Q.: I recently poured an exposed aggregate driveway. The concrete came with retarder in the mix and I also sprayed retarder on top. The next morning some spots were still wet at the surface and the rest of the load was completely set. This was a 90-degree day with high humidity. What happened?
A.: Aside from the two obvious possibilities (incomplete mixing of the concrete and improper application of the top surface retarder—probably overdosing), one possibility is segregation. We have never heard of this bad a case, but sometimes, especially with gap-graded aggregate mixtures, the rocks go one way and the mortar goes another. The rocky areas have a low water content and set up rapidly; the over-mortared areas have a high water content and set up slowly. Of course, it could also be a sand ball that never got any cement in it.
Of the many variables possible, another comes to mind—using retarders in the mix on short hauls, especially on very hot days. In some cases, the retarder will not let the concrete take a set once placed on these short hauls, but the ambient conditions allow the concrete to dry. This results in uneven (spotty) finishing/curing and, even worse, severe plastic shrinkage cracking as a result of the concrete not attaining sufficient early strength to withstand the forces of shrinkage.
Ultimately, the retarder works by way of the cement particles adsorbing on their surface the admixture anions and molecules. This serves to inhibit for a given period the normal hydration reaction. Dose rates and mixing are therefore critical. Check your dose rates with the manufacturer to see if you have used enough, based on total cement content. Check your mix cycle. Check on the effect, if any, of the presence of other ultrafines. We have consistently found that inadequate mixing (short hauls add to the problem) is most often the culprit.