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What Do You Mean By ‘Polyurethane?’

All photos courtesy Quest Construction Products

by Steven Heinje

‘Polyurethane’ is not a useful specification term on its own. It is akin to saying, “I want a form of transportation,” as opposed to something much more specific, like, “I want a four-cylinder sedan.” Getting to the point where a manufacturer or supplier will only offer products meeting a specific project’s needs will require more precise and non-proprietary terminology that makes performance the outcome.

The term ‘polyurethane’ simply means many repeated units of urethane, much in the same vernacular as ‘polypropylene’ or ‘polyester.’ Often, these coatings are referred to as ‘urethanes,’ similar to the way terms like epoxy or acrylic are employed. They are all polymers of the key reactive or function group. The urethane group confers many of the traits of a polyurethane—most notably, adhesion—in much the same way most acrylic paints share ultraviolet (UV) resistance.

Typically, a Technical Data Sheet (TDS) for a polyurethane coating will include a long list of tests, often with some impressive numbers. A few of these will find their way into various specifications. Perhaps they will include a benchmark product and later a few ‘equals’ will be added—such a specification is representative of the standard of care for using a polyurethane in today’s construction marketplace.

This article seeks to add some background and a few key details so specifiers understand what type of product is actually being offered, what are its intrinsic performance attributes, and whether it truly has adequate offsets. (The focus is on coatings; polyurethane foams for insulation are outside this article’s scope.) A little chemistry is required to fully grasp the concepts, but the desired outcome is for design professionals to know what questions to ask and ultimately to be able to write better specifications and avoid failures.

This 100 percent aliphatic polyurethane has exceptional ultraviolet (UV) resistance needed to hold this deep blue color on the very visible roof of Marin County Civic Center (San Rafael, California).

The flexibility and hydrophobicity of a specific polyurethane was chosen over an epoxy alternative for this plant near Shanghai.

Why use a polyurethane?
Polyurethanes have the broadest range of any product group: they can be soft as a baby’s skin or hard enough to be machined and tooled. Some may yellow and chalk severely, while others are among the most light-stable coatings available. Chemical resistance, including water resistance, is similarly variable.

Polyurethanes combine toughness and flexibility—a unifying trait that comes from the urethane linkage itself. Urethanes form a molecular spring based on the intense attraction of the urethane groups to each other, so they form hard domains within a softer and more flexible matrix. This link is flexible, and more importantly, it is recoverable; this leads to products that combine high tensile and high elongation, and even maintain high hardness and flexibility at low temperatures (down to −30C [−22 F]) and up into higher temperatures (75 C [167 F]). This feature allows them to be used as high-performance coatings, as well as durable foams.

In most cases, another key trait is adhesion. As a rule, urethanes bond to various substrates, often better than styrene-ethylene/butylene-styrene (SEBS), acrylic, silicone, and even butyl. Adhesion is important to consider because maintenance coatings are often used over unknown, older surfaces, or even multiple surfaces giving the contractor or specifier a good reason to specify a polyurethane above other more-restrictive options.

Beyond toughness and flexibility, all other traits—gloss retention and resistance against abrasion, water, UV, solvents, and acids/alkalines—depend on the ‘backbone,’ as defined later in this article. Application traits are another mixed bag, relying on the nature of the curing chemistry, which is referred to as the ‘curative.’ With the appropriate formulation, there is no other class of product that has a better balance of adhesion, UV resistance, abrasion resistance, and flexibility. By contrast, epoxies are too hard, acrylics exhibit poor abrasion performance, and silicones show relatively weak physical properties as a product class. There are few properties a urethane cannot be formulated to achieve.

However, high heat resistance is an overall weakness. Below 75 C is urethane territory: applications include roofing and most industrial and sealant applications. While some specialty urethane products can perform long-term above 100 C (212 F), these high temperature applications typically require silicone and epoxy products.

This polyurethane system has exceptional fire retardancy for use on sloped roofs.

A low-toxicity polyurethane coating was used on the structural concrete for this wastewater treatment plant.

Types of urethanes: Varnish to airplane components
ASTM D16, Standard Terminology for Paint, Related Coatings, Materials, and Applications, defines six types of polyurethanes.

Type I: One-package (1K) urethane alkyds¹
Like all alkyds, or ‘oil-based paints,’ these cure by oxidation of a drying oil and solvent evaporation. They are used as varnishes, floor finishes, and abrasion-resistant paints. They do not cure by a urethane reaction. In this category, the oil-based paint becomes a platform for using an aromatic polyurethane to enhance the former’s performance. (This is also done with acrylics and epoxies, but in those cases they are not referred to as ‘acrylics’ or ‘epoxies’ in the way varnishes and paints are called ‘polyurethanes.’) Looking at toxicity, they have no free isocyanate, which means they are no more toxic than other alkyds.

Type II: 1K moisture-cured paints and industrial coatings
This important type is often employed in high-performance thin film (i.e. 25 to 75-µm [1 to 3-mil]) floor and industrial paints. This class also includes higher-build (i.e. 0.5 to 2-mm [0.02 to 0.08-in.]) elastomerics such as those described in ASTM D6947/D6947M, Standard Specification for Liquid-applied Moisture-cured Polyurethane Coating Used in Spray Polyurethane Foam Roofing System. This class uses atmospheric moisture to act as the curative, allowing them to be reactive 1K products. This can cause problems and limitations along with advantages.

Unreacted isocyanates, such as those found in Type II urethanes, react with many other chemical species found in wood, metal, epoxies, and acrylics—this gives these resins exceptional adhesion. Moisture cures are often high in solids, relatively low in viscosity, and have moderate volatile organic compound (VOC) content. When they cure, however, they release a lot of carbon dioxide (CO₂) gas. Consequently, Type IIs used in flooring and corrosion protection need to be applied thinly, so not too much gas is evolved from curing. Elastomerics have a lower concentration of isocyanate, allowing them to tolerate thicker films before they exhibit the same foaming problems.

The moisture cures require experienced contractors because bubbles, foaming, and blisters present high risks. Most of these Type II products are aromatic, which means they will yellow. ASTM D6947 moisture-cured coatings used in roofing are typically blended with asphalt. These sealants and coatings are not UV-stable, but when used for roof details and crack-bridging or caulk applications at thicknesses approaching 2 mm, they can weather for more than a decade.

Type III: 1K heat/oven-cured elastomers and industrial coatings
These elastomerics are generally not used as maintenance coatings. Instead, they are reserved for industrial products, car bumpers, or technical fabrics for sportswear.

Type IV: Two-package (2K) products
These materials are similar to the Type IIs; in fact, some may simply be a Type II product offered with a liquid curative. This eliminates the need to rely on atmospheric moisture to cure the resin, and it avoids the release of CO₂ gas. They generally have higher solids content and moderate pot-life in the range of an hour. They are still prone to problems with CO₂ gas when used carelessly.

As an example of diversity within polyurethanes, there is a Type IV industrial that uses a pure aliphatic resin cut in solvent and is cured as a polyurea—it achieves 34,475 kPa (5000 psi) tensile and 400 percent elongation, is easily sprayed, and has a two-hour pot-life. Further, as a polyurea, it is not affected by moisture at all.

Another case is a sub-group used in roofing and flooring: moisture-triggered (oxazolidine) cured aliphatic urethanes. They are described in ASTM D7311/D7311M-10, Standard Specification for Liquid-applied, Single-pack, Moisture-triggered, Aliphatic Polyurethane Roofing Membrane. This is another example of a specific urethane chemistry not prone to gassing even in thick films due to the special chemistry of the oxazolidine curative.

