This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Figs. 9 and 10 show the concrete base a, and stepped-up brick footing courses b. In Fig. 9, each course of brickwork sets back 1 1/2 inches for each course, while in Fig. 10 the courses are set back 3 inches for each two courses. At c is shown a 20-inch brick foundation wall resting on the stepped-up brick footing.
44. Fig. 11 illustrates stone footings a, composed of three courses of flat stone, each course being 8 inches thick. The top course has a projection of 6 inches on each side of the 20-inch brick foundation wall b, and the middle and bottom courses each project 3 inches, making the width of the bottom stone 3 feet 8 inches.
Fig. 12 shows a stepped -stone footing a, similar to those shown in Fig. 11, but supporting a 24-inch foundation wall. Each base course advances in stages of 3 inches.
Fig. 13 shows a footing consisting of a single course of stone a, 8 inches thick and 2 feet 4 inches wide, carrying the stone wall b, 20 inches thick.
Concrete, for footings, should be made of 1 part good cement, 3 parts clean, sharp sand, and 5 parts sharp, broken stone. In very important work, such as bridge piers, and the footings of very high buildings, chimneys, etc., a proportion of 1 of cement, 2 of sand, and 4 of broken stone is generally used. The New York building laws call for 1 of cement, 3 of sand, and 5 of broken stone.
None of the stone used in making concrete should be larger than will go through a 2-inch ring. In localities where stone cannot readily be obtained, broken brick or terra cotta may be used in the same proportion as stone, taking care to use good hard-burned material.
Well broken foundry slag and scoriae, steam-boiler ashes from anthracite coal, and clean-washed gravel, mixed in the proportions given, make good concrete, though gravel, being rounded and smooth stone, does not adhere to the mortar as well as broken stone, slag, brick, or scoriae does.
46. In preparing concrete, the material should be worked on a platform of boards, with sides about 10 inches high, battened on the back and laid on the ground near the work. The platform is necessary in order that no loam or clay may contaminate the concrete, the effect of this being a loss of strength in the concrete, as the clay adheres to the stone and prevents close contact with the mortar. The sand and cement should first be thoroughly mixed by shoveling them together while dry, at least twice, so that there will not be an unequal proportion of sand to cement in different parts of the heap. The broken stone, or whatever material is used for the aggregate (as the stone, slag, or other coarse material is called), should then be added, the mixture being kept wet all the time and thoroughly shoveled together, so that every portion of the stone or other material may be perfectly coated with the mortar.
When extensive works are carried on, the concrete is often mixed by the wet, or machine process. The cement, sand, and broken stone, or other aggregate, are placed in a cylinder of iron, or wood lined with iron, containing a long augerlike screw, laid longitudinally, over which a perforated pipe sprinkles water into the trough, or mixer. By the revolution of the screw, the material is thoroughly mixed.
No concrete should be made unless it is to be used at once, because the cement, forming its most essential part, sets or hardens quickly, and if it sets before being placed in the footing trenches, it is valueless.
47. As soon as the concrete is thoroughly mixed, it should be conveyed to the footing trenches and put down in layers of from 6 to 8 inches thick. As each layer is put down, the concrete should be well rammed with a wooden rammer, until the cement flushes, or shows on top of each layer. This method causes the different layers to unite, and make one solid, homogeneous mass, and is preferable to throwing the concrete from a platform into the trenches, as by the latter method the concrete does not become consolidated.
Should one layer have become partly set before another layer is put down, the concrete should be swept clean, scratched with a rake, and well wet before the next layer is put in place.
Sometimes it becomes necessary to lay concrete in running water, and unless some means is devised to protect it during the laying, the water will wash the cement away from the concrete, and weaken it. By making large bags of oiled cotton and filling them with concrete, and then lowering them into the excavation, the concrete will set before the water can wash the cement out.
48. As before stated, qiiicksand, when confined, can be safely built upon. Fig. 14 shows a method of confining quicksand by sheet piling, and placing concrete between the piling. The sheet piling shown at a is placed, in this case, 4 feet apart; the concrete shown at b is 2 feet thick and extends the full width of the piling; the quicksand, through which the sheet piling is driven, is shown at c, and the 20-inch brick foundation wall, at d.
49. Fig. 15 gives an example of a footing composed partly of timber. This was placed near the water line of a marsh in New York state, to carry a large factory building 50 ft.x 80 ft. and 40 ft. high. The soil is a stiff, black muck, and at a depth of about 5 feet, water-soaked sand was found. After the trenches were dug a bedding of concrete a, 12 inches thick, was laid. On top of this, 2-inch spruce plank b were placed crosswise, followed by 8"x 8" timber c, laid parallel with the trenches, filled in between with concrete. On this are laid the base stones d, and on top of these is built a 20-inch foundation wall c. The trenches on each side of the wall were filled in with sand, rammed down, as shown at f.
The factory has an engine, shafting, boiler, and machinery, and over one hundred operatives are employed. No settlement has occurred, though the factory has been built several years.
50. Stone-footing courses should be laid with large flat stones not less than 8 inches thick. If more than one course is laid, as shown in Figs. 11 and 12, the joints should never come over each other, as this would defeat the object of bonding, which is to firmly tie together the parts of the wall.
All stone footings should lie on their natural, or quarry beds, and all the joints and spaces between the stone must be well filled with mortar, because the mortar acts as a bedding between the stone, and unless this were interposed, the uneven pressure of one stone on another might cause a fracture of the lower one and produce settlement.
51. All footing courses, as indeed all mason work below ground level, should be laid in' cement mortar, although in dry, well drained soil, lime and cement mortar may be used. The usual proportion of cement and sand for cement mortar is 1 part of cement and 3 parts of sand, and should be used immediately after being mixed. The proportions of cement and lime mortar are 1 of cement, 1 of lime, and 3 of sand. The above proportions are those given in the building laws of New York, Chicago, and Boston, and have been found to be suitable for general mason work.
52. Stepped-up brick footings are often used, having concrete and stone bases, as shown in Figs. 9 and 10. The pyramidal form of stepped-up brickwork carries the load of the superstructure more evenly to the footings and reduces the risk of settlement or fracture. This form of footing is used very extensively for piers supporting iron columns. Nothing but good, hard, well burned brick should be used; and they should be laid in cement mortar, and should break joints - that is, no two joints should come over each other.