360. Effect Of Consistency Of Mortar On Tensile Strength

The results in Table 73 are from briquets of Portland cement with two parts "Standard" crushed quartz. The consistency of the mortars varied from a "trifle dry," in which water rose to the surface only after continued tamping, to a wet mortar which would just hold its shape when placed in a heap on the slab. Half of the briquets were immersed, while the remainder were stored in the air of the laboratory. The air hardened specimens gave higher results in all cases than those hardened in water. The highest strength was given in general by the dry est mortar, but the differences in strength decrease as the age of the specimen increases.

Table 73. Effect Of Consistency On The Strength Of Portland Cement Mortar Hardened In Water And Air

Ten-silk Strength, Pounds per Square Inch.

Cement: Brand R, Sample 18 R, with two parts "Standard" sand. Consistency: a, trifle dry; b, O.K.; c, moist; d, very moist; e, would just hold shape.

361. Tables 74 and 75 give similar results for Portland and natural cement mortars, respectively, all specimens having hardened in water for three months. The amount of water used in gaging had a wide range, giving mortars of all consistencies from very dry to very moist. The richness of the mortar was also varied, from neat cement to five parts sand. A comparison of the results in these two tables indicates that the highest strength is usually given by mortars a trifle dryer than that considered right for briquets; that an excess of water is less deleterious to rich mortars than to lean ones, and to Portland cement than to natural cement.

362. Conclusions

Although all of these tests indicate the superiority of dry mortars, in considering the effect of consistency from a practical standpoint, one must not fail to consider the difference between the conditions existing in the actual use of mortars and in laboratory tests. When mortar is used in masonry, the stones or bricks, even though they be dipped, or sprayed with a hose before setting, are very likely to press out or absorb considerable moisture from the mortar. To realize this one has only to raise a heavy stone just after it has been bedded; and the greater ease of setting either stone or brick, and obtaining a full mortar bed, with a rather wet mortar, is appreciated by all masons. In the laying of concrete, the difficulties of obtaining a compact mass with a dry mortar are also not to be overlooked, but this point is discussed elsewhere.

Table 74. Variations In Consistency Of Mortar. Effect On Strength Of Portland Mortar At Three Months

Consistency — Significance of numbers : Increasing per cent, water used for higher numbers.

1 — Very dry ; little or no moisture appeared on surface of briquets. 5—About proper consistency for briquets. 9 — Very moist; mortar would barely hold shape and shrank in molds in hardening.

Table 75. Variations In Consistency Of Mortar. Effect On Strength Of Natural Cement Mortar At Three Months

Consistency—Significance Of Numbers

1 — Very dry; little or no moisture appeared on surface briquets. 5 — About proper consistency for briquets.

9 — Very moist; mortar would barely hold shape and shrank in hardening.

363. Effect Of Aeration On The Tensile Strength Of Cement

Portland cements that are not perfect in composition and, burning, and that therefore contain free lime, may sometimes be rendered sound by exposing them to air, and such exposure: was at one time considered almost essential in Portland cement manufacture.

Fresh Portland cements that are slightly defective may have their properties quite radically changed by such treatment; their rate of setting becoming first more rapid, and then, by further aeration, slower, and their tendency to expand overcome or ameliorated. Portland cements that are perfectly sound suffer some loss in. specific gravity by the absorption of carbonic acid and water from the atmosphere, but moderate aeration has no radical effect upon their strength, and Port-lands deteriorate but very slowly by storage, provided the. cement is kept dry and does not cake in the package.

Natural cements, however, usually suffer by aeration, and this is illustrated by tests on several samples of one brand; given in Tables 76 and 77. Of the four samples in Table 76, NN and 00 showed an improvement by two weeks' exposure to air, spread out in a thin layer, but longer exposure resulted in a serious loss of strength. Of the other two samples, SS was greatly improved by five weeks' aeration, but longer exposure was detrimental, while sample QQ showed a continuous improvement up to the limit of eleven weeks' exposure to air.

Table 76. Effect Of Aeration On Four Samples Of Same Brand Natural Cement

Cement: Brand Gn; Sand, two parts crushed quartz to one cement. All briquets of one sample were made by one molder and same percentage water used.

In Table 77 the effect of aeration on five samples of the same brand is shown. One of these samples was overburned and was rendered practically worthless by fourteen weeks' exposure to air. Nearly all of the samples in this table were seriously affected by six weeks' aeration.

Table 77. Natural Cement. Effect Of Aeration

a — Fineness expressed as per cent, passing holes .0046 inch square. b— Time setting fresh cement, time to bear 1/12 inch 1/4 lb. wire.