2. Blocks poured or tamped from wet concrete, and allowed to remain in the mold until hardened.
Tamped Blocks From Semi-wet Mixture.
Various shapes and sizes of blocks are commonly made; the builders of the most popular machines have, however, adopted the standard length of 32 inches and height of 9 inches for the full-sized block, with thickness of 8, 10, and 12 inches. Lengths of 24, 16, and 8 inches are also obtained on the same machines by the use of parting plates and suitably divided face plates; any intermediate lengths and any desired heights may be produced by simple adjustments or blocking off.
Blocks are commonly made plain, rock-faced, tool-faced, paneled, and of various ornamental patterns. New designs of face plates are constantly being added by the most progressive machine makers.
Block Machines.
Blocks Poured From Wet Concrete.
The well-known cast-stone process consists in pouring liquid concrete mixture into a sand mold made from a pattern in a manner similar to that in which molds for iron castings are produced. The sand absorbs the surplus water from the liquid mixture, and the casting is left in the mold for 24 hours or longer until thoroughly set. This process necessitates the making of a new sand mold for every casting, and is necessarily much less rapid than the machine method. It is less extensively used for building blocks than for special ornamental architectural work, sills, lintels, columns, capitals, etc., and for purposes of this kind it turns out products of the highest quality and beauty.
Tamping Of Concrete Blocks.
Hardening And Storage.
Blocks should not be used in building until at least 4 weeks from the time they are made. During this period of seasoning, blocks will be found to shrink at least 1/16 inch in length, and if built up in a wall when freshly made, shrinkage cracks in the joints or across the blocks will surely appear.