134. Strength of Mortar

The exact strength of mortar to resist compression is not of very great importance, as it seldom, if ever, fails in this way. The tensile and adhesive strength of mortar is more important, particularly the latter, as whenever a building has fallen from using poor mortar it has generally been on account of the failure of the mortar to adhere to the bricks or stones. Whatever kind of mortar is used, it should be made rich and well worked, as the saving by using more sand is but a small percentage at most, and it is never safe for an architect to allow poor mortar to be used in his buildings.

The safe crushing strength of Portland, Rosendale and lime mortars used in -inch joints should equal the following values in tons per square foot:

Portland cement mortar,

1

to

3,

3

months,

40

tons;

1

year,

65

tons.

Rosendale " "

1

to

3,

3

months,

13

tons;

1

year,

26

tons.

Lime mortar,

1

to

3,

3

months,

8.6

tons;

1

year,

15

tons.

From these values we see that for granite piers, heavily loaded, only Portland cement mortar should be used. For all piers loaded with over 10 tons per square foot, and not exceeding 20 tons, Rosendale cement mortar should be used. Lime mortar should never be used for piers that are to receive their full load within six months.

135. "The adhesion of mortars to brick or stone varies greatly with the different varieties of these materials, and particularly with their porosity. The adhesion varies also with the quality of the cement, the character, grain and quantity of the sand, the amount of water used in tempering, the amount of moisture in the stone or brick, and the age of the mortar."

Mortar adheres to both stone and brick better when they are wet (unless the temperature is below the freezing point), and the architect should always insist on having the bricks well wet down with a hose before laying. A dry brick absorbs the moisture from the mortar so that it cannot harden properly and destroys its adhesive properties. The wetting of the brick is fully of as much importance as the quality of the mortar in brickwork. The adhesive strength of the cements and lime are as a rule in proportion to their tensile strength. Therefore where great adhesive strength is desired to prevent sliding, as in arches, etc., either Portland or Rosendale cement should be used, according to the importance of the work and stress to be resisted. Some years ago the walls of a brick building in New York City were pushed outward by barrels of flour piled against the walls, so that the walls suddenly fell into the street. An examination of the mortar showed that it was of poor quality, with little adhesion to the bricks. Had good mortar been used and the brick well wet, the failure (it should not be called an accident) would not have occurred. The adhesive and tensile strength of mortar is also of great importance in resisting wind pressure and vibration.