Concrete Field and Laboratory Testing: Fresh and Hardened Properties
Fresh Concrete Tests
Understand slump (ASTM C143) as a measure of consistency, not strength; air content by pressure meter (C231) or volumetric method (C173); temperature; and density/yield (C138). Know that test methods specify the sample must be obtained and tests started within defined time limits after mixing, and that specimens must be molded, cured initially in the field, and transported without damage. Strength specimens (C31/C192) are cylinders or beams; cylinders are typically 6 by 12 in. or 4 by 8 in.
Hardened Concrete Tests
Know compressive strength (C39) as the primary acceptance test, plus flexural strength, and nondestructive methods such as rebound hammer and ultrasonic testing as indicators, not acceptance substitutes without correlation. Familiarize yourself with the concept of acceptance based on averages of consecutive strength tests and the requirement to investigate low results.
Traps
Questions often test unit confusion (slump in inches, air in percent, temperature in degrees), the difference between cylinder strength and in-place strength, and whether a test is acceptance or informational. Another common trap is forgetting that field-cured cylinders represent the structure's early curing conditions while standard-cured cylinders represent potential strength.
Worked Example
Three cylinders from one sample test at 3,850, 4,100, and 4,250 psi. Average = 4,067 psi. If the specified strength is 4,000 psi, the average passes, but also check whether any single cylinder falls more than the permitted amount below the specified value - a low individual result can trigger investigation even when the average is acceptable. Rule of thumb: evaluate both the average and the individual results.