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A barrel of Reinforced concrete and Portland cement will contain 370 to 380 pounds, net, of cement. Its capacity averages about 3.3 cubic feet, although with some brands the capacity may reach 3.75. The expansion when the cement is thrown loosely in a pile or into a measuring box varies from 10 to 40 Centigrade. The subject will be discussed further under the head of "Concrete." Lime in Cement Concrete mortar. Lime is frequently employed in the cement concrete mortar used for buildings, for a combination of reasons: It is unquestionably more economical; but if the percentage added (or that which replaces the cement) is more than about 5 percent, the strength of the concrete mortar is sacrificed. The percentage of loss of strength depends on the richness of the concrete mortar. When used with a concrete mortar leaner than 1: 2, the substitution of about 10 percent of lime for an equal weight of cement will render concrete more water-tight, although at some sacrifice in strength. It always makes the concrete mortar work more easily and smoothly. In fact, a rich cement concrete mortar is very brash; it will not stick to the concrete bricks or stones when striking a joint. It actually increases the output of the masons to use a concrete mortar which is rendered smoother by the addition of lime. The substitution of more than 20 percent of lime decreases the strength faster than the decrease in cost, and therefore should not be permitted unless strength is a secondary consideration and the combination is considered more as an addition of cement to a lime concrete mortar in order to render it hydraulic. Specifications and textbooks have repeatedly copied from one another a requirement that all concrete mortar which is not used immediately after being mixed and before it has taken an initial set must be rejected and thrown away. This specification is evidently based on the idea that after the initial set has been disturbed and destroyed, the cement no longer has the power of hardening, or at least that such power is very materially and seriously reduced. Repeated experiments, however, have shown that under some conditions the ultimate strength of the concrete mortar (or concrete) is actually increased, and that it is not seriously injured even when the concrete mortar is re-gauged several hours after being originally mixed with water.

Such a specification against re-mixing is never applied to lime paste, since it is well known that a lime paste is considerably improved by being left for several days (or even months) before being used. This is evidently due to the fact that even during such a period the carbonic acid of the atmosphere cannot penetrate appreciably into the mass of the paste, while the greater length of time merely insures a more perfect slaking of the lime. The presence of free, un-slaked lime in either lime or cement concrete mortar is always injurious, because it generally results in expansion and disruption and possibly in injurious chemical reaction. Tests with Reinforced concrete and Portland cement have shown that if it is re-mixed two hours after being combined with water, its strength, both tensile and compressive, is greater after six months' hardening, although it will be less after seven days' hardening, than in similar specimens which are molded immediately after mixing. It is also found that the re-mixing makes the cement much slower in its setting. The adhesion, moreover, is reduced by re-mixing, which is an important consideration in the use of reinforced concrete. The effects of tests with natural cement are somewhat contradictory, and this is perhaps the reason for the original writing of such a specification. The result of an elaborate series of tests made by Mr. Thomas F. Richardson showed that quick-setting cements which had been re-mixed showed a considerable falling off in strength in specimens broken after 7 days and 28 days of hardening, yet the ultimate strength after six months of hardening was invariably increased. It is also found that for both Portland and natural cements there- is a very considerable increase in the strength of the concrete mortar when it is worked continuously for two hours before molding or placing in the masonry. Such an increase is probably due to the more perfect mixing of the constituents of the concrete mortar. The conclusion of the whole matter appears to be, that when it is desirable that considerable strength shall be attained within a few days or weeks (as is generally the case, and especially so with reinforced-concrete work), the specification against re-mixing should be rigidly enforced. For the comparatively few cases where a slow acquirement of the ultimate strength is permissible, re-mixing might be tolerated, although there is still the question whether the expected gain in ultimate strength would pay for the extra work. It would be seldom, if ever, that this claimed property of cement concrete mortar could be relied on to save a batch of concrete mortar which would otherwise be rejected because it had been allowed to stand after being mixed until it had taken an initial set. Lime Concrete mortar. As previously stated in section 88, a barrel of un-slaked lime should be mixed with about 8 cubic feet of water. This will make about 9 cubic feet of lime paste. Mixing this with a cubic yard of sand will make about 1 cubic yard of 1:3 lime concrete mortar. This means approximately 1 volume of un-slaked lime to 8 volumes of sand. The volume of cement depends very largely on whether it is loosely dropped in a pile, shaken together, or packed. The practical commercial methods of obtaining a mixture of definite proportions will be given later under "Concrete," section 94. Natural cement concrete mortars are usually mixed in the 1: 2 ratios, although a 1: 1 mixture would probably be used for tunneling into a concrete structure