Positioning matters. The design of the structure is based on having the steel in the right place. Incorrect reinforcing steel placement can and has led to serious concrete structural failures. For example, lowering the top bars or raising the bottom bars by ½ inch more than that specified in a 6-inch-deep slab could reduce its load-carrying capacity by 20%.
Placing reinforcement atop a layer of fresh concrete and then pouring more on top is not an acceptable method for positioning. You must use reinforcing bar supports, which are made of steel wire, precast concrete, or plastic. Chairs and supports are available in various heights to support specific reinforcing bar sizes and positions. In general, plastic accessories are less expensive than metal supports. The Concrete Reinforcing Steel Institute’s Ready Reference Reinforcing Steel Resource Guide or the classic Placing Reinforcing Bars has three tables that show most of the currently available supports in the various materials and describing the situation where each is most effectively used.
