Structural precast concrete handbook
Precast concrete wall systems are most often constructed as a curtain wall or veneer, in which no building loads are transferred to the concrete panels. Most typically the precast concrete wall system must resist lateral loads directly imparted on it, such as from wind and earthquake ; as well as vertical loads resulting from the self weight of the precast wall system. These loads must be transmitted through the wall system and secondary structural elements to the building's structure.
Other loads such as erection, impact, construction related, and transportation must also be taken into account in the design. It is important to evaluate the design, detailing and erection of precast panels in order to avoid imposing unwanted loads onto the panels.torosuk.com/libraries/2020-06-05/cur-iphone-6.php
Concrete Construction Engineering Handbook - CRC Press Book
Steel elements of a wall system are designed in accordance with AISC specifications for steel construction. Joints between panels must be wide enough to accommodate thermal expansion and differential movements between panels. Joints between panels are most commonly sealed with sealant to prevent water penetration in the wall cavity.
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The wall cavity space and back up wall which is usually covered with a water resistant membrane provide a secondary line of protection against water penetration into the building. Precast wall panels derive their thermal performance characteristics primarily from the amount of insulation placed in the cavity or within the backup wall, which is commonly a metal stud wall in commercial construction.
Precast Concrete Structures - Hubert Bachmann
The most common moisture protection system used with precast concrete wall systems is a barrier system incorporating an adequate joint seal. In some cases where additional moisture protection is needed, the application of a sealer or a concrete coating is also used. Sealers can be either clear or pigmented if used as an enhancement of the precast appearance. Film-forming coatings usually offer a higher level of performance but will have a significant impact on the appearance of the precast concrete unit.
Precast Design & Engineering Handbook
The precast concrete panel should also be designed to provide the appropriate level of durability for the planned exposure. Durability can be improved by specifying minimum compressive strengths, maximum water to cement ratios, and an appropriate range of entrained air. Precast concrete wall systems are not considered to provide any improvement in fire safety over cast-in-place concrete. In fact, for high-rise buildings precast concrete panels can pose a serious safety hazard when a fire occurs that damages the panel connections and causes a panel to then fall from the building.
See Cast-In-Place Concrete Wall Systems for additional information, as well as the information included under Resources in this section. A precast concrete wall system and cast-in-place facade will provide similar performance regarding sound transmission from the exterior to the interior of the building. However, distressed and open joints between panels can provide a condition in which sound transmission to the interior may be increased.
Precast concrete panels used in wall systems have many different finishes and shapes. Often the finish will include the abrasion or modifying of the surface by sandblasting, exposing aggregate, acid washing, bush-hammering, or other techniques. Each of these finishes presents a different challenge in producing a durable precast concrete panel. Sandblasting a concrete surface can produce a surface that is less resistant to moisture penetration.
As a result, a surface treatment, such as a sealer, should be considered where this technique is used for finishing. A precast panel with a highly architectural surface will present challenges in development of a concrete mix and placement of reinforcing steel.
More complicated profiles in the surface of the panel usually require more workability in the concrete mix, better consolidation techniques, and often more post-production surface repairs. Precast panels with differing depths of surface profiling also require more care in maintaining sufficient concrete cover over the embedded reinforcing steel.
In summary, the more complicated the appearance of a precast concrete panel, the more challenging and important the review and approval process and quality control program.
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Most distress and deterioration encountered with precast concrete wall systems can be attributed to problems during erection, anchors used to attach panels to the structure, or corrosion of the embedded reinforcing steel. Panel cracking, displacements, or other distress conditions can occur at locations where anchors are inadequately or improperly connected. Poor construction is often the result of poor quality control and out of tolerance fabrication or erection of the panels.
Also, damage from handling during construction can result in panel cracking, some of which may not become evident for several years. Evaluation of future precast concrete durability is performed in several ways. Often requirements are specified air entrainment, maximum absorption, minimum compressive strength, etc.
History of the concrete mix and finish can also provide useful information. ACI specifies various criteria for acceptance of a concrete mix. In addition, water-to-cement ratio, minimum compressive strength, air entrainment range and other criteria are also listed.
In addition to a precast concrete mix meeting the requirements and recommendations of ACI , evaluation and study of the historic performance of a particular concrete mix in a similar exterior environment can also be performed. Petrographic evaluation ASTM C is also commonly used to evaluate aggregate in an effort to identify the mineral composition of the concrete and particularly the aggregate, and based on these observations and past knowledge of those characteristics, to predict future performance.
Another method of evaluation is to expose samples of the concrete to an accelerated weathering procedure, and evaluate physical and mechanical properties for changes. When properly constructed precast concrete panels systems require some maintenance. The version will be updated by Worksafe New Zealand.
The web sites of both these organisations also offer a range of free Technical Brochures covering many aspects of precast concrete. Hollowcore Test Report is a University of Canterbury paper on performance of connection and seating details for Hollowcore floor systems. Header Widget. Home Members Why Be a Member? Standards And Design Guides. They are available, together with updates and amendments, and many other concrete related standards from Standards New Zealand NZS — Concrete Structures Standard Rules applying to design of concrete structures in New Zealand.
NZS Specification for Concrete Surface Finishes Standard surface finish details for off the form finishes, exposed aggregate surfaces, floors, etc. Members Why Be a Member? Membership Information Members Only Content.