The Broken Cube, the Wet Pallet, and the Expired Certification
A technician samples 8-in. CMU for a project specifying a 1,900 psi net-area compressive strength with ASTM C90 compliance. The delivery has stood on site two weeks in the rain. He selects five units from a broken cube at the bottom of the stack; two have corner breaks and one has a hairline crack. In the laboratory, the 24-hour absorption of one unit is 9.0 lb/ft3 against an 8.0 lb/ft3 limit, and the boiled absorption is 10.5 lb/ft3; the sample average is 8.6 lb/ft3. Net-area compressive strengths average 2,050 psi, with one specimen at 1,700 psi. The technician's ACI certification expired three months ago, and the report was signed by a laboratory manager who never visited the site.
1. Is this a valid sample, and how should the damaged units be handled?
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The sample is not defensible. Units pulled from a broken cube at the bottom of a stack that has stood in rain for two weeks are neither randomly selected nor representative of the lot, and they have seen moisture and damage the delivered units did not. Corner breaks and a crack disqualify compressive specimens because bearing area is reduced and failure initiates at the defect, so a low result would say nothing about the lot.
The technician should confirm the required sample size and sampling procedure from the specification and the unit test method before testing anything, replace damaged units with a fresh random selection, and document pallet location, exposure, and every substitution. Recomputing statistics from a bad sample is not a remedy; resampling is.
2. How should the strength and absorption results be evaluated, and what is the significance of the saturation coefficient?
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Evaluate against the specification's acceptance basis, not unit by unit. Compressive strength is computed on net area, and acceptance is typically based on the average of the set with some allowance for individual units; an average of 2,050 psi against 1,900 psi with one 1,700 psi specimen may or may not comply depending on that criterion. The technician reports the values and the criterion; the specifier decides.
The absorption results are more clearly unfavorable: 9.0 lb/ft3 exceeds an 8.0 lb/ft3 limit and even the 8.6 lb/ft3 average exceeds it. The saturation coefficient, 9.0 divided by 10.5, is about 0.86, which is high against a typical durability limit near 0.80 and suggests poor freeze-thaw resistance. Report every value, the average, the computed coefficient, and the specific failing units; do not average a failure out of existence.
3. What is wrong with the reporting and qualification, and what corrective action is required?
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There are two separate problems. First, content: the report must identify the standard and method used, sampling location and date, specimen identification, dimensions and net area, test age and dates, individual and average results, all calculations, deviations, and the name and certification of the technician who did the work. Second, qualification: masonry field tests are to be performed by technicians qualified under the project's quality requirements, normally holding current certification. An expired certification and a signature from a manager who never visited the site leave the results open to rejection regardless of their technical merit.
Corrective action: disclose the lapse promptly, arrange for the work to be repeated or witnessed by a currently certified technician, and revise the report so that the signature belongs to the person actually responsible for the testing.