Specifications for Design of Concrete Bridges

Mixing. Mixing of materials shall be done by ma chine whenever practicable, and always in batches. When mixed by hand, the cement and sand shall be first mixed dry, and then made into mortar by gradually adding clean water and hoeing or otherwise working until a uniform mixture is secured. The stone shall be spread upon a suitable floor to a depth of about 6 inches, and thoroughly wetted, and the mortar evenly spread over it. The whole mass shall then be turned over four times, and raked to secure complete and uniform mixture. If the Contractor desires to use some other method, he must submit it for approval. Concrete shall be kept in motion until deposited in place. Should any be permitted to set before placing or tamping, it shall be removed and not used. Hand-mixed batches shall not be larger than 1 cubic yard in volume.

Preparation of Surfaces.

Surfaces upon which con crete is to be deposited, whether of old masonry or of concrete layers, shall be thoroughly cleaned and wetted, and coated with a complete layer of cement mortar.

Depositing.

Concrete shall be deposited in layers not more than 6 inches in depth, and compacted by tamping if necessary until flushed. When used for foundations, it shall, where practicable, be deposited and rammed against the sides of the excavation. In the abutments of arches, the layers shall be sloped as nearly as possible normal to the direction of the line of thrust of the arch. In other abutments and retaining walls, the courses shall be sloped so as to shed water to the back. Steps at back shall be sloped from the wall 1 inch per foot to shed water.

When concrete is marked on the face into courses, each requiring two or more layers, the layers shall follow each other in close succession, before setting, so as to avoid visible joints in the face of a course.

Joints.

In work above ground, the top surface of each course shall be scraped within 24 hours, and swept to remove "laitance" or scum, and insure the adhesion of the next layer.

Embedded Stone.

In foundation concrete. approved hard flat stone equal to the best Conshohocken stone of one-man size or larger may be embedded in the concrete, the stones to be separated sufficiently to be each com pletely encircled by the concrete, and to be not closer than 2 inches to any face of the foundation. In neat work,

similar stones, but not larger than two-man size, shall be embedded in each course so as to project up and form dowels bonding the courses together; and no such stone with flat vertical surface shall be closer than 6 inches to the face of the work.

All

embedded stone shall have the concrete deposited first, and shall then be embedded by joggling the stone down in the concrete.

Protection. Finished concrete work shall be protected against disturbance until thoroughly hardened, by pre venting working or building upon it; and against the sun, if necessary, by covering and wetting for at least 48 hours.

Granolithic Surfaces. All showing faces of concrete work shall have a granolithie surface. Granolithic sur facing shall be composed of 1 part cement, 1 part coarse sand or gravel, parts granolithie grit, made into a stiff mortar. Granolithic grit shall be granite or trap rock crushed to pass a IA-inch sieve, and screened of dust. For vertical surfaces the mixture shall be deposited against the face forms to a least thickness of 1 inch, by skilled workmen, as the placing of the concrete proceeds, and thus form a part of the body of the work. Care must be taken to prevent the occurrence of air-spaces or voids in the surface. The face forms shall be removed as soon as the concrete has sufficiently hardened, and any voids that may appear shall be filled with the mixture. The surface shall then be immediately washed with water un til the grit is exposed and rinsed clean, and protected from the sun and kept moist for 3 days. For bridge seat courses and other horizontal surfaces, the granolithic mixture shall be deposited on the concrete to a least thick ness of inches, immediately after the concrete has been tamped and before it has set, and shall be troweled to an even surface; and, after it has set sufficiently hard, shall be washed until the grit is exposed.