418. The compressive strength of cement mortar is from five to ten times the tensile strength. As the result obtained in tests of either compression or tension depends upon the shape and size of the specimen, no definite value can be assigned to the ratio of compression to tension. Comparative tests have indicated in a general way that the cements giving the best results in tension show also the highest compressive strength; but with variations in treatment, different kinds and brands of cement do not give the same variations in the ratios of the two kinds of strength.

Mortar is not usually employed alone in large masses. It more frequently forms the binding medium between fragments of other substances, such as brick and stone. The dependence of the strength of masonry upon the strength of the mortar increases with the roughness of the stone or brick, and the thickness of the bed joints. In fine ashlar masonry this dependence is comparatively small, in brickwork it is important, and in concrete any increase in the strength of the mortar increases the strength of the concrete in nearly the same ratio.

Piers of brickwork may give a crushing resistance either greater or less than the strength of cubes made from mortar of the same composition as that used in building the piers. Thin beds of mortar between strong materials resist high compressive stresses, while in walls or piers built with weak blocks, the mortar is destroyed by the cracking of the blocks at a lower stress than the mortar would withstand in a cube pressed between steel plates. Since in brick and stone masonry the mortar forms but a small part of the structure, it is not economical to use a poor quality of mortar with good brick and stone.

419. Ratio Of Compressive To Tensile Strength

M. E. Candlot has made many experiments showing the effect of certain variations in the preparation of mortars upon the compressive and tensile strength. A few of the results of one series are presented in Table 128. The reduction from the metric system has been made, and a column added giving approximately the number of parts of sand to one of cement by weight, the accurate proportions appearing in the form of weight of cement to one cubic yard of sand. These results indicate that the ratio of the strength in compression to that in tension increases with the age of the mortar and also with its richness.

Table 128. Resistance Of Cement Mortars To Tension And Compression, With Varying Proportions Of Normal Sand

Specimens Hardened in Fresh water [From Ciments et Chaux Hydrauliques, par M. E. Candlot].

From a study of the results of nearly three thousand tests made by Professor Tetmajer, the late Professor J. B. Johnson concluded that for mortars containing three parts sand to one cement the ratio of the compressive strength to the tensile strength is equal to 8.64 + 1.8 log. A, where A is the age of the mortar in months. It is shown above that the ratio increases with increasing proportions of sand.

420. Table 129 gives some results obtained at the water-town Arsenal in tests of cement mortar cubes.1 The mortars contained one, two and three volumes of sand to one of natural cement, and two to four parts sand to one volume of Portland.

1 Prepared by Mr. George W. Rafter for the State Engineer of New York.

Table 129. Compressive Strength Of Cement Mortar. Portland And Natural

Tests of 12 Inch Cubes, Twenty Months Old, Made at Watertown Arsenal for State Engineer of New York.

1 Result interpolated.

2 2,043 omitting interpolated result.

The proportions of water used were such as to give mortars of different consistency, "dry," like damp earth, "plastic," of the consistency usually employed by masons, and "excess," quaking like liver with slight tamping. The specimens were twelve inch cubes and four methods of storage were used, as indicated.

Comparing the results with similar tests of tensile strength, it appears that the strength in compression decreases more rapidly as sand is added than does the tensile strength. The same conclusion was drawn from Table 128.

The strength of the Portland mortar with four parts sand is about equal to the strength of the natural with two parts. The dry mortar gives the highest strength with natural cement, but with Portland the "dry" and "plastic" give about the same result.

Concerning the consistency, it has already been pointed but that the conditions of the actual employment of mortar are such as to favor, in general, the use of a wetter mixture than that which gives the best results in laboratory tests of mortars. As to storage, the specimens kept in water for three or four months after made, give the highest results with natural cement. There seems to be no choice between the other three methods of storage.