Inspection, Testing, and Quality Control
Core concepts
Know sampling and testing: slump (workability), air content (freeze-thaw durability), temperature, and compressive strength via cylinders or cores. Understand that acceptance is typically based on specified strength at a given age (often 28 days) and that cylinders must be made, cured, and tested per standard procedures. Know statistical acceptance concepts-averages and variability-and that a single low test may trigger investigation rather than automatic rejection. Recognize the role of pre-placement inspection (forms, reinforcement, embedments) and documentation.
Traps
Questions test sample location (samples taken at discharge, not from the plant) and curing conditions for test cylinders (standard moist curing versus field curing). A common trap is treating one low cylinder as proof of nonconforming concrete; specifications often use statistical criteria. Another is confusing slump test purpose (consistency/workability) with strength. Watch for air-content tolerances and the rule that tests must be representative and properly handled.
Worked example / rule of thumb
If three cylinders average below design strength but within statistical limits, investigate rather than reject outright; cores may resolve disputes. Rule of thumb: sample at the point of discharge, cure test cylinders under standard conditions, and record slump, air, and temperature for every significant placement.