Strength of Mortar

Compressive Strength.

Not nearly as many experiments have ben made upon the compressive strength of mortar as upon the tensile strength, partly because of the greater difficulty of mould ing the test specimen, and partly because of the uncertainty intro duced by the smoothness of the pressed surface, and partly because of the larger and more expensive testing machine required. Experi ments seem to show that the crushing strength of cubes is about 8 to 10 times the tensile strength of the same mortar at the same age determined in the usual manner. This ratio increases with the age of the mortar and with the proportion of sand, and decreases with the wetness of the mortars; and varies for different cements and different sands. Several formulas have been proposed to express the relation between the crushing and the tensile strength of cement mortar, but none are reasonably general. The standard German specifications require that the compressive strength of cement mortar shall be at least 10 times its tensile strength.

Data determined by submitting cubes of mortar to a compressive stress are of little or no value as showing the strength of mortar when employed in thin layers, as in the joints of masonry. The strength per unit of bed area increases rapidly as the thickness of the test specimen decreases, but no experiments have ever been made to determine the law of this increase for mortar.

Adhesive Strength.

Although the adhesion of cement mortar is as important for purposes of construction as its cohesive strength, very few experiments have been made to determine the power with which mortars stick to brick, stone, etc.; and, unfortunately, the results of the few experiments that have been made differ very greatly. All tests for adhesive power are subject to the same causes of variation as cohesive tests, and in addition are subject to variations because of differences of the test specimens in absorptive power, in porosity, in smoothness of the surface, to variations in the pressure upon the specimens, etc., and apparently differences in fineness of the cement and in the character and fineness of the sand make more difference in the tests of adhesion than in the tensile tests. Further, in some of the methods that have been employed to determine adhe sion, errors due to eccentricity of stress are proportionally much greater than in ordinary tensile tests.

Two methods have been employed in making tests of the adhesive power of mortars; viz.: (1) cementing two bricks or pieces of stone together, and then pulling them apart; and (2) moulding a piece of the material to be tested in the middle of a briquette, and then testing the briquette in the usual way.

1. When two bricks are cemented together crosswise, the results are so variable as to be of little value—partly for the reasons men tioned in 256, but chiefly because with so Large a surface of contact there are nearly certain to be large eccentric stresses which greatly reduce the results. In one set of twenty-five experiments using

both natural and portland cement the ratio of the tensile strength to the adhesion to soft brick varied from 3.9 to 26; and decreased somewhat uniformly from 28 days to 6 months, and increased from neat cement to 1 : 3 mortar.* The wide range of these results, and the surprising way in which they varied with age and the richness of the mortar, is characteristic of this method of making tests of adhesion.

A set of 1200 experiments t made in 1882 by cementing two pieces of stone 1+} inches long by 1 inch wide showed not much difference between the adhesion of hydraulic cement to polished plate glass and to chiseled granite or sawed limestone. The results for the adhesion to sawed limestone of a neat portland mortar having a tensile strength of 425 lb. per sq. in. at 7 days, are: at 7 days, 61 lb. per sq. in.; at 28 days, 84 Ib. per sq. in. The finer the cement the greater the adhesion. There seems to be no constant relation between the cohesive and the adhesive strength of a cement.

2. The French have a standard method of testing the relative adhesion of different cements and also of testing the adhesion of a cement to different surfaces. To determine relative adhesion of • different cements, a half briquette is moulded of standard mortar, and after it has hardened the other half briquette is moulded against the first half, and then the briquette is tested in the ordinary cement-testing machine. Results by this method show great differ ences for different cements, and no apparent relation between mor tars of different proportions.$ To determine the adhesion of a cement to different surfaces, a plate of the material to be tested is moulded into the middle of the briquette. Results by this method show an adhesion of a 1 portland-cement mortar at 28 days to sandstone of from 78 to 125 lb. per sq. in.¶ Mr. L. C. Sabin made some tests of adhesion by inserting thin plates of soft dolomitic limestone in the middle of the ordinary bri quette mould and then filling the mould with mortar in the ordinary way.** The results show a ratio of cohesive to adhesive strength of portland mortar from 2.03 to 3.05. In both cases there is no practical difference in the ratio for 28 days and 6 months, nor be tween neat cement and a 1 mortar. The greatest adhesion is given by a mortar considerably wetter than that which gives the highest tensile strength.