Economics and Use of Lightweight Concrete in Prestressed Structures

Abstract

•I n many structures it is economically advantageous to use lightweight concrete in place of normal weight concrete. This economy is generally reflected in the foundation design due to a 20 to 30 percent lighter dead load in the superstructure. The degree of savings varies greatly depending on: (a) Site conditions which dictate the type of superstructure design. (b) Availability and cost of lightweight aggregates. (c) Cost of high tensile steel (if the concrete is prestressed) which may be required to counteract the relatively large prestress losses in the lightweight concrete induced by creep and shrinkage. It should be noted that the properties of lightweight concrete vary significantly from place to place depending on the source and manufacture of the aggregate. However, once the design engineer is aware and familiar with the particular product, he can predict fairly accurately the creep and shrinkage characteristics of the lightweight concrete in the design of his structure. With this recognition, the design should be no more difficult than that for normal weight concrete. Therefore, it is good practice to investigate the use of lightweight concrete in the preliminary design stages of any structure. Two recent notable examples showing economy in cast-in-place structures built with lightweight concrete in America are the Napa River Bridge and the Parrotts Ferry Bridge, in California. The latter bridge has a center span of 640 11 (195 m). While lightweight concrete is often found economical in cast-in-place structures, it is found that structures employing precast elements are usually economical in lightweight concrete but for different reasons. Precast concrete structures may also