. Cement and concrete. arse and fine particles, a compact mortar may bemade with one part cement to three parts sand, while to thesame mortar we may add at least five or six parts of brokenstone, and the resulting concrete will be nearly as impervious asthe mortar alone. With ordinary sand it will be found neces-sary to use one part cement to not more than three parts sand toassure impermeability, but of well-graded gravel as much as fiveparts to one cement may be employed. With artificial crushedproducts, that is, broken stone, one cement to two and a halfor three parts sand by weight makes a
. Cement and concrete. arse and fine particles, a compact mortar may bemade with one part cement to three parts sand, while to thesame mortar we may add at least five or six parts of brokenstone, and the resulting concrete will be nearly as impervious asthe mortar alone. With ordinary sand it will be found neces-sary to use one part cement to not more than three parts sand toassure impermeability, but of well-graded gravel as much as fiveparts to one cement may be employed. With artificial crushedproducts, that is, broken stone, one cement to two and a halfor three parts sand by weight makes a good mortar. The quan-tity of mortar to be used should somewhat exceed the voids inthe broken stone. If no broken stone is employed, the proportionof cement required to secure imperviousness is so large as tomake the blocks expensive. This cost may be reduced byusing silica cement (§33). The use of hydrated, or slaked, limehas also been recommended to assist in filling the voids; the CONCRETE BLOCKS : MANUFACTURE 523. ORNAMENTAL BRACKET CAST IN SAND MOLD. 524 CEMENT AND CONCRETE possibility of efflorescence from such use is sufficient to dictatecaution, however, until it is proved to be unobjectionable.(Articles 34, 40, and 61 should be read in this connection.) 792. Special Forms. — In the construction of a building manyspecial forms are required, such as sills, lintels, columns, steps,etc. Block machines are now made to mold many of theseforms when of moderate length. Special molds of steel withhinged sides are also made into which the concrete is the size of block required is beyond the capacity of themachine in use, wooden molds may be readily made. Unless forsome form which is not to be duplicated, such wooden moldsshould be so made that they may be used repeatedly. Tomake a convenient form, fasten one plank on edge along one sideof the bottom piece or pallet, thus making an L; the remainingside piece is not fastened, but is held in place by a strong ironcl
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