Why use precast concrete?
Precast concrete provides resilience, efficiency, and versatility both in the design and construction stages of the project. Precast concrete buildings have great fire ratings and often eliminate the costly process of fireproofing. Precast insulated wall panels are highly energy efficient and since these are a main component of the building envelope they are the main contributor to energy savings. Using a precast concrete roof system greatly increases the total energy efficiency of a building.
Concrete construction is extremely fast and very durable. Buildings can be erected in any weather condition including harsh winters. Prefabricated concrete buildings have great fire ratings and are very sound and versatile.
Concrete construction is durable and long lasting, and retains its value throughout time. Precast concrete construction lessens the construction process which saves money on financing costs. Eliminate many of the dangers associated with on-site construction by providing a controlled, off-site fabrication environment. On-site, precast requires less trades for construction, reducing the number of people on site and the number of opportunities for accidents.
Precast products will compress construction schedules because precast concrete is manufactured under roof while the site is prepared, there is no waiting time for curing cement or delays for harsh weather. Products are delivered when needed for quick, one-pass installation and minimal site disruption. Compressed schedules, fewer on-site trades, and eliminating weather delays add up to reduced project costs.
Prestressed concrete has very good crack control and strength to weight ratio. Precast concrete is able to create longer, thinner spans than poured concrete.
How flexible is precast concrete design?
Whether you use our wall panel system, or our hollowcore floor system, or if you build a total precast building, the design possibilities of concrete construction are endless. Call us today to learn more. You’ll be surprised by precast concrete’s versatility!
Why is concrete so heavy?
Concrete is actually 31% of the weight of structural steel. Concrete weighs 150lbs/ft3, steel is 490lbs/ft3. In office structures, a completed concrete structure (floors, walls, roof, beams, columns etc.) could be as much as 19% lighter than a composite steel frame and concrete slab building.
What makes concrete so durable?
The primary ingredients of concrete — sand, gravel, and cement — are mineral based. When mixed with water, the cement molecules chemically combine with the water to create a crystalline matrix of high compressive strength. This matrix binds the sand and gravel together, creating what is sometimes known as “liquid stone.” Unlike other construction materials that rust, rot, or otherwise degrade when in the presence of moisture, concrete actually gets stronger.
Precast concrete panels provide a long service life due to their durable, low-maintenance surfaces. Insulated wall panel construction with concrete exterior and interior walls provides long-term durability inside and out, and precast concrete construction also provides the opportunity to move and reuse panels to refurbish the building, rather than tear it down, should its use or function change.
Quality Control / Certification
Is your product manufactured in a certified plant?
Yes. Our products are manufactured in a Precast Concrete Institute (PCI) Certified Plant.
What kind of Quality Control/Quality Assurance is in place in the precast industry?
PCI has three different certification programs:
Plant Certification – First introduced in 1967, covers the overall management of the quality system within precast/prestressed plants.
Personnel Training and Certification – First introduced in 1985, covers the qualification and certifications of the people doing the product inspections and managing the Quality Assurance departments within precast/prestressed plants.
Erectors Qualification/Certification – First introduced in 1999, covers the training and qualification of field services personnel.
For more information on the PCI Certification program, refer to PCI Manuals for Quality Control – MNL116 or 117 that can be found on the PCI website at www.pci.org.
How important is PCI certification for me as a designer?
We are certain that you, as well as your client, expect your building’s exterior to become the high quality realization of your design’s intent. And the best assurance that your project will be a high quality result is to rely upon the industry’s only truly independent certification program – PCI’s plant certification program.
How is precast concrete made?
Precast concrete is made in our plants, where a dedicated batch plant produces a specially designed concrete for precast concrete products such as structural beams, columns, and double tees; architectural cladding; and wall systems. Aggregates usually come from nearby quarries and cement and other ingredients are often supplied by local manufacturers.
The mixed concrete is placed into a form around reinforcement and, often, prestressing strands that provide load-resisting camber to the finished precast concrete member. After the member is cured, the precast concrete product is stripped from the form and moved to our plant’s yards for finishing and storage until it is ready to ship to the jobsite. Using prestressing strand reduces the amount of conventional reinforcement, resulting in concrete in compression which greatly reduces cracking and provides elastic benefits to the member.
