What Is Precast Concrete?
Precast Concrete consists of concrete. A mixture of Cement, Water, Aggregates and Admixtures. A method called concrete shuttering (Typically shapes constructed of wood or steel) is used to form the concrete to the desired shape as specified by the Customer. Once cured, these components are then stripped from the mold (usually the next day) and transported to site for installation.
Precast concrete components are reinforced with either conventional reinforcing bars, Pre-tensioned strands with high tensile strength, or a combination of both. Pre-tension being were the strands are stretched before the concrete is poured, once the concrete has cured to the specified strength the strands are cut (de-tensioned). As the strands have bonded to the concrete, the attempt to regain their original un-tensioned length causes a compressive force. This ‘pre-compression’ increases load-carrying capacity to the components and helps control cracking to specified limits allowed by building codes.
Precast components are used in various applications and projects of all types. Key components include:
There are many benefits to precast concrete and the production methods. These Include:
Design Flexible
Architectural precast concrete panels can be sculpted to resemble a wide range of finish materials, including limestone and brick. Company names, emblems, and other custom touches can be cast into panels, creating unique accents.
Fig;12 Ref; 2
STRUCTURAL CAPACITY
Structural precast concrete panels are a strong, durable and versatile building material, particularly suited to harsh climatic extremes and requiring minimal maintenance. They can be engineered to meet the structural needs of every type of domestic construction.
Concrete is flood and fire resistant and doesn’t shrink, rot or distort. It gains strength as it ages and in structural terms is ideally suited to the unpredictable conditions associated with climate change.
The inherent structural properties of precast walls mean that they do not require additional bracing to resist racking loads and that simple design of cross walls and junctions can provide adequate lateral bracing. Tie-down inserts to resist cyclonic winds are simple to design, install and connect.
Precast finishes are highly impact resistant, withstand wear and tear, and require minimal repairs and maintenance.
Fig;13 Ref; 2
THERMAL MASS
Precast delivers high standard internal finishes off the form (mould providing internally exposed thermal mass at a low cost without the need for insulation or other linings.
At 2060 kJ/m2Kelvin (K), dense concrete has the highest volumetric heat capacity of any commonly used housing material (e.g. brick 1360 kJ/m2K).
Fig;14 Ref; 2
INSULATION
High insulation levels are simply achieved.
Insulation design and detailing should avoid thermal bridging by ensuring continuous insulation between the internal and external skins in all external walls. Bridging through continuous concrete ribs and edge beams or failure to ensure all junctions have continuous insulation is a common design fault.
Fig;15 Ref; 2
SOUND INSULATION
Precast concrete provides one of the highest levels of acoustic separation of any common housing construction system for both internal and external walls.
Joints and openings must be detailed properly to maintain sound ratings. Precast concrete houses have fewer joints, and dimensional accuracy allows for the snug fitting of acoustically sound windows and doors.
Because services outlets are cast into wall panels, sound transmission weakness around switches and power outlets is avoided and transmission through cavities or air spaces is prevented
Fig;16 Ref; 2
FIRE RESISTANT
Precast panels are highly fire resistant with zero flammability on both external and internal surfaces. In fire, concrete performs well ‘ both as an engineered structure, and as a material in its own right. It has the highest fire resistance classification (class AI) under EN 13501-1:2007- A1:2009. In most cases, concrete does not require any additional fire-protection because of its built-in resistance to fire. It is a non-combustible material (i.e. it does not burn), and has a slow rate of heat transfer. Concrete ensures that structural integrity remains, fire compartmentation is not compromised and shielding from heat can be relied upon. Because of concrete’s inherent material properties, it can be used to minimize fire risk for the lowest initial cost while requiring the least in terms of ongoing maintenance.
Fig;17 Ref; 2
DURABILITY AND MOISTURE RESISTANCE
Dense precast concrete is generally not subject to rising damp or structural damage from condensation or dew-point formation. However, condensation problems can arise in any building due to temperature, humidity and ventilation conditions that encourage dampness and mildew.
No type of construction is immune but precast concrete is rarely affected by internal or interstitial condensation (i.e. from water vapour passing through the material).
Surface condensation or dew-point formation is more common and, while often causing unsightly mildew and stains, structural damage in precast concrete is unlikely.
Insulation, ventilation and appropriate heating reduce this risk.
Fig;18 Ref; 2
DESIGN PROCESS
The design process will start with the customer (usually industrial, residential and private contractors) contacting the sales team at Creagh Concrete with an enquiry about a precast concrete product, in this case a sandwich panel system for a new apartment development. The developer (customer) in turn will explain to the Creagh Concrete salesperson what is needed for the development. The sales person will advise as to what products are available, which products are most suitable and what overall is achievable with precast concrete products and what the benefits are and what the limitations are. ( Limitations include panel size required, weight of panel, panel span, production speed, transportation, storage space if needed, room on site and site access ) The sales person will collect information about site location, access, size and time scale of the project and if there are any budgets to adhere to.
