Q. We are planning to supply mortar for a project. The architect has specified the mortar aggregate ratio method from ASTM C780 Annex A4 as a quality assurance test during construction, instead of using mortar cubes.
What is this test, and how is it performed?
A. The mortar aggregate ratio method is a test method described in ASTM C780, “Standard Test Method for Preconstruction and Construction Evaluation of Mortars for Plain and Reinforced Unit Masonry.” It does not measure strength.
Using this procedure, technicians determine the aggregate to cementitious ratio of a fresh mortar sample. The ratio defines the volume of aggregate (sand) to that of the cementitious materials in a given mortar sample. (Cementitious materials include portland and blended cements, masonry, mortar cement, and lime.)
By comparing the values of the aggregate ratio from samples taken during construction, producers, architects, and mason contractors have a good quality control tool. They can evaluate if a particular mortar sample has been properly mixed by comparing test results to the ratio of the mortar's approved mix design. By reviewing the results from a number of different samples over a period, they can also determine if the batching and mixing process has been consistent.
The aggregate volume is determined by wet-sieving a split of the fresh mortar sample over a No. 100 sieve, then oven-drying the retained material to find a dry weight. Technicians also perform a water content test outlined by ASTM C780 Annex A5 to test on a companion mortar sample. Here, they first carefully flash off the alcohol from the sample that was at the jobsite to retard hydration. They then oven-dry the sample to find the weight of cementitous ingredients.
Using these two results, technicians follow the calculation procedure that determines the volume ratio of the two portions.
One useful advantage of the mortar aggregate ratio test, compared to mortar compression testing, is its speed. Technicians can determine the ratio using the procedure in hours. Compression tests using mortar cubes or cylinders can take several weeks to perform since the samples must be cured, typically in three, seven, or 28 days. Even then, results from compression tests are highly dependent on the method of making and curing the cube samples.
But there is a drawback. Because the mortar aggregate ratio test method uses a sieve analysis to measure the amount of aggregate, it is possible that some hydration of the cement may have occurred with the passage of time. These larger cement particles or clumps can be retained on the sieve. They then would be included erroneously with the sand volume, rather than included in the cementitious materials figure. The amount of clumping would depend on the materials and the conditions at the site where the mortar is used.
To resolve this question, the Portland Cement Association established a research project with the National Concrete Masonry Association to refine the procedure outlined in the ASTM document. Their work discovered three refinements that directly reduced the hydrated cement concern. They recommended using a 500-gram sample size; limiting the testing window to within four hours of batching; and requiring that the jar containing the mortar sample and alcohol be thoroughly agitated.
The sources for the answer are “Ruggedness Testing of the Mortar Aggregate Ratio Procedure,” Jeffrey Greenwald, Portland Cement Association, SN2767, 2004; and National Concrete Masonry Association, “Sensitivity Analysis of the Mortar Aggregate Ratio,” Portland Cement Association, SN2842, 2006.