How much cement is in precast concrete?
Typical concrete contains approximately 10% to 12% cement by volume. The cement chemically reacts with water to bind together the aggregates and other ingredients of the concrete. According to the Department of Energy (DOE), cement production contributes between 1% and 2% of global carbon dioxide emissions through the burning of fossil fuels and process-related emissions.
The amount of cement used in precast concrete may be reduced by up to 60% through substitution by supplementary cementitious materials (SCMs). The amount of cement substitution possible is affected by the mixture design requirements, the products and processes of individual precast concrete manufacturers and plants, and the local availability of materials.
Products - Hollowcore
What is hollowcore?
Hollowcore is a prestressed concrete slab with continuous voids provided to reduce weight and cost. Hollowcore is primarily used as a floor or roof deck system.
What are the benefits of hollowcore?
Hollowcore slabs provide economical and efficient floor and roof systems. Structurally, a hollowcore slab provides the efficiency of a prestressed member for unsurpassed load capacity, span range and deflection control. In addition, the grouted slab assembly provides a basic diaphragm for resisting lateral loads. Excellent fire resistance is another attribute of the hollowcore slab system. Used as floor-ceiling assemblies, hollowcore slabs have the excellent sound transmission characteristics associated with concrete.
Will the hollowcore provide a perfectly smooth surface?
Hollowcore slabs will be cambered as with any other prestressed flexural member. The amount of camber is affected by span, prestressing forces, concrete differences and curing variations. Wells recommends that the erector adjust the bearing elevations and level adjacent slabs to minimize the camber differential. Also, as with any structural concrete component, minor imperfections in the surface should be anticipated.
What is done to finish hollowcore at the project site?
The top surface can be finished with a structural, composite concrete topping or by simply ‘feathering’ the joints with latex cement. We do not recommend carpet direct. The underside can be used as a finished ceiling as installed, by painting, or applying an acoustical spray.
What openings can Wells put in the hollowcore?
The drycast system used in the manufacturer of the hollowcore slabs is not conducive to providing finished openings similar to those available in other Wells products. We will provide rough square or rectangular openings through the hollowcore slabs required by other trades – which are 100 square inches or larger. Wells will review required openings in the hollowcore slabs and if necessary, introduce steel header frames to achieve the openings. In some instances, openings may have to be field cut by others to ensure the structural integrity of the hollowcore slabs during manufacture, shipping and installation.
What needs to be done to finish the openings?
Plant provided openings are ‘rough openings’. If openings through the hollowcore slabs are to be left in an exposed condition, others may provide concrete patching or trimming of the opening with another building material.
What needs to be done for openings less than 100 square inches?
Holes smaller than 100 square inches should be core drilled by the trade requiring them after the slabs have been erected and grouted. The drilling should be done only with the express written approval of Wells’ engineering department.
Are narrow width slabs available?
Wells can provide hollowcore slabs narrower than the 4’-0” modular width. A longitudinal saw cutting process after the slab is cured and removed from the casting facility achieves this. To ensure the structural adequacy of the slab, there are restrictions on the widths of slabs that are available. Contact Wells for further information on the range of narrow widths available.
Will the hollowcore cover the entire floor area?
The hollowcore slabs will be laid out on Wells’s drawings to minimize the requirements for field pour strips. However, in some instances, pour areas adjacent to walls or around openings may be required. These will be clearly indicated on our layout drawings and details and are the responsibility of others.
How is the hollowcore erected?
Hollowcore should be erected by personnel skilled in such work utilizing lifting devices and cranes capable of safely handling the slabs. The slabs are to be positioned and connected to the structure in accordance with Wells drawings and details. The slabs are erected one at a time with slings and spreader bars specifically suited for hollowcore. Your Wells representative can provide you with the information sheet “Procedure for the Proper Installation of Precast Hollowcore Slabs.”
How is hollowcore connected to my building?
