Concrete Field and Laboratory Testing: Sampling, Fresh Properties, and Strength
Sampling and fresh-property tests
Field testing starts with a representative sample: composite samples taken from the middle of the discharge stream, never the first or last portion, and not from a stationary hopper unless specified. Standard test methods (slump, air content, temperature, density/unit weight, and strength specimens) each define sample size and time limits - know that fresh-property tests should be completed promptly after sampling and that the sample must be remixed before each test.
Slump and air content
Slump measures consistency, not strength or workability in a broad sense; a low slump does not automatically mean low strength, and adding water on site changes w/c ratio and is controlled by specification. Air content is critical for freeze-thaw durability; test method choice depends on aggregate type (pressure method is unsuitable for highly porous or lightweight aggregate - use volumetric). Traps: reporting a single slump as "the" workability, or accepting air results from the wrong method.
Strength specimens
Cylinders are cast in specified molds, consolidated per method, capped, and cured under controlled conditions until tested. Track specimen identity, casting time, storage location, and transport. Acceptance is based on specified age (commonly 28 days) and statistical criteria in the specification - not a single cylinder.
Worked example
Specification requires 4×8 in. cylinders, three per set, moist-cured at 73 ± 3 °F until testing. A set arrives at the lab at 26 days in a dry box. Correct action: note the curing nonconformance on the chain-of-custody, test as required, and report the deviation - do not discard or silently re-cure.
Rule of thumb
Sample where the concrete is used, protect the sample, and let the specification - not convenience - set curing and test age.