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November 2007

Cast Stone

Architectural Precast Concrete Vs. Cast Stone

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.

A common misconception is that cast stone is far different — and better — than architectural precast.

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 the product you specify, you are purchasing a pretender that, most likely, will not perform to your expectations.

Masonry Magazine

Checkerboard precast (above): This job depicts extremely poor quality control produced by a plant that was not certified by APA or PCI.

Right: This image depicts inconsistent color and texture of trim units that are result of poor quality control by the precaster (note the center piece of trim). The owner should have rejected this job.

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!

Another recent case-in-point happened in Arizona, when the lowest bid was 40 percent lower than the next highest price. What a deal, right? An informed masonry contractor stepped in and stopped the architect from accepting the low bid and, potentially, ruining the job.You see, the contractor had past experiences with the precaster in question. The quality of the precast was historically so poor that the mason feared it would crumble during installation and would be so dimensionally imperfect that he would have to trim many of the pieces to ensure a proper fit.

The contractor simply told the architect that his bid would substantially increase if the low-cost precast was accepted. So, how do you ever know if a great price will ensure exceptional quality, especially if you are unfamiliar with the precaster? It is just not feasible to inspect each precast plant and personally oversee production. Besides, there are others who do that for you. Read on.

Hire a certified plant
The first and most important part of the precaster selection process begins with job specifications. The specs should demand that the producer of your architectural precast be certified by a reputable organization. The Architectural Precast Association (APA) and the Precast/Prestressed Concrete Institute (PCI) provide certification programs, and certification is a prerequisite of membership. Both organizations conduct unannounced inspections of their members twice per year using registered professional engineers as inspectors. A comprehensive quality control (QC) manual is also required. If your potential supplier does not have an APA- or PCI-approved QC manual in place, my advice is to pass him by. Ask for the manual, and read it. By specifying an APA or PCI member, you have the guarantee that the producer of your precast has the ability to manufacture the highest quality precast, regardless of price.

Both examples of questionable quality described in this article were produced by companies that claimed to be certified. However, the announced inspections, which forewarned of the impending inspection, occurred only once every two years and no QC manual was required. A sample legitimate specification can be found at www.archprecast.org.

Architectural precast concrete and cast stone can last over a lifetime if produced properly. Don't let the names perplex you. They are equal in performance and finish. If the curing is done properly, and structural considerations are met, choose any name you wish. And, by specifying the proper plant certification program, many of your worries can be laid to rest, and price will cease to be a confounding issue.

  • Georgia Peach

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  • Georgia Peach



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