Type V: 2K urethanes
Important commercial products used in industrial applications (and to a lesser extent in roofing), these urethanes are typically used in a 1:1 volume ratio with plural component equipment.²

Most are 100-percent-solids elastomers or coatings. A sub-group, called ridged polyurethanes, is specifically devised for tank, pipe, and petrochemical use. Some have enough pot-life to be rolled or squeegeed, but most require plural component equipment, in-line heat, and will cure within minutes. Also within this group are the so called polyureas, which represent another approach of avoiding the issue of atmospheric moisture and CO₂ gassing by using curatives that produce an almost instantaneous reaction. These require the use of an impingement mixing plural component spray gun. The reality is most polyureas are hybrids of both polyurea and polyurethane reactions and should be treated as another flavor of the Type V polyurethanes.³

Type VI: One-package solvent-based urethane lacquers
VOC restrictions have largely eliminated this type of polyurethane from commerce, but they may still be found in adhesive applications. (Water-based urethanes, or ‘urethane latex,’ could also be included here.) Often blended with acrylic emulsified resins, these are finding use in floor coatings, primers, and other high-performance and specialty applications.

There are a few words of caution related to the use of the term ‘urethane,’ which can be a marketing selling point. There are examples where a urethane material has been used to thicken an acrylic latex product, which is then illegitimately branded as a urethane. In other cases, a low level of a legitimate urethane resin may be added for marketing purposes. A urethane’s key properties—being flexible and tough—does not ‘play well’ with the high levels of fillers typically included in vinyl, styrene-acrylic, and acrylic paints. If a product has a high density (i.e. > 1.4 g/L [11.7 lb/gal]), then it is functionally not a urethane.

For this underground concrete storage tank, a polyurethane able to resist alkaline patching materials was required.

Form follows function: A primer on the chemistry
With so many different types of polyurethanes, a specifier has many factors to consider in selecting the right product for the job. The three key components to understanding what a polyurethane provides in terms of performance are:

isocyanate used;
curing chemistry; and
backbone.

For most specifiers, the best understood difference between types is aliphatic (highly UV-stable) versus aromatic (less UV-stable) urethanes, which is based on the type of ‘iso’ they have. The curing chemistry is generally recognized and understood in terms of application by the experienced contractor.

Less obvious is how the curing chemistry also has an impact on the film or membrane’s long-term performance. Most specifiers understand urethane varnish acts and bonds like a varnish. They know 2Ks and moisture cures tend to have good adhesion (due to unreacted isocyanate) and they know ‘polyureas’ require specific equipment and are not very moisture-sensitive. However, less visible to specifiers, is the ‘backbone.’

The backbone is the softer portion that forms a matrix around the urethane groups. The backbone is itself a polymer, and so it bring its own set of strengths and weaknesses. For example, if the backbone is not UV-stable, even an aliphatic urethane may not exhibit good weathering. This level of detail gets lost in technical data and specifications; it is rarely adequately captured by the test reports—and this is where things can go wrong.

While the list of materials shown in Figure 1 covers most products, it is not comprehensive—there are more than 300 potential combinations, and each could perform quite differently. Further, this list does not take into account the fact coatings are compounded products that allow blending of these materials in various ratios. Fortunately, a much smaller subset sees use as coatings.

Figure 2 lists polymer backbones representing at least 95 percent of the commercial volume of coatings and their typical application. Drilling down a little deeper, the resistance properties in Figure 3 make product selection clearer and more specific. For example, an aliphatic acrylic is a step above most polyester-based urethanes, and so they find use in automotive applications. Castor is a bit weak physically, but its water-resistance and low viscosity mean it can be used to good effect in a 100 percent solids adhesive.

Castor backbones can be found blended with other agents to achieve a lower VOC. This is significant because increasingly restrictive VOC rules favor materials inherently lower in viscosity. One hears more about polyethers, castor, aspartic esters, and polyetheramines (as polyureas) being used to meet VOC restrictions. Often, the backbones that require the most solvent are the toughest. Materials including polytetrahydrofuran (PTHF) and acrylic polyols are seeing less use. The material polycaprolactone, which once might have found use in a high-end roofing product, will now be used as a substitute for an acrylic in a less cost-sensitive industrial paint application as it moves up the VOC and cost ladder. Products are changing in response to regulation, and achieving lower VOC content is linked to changes in the backbone and curative chemistry.

For this reason, one should always specify with reference to the backbone. When a certain product worked as a low-build industrial finish for metal, and was an aliphatic acrylic, then “aliphatic with acrylic backbone” should be accordingly specified. It is important not to overlook referencing the material safety data sheet (MSDS) as a possible source for this information.

At the same time, a specifier will be forced to accept compromises to meet VOC restrictions; he or she must be careful about these inevitable substitutions.

Urethanes in systems
Another area where urethanes excel is to serve roles within coating systems to meet specific challenges. A common roofing practice is to use an aromatic primer or base coat with an aliphatic top coat. These pairings can also be done with other properties.

Today, a manufacturer might use a primer based on aromatic isocyanate and castor that will have lower costs, reduced VOC content, and excellent corrosion resistance, and then pair it with an impact-resistant, aliphatic polyester top coat that meets a lower VOC rule—together, they cover the needs.

A traditional combination is to use an epoxy primer and an aliphatic urethane surface coat. This works because the epoxy primer typically has unreacted amines available to the isocyanate for reaction—a single molecule between the primer and surface coat. The point is these products work as a system, and substitutions that are not specific cannot be expected to perform the same.

Specifications, testing, and what data sheets do not reveal
In the paint industry, there is little standardization and few requirements for third-party testing. In roofing, there are more specific protocols, but not many for polyurethanes. When looking at a technical data sheet, the reality is the values shown—for example, “250% elongation per ASTM D412”—may state a method, but are not well-defined and often lack sufficient context for the public bid process and apples-to-apples comparisons.

Although an ASTM test is cited, the values reported could have been derived in many ways and under different conditions. For example, manufacturers could run the same test methods under two conditions to get optimal values for tensile and elongation. This is not to suggest TDSs are all useless or deceptive, but if one were to compare products between manufacturers based on independent testing and technical data, the information may be nebulous. Third-party testing organizations tend to maintain a more consistent practice and should be requested.

The more accurate way to compare data is to reference a specific protocol. In roofing, these would be:

ASTM D6497, Standard Guide for Mechanical Attachment of Geomembrane to Penetrations or Structures;
ASTM D7311; or
ASTM WK9048, Elastomeric Coatings Used in Spray Polyurethane Foam Systems (the current designation for the new classification due to be published next year).

These material standards include specific test methods: they define how the test is run, how the sample is cured, how thick it is, and other parameters allowing the data to be used for commercial comparisons.

Another hard truth about testing is it is often not demanding. In many cases, procedures are run without comparison to some type of control or to unfavorable conditions; consequently, they are unlikely to be run until failure. For example, it is not unlikely for a urethane to sport a D412 tensile of 13.8 MPa (2000 psi). That same product tested after immersion in water for 28 days might only have a tensile of 2758 kPa (400 psi), but this would not be reported on the TDS. Unfortunately, the post-immersion tension value is the more appropriate one for a deck coating. Another product may note a 6895 kPa (1000 psi), but may have a 8274 kPa (1200 psi) after 28 days of immersion. In short, traditional paint testing does not predict service life.

