Masonry Magazine November 2007 Page. 37
Frustrated architects call my office almost weekly to ask the difference between architectural precast concrete and cast stone, having received confusing information that causes uncertainty regarding the kind of architectural precast to specify.
Myth:
A common misconception is that cast stone is far different and better than architectural precast.
Fact:
The truth is that cast stone and architectural precast are essentially the same.
Cast stone and architectural precast are comprised of cement, sand, water, pigment and additives to achieve a finish that simulates natural stone. If these mix components are not in this production provides the owner pretender that, most likely, will not perform to with a beautiful, lower cost structure your expectations. It is safe to say that cast stone is viewed as smaller, non-structural pieces that are hand set by masonry contractors. Architectural precast is thought of as large, expansive panels with structural properties that require a crane for installation. The durability, longevity and finish of both items should be of equal quality. If they aren't, the fabricator has cut some corners, and the owner will suffer. The architectural concrete manufacturing process does not create natural stone, which, judging from the phone calls I've received, may come as a great surprise to many. To the general public, simulated limestone or other finishes are viewed as natural stone cut from a quarry, but to the educated professional, the difference is evident. The factor most key to the purchase of natural or manufactured stone is price. The production of architectural precast and architectural cast stone is an art form. When done properly, this product provides the owner with a beautiful, lower cost structure that will maintain its aesthetic properties for many decades.
Wet cast versus dry tamp
THE TWO METHODS of producing genuine architectural precast products are the wet cast and vibra dry tamp methods. Wet cast is plastic or pourable concrete, the viscosity of which can be measured by a slump test. The finishes of wet cast are virtually unlimited, and its products can sustain structural characteristics. Dry tamp is, as the name implies, an extremely dry mix with no measurable slump that is placed in a form and compressed with pneumatic rammers. The finish of dry tamp is generally restricted to that of simulated limestone with limited structural properties. The pounds per square inch (psi) of architectural precast should be 5,000, and our specification calls for 6,500 psi for cast stone. Be sure you confirm this before you buy. Ask for proof. The fabricator should be keeping records. Aside from the physical properties and forming techniques, curing is the main difference between wet and dry production. Following pouring, wet cast usually sits indoors, overnight, in a climate-controlled environment and is stripped the next day. Test cylinders are made at the time of casting to ensure proper strength. Dry tamp concrete is immediately removed from the forms after compaction and moved to a curing room, where humidity levels are kept at nearly 100 percent for 24 or more hours. The absence of this curing method accelerates the hydration of the concrete, which severely compromises quality. You likely won't notice the poor quality until after it is in place for several months or after a few freeze/thaw cycles. Ask your dry tamper about his curing procedures.
Quality, price and your profit
A HIGHER PRICE does not necessarily guarantee the best quality, nor is the best price always the best deal. Let's face it: We all want to get the most for our labors, which equates to whatever the market will bear. But, as demand subsides (as is the case today), select precasters may be tempted to "scrimp" a little to become more competitive. For example, a precaster may use less cement or more sand; water-down mixes, ship before curing is adequately completed; or even leave out the reinforcing steel, as was recently the case in a job out West, in which the concrete was literally falling off the building. But, it was a great price!