Masonry Magazine September 2002 Page. 29
What we find is that two of those factors-proportions and properties actually govern the third, composition. According to Brian Carney, vice president and general manager of Spec Mix, Mendota Heights, Minn., "You need to be concerned about what the requirements are because there are two different specifications in ASTM C 270, the property specification and the proportion specification. Some architects specify property and other proportions. It does make a difference in what you end up with."
Let's say the specification calls for Type S mortar. However, if it calls for proportion Type S, that's telling you to follow the specific formula, "This is the recipe that I want you to use to make my Type S mortar." So, for Type S you take one part of cement-one "part" equals one bag equals one cubic foot-and add in a half part of lime. Then you add up the volume of those two components (1.5), multiple by three and that tells you how many parts of sand to put in the mixer-in this case, 4.5 parts sand.
"We tell the architect that we'll put in our 3000 pound bulk bag of Spec Mix the equivalent of one bag of cement, half bag of lime, and 4.5 cubic feet of sand by volume," says Carney. "That means that you're probably going to have a mortar with a laboratory determined compressive strength at anywhere from 3,500 psi to 4,000 psi, maybe higher. While the designer intended for 1800 psi type S mortar, we have produced a mortar with approximately twice the compressive strength of what was intended, creating a possible compatibility mismatch between the mortar and the masonry unit."
Now, if the architect specifies Type S property mortar, the methodology changes, often for the better. Carney says, "It still has to be made from Portland cement but we can design the mix and formulate the recipe to what we need to achieve the properties of Type S-for example, the requirements of 1800 psi. Often masons and architects are thinking that one bag of cement and half a bag lime is the best mix for a Type S mortar, but what they end up making is in excess of a 'property Type M' mortar."
Chris Darner, Construction Products sales representative at Quikrete, Atlanta, Ga., explains, "Type M mortar is generally intended for below grade construction-foundation walls and heavy load bearing walls. Type S mortar is generally used for above grade block construction and some brick, depending on the architect. Type N is used in brick construction. They have different consistencies because they are made with different strength cement. The proportion that is added to the mix stays the same but the bond strength of the cement is greater in the M as opposed to the N and S."
As Darner says, Type M is a very strong mortar that might not be right for the integrity of the masonry assembly. When the mortar is too hard, masonry walls won't move in a thermal dynamic plane properly, as they were designed to do.
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The Voice of the Mason Contractor September 2002
Masonry 27