The main reasons a sandwich wall panel would be used is because of they are Energy Efficient as A key benefit of a concrete structure is its ‘thermal mass’, its ability to absorb and distribute heat. They have a Reduced Life Cycle Costs, Energy and maintenance costs over a building’s life can be significantly high, Precast erection speed helps bring the structure on-line quickly as high-performance wall panels can be pre-glazed at the plant or window installation can be immediately completed in the field resulting in quick enclosure of the shell in any weather condition. Design Flexibility or Functionality Combining loadbearing wall panels with an economical precast structural floor system creates long, clear unobstructed spans with large, open bays with interior heights up to 55 feet. In most cases, the height is restricted only by what can be transported and delivered to the site. For offices, 45 to 50-foot spans are optimal because beyond that length, bays become so deep that daylighting is compromised.
Reduction in Trade labor, with early coordination, panels can be designed with electrical conduits and outlet boxes cast right into the panel, eliminating additional interior furring and material, Precast concrete walls usually do not need additional treatments to provide adequate sound insulation, as mass is concrete’s greatest asset when used as a sound insulator.
After all aspects are taken into consideration the sales team will explain to the design team what is needed for the project, to what specification and any relevant information that they have obtained. The design team will visualize these details with a drawing of the structure taking in to consideration, load weight, load size, moving of the product, distance to site, safety restraints, time and cost.
Each batch of concrete used to produce the molds will be lab tested and ID’ed before being used in the manufacturing of the sandwich panel so it can be traced back to all the components that may have been molded by that batch. Once the precast meet all required approval and safety standards are met, it can be released. The main testes carried out for approvals are; Visual examination by an experienced technician, Strength & Density properties, Permeability to liquid and gasses and movement.
BS EN 12390 series, Testing hardened concrete
BS 1881 series, Testing concrete
BS 8204 series, Screeds, bases and insitu floorings
ASTM C457 and C856, Microscopical examination Also apply for approvals.
The main objective of quality assurance is to identify problems as soon as possible or to avoid and eliminate them. Quality assurance is important because it is cheaper for a company to correct problems earlier in the development process than to correct them later on.
Principles of Sandwich Structures Typical sandwich materials always exhibit a particular fundamental pattern ” two faceplates (facings), which are comparatively thin but of high strength and stiffness, enclosing a core structure, which is relatively thick but light-weight, and possesses sufficient stiffness in the direction normal to the plane of the faceplates. The components of the sandwich material must also be bonded together, using either adhesives or mechanical fastenings, such that they can act as a composite load-bearing unit. In principle, the basic concept of a sandwich panel is that the faceplates carry the bending stresses whereas the core carries the shear stresses.
WHY CAD?
There are many reasons why the use of cad is so popular. The advantages of using CAD;
- Organised Drawing Info, each element in CAD can be categorised in layers and blocks making it easy to choose separate aspects of the drawing as and when required
- Draw Efficiently, CAD has a variety of drawing tools and commands that help to increase the speed of drawing
- Draw Accurately, CAD has a several methods to obtain exact dimensions and coordinates
- Views of Your Drawing, commands such as zoom and pan make it easier to draw detailed sections of a drawing
- Modification of Drawings, CAD allows the user to erase, copy or move any element of their drawing as and when required.
- File Sharing, drawing files can be shared to other users by email or if they are save in a sever in the work place
DISADVANTAGES OF CAD?
- Cost of CAD, for individual user programs such as AutoCAD are very expensive.
- Training, every new release of CAD software usually requires the user to update their skills. Improper use can make CAD files and drawing very muddled and hard to interpret for other users
- Safe Keeping of Files, if files are not properly stored and backed up they can be difficult to retrieve if a file becomes corrupt, server fails or a computer becomes broken
Referencing
1. concrete, C. (2017). Precast Concrete Cross Wall Frames. [online] Creaghconcrete.co.uk. Available at: http://www.creaghconcrete.co.uk/products/precast-frames/concrete-crosswall-frames.html? p=197#prettyPhoto [Accessed 21 Sep. 2017].
2. Anon, (2017). [online] Available at: https://www.pci.org/ [Accessed 21 Sep. 2017].
3. Materials, C. and Testing, H. (2017). Hardened Concrete Testing – Sandberg. [online] Sandberg.co.uk. Available at: https://www.sandberg.co.uk/laboratories/construction-materials/hardened-concrete-testing.html [Accessed 22 Sep. 2017].
4. Slideplayer.com. (2017). Lecture 2 SANDWICH COMPOSITES – ppt video online download. [online] Available at: http://slideplayer.com/slide/3970949/ [Accessed 22 Sep. 2017].
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