Most troubling is how this data gets used in commerce. A bid may be lost for $0.20 on a basis of $30 per gallon, which is less than one percent of the total. This process rests on the assumption testing values are the most relevant factor, and they allow for direct comparison between different products. The tests are not nearly as precise as the accounting, and worse, the tests typically are not run in the same manner. If the product is only described as a ‘polyurethane,’ the chances are good one will be picking a product based on only the tensile value under ASTM D412, Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers.

There are many other important product attributes that need to be considered beyond this limited view—for example, ASTM D471, Standard Test Method for Rubber Property: Effect of Liquids, which tests the effects of water, is singularly useful. It is important to bear in mind any number without context is not necessarily good information and a low test value is not reason enough to reject a product with a long track record of performance. “Or equal” substitutions in specifications involving polyurethanes are intrinsically risky.

Construction specification documentation needs to be better for this group of products. Using the ASTM D412 test for tensile/elongation as an example, instead of accepting:

ASTM D412 > 23.8 N/mm (2000 psi)

one should call for:

ASTM D412 Tensile initial and after 1000 hours of ASTM D4798 weathering at 0.35 watts/m² @340 nm.

Part of a material standard can be called out even when it does not quite fit, such as ASTM D6497. When the application is critical, a ratio—rather than a minimum value—should be specified. This example adds specific parameters to the test method, along with a ratio:

less that 30% loss of the original value when run per ASTM D-6497.

Another approach is the addition of a basic chemical description, such as:

aliphatic polyurethane using polyurea curative, IPDI and polyether chemistry per ASTM D6497.

Conclusion
Polyurethanes are a useful and necessary part of coatings technology, but they are often misunderstood—the confusion can ultimately lead to failures. Specifying a polyurethane by its backbone chemistry is the most important and necessary step to ensure the right product is being chosen for the job.

Before selecting a product, specifiers must ask themselves what does it really need to do, where do these products go wrong or how do they fail, and are the properties that play into these failures really reported?

Knowing which tests and standardized test methods, or ratios, to use for a specific application can provide the same benefit: a specification that links the track record and expected service life with the specific parameters required for the application. The best approach, however, is not a long list of random test methods, but rather specific testing protocols such as those cited in this article that give the type of comparable data needed for use in open bids.

Notes
¹ ‘One-package’ (also written as 1K for ‘Kompent,’ the traditional abbreviation) means the entire product is a single can that does not require mixing in or another catalyst, agent, or Part B. (back to top)
² The SSPC: The Society for Protective Coatings has information on the various types of Type V urethanes. Molded parts can even be made with these products. Search www.sspc.org for more data. (back to top)
³ Design professionals can learn about these products from the Polyurea Development Association (PDA). Visit www.pda-online.org. (back to top)

Steven Heinje is the technical service manager for Quest Construction Products, headquartered in Phoenix, Arizona. He has degrees in biology and chemistry, along with an MBA. Heinje has 30 years of experience in roof coatings, specializing in acrylic elastomers and urethane coatings. He is a vice president and board member of the Roof Coatings Manufacturers Association (RCMA), and leads several task groups in ASTM D08 roofing, as well as maintaining active memberships with American Society for Quality (ASQ), RCI, Reflective Roof Coatings Institute (RRCI), and the American Chemical Society (ACS). Heinje can be reached at heinje@quest-cp.com.

Protecting Infrastructure from Major Floods

All photos courtesy Kryton International

by Jeff Bowman, B.Sc., Greg Maugeri, and Sarah Rippin

Major flooding has dominated international news in recent years. While it may not be occurring more frequently than it was 50 years ago, due to growing infrastructure, the impact is certainly greater. Since 1949, the U.S. population has doubled, leading to a rapid increase in the construction of the urban environment. Most cities are unprepared for these rare increases in water levels—initiatives to protect infrastructure from major damage too often occurs only after the destruction.

Concrete is the modern world’s most commonly used building material, employed twice as much as other major building materials—steel, aluminum, wood, and plastic—combined.¹ Concrete walls naturally protect against structural damage that can be caused by the effects of nature.

Looking back in history, many century-old structures stand longer than those erected in the last 50 years. This is largely due to reinforcement methods—instead of using solid stone, most U.S. infrastructure contains reinforcing steel embedded within poured concrete. As the priorities of construction methods shift to increase productivity and streamline scheduling, long-term durability often takes a backseat.

Feeling the effects of flood damage
In 2012, Hurricane Sandy ravaged the Atlantic coastline. The ferocious storm was among the worst disasters to ever hit the United States, causing tens of billions of dollars in damages and losses. The storm surged more than 4 m (13 ft) above the average low tide, leaving millions without power, causing severe flooding, and leaving properties destroyed.

One such flooded building happened to house the server farm for a well-known company’s global financial transactions. The server farm was located in a large warehouse building on West Street in Manhattan, more than two blocks from the Hudson River.

The most damaging issue in the storm’s aftermath was the street-level power plant running the server infrastructure was dramatically flooded. The mechanical room took 750 mm (2 ½ ft) of flooding during the hurricane, which was foreseen by neither owners nor builders when the servers were installed.

After the waters receded, engineers worked with waterproofing experts to come up with a way to protect the vital systems from potential flooding in the future. They opted to surround critical systems with waterproof concrete half-walls that could stand up to hydrostatic pressure.

For this Manhattan server room project, the concrete-to-concrete joints of the walls were constructed using the same crystalline technology to fully tank the room. To provide extra protection, a slurry coat containing the same crystalline properties was applied to the outside of the walls.

Even under normal circumstances, if soil around a below-grade structure’s foundation is saturated with water to 1 m (3 ft) or more above the ground level, the water’s force causes pressure on the concrete. The walls absorb the moisture like a sponge, leading to cracks and ensuing water infiltration, rebar corrosion, and mold. In a flash flood, similar water pressure is applied to concrete structures.

Waterproofing concrete from the inside out
The project team knew waterproofing the server farm would require a robust assembly that could withstand the sudden onset of a large volume of water. Typical concrete structures built in the past 50 years are waterproofed solely on the positive side (i.e. wet side) using surface-applied paints or membranes. These surface barriers are vulnerable to damage during construction, which can lead to waterproofing failure. In the case of the server farm, the water pressure of a flash flood (and the debris it carries) could damage a membrane and leave the servers vulnerable at the most critical moment.

The slow effects of water damage to an aging building can be accelerated by flooding, which adds force to aging materials and pushes moisture into concrete. This moisture stays within the concrete even after the building is cleaned up, and can cause mold and mildew growth over time. The moisture can also collect around the steel reinforcing rebar within the concrete, causing corrosion that spreads throughout the structure. Additionally, the rebar expands as it corrodes, cracking the concrete and adding to the deterioration.

To make concrete truly ‘waterproof’—which means both preventing water passage and resisting hydrostatic pressure—many contractors have embraced a long-term, permanent approach. Using an integral system, usually in the form of a powdered admixture added directly to the concrete itself, the entire mass of concrete can be made the waterproofing barrier. The system should include properties which work to protect steel from corrosion, saving structures which incur water damage fewer repair or replacement costs down the road.

In late 2010, the American Concrete Institute (ACI) published a new report on chemical admixtures for concrete called ACI 212.3R-10, Report on Chemical Admixtures for Concrete. This document contains a new chapter focused entirely on permeability-reducing admixtures (PRAs). Chapter 15 embodies research dating back nearly five years, with volunteers from across the industry spending countless hours and untold effort into its development.

