Tag Archives: specifications

Architectural Project Delivery Goes Back to the Future

Finding BIM’s place
by H. Maynard Blumer, FAIA, FCSI

An active CSI member since 1962, I spent years authoring articles in my local chapter’s newsletter, based on what I had learned while writing specifications and managing an architectural studio—sharing knowledge with my peers in the old-fashioned spirit of the Construction Specifications Institute. Now, a few decades later, I’m ‘coming out of retirement’ to write one more article after I sat in on a presentation at a chapter meeting given by a contractor about building information modeling (BIM).

This is because it became clear how BIM could help answer many problems I had worked to solve over my 50-year career. The technology improves architecture and construction, increases value, and reduces costs.

By incorporating BIM into a system of project delivery documents, architects will return to being the designer, the specifiers, and the arbiters of tradition, rather than computer operators. Shades and shadows will return to design; there will be watercolor, charcoal, and pencils. Contractors armed with BIM will work with their subcontractors. Graphic portions of shop drawing submittals will replace architect-generated detailed construction drawings. Materials suppliers with manufacturers and subcontractors will employ detail designer-draftspersons who will move their employment closer to real construction—perfecting details, eliminating duplication, and reducing construction costs.

Architectural services will provide the starting place with design concepts, complete specifications, and pilot details, constituting the control documents. Contractor-provided BIM documents will replace shop drawings and will be monitored by architects for concept and specifications compliance. Change orders will keep documents in contractual order while incorporating supplier and subcontractor suggested economies. We will be back to the traditional architectural project delivery. Design will have been snatched from the computer and returned to the architect.

By attaching American Institute of Architects (AIA) A201, General Conditions of the Contract for Construction, to appropriate agreements, insurance attorneys and bondspersons know who is covered and who is responsible in accordance with construction case law, as it has been for these years. Ethics and intellectual property will be not be confused. Supplementary Conditions can be provided to cover who does what, as needed, to any project delivery system.

More than a quarter-century ago, I wrote two pieces for The Construction Specifier—September 1989’s “Brand Name Specification” and April 1986’s “Prior Approval” (for materials substitutions). I believe those concepts, along with BIM and AIA A201, provide the keystone for ethical and competitive construction documents that deliver value-added projects when incorporated within any architectural project delivery system.

Maynard Blumer, FAIA, FCSI, is a retired architect and landscape architect living in Paradise Valley, Arizona. He received his bachelor of architecture from Oklahoma State University (then Oklahoma A & M College) in 1953. Blumer directed the production studios of GSAS Architects for 20 years, and practiced as a consulting architect for 27 years in Phoenix, Arizona. He can be reached at bluehmaynard@q.com.

Intent And Interpretation: What I Meant Was…

David J. Wyatt, CDT

When a contractor’s interpretation of a contract requirement differs substantially from the design professional’s initial intent, the assumption is often that he or she is mistaken, and will disrupt the project if allowed to continue uncorrected. To affirm this belief, specifiers will check the documents to verify they are consistent and correct.

If a conversation fails to put things aright and there is a need for formality, then an Architect’s Supplemental Instruction (ASI) is written up to clarify intentions.

The contractor might see this as damage control, keeping the issue open for dispute. In some situations, the ASI may be regarded as a concession of vagueness, and the determined stakeholder may exploit it in that light.

A vague or ambiguously expressed intent makes it difficult for design professionals to prevail in such situations. If a contractor can demonstrate his or her interpretation is a reasonable one, then a mediator, arbitrator, or judge might agree. The author of a document is normally blamed for any ambiguities it contains.

Requests for Interpretation (RFIs) during the project’s procurement stage provide clues to the strength of the design intent. Despite the inconvenience, document revisions will be far less disruptive at this stage than during the construction stage. The hope is discoveries of this sort surface no later than submittal review, when they may have only moderate schedule impact. However, once construction starts, interrupting the schedule to work out problems will likely have an acute effect on several project stakeholders.

