Interpreting Compressive Strength Testing Results for Masonry Mortar

Undoubtedly if you’ve been involved in a project that involves masonry construction, then you’ve probably dealt with a question that has puzzled many: “why do the compressive strength tests of mortar continue to ‘fail’?” The answer to the question is somewhat complicated, but in the end it is generally what all project participants want to hear: “well, it doesn’t necessarily fail.” The process of getting from this question to the answer is worth an explanation.

Mortar that is designed to be used with concrete masonry units (CMU) for structural applications is an important building element. For states that model their building code off of the latest version of the International Building Code, verification of masonry strength (f’m) is required for all masonry materials by ACI 530.1. This includes compressive strength testing for mortar. Prior to the field testing, mortar mixes should first be designed in a laboratory in accordance with either the Proportion specification or the Property specification of ASTM C270, “Specification for Mortar for Unit Masonry.” When the Property specification is used (and in many areas it is the only specification possible due to the locally available materials) the mortar mix is tested under laboratory conditions to prove that it meets certain properties.

As it would seem obvious, laboratory conditions are usually not the same as field conditions. Mortar mixed in the laboratory generally uses much less water as compared to what is used in the field. Unlike concrete where water does not evaporate from the mix, water can and does evaporate from a mortar mix in the field. To account for this, experienced masons know the right amount of water to add to a mortar mix in order to make it “workable.” To an extent, they can also re-temper a mortar mix with more water to account for the evaporation. The variation in the water content of the mortar plays a major role in the compressive strength of mortar specimens. Simply put, because more water is used in the field than in the laboratory, the 28-day compressive strength results of the mortar mixed in the field will be lower than the mortar mixed in the laboratory. Therefore, if a mortar is designed in the laboratory to have a compressive strength value of 1,800 psi at 28-days (typical of most Type S masonry cements) then it can (and should) be expected to have a lower compressive strength value in the field.

This now begs the question of how should one determine acceptance of field mixed mortar. Instead of focusing on a “target” compressive strength value from the beginning, the emphasis should be placed on making sure that the mortar is mixed properly (i.e. the way it was mixed during trials in the laboratory). This means ensuring that the same materials are being used from the sand to the cement. At the beginning of masonry construction it is also helpful to perform other ASTM C780 tests to establish baseline field-mixed mortar parameters.

These include tests for cone penetration, mortar water content, and mortar aggregate content. This establishes “base line” values that subsequent batches of mortar should intend to replicate throughout the project. The compressive strength results of this “baseline” mortar mix now become the “target.” The expectation is that future mortar compressive strength tests will also have this same value. If they don’t, then it likely means that there was an inconsistency in the mix of the mortar which should then be explored.

For an example, let’s say that a Type S masonry cement will be used to construct an 8-inch load-bearing CMU wall. Laboratory mixes show that the design compressive strength of the mortar is 1,800 psi when mixed at a 2.5:1 ratio (i.e. 2.5 cubic feet of sand to 1 cubic foot of Type S masonry cement). In the field, the same sand source and cement are used at the same proportions. ASTM C780 tests for the field mixed mortar show that it has a cone penetration value of 52 mm, a wet mortar water content of 13.3%, and a mortar aggregate ratio of 2.5:1. The average 28-day compressive strength value for this mortar is 1,300 psi. This is now the baseline data for this project. Because all of the properties established in the laboratory were met, then future compressive strength testing results of the field mixed mortar should expect to have a value of 1,300 psi.

As demonstrated by this example, what is important to mortar is that proper proportions and properties are met in the field. Because the greatest variable from the laboratory to the field is the amount of water in the mix, the compressive strength test results should not be expected to be identical.