Concrete is tested to ensure that the material that was specified and bought is the same material delivered to the job site. There are a dozen different test methods for freshly mixed concrete and at least another dozen tests for hardened concrete, not including test method unique to organization like the Army Corps of Engineers, the Federal Highway Administration, and State department of transportation.
Slump, air content, unit weight and compressive strength tests are most common tests.
Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete can’t flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won’t stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressure, finishing delays and segregation.
Air content measure the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation Consolidating, placement and finishing. Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure.
Unit weight measures the weight of a known volume of the fresh concrete.
Compressive strength is testes by pouring cubes of fresh concrete and measuring the force needed to break the concrete cubes at prescribed interval as they harden. According to Building Code Requirements for reinforced concrete (ACI 318) as long as no single test more than 500 psi below the design strength and the average of three consecutive tests equals or exceed the design strength then the concrete is acceptable. If the strength tests don’t meet these criteria, steps must be taken to raise the average.