Documents prone to multiple interpretations probably lack clarity. Although this problem may not rise to the level of ‘professional negligence,’ it certainly sinks to the level of ‘professional mediocrity.’ It is far easier to prevent this problem than to fight it and win. We can do this by narrowing what famed photographer Diane Arbus called “the gap between intention and effect.”

Pictures, diagrams, graphs, and tables deliver some types of information many times more efficiently than written words—this has scientific basis. It is well to consider when an image can do a better job of delivering information than text.

In non-business interactions, language is used in a less-than-precise manner, relying on other aspects of interpersonal communication—such as body language and tone of voice—to convey intentions. However, this leads to trouble when such habits are brought into the realm of document preparation, where the goal is absolute precision.

A statement requiring more than a couple of readings to understand may become problematic at a critical point. It should be re-expressed in a simpler, clearer way. To do this, it is better to adopt a style of ‘lists’ rather than ‘paragraphs’ when writing a set of requirements, making it easier to see the information, and to expose errors.

Inconsistencies lurk in long sentences and paragraphs where even the author does not notice them. It is a good idea to limit sentences to single lines and paragraphs to three sentences. To the most practical extent, paragraphs of more than three sentences should be divided

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into single-sentence sub-paragraphs.

A partially expressed requirement complements other special knowledge the author has, so it seems complete, but not so to other participants. Therefore, technical requirements should be expressed completely so one without special knowledge can understand the full intent. Authors must leave nothing in their heads.

When specifiers do not know what something means, they must either gain sufficient knowledge of it or take it out of the document. Problems occur when we rely on guide specifications provided by outside sources that do not account for a particular project’s special conditions.

Much has been written to tout the value of peer reviews, yet it is one of the most underused processes available to design professionals. (This is often explained away as ‘not being in the design budget or schedule.’) Many have observed an effective peer review can be done inside the design professional’s office as long as the reviewer is someone not involved in the project.

In The Elements of Style, William Strunk Jr. wrote, “A sentence should contain no unnecessary words, a paragraph no unnecessary lines, and a machine no unnecessary parts. This requires not that the writer make all his sentences short, or that he avoid all detail and treat his subjects only in outline, but that each word tell.”

The commonality in most of this takes us back to the 4Cs of specifying—clear, concise, correct, and complete—as outlined on page 35 of CSI’s Specifications Practice Guide. The stylistic points of concision and clarity help us expose potential errors and ambiguities in our documents. In turn, they help us determine whether a document is complete and correct.

David J. Wyatt, CDT, is the specifications writer for TC Architects (Akron, Ohio), where he is responsible for product research, technical specifications, bidding documents, preparation of project manuals, construction contracts, construction bulletins, shop drawing review, and contract close-out for all project. With the late Hans Meier, Wyatt co-authored Construction Specifications: Principles and Applications. He can be reached at dwyatt@tcarchitects.com.

Commissioning a LEED Platinum Science Building

Photo courtesy Bohlin, Cywinski, Jackson

Photo courtesy Bohlin, Cywinski, Jackson

by Bo Petersson, PE, LEED AP

Design and construction of high-performance buildings involves control systems that are increasingly more advanced. To get the facility to truly perform to its potential, these systems have to work together in an optimal way, interacting to ensure startup and commissioning do not slow down the project’s acceptance-and-completion phase.

This article describes the importance of high-quality contract documents, highlighting important aspects of the specifications to give the commissioning and closeout team a useful tool to help ensure performance as the owner takes over the building. The content is based on this author’s experience with multiple projects, particularly the Dartmouth College Class of 1978 Life Science Center (LSC).

Completed in late 2011, the LSC is a 16,400-m2 (176,000-sf) building consisting mostly of laboratories, auditoriums, and offices. It was designed by architect Bohlin, Cywinski, Jackson (Pittsburgh, Pennsylvania) along with mechanical/electrical/plumbing (MEP) firm, VanZelm Haywood and Shadford Engineering (Hartford, Connecticut).

