Masonry Magazine February 1994 Page. 43
Mortar mixed to a laboratory flow consistency of 110±5 percent as required for determination of compressive strength according to ASTM C270 property requirements (illustrated top left and bottom left) approximates the consistency of mortar after it has been in contact with absorptive masonry units. Mortar mixed to a workable consistency suitable for construction (top right and bottom right) has a flow that is much higher often over 130 percent. Compressive strength tests of mortar mixed to that higher job flow consistency are not required to meet the strength minimums of the property specifications of ASTM C270.
Masonry Mortar Performance Requirements
Comparisons between masonry mortars and concrete are often misleading. While these materials have some similar ingredients, their structural properties differ. Materials and methods that produce strong and durable concrete do not necessarily produce mortar or masonry of similar quality.
As an example, it is recognized that high strength concrete requires a low water-cement ratio. Conversely, quality masonry mortar requires comparatively higher water-cement ratios during placement. Higher water content is necessary for workability and to satisfy the absorptive characteristics of the masonry units. That high water content produces a workable mortar mix that readily flows into the irregular surfaces of masonry units assuring intimate contact between mortar and unit. Since some of the mixing water is absorbed by the masonry unit, in-place mortar has a much lower water-cement ratio than the freshly mixed mortar.
The fact that masonry mortar is placed in relatively thin layers between absorptive units means that its flowability and water retentive characteristics are of primary importance to the construction of quality masonry. These properties of the plastic mortar are often more important than its relative potential compressive strength when molded in non-absorptive cube or cylinder molds. Higher strength mortar does not necessarily produce better or stronger masonry. Mortar properties should be compatible with the units used on a project to achieve optimum performance. Measures to increase the compressive strength of mortar that reduce the workability of the mortar (such as reducing water content or arbitrarily increasing the cement content) can have an adverse effect on the completed masonry. For this reason changes to mortar mix designs should not be based on field test results of compressive strength. The need for changes in mix design should be confirmed by laboratory controlled tests or a combination of laboratory and field tests.
Field Test Procedures
ASTM C780 outlines several test procedures for measuring various properties of field sampled mortar. Each procedure is listed as a separate annex to the standard, as indicated in Table 1. Annex A9 outlines a report form indicating the information to be recorded and reported for each test procedure.
Of the eight test procedures outlined, six are conducted on samples of the mortar while it is still in its plastic state. Two, the compressive strength test and the splitting tensile strength test, are measures of hardened properties of mortar specimens prepared from plastic mortar samples. Of these procedures, only one (the compressive strength test) is commonly performed on masonry mortars. Unfortunately, it is dependent on many related variables (as indicated in Table 1) and test results are not available until 7 or 28 days have elapsed from the test date. These weaknesses greatly diminish the effectiveness of field compressive strength tests of mortar for quality control purposes.
When compressive strength tests are conducted, the procedures outlined in ASTM C780 need to be carefully followed. Particular care should be taken to assure that the proper equipment is used and that specified sampling, curing, and testing procedures are followed. The report form contained in Annex A9 of ASTM C780 should be used.