458. Shearing Strength

The shearing strength of mortars and concretes is of importance not only because of its intimate relation to the compressive strength, but because of the shearing stresses to which these materials are subjected in structures reinforced with steel. But few tests of shearing strength have been made, however, partly because of the lack of appreciation of their value, and partly because it is difficult to subject a specimen to a purely shearing stress. It is frequently stated that the shearing strength is somewhat in excess of the tensile strength, perhaps as much as twenty per cent.

Table 158 gives the results of a series of tests made by Prof. Bauschinger in 1878.1 The values in shear are very closely twenty per cent, in excess of the tensile strengths of similar mortars tested at the same time.

Table 158. Shearing Strength Of Portland Cement Mortar Cubes Hardened In Air

Note : — Cement, each result mean of four brands. Sand, medium grain, clean. Mortars hardened in dry air. Tests by Professor Bauschinger, 1878.

459. A distinction should be drawn between the resistance offered by a thin mortar bed to the sliding of one stone or brick on another and to shear of the mortar itself. The former resistance involves the adhesion of the mortar to the surface of the brick or stone, and the values for this resistance are usually much less than the shearing strength, and not greatly in excess of the adhesive strength. The one is of importance in the determination of the stability of masonry dams, retaining walls, etc., but the latter is the resistance in question in the design of monolithic concrete structures.

1 Quoted by Mr. Emil Knichling in a Report on Cement Mortars.

460. Resistance To Abrasion

The resistance of cement mortar to abrasion depends on the quality of the sand as well as the cement. The abraiding surface wears away the cement or pulls the particles of sand out of their beds in the cement matrix. If the adhesion to the sand grains is strong, the sand particles receive the wear and withstand it until nearly worn away. With hard sand particles, therefore, the resistance to abrasion should increase as the proportion of sand increases, until the volume of the cement matrix becomes relatively too small to thoroughly bind the sand grains together. This limit is reached, however, when the mortar contains not more than two parts sand. With soft sand grains, the neat cement will usually give the highest resistance to abrasion, at least in the case of Portland. It has been found that specimens hardened in the air are brittle and wear more rapidly than those hardened in water.

461. Table 159 gives the results of several tests made to determine the relative wearing qualities of different mortars for such uses as sidewalk construction. The specimens were two-inch cubes, hardened in water and dried for a few hours just before grinding. An emery plate, set horizontally, was used in most of the tests. The results in any given line of the table are comparable, but, owing to changes in the grinding plate and in the methods used, the results in different lines are not all intercomparable. It is seen that when soft sand is used, such as limestone screenings, the greatest resistance to abrasion is offered by the neat cement mortar, and the resistance decreases constantly as the amount of sand is increased. When hard sand, such as the siliceous river sand, from Point aux Pins ("P.P." in the table) is employed, the greatest resistance is offered by mortars containing about equal parts of sand and cement. A comparison of lines 5 and 10 indicates that rich natural cement mortars lose about twice as much as similar mortars of Portland, but natural cement mortars containing more than two parts by weight of sand do not give relatively as good results.

Table 159. Abrasive Tests Of Cement Mortars