CONCRETE BUILDING-BLOCKS.
Under this head will be considered briefly the method of manufacture and the uses of comparatively small blocks of concrete employed as substitutes for brick or cut stone in buildings. Such blocks are usually hollow, and the term hollow concrete building. blocks is frequently used to designate this form of material. Owing to the thinness of the walls of many of the hollow blocks, it is impos sible to use a coarse aggregate, and consequently the material is cement mortar rather than concrete; and therefore building-blocks are called cement blocks nearly as frequently as concrete blocks. Attempts have been made to introduce cement blocks of the size of ordinary brick; but the size is too small for economy and has no compensating advantage. Such material is appropriately called cement brick.
The concrete building-block is of quite recent origin, but it has developed very rapidly within the past few years, and has reached the position of an important building material. The two qualities which make the ordinary concrete blocks a valuable building material are their cheapness and the ease with which blocks of any size or form may be moulded. In many localities concrete blocks are cheaper per unit of volume than either brick or cut stone; and on account of their larger size concrete blocks are superior to bricks, either burned-clay or sand-lime, since the large size requires less skill and also costs less to lay, and secures a more uniform bearing and hence a stronger wall for materials of approximately the same strength.
Originally the concrete-block industry was stimulated by the numerous manufactures of patented machines for moulding the blocks. It was represented that any one could make concrete blocks;
and as a result, the manufacture fell largely into the hands of men with out any knowledge concerning either the selection of the materials or their combination, and consequently many poor blocks were put upon the market, which greatly injured the reputation of concrete blocks. Further, owing to inattention to the principles of correct design, the artistic possibilities of concrete have been underestimated. Notwithstanding the mistakes and failures in the early history of the industry, concrete blocks are a valuable building material. The concrete block has an advantage over concrete built in situ, in that (1) the block can be moulded on the ground under factory conditions, (2) requires much less expense for forms, and (3) is simpler to erect. Of course, block construction can not compete with mass concrete in strength or cost, and can not be used where subjected to any considerable transverse stress.
There is no standard size of concrete building-blocks. The length iH 8, 16, 24, or 32 inches, the second or third being the most common; the height is 8 or 9 inches; and the thickness 8, 10, or 12 inches, according to the thickness of the wall desired. The smaller sizes are produced in the moulds for the larger sizes by inserting partitions or filling blocks. In mould ing sills and lintels for buildings, and ring-stones and stones for the parapet walls of concrete arches, much larger sizes than the above are made.