Question: Before placing the concrete for the last pier of a drilled-pier foundation job, the foreman decided to add water to the ready mix truck. The inspector didn't like the looks of the watered-down concrete and took test cylinders that represented that one pier. The specifications call for a 28-day strength of 3000 psi. After the lab broke the seven-day cylinders, the cylinder from the pier with added water broke at 1980 psi. The other seven-day cylinders were as high as 2620 psi. The engineer is concerned that the concrete will not meet the specified strength. I realize that adding the water was the wrong thing to do, but I don't want to remove the pier if it is of adequate strength. Will it reach the specified 3000 psi?

Answer: As this case shows, it is often useful to extrapolate 28-day strengths from seven-day strengths. Of course, the amount of strength gain varies between the seven-day and the 28-day tests. Cement type and curing conditions are two factors that affect the amount of strength gain to be expected. Concrete, by Mindness and Young, gives a general rule: The ratio of 28-day to seven-day strength lies between 1.3 and 1.7 and generally is less than 1.5, or the seven-day strength is normally between 60% to 75% of the 28-day strength and usually above 65%. The cylinder that broke at 1980 psi is 66% of the specified 3000 psi. According to Mindness and Young's rule, it should meet the specified strength at 28 days. Most likely, the mix wasn't designed for 3000 psi but for a higher compressive strength to account for variability. By adding the additional mix water you raised the water-cement ratio which, in turn, reduced the strength. The piers placed before the water was added will probably have strengths higher than the specified 3000 psi. The pier in question, however, will most likely meet the specified strength. If after 28 days the cylinders still do not meet specified strength, take cores to verify the strength before implementing a costly pier removal.