Q: At our ready mix concrete plant we test cylinders for our own quality control purposes. I want to use neoprene pads on cylinder ends instead of capping compound. However, there's not enough crosshead movement in our testing machine to allow room for the cylinder plus pad. I thought of cutting off a half inch from the cardboard mold to solve the problem. However, I heard that cylinder length affects strength. How much would the strength be changed by reducing length by 1/2 inch?

Not much. As the length-to-diameter ratio decreases, apparent strength increases slightly. To correct for this effect, the calculated strength is multiplied by a correction factor. ASTM C 42, Standard Method of Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, gives the following correction factors for cores with strengths from 2000 to 6000 psi:
Ratio of Cylinder
Strength correction
1.75 0.98
1.50 0.96
1.25 0.93
1.00 0.87
The same factors can be used for cylinders. For your 11 1/2-inch-long cylinder, the length-to-diameter ratio is 1.92 and the correction factor is 0.99.