STRENGTH OF MORTAR. The strength of mortar is dependent upon the strength of the cementing material, upon the composition, fineness, etc., of the sand, and upon the adhesion of the former to the latter. The kind and amount of strength required of mortar depends upon the kind and purpose of the masonry. If the blocks are large and well dressed, and if the masonry is subject to compression only, the mortar needs only hardness or the property of resisting pression; hard sharp grains of sand with comparatively little ing material would satisfy this requirement fairly well. If the blocks are small and irregular, the mortar should have the capacity of adhering to the surfaces of the stones or bricks, so as to prevent their displacement; in this case a mortar rich in a good cementing material should be used. If the masonry is liable to be subject to lateral or oblique forces, the mortar should possess both adhesion and cohesion. Tensile Strength. For the tensile strength of neat cement and of cement mortar made with standard sand, see Table 14, page 82. The results in Table 14 were obtained with standard sand, but the better natural sands used in actual practice would, under the same conditions, give higher results on account of the smaller per cent of voids. On the other hand, in actual practice the mortar is not likely to be as thoroughly mixed nor seasoned under as favorable conditions as in the laboratory, and hence it is not likely that the mortar used in ordinary practice will have as great strength as shown by laboratory tests. Further, mortar which sets under even a moderate pressure may be something like one third stronger than that which sets without pressure; and in practice mortar nearly always sets under more or less pressure.
Effect of Time. Fig. 7 shows the effect of time on the strength of mortars.* The curves for neat and the 1 : 3 port land cement represent the average of over 150,000 briquettes; while the lines for the, other portland-cement mortars are based upon 300 to 500 tests each. The curves for natural cement represent
seven different sections of the United States. The line representing lime is based upon only a few experiments by the writer, and repre sents the value obtained by exposing standard briquettes of mortar freely to the air; but this line is not well determined.
Note that the portland cement both neat and with sand gains its strength proportionally more rapidly than natural cement, not withstanding the fact that the latter will usually attain "hard set" earlier than the former.
Notice the sag in the curves for the neat and the 1 : 1 portland cement mortars. This is due to the hardening action of some of the constituents of the cement not being permanent—see § 158.
Notice that portland cement both neat and with sand does not gain much strength after 28 days, while natural cement both neat and with sand continues to gain strength for at least a year. Tests extending over a longer time usual ly show a falling off in the strength of the neat and rich portland mor tars, while natural cement, either neat or with sand, gains strength continuously for at least four or five years. Some natural cements after four or five years are nearly as strong as some portlands.* The chief advantage of port land cements over naturals is that they are more uniform in quality and gain their strength earlier; and the fact that they lose a small part of their strength is not serious.
Effect of Sand. Fig. 8 shows the strength at six months of a portland and a natural cement mixed with various proportions of natural sand. Such relations vary with the fineness of the cement and of the sand, but the above is believed to be fairly representative except that the cement is a little coarser than required by the present standard specifications. A diagram like Fig. 8, is useful in dis cussing the question of relative economy of natural and portland cement (§ 259), in which case it should be made for the particular brands of cement to be considered and with the sand to be used in practice.