Sampling and Fresh Concrete Testing
Sampling and fresh concrete testing covers ASTM C172 sampling and the field tests that decide whether concrete can be placed: slump (C143), air content (C231 or C173), temperature (C1064), and unit weight (C138). The exam tests procedure, timing, and reporting precision.
Sampling - C172
Take a composite sample of at least 1 ft³ from at least three points across the discharge, after the first 10% and before the last 10%. Remix in a wheelbarrow before testing. Begin tests promptly; C172 requires starting slump, air, and temperature tests within 15 minutes of obtaining the sample.
Field tests
- Slump, C143: three equal layers, 25 rod blows each, strike off, lift the cone vertically in 5 ± 2 s, measure to the nearest 1/4 in.
- Air content, C231: pressure method; do not use it on lightweight or highly porous aggregate concrete. Report to 0.1%.
- Air content, C173: volumetric method with isopropanol; useful where the pressure method is unsuitable.
- Temperature, C1064: probe at least 3 in into the concrete, leave 2–5 min, read to 1°F.
- Unit weight, C138: rod each layer 25 times, strike off, weigh; unit weight = net mass ÷ mold volume.
Worked example
A 6 x 12 in mold has a volume of 0.1963 ft³. The filled mold weighs 28.45 lb and the empty mold weighs 0.50 lb, so net concrete mass is 27.95 lb. Unit weight = 27.95 ÷ 0.1963 = 142.4 lb/ft³. Report 142.4 lb/ft³, not the raw scale reading.
Traps
Sampling the first or last part of the discharge; testing a sample that has been sitting; recording slump to 1/8 in; using the pressure meter on porous lightweight concrete; and reporting air content to 0.01% when the method supports 0.1%.