During the design phase, the entire team focused on meeting an energy goal of no more than 310 kWh/m2-yr (100,000 Btuh/sf-yr)—a tall order for a science building. This number included all energy used in the building: lighting, elevators, plug loads, and a 344-m2 (3700-sf) greenhouse located on the roof. The building was awarded Platinum under the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) program, in no small part due to its energy performance. Since occupied, the actual building energy use has tracked close to the model. What role did the specifications play in this feat?

Brief building description
For the LSC to achieve these results, new collaborative thought processes and the best affordable technologies were required. Its main aspects of the design include:

  • a high-performance building enclosure with sprayed polyurethane foam (SPF);
  • triple glazing and a conscious design effort to reduce the amount of glass to an appropriate level;
  • natural ventilation wherever possible;
  • high-performance enthalpy heat-recovery wheels with low face velocities for improved efficiency;1
  • laboratory equipment waste-heat recovery for outside air preheat;
  • demand-control ventilation for laboratories, offices, and classrooms;
  • chilled beams in laboratories, with radiant cooling and heating in most other spaces;
  • two chilled water systems—a low-temperature system in the air-handling units (AHUs) and a medium-temperature system for chilled beams, radiant panels, and floors with radiant cooling;2 and
  • high level of lighting control with occupancy sensors and daylight-harvesting controls.

There are numerous systems, with a proprietary building controls assembly as the hub. This building management system (BMS) technology interacts with the laboratory controls and the demand control ventilation system. The highly accurate sensors of the latter both monitor carbon dioxide (CO2) levels in classrooms and auditoriums and detect laboratory spills. It also measures relative humidity (RH) in the AHUs and spaces with non-condensing cooling systems. The BMS also interacts in a limited way with the greenhouse and lighting control systems. Additionally, the lighting system is fully computerized and addressable.

Importance of specifications
New construction projects are exciting—there is large equipment onsite, tight schedules, and interesting characters working on intriguing challenges and solutions. Well-thought-out and complete construction documentation is crucial for success. After all, there are often projects where the finer details make the difference between a few minor change orders or an endless stream of change orders and bulletins with resulting cost overruns.

The language in the specifi cations for this space clearly defi ned ‘readiness,’ and the testing was rescheduled without any cost impact to the owner. Photos courtesy Cornerstone Commissioning

The language in the specifications for this space clearly defined ‘readiness,’ and the testing was rescheduled without any cost impact to the owner. Photos courtesy Cornerstone Commissioning

The importance of getting the specifications correct cannot be overstated—not just from a commissioning agent’s perspective, but from the owner’s perspective as well. Additionally, a well-defined project scope makes the entire project easier for everyone involved. A project with fewer change orders reflects well on the whole team.

Time invested up front in the design phase will pay dividends throughout the project’s life, both in energy and in avoided change orders.

Division 01
Division 01−General Requirements is where much of the contractual, procedural, and definition language can be found. Sometimes, it feels as if few people want to venture into this part of the section to work on clarifications and definitions.

Submittal procedures
This process currently happens electronically on most projects. To improve the review process, one should request submittals be sent in a searchable format. The reviewer can look for key words and important features more efficiently.

Substantial completion
Most specification sections define substantial completion as ‘when most finishes have been completed and furniture is ready to move in.’ However, what may seem more difficult to define, and is therefore often ignored, is ‘completion of systems.’

When it comes down to the fundamentals, this is rather straightforward. It is advisable to recommend the testing and balancing work be complete, and the commissioning authority have been able to complete the functional-performance testing (at least the first series of tests) on all vital systems.

Operating and maintenance manuals
Operating and maintenance manuals (O&Ms) are often provided months after the owner has taken possession of the building. However, these documents should be ready before any equipment has been started up. The best way to ensure success for the early delivery is to connect it to the startup of systems and payments.

Use of permanent equipment
Permanent equipment will almost always be necessary to run during the end of construction. It is important all aspects of use are defined or problems may arise. Specifiers need to define who is responsible for upkeep, how it is documented, and what shape the equipment needs to be in when turned over to the owner.

MEP coordinator
On complicated projects, the ability, experience, and clout of the MEP coordinator are the keys to successful completion. While some of this is hard to quantify in writing, there are ways to do it. One should consider requirements for the individual’s background, experience on similar projects, years of experience in the trades, and where this individual exists in the organization chart.

