Loading Rate and Machine Compliance on a 3,000 psi Mix
A 3000 psi design mix is being tested at 28 days. The testing machine has a 250,000 lbf capacity and a spherical seat that is slightly stiff. The technician sets the loading rate at 60 psi/s because the machine's pump runs faster at low loads. The first cylinder breaks at 2850 psi with a fracture pattern that is mostly vertical and explosive. The second cylinder breaks at 3100 psi with a cone-shaped fracture. The third cylinder breaks at 2700 psi. The project specifications require a minimum average of 3000 psi. The technician claims the mix is deficient.
1. Evaluate whether the loading rate and machine setup comply with ASTM C39.
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ASTM C39 requires a loading rate within 20-50 psi/s for the expected strength range; 60 psi/s exceeds the upper limit and can artificially increase or scatter results. The spherical seat must be free to rotate to ensure uniform load; a stiff seat can cause eccentric loading. The explosive vertical fracture on the first cylinder suggests a loading rate or seating problem, while the cone fracture on the second is a normal failure mode. The third cylinder at 2700 psi may reflect the same rate issue. The average of 2850, 3100, and 2700 is 2883 psi, below 3000 psi, but the non-compliant loading rate invalidates the test. The technician's claim of mix deficiency is premature.
2. What is the correct testing procedure and how should the results be interpreted?
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The technician must adjust the loading rate to within 20-50 psi/s, ideally 35 psi/s, and verify the spherical seat is properly seated and free to rotate before loading. The machine should be calibrated. Retest the cylinders at the correct rate. If retest results average at least 3000 psi with no single cylinder below 2500 psi (500 psi below f'c), the mix is acceptable. If the retest still averages below 3000 psi, then evaluate for acceptance per the project criteria and consider core testing per ASTM C42. Document loading rate and failure mode for each cylinder. The explosive fracture should be noted as a possible sign of high rate or end condition.