Since concrete is a manufactured product, it requires specific control tests
and evaluations during the manufacturing process and at the job site to ensure
high-quality concrete. Some of the important concrete properties measured by
basic quality control tests are strength, temperature, slump, air content, and
unit weight. Each test helps to determine the quality of concrete and should be
performed in accordance with American Society for Testing and Materials (ASTM)
standards.
Sampling Fresh Concrete (ASTM C 172)
Concrete used for testing should be representative of the entire
batch. A good practice is to take two or more portions from the middle-third of
the truckload and combine the portions. Take samples after all water has been
added. Obtain a minimum 2-cubic-foot sample size if cylinders will be made.
Slump Testing (ASTM C 143) measures the consistency of concrete
and often reflects a change in the amount of mix water, temperature, setting,
and hydration. This test should be done in the field.
Air Content (ASTM C 231) Testing measures the amount of air
bubbles in fresh concrete. Entrained air is purposely added to concrete for
freeze-thaw resistance. Too much air results in low strength. This test should
be done in the field.
Strength Testing - Making Cylinders (ASTM C 31) Cylinders are made
and tested to determine compliance with strength specifications. Test specimens
must be cast, handled, cured and tested in accordance with ASTM specifications
dealing with both field and laboratory test specimens. Cylinders are made in
the field and tested in the lab.
To make a cylinder:
1. Place cylinder mold on a level, rigid, horizontal surface, free
from vibration and other disturbances and as close as possible to the location
where it will be stored for the first 24 hours.
2. Fill the mold in three equal layers.
3. Rod each layer 25 times, penetrating into the previous
layer about 1 inch. Tap side of the mold lightly after each lift to close
voids.
4. Strike-off the surface and cover immediately to prevent
evaporation.
5. Store cylinders made to check strength on site for 16
to 24 hours at a temperature of 60 to 80° F then ship to the testing Lab.
6. Protect cylinders from damage during storage and shipping.
Low Test Results
Low test results can occur for a number of reasons. Not following prescribed
standard ASTM procedures can cause low results:
• Cylinder and beam test results will represent only about 20% of
the actual strength of concrete in place.
• Certain non-standard conditions can reduce test
strengths by as much as 75%. Non-Standard conditions comprise improper casting,
handling, curing and testing both in the field and in the testing laboratory. (For a complete list, see "American Society of Civil Engineers Document, Vol. 3 No. 2 Table # 1, May 1991.")
Were the cylinder test results evaluated properly?
• The average of two cylinder strengths made from the same sample of
concrete are required for one individual test.
• The average of any 3 consecutive tests is required to be above the
specified strength.
• No individual strength test should fall below the specified
strength by more than 500 psi.
• Strength test failing to meet the criteria will occur occasionally
(probably about once in 100 tests). Allowances should be made for such
statistically expected variations in deciding whether the strength level being
produced is adequate.
Was it the approved mix that was delivered and sampled? You must determine:
• Whether the right mix was ordered and dispatched.
• Whether the mix was batched correctly—check for possible omission
of water reducer, excessive air entrainment, or low cement content.
• Whether any unauthorized or unrecorded water was added on the
jobsite.
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