The result is a valuable resource for concrete users, with the greatest value of the new chapter being its clear categorization of permeability-reducing admixtures into two divisions:

permeability-reducing admixtures for non-hydrostatic (PRAN) conditions; and
permeability-reducing admixtures for hydrostatic (PRAH) conditions.

Dampproofing admixtures
Used for PRAN conditions, dampproofing admixtures reduce water absorption via treatment with repellent chemicals (e.g. soaps or oils) or partial pore-blocking (i.e. fine particle fillers). Since resistance to water under pressure is limited or non-existent, these admixtures are not suitable for concrete exposed to this situation.

For the Manhattan data center project, the critical systems were surrounded by waterproof concrete walls, which incorporated a crystalline concrete waterproofing admixture.

Waterproofing admixture
Waterproofing admixtures are specified for PRAH conditions. They reduce water penetration via a pore-blocking mechanism (e.g. crystalline growth or polymer plug). Given these materials are sufficiently stable to resist water under pressure, they are suitable for use in watertight construction, such as basements and water tanks.

Corrosion considerations
Not all flood damage to concrete structures is immediately apparent. Even after the water recedes, there is still a risk of rebar corrosion, especially if the flooding involved salty ocean water.

Corroded rebar can weaken a buildings vital support network, and the damage can quickly spread. Reducing water permeability effectively lowers opportunity for this, thereby increasing the structure’s longevity. Of course, not all corrosion-inhibiting admixtures (waterproofing or otherwise) are created equal.

The University of Hawaii recently released the results of a 10-year study on the corrosion of reinforced concrete exposed to a marine environment.² Concrete panels were placed in the tidal zone of Honolulu harbor—a highly corrosive environment due to chlorides in the ocean water, as well as constant weather fluctuations.

Corrosion can be prevented in concrete in two primary ways. If the permeability of the concrete is very low, the penetration of water and chlorides will be minimized, preventing corrosive conditions from developing. Alternatively, the concrete can be treated with corrosion-inhibitors that act to chemically inhibit corrosion at the surface of the steel once corrosive conditions develop. For real projects, both methods can be used, but for research purposes only one additive was used in each test mix.

The Hawaii program used a good quality, control concrete (water/cement [w/c] ratio of 0.40) that would be considered durable. Even so, the control (i.e. plain concrete) showed corrosion induced cracking and rust residue after 10 years. The other materials evaluated included two supplemental cementitious materials (SCMs)—fly ash and silica fume—as well as four corrosion-inhibitors and three PRAs.

By implementing long-term waterproofing solutions to key areas, the Manhattan server room will withstand high water exposure caused by any future massive flooding.

At the study’s conclusion, the report published in 2012 made the following recommendations to minimizing corrosion:

Use a W/C ratio as low as possible, but not greater than 0.40.
Include fly ash with at least 15 percent replacement of cement, or silica fume with at least five percent replacement of cement. Mixing must ensure the fly ash and silica fume, in particular, are well-distributed throughout the concrete.
Include a calcium nitrite admixture at minimum dosages of 20 L/m³ (4 gal/cy).
As added protection, consider including a proprietary hydrophilic crystalline product at two percent by weight of cement.

These findings are particularly relevant because they are based on field exposure in a harsh costal environment. This is the best type of testing because the exposure simulates the actual service conditions of a real structure. Laboratory tests are generally designed to provide accelerated results using conditions that do not always model real life. Many products may perform well in a short-term laboratory experiment, but perform poorly over the long term in actual conditions.

Wall joints and entryways
Certain water entry points are particularly sensitive when exposed to hydrostatic pressure. For example, water can easily penetrate through the joints where walls meet in a corner, or where the wall meets the floor. Tie-holes should also be treated as possible leakage points, and with a product that is effective under hydrostatic conditions, especially if in a below-grade location.

It can be difficult to predict how vulnerable a concrete joint is until it is too late, so it is important to mitigate any concerns well in advance by administering a permanent jointing system that can permanently withstand high water pressure, without breaking down and becoming an entry point for moisture.

Unsuspecting entryways are another issue. Insufficient waterproofing of the concrete is not the only way water can infiltrate an interior space during a major flood. In many cases—including the aforementioned building housing the server farm in New York—there are strong enough winds and water pressure to simply break through windows or doorways.

Boarding up these entrances is an effective protection measure in many cases, but this is time-consuming and, in some cases, impossible. Installing flood-resistant doors, gates, and window protectors upon the construction of the building helps ensure the structure can be protected quickly and efficiently when facing a flood.

If all entrances cannot be flood-proofed, protecting below-grade concrete from water damage from both the positive and negative side is a vital step in ensuring a building’s longevity.

Not all flood damage is immediately apparent. Even after the water recedes, there is still a risk of rebar corrosion for concrete structures.

The concrete barriers, designed by engineers for the server building in Manhattan, contained a system that would work with water under pressure, rather than against it. Hydrophilic technology (from the Greek hydros, meaning water and philia, meaning friendship) absorbs water rather than repelling it, effectively using the water contact to its advantage.

This process is employed in crystalline admixtures and surface-applied cementitious crystalline products, which transform water into microscopic crystals. These crystals permanently block the pores of the concrete, preventing water from penetrating and moisture from remaining within the wall. As time goes on, the crystalline product remains dormant in the concrete, and reactivates upon the presence of moisture, throughout the entire life of the structure.

For the New York project, the finished wall took on crystalline waterproofing in three different applications:

as an admixture added directly to the concrete mix;
as a surface-applied brush-on (i.e. to the positive side—on the outside of the walls facing any potential water penetration into the building); and
within the concrete joints as a two-part physical and chemical barrier.

By using these extra enforcements, engineers could guarantee the protection of this vital server farm should a flooding event at the level of Hurricane Sandy occur again.

Preparing for the future
With climate change and the increases in water-related damage to cities and areas around the world, it is becoming more apparent drastic steps need to be taken regarding the way we build, and the materials we use. In order to gain full insight, it is critical to pay attention to new innovations based on proven techniques, which reflect both sustainability and durability. Staying up to date with the new tools, research, and reports can help us to know what we can do to protect our structures from costly damage and early deterioration.

Notes
¹ Visit cementtrust.wordpress.com/a-concrete-plan. (back to top)
² For more see the report, “Performance of Corrosion-inhibiting Admixtures in Hawaiian Concrete in a Marine Environment,” by Joshua Ropert, MS, and Ian N. Robertson, PhD, SE. Visit www.cee.hawaii.edu/reports/UHM-CEE-12-04.pdf. (back to top)

Jeff Bowman, B.Sc, is a technical manager at Kryton International Inc. He earned his bachelor’s degree in chemistry from the University of British Columbia, and has extensive experience in the development and use of various construction products for the repair and waterproofing of concrete, specializing in crystalline admixtures and repair materials. Bowman has written numerous articles on waterproofing technologies and has contributed to international publications on this topic. He can be reached at jeff@kryton.com.

Gregory Maugeri is the CEO and managing partner of New England Dry Concrete, which specializes in solving cementitious waterproofing problems with crystalline materials. He has decades of construction and waterproofing experience; his company has been awarded the (ICRI) Project of the Year. Maugeri can be contacted via e-mail at greg@dryconcrete.com.

Sarah Rippin is a multimedia coordinator for Kryton. Over the past eight years, she has worked within the construction and marketing industries, and now uses both visual and written communications to promote and draw awareness to the importance of concrete waterproofing. She can be contacted at srippin@kryton.com.

