ACCEPTANCE OF CONCRETE CUBE TEST RESULTS
ACI-5.6 2008 requires that all the tests to be performed on fresh, hardened or old concrete are to be performed by qualified field testing technicians. They should collect and prepare all the specimens for testing and should record temperature and other important information about the fresh concrete.
1. Frequency Of Testing
A strength test is considered as the average of the strengths of at least two 150 by 300 mm cylinders or at least three 100 by 200 mm cylinders made from the same sample of concrete and tested at 28 days or at test age designated for determination of fc’.
As mentioned in the earlier chapters, samples for strength tests of each class of concrete placed each day must be taken not less than all of the following:
a) Once a day
b) Once for each 110 cum of concrete
c) Once for each 460 sqm of surface area for slabs or walls. Only one side of the slab or wall should be considered in calculating the area. This criterion requires more frequent sampling than once for each 110 m3 of concrete placed if the average wall or slab thickness is less than 240 mm.
d) At least five randomly selected samples for a given class of concrete.
e) According to ACI, when total quantity of a given class of concrete is less than 38 m3, strength tests are not required when evidence of satisfactory strength is submitted to and approved by the building official.
2. Acceptance Based On Standard-Cured Specimens
Strength of a new concrete determined by standard-cured specimens is considered satisfactory if both of the following conditions are satisfied:
a) Every arithmetic average of any three consecutive strength tests equals or exceeds fc?
b) No strength test falls below fc? by more than 3.5MPa when fc? is 35 MPa or less; or by more than 0.10 fc? when fc? is more than 35 MPa.
If the above conditions are not satisfied, steps must be taken to increase the average of subsequent strength test results.
3. Acceptance Of Field-Cured Specimens
Field-cured test cylinders should be molded at the same time and from the same samples as laboratory-cured test cylinders. These results are not directly used as acceptance criterion but give idea about the field curing procedure. Procedures for protecting and curing concrete are to be improved when strength of field cured cylinders at test age designated for determination of fc? is less than 85 percent of that of companion laboratory-cured cylinders. The 85 percent limitation need not to be applied if field-cured strength exceeds fc? by more than 3.5 MPa.
4. Investigation Of Low-Strength Test Results
If the strength tests of laboratory-cured cylinders do not satisfy the criterion for acceptance or if tests of field-cured cylinders indicate deficiencies in protection and curing, immediate actions are required to avoid under-strength construction. If it is almost confirmed that the concrete may be of low strength, three cores must be taken for each under-strength test. Cores are tested after 48 hours but not, later than 7 days after coring unless approved by the licensed design professional. The strength of concrete may be considered satisfactory if the average of three cores is equal to at least 85 percent of fc? and if no single core is less than 75 percent of fc?. If these criteria are not satisfied, it is allowed to extract and test additional cores. Cores taken to confirm structural adequacy are usually taken at ages later than those specified for determination of fc?. If the core test also fails and the doubt about the concrete strength still exists, structural strength evaluation / load test may be recommended.