How to read a compressive strength report before you sign it
Three things to check in the report: the reference standard, the area used in the calculation, and the difference between the average and individual values.
Standard first: C90 or C129?
ASTM C90 covers load-bearing concrete masonry units, with minimums of 13.1 MPa average and 11.7 MPa individual on net area. ASTM C129 covers non-load-bearing units, with minimums of 4.14 MPa average and 3.45 MPa individual.
A common mistake is comparing a C129 result against the C90 limit and rejecting the product — or the reverse, accepting a non-load-bearing product in a load-bearing location.
Area: net or gross?
Compressive strength is calculated by dividing the load by the area, and the difference between net and gross area is large for hollow block. ASTM C90 reports are calculated on net area, and this must be stated explicitly in the report.
Average and individual values
A report usually covers three or more samples. It isn’t enough for the average to exceed the limit; no individual sample may fall below the individual limit. One weak sample is enough to reject the batch.
Date and size
The report applies to the sample tested, at its size and date. A report for 20×20×40 can’t be used for 25 or 30 cm blocks, and a report from years ago doesn’t describe current production.
Finally: who issued the report? An accredited third-party lab gives an independent result; the factory’s self-certification is no substitute.