The Rising Artistry of Tilt-up

All photos courtesy Tilt-up Concrete Association

by Kristin Dispenza, CSI

Materials with origins in engineering or industrial applications often become embraced for their artistic or architectural potential. For example, weathering steel was developed in the 1930s for railway coal wagons, but eventually began appearing as cladding on high-profile buildings in the 1960s—its signature brand name, Cor-ten, is now universally recognized among designers.

Such break-out success is not uncommon in architecture. After all, experimenting with the aesthetic properties of an otherwise basic or practical building material has always been a hallmark of the discipline; this tendency only gained momentum in the 20^th century when the concept of functionalism was introduced. In the modern and post-modern eras, the line between form and function has been frequently crossed.

In Austin, Texas, Dalchau Service Center Building D (STG Design) melds tilt-up concrete with various metal and glass components to achieve visual balance. Construction work was performed by American Constructors Inc.

Surprisingly, tilt-up construction, a method regularly selected for industrial and other plain applications following its surge into mainstream building during the 1960s and 70s, did not begin as such a general construction solution. California architect Irving Gill pioneered the technique in the 1920s, reportedly inspired by factory assembly lines. In the true modernist tradition, he refined a planar, undecorated aesthetic based on engineering efficiency.

In short, tilt-up construction was identified as an efficient and effective method for raising a complex concrete façade without the tedious vertical forming process. Gill’s contemporaries capitalized on his method of erecting modern concrete façades with even greater efficiencies. Wartime economies, however, tipped the scale in favor of tilt-up construction’s ease of use, speed of erection, and low cost. By mid-century, it had come to be used almost exclusively for low-cost housing and big-box buildings—its reputation as a ‘warehouse’ material became entrenched.

Application evolution

There are good reasons for tilt-up construction’s dominance in the building of rigid-wall, flexible-diaphragm large-box structures. It is easy to use the vast floor slabs of these structures for casting wall panels, and simple for cranes to then hoist them into position on buildings that have relatively plain perimeters and uncomplicated wall envelopes.

Nevertheless, tilt-up construction also has the capacity to deliver multi-story, irregularly shaped, complex projects—a fact increasingly embraced by architects over the past two decades. The ability to easily and creatively deliver form is contributing to the resurgence of craft—essentially, because one of the greatest benefits of tilt-up is speed, there is more time to focus on quality.¹

Tilt-up is now regularly considered for almost every project type, depending on the region, says Jeffrey Brown, AIA, of Powers Brown Architecture.

“From warehouses and functional origins, tilt wall has begun to transverse building types at an amazing rate,” he explains.

The Tilt-up Concrete Association (TCA) recently conducted its 23^rd annual achievement awards, honoring projects representing an increasing diversity of building types constructed with site-cast tilt-up concrete. A select few, profiled in this article, provide an understanding of both the visual and functional possibilities attainable with this delivery method.

RMW Architecture and Interiors designed large wing wall panel extensions to create shadow and relief along the façade of the Metropolitan Van & Storage building in Napa, California. Construction work was performed by Panattoni Construction, Inc.

“Inherent to the material and construction method, tilt-up establishes an incomparable blend of structure and aesthetics,” says Jim Baty, TCA’s technical director. “Where tilt-up has been, and where it is currently headed, is a dynamic blend of architects desiring form and function that are unified, engineers capitalizing on the plasticity and componentry of the raw materials, and contractors visualizing the modification of both surface and space to produce buildings no longer constrained by the challenges of securely assembling material components at unsafe heights or in awkward positions.”

New levels of artistry

Tilt-up’s familiar planar panels lent themselves to early modernism because of their inherent ability to define sleek, machine-inspired forms. As tilt-up construction furthers its inroads into high design, it is this ability to shape a planar composition that is most often explored.

Chico’s National Store Support Center

Volumes made with tilt-wall panels can be interspersed with contrasting forms, as Gora/McGahey Architects did for the Chico’s National Store Support Center (Building 10), a 13,615-m² (146,555-sf) office project in Fort Myers, Florida. The building’s exterior features cantilevered members, tapered panels and compound shapes.

The building also sports bright colors and a modern, smooth surface finish to emphasize its contemporary character. Panels were designed to remain in an ‘as-cast’ state to offer an exposed concrete look; nearly all were coated with a clear sealer, which required the panel surface to be flawless.

Dalchau Service Center Building D

Where tilt-wall panels are applied to structures having less differentiation in massing, they can be combined with different building materials, as was successfully done by STG Design on the Dalchau Service Center Building D, a 7335-m² (78,952-sf) office building in Austin, Texas. Certified Silver under the Leadership in Energy and Environmental Design (LEED) program, the building melds tilt-up concrete with various metal and glass components to achieve visual balance.

Grandview Business Centre

North of the border, the Grandview Business Centre (Surrey, British Columbia), combines tilt-up concrete with glass and steel. Designed by Ionic Architecture, construction work was performed by Double V Construction Ltd.

North of the border, the Grandview Business Centre in Surrey, British Columbia, also marries tilt-up concrete with various building materials. The 6689-m² (72,000-sf) multi-tenant commercial office building was designed by Ionic Architecture. It is located two blocks from Grandview Corners, the largest unenclosed retail development in British Columbia.

This retail project needed a design that was volumetrically dynamic, so the structure’s basic rectilinear form was stepped back at the southwest corner. Glass and steel were used on this portion of the building, as well as on the entrance tower, enriching the material palette. The tilt-up structure interfaces seamlessly, both functionally and aesthetically, with the glass and steel.

Gordon Holdings Building

Tilt-up planes on a building’s façade interface well with curtain walls, ribbon windows, spandrel panels, and punched openings. Consequently, there are many opportunities to employ a complex architectural vocabulary. Intergroup Architects, designers of the Gordon Holdings Building, used tilt-up panels to break up large wall surfaces. For the 11,092-m² (119,397-sf) industrial building in Englewood, Colorado, a 7.6-m (25-ft) long spandrel panel frames a dramatic glazed cantilever at the entry.

Metropolitan Van & Storage

Tilt-wall panels are not limited to defining closed volumes of space; they can be overlapped or applied as freestanding elements, helping to further differentiate a building’s massing. RMW Architecture and Interiors designed large wing-wall panel extensions to create shadow and relief along the façade of the Metropolitan Van & Storage building, a 9980-m² (107,424-sf) warehouse building in Napa, California. The wing walls are finished to resemble cast-in-place concrete and are further enhanced with 0.6-m (2-ft) edge returns and Cor-ten weathering steel accents.

North Fraser Corporate Centre

For the North Fraser Corporate Centre, an 11,584-m² (124,693-sf) office building in Burnaby, British Columbia, tilt-up panels were used as façade elements to divide the building’s massing. Despite the technical challenge of having to construct the building on 23-m (75-ft) piles—more than 900 were used in its overall construction—Beedie Development Group was able to include detailed architectural features that are not standard for industrial buildings.

SiteCast Construction Corp. built two model homes in Abu Dhabi. Traditional United American Emirates (UAE) architecture was mimicked using load bearing tilt-up.

Angled tilt-wall panels extend outward from the building and articulate a series of bays. These vertical wedges of concrete are counterbalanced by window units, which form horizontal projections within each bay. Architectural detailing—such as reveals, recesses, and colorful projecting steel channels—complete the composition.