There are numerous methods to connect the hollowcore slabs to the building frame. The type of connection is dependent on the building material the hollowcore bears on or connects to. Wells has a library of cost-effective connection details for concrete, masonry, steel and precast concrete construction.
Structural / Engineering
What happens when a prestressed strand is cut in a finished piece of concrete?
Visually or safety wise, nothing immediate would happen. The structural capacity of the member could be affected if any type of reinforcing is cut (prestressed or mild reinforcing). A structural engineer should be consulted before cutting any type of reinforcement.
How are fire ratings calculated in concrete?
The most common practice is to follow the PCI Manual MNL 124, “Design for Fire Resistance of Prestressed Concrete.”
What is the relationship between the Engineer of Record and the Precast Specialty Engineer?
The Engineer of Record is responsible for the structure in its entirety and delegates the design of the components to the specialty engineers involved in the project. This is the case in precast/prestressed concrete as well as many other products (wood, steel etc.). This relationship is well defined by the National Council of Structural Engineers Association (NCSEA).
What are the benefits of precast in terms of fire?
Precast/prestressed concrete is non-combustible and provides endurance against fire as in any type of concrete. The use of precast/prestressed concrete floors and walls provides excellent compartmentalization in multi-family or office structures.
What are some key things to look for when inspecting precast?
- Good bearing conditions i.e. bearing pad is in correct location
- Surface is uniform
- Structure is plum, well aligned
- Welding of connections is appropriate and complete to stage of construction
- Loads being applied at time of inspection are appropriate to intended design
For further detail, refer to PCI’s Erection Manual MNL 127 and Tolerance Manual MNL 135. Any specific questions should be answered by a structural engineer.
What kind of testing has been done in cracking of precast?
Refer to PCI Committee on Quality Control Performance Criteria “Fabrication and shipping cracks in prestressed…” PCIJournal Volume 28 and 30. For a more complete list see PCI’s Design Handbook MNL 120.
What are the minimum requirements for joints between precast products?
Criteria for joint sizes can vary considerably depending on the product type, size and building movements. Typically, joints in structural products are 1” and architectural products are ¾”. However, there are a number of considerations like building expansion joints, building isolation joints etc. that will dictate joint widths. Please see PCI Design Manual MNL120 and PCI Tolerance Manual MNL 135 for further information.
Sustainability and Precast Concrete
Is precast concrete a green building material?
Precast concrete contributes to green building practices in significant ways. Its inherent strength – 35 to 50 MPa – means precast concrete extremely durable. Its mass can shift heating and cooling loads to help reduce mechanical system requirements. Because it’s factory-made, precast concrete reduces construction waste both in the factory and on the job site, and does not add to indoor air quality (IAQ) concerns. The load capacity and long spans of precast members help eliminate redundant structures and precast readily accommodates recycled content.
Are precast concrete structures energy efficient?
An inherent characteristic of precast concrete is its natural resistance to mold, greatly reducing health concerns from VOCs and off gassing. With these environmentally friendly advantages, precast concrete satisfies a growing demand for sustainable design and construction. Additionally, precast concrete structures are completely recyclable making their impact on the environment minimal.
Energy efficiency is part of the design of precast structures. The thermal mass of precast concrete can absorb and release heat slowly, shifting air conditioning and heating loads to allow smaller, more efficient heating, ventilating, and air conditioning (HVAC) systems. Insulation can be incorporated in architectural exterior wall panels to increase thermal efficiency and provide continuous insulation (CI) in walls. The savings can be significant – up to 25 percent on heating and cooling costs. See the reports by U.S. Department of Housing and Urban Development (HUD) and the National Institute of Standards and Technology (NIST)
Does precast concrete contain recycled materials?
Precast concretes fresh and in-place performance can improve when several common industrial byproducts are added. Fly ash, slag, and silica fume, which would otherwise go to landfills, can be incorporated into concrete as supplementary materials. These by-products can also reduce the amount of cement that is used in concrete. Reinforcement is typically made from recycled steel. (Steel is one of the most recycled building materials, and can be reused again and again.) Insulation and connections within the precast concrete also contain recycled content.
