The materials and proportions used in the manufacture of concrete blocks vary so widely that it is difficult to give any general statements concerning their strength. With a knowledge of the proportions and ingredients, Chapters XVI and XVII will give a good idea of the strength to be expected. On page 304, tests of 12-inch cubes of mortar containing 4 volumes sand to one cement showed a compressive strength of 1,500 to 1,700 pounds per square inch at 20 months. At 28 days the strength should be at least one-half of this, or, say, 800 pounds per square inch. These specimens were made of so-called " dry " mortar, the consistency of damp earth, well tamped into molds. For a part of the specimens the storage was similar to the curing of blocks. The specimens are therefore comparable with concrete blocks of the same proportions. The tests on pages 306 and 307 show that concrete made with mortar containing 3 parts sand to one cement, the voids in stone being filled, has a strength of 1,800 to 2,000 pounds per square inch. A wall of concrete 100 feet high would give a load of 100 pounds per square inch. If in a building four stories high, ten square feet of space on each floor, loaded with 60 pounds per square foot, were carried by each linear foot of wall four inches thick, the resulting load on the concrete would be about 100 pounds per square inch.
Walls of concrete blocks properly made and laid have little to fear from direct crushing by a fairly applied load. Many blocks, however, are made with more than four parts sand to one cement, and the quality of the materials is frequently not good. Some of the greatest sources of weakness, however, are poor design, whereby the load from girders and beams is applied eccentrically, and carelessness in laying resulting in lack of uniform bearing on bed joints. The thickness of the shells of concrete blocks might be made less so far as the strength of the block itself is concerned, but with less than a two-inch shell the danger of poor bed joints is too great. Improper laying may subject a block to transverse stress, and on this account, as well as because the cross-bending test is very easy to make, the transverse strength is sometimes specified in building regulations.
The rapid development of the concrete block industry has made it necessary for some of the more progressive cities to frame building regulations governing the use of this material. Denver, Minneapolis, Chicago, and Newark have adopted such regulations, but those of Philadelphia are very complete and are reproduced below.
1. Hollow concrete building blocks may be used for building six stories or less in height, where said use is approved by the Bureau of Building Inspection, provided, however, that such blocks shall be composed of at least one (1) part of standard Portland cement and not to exceed five (5) parts of clean, coarse, sharp sand or gravel, or a mixture of at least one part of Portland cement to five (5) parts of crushed rock or other suitable aggregate. Provided further that this section shall not permit the use of hollow blocks in party walls. Said party walls must be built solid.
2. All material to be of such fineness as to pass a half-inch ring and be free from dirt or foreign matter. The material composing such blocks shall be properly mixed and manipulated, and the hollow space in said blocks shall not exceed the percentage given in the following table for different height walls, and in no case shall the walls or webs of the block be less in thickness than one-fourth of the height. The figures given in the table represent the percentage of such hollow space for different height walls.
1 and 2.......
3 and 4.......
5 and 6.......
3. The thickness for walls for any building where hollow concrete blocks are used shall not be less than is required by law for brick walls.
4. Where the face only is of hollow concrete building block and the backing is of brick, the facing of hollow concrete blocks must be strongly bonded to the brick either with headers projecting four inches into the brickwork, every fourth course being a heading course, or with approved ties, no brick backing to be less than eight inches. Where the walls are made entirely of hollow concrete blocks, but where said blocks have not the same width as the wall, every fifth course shall extend through the wall, forming a secure bond. All walls where blocks are used shall be laid up in Portland cement mortar.
6. Wherever girders or joists rest upon walls so that there is a concentrated load on the block of over two (2) tons, the blocks supporting the girder or joists must be made solid. Where such concentrated load shall exceed five (5) tons the blocks for two (2) courses below, and for a distance extending at least eighteen (18) inches each side of said girder, shall be made solid. Where the load on the wall from the girder exceeds five (5) tons, the blocks for three (3) courses underneath it shall be made solid with similar material as in the blocks. Wherever walls are decreased in thickness the top course of the thicker wall to be made solid.
7. Provided always that no wall or any part thereof composed of hollow concrete blocks shall be loaded to an excess of eight (8) tons per superficial foot of the area of such blocks, including the weight of the wall, and no blocks shall be used that have an average crushing strength less than 1,000 pounds per square inch of area at the age of twenty-eight days, no deduction to be made in figuring the area for the hollow spaces.
8. All piers and buttresses that support loads in excess of five (5) tons shall be built of solid concrete blocks for such distance below as may be required by the Bureau of Building Inspection. Concrete lintels and sills shall be reinforced by iron or steel rods in a manner satisfactory to the Bureau of Building Inspection, and any lintels spanning over four feet six inches in the clear shall rest on solid concrete blocks.