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What happens when a pre-stressed strand is cut in a finished piece of concrete?

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 (fpsfse)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.

Dustin Jones, P.E.
Plant Engineer


National Safety Month

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.

  1. 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.
  2. 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.
  3. 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.
  4. 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:

  1. Measuring silica exposure associated with various work areas and work processes
  2. Implementing engineering modifications and administrative controls to reduce exposure
  3. Training employees and supervisors on our Exposure Control Plan; a written set of procedures required under the OSHA rules
  4. Developing medical surveillance and record keeping processes called for under the rules


The Benefits of Using a CPCI/PCI Certified Precaster

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.

Art Macaw, PE
Sales – Canada


As the summer heats up, so does your concrete

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
  • Increased permeability

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
  • Wind speed

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.

Steve Kloos
VP – Quality Control


5 Advantages of Precast Concrete You May Have Forgotten

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

1. DURABLE

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.

Jace Rossow
Sales


They’ll come back. Just like bell bottoms.

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.”

 

I hope he’s right.

Ryan Garden
Vice President – Drafting/Engineering


Precast Benefit: Design – Construction – Interior Finishing

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.

Design

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.

Construction

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.

Interior Finishing

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.

 

Jesse Hiller/Eric Edstrom


The ‘Prefabrication’ Push and Precast Concrete’s Impact

The construction industry as a whole is experiencing an amazing stretch of long backlogs and exciting market forecasts!  However, as with most things, there is another side to that coin.  The labor shortage is becoming more and more apparent with each passing year and we are not seeing any data that would tell us things are going to improve in the near future.  This is true of both labor in the field as well as skilled labor on our manufacturing facilities.  Is there a solution?

Builders and contractors are finding ways to continue to put up new structures and doing so with some innovative pre-planning and using as many prefabricated/preassembled building components as they can.  We have seen this type of construction really take a strong hold in the prison and hotel markets with modular units being completely constructed offsite and brought in on truck to be stacked (like Legos) onsite.  This has significantly reduced the number of laborers needed onsite as well as shortened construction durations.  We are starting to see this ideology grow into more and more building types and it does not look like this method of manufacturing and construction is going to fade away.

As a precast/prestressed concrete manufacturer Wells Concrete has gotten on board with this mindset and asked ourselves what we could do to help our clients.  As it is, insulated precast wall panels already encompass several components needed in an exterior wall assembly: the load bearing element, the interior finish of the conditioned space, the insulating wythe, vapor and air barrier, as well as the architectural color and finish options for the exterior.  Once onsite all it takes is a crane and a few ironworkers to get the panels in place and welded off to the other structural elements of the structure.

The usual next step in the process is bringing in a glazing contractor to install the windows into the precast openings.  This is where Wells Concrete saw an opportunity to save some time and labor needs at the jobsite.  We have teamed up with Marvin Windows and are now offering preinstalled windows as a part of our finished product.  These windows are installed and glazed at our facility so that when the panels are erected at the site the exterior wall assembly is completed in one simple step!!  Once the wall panels are in place, the roof can go on and the building can be weather tight in a matter of weeks vs. several months…and with a very small amount of field laborers needed.

As you can see the Insulated precast wall panel can solve all of your exterior wall assembly needs, be produced/procured offsite and many months in advance of the site being ready.  With the addition of pre-installed windows it is pretty clear that precast concrete components are ready and capable of being an innovative partner in the ever changing world of modular and prefabricated construction ideology!

Curt Gear
Director of Business Development


Graphic Concrete Technology

Throughout the past few years Wells Concrete has been perfecting their architectural capabilities and also expanding their product line to stay current with modern architectural demands. The newest product line we are soon to be offering is Graphic Concrete imaging. This will expand the clients’ and architects’ imaginations and provide an opportunity to put a personal stamp on buildings of the future.

 

 

 

 

What is Graphic Concrete?

Graphic Concrete provides technology for use by architects in the design process, and also for the precast concrete industry to manufacture high-quality patterned concrete surfaces. Builders use these surfaces in industrially-produced façades, floors and walls. Graphic Concrete is a patented technology invented by interior architect Samuli Naamanka, and the technology is a proven concept within the precast concrete industry. An extensive number of completed projects from all over the globe demonstrate the vast range of designs in which architects can use Graphic Concrete, be it public, residential or industrial. When an architect decides to use Graphic Concrete in the design of a concrete surface, the visual quality of the traditional surface is radically altered.

