. Cement and concrete. well tamped and give good results, but place brokenstone in the same mixture, and the mass is likely to bridge CONCRETE BLOCKS: MANUFACTURE 513 and cannot be so well compacted. More water lubricates themixture and insures close contact of mortar and stone at allpoints. Strength is not, however, the only requisite in con-crete blocks; impermeability, or the quality of being water-proof, is also essential for the best results. It has been statedin Art. 61 that mortars mixed either very wet or very dry aremore permeable, less waterproof, than those of ordinary con-sistency,
. Cement and concrete. well tamped and give good results, but place brokenstone in the same mixture, and the mass is likely to bridge CONCRETE BLOCKS: MANUFACTURE 513 and cannot be so well compacted. More water lubricates themixture and insures close contact of mortar and stone at allpoints. Strength is not, however, the only requisite in con-crete blocks; impermeability, or the quality of being water-proof, is also essential for the best results. It has been statedin Art. 61 that mortars mixed either very wet or very dry aremore permeable, less waterproof, than those of ordinary con-sistency, but since in the case of concrete large voids are likelyto result from insufficient tamping, an excess of water is betterthan a deficiency. A rather free use of water thus tends toprevent the occurrence of large voids, but results in a largernumber of small voids through which water does not easilypermeate. 784. TAMPED BLOCKS. — This method was the first tocome into prominence in the manufacture of concrete blocks,.
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