Q.: Part of a 4 1/2-inch-thick concrete deck that is to be post-tensioned is not hardening, though it is 19 hours old. Only one truckload is involved, and the troublesome area is surrounded by good concrete. All the concrete contains a retarder. Can we do something to make it set or must we take it out?
A.: It sounds as though you've had an overdose of retarder in that truckload. This can happen through a batching error or when an admixture dispenser malfunctions. Can you wait a few days? The concrete will most likely set up if given enough time and should ultimately gain its full strength. If the concrete never does set (which is unlikely) you may want to get a testing laboratory to find out what got into the mix to retard it so badly. If it does set in a few days it may be wise to core the slab at age 28 days to see if the proper strength has developed.