How does Graphic Concrete work?

Graphic Concrete technology is the printing of a surface retarder on a special membrane. Concrete is cast on top of the membrane and the retarder slows the setting (hardening) of the concrete in the selected areas. The unset, softer concrete surface is high pressure washed, revealing the fine aggregate finish. The actual pattern (or image) on the surface of the concrete then results from the contrast between the smooth (or fair-face) finish and the exposed aggregate. Color pigments can be added to the cement and aggregates of different colors can also be used to further enhance a pattern or design. Casting of the concrete elements takes place in a precast plant; the membranes are ordered through a certified vendor and delivered to the precaster specifically chosen for that project by the architect, developer or builder.

Wells Concrete is excited about this new technology and the doors it opens for architects and their creative minds. We hope to be a part of this exciting finish development and will display these finishes once we have a few under our belts. For now, stay tuned!

Scott Monzelowsky
Sales


Lesson of the Day

It is typical and has been for many years to hear the word “cement” used in place of “concrete.” We hear of cement blocks, cement driveways, and cement buildings… It was back in the late 70’s when I was in my Engineering 101 class in college that the instructor, after one of the exams, offered an opportunity for extra credit. He informed us that one could get an extra point for each thing he could list that was made out of cement in the next two minutes. As the entire class rushed to write down as many things that came to mind in two minutes I learned something I will never forget.

Only one person had the correct answer that day: you can make concrete out of cement with the combination of aggregates and water. You see, cement is only an ingredient in concrete – it is a dried powder on its own. In this form, it would make a pretty worthless cement block or cement driveway; the powder would merely blow away in the wind.

So the next time you hear someone talk about cement buildings you have an opportunity to educate them on the differences between cement and concrete.

Gregg Jacobson
VP – Operations


5 reasons general contractors should consider promoting precast on their next jobsite

  1. Control over your jobsite and schedule. With offsite production, pieces are poured and cured in a controlled environment and then stored offsite until they are headed to a particular jobsite. This means you get a high-quality product that requires less onsite labor and has less impact on the construction site. Remember, “Time Is Money.”
  2. Precast building systems can be installed by a relatively small crew in a timely manner. This means fewer trades on site and fewer safety risks. Precast also takes some of the worker-safety liability away from the general contractor thanks to off-site production.
  3. Along with our high quality products, Wells provides a high level of service. With each project Wells designates a specific project manager to see projects through, starting with contract documents all the way until the final checklist has been complete. This results in smooth supplier coordination and logistics, good communication, product knowledge, and product service.
  4. Structural & architectural precast products are typically hoisted from a transport truck by crane and set into final position ready for service, thus eliminating the need for on-site laydown areas, where space is precious.
  5. Precast concrete virtually eliminates weather-related construction delays. Due to our geographic location weather is one of a general contractor’s greatest nemeses. It is uncontrollable and can completely ruin a project schedule. Since a big part of being a GC is managing risk, it makes a lot of sense to eliminate some of the uncertainty.

 

Zac Zimmerman
Account Manager


Safe + Sound

Last June OSHA launched its inaugural “Safe + Sound Week”.  OSHA described the initiative as “…a new nationwide effort that calls on organizations of all sizes in a wide range of industries to raise awareness of the value and importance of workplace safety and health programs.”  They have targeted the week of August 13 – 19, 2018, for this year’s “Safe + Sound Week”.

In keeping with OSHA’s “Safe + Sound Week” launch, last June Wells debuted an internal reporting and tracking system designed to raise the awareness of workplace safety.  The reporting and tracking system, dubbed S.E.E. for Safety Enhancement Effort, asked first level supervisors to pay special attention to their work environment and work crews to either (1) identify potential safety issues to remedy, or (2) recognize co-workers for demonstrating safe work practices.  The system is web-enabled and in cases where potential safety issues are identified, appropriate personnel are automatically notified (e.g., safety / environmental professionals, operations managers, company officers, etc.) and corrective measures are put in motion.  The corrective measures may include an immediate shut-down of operations depending on the nature of the potential hazard.

So far, so good.  The system has heightened all employees’ level of awareness of their work surroundings.  We’ve seen a drop in worker injuries and an increase in preventative activities to make the workplace a safer place.

Visit the OSHA website for more information about Safe + Sound Week.

Mark Del Vecchio

VP of Human Relations and Safety