These large mechanical rooms have complicated piping. The specifications called for color-coded piping which has made the facility’s operation easier for the operators.

These large mechanical rooms have complicated piping. The specifications called for color-coded piping which has made the facility’s operation easier for the operators.

This section is important for the construction-and-acceptance phases. The commissioning section needs to define who is responsible for what, and how the commissioning process will take shape. It is important to define labor hours for the trades to complete Integrated System Testing (IST) after substantial completion.

It is often the case commissioning time is planned for construction, but not after substantial completion. By defining the hours required by each trade after substantial completion, this potential change-order discussion can be eliminated.

Divisions 07 through 09
In high-performance buildings, enclosure performance is increasingly an important part of the HVAC system design. It is vital for how the systems will be sized, operate, affect occupant comfort, and make energy model predictions come true.

Minimum thermal and solar performance for doors, windows, and skylights must be defined; allowable air leakage is equally important. There are American National Standards Institute (ANSI) standards that can be used to define the building enclosure performance. Building enclosure testing is a little more difficult, but there are many skilled and experienced companies that can be used for air-leakage testing.

The team needs to define whether the air-leakage testing should be performed on a mockup, on part of the actual building, or, in the most extreme cases, on the entire building. Each approach has positives and negatives, and there is no single method that fits every project, budget, and schedule.

It is often forgotten that buildings such as laboratories and hospitals require internal pressure relationships that cannot be fulfilled unless the separating walls are properly installed. Once all penetrations have been made and sealed, it is important internal spaces are tested for airtightness. This is considerably easier than testing the building exterior enclosure. It is critical this work is scheduled late enough such that walls are nearly finished, but early enough to allow time to address systemic problems.

Divisions 11 through 14
The main energy impact issue is in Division 14−Conveying Equipment. Vertical transportation, especially elevators, can be a big part of a building’s electrical usage. Currently, there are many ways to accomplish energy-efficient vertical transportation. Depending on what systems are being selected, one should look for simple, yet effective ways to reduce the electrical consumption. More mundane hydraulic systems may benefit from simple items such as variable frequency drives (VFDs) and premium-efficiency motors.

Divisions 22 and 23
For plumbing (Division 22) and HVAC (Division 23), there is a dizzying array of considerations when writing the specification. Many of these will have a big impact on the final product, operating efficiency, and cost.

For instance, it is critical to ensure the energy efficiency metric cited is relevant and applicable to the particular project. The selection of condensing boilers for use in systems designed with 82-C (180-F) supply temperatures provides one example. The boiler will not operate in the condensing mode, making its rated efficiency irrelevant. While it is tempting to specify a boiler that is as efficient as possible for lower temps, it may not be as efficient as other options when considering the actual operating temperatures. Often, the design team can be challenged to construct a system that uses lower water temperatures allowing for the efficiencies of condensing boilers.

Another consideration involves specifying the efficiencies of the largest energy-users where the ‘biggest bang for the buck’ can be realized. Chillers, for example, are obvious components where the specifiers should consider factory testing to ensure the best possible performance.

Today’s buildings often contain multiple systems of electronics that need to interface with each other. While many find this side of specifications writing confusing and complicated, it is extremely important to get this right. Therefore, it is crucial someone in the design team with experience in control system architecture and how equipment can interface with each other, document how it should be done in the contract documents. Considerations such as self-contained controllers versus active controlling by the BMS need to be determined.

There are often two sides to this. It can be advantageous to take control of a device so one can clearly see in the BMS what it is doing and make adjustments. However, it can also be best to leave this to the device manufacturer, as it will know its system the best. This is a case-by-case judgment call.

Division 25
It is important control specifications are written correctly to ensure the end result everyone wants. The control sequences have to be reviewed carefully to make sure they meet CSI’s 4 Cs; that is, that they are clear, concise, correct, and complete. Further, they must be as straightforward as possible—complex sequences are often misunderstood and are more likely to be overridden with high-energy use over time as a result.

