Q.: As I understand it when a superplasticizer is added to a truck mixer at the jobsite, the slump immediately increases substantially and, for at least a short time, the concrete becomes very fluid and easy to place. Does this make the 28-day strength any higher or lower than it would otherwise have been?

A.: It doesn't decrease the strength. And even though in this case no change was made in the water-cement ratio, the 28-day strength may be slightly higher than without the superplasticizer. This is said to be true whether the test cylinders are compacted by vibration or not.