Many apparent problems are actually caused by how a material is tested or what the results are compared with, rather than by the real strength of the masonry in the wall. If both the testing lab and the contractor understand where errors can creep in, many problems can be averted.
This article discusses testing before and during construction.
Mortar, grout, and masonry assemblages may require preconstruction testing. For any of these, there are two ways to specify. The only reason to do preconstruction testing of mortar, grout, or prisms is to establish the specified properties. If properties are not specified, then there is no point in testing. On the other hand, if mortar, grout, or masonry is specified based on performance, then mix proportions or unit/mortar combinations must be developed to achieve those properties. Tests must then be done to verify that the specified properties are achieved.
The compressive strength of masonry can also be specified by either the materials (the Unit Strength Method) or the performance (the Prism Test Method). These requirements are embodied within the building code (or the Masonry Standard Joint Committee's Specification for Masonry Structures). Again, masonry specified by the Unit Strength Method does not necessarily have to be tested with prism tests, although prism testing is often still required.
ASTM C 780 is the standard field test method for mortar. The problem is that mortar made in the lab has a much lower water-cement ratio than mortar made for actual construction. Mortar samples made with the construction mortar will therefore have a much lower compressive strength.
Another potential error with mortar testing is extreme heat or cold. Transport and curing of prisms is another potential problem area.
One potential error in prism testing is to disregard the aspect ratio of the specimen. The ratio of height-to-thickness of the prism needs to be considered when comparing compressive strength results.
One final source of errors is in the calculations.
This article also includes a table relating the property being tested with the ASTM or UBC standard that applies.