Effects of Aggregate Grading on Properties of Masonry Mortar
Authors: Bloem DL 
Published: March 2009

Cooperative investigations by several laboratories have been made on behalf of Subcommittee IV on Specifications for Aggregates for Mortar of ASTM Committee C-12 on Mortars for Unit Masonry in an effort to improve grading limitations in ASTM Specifications for Aggregate for Masonry Mortar (C 144). This paper summarizes the results of those researches.

The first test program, in which four laboratories participated, involved comparisons between naturally graded masonry sands and the same sands regraded to a specific size distribution, essentially within the limits of Specifications C 144. The regimen of mortar tests was not identical in all laboratories but included measurements of water requirement, water retention, compressive and tensile strength, plasticity, and shrinkage.

It was found that water requirement of masonry mortar for a given sand grading varied greatly among the different sources. For any given source, the water-cement ratio was an inverse function of the fineness modulus; hence, the effect of regrading depended upon whether the original fineness modulus was higher or lower than that of the standard grading used as the basis for comparison. Mortar strength decreased as water-cement ratio increased and was relatively independent of the source of sand. It followed that, for the fixed proportions of solids, strength decreased as the aggregate became finer. The tests for water retention, plasticity, and drying shrinkage gave no evidence that sand grading, within the limits studied, was of significance to the quality of the mortar.

A second investigation, with five laboratories participating, was aimed more directly toward evaluation of sand grading per se. In this case, each laboratory tested mortars with both low and high air contents using the same sand, and in some cases a second sand, in five stipulated gradings ranging from the coarsest to finest permitted by Specifications C 144. All laboratories tested the mortars for water requirement, water retention, air content, and compressive strength. Individual laboratories made tests for plasticity, shrinkage, and density.

It was observed that: (1) The coarsest sands permitted by Specifications C 144 produced unworkable mortar which would not be considered acceptable for masonry construction; (2) as the sand became finer, water requirement increased, causing reduction in strength, reduction in water retentivity, and slight increase in drying shrinkage; (3) high air content of mortar improved water retentivity and plasticity but increased drying shrinkage slightly and reduced strength; (4) the amount of air entrainment in mortar was affected by the intermediate sizes in the sand; (5) sands from four different sources produced mortars differing in water requirement and air content, thus indirectly affecting strength, water retentivity, and drying shrinkage.

The researches were not sufficiently comprehensive to provide a firm basis for delineating accurately the boundaries of acceptable aggregate gradings for masonry mortar. They did suggest, however, that the gradings preferred by the building trade on the basis of manipulability of mortar may not be best from the standpoint of structural performance.