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Acceptance of Concrete Test ResultsAs per ACI-5.6 (2008) it is recommended to perform the test on concrete in fresh, hardened or old state by well qualified field-testing technicians. The respective technicians are supposed to collect and prepare the test specimens according to the specifications provided. Here, we will discuss some of the factors based on which the acceptance of concrete cube test, cylinder test and core test results is taken into consideration.
1. Acceptance Based on Frequency of Testing the specimensThe frequency at which the specimens are tested have an important role in justifying the acceptance of test results. Generally, the strength test is taken as average strength of minimum two cylinders of 150 x 300mm or three 100 x 200mm. These samples are made out of same concrete. The test is conducted after 28 days or the testing date as specified by the standard codes. This testing age will give the value of compressive strength of concrete fc’. The frequency of selecting the test samples for conducting the strength test of each class concrete, must not be less than the following:
- Once a day
- Once for each 110 cum of concrete
- 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.
- At least five randomly selected samples for a given class of concrete.
- 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 SpecimensIf the strength of a newly prepared concrete is determined by means of a standard-cured specimen, then the following two factors are considered to accept the test results.
- The test result is accepted, if the arithmetic average of any 3 of the consecutive strength test is equal to or greater than fc’.
- 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
3. Acceptance Based on Field Cured SpecimensThe samples prepared for field testing (test cylinders) have to be prepared with the same sample material and time as that of the laboratory testing. The two test results won’t be considered as a acceptance criterion, but will provide us an idea about the curing procedures conducted in the field. When the strength that is designated for determining fc’ for the field cured cylinder is less than 85% that mentioned for laboratory cured cylinder, then focus must be given to protect and cure the concrete to improve its strength in the field. If the strength of field cured concrete strength exceeds the laboratory tested sample by an amount of 3.5MPa, we need not consider the limitation of 85%.
Low Strength Test ResultsCertain criteria are to be followed when the test results obtained does not come under the above mentioned acceptance conditions. If the tested cylinders are deficient in strength or curing, then steps are to be taken in order to avoid under construction. When a particular concrete is found as low strength, then three cores have to be taken from the structure to perform the under strength test. The core samples have to be tested after 48 hours but not greater than 7 days. This must be properly guided by the liscensed design professional in charge. The strength of concrete is taken as acceptable,
- If the average of the three cores strength is equal to minimum of 85% of fc’.
- And, The strength of single cores is not less than 75% of fc’