The specifications must also include monitoring and alarming of the smaller items that are often overlooked such as critical lab equipment and feed and condensate pump set alarm points. The reviewer shall also carefully consider what devices will benefit from features such as end switches and proper feedback loops.

This photo shows a dirty chilled beam. Thanks to the clear language in the specifications, the contractor had to clean them thoroughly.

This photo shows a dirty chilled beam. Thanks to the clear language in the specifications, the contractor had to clean them thoroughly.

One must define sensors that are of high quality and that will not drift over time, wasting energy due to sensors out of calibration. When a CO2 sensor is off by 20 percent, it can make a huge difference in how much outside air a system brings in.

Specifications should also include language for ample test time with the commissioning authority, and verification of sensor and actuator accuracy.

Divisions 26 through 28
The electrical specifications have many aspects that can affect the building’s energy performance over time. Light fixtures are an obvious example. The reviewer should consider what will likely be the best available technology at the time of construction. Light-emitting diode (LED) technology is rapidly evolving—what may be rather expensive today could be less expensive at the time of construction.

In this same vein, lighting controls have come a long way. There are new computerized systems with amazing capabilities. However, the application needs to be carefully discussed with the end users. Complex lighting control systems may be too hard to operate; for a particular project, it may be more suitable to use smaller and simpler lighting control systems.

The reviewer should also be informed of, and look for, the most efficient transformers.

There are many projects with lofty energy goals that may never be able to perform as intended because the specifications were not clear or developed enough to produce the end results that everyone involved wanted to see. While design reviews are important and often performed rather well, it is imperative the reviewer also spend a considerable focus on the project specifications.

1 For more on enthalpy wheels, see the article, “Reducing Building HVAC Costs with Site-recovered Energy,” by Stephen J. Pargeter, in the March 2012 issue of The Construction Specifier. Visit www.constructionspecifier.com and select “Archives.” (back to top)
2 The low temperature is supplied by the campus system. Medium temperature is generated from a local high-performance chilled water system. (back to top)

Bo Petersson, PE, LEED AP, is the director of engineering services at Cornerstone Commissioning. He oversees the development and maintenance of all technical processes and documentation, and manages the commissioning staff. Petersson has a master’s degree in mechanical engineering and has been working in the HVAC industry for 25 years. He has been a speaker at conventions and conferences including Labs21, Lab Design, and chapter-level (APPA) and American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) meetings. Petersson can be reached via e-mail at bpetersson@cornerstonecx.com.

Specifying Steel Fibers for Concrete Floors

Photo © BigStockPhoto/Jacek Sopotnicki

Photo © BigStockPhoto/Jacek Sopotnicki

by George Garber

Thin, short strands of steel fiber are being specified more and more as reinforcement in concrete floors. Sometimes, these fibers are used on their own, and sometimes they are used in conjunction with conventional reinforcing steel. They appear in ground-supported slabs and in composite steel deck slabs.

In ground-supported slabs they are used to control cracks, to allow greater joint spacing, and to justify thinner slabs—though the last goal is controversial because it involves properties of fiber-reinforced concrete that experts disagree on. In composite steel deck-slabs, fibers can replace traditional wire mesh to control shrinkage cracks.

Structural engineers are still figuring out how best to design floors with steel fibers. American Concrete Institute (ACI) 360R-10, Guide to Design of Slabs-on-ground, offers guidance on their use in ground-supported floors. Steel Deck Institute (SDI) C-2011, Standard for Composite Steel Floor Deck–Slabs, gives basic rules for using them in composite steel decks. However, neither document represents the last word on the subject, so the research continues. Meanwhile, specifiers need to think about how to define this material in contract documents.

Fibers are put into concrete are batched by mass, so volume-based specifications must be converted. This table shows equivalents for specified doses. Images courtesy George Garber

Fibers are put into concrete are batched by mass, so volume-based specifications must be converted. This table shows equivalents for specified doses. Images courtesy George Garber

Whenever people learn a job will include steel fibers, the first question is always some variant of ‘how much?’ It seems everyone wants to know the fiber dosage, which is usually stated as the mass added to each unit volume of concrete. Typical units are kilograms per cubic meter (kg/m3) or pounds per cubic yard (lb/cy).