Can precast concrete members be reused?
Precast concrete members are unique in that they are individually engineered products that can be disassembled. Designers can easily plan future additions to buildings, because the precast concrete components can be rearranged. Once removed, precast concrete members may be reused in other applications.
Precast concrete is also friendly to downcycling, in which building materials are broken down, because it comes apart with a minimum amount of energy and retains its original qualities. An example of downcycling would be the use of crushed precast concrete as aggregate in new concrete or as base materials for roads, sidewalks, or concrete slabs.
What steps are precast operations taking toward sustainability?
As a PCI Producer Member, we meet local and state ordinances and emissions requirements. Initiatives within the industry include:
- Use of local materials in all mixtures, including local aggregate resources
- Water reduction, reclamation and recycling
- Reducing cement requirements by lowering water-cement ratios
- Admixtures such as hardening accelerators to eliminate applied heat in curing
- Use of self-consolidating concrete (SCC) for quicker placement, no vibration, and reduced surface defects
- Use of environmentally friendly thin brick in place of conventional brick in precast concrete systems
- Carbon-fiber reinforcement that allows lighter and larger concrete sections with less embedded energy and no corrosion
- Use of supplemental cementitious materials (SCMs) to reduce cement consumption; participation in Cool Climate Concrete
- Enclosed sandblasting facilities with 100% process-waste control
- Standardizing wood form parts for multiple reuse; recycling discarded forms into mulch or fuel
- Recycling all scrap steel and reinforcement
- Reducing and reusing product packaging received in facilities
How does concrete affect the environment compared to wood and steel?
Concrete is essentially inert; it does not rot, burn, off-gas or rust, and provides durability that significantly outlasts many other building materials including wood and steel. The cement industry utilizes industrial by-products like fly ash and consumes less energy than its competitors. According to the Canadian and U.S. Departments of Energy, cement production accounts for 0.33 percent of energy consumption – lower production levels than steel production at 1.8 percent and wood production at 0.5 percent. In addition, it places less stress on the environment to acquire the raw materials for concrete than for steel or wood. Thus, concrete is an excellent choice for sustainable development.
What is the urban heat island effect and how does concrete fit in?
Scientists have observed that urban areas with more buildings and paving and less vegetation are typically warmer than surrounding rural areas. This is partially attributed to the dark surfaces of roofing and paving used to create the built environment. Temperature increases have been measured as high as 80 degrees F. This additional heat causes air conditioning systems to work harder and consume more energy. The additional heat also contributes to the creation of smog. Concrete’s natural light color can reduce urban heat islands. Light-colored concrete reflects more solar energy than dark-colored materials, such as parking lots, driveways, or sidewalks, thereby reducing the high temperatures.
What is being done about CO2 emissions during the cement-manufacturing process?
Since 1975, the cement industry has reduced CO2 emissions by 33%. Today, cement production accounts for less than 1.5% of U.S. carbon dioxide emissions, well below other sources such as electric generation plants for heating and cooling the buildings and buildings we live in (33%) and transportation (27%). In 2000, the cement industry created a new way to measure CO2 emissions. Recently introduced guidelines allow for greater use of limestone as a raw material in cement, ultimately reducing CO2 by more than 2.5 million tons per year. By the year 2020, plans call for further reduction of CO2 emissions to 10% below the 1990 baseline through investments in equipment, improvements in formulations, and development of new applications for cements and concretes that improve energy efficiency and durability.
Is precast manufacturing environmentally friendly?
When compared to jobsite operations, precast manufacturing is definitely more environmentally friendly. Less waste is generated, less material is used within comparable products, forms or molds have a longer service life, noise is reduced and both quality and safety are improved.
