WATERPROOF MORTAR. A non-absorbent and impermeable mortar is important in all forms of masonry construction, and in some cases such a mortar is vitally essential. If the mortar is porous, it will absorb water, which may freeze and cause disintegration; and if the mortar is permeable, it may permit water to percolate or flow through it, which will make it useless for some purposes.
To make a non-absorbent and impermeable mortar, use sand containing a small per cent of voids, that is, sand containing a proper proportion of grains of various sizes, and enough fine-ground cement to completely fill the voids in the sand; and mix the mortar very thoroughly, making it neither very wet nor very dry. There are a number of foreign ingredients that are sometimes mixed with mortar to make it impervious, but usually it is both better and cheaper to use a richer mortar than to add the foreign substances. The method of making mortar impervious by filling the voids with some foreign ingredient is substantially the same as for concrete (§ 369-76) FREs8ING OF MORTAR. The freezing of mortar before it has set has two effects: (1) the low temperature retards the setting and hardening action; and (2) the expansive force of the freezing water tends to destroy the cohesive strength of the cement.
Owing to the retardation of the low temperature, the setting and hardening may be so delayed that the water may be dried out of the mortar and not leave enough for the chemical action of harden ing; and consequently the mortar will be weak and crumbly. This would be substantially the same as using mortar with a dry porous brick. In ordinary practice cement mortar is always mixed with considerably more water than is required for the chemical combina tion; but when mortar is likely to be exposed to frost, it should be mixed dryer than usual to hasten the set, and hence the drying out may seriously injure the strength of the mortar. Whether the water evaporates to an injurious extent or not depends upon the humidity of the air, the temperature of the mortar, the activity of the cement, and the extent of the exposed surface of the mortar. The mortar in the interior of the wall is not likely to be injured by the loss of water while frozen; but the edges of the joints are often thus seriously injured. In the latter case the damage may be fully repaired by pointing the masonry (§ 565) after the mortar has fully set.
On the other hand, when the cement has partially set, if the ex pansive force of the freezing water is greater than the cohesive strength of the mortar, then the bond of the mortar is broken, and on thawing out the mortar will crumble. Whether this action will take place or not will depend chiefly upon the strength and activity of the cement, upon its hardness at the time of freezing, and upon the amount of free water present. Further, cement in setting generates some heat (§ 348) which tends to prevent freezing.
The relative effects of these several elements are not known certainly; but it has been proven conclusively that for the best results the following precautions should be observed: 1. Use a
quick-setting cement. 2. Make the mortar richer than for ordinary temperatures. 3. Use the minimum quantity of water in mixing the mortar. 4. Prevent freezing as long as possible.
There are various ways of preventing freezing: 1. Cover the masonry with tarpaulin, straw, manure, etc. 2. Warm the stone and the ingredients of the mortar. Heating the ingredients is not of much advantage, particularly with Portland cement. 3. Instead of trying to maintain a temperature above the freezing point of fresh water, add salt to the water to prevent its freezing. To prevent water from freezing down to 0° F., add salt equal to 1 per cent of the weight of the water for each 1° F. below freezing. A common rule which has been much used to keep mortar from freezing is: "Dis solve 1 pound of salt in 18 gallons of water [practically 150 pounds] when the temperature is at 30° F., and add 1 ounce of salt for each 1° of lower temperature." This rule does not give as much salt as the first one—at 31° only about two thirds as much and at 20° only about one tenth as much,—and it gives either too much salt for temperatures only a little below freezing or too little for temperatures near zero. The fact that the second rule has been successfully used to prevent damage to mortar at atmospheric temperatures 10° or 15° F. below freezing, seems to show that with mortar it is not necessary to use the full amount of salt required to keep the water from freez ing. Apparently then a safe rule would be: "Use salt equal to 3 of 1 per cent of the weight of the water used in making the mortar for each 1° F. below freezing." This rule is better than the second one above because it gives the correct relative proportions of salt at all temperatures; and below 28° the last rule gives more salt than the second rule, and therefore is more safe at low temperatures where most needed.
Alternate freezing and thawing are more damaging than continuous freezing, since with the former the bond may be repeatedly broken; and the damage due to successive disturbance increases with the number.
Practice has shown that portland-cement mortar of the usual proportions laid in the ordinary way is not materially injured by alternate freezing and thawing, or by a temperature of 10° to 15° F. below freezing, except perhaps at the exposed edges of the joints. Under the same conditions natural-cement mortar is likely to be materially damaged.
By the use of salt, even in less proportions than specified above, or by warming the materials, masonry may be safely laid with portland-cement mortar at a temperature of 0° F.; and the same may usually be done with natural cement, although it will ordinarily be necessary to re-point the masonry in the spring. Warming the materials is not as effective as using salt.