Concrete building blocks are formed by molding plastic mortar or concrete into shapes resembling blocks of stone used in building construction. The blocks are usually hollow, but certain special pieces, such as sills and lintels, are frequently made solid, and in other cases the blocks for the body of the wall are in themselves solid, but of such form as to make a hollow wall when laid.
The manufacture of such blocks for use in the construction of buildings has recently had such a remarkable development, winning for itself a place as a new industry, that we are wont to regard the method as a very recent invention. In fact, however, it is the result of a natural evolution in the application of concrete, although the present generation may justly claim credit for its development. The building of a concrete mass within a timber form, and the use of concrete for the backing of a wall laid in stone or brick, apparently suggested the idea of replacing the wood, which has to be removed, or the expensive stone facing, by a wall built up of concrete blocks which should form a part of the completed structure. To omit the concrete backing entirely and to make the blocks hollow for greater ease in setting, were short steps to the modern hollow concrete block.
A patent for a hollow concrete block to be used as a form for mass concrete was granted in England in 1850, and although the inventor contemplated filling the block with plastic mortar after it was in place, yet it shows the application of the idea. In 1866, 1868, and 1874 patents were granted in the United States to C. S. Hutchinson, T. J. Lowry, and T. B. Rhodes, respectively, on forms of hollow block which strongly resemble the blocks made under more recent patents and certainly involve the main principle of such construction. Although some of the companies furnishing concrete block molds make broad claims for their patents, the fact that suits for infringement do not appear to be vigorously pressed indicates a lack of faith in these claims. Although it is very questionable whether the general principle of making hollow concrete blocks is now covered by valid patents, certain special features of blocks and molds undoubtedly are so covered, and one who contemplates going into the business of manufacturing them should assure himself that the machine or method he pays for has not a lawsuit attachment. For a brief account of the main features covered by the patents of some of the best known types of block, the reader is referred to the paper by Mr. Wm. M. Torrance, published in " Engineering News " of Oct. 12, 1905, and " The Cement Age " of November, 1905.