I would like some technical information pertaining to sidewalks, particularly with regard to monolithic cast and placing walks in two lifts-a base of 4 inches and a topping of 2 inches. In the event there is a need to repair a two-lift installation, what are the merits of breaking out the topping and replacing it? If replacing the topping is a correct procedure, what precautions must be taken to achieve bond and what mix characteristics give the best results?
In general, monolithic sidewalk construction is more economical than a two-course job. A satisfactory mix should contain 6 sacks cement per cubic yard concrete (1-2 1/2- 3 1/2); fine and coarse aggregate of 1 inch maximum size; 4 to 6 percent entrained air and no more water than necessary to make a workable concrete having a slump of 2 to 4 inches. The concrete shall be well compacted on the properly prepared base. A wood-trowel finish will result in a non-slip surface. Final finishing operations should be delayed until the concrete has stiffened somewhat (no free water appearing under thumb pressure). Very important is the protection of the freshly placed concrete against drying out. The concrete must be kept wet at least 3 days. Control joints at 20 foot spacings are suggested. Two-course job: The surface of the base concrete must be clean and free from loose particles, dust or stains. It should be reasonably rough in order to obtain a good bond between topping and base. The topping mix must contain coarse aggregate of a size not exceeding 1/3 of the topping thickness. The mix shall be 1:1 1/2:2 1/2 air-entrained concrete. Before applying topping to an old, dry base, the base concrete must be wetted (no free water on the surface) and placing of topping preceded by applying a slush coat of a 1:1 cement-sand mix having the consistency of thick cream. Placing, finishing and curing are the same as for monolithic concrete.