Fresh Concrete Tests
Testing concrete during placement provides immediate information about quality and workability. , and proper compressive strength testing procedures should be verified during constructionThe slump test, performed according to ASTM C143, measures the consistency of fresh concrete by filling a cone-shaped mold with concrete in three layers, each rodded 25 times, then lifting the mold and measuring the vertical settlement. The slump indicates the water content and workability of the concrete mix, with typical structural concrete having a slump of 3 to 5 inches. A slump significantly different from the design value indicates a change in water content that will affect strength and durability.
Air content testing by the pressure method measures the volume of air voids in fresh concrete. Entrained air in the range of 5 to 8 percent is required for concrete exposed to freeze-thaw cycles. The air content affects concrete durability, workability, and strength. Each 1 percent of entrained air reduces compressive strength by approximately 5 percent, so the air content must be carefully controlled to provide freeze-thaw protection without excessive strength loss. The pressure meter test is quick and reliable when performed according to ASTM C231.
Concrete temperature measurement ensures that the concrete is within acceptable limits for the prevailing weather conditions. Hot weather limits are typically 90 degrees Fahrenheit maximum, while cold weather limits require concrete temperature above 50 degrees Fahrenheit at placement. Unit weight measurement by the gravimetric method verifies that the concrete density matches the design assumptions. The temperature and unit weight tests are simple but important elements of the quality control program.
Compressive Strength Testing
Compressive strength is the primary acceptance criterion for structural concrete. Standard 6 by 12 inch cylinders or 4 by 8 inch cylinders are cast from fresh concrete samples taken at the point of placement. Understanding concrete air content testing is essential for achieving quality results in this aspect of construction. Understanding statistical quality control is essential for achieving quality results in this aspect of construction.Understanding non-destructive testing methods is essential for achieving quality results in this aspect of construction.ASTM C31 specifies the procedures for sampling, molding, and initial curing of test cylinders. Field cylinders must be stored in controlled conditions, protected from vibration, temperature extremes, and moisture loss during the first 24 hours after casting.
After initial curing, cylinders are transported to the testing laboratory and stored in standard conditions of 73.5 degrees Fahrenheit and 100 percent relative humidity until tested. The 28-day compressive strength is the standard acceptance criterion, though early-age tests at 3, 7, or 14 days provide indications of strength development. The testing machine applies load at a specified rate until failure, and the maximum load divided by the cross-sectional area gives the compressive strength. A minimum of two cylinders from each test represents the strength of the concrete placed.
Non-Destructive Testing Methods
Non-destructive testing methods evaluate concrete quality without damaging the structure. The rebound hammer measures surface hardness by recording the rebound of a spring-driven hammer impacting the concrete surface. The rebound number correlates with compressive strength but is affected by surface moisture, carbonation, and aggregate type. Rebound hammer testing is useful for comparative assessments and identifying areas of potential weakness but should not be relied upon for precise strength determination.
Ultrasonic pulse velocity testing measures the time required for a sound wave to travel through concrete. The pulse velocity correlates with concrete quality, with higher velocities indicating better quality and fewer internal defects. This method is effective for detecting internal voids, cracks, and deterioration. Combining rebound hammer and ultrasonic pulse velocity measurements in the SonReb method provides more reliable strength estimates than either method alone.
Statistical Quality Control
The statistical evaluation of concrete test results provides a rational basis for acceptance decisions. The average strength of all test results must exceed the specified design strength by a margin that accounts for normal variability. ACI 318 requires that the average of three consecutive tests exceed the specified strength and that no individual test falls more than 500 psi below the specified strength. The standard deviation of test results measures the consistency of concrete production, with typical values ranging from 400 to 600 psi for well-controlled production.
When test results fall below acceptance criteria, the engineer must evaluate the structural adequacy of the affected concrete. Core tests drilled from the structure provide direct strength measurements of the in-place concrete. The location and number of cores should be determined by the engineer based on the nature and extent of the low-strength concrete. If core tests confirm inadequate strength, the engineer must evaluate whether the structure can safely support design loads or whether remedial measures such as strengthening or replacement are required.