MOLDING CONCRETE CHIMNEYS, SLATE AND ROOF. TILES So much has been written on the merits of concrete chimneys and roofs of monolithic concrete, as well as tiles of the same building material, that a discussion of the advantages would not be of interest to the average reader, aside from the methods of construction that are necessary to secure success. REQUISITES OF CHIMNEY CONSTRUCTION The foundation of every concrete chimney should be ample to withstand the load with a factor of safety of not less than four; for while this precaution may be considered needless by many, yet a large percentage of the failures in concrete chimneys may be attributed to defects in an insecure foundation. The foundation should extend below frost level, and always rest upon a firm foundation soil or rock if possible. The foundation should be amply reinforced and the vertical reinforcing rods placed within the wall of chimney should be tied or anchored to the steel imbedded into the concrete, thus bringing the load or "pull" more directly upon the foundation. 14 MOLDING CONCRETE CHIMNEYS The concrete that is placed in foundation should be as carefully mixed and placed as in any portion of the work, for the reason that the weight or compression stress placed upon the foundation is but a part of the load it must withstand. The concrete employed should be mixed wet, as a dry or semi-dry mixture will not give the same strong adhesion or bond to the reinforcing steel as is secured by a wet mixture. The proportions should not be leaner than 1: 2: 4 of cement, well graded sand and crushed stone of " in diameter. Where an aggregate of sand alone is employed this should be mixed 1:3, using a sharp, clean, and well-graded sand. In ascertaining the wind pressure upon the chimney, the maximum velocity of one hundred miles per hour should be used; as it is safe to say that it will not be exceeded. This is given as forty pounds per square foot against a vertical plane, with one-half the pressure against a cylindrical surface; thus in reinforcing to withstand a load of forty to fifty pounds to the square foot of surface, we are precluding any danger from the most violent wind load. It must also be considered that in the construction, the height prevents accurate placing of the reinforcing and concrete to a great extent, hence any slight imperfections of that kind are remedied by the ample reinforcing. Enough of the vertical reinforcement should be provided so that it will amply withstand the pull without exceeding fifteen thousand pounds tension to each square inch of the reinforcing steel. The walls of the chimney should be designed so that the compressive strength is ample to withstand the load with a factor of safety of not less than 4; thus in estimating the weight of the concrete at one hundred and fifty pounds to the cubic foot and the compressive strength at two thousand four hundred pounds to the square inch, we have a safe unit value in using six hundred pounds as the compressive strength of each square inch. The horizontal rods should be small in size and placed closely together, to provide for the vertical shear as well as the expansion caused by the heat, when the chimney is in use. A safe unit for the shearing strength of the concrete is sixty pounds to the square inch. The concrete should be well bonded at all points where the layers or courses join. The reinforcing material should be as accurately placed as conditions will permit, and guides erected so that the displacement of the steel will not be possible when the concrete is placed. The concrete should be placed with care around the reinforcing material, so that it will bond securely to same; which can be insured if a wet mixture is employed and this is well tamped at all points. Where the thickness of the wall is abruptly changed, special reinforcement should be used; also such points in the walls as will be subjected to very marked changes of temperature. In erecting forms for chimney construction every |