Voided "Two-Way" Flat Slabs

Abstract

Voided slabs have been used in construction of reinforced concrete floor systems for over 100 years. This article offers an historical perspective on this technology. A discussion of early applications thru several case studies is followed by on overview of traditional systems still in use and concludes with a discussion on new technological advancements. The improvements on this “old” concept have taken the efficiency of voided slab systems to the next level with economical spans reaching 50 feet. In voided two-way slabs, a hard shell with struts is created through the use of appropriately located cavities formed by void formers. This is achieved by placing hollow spheres made of recycled plastic between the upper and lower static reinforcement of the concrete slab while displacing concrete where it has no structural benefit. The effect is to decrease the overall weight by as much as 35% when compared to a solid slab of the same bearing capacity. This paper describes traditional two-way slab systems as well as new technological improvements on the design and construction of two-way slab systems. WAFFLE JOIST SYSTEM A two-way joist system, or waffle slab, is composed of evenly spaced concrete joists spanning in both directions and a reinforced concrete slab cast integrally with the joists. The voided floor system is formed with domes that are 30 in., 41 in., and 52 in. wide, resulting in 3-ft, 4-ft, and 5ft modules, respectively. Systems with 3-ft and 5-ft modules are considered in this publication. See Figure 1. The main advantages of waffle slabs are that they are economical for long spans with heavy loads, the dome voids reduce the dead load, and electrical fixtures can be placed in the voids. Additionally, waffle systems provide an attractive ceiling, which results in cost savings, since no additional architectural finishes are required. Two-way joist systems are commonly used in office buildings, warehouses, and industrial plants. Approximately 60% of the total floor system cost can be attributed to the formwork. The concrete material, placing, and finishing costs are about 25%, and the material and placing costs of the mild reinforcement are 15%. (REF. 1)