Tag Archives: Durability

Durability of