravel sizes.

Our local concrete aggregates are produced from river gravel and contain a low (30% to 40%) proportion of crushed particles. Despite the use of similar concrete mixes, problems with rocking stones occur only from time to time. This has led to the conclusion that they are more of a finishing defect than a mix design problem. On the occasions when rocking stones have been noted, they have been associated with high evaporation rates, cool temperatures, and increased setting times (that is, winter construction). Blistering also has been noted on these jobs, which suggests that there was some crusting of the concrete that resulted in premature finishing.

To date, this problem has not been pervasive enough to require special measures other than good finishing practices (including appropriate placement temperatures).

- J.D. (Dave) Robson, P.Eng. EBA Engineering Consultants Ltd., Edmonton Alberta