Our plants monitor their operating processes and implementing changes to ensure:
- Effective safety programs are in place to safeguard workers and assets
- Effective maintenance programs are in place to ensure equipment safety and efficiency
- Effluent water is adequately detained and neutralized for discharge to the receiving environment and/or recycled for re-use
- Air quality is monitored and controlled; dust from cement silos, mixer operations and sandblasting is minimized, unpaved road dust is addressed, welding operations are adequately vented
- Solid waste is monitored and controlled, excess concrete is effectively used, culls are minimized, waste product is crushed into re-usable road-base material, steel is separated and recycled, wood forms and steel forms are recycled, paper use is minimized
- Energy use is monitored and controlled, heat curing is done in a closed system, process heat is adequately controlled, flue gases are monitored and energy sources are properly tuned with heating pipes and conduits insulated, hydro power factors and demand are monitored and adjustments made to minimize consumption
- Fuel and oil tanks are adequately contained to prevent any ground contamination from possible spillage
- Effective continuous improvement programs are in place to ensure that tomorrow’s performance will be even better than today’s
Architectural Precast - Finishes, Features and Colors
What are the keys to color uniformity?
There are numerous ways in which precast color uniformity can be enhanced.
Use a retarded finish. This exposes the coarse aggregate, permitting the uniform natural color to “carry” the panel’s color. It is important to use a color compatible matrix (sand/cement/pigment), in order to mask any uneven coarse aggregate dispersion.
Use white cement instead of gray cement whenever possible. White cement’s color control is excellent. Gray cement manufacturers do not attempt to control color. Thus, gray cement color can vary widely even within a single supplier, causing significant precast color variations.
Remove a sufficient amount of the as-cast concrete’s surface. All concrete is blotchy when left as-cast. The surface “paste” or “skin” must be removed in order to reveal the true concrete color. For example, light acid etching is apt to result in a blotchy or shaded appearance because the acid does not remove all of the surface “paste” or “skin.” Therefore, acid etched finishes should be deep enough to reveal the tips of the coarse aggregate. Likewise, lightly sandblasted finishes should reveal some coarse aggregate, in order to appear reasonably uniform.
Avoid large planes of smooth, uninterrupted surfaces. If the eye has nothing to focus upon except large expanses of plain, smooth concrete surfaces, it will perceive minute panel to panel color differences. If such large surfaces are interrupted by rusticated joint patterns, plane changes, reveals and/or mix/finish changes, the eye will be drawn to these features making slight color variations almost indistinguishable.
Avoid using pigments in very small dosages. Pigment content as a percentage of the total ingredients in a concrete batch is very small. As that percentage diminishes, the likelihood of increased pigment content variation from batch to batch goes up dramatically, causing noticeable color variation.
Will using multiple mixes and/or finishes in a single project increase my precast cost dramatically?
Multiple mixes and finishes have been used successfully but, require consultation with precast representative. Multiple mixes will increase cost. Consult your Wells representatives before finalizing your design because some mix and/or finish combinations require more additional labor than others.
What should I do to ensure the look I want at an affordable cost?
The most important thing you can do is call a Wells representative as early as possible in the development of your project’s architectural precast application. Our sales representatives are trained to help designers maximize the value of their precast design. For example, how a precast exterior is panelized can affect cost significantly with little or no difference in appearance. Also, a designer can employ important features such as reveals, rustication joints, medallions, form liner patterns, etc. at very little additional cost, but only if such features are used repeatedly (avoiding costly, frequent form changes). It’s important to know what design techniques to use and when to use them.
Why should I design with architectural precast, and what makes it my best option?
There are four (4) compelling reasons to choose architectural precast concrete for your building’s exterior:
- Customized design. Architectural precast is the only cladding material that permits you, the designer, to custom design shape, color, texture and pattern – only you control your building’s unique, custom exterior look.
- Enclose your building sooner. Architectural precast enables you to enclose your building’s exterior in a small fraction of the time versus most all other cladding materials – this reduces construction cost dramatically and provides your client much earlier occupancy.
- Proven low initial cost. Architectural precast concrete is used widely by office building developers nationwide. They tell us it is their cladding material of choice, because it offers a high quality “look” at a very economical price.
- Long term maintenance is almost negligible. All you need to do is re-caulk the precast joints after 15 to 20 years.
Can architectural precast be used as a load bearing element?
Certainly! We encourage you to do so. For a minimal additional cost (some additional reinforcing and minor additional connection cost) architectural precast concrete can become a terrific load bearing element.