Beyond planes

Tilt-up is a form of precast concrete construction. However, unlike the constrained modularity of plant-cast panels, it is adaptable to non-modular dimensions. Not only have designers used it to clad irregularly shaped building envelopes, but they have also employed tilt-up construction methods to create architectural elements such as pilasters, achieving looks often associated with steel framing but with improved economy.

It is even possible for highly sculptural building forms to be executed using tilt-up construction. The contour of a tilt-up panel can be manipulated in plan, elevation, or both. Consequently, curved and shaped panels have come into common use, offering designers flexibility not economically practical on most projects.

Abu Dhabi villas

Showcasing this fact, 2013 TCA award-winners included vernacular buildings styles and even traditional church architecture, complete with domes and arches. SiteCast Construction Corp., built two model villas in Abu Dhabi, United American Emirates (UAE); the 510-m² (5500-sf) single-story homes feature traditional UAE architecture, but were constructed using load-bearing tilt-up. This effort at reproducing local design was accomplished by paying special attention to forming techniques, panel joint locations, and interior wall treatments.

“Tilt-up allowed the local architecture to be enhanced by the durability and versatility of the concrete,” said the firm’s president, Shawn Hickey.

Saint Mary Coptic Orthodox Church

For the Saint Mary Coptic Orthodox Church (Delray Beach, Florida), api Group Architecture took a design that initially called for reinforced masonry with metal gauge framing, lath, and stucco, and converted it to tilt-up. Houlihan Construction was also part of the project team.

For the Saint Mary Coptic Orthodox Church, a 1277-m² (13,750-sf) church in Delray Beach, Florida, api Group Architecture took a design that originally combined a reinforced masonry structure with metal-gauge framing, lath, and stucco at the façades, and converted it to use tilt-up construction.

The church features a full dome—9 m (29 2/3 ft) in diameter and with a weight of 46,221 kg (101,900 lb)—that was cast monolithically and lifted using tilt-up systems. Tilt-up panels also form the two church towers. Layered panels are employed to create traditional building elements such as the stepped-down, vaulted archway at the church entrance. This project demonstrates the creativity of form that can be developed using site cast panels and layering as well as structural variety to deliver intimate spaces and dramatic open spans.

Finish treatments

The varieties of finishes possible with tilt-up concrete also provide a wide range of opportunities for making a building distinctive.

Tooele Applied Technology College

Architects with MethodStudio modified the thickness of cast panels used on the Tooele Applied Technology College to achieve a more craftsman-like feel. Materials specified for this 6852-m² (73,751-sf) educational building in Tooele, Utah, included reclaimed wood from the Great Salt Lake, corrugated metal panels, mineral rocks, and hanging pendant lights—all with symbolic importance to the community.

Tapering the panels and beveling their edges, as opposed to leaving the panels at a uniform thickness, lent a handcrafted feel to the architecture. The architects also used form-liners to impart the appearance of cedar boards to some of the panel surfaces.

To obtain a ‘granite’ look for the St. Louis Art Museum New East Building (David Chipperfield Architects), panels underwent a unique polishing process. They were also poured oversized and cut to exact dimensions to effectively reveal the aggregate.

A gap yields the appearance of floating panels, as they are attached to the structural steel of the building. Panels were installed by Fenix Construction Company.

St. Louis Art Museum New East Building

Panels have often been finished using sandblasting or retarders to expose the aggregate. However, to achieve a final finish similar to granite for the St. Louis Art Museum New East Building—a 19,974-m² (215,000-sf) addition in St. Louis, Missouri, designed by David Chipperfield Architects—the panels needed to receive a polished finish.

Panels were therefore cast with the exterior face up to facilitate higher quality grinding and polishing prior to erection. Lifting inserts installed on the exposed face of the panel had to be patched to match the surrounding finish. Building corners with 90-degree return legs were cast monolithically, requiring a 12 mm (1/2 in.) of material to be saw-cut from the finished face to expose a cross-section of the aggregates.

Panel joints were unique for a tilt-up envelope as they were designed without joint sealant and intentionally exposed at the edges. Panels had to be poured oversized and cut to exact dimensions in order to effectively reveal the aggregate. At (¾ in.), the gap left open between panels accents their rectilinear form to produce the appearance of ‘floating’ panels—they are not set on a foundation, but rather attached to the building’s structural steel. The museum addition achieved LEED Gold certification.

For thus Strathman Sales warehouse in Topeka, Kansas, a formliner representing split-face block was employed, along with formed quoins. Panels were installed by Seretta Construction-Atlantic.

Renaissance Charter School of Manatee

Form-liners are often relied on to impart texture to tilt-up panels, including the look of modular building materials such as stone, brick, or even quoins. For the 5960-m² (64,146-sf) Renaissance Charter School of Manatee in Bradenton, Florida, a form-liner was used to create the look of brick veneer with reveals at section borders. This provided the client with the desired look of a traditional school..

Strathman Sales Warehouse

For the 5290-m² (56,950-sf) Strathman Sales warehouse (Topeka, Kansas), a form-liner representing split-face block was used along with formed quoins to achieve a classic look.

Grenville Mutual Insurance

The use of shapes and graphics embossed into panels can also be used to reinforce a client’s corporate identity. This can be done in a literal way, by replicating a company logo, or it can be as subtle as establishing, on the larger canvas of a building façade, a recognized, brand-associated pattern.

The potential for panel finish customization was fully explored for the Grenville Mutual Insurance building in Kemptville, Ontario. Architect Gerry Shoalts (Shoalts and Zaback Architects Ltd.) wanted to create elevations that would reflect the site’s natural setting. Therefore, in addition to a façade featuring handset natural stone, the building team created a customized embossed ‘forest’ pattern.

The form-liner pattern was developed from a still image, repeated to achieve a similar appearance. The customized panels are on the building’s exterior as well as being carried into the office’s open-air courtyard. To add depth and shading, building panels for the 1672-m² (18,000-sf) office were treated with natural multi-depth paint tones. The tilt-up concrete construction technique also complements the building’s traditional metal-sloped roof and chimney.

For Grenville Mutual Insurance (Kemptville, Ontario), Shoalts and Zaback Architects wanted elevations to reflect the site’s natural setting.

In addition to a handset natural stone façade, there is a customized embossed ‘forest’ pattern created by formliners. SiteCast Construction Corp. installed the panels.

Beyond traditional aesthetics

Whether embossed, embedded, layered, or attached, the creativity of form manipulation with tilt-up is expressed in an ever-broadening palette of opportunity for designer and contractor. Visual enhancements like grooves and reveal patterns rank high on the list of popular techniques for using tilt-up construction to serve the architectural goals of a project. Metals and natural materials such as stone often enhance the building aesthetic, introducing warmth and smaller scale with the added benefit of reduced maintenance over painted features.

Tilt-up panels also serve as the canvas for graphic expression through the use of exterior insulation finish systems (EIFS). A concrete panel establishes a monolithic structural surface for the attachment of these systems in various ways. Projects featured in the TCA’s award program express banding, layering, and highlighting created by these applied techniques to further define the building’s look.

This year’s TCA Achievement Award winners show efficient, time-saving building methods like tilt-up construction support, not hinder, great architecture. The delivery method offers the benefit of cost-effectiveness combined with material plasticity, so designers will continue to push the envelope in using it to explore the making of forms.

