Denver-based High Concrete Group LLC has been awarded a contract from Boonton, N.J.-based Phelps Construction Group for insulated architectural precast panels for the new Statue of Liberty Museum on Liberty Island in New York Harbor.
1. What is the most effective way to handle wind and seismic loads in a precast parking structure?
For most structures, shear walls are a better solution than moment frames. Shear walls can be provided with little or no cost impact to a structure, since these walls can also be designed to carry gravity loads, thus replacing beams and columns. An example of this is an interior "light" wall, which is a cost-effective way to support tees on a ramp, but also, the "light" wall in an effective shear wall to resist loads in its long direction. The "light" wall has an added benefit as well in that it is more open than a system of columns and crossing spandrels.
2. What is the best method to ensure longevity of a precast parking structure?
The most effective way is to ensure that the deck members are cast with good quality concrete. A low water-cement ratio is extremely critical in providing a long lasting durable parking deck. We typically recommend a water-cement ratio of .36 or less. This is a more effective method than use of corrosion inhibitors. Galvanized or epoxy coated reinforcing is not recommended, and may, in fact, reduce the service life of a garage.
3. Can tees be warped to improve drainage on a parking structure?
Yes, we typically warp tees by 3/16" per foot of width for a 60' long tee. Tees that are warped by more than this can crack; transverse tensioning is being used in some garages where this criteria needs to be exceeded.
4. What is the maximum length you would recommend for a garage without an expansion joint?
Care should be taken in keeping the stiff elements away from the extremities of a garage, but when this is done, it is possible to build a garage up to 325 feet in length without the need for an expansion joint. Attention must also be given to diaphragm loads, and the spacing of shear walls.
5. What floor to floor height is recommended in a parking structure?
When the overall height of the structure is not a problem, we recommend floor to floor heights of 10' - 4" as a minimum. This will provide a clear height of 7' - 6" for each level within the structure, and will help in making the garage feel more open as well as improving the lighting and the visibility of signage. Additional clearance is sometimes required to meet ADA requirements.
6. Can curved panels, or unusually shaped panel profiles be created economically?
Yes, but only if shapes are repetitive. It costs considerably more to create an original mold (or form) for complex shapes/profiles. However, if 20 to 30 castings are made from a single mold, the cost is amortized over all of the castings, reducing the premium to a nominal amount per casting.
7. What are the keys to color uniformity?
There are numerous ways in which precast color uniformity can be enhanced.
Use a retarded finish. This exposes the coarse aggregate, permitting the coarse aggregate's uniform natural color to "carry" the panel's color. In doing so, be certain to use a color compatible matrix (sand/cement/pigment), in order to mask any uneven coarse aggregate dispersion.
Use white cement instead of gray cement whenever possible. White cement's color control is excellent. Gray cement manufacturers do not attempt to control color. Thus, gray cement color can vary widely even within a single SUPPLIER, causing significant precast color variations.
Remove a sufficient amount of the as-cast concrete's surface. All concrete is blotchy when left as-cast. The surface "paste" or "skin" must be removed in order to reveal the true concrete color. For example, light acid etching is apt to result in a blotchy or shaded appearance because the acid does not remove all of the surface "paste" or "skin". Therefore, acid etched finishes should be deep enough to reveal the tips of the coarse aggregate. Likewise, lightly sandblasted finishes should reveal some coarse aggregate, in order to appear reasonably uniform.
Avoid large planes of smooth, uninterrupted surfaces. If the eye has nothing to focus upon except large expanses of plain, smooth concrete surfaces, it will perceive minute panel to panel color differences. If, instead, such large surfaces are interrupted by rusticated joint patterns, plane changes, and/or mix/finish changes, the eye will be drawn to these features making slight color variations, panel to panel, almost indistinguishable and unimportant.
Avoid using pigments in very small dosages. Pigment content, as a percentage of the total ingredients in a concrete batch is very small. As that percentage diminishes, the likelihood of increased pigment content variation, batch to batch, goes up dramatically causing noticeable color variation.
8. Will using multiple mixes and/or finishes in a single project increase my precast cost dramatically?
Multiple mixes will increase cost. Consult your High representatives before finalizing your design, because some mix and/or finish combinations require more additional labor than others.
9. Should I avoid creating small precast pieces, or, instead, make them part of larger pieces where possible?
Small precast pieces usually cost much more per square foot than large pieces (pieces 100 sq. ft. or larger) because handling costs (i.e., stripping, finishing, yarding & loading, hoisting and connections) are similar for small and large pieces. The cost for handling small pieces is distributed over fewer square feet per piece than in large panels so that the handling cost per square foot for small panels is greater than for large panels. Where possible, combine small pieces with adjacent precast pieces.
10. What should I do to insure the look I want at an affordable cost?
The most important thing you can do is call 1.800.PRECAST to involve your HCG representative as early as possible in the development of your project's architectural precast application. Our sales representatives are trained to help designers maximize the value of their precast design. For example, how a precast exterior is panelized can affect cost significantly with little or no difference in appearance. Also, a designer can employ important features such as reveals, rustication joints, medallions, form liner patterns, etc. at very little additional cost, but only if such features are used repeatedly (avoiding costly, frequent form changes). It's important to know what design techniques to use and when to use them. Contact your High sales representative.
11. How important is PCI certification for me as a designer?
We are certain that you, as well as your client, expect your building's exterior to become the high quality realization of your design's intent. And the best assurance that your project will be a high quality result is to rely upon the industry's only truly independent certification program - PCI's plant certification program.
12. Why should I design with architectural precast, and what makes it my best option?
There are four (4) compelling reasons to choose architectural precast concrete for your building's exterior:
1. Architectural precast is the only cladding material that permits you, the designer, to custom design shape, color, texture and pattern - only you control your building's unique, custom exterior look.
2. Architectural precast enables you to enclose your building's exterior in a small fraction of the time versus most all other cladding materials - this reduces construction cost dramatically, and provides your client much earlier occupancy.
3. Proven low initial cost. Architectural precast concrete is used widely by office building developers nationwide. They tell us it is their cladding material of choice, because it offers a high quality "look" at a very economical price.
4. Long term maintenance is almost negligible. All you need to do is re-caulk the precast joints after 15 to 20 years.
13. Can architectural precast be used as a load bearing element?
Certainly! We encourage you to do so. For a minimal additional cost (some additional reinforcing and minor additional connection cost) architectural precast concrete can become a terrific load bearing element. Call 1.800.PRECASTor contact your HCG sales representative to determine how to gain this additional benefit.