PCA - Mortar Compressive Strength(PCA)(Print Article)(Mortar Performance)

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Mortar Compressive Strength Masonry Home > Products and Properties > Mortar & Grout > Mortar Compressive Strength

Q: Our field tests are not meeting the specified comp ressive strength formasonry mortar, even though w e used the mix specified by the architect.Whats wrong?

Compressive strength specimens formortar are 2-in. cubes

A: Unfortunately, this is an all-too-commonoccurrence: trying to compare a lab-specifiedstrength with a field-tested mortar. Quite simply,what is wrong is your expectation: for a number of reasons that are outlined here, the field

compressive strength is not expected or required tomeet the specified mortar strength.

Masonry mortars are specified by ASTM C 270,Specification for Mortar for Unit Masonry. Mortarsare evaluated by ASTM C 780, Preconstruction andConstruction Evaluation for Mortars for UnitMasonry. The two documents are designed to worktogether, but have distinct purposes.

The two mortars (field vs. lab) would not be expected to exhibit the same strength. TheONLY way to make any comparison between the two is to run preconstruction tests onthe (C270) mortar, recognizing that this mortar will be proportioned with a higher watercontent (higher flow) in actual field conditions.

Based on compression strength tests alone, its not possible to say that your particularmortar meets or does not meet the project specification. However, comparing C 270requirements and C 780 results is NOT acceptable. The approach is flawed.

In a lab, C 270 mortar is mixed to a drier mix consistency than you would have in thefield. A wetter mix, C 780 as mixed on the job, has a higher water content and higherwater-cement ratio, which lead to lower strength results on mortar specimens, especiallysince the field mortar is being placed into non-absorbent molds. On the actual project,mortar is being placed between absorbent masonry units that will draw water out of themortar, lowering its water content and increasing its strength. In addition, the aspectratio (height to thickness) of a specimen affects its apparent/measured strength. A 2-in.cube will exhibit a lower strength than a 3/8-in. joint even when made from the exactsame material. The bottom line is that field mortars are frequently just as strong if notstronger than the lab mixes they are based on.

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There are numerous documents written to address this exact misinterpretation of theuse of C 270 and C 780. ASTM developed a guide to the correct use of C 270 and C 780.That document, C 1586, is entitled Guide for Quality Assurance of Mortars. The flowchart below shows that C 270 is used to establish mortar mix proportions and that C 780is used to evaluate site proportioning. It bears repeating that if compressive strengthtesting is used:

1. it's recommended to have preconstruction testing to establish baseline values2. compressive strength is not to be used for compliance but only mortar uniformity,

and quite specifically3. the field compressive strength is not required to meet C270 minimum

compressive strength.

Both C 270 and C 780 contain statements to discourage theirimproper use. In C 270, Section 3.1, Specification C 270 is nota specification to determine mortar strengths through fieldtesting. And also in Section 3.3, The compressive strengthvalues resulting from field tested mortars do not represent thecompressive strength of mortar as tested in the laboratory, northat of the mortar in the wall. Physical properties of fieldsampled mortar shall not be used to determine compliance tothis specification and are not intended as criteria to determinethe acceptance or rejection of the mortar.

In C 780, Section 1.2, This test method is also used to



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After curing, cubes arecrushed to determinestrength

establish baseline values for comparative evaluation of fieldmortars. And in Section 1.4, The test results obtained underthis test method are not required to meet the minimumcompressive values in accordance with the propertyspecifications in Specification C 270.

The same issues are covered in a continuing education sectionon mortar quality assurance, RP415 , highlighted under theheading "Avoid the Most Common Testing Mistake.

Consider using the mortar aggregate ratio test as a more timelyand effective job site quality control test than compressivestrength of mortar.

Additional resources:

q The Road Map to Quality Mortar: ASTM C 1586 (PL388)q Masonry Mortars: Developing a Quality Assurance Program (RP415)q ASTM C 270, C 780, C 1586

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PCA - Mortar Compressive Strength(PCA)(Print Article)(Mortar Performance)

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