Notes

¹ For more on this line of thinking, see the cover story from the June 2013 issue of The Construction Specifier—“Tilt-up: An Opportunity for the Resurgence of Craft,” by Mitch Bloomquist. (back to top)

Kristin Dispenza, CSI, is an architectural/engineering/construction editorial specialist with Constructive Communication. She has more than 20 years of writing and editorial experience with industry trade publications. Dispenza holds a bachelor’s of science degree from the Ohio State University College of Engineering/School of Architecture. She can be reached at kdispenza@constructivecommunication.com.

Designing with Activated Fly-ash Cement Concretes

Photos courtesy CeraTech USA

by Ivan Diaz-Loya, PhD

With evidence of climate change bringing awareness to greenhouse gas (GHG) emissions, the sticking point for concrete construction is the amount of embodied carbon in the material. Most of this embodied carbon comes from production of portland cement—the fine powder that hydrates and ‘glues’ sand and stone together to produce concrete.

Approximately 600 kg (1322 lb) of carbon dioxide (CO₂) is released into the atmosphere for every ton of cement produced.¹ This may not sound like much, but when the expected global cement output is 3.4 billion tons for 2013, it means roughly 2 billion tons of CO₂—five percent of all manmade GHG emissions—will be released to the atmosphere as part of the cement production process.²

To bring this to the specifier’s perspective, when a cubic yard of portland cement concrete is specified to have a cement content of 249 kg (550 lb), that cubic yard of concrete already has around 149 kg (330 lb) of embodied carbon generated from cement production alone.

This figure shows typical shrinkage values of activated fly ash cement concrete tested per ASTM C157, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, at 50 percent relative humidity (RH).

Concrete carbonation
Part of these emissions is recaptured throughout the concrete’s life when hydrated portland cement reacts with the CO₂ in the atmosphere to form calcium carbonate—a process called carbonation. However, it is uncertain how much CO₂ is reabsorbed into the concrete. Washington State University’s associate professor Liv Haslebach states in the article, “Concrete as a Carbon Sink,” published on Portland Cement Association’s (PCA’s) website, that although there have been numerous studies, no one has been able to put an accurate number to the amount CO₂ recaptured though carbonation.³

A 2007 study by Claus Pade and Maria Guimares, “The CO₂ Uptake of Concrete in a 100-year Perspective”, estimates the amount of CO₂ recaptured through carbonation is between 33 to 57 percent over a century.⁴ Whatever the actual number, it only stands to reason by the time the hydrated cement in concrete fully carbonates, the structure will have reached the end of its service life, and another structure will be built using more portland cement concrete to replace the old one. Therefore, even in a perfect scenario where 100 percent of the CO₂ is recaptured through carbonation and no new growth occurs, a revolving amount of CO₂ is being maintained in the atmosphere as old structures decay and new ones are built.⁵

This graph shows moisture vapor emission rates (MVERs) of portland cement and activated fly-ash cement concrete tested per ASTMF 1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.

Alternative binder systems
PCA calls portland cement the “building block of the nation’s construction industry.” While portland cement will likely continue to be used for generations, there are alternative binder systems available for the production of concrete. One of these alternatives is fly ash—the fine powder filtered out from the flue gases resulting from the combustion of coal. Fly ash is typically rich in silica, alumina, and calcium oxide (CaO) in many cases. During the power production processes, fly ash moves rapidly from combustion to room temperature, forming tiny spheres composed mainly of reactive glass; the material has an appearance and fineness similar to portland cement.

This image shows placement of activated fly-ash cement concrete at the Port of Savannah—an application where concrete will be subjected to abrasion and loads of 55,791 kg (123,000 lb) from overhead mobile gantry cranes.

Fly ash can be easily activated using a small amount of alkalis in the form of hydroxides or pH-neutral hydro-carboxylic acid salts to form a strong and durable binder that can replace portland cement in the production of concrete. Although fly ash has long been used to partially replace portland cement (sometimes up to 30 percent), total replacement of portland cement virtually eliminates all greenhouse gas emissions associated with the production of the cementitious binder in concrete.

Fly ash is typically produced from the combustion of coal to produce electricity, less than 35 percent of which is repurposed for beneficial use. The “2011 Coal Combustion Product (CCP) Production and Use Survey Report,” released by the American Coal Ash Association (ACAA), recorded only 23 million tons of fly ash were reused out of the 60 million tons produced in 2011.⁶ As such, approximately 37 million tons of fly ash could have been employed to produce activated fly-ash cement concrete instead of being sent to landfills. While some of this fly ash may have been lower quality, much of it could have been blended with high-quality material rather than disposed of.

Benefits of activated fly-ash cement
Activated fly-ash cement concrete not only offers environmental benefits, but also performance benefits, including:

improved volume stability;
mix design flexibility;
high early strengths;
low water vapor transmission at early ages; and
resistance to corrosive and high temperature environments.

Mix design flexibility
Concrete mixtures using activated fly-ash cement are proportioned in the same way as their traditional counterparts; in many cases, they even follow the same guidelines given by the American Concrete Institute (ACI). After all, it is still concrete with only the powder being different. The main difference is the addition of the activating liquid; at typically less than 24,815 mL/m³ (5 gal/yd³) of concrete, it is less than two percent of the total volume of the concrete mixture.

The type of activator may vary, but sodium hydroxide and sodium silicate are typically used in combination to activate low or moderately low calcium fly ash (i.e. <15 percent CaO). Hydro-carboxylic acid salts, which have a pH close to neutral, can be used for fly ash with higher calcium contents. Since portland cement mixtures can have a pH higher than 13, concrete practitioners are already accustomed to the level of alkalinity that fresh sodium hydroxide/sodium silicate-activated fly-ash mixtures can exhibit.

This graph shows the weight loss of portland cement and activated fly ash cement concretes exposed to different acids, tested per ASTM C267, Standard Test Methods for Chemical Resistance of Mortars, Grouts, and Monolithic Surfacings and Polymer Concretes. Image courtesy American Concrete Institute’s (ACI’s) May 2013 Concrete International

Nevertheless, handling concentrated solutions, especially of sodium hydroxide, may be challenging to certain ready-mixed concrete operations because of its high alkalinity. Hydro-carboxylic acid salts on the other hand, are neutral by pH and produce fresh activated fly ash cement concrete with a pH of around eight. Once the concrete hardens, the pH raises to around 12, which is necessary to protect reinforcing steel from corrosion.

Hydro-carboxylic acid salts activate (or modify) high calcium fly ash by allowing the formation of a solution equilibrium, and the solubility of fractions of the fly ash that do not normally react when mixed with water alone. Then, when calcium aluminosilicate hydrates nucleate and grow, they produce a predictable set time and normal, but accelerated, strength development.

Volume stability
Another benefit of using fly-ash cement is the reduction of water necessary to achieve workable concrete mixtures compared to portland cement mixtures. This allows for fly-ash cement concretes with low water-to-cementitious (w/c) ratios, and the main reason the concrete exhibits low drying shrinkage. Drying shrinkage values of activated fly-ash cement concrete range from 100 to 300 millionths, while regular concrete values range from 400 to 800 millionths after 28 days. This important feature reduces the potential for cracking and can speed up construction times for concrete flatwork and foundations.

