Well, they're not building any more land onto Manhattan island, so ultra-expensive real estate must look toward the heavens. Thanks to high-strength concrete, residential towers in New York City are not only growing taller but slimmer as well. One challenge in building the ultra-slender towers is making sure its "structure and stiffness" are not sacrificed. The use of high-strength concrete is key to building these towers, according to Silvian Marcus, the director of building structures at WSP Architectural Record magazine. According to Marcus, high-strength concrete (around 14,000 psi) is essential in slender towers. Concrete mixes have improved a great deal in recent years, he says, with additives enhancing workability. Fly ash, a byproduct of coal combustion, is increasingly used to replace a large portion of Portland cement in these mixes, because it offers strength without the substantial heat normally generated during cement-curing, which can cause cracking.