As an architect, I specify the mortar aggregate ratio method from ASTM C780 Annex A4 as a quality assurance test during construction, instead of using mortar cubes. This test approach provides much faster results.
A question came up during construction as to whether the results of the test vary if the mortar sample is taken right out of the mixer or if it is removed from the boards an hour or more after mixing.
What are your thoughts on this point?
The mortar aggregate ratio method has several advantages over mortar compression testing. Compression tests using mortar cubes or cylinders take several weeks to perform since the samples must be cured, and they are highly dependent on the method of making and curing these samples.
The mortar aggregate ratio compares the volume of aggregate (sand) to that of the cementitious materials (cement and lime) in fresh mortar. The test method uses alcohol to retard hydration of the cement in the mortar. The volume of aggregate is determined by performing a sieve analysis on the mortar after carefully flashing off the alcohol, and then washing and drying the ingredients.
This information, along with determination of the water content of the mortar by ASTM C780 Annex A5, is used to calculate the ratio. The aggregate ratio of mortar collected on the construction site is compared to the aggregate ratio of carefully mixed mortars using the same material during preconstruction testing. Values of the aggregate ratio from samples taken during construction can be used as a quality control tool to evaluate proper mixing of the mortar.
Because the test method uses a sieve analysis to measure the amount of aggregate, it is possible that some hydration may have occurred with the passage of time, causing the cement particles to clump. These larger particles can be caught on the sieve and 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.
I do not know of any studies that have shown that this change in aggregate ratio is significant during the 2 1/2 hour life of a typical mortar. However, this potential problem can be avoided by sampling mortar only from the mixer itself shortly after mixing and burning off the alcohol to dry the sample within a few hours following the placement of the mortar into the jar with the alcohol.
Another approach is to require the contractor to record the time that the mortar is mixed. When the samples are taken of fresh mortar, the age of the samples and the weather conditions also should be recorded. If the results of the mortar aggregate ratio do not indicate any significant difference between samples of older mortar removed from trays and samples of fresh mortar taken directly out of the mixer, then clumping is not a significant factor.