Concrete high-rise buildings offer lower costs over the entire life of the structure. In comparison with other structural systems, site-cast concrete buildings can be more economical to finance, construct, operate, and maintain. The concrete advantage starts at the time of the notice to proceed. Ready mixed concrete and reinforcement--and craftsmen skilled in their use--are readily available on short notice in most parts of the United States. So construction starts promptly. Construction time also is often shorter with concrete than with steel frames. High-strength concrete and post-tensioning permit longer spans and fewer and smaller columns, leaving more useable floor space. The greater mass of a concrete building is an advantage in heating and cooling. Because concrete exteriors are inherently resistant to weather, temperature, and chemical attack, maintenance costs are low.