luffymuromnew
Joined: 19 Feb 2012
Posts: 13
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 Posted: Sun Feb 19, 2012 3:37 am Post subject: Stone Veneer-Faced Precast |
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Natural stone has been used widely in building construction for centuries due to its strength,
durability, aesthetic effect, availability and inherent low costs for maintenance. In the 1960s,
the practice of facing skeleton-frame structures with large prefabricated concrete components
to decrease construction time and reduce costs resulted in a combination of the rich beauty of
natural stone veneer and the strength, versatility and economy of precast concrete.
Stone veneer-faced precast concrete panels offer many benefits. These include:
1. Veneer stock can be used in thinner sections because anchoring points may be placed
closer together.
2. Multiplane units such as column
covers, spandrels with integral soffit
and sill sections, deep reveal window
frames, inside and outside corners,
projections and setbacks, and parapet
sections are more economically
assembled as veneer units on
precast concrete panels (Fig. 1).
3. Precast concrete backup systems
permit faster enclosure, allowing
earlier work by other trades and subsequent earlier occupancy, because each of the
larger panels incorporate a number of veneer pieces.
4. Veneered precast concrete panels can be used to span column-to-column, thereby
reducing floor-edge loading and eliminating elaborate temporary scaffolding.
The purchaser of the stone should appoint a qualified individual to be responsible for
coordination. This person should oversee delivery and scheduling responsibility and should
ensure acceptable color uniformity. Color control or blending of the stone veneer should take
place at the stone fabricator’s plant, where ranges of color and shade, finishes and markings
such as veining, seams and intrusions are viewed most easily. Acceptable stone color should
be judged for an entire building elevation rather than as individual panels. The responsibility for
stone coordination should be written into the specifications so its cost can be bid. The owner,
architect and precaster should visit the stone fabricator’s plant to view the stone veneer and
establish criteria and methods for color range blending on the project.
Separate subcontracts and advance awards often occur in projects with stone-veneered
panels. While these procedures may affect normal submission routines, it is not intended that
responsibilities for accuracy should be transferred or reassigned. The precaster is responsible
for precast concrete details and dimensions, while the stone-veneer fabricator is responsible
for stone details, dimensions and drilling of anchor holes.
Stone Veneer-Faced Precast
Offers Cost Efficiencies
PCI’s Architectural
Precast Concrete
Services Committee
highlights how precast
can achieve the same
goals as stone façades
General Considerations
State Office Tower II, in
Columbus, Ohio, has 3 cm of
granite on 5- and 7-inch-thick
precast concrete backup.
(Architects: Bohm-NBBJ)
Fig. 1 Typical spandrel and column cover panels.
Reinforcing
Steel
Reinforcing
Steel
Stone
Veneer
Bond
Breaker4 1
The production of stone veneer panels requires adequate lead time in order to avoid
construction delays. Therefore, it is important that approvals for shop drawings are obtained
expeditiously. Furthermore, it is recommended that the designer allow the submission of shop
drawings in predetermined stages so manufacturing can begin as soon as possible and ensure
there is a steady and timely flow of approved information to allow uninterrupted fabrication.
The precast concrete producer must provide the stone quantity and sequence requirements
to meet the erection sequences. The precaster and stone fabricator should coordinate
packaging requirements to minimize handling and breakage. Extra stone (approximately 2 to 5
percent) should be supplied to allow immediate replacement of damaged stone pieces,
particularly if the stone is not supplied from a domestic source.
Because of the difference in material properties between natural stone and concrete,
veneered panels are more susceptible to bowing than all-concrete units. However, panel
manufacturers have developed design and production procedures to minimize bowing.
The panel manufacturer and designer should consider the following in design and production
in order to minimize or eliminate panel bowing:
1. The temperature differential (exterior to interior).
2. Coefficients of expansion of the materials.
3. Ratio of cross-sectional areas of the materials and their moduli of elasticity.
4. Amount, location and type of reinforcement in the concrete panel.
5. The use of prestressing.
6. Type and location of connections to the structure.
7. Rigidity of connection between stone veneer and concrete backup (too rigid
may cause problems).
8. Shrinkage of the concrete.
Minimum thickness of backup concrete on flat panels that will control bowing or warping is
usually 5 to 6 inches, but 4 inches has been used where the panel is small, or it has adequate
rigidity obtained through panels shape or thickness of natural stone. See Fig. 2 for mold
considerations. Cover depth of reinforcement must be a minimum of 3/4 inch at the veneer
surface. Non-corrosive spacers such as plastic should maintain this cover.
The strength of natural stone depends on several factors: the size, rift and cleavage of
crystals, the degree of cohesion, the interlocking geometry of crystals, the nature of natural
cementing materials present and the type of crystal. The stone’s properties will vary with the
locality from which it is quarried. Therefore, it is important that current testing is performed
for stone quarried for a specific project.
Sedimentary and metamorphic rocks, such as limestone and marble, will exhibit different
strengths when measured parallel and perpendicular to their original bedding planes. Igneous
German limestone is anchored
to precast panels on the Terry
Sanford Institute of Public
Policy at Duke University in
Durham, N.C. (Architect:
Architectural Resources
Cambridge Inc.)
Stone Strength4 2
rocks, such as granite, may
or may not exhibit relatively
uniform strength characteristics on the various
planes. In addition, the
surface finish, freezing and
thawing, and large temperature fluctuations will affect
the strength and in turn
influence the anchorage
system.
Information on the
durability of the specified
stone should be obtained
through current testing in
conjunction with observations of existing installations of that particular stone. This information should include such factors as tendency to
warp, reaction to weathering forces, resistance to chemical pollutants, resistance to chemical
reaction from adjacent materials and reduction in strength from the effects of weathering or
wetting and drying.
Prior to awarding the precast concrete contract, tests should be performed to determine
the physical properties of the stone being considered. The testing should be done on stone
with the same finish and thickness to be used on the structure. Flexural tests (ASTM C880)
should be used to evaluate the physical properties and obtain design values. Absorption
testing (ASTM C97) helps evaluate freeze-thaw durability. These properties, along with
properties of the anchor system, should be used to ensure adequate strength of the panel
to resist loads during handling, transportation, erection and in-service conditions.
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