We placed 400+ yards of a 5000 psi high-early strength concrete on an elevated slab with the temperatures in the 90s. The mix had superplasticizer, water reducer, and chilled water. The slump range was 6 to 8 inches. The finishing was a light broom and when the finishers got off of the slab it looked real good. The next day we walked the slab and noticed it had numerous ripples throughout. The slab was 8 inches thick and the re-bar was at minimum ¾ inch below the surface. My question is what would cause this rippling effect overnight?

We asked Jerry Holland of Structural Services Inc., Bethlehem, Pa., to comment. He noted first that the writer had not indicated if the ripples were straight. But assuming they are then the ripples are probably directly above the rebar. With this mix, using lots of water reducers and cold water, the set could be delayed making it susceptible to crusting and settlement over the rebar. One way this can happen is if the workers are stepping on the bars in one placement when the adjacent section is complete. This can cause some vibration in the bars, leading to what amounts to extra consolidation around the rebars. This phenomenon is also sometimes seen in mixes that have fly ash, since the mix sets more slowly and there’s time for settlement above the bars.