Dosage matters, of course, but it is just a start, because not all fibers are alike. If other key details are not specified, the result is concrete that contains the specified mass of fibers, but does not fulfill the designer’s intentions.

Steel fibers in specifications
Since steel fibers can be considered a kind of reinforcement, it is tempting to stick them in MasterFormat Division 03 20 00–Concrete Reinforcing, with rebar and wire mesh. However, fibers are better handled in Division 03 30 00–Cast-in-place Reinforcing or Division 03 24 00–Fibrous Reinforcing. If fibers are put in their own section, it should be referred to in Division 03 30 00–Cast-in-place Concrete as this is where the concrete contractor and ready-mix supplier will look. If the specifications include a special section for the concrete floor, then that would be a good place for the steel fibers.

Every steel-fiber specification should incorporate, by reference, ASTM A820, Standard Specification for Steel Fibers for

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Fiber-reinforced Concrete. This document lays down rules for strength, bendability, dimensional tolerances, and testing that apply to all kinds of steel fibers commonly used in concrete floors. Fibers must have an average tensile strength of at least 345 MPa (50,000 psi). They must be flexible enough to be bent 90 degrees around a 3-mm (1/8-in.) rod without breaking. They cannot vary from specified length or diameter by more than 10 percent. (This does not need to be put in the project specifications, because ASTM A820 does the work for you.)

ASTM C1116, Standard Specification for Fiber-reinforced Concrete, can also be incorporated into specifications. This standard regulates how fibers are added to the concrete mix.

Citing ASTM A820 and ASTM C1116 is never enough, however, as these standards explicitly leave important decisions to the designer. A complete specification covers all these points:

  • dosage;
  • type;
  • length;
  • effective diameter or aspect ratio; and
  • deformations.
From top: Type I fiber 50 mm (2 in.) long, Type II fiber 25 mm (1 in.) long, and Type V fiber 35 mm (1.3 in.) long.

From top: Type I fiber 50 mm (2 in.) long, Type II fiber 25 mm (1 in.) long, and Type V fiber 35 mm (1.3 in.) long.

This photo shows deformations on hooked ends and continuous.

This photo shows deformations on hooked ends and continuous.









Fiber dosage
The amount of fiber is usually specified by mass of fibers per unit volume of concrete—this is measured in kg/m3 or lb/cy. An alternative is to specify the fiber volume as a percentage of the concrete volume. This makes a lot of sense, especially during the design stage. A volume percentage is easier to visualize, and it stays the same across all measurement systems. However, the workers who actually put the fibers in the concrete have no way to batch by volume. They can only batch by mass, so any volume-based specification will have to be converted along the way. Figure 1 shows equivalents for some specified dosages.

Fiber dosages generally range from 12 to 42 kg/m3 (20 to 70 lb/cy). Dosages below that range are occasionally specified when fibers are used to replace light-gauge wire mesh. Dosages above that range are rare.

Setting fiber dosage is not an exact science, but ACI and SDI offer guidelines. According to ACI’s Guide to Design of Slabs-on-ground, the fiber dosage in ground-supported slabs should never be less than 20 kg/m3 (33 lb/cy). When the purpose of the fibers is to allow a wider joint spacing, this guide recommends at least 36 kg/m3 (60 lb/cy). SDI’s Standard for Composite Steel Floor Deck–Slabs, has a short, simple recommendation for steel fibers in composite steel deck-slabs: use at least 15 kg/m3 (25 lb/cy). In the end, though, the decision is up to the floor designer, who may rely on experience or on recommendations from one of the steel-fiber manufacturers.

ASTM A820 divides steel fibers into five types, based on how they are made:

  • Type I—cold-drawn wire;
  • Type II—cut-sheet steel;
  • Type III—melt extract;
  • Type IV—mill cut; and
  • Type V—cold-drawn wire, shaved into fibers.

