Archive through November 11, 2014

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Author Message John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 718 Registered: 04-2002 Posted on Wednesday, October 29, 2014 - 04:04 pm:    I have recently taken on specs for hotels and perhaps (?!!) multi-unit residential projects. These project have "precast" ornamental elements incorporated into the exterior finish. I have been challenged to explain the differences and why expensive precast concrete ornaments must be used. Why not use the same spec as for parking bumpers and splashblocks? So, help, please. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1603 Registered: 03-2002 Posted on Wednesday, October 29, 2014 - 04:41 pm:    For one thing, if a splash block or bumper deteriorates, you just buy a new one and plop it down. Not so for precast elements incorporated into a masonry veneer, those are extremely expensive to replace once in place. These must last at least as long as the rest of the exterior enclosure, which is many times that of a parking bumper. You want the precast to last as long as the surrounding brick. After all, the envelope lasts only as long as the poorest quality component in it. In addition, there are probably structural loads imposed on these elements that don't exist for a bumper. Finally, aesthetics are important, too. You want uniform color, and texture, and you want it to stay that way for 50 or more years. J. Peter Jordan Senior Member Username: jpjordan Post Number: 780 Registered: 05-2004 Posted on Wednesday, October 29, 2014 - 07:57 pm:    It is difficult to add to Mr. Bunzick's succinct reasoning. The standards for the decorative stuff are indeed more rigorous. I would argue that GFRC is something else altogether, but it fundamentally isn't, just reinforced and fabricated to be thinner and lighter. Earlier this year, I got involved in developing specifications for repairs to a 45-year old building here in Houston that had a precast architectural concrete screen on the facade. What became apparent is that a good deal of the reinforcing in these screen members on the facade did not have adequate coverage so it had been deteriorating almost from the beginning with a lot of very poor patch and repair work done over the years. The fix is going to be very expensive, but since chunks of concrete are falling off the building, it has to be done. The current standards for architectural concrete take into account not only aesthetic quality, but also are more rigorous in the technical aspects of quality as well. Cast stone is basically architectural precast on steroids. GFRC is nice since you can get lots of neat stuff molded and doesn't have any steel to corrode. John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 720 Registered: 04-2002 Posted on Tuesday, November 11, 2014 - 01:53 am:    After considerable research and fussing to produce a new-to-me “prototype” specification for ornamental concrete fabrications, I produced Section 03 4919 – Glass Fiber Reinforced Concrete (GFRC) Fabrications. This Section supersedes Section 04 7200 – Cast Stone. I decided on using GFRC as the product and title because it fits best the product for ornamental “precast concrete”. To summarize what I have learned: The problem at its root is that the term "precast concrete is used too loosely." It's used like slang for several types of products that are cementitious and precast (not cast in place). There seem to be three products for “ornamental’ precast concrete: - Cast Stone: The concrete is a mix of fine sand and actual stone aggregate, so it produces an appearance close to carved natural stone, including limestone and sandstone. It is heavy. Think fireplace surrounds and mantels. - Glass Fiber Reinforced Concrete: Lighter than cast stone. It is manufactured by pneumatically placing a mix of portland cement, fine aggregates, additives and finely chopped glass fibers into a mold to a thickness of about 1”. After some curing of the glass fiber reinforced concrete (for handling) The mold is then filled with conventional concrete with small aggregates and embedded steel reinforcing and anchoring devices. - Coated Foam Fabrications: These are the lightest. Somethimes expanded polystyrene foam shapes are embedded in a GFRC fabrication to make it lighter. Often the foam shape is coated with a thin concrete (really a thick plaster) mix which is troweled and finished, yielding the desired “precast” shape. Since the surface is not cast against a mold, the finish is not as fine and details are not sharp as cast stone or GFRC. I'm sure there is more to say ... much more. But this is my simplistic understanding, now. I'd welcome comments and amplification on ornamental concrete fabrications. I think I'll continue to face this subject on future hospitality and residential projects. Robert E. Woodburn, AIA, CSI, CCS, CCCA Senior Member Username: bob_woodburn Post Number: 118 Registered: 11-2010 Posted on Tuesday, November 11, 2014 - 10:09 am:    I remember a sprawling 1970s steel-framed office building designed to have deep channel-shaped bay-long precast concrete spandrels. After construction was under way, the contractor realized that due to the building width and site constraints, there was no way they could position a crane with the reach and load capacity to lift them from the truck to some portions of the facade. So they were all redesigned in thin-shell precast, with a cold-formed framing structure covered with about 1-inch thick concrete. I believe the concrete shell was glass-fiber reinforced. It was not concrete-filled, so it was far lighter than the normal 4-inch thick precast shell originally intended, but it was too late to realize the savings that would have been gained by using a much lighter steel structure, which was still designed for the heavy precast. Good case for timely constructability reviews... Another related material is cast ornamental plaster, which would be the material of choice for intricate shapes, particularly for matching existing decorative features (such as sculptured terra cotta), using flexible silicone molds that can be made from almost anything. It's a specialty craft, an almost-lost art that has been revived by the new technology, but there are probably firms in most major cities that do it. Robert E. Woodburn, AIA, CSI, CCS, CCCA Senior Member Username: bob_woodburn Post Number: 119 Registered: 11-2010 Posted on Tuesday, November 11, 2014 - 10:17 am:    Come to think of it, I've always thought of GFRC as a thin-shell product, about 1 inch thick, max. Never heard of filling the rest of the mold with conventional concrete, or even very lightweight concrete. Is that done? What is gained by using the GFRC surface layer? Brett Scarfino (Unregistered Guest) Unregistered guest Posted on Tuesday, November 11, 2014 - 10:05 am:    See PCI MNL-128 (Recom. practice for GFRC) MNL-130 (QC for production of GFRC). A good resource for spec development. GFRC is sprayed into the molds. It was an interesting process to see. A large bundle of e-glass fiber is continuously fed to top of the spray gun, where a chopper cuts them to approx. 1.5-inch lengths...co-current with the concrete mix spray. Our design thickness was 3/4" total; it was surprisingly tough at this thickness. GFRC is typically anchored with bent rods, aka flex anchors. After the panels are sprayed up to the design thickness, the rods are embedded to the back of the panels with a large handful of reinforced mix. Rods are commonly welded to some sort of steel strong-back frame for handling/erection. The panels end up being mostly hollow and light relative to solid precast. Paul Sweet (Unregistered Guest) Unregistered guest Posted on Tuesday, November 11, 2014 - 01:01 pm:    I believe that cast stone is, or used to be, produced using a fairly dry mix that is pressed into the molds. Architectural precast concrete is more like structural or utility precast concrete, with colors added (pigments and/or specially selected sand & asggregate) and tighter controls on color and finish. All the GFRC I've seen is hollow on the back.