. Cement and concrete. , moist atmos-phere, the room in which the curing is done is sometimes filledwith steam. If the cement is sound and the block well made,this should have no injurious effect, and would greatly hastenthe time when the block could be used. CONCRETE BLOCKS: MANUFACTURE 517 For the manufacture of blocks in temperatures below freez-ing, a heated building is essential for making and curing if perfe ctblocks are sought. Good work may be accomplished with massconcrete in low temperatures, but with small, hollow blocks it isnot the same; there are too many points of attack, and va
. Cement and concrete. , moist atmos-phere, the room in which the curing is done is sometimes filledwith steam. If the cement is sound and the block well made,this should have no injurious effect, and would greatly hastenthe time when the block could be used. CONCRETE BLOCKS: MANUFACTURE 517 For the manufacture of blocks in temperatures below freez-ing, a heated building is essential for making and curing if perfe ctblocks are sought. Good work may be accomplished with massconcrete in low temperatures, but with small, hollow blocks it isnot the same; there are too many points of attack, and variationsin temperature and rate of hardening introduce internal stressesin the hollow structure, to say nothing of the difficulty of obtain-ing a g®od face under such conditions. 787. PRESSED BLOCKS. — Since the presence of cores wouldinterfere with the application of pressure, the method involvingsuch application is practically confined to blocks of the so-called two-piece system. The most common block used in this. BLOCKS OF TWO-PIECE SYSTEM method is in the form of a T, although L and U forms are alsomade. On account of the heavy pressure used in the manu-facture, it is possible to employ concrete more moist than is usedin tamped blocks, and it is usually considered that on this 518 CEMENT AND CONCRETE account a larger proportion of broken stone or gravel may beemployed. These blocks are molded in heavy metal molds. These arefilled with the mixture, a face plate is laid on the concrete,and pressure is applied, by either hand or machine press. Theblock is removed from the mold immediately and cured in thesame way as tamped blocks. 788. POURED BLOCKS. — The method employed in mak-ing blocks of wet concrete is quite different from those describedabove, inasmuch as neither tamping nor pressure is used tocompact the mass, and the block cannot be removed from themold until the concrete has set. This does away with much ofthe so-called machinery used in other methods, but nece
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