Only Types I, II, and V are currently being used in concrete floors.

Deformations including hooked end, flat end, and continuous are shown here.

Deformations including hooked end, flat end, and continuous are shown here.

Predictably, fiber manufacturers disagree on which type works best. From the user’s point of view, the main issue is certain properties may not be available in all types. For example, in the current market the only fibers being made with hooked ends are Type I.

When discussing fiber type, one must watch out for confusion between ASTM A820 and ASTM C1116. ASTM A820, which deals only with steel fibers, divides them into the five types listed above. In contrast, ASTM C1116, which deals with all kinds of fibers, divides fiber-reinforced concretes into four types, depending on what kind of fibers they contain. In ASTM C1116, concrete with steel fibers is called Type I. Types II, III, and IV contain glass, plastic, and cellulose, respectively.

Thanks to the two different classifications, one can end up with a Type I concrete mix that contains, say, Type II steel fibers. It is important to remember the classification in ASTM A820 covers fibers, while the classification in ASTM C1116 covers concrete mixes.

Fiber length
The steel fibers used in concrete floors range in length from 25 to 65 mm (1 to 2 1/2 in.).

While it is usually agreed length matters, there is no consensus as to which length is best. It depends on what the fibers are expected to do. Engineers who rely on fibers’ ability to limit the widening of cracks after they have formed—a property called residual strength, ductility, or flexural toughness—tend to prefer longer fibers. Those who rely on fibers’ ability to prevent visible cracks tend to prefer shorter ones, because they result in higher fiber counts and less distance between fibers. Concrete workers also like shorter fibers, which are less likely to tangle and stick up above the floor surface.

Both piles have the same mass, but the 25-mm (1-in.) fibers outnumber the 50-mm (2-in.) fibers almost eight to one.

Both piles have the same mass, but the 25-mm (1-in.) fibers outnumber the 50-mm (2-in.) fibers almost eight to one.

There are limits in both directions, however. The upper limit seems to be close to 65 mm, and any longer will clump to form balls. Even fibers in the 50 to 65-mm (2 to 2.5 in.) range can tangle, and to prevent that problem are sometimes sold in collated form—stuck together with a weak glue that dissolves as the concrete is mixed. The lower limit has not been well-established, but fibers less than 25 mm long are seldom used in concrete floors today. Researchers are working with even shorter fibers, though, so floor designs that rely on lengths below 25 mm may be seen eventually.

If a design is based on fibers of a particular length, the specification should require that length. Length is specified as a single target value (not a maximum or minimum), with an implied tolerance, according to ASTM A820, of ±10 percent.

Effective diameter or aspect ratio
For a fiber with a circular cross-section, the effective diameter is that of the circular section. For a fiber with a cross-section of any other shape, the effective diameter is that of a circle equal in area to the actual section.

For fibers of Types I to IV, the effective diameter is specified as a single target number, with an implied tolerance of ±10 percent. Type II fibers, which are rectangular in section, can be specified by width and thickness instead of effective diameter. Type V fibers are supposed to be specified differently. Since the manufacturing process for Type V results in substantial variation in effective diameter, ASTM A820 suggests specifying a range with upper and lower limits, not a target. However, this rule is not universally followed. Some makers quote a single effective diameter for their Type V fibers.

People sometimes talk about a fiber’s aspect ratio instead of, or in addition to, its effective diameter. Aspect ratio is length divided by effective diameter. Since any two of those properties determine the third; all three do not need to be specified. When aspect ratio is specified, be aware ASTM A820 allows the measured value to vary by ±15 percent from the specified target.

In today’s marketplace, effective diameters range from 0.58 to 1.14 mm (20 to 40 mils). As with length, choosing the diameter involves trade-offs. Thicker fibers are less likely to tangle while thinner result in higher fiber counts.

Many people worry steel fibers will show at the floor surface, making the floor look worse. This floor, made with colored concrete and Type II fibers, 25 mm (1 in.) long, shows the steel fibers need not affect appearance.

