So far as possible, the work shall be carried on con tinuously—that is, with the least number of joints prac ticable; and in no case must the concreting be stopped within eighteen inches of the top of a wall.
Whenever it becomes necessary to allow one layer of concrete to set before the next one is placed, the exposed faces adjacent to the form shall be leveled off with a trowel or by setting in a straight plank so that the joint will be parallel to the cracks in the form. Back from the face, the top of the concrete should be left rough and irregular. Boulders, heaps of concrete, or depressions made by inserting timbers, should be left on this surface to insure a good bond.
Before the next layer of concrete is started, the form shall be thoroughly tightened to prevent overhanging joints; and the inside surface of the forms shall be cleaned. The old surface of concrete should be thorough ly wet, and a layer of mortar placed over it, before de positing additional concrete. Particular care should be taken to use mortar along the face so that no joint will be perceptible.
The footing and neatwork must be thoroughly bonded together in a manner similar to that described above for other horizontal joints.
Where it is necessary to stop placing concrete before any course is finished, use a bulkhead of boards at the stopping place, to obtain a vertical joint.
Where the laying of concrete is stopped on the slopes of abutment wings, a must be avoided by making a joint at right angles to the slope. This can be accomplished by using a plank properly placed, behind which the concrete must be thoroughly rammed.
Bridge-seats and all places requiring a bearing surface should be leveled off with a float, and troweled, to secure a sidewalk finish.
12. Freezing Weather. Only concrete that is laid in large masses should be placed in freezing weather, and no concrete work of any kind should be carried on when the temperature is lower than 15 degrees above zero, except where it is deposited in water or below the surface of the ground. Whenever concreting is done in freezing weather, the water, sand, and broken stone must be heated and all frost removed before they are used; and concrete, when deposited in the forms, must be protected from alternate freezing and thawing by some method ap proved by the Engineer-in-Charge.
13. Reinforced Concrete. Where concrete is placed in connection with reinforcing metal, care must be taken to locate accurately all bars in strict accordance with the plans, and they should be wired together in such a man ner as to avoid any possibility of their being displaced when the concrete is deposited.
The steel used shall be clean, and free from dirt, oil, paint, or injurious rust.
In general, no reinforcing bars larger than one inch in diameter will be used; and these may be bent cold, when necessary, about a radius of not less than six inches.
Reinforced concrete work should be as far as possible monolithic; but when joints are necessary, they shall be made at such points and in such a manner as to meet the approval of the Engineer-in-Charge.
In reinforced beam work, such as floor beams, floor slabs, or culvert covers, the bars should be first entirely surrounded with a cement mortar composed of one part cement to two parts sand, with the concrete deposited immediately afterwards.
14. Forms. The following sizes of lumber are to be used in the construction of forms for concrete masonry: