In the construction world there are a lot of unknowns that take place and if a General Contractor can find any avenue that they know will be reliable and make their life easier, that avenue would be using precast concrete components. Going with precast concrete components guarantees a smooth route for owners and designers in both the immediate and distant future. Three keys to making your project a success while using precast concrete components include maintaining schedule, safety of personnel onsite, and the quality of products and construction.
Maintaining schedule: Every project has a life cycle and every General Contractor wants to make sure that their milestones are met if not superpassed. Precast concrete is produced offsite and is cured in a controlled environment, then stored until the jobsite is ready for installation to take place. Installation can take place year round, so once the project’s jobsite is ready, the precast installer can begin installation at a much faster pace than other structural/architectural components that are built at the jobsite. As the old saying goes “time is money,” and every day that is saved on the back end of the general contractor’s schedule helps their bottom line.
Safety of personnel onsite: Making sure that your subcontractors and crews leave the jobsite the same way as they were when they arrived is important to us all. Luckily, the precast panel systems take some of the worker-safety liability away from general contractors due to our off-site production and small crews to install onsite; which means fewer trades and safety risks onsite.
Quality of products and construction: There are many variations of design and architectural finishes and mixes that can be achieved with precast concrete systems. From very smooth concrete, which is great for food processing, all the way to a glossy polish, which helps make your building pop aesthetically above all others. Installation crews do a fantastic job in making sure everything is installed to plan. They also help to coordinate with other trades onsite to ensure the construction life of the project continues moving forward to meet or surpass schedule expectations. Wells Concrete makes it a priority to ensure that the quality of our products meet or exceed the owners’ and designers’ expectations.
Today’s mid-rise and multi-use buildings can include retail space, office space and residential living units and parking.
The challenge with having multiple users is finding a single structural system that will accommodate the very different floor plate layouts for each without placing load bearing columns and walls in odd areas that will hinder the use of the space.
The parking area requires an open drive lane that is usually 24’ to 26’ wide to support two way traffic with 18’ to 20’ parking stalls on either side. The layout of the housing is driven by the location of the public corridor and gathering areas. Sandwiched between these two areas is often a retail or office space looking for as much open area as possible to customize their floor layout.
The solution is to use a long span precast truss system (ER-POST ™) that spans the width of the Building. The trusses are supported by precast concrete columns at the perimeter or outside of the structure. Hollow core precast plank bears on the top and bottom cords of the precast truss.
The precast trusses can span the width of the building up to 80 feet while the hollow-core can span from truss to truss up to 44 feet. Because the truss is both top and bottom cord bearing, they are only required at every other level leaving the levels between completely open. Lateral support can come from the precast stair and elevator walls.
This system gives users the freedom to layout their space without designing around interior columns while taking advantage of the benefits precast concrete. Fire rating values from 1.5 to 3 hours, low sound transition, durability, speed of construction and all weather construction.
Take it a step further and incorporate a one of our architectural insulated precast exterior wall systems to enclose the structure.
Precast concrete has the capability of incorporating other trades in a plant fabricated atmosphere. Components can be cast into the concrete, or it can be used as a platform to prefabricate other trades. This can take work off the jobsite, reducing congestion and shortening construction time. Additionally, plant fabrication can offer productivity and quality gains, and while some elements added to precast are tried and true, others are new.
Board insulation in sandwich wall panels has been around for many decades. A layer of concrete is cast, insulation is placed on top of that, and another layer of concrete is cast on top of the insulation (picture an oreo cookie). The two concrete layers are tied together with a pin (a.k.a. wythe tie) that passes through the insulation.
Some new items used in plant-installed insulation are non-conductive wythe ties and new insulation types. A variety of wythe ties that don’t cause a thermal break through the insulation are available from various manufacturers. These are becoming increasingly common as a greater focus is put on energy efficient buildings. Plant-applied sprayfoam insulation has been used by some precasters for more than a decade and is becoming more common. Currently, this means sprayfoam is applied to the interior of a precast panel. Sprayfoam and other high insulating materials may one day be used in sandwich type precast wall panels, offering greater R values or productivity.
Wells Concrete has experience with plant installation of electrical items and windows. The typical electrical item is vertical conduit and a box cast into the concrete of a wall panel. This hides the electrical elements in the wall. Wells Concrete has begun developing a stud-frame-backed precast wall that has potential for making plant installation of electrical elements even more efficient.
Plant-installed windows have been on the rise at Wells Concrete, as well as the precast industry as a whole, over the last few years. After a wall panel is cast, the windows are installed in the plant or storage yard using the same details and methods as field installed windows. The precast is then shipped to jobsite with the window installed.
In the precast industry, some precasters have developed flooring systems that incorporate plumbing, electrical, and other utilities. The utilities are plant-installed within a floor piece that may typically be 12ft x 30ft. The utilities are spliced together at the construction site after installing the precast. Other precasters have developed floor systems that serve as a modular platform. Various elements can be plant-installed on top of the floor piece. Many of the elements in a room have been installed in prototypes: stud wall, sheet rock, electrical wiring and fixtures, plumbing with toilet and sink, and cabinets. The assembly is shipped to the job site and craned into place.
Tim Edland, P.E.
Research and Development Director
Concrete buildings from hundreds of years ago are still in use today. Some say concrete can last up to 2,000 years, and there are certainly many structures around that are well on their way to such a ripe old age. Why? Because it gets stronger every day!
Unlike most other materials, precast concrete increases in strength over time. Concrete is designed to provide a reliable structure for many years to come, it is not uncommon for designers to design concrete building for a 75 to 100-year life cycle.
The Ancient Romans were the first to develop concrete as a building material. They accomplished this by mixing lime, water, and volcanic ash.
Concrete is used more than any other man-made material on the planet.
Six billion tons of concrete are produced every year.
The world’s largest concrete structure, Three Gorges Dam in China, consumed over 35 million cubic yards of concrete. Hoover dam consumed 5 million cubic yards.
Reinforced concrete is the only building material that is highly resistant to both water and fire.
Concrete is the best material for road construction. The first concrete road was built in 1909, and 30% of interstate highways are built with concrete.
The British Army used concrete to detect enemy aircrafts, and before radar, concrete was used to construct acoustic mirrors.
Concrete and cement are not the same thing. Concrete is a mixture of cement, sand, gravel and water.
Christ the Redeemer – Brazil
Reinforced concrete is the only building material used for underwater structures such as damns, piers, and sewer works.
Concrete has an incredibly high compressive strength: typical (psi) compressive strengths range from 3,000-7,000 psi.
The statue of Christ the Redeemer in Brazil was constructed using concrete and soapstone. The entire project took nine years and 635 tons of concrete to finish.
According to the Washington Post, China has used more cement during the years 2011-2013 than the United States has in the entire 20th century.
Wells Concrete has just finished manufacturing our first project using 3D printed molds. The project name is the University Of Minnesota Pioneer Hall in Minneapolis.
Advantages of 3-D printing in the construction industry:
Complex designs – 3D Printing allows complicated, complex models to be developed and used in our manufacturing process; whereas wood and plastic would have been used in the past and limited the possibilities of designs. 3D printing can build curvilinear structures (rather than rectilinear forms).
Durability – the amount of scrap from wood and plastic would be significantly reduced as a typical mold can be used over and over whereas wood and plastic have to be replaced and repaired every few uses.
Improved Project Planning – an important part of every project plan is the design. With 3D printing, companies will be able to quickly create models to have a visual representation of the project as well as help pinpoint problem areas and avoid delays.
Economical – as the technology continues to improve, 3D printing costs will continue to decrease allowing for wide spread use of this technology. As labor is a limited resource, 3D printing can eliminate some of the labor required on creating elaborate forming buildups.
The two pictures below are the 3D molds (black window sills) in the form and the finished product.
Figure 1. Gustave Mangel’s famous stack of books theory
To understand what happens to a pre-stressed strand once it is cut in concrete, one must first understand its purpose. Concrete is very strong in compression and weak in tension. Compressive strength of concrete is the maximum compressive load (stress) it can carry per unit area. Tensile strength of concrete is usually considered about one tenth of its compressive strength. So how can we take advantage of concrete’s compressive strength to improve tensile strength? By adding compression at locations of tension, in order to obtain a tension that is below the cracking limit.
Professor Gustave Mangel developed the concept of pre-stressing in the 1940s and brought the ideas to America in 1946. He explained the concept of pre-stressing using his famous stack of books theory (figure 1). The books at the bottom are like pre-compressed concrete. Utilizing this compressive force, they support their own weight, plus additional superimposed loads, simulated by the stack of books on top. So by adding a squish on the lateral stack of books, they stay in compression and can support the stack of books on top, without falling apart.
Figure 2. 7 wire 1/2″ 270 ksi Lo-Lax strand
How can we accomplish this in a concrete member? First, we specify the type of prestressing strand we want to use to add the squish. Typically, 7 wire ½” diameter 270 ksi Lo-Lax is used in precast structural members (figure 2). The area of ½” diameter strand is only 0.153 square inches, between the area of a #3 and #4 bar. Because of its higher ultimate strength, this strand can be pulled to a maximum of 31,000 lbs, which is 2.5 times the maximum tensile strength of #4 bar.
Figure 3. Typical pre-stressing setup
To get this force into the concrete member, first we must anchor the strand to a form, capable of handling the total prestress force without buckling (figure 3). Hydraulic jacks are used to pull each ½” 270 ksi Lo-Lax strand to 31,000 lbs. Strand elongation readings are taken, to ensure PCI tolerances, and then concrete is poured into the form, around the prestressed strands (figure 4). When a strand is stretched, the diameter of the strand decreases a little bit. So upon cutting it, the diameter will increase a tiny bit as it wants to shrink back to its original length, like a rubber band, slightly recessing into the concrete member at each end. This creates an added compression force, or squish, to the concrete member.
Figure 4. Pre-stressed 24″x24″ column setup
When can we cut the strand to transfer its prestress force to the concrete in the form of a compression squish? ACI 318-14 specifies a maximum stress of 0.6f’ci’, however, PCI states that common industry practice allows us to increase this to 0.7f’ci’ . This means that however much compression stress we are adding to the concrete member by all the tensioned strand, while considering self weight of the member, we must have a minimum of 1.43 times that compressive stress in the concrete when the strand is cut. Once cylinders have been broken to verify the concrete’s compressive strength meets this requirement, only then can we cut strand and strip the pre-stressed concrete member.
Figure 5. Variation of strand along development length, PCI 8th Edition
So now that we have cut the strand, what stops the strand from shrinking to its original length and how does the initial prestress force get transferred to the concrete member? By its bond to the concrete. The length required to bond the initial prestress force of the strand is called the transfer length. ACI 318-14 defines this transfer length as dbfse / 3 where db is the diameter of the strand in inches, and fse is the effective prestress, in ksi, after all losses (figure 5). If we have one strand pulled to 31,000 lbs, the transfer length is the length required to put the 31,000 lbs, minus some losses, into the concrete member. So, fse is the effective stress put into the concrete, by the strand, that decides whether the concrete member will be uncracked, in transition to cracking, or a cracked member, under a specified loading condition, after all losses are taken into account.
What exactly are these losses that keep getting mentioned? Loss of prestress force is the reduction of prestress force in strand due to elastic shortening, shrinkage, creep, and other external factors. So if we pull 31,000 lbs on a strand, we don’t necessarily get 31,000 lbs into the concrete at final service loading conditions.
In elastic shortening, the concrete around the strands shortens as the prestress force is applied to it.
During plastic shrinkange, the hydration of cement results in a reduction in the volume of the concrete.
Drying shrinkage appears after the setting and hardening of the concrete mixture due to loss of capillary water.
Creep is the time dependant deformation of hardened concrete due to a permanant force.
In all these losses, strand goes along for the ride and shrinks as well. Creep in the strand, also known as relaxation, is a reduction of force, in the strand, under constant strain. Once the strand is pulled, then released, the extra length required for the strand chuck to properly seat and hold the strand is a type of external loss. All these losses add up, to reduce the initial pull, or initial stress, to the actual pull, or effective stress, fse.
But what if we want to know when the strand will break? This limit is called ultimate strength or nominal strength. How much more bond length is required? The total length required to develop a strand’s ultimate strength is called the development length. ACI 318-14 defines the development length as (fps – fse)db plus the transfer length, where fps is the stress in the strand when a concrete member fails in bending. The transfer length is always a portion of the development length (figure 5). In figure 6, the transfer length can be seen going from zero to the dotted fse = 170 ksi effective stress line, and the full development length goes all the way up to the dotted fpu = 270 ksi line, where fpu is the specified ultimate strength of the strand. For our ½” strand pulled to 31,000 lbs, and assuming 15% losses, we get an effective stress of around 170 ksi. The transfer length to apply 170 ksi to the concrete is approximately 27” and the devleopment length to break the strand at 270 ksi is approximately 78”. Notice in figure 6, that a partially debonded strand’s transfer and development length are doubled. This occurs when strand is intentionally debonded with sheathing, goes through an opening where strand is removed, like a window, or when openings are cut through strand and into the member at a later date.
Figure 6. Design stress for underdeveloped strand, PCI 8th Edition
We have learned that the effective prestress, fse, is what keeps a pre-stressed concrete member in an uncracked state under service loads and fps is the stress in the strand at the members ultimate bending strength, when failure occurs. So what happens inbetween? Usually, pre-stressed concrete members will remain uncracked under service loads, and have have some wiggle room to take more loading prior to strand breakage, to ensure a ductile failure. That way, we can detect impending failure by serious deformation and serious cracking, long before failure will occur.
Figure 7. MSUM 2018 PCI Big Beam Contest testing
Some minor cracking can be normal under service loading as well, depending on the size of the member, span of the member, number of strand, the loading criteria, etc. What holds the pre-stressed concrete member together after cracking is the leftover tensile strength of the strand, and compression at the top of the member, (in typical positive moment bending). Figure 7 shows a beam designed by students from Minnesota State University, Mankato, for the PCI big beam contest. The contest specifies a loading condition to remain uncracked, and a higher loading condition to break. This beam has been loaded well above its cracking limit and below its ultimate bending strength. The beam still takes on loading, due to the tension in the strand crossing the cracks at the bottom of the member and the compression at the top of the member, informing us of an impending failure.
Figure 8-1. Samaritan Bethany
Figure 8-2. Samaritan Bethany
Now we know that by cutting strand, a squish is added to a concrete member, similar to Gustave Mangel’s stack of books. The strand must be tensioned in a concrete form, cut at a minimum concrete strength, enough effective stress applied to keep the member uncracked with all losses taken into account given enough transfer length; and a ductile failure will show serious cracking prior to member failure at ultimate bending stress given enough development length. Cutting strand is what turns the squished books into multi-level total precast pre-stressed concrete buildings.
The Department of Labor has issued a rule reducing permissible exposure limit of respirable crystalline silica from 250 to 50 micrograms per cubic meter averaged over an eight hour work shift. This rule goes into effect for OSHA enforcement in the construction industry on June 23, 2017.
Complying with this rule will be a big challenge to construction in general. While precasters face challenges, they also have advantages against comparable building systems.
Precast concrete is manufactured in a factory and shipped to a construction site for assembly. That means all manufacturing processes creating dust occur at our factories, not the job-site. We can apply engineering controls during manufacturing to capture dust in a factory setting under reproducible conditions rather than a job-site.
Most openings are formed into our pieces rather than by cutting concrete. This means the additional dust from cutting openings at the job-site is not created.
Precast concrete pieces are custom designed. We can adjust piece layout and add reinforcing to minimize the amount of cutting required at the job-site.
Precast has a short construction duration. Coordination of subcontractors to prevent silica dust contamination between trades is likely to become a real concern. Our short construction duration means our crews create dust fewer days and have to be coordinated with the dust producing activities of other trades fewer days.
So what have we done in preparation for this new change? A lot. We’ve made significant investments to make sure we’re in compliance, such as:
Measuring silica exposure associated with various work areas and work processes
Implementing engineering modifications and administrative controls to reduce exposure
Training employees and supervisors on our Exposure Control Plan; a written set of procedures required under the OSHA rules
Developing medical surveillance and record keeping processes called for under the rules
Wells Concrete is a member of both PCI (Precast Concrete Institute) and CPCI (Canadian Precast Concrete Institute). If you are an engineer, architect, contractor, or a building owner and are considering using precast concrete, the question as to why this is important may cross your mind.
Aside from the most obvious reason, quality assurance for all of the products we manufacture and install, there are many beneficial reasons to insist on requiring your precaster be a member of CPCI or PCI.
Both PCI and CPCI have multiple roles other than the certification of our plants, the first being to promote the use of precast concrete in all aspects of construction. This is rather self-serving, but how we implement this is of great benefit to all who are involved in the design, construction and end use of any building system; the promotion of the use of precast concrete requires more than just saying “it’s the best.”
To that end, another major role of both institutes is to build the precast body of knowledge. We accomplish this in many ways, including:
Industry experts from all over Canada and the US work together on committees developing specifications for the many products and building systems we provide.
Providing funding to colleges and universities all over Canada and the United States for research into leading edge use of precast concrete.
Funding of best practice guides for building envelopes, architectural precast and thermal performance.
The promotion of precast studios, with industry professionals providing teaching assistance to colleges and universities.
The certification of installation, a very important life safety issue.
The promotion of the Master Precaster program, ensuring that our plants employ experts in precast manufacturing.
In addition, as an active member of PCI, Wells participates in benchmarking groups. In simple terms a benchmarking group consists of non- competing precasters collaborating and perfecting the best practice use of precast concrete.
By using a PCI or CPCI member, all of this vast and ever-growing wealth of information will be incorporated into your building. This guarantees a state-of-the-art, beautiful building that will stand the test of time.
As we all start to enjoy the warm summer days we need to be aware of the problems that come with concrete temperatures rising with the summer heat. According to the ACI building code we should never pour concrete when the concrete temperature is greater than 90 degrees F and potential concrete problems in hot weather are likely to include:
Increased water demand
Increased rate of slump loss
Increased rate of setting
Increased tendency for plastic-shrinkage cracking
Increased difficulty in controlling entrained air content
Decreased 28-day and later strengths
Increased tendency for differential thermal cracking
Greater variability in surface appearance
ACI 305 “Hot Weather Concreting” defines hot weather concreting as any combination of the following conditions that tends to impair the quality of the freshly mixed or hardened concrete:
High ambient temperature
High concrete temperature
Low relative humidity
The success of many hot-weather concreting operations depends on the steps taken to slow the cement hydration reactions within the concrete and to minimize the rate of evaporation of moisture from the freshly mixed concrete.
The graph below is another good guide that can help you determine the evaporation rate of concrete at any given temperature, humidity and wind velocity levels.
Your concrete needs to be taken care of just like we take care of ourselves in the dog days of summer. If you need to be cooled down just imagine what your concrete is going through as well.
By now, most people know the main advantages of using precast/prestressed concrete on a project. Short construction schedules, all weather construction, plant controlled conditions for pouring high quality concrete, long shallow spans, and the endless number of exterior finish options are the advantages that get all of the publicity. There’s a reason these are the most well-known so I’m not trying to diminish their importance, but today I thought I’d provide a list of the “Next Top 5” advantages of using precast concrete that don’t always get the credit they deserve.
The Roman Colosseum – still standing
Concrete buildings from hundreds of years ago are still in use today. Some say concrete can last up to 2,000 years, and there are certainly many structures around that are well on their way to such a ripe old age. Why? Because concrete gets stronger every day. Unlike most other materials, precast concrete increases in strength over time. What’s more, concrete is designed to minimize the effects of creep and shrinkage, providing a reliable structure for many years to come.
