low concrete test specimen breaks
If you want to get the full attention of everyone involved with a construction project, send an email with a subject line that says "Low Cylinder Breaks." This is because the cost of low breaks could be tremendous if replacement is the only acceptable remedy. But is that true? What is a "low break?' And when is a low break a real concern?
First, the acceptance of concrete, when a strength requirement is specified, is determine by using standard cured test specimens which are typically tested at an age of 28 days. Because of the nature of any form of test, the result should never be based on one specimen break but rather on the average of 2 or 3 breaks. Because of normal variations in concrete and testing, ACI 318 and ACI 301 both recognize that 1%-2% of test results will be usually be low, just do to the statistical nature of sampling and testing.
Because of this statistical variation, concrete with specified strength of less than 5000 psi (35 MPa) is acceptable if:
There are many reasons why strength results could be low. But low breaks are usually do to one of the following factors:
When low-breaks, beyond what is deemed "acceptable" by ACI 318 and ACI 301, the cause of the low break should be determined.
If issues are discovered with regard to testing issues such as those above, then consider the fact that the test results are not likely to reflect the true potential strength of the concrete in question.
Next, review the history of slump and air content and determine if the batch-to-batch variability is acceptable. Then compare it to the batch in question.
Finally, consider the actual required strength and criticality of the strength of the location in question to decide how to proceed. And are there acceptable options other than removal.
Some options are:
First, the acceptance of concrete, when a strength requirement is specified, is determine by using standard cured test specimens which are typically tested at an age of 28 days. Because of the nature of any form of test, the result should never be based on one specimen break but rather on the average of 2 or 3 breaks. Because of normal variations in concrete and testing, ACI 318 and ACI 301 both recognize that 1%-2% of test results will be usually be low, just do to the statistical nature of sampling and testing.
Because of this statistical variation, concrete with specified strength of less than 5000 psi (35 MPa) is acceptable if:
- No single test is lower than the specified strength by more than 500 psi (3.5 MPa), and
- The average of three consecutive tests equals or exceeds the specified strength
There are many reasons why strength results could be low. But low breaks are usually do to one of the following factors:
- Improper handling of test specimens during curing and testing
- The addition of too much water on the jobsite
- Production errors
When low-breaks, beyond what is deemed "acceptable" by ACI 318 and ACI 301, the cause of the low break should be determined.
- Review the history of test results. The strength variation between test specimens from the same sample should not usually vary by more than ~10% of the average strength. If higher specimen to specimen variation exists, investigate the testing procedures.
- Consider the ambient conditions and the fact that the test specimens should be standard cured. Were the test specimens cured under the appropriate standard conditions? How long were specimens left in the field? In mild weather, deviations may not have much of an effect. In hot or cold weather, the impact of non-standard during conditions could be significant. Once convenient way of providing the ideal curing environment on a jobsite is by using an environmentally controlled, heated and cooled curing box such as the MasterCure by RPX Technologies.
If issues are discovered with regard to testing issues such as those above, then consider the fact that the test results are not likely to reflect the true potential strength of the concrete in question.
Next, review the history of slump and air content and determine if the batch-to-batch variability is acceptable. Then compare it to the batch in question.
Finally, consider the actual required strength and criticality of the strength of the location in question to decide how to proceed. And are there acceptable options other than removal.
Some options are:
- Test the in-place concrete. The traditional "coring" method is usually used. But nontraditional testing such as "pullout testing" (e.g. LOK Test per ASTM C900, BS 1881:207, or EN 12504-3, sold by Germann Instruments) may be useful and much more convenient.
- Give the placement area in question a longer time to cure prior to critical construction operations.
- Make an engineering determination that the placement strength in that area is adequate.
RESOURCES
ASTM C31 - SIcerts ON CREATING SPECIMENS IN THE FIELD