Excerpt from the December 2019 issue of Concrete Plant International by Michael Khrapko
The precast technique is practical and economical. This is proven by the very existence of the precast concrete industry and the numerous successful building projects achieved using precast concrete. A number of aspects make concrete precast different from in-situ concrete. Precast elements must be joined with each other to form a complete structure. A precast concrete structure is an assemblage of precast elements which, when suitably connected together, form a 3D framework capable of resisting gravitation and wind (or even earthquake) loads. Another unique concrete precast feature is vertical patterned texture, achieved by using formliners, which are essentially molds for giving texture and design.
New Zealand is a relatively young country. Europeans only started settling in New Zealand in any significant numbers of the past one hundred years. Being isolated geographically and having no cultural traditional building systems to change, New Zealand has been quick to adopt innovative precast building systems, which now enjoy a relatively high market share. Some of these innovative building systems, such as on-site precasting and moment-resisting precast building frames, have evolved their own style and character to meet New Zealand's unique needs. Over the years, innovative and unique systems to support precast concrete construction industry have been developed.
The precast concrete industry controls about 25 percent of the multi-story commercial and domestic building marketing, including frames, floors and cladding (facades). Precast concrete has many advantages over in-situ concrete and other materials. Precast concrete components are produced in controlled conditions that enable to manufacture units to tight tolerances, varying shapes and highly attractive architectural finishes. Controlled production processes allow for faster and most effective implementation of advanced material technologies, like self-compacting concrete and fiber composites. Compared with other materials, precast concrete can provide benefits in fire resistance, durability, thermal and acoustic properties, installation time and can perform its function immediately upon arrival at a construction site, therefore eliminating on-site curing time.
Precast concrete production and construction require efficient, effective and safe lifting and transporting methods. Growing concerns about safety on construction sites, together with escalating demands on cost efficiency, encouraged new developments in this area. Efficient and safe lining systems have been designed and successfully used.
One of the characteristic features of precast elements is that they must be joined together to form a complete structure. The connections for precast concrete are important components of the building envelope and frame systems. The primary purposes of the connection are to transfer load to the supporting structure and provide stability. Connection of precast elements becomes an essential component for construction in seismic areas like New Zealand. Precast connections for seismic resistance is another area where innovative systems have been developed.
Architectural facades using formliners
One of the greatest advantages of working in concrete is its versatility. When viewed as an artistic medium rather than simply a construction component, the material offers infinite possibilities for creativity. Many tools for expressing this creativity have been around for a long time, but they are finding new uses. One proven system receiving renewed attention is formliners.
Formliners are essentially molds for giving texture and design to vertical concrete surfaces. Formliners can be described as "reverse stamp." Instead of pouring the concrete and applying a texturing tool, the tool (the formliner) is attached to the form and concrete is poured onto it. Formliners have been widely used for years to beautify buildings and otherwise ordinary structures such as highway walls, sound barriers, bridge supports and retaining walls. This market continues to grow as more and more communities demand beauty as well as functionality from their buildings and highway systems. In many cases, budgets for these projects include a required amount of art, a requirement that can be met with form liners. Decorative formliners have been further developed in recent years, and certain times can be reused 100 times or more. This is partially due to formliner quality and improvements in the technology of adhesives and concrete release agents. Significant advancements in concrete admixture technology have played an integral role in producing concrete mixes that minimize surface blemishes, enabling the production of the specified surface finish. The new generation of polymer-based admixtures have ensured that concrete placing is easier than in the past, enabling quicker, continuous pouring. It is possible to achieve a high-quality, dense surface finish using self-compacting concrete (SCC). This further increases the life of the formliner as vibrating apparatus is not used.
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