The objective of acceptance testing
The objective of acceptance testing is to ensure that concrete conforms to the requirements of the buyer. Tests are performed on samples of concrete, taken as-delivered to a job site. Acceptance testing includes tests on plastic (unhardened) concrete for slump, air content, density, and temperature. It also includes tests on hardened concrete for strength and durability.
Hardened concrete is tested for acceptance in accordance with standard procedures to determine whether it has the potential to develop the desired properties. It is not the intent of these test results to reflect the actual properties of concrete in the structure. During construction, numerous variables will influence the properties of in-place concrete. These variables are beyond the control of the ready mixed concrete supplier.
Testing and Responsibilities
The responsibilities of all involved parties for proper sampling, specimen storage, handling, transportation, jobsite sample disposition and laboratory testing should be clearly defined prior to the start of a project. Contractors are legally obligated to facilitate or conduct acceptance testing by jurisdictions that follow model codes like the International Building Code. These model codes in turn refer to the ACI 318 Building Code.
All parties involved should also realize that the results of the testing have substantial implications on the project schedule, cost to participants, and may impact the safety of the structure — and its inhabitants.
Acceptance testing must be conducted by certified technicians. All acceptance testing of concrete must be conducted in accordance with established standards referenced in contract documents. Any deviation from standard procedures is adequate reason for invalidating test results.
Samples
Samples of concrete from concrete delivery vehicles for acceptance tests should be obtained in accordance with ASTM C 172. The sample should be obtained at the end of the truck chute. Two or more portions of concrete as discharged from the middle portion of the load are composited to obtain a sample that is representative of the load.
Slump and Air Content
If the slump and air content measured on the initial sample are lower than specified, it is permissible to make jobsite adjustments with water or admixtures — followed by adequate mixing.
If slump and air contents are higher than specified, a retest is made immediately. If the retest fails, then the concrete is considered to have failed the requirements of the specification.
Concrete slump is measured in accordance with ASTM C 143. The tolerance on slump varies by slump level as ordered or specified. Slump tolerances of ASTM C 94 are summarized in Table 1.
The air content of concrete is measured in accordance with the pressure method, ASTM C 231 or by the volumetric method, ASTM C 173. These standards are for lightweight concrete or for aggregates with high absorptions. For air-entrained concrete, the tolerance on air content as ordered or specified is ±1.5%.
Density and Yield
For strength tests, ASTM C 94 requires measuring the density of the concrete in accordance with ASTM C 138. This can be done by determining the weight of the air meter container after the sample has been prepared. The minimum container size based on the nominal maximum size of the aggregate in the concrete mixture should be followed. Density measurements can also be correlated with air content measurements and can be an indicator of the water content in the mix.
Temperature
The temperature of concrete is measured in accordance with ASTM C 1064. Temperature is measured to determine conformance to temperature limits in a specification. Temperature testing is required when strength test specimens are prepared.
Hardened Concrete Tests
ASTM C 31 describes the procedures for preparing cylinders and beams for compression and flex strength tests. It describes the procedures for storing specimens at the jobsite, and transporting them to the lab. ASTM C 31 requires maintaining test specimens in a moist condition, in a temperature range of 60 to 80°F in the field. For high strength concrete with a specified strength greater than 5,000 psi, storage temperature limits are tighter at 68 to 78°F. Maintain a record of temperature conditions during field storage of the concrete test specimens. These should not be stored at the jobsite for longer than 48 hrs. Transportation time should not exceed 4 hrs.
The contractor is generally responsible for providing adequate storage facilities at the jobsite for specimens, but it is also the responsibility of the testing technicians and the person certifying test results to ensure that standard procedures are followed during testing.
In the early stages of concrete’s development, temperature and moisture affect its quality, which is why deviations from standard protocols should be regarded as grounds for rejecting results as they increase the likelihood of failing tests of acceptable concrete.
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