Q: I received a submittal for sand that had a gradation significantly outside the limits of ASTM C144. The masonry subcontractor stated that his company had used the sand successfully on many other projects and had no complaints. The contractor liked the finer sand because it was easier to tool and he preferred the appearance.
What problems can occur when fine sands are used?
A: ASTM C144, "Standard Specification for Aggregate for Masonry Mortar," permits the use of sands outside the gradation limits if mortars made from these products comply with the aggregate ratio, water retention, and compressive strength requirements of the property specifications listed in ASTM C270, "Standard Specification for Mortar for Unit Masonry."
The required range for the aggregate ratio listed in Table 2, "Property Specification Requirements of ASTM C270," is between 2.25 and 3 times the volume of cementitious materials. This figure is the ratio of sand to the sum of the portland cement, lime, and mortar cement or masonry cement in the mortar. The ratio is to be measured when the sand is in a damp, loose condition.
Water retention is a measure of the change in flow rate after water has been removed from the mortar following suction. It is intended to simulate the effect that masonry units have on the mortar. This value is determined using the procedure described in ASTM C91, "Standard Specification for Masonry Cement." The minimum value for water retention listed in Table 2, "Property Specification Requirements of ASTM C270," is 75 percent.
Compressive strength requirements are determined from laboratory prepared mortar mixes made from the materials that will be used in construction, but are mixed with water that produces a flow of 110 +5 percent when tested in accordance with the procedure described in ASTM C109/C109M, "Standard Test Method for Compressive Strength of Hydraulic Cement Mortars." The flow, and consequently the quantity of water used to mix this laboratory mortar, is less than that needed to produce a workable mortar during construction. The amount of water, however, is reduced to approximate the properties of field prepared mortars after they have been placed in the wall and water has been drawn out of the mortar by absorption of the masonry units.
The average compressive strength (measured in pounds per square inch) at 28 days must be a minimum of 2,500 for type M mortar, 1,800 for type S mortar, 750 for type N mortar, and 350 for type O mortar.
Although sands not meeting the gradation limits of ASTM C144 are permitted under this specification if they satisfy the three requirements of the property specifications listed in ASTM C270, there may still be problems.
I have been involved with projects where the use of sands significantly finer than those listed in ASTM C144 contributed to increased mortar shrinkage. Fine sands can increase the water demand of the mortar during mixing and placement. A high water to cement ratio leads to greater drying shrinkage, with cracks typically appearing as regularly spaced transverse cracks. There may also be an increase in separations between the mortar and the brick with high water to cement ratio mortars.
Whenever possible I recommend using sands that meet C144 gradation limits. However, in several areas of the country it is difficult to find sands that meet these limits.
Before using sands containing excessive fines, I recommend reviewing several masonry projects constructed using mortars made from the sand in question. The buildings should be two to three years old. To provide a good comparison, the mortar should be the same type and the brick units should have a similar initial rate of absorption to those that will be used. Closely examine the mortar joints for shrinkage cracks or separations before proceeding with the use of the proposed mortar.