Effects Of Improper Field Testing Practices

Air content by pressure method (ASTM C 231)

Improper Practice Effect on Test Result
Failure to use an aggregate correction factor. Incorrect high air-content reading if the aggregate is porous.
Use of the pressure meter to measure air content of lightweight concrete. Highly variable and inaccurate results. The test method isn’t applicable to concretes made with lightweight aggregates.

Sampling (ASTM C 172)

Improper Practice Effect on Test Result
Excessive prewetting of sampling container, such as a wheelbarrow, leaving surplus water in the container. Mixing a cup of water (8 fluid ounces) into a one-cubic-foot sample of concrete is equivalent to adding nearly 2 gallons per cubic yard. This increases slump and can cause a strength loss as high as 400 psi.
Sampling from the form into which the concrete has been placed. Concrete from the form is unlikely to represent concrete from the truck. When concrete is placed in the forms, bleedwater comes to the surface as aggregate and cement particles settle.
Failure to remix the concrete sample in the sample container before starting the tests. Discharging concrete from the truck chute into a sample container often causes segregation, with mortar collecting nearest the chute and coarse aggregate farthest from the chute. Tests run without remixing can show erroneous slump, air content and strength results.

Slump test (ASTM C 143)

Improper Practice Effect on Test Result
Failure to firmly support the base plate. Stepping off the base may cause it to shake, increasing the apparent slump.
Base plate not large enough, allowing high-slump concrete to spill over the edge of the plate. Most often a problem with superplasticized concrete, it increases the apparent slump.
Base not level With an inclined base, gravity tends to increase the lateral sample spread and the apparent slump.
Dampening the slump cone and base leaves a puddle of water on the base surface. Free water on the base surface increases the lateral spread and the apparent slump. The surface should be moist, not wet.
Measuring the difference between the top of the cone and the highest or lowest point on the slumped specimen Either an incorrect low or high value. Slump measurement must be made over the original center of the top surface of the specimen.

Air content by volumetric method (ASTM C 173)

Improper Practice Effect on Test Result
Failure to dislodge all concrete in the base before rolling and rocking the air meter. Incorrect low air-content reading. The meter must be repeatedly inverted and agitated to free concrete from the base. When the concrete has broken free, the technician will be able to hear aggregate moving in the air meter.
Failure to roll and rock the air meter long enough for all air to rise to the surface of the graduated neck. Incorrect low air-content reading. The technician should vigorously roll and rock the air meter for about a minute, then set the unit upright and allow it to stand while air rises to the top and the liquid level stabilizes (doesn’t change more than 0.1% within a one-minute period).