2. HIGH MARGINS OF DESIGN SAFETY
The strength and resilience of precast concrete structures mean that extra safety is always built in, often well beyond what is required by design codes. Precast concrete storm shelters can easily be installed for protection in tornado-prone and hurricane-prone areas. Whether for homes, businesses, schools, bank vaults and increasingly in prison construction, precast is secure against break-ins and breakouts. High strength reinforced concrete is extremely difficult to penetrate and is impact resistant. In some cases, this benefit could be a life-saver.
That middle-age sag
3. LESS STRESS RELAXATION
Keeping in shape is important, and precast concrete is no exception. Some materials “relax” over time, which can be difficult to account for in design. Not precast. A precast concrete structure maintains its shape, size and properties and won’t suffer from middle-aged sag.
4. DAMPENS VIBRATION
Structures like sports stadiums and concert halls are particularly susceptible to vibration from noise and crowd movements, which in some cases can be disturbing to people using the facility. Precast concrete can be used to dampen these vibrations due to its mass, which makes it the material of choice for modern stadium and concert hall construction.
5. DOES NOT RUST
Precast concrete is corrosion resistant and can be used with confidence in very aggressive environments. For example, precast concrete piers are resistant to microbial attack experienced in some marine environments. And tight quality control in production means that cover levels for rebar in any application are ensured.
In the end, there are a lot of factors that go into deciding what materials to use on a building. Whether you’re looking for the often talked about advantages that precast concrete can bring to a project, or the lesser-known ones, the fact remains that precast concrete has quite the list of merits on its side. It’s hard to beat durability, safety, and a lifetime of being the envy of all the other buildings on the block.
Recently, I took part in a review of the 8th Edition of the PCI Design Handbook. These sessions take place whenever a new edition is released for public use. Generally, these reviews would fall into the “watching paint dry” spectrum on the interesting meter; there will be some fairly minor design changes or possibly a reorganization of chapters but nothing earth shattering. That was not the case this year. For this engineer (who received the 3rd edition on my first day of employment), the most noteworthy of the changes in this 8th edition was the REMOVAL of the double tee wall panel design example.
Prior to today’s unlimited architectural shapes and finishes on flat wall panels, there were raked and broomed finishes on flat wall panels. Prior to that, the double tee wall panel was the bread and butter for precasters in this region. They were used for warehouses, gymnasiums, manufacturing facilities, office buildings, etc. Heck, even the Imax theatre at Valley Fair was constructed using double tee wall panels. They’re lighter, stronger, and much easier to erect than flat wall panels. More cost effective also. I’ll grant you that aesthetically there are more options with a flat wall panel, but from a pure functionality standpoint, nothing beats a DT wall panel and it saddens me that its popularity has fallen so far as to warrant exclusion from the Design Manual.
I shared the news with a couple of retired engineers and one of them sent a message back. It read:
“Just think, in a few years we’ll have a
unique economical product that
architects will gush over.”
Precast concrete provides many areas in which to save for a project. Some of these saving are in accelerated design, construction, and finishing processes, areas that aren’t always considered when doing the initial budgeting.
Repetition in precast panels allows for the design process to move fairly quickly through the shop drawing stage. A lot of the time while precast is being designed, the other phases of a job are still being designed. Repetition in panels helps the design process go more quickly, and it also helps in forming requirements that allow for continuous production without changing the bed.
Precast can be erected very quickly, which can knock off days, weeks, or even months from a project’s schedule. Contractors appreciate this because they can get a roof on faster and start interior work without having to worry about the exterior elements.
Since precast panels are produced in a controlled environment, they can be erected all year long. The nasty winter weather doesn’t have an impact on the production schedule or the quality of the product.
Precast panels provide a finished interior wall which eliminates time and cost of framing, dry-walling and taping/mudding. Electrical conduit and boxes can also be cast into a panel. With this benefit of precast you can produce a wall with one trade instead of a combination of five or six for other materials.