Vapor emission rates at early ages for floorcoverings
For floorcoverings and coatings applied over concrete, activated fly-ash cement offers a tremendous advantage given its inherent rapid drying. When flooring materials are applied over concrete with high moisture vapor emission rates (MVER), moisture can condensate beneath the floor covering and dissolve the alkalis in concrete, raising the pH. These conditions can break down the adhesive and cause failures in the flooring such as blistering and delamination. Months can pass from the time the concrete is placed until it has acceptable MVER levels (typically 1.45 kg/100 m² [3 lb/1000 sf] daily) to apply a low MVER floor covering. This can leave installers with the choice of meeting the construction schedule or waiting for the concrete to dry. Activated high-calcium fly-ash cement concretes do not require wet curing and dry quickly, allowing flooring systems with the most stringent placement requirements to be placed in less than three weeks.

The picture shows the placement of activated fly-ash cement concrete for a high-temperature application at a steel fabrication business in Bakersfield, California. Photos courtesy CeraTech USA

Resistance to corrosive acids
Activated fly-ash cement concrete also offers high chemical resistance, mainly due to a combination of two factors:

its low w/c ratio, which can be 40 to 50 percent less than typical portland cement mixtures, resulting in a denser concrete (approximately 80 kg/m³ [5 lb/y³] more than portland cement concrete) that restricts the ingress of destructive agencies; and
calcium in activated fly-ash cements is combined in a less soluble form than in portland cements.

Portland cement concretes contain a high proportion of calcium hydroxide, which is highly soluble in most acids. This causes the material to quickly decompose. Conversely, activated fly-ash cement concretes have no excess calcium hydroxide, making them more resistant to corrosive acids.

Resistance to heat and temperature changes
The reaction products of activated fly-ash cements decompose at higher temperatures than those of portland cement, making them able to withstand such conditions. Additionally, because the overall thermal properties of concrete also depend on the characteristics of the aggregate being used, it is important how heat travels into, and through, the material.

Many heat-related failures in concrete happen not because hydration products decomposed, but rather due to temperature changes in the structure, which create a tremendous amount of thermal stresses. An example is when a heated glass jar bursts when exposed to ice-cold water. The higher density of activated fly-ash cement generates two important changes in the thermal properties of concrete: higher heat capacity and thermal conductivity.

Although low thermal conductivity is one of the most sought-after features in most construction materials, it is not as desirable in more brittle (and thus thermal stress-susceptible) materials like concrete. Activated fly-ash cement concretes absorb heat more slowly, and dissipate heat more rapidly through its concrete structure than portland cement concrete because of thermal conductivity which effectively reduces the thermal stress on the concrete. For example, an activated fly-ash cement concrete slab raises its temperature more uniformly, having a lower temperature gradient between the top and bottom of the slab. These features are not only desirable in high temperature exposure applications, but also in regular concrete flatwork applications as they minimize the potential for curling and cracking caused by daily temperature fluctuations.

Placement of a pedestal base for a nitric acid absorber using activated fly-ash cement concrete in Tuscumbia, Alabama, to alleviate corrosion damage.

In the past, activated fly ash cement concretes were only accomplished in highly controlled lab conditions and were impossible for field applications. While this perception still remains for many, the reality has changed. Activated fly-ash cement are now being used as a one-for-one replacement for portland cement concretes in many applications.

In 2009, Missoula Federal Credit Union (MFCU) built the first Leadership in Energy and Environmental Design (LEED) New Construction (NC) v2.2 Platinum-certified building in Montana using 100 percent fly-ash concrete (i.e. portland cement-free) as the main building material. The concrete mixture incorporated recycled glass as aggregate, and was used to build the footings, foundation walls, floor slabs, interior components, column surrounds, and roof beams. Montana State University developed the concrete mixture and worked with local contractors and architects to design and build the new branch office for MFCU. Researchers at Louisiana Tech University have also been running field tests since 2008 to create confidence in activated fly-ash cement concrete.

These ingredients can be easily introduced to existing ready-mixed concrete plants. Activating liquids can be plumbed into the plants in the same way most admixtures are currently used and given most plants already have a dedicated silo for fly ash, there is no need for setting up additional storage for it. Leading designers and specifiers have started taking notice of the capabilities and advantages activated fly ash cement concretes provide to their clients.

In July 2012, the Georgia Ports Authority used activated fly-ash cement to replace part of the lane where overhead mobile gantry cranes operate in the Port of Savannah. The Federal Highway Administration (FHWA) is now looking at the potential use of activated fly-ash cement concrete in rigid pavements and other transportation structures by signing cooperative research agreements (CRADAs) with leading producers of activated fly-ash concrete.

From start to finish, the reconstruction of a military base’s “hush house” for engine testing was recently completed with activated fly-ash cement concrete to alleviate deterioration from high, intermittent temperatures.

The Department of Defense (DOD) has also been using activated fly-ash cement concrete regularly to the replace concrete slabs that receive impingement heat from vertical takeoff and landing aircraft. Military bases throughout the United States, and internationally, have successfully been using activated fly-ash cement concretes for more than a decade now in the construction and rehabilitation of surface and aviation transportation infrastructure.

This graph shows typical strength development curves of concrete using high calcium fly-ash activated concrete with hydrocarboxilic acid salts as binder. Data courtesy Patel et al’s, “Green concrete using 100 per cent fly ash-based hydraulic binder,” Proceedings of the 2012 Concrete Sustainability Conference (Seattle).

Since activated high-calcium fly-ash cement relies on its hydraulic activity to harden and gain strength, it can be considered hydraulic—this a candidate for meeting ASTM C1157, Standard Performance Specification for Hydraulic Cement. Conversely, for fly ash with a lower calcium content—typically Class F per ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete—ASTM Committee C01 on Cement recently formed a task group to develop standard test method for testing compressive strength of alkali-activated fly ash and natural pozzolan cement. This test method is expected to be the base for the development of future specifications.

Conclusion
The current social and cultural forces shaping the construction industry are increasingly calling for materials in addition to meeting performance requirements and being produced with environmental responsibility. With regulations looming, it is difficult to predict what will happen in five, 20, or 50 years. On the bright side for fly ash, the EPA is now considering other options, none of which label the material as hazardous waste. ⁷

Activated fly-ash cement offers a solution by providing a value-added product while reducing the use of portland cement and associated emissions, and recycling a coal combustion product into beneficial construction materials, reducing landfill and disposal facilities.

Notes
¹ For more information, visit www.globalcement.com/news/item/1806-decoupling-carbon-emissions-from-cement-production. (back to top)
² Visit www.nature.com/news/green-cement-concrete-solutions-1.12460. (back to top)
³ Visit www.cement.org/tech/carbon_sink.asp. (back to top)
⁴For more information, see Claus Pade and Maria Guimares’ “The CO₂uptake of concrete in a 100-year perspective” from the International Symposium on Sustainability in the Cement and Concrete Industry held in Lillehammer, Norway in 2007. (back to top)
⁵ For more, visit www.nrmca.org/greenconcrete/concrete%20co2%20fact%20sheet%20june%202008.pdf. (back to top)
⁶ Visit, www.acaa-usa.org/associations/8003/files/Final2011CCPSurvey.pdf. (back to top)
⁷ Visit www.epa.gov/epawaste/nonhaz/industrial/special/fossil/ccr-rule/ccrfaq.htm#3 (back to top)

Ivan Diaz-Loya, PhD, joined CeraTech Inc., in 2011 as a research engineer to help develop a better understanding of activated high-calcium fly-ash cement concrete. He holds a PhD in engineering from Louisiana Tech University, where his research focused on the material. Diaz-Loya is a member of American Concrete Institute (ACI) and ASTM where he chairs a task group developing a test method for compressive strength of alkali-activated fly ash cement mortars. He can be contacted by e-mail at ivan.diazloya@ceratechinc.com.