Q.: We accidentally added 6% calcium chloride by weight of cement to a truckload of concrete that went into a residential driveway placed during cold weather. Though the contractor had no trouble placing and finishing the concrete, he is concerned about low concrete strength. He heard that using more than 2% calcium chloride can cause strength loss at later ages. Is this true?

A.: Accelerators increase early strength at the expense of later strength, so a similar concrete without the accelerator would gradually reach a higher strength than the concrete with an accelerator.

If the contractor is concerned that strength at, say one year will be less than the 28-day strength, that situation is unlikely. We could find no data showing a 6% calcium-chloride dosage causing strength retrogression with age. But if there are no 28-day strength results because cylinders weren't made, you could core the driveway to determine strength of the in-place concrete. A 3000-psi or higher core strength should be adequate for a driveway in a cold climate. The driveway owner may not like the appearance of patched core holes, however.