How much strength do we forfeit for each additional inch of slump?

The concrete in question has the following design criteria:

28-day compressive strength

4,000 psi

Air content

5 to 7 percent

Maximum size of coarse aggregate

3/4 inch

Maximum water content

298 pounds per cubic yard

Minimum cement content (Type II portland cement)

610 pounds per cubic yard

This kind of question can be best answered by reference to curves of strength based on water-cement ratio but additional empirical information is used beforehand.Since your question is phrased in terms of slump the first thing that must be determined is how much change in water-cement ratio will be effected by a change in slump of 1 inch. This amount will vary somewhat, depending on the materials being used in our particular concrete. If you have good mix records you can undoubtedly obtain a reliable factor to use with your materials.From the Portland Cement Association Engineering Bulletin "Design and Control of Concrete Mixtures, Eleventh Edition," it can be noted in the first footnote on page 47 that for air entrained concrete an increase in slump will increase the water content (and hence the water-cement ratio) by about 3 percent. If you should not have adequate records for your materials and were willing to assume that this figure holds true for them we would find that your water-cement ratio would be increased from 0.49 to about 0.505.From Figure 32 on page 44 of the same publication, following a curve for 28-day compressive strength versus water-cement ratio, it can be seen that strength might decrease from 4,000 psi to about 3,830 psi, very roughly. You might then estimate that for this quality of concrete you could lose about 170 psi strength at 28 days for each 1-inch increase in slump.Although the estimate just made is based on Type I portland cement the effect at age 28 days for Type II portland cement would be almost identical.