Many people worry steel fibers will show at the floor surface, making the floor look worse. This floor, made with colored concrete and Type II fibers, 25 mm (1 in.) long, shows the steel fibers need not affect appearance.

Fiber count
Fiber count—the number of fibers per pound or kilogram—is an important factor in the effectiveness of steel fibers as concrete reinforcement. Higher counts result in less distance between fibers, and that generally means better performance. A floor design based on a particular fiber count may not work as well with a lower count, even if the mass of fibers stays the same.

Though fiber count is never directly specified, it is determined by two properties that are specified: length and effective diameter (or length and aspect ratio, depending on preference). Since shorter fibers are usually thinner, too, reducing the length dramatically raises the fiber count. In this figure, both piles have the same mass. The fibers on the right are 50 mm (2 in.) long and have an effective diameter of 1.14 mm (0.04 in.). The fibers on the left are 25 mm (1 in.) long and have an effective diameter of 0.58 mm (0.02 in.). The shorter fibers outnumber the longer fibers, almost eight to one.

Fiber count can be determined from the following equations:

In metric units:

c = 1/[(7.9 x 10-6)Lπ(d/2)2]

Where c = fiber count per kilogram
L = length of fiber in millimeters
d = effective diameter of fiber in millimeters

In U.S. customary units:

c = 1/[(0.29Lπ(d/2)2]

Where c = fiber count per pound
L = length of fiber in inches
d = effective diameter of fiber in inches

Fiber counts range from about 2500 to 20,000 per kilogram (1100 to 9000 per pound).

The earliest steel fibers were smooth, straight pins, and ASTM A820 still recognizes that shape as an option. In practice, though, the fibers used today are all deformed so concrete can grip them better. Deformations take one of three forms: continuous, hooked ends, and flat ends.

Steel fibers are being loaded into a ready-mix truck. Fibers are usually added at the concrete plant, but can also be added onsite. [CREDIT] Photo courtesy Mike McPhee

Steel fibers are being loaded into a ready-mix truck. Fibers are usually added at the concrete plant, but can also be added onsite. Photo courtesy Mike McPhee

A continuously deformed fiber has waves or bumps running down its whole length, much like ordinary steel rebar. A hooked-end fiber has a bend—or multiple bends—at each end. A flat-end fiber has its ends squashed flat, somewhat like a kayaker’s double-ended paddle.

While dosage, length, effective diameter, and deformation are the essential features every steel-fiber specification should cover, a few other details are worth considering.

Consider requiring fibers be delivered in containers marked to show the mass. Some specifiers go further and demand containers that include the exact quantity going in each cubic meter or cubic yard of concrete. If the specified dosage is 20 kg/m3 (33 lb/cy), each box or bag would have to contain exactly 20 kg (33 lb). This simplifies batching and reduces the risk of error. Some suppliers may have trouble packaging fibers in anything other than standard quantities.

Fibers should be stored under cover, protected from rain and snow. Left outdoors, boxes can disintegrate and fibers can rust.

Last, it is a good idea to insist all concrete tests, including those needed for approval of the mix design, be made after the addition of fibers. That may seem like common sense, but it will not always happen without a reminder.

Of course, it takes more than the right specification to make a successful steel-fiber-reinforced floor—it also takes a smart designer and a careful contractor. Nevertheless, a full, accurate specification is an essential part of the job when the floor is expected to fulfill the designer’s intentions.

George Garber is the author of Design and Construction of Concrete Floors, Concrete Flatwork, and Paving with Pervious Concrete. Based in Lexington, Kentucky, he consults on the design, construction, and repair of concrete floors. Garber can be reached by e-mail at ggarber@iglou.com.



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Colin Gilboy, founder of 4specs website, estimates there are only 800 W-2 employed and 1099-independent specification writers currently working in the United States. (By his reckoning, it takes 30 to 50 employees to support a full-time specifier.) The rest of the specification writing is done by people for whom specification writing is not their primary task, such as project architects. Although this number may be relatively small when compared to similarly situated specialists serving other professions, specifiers are a stable and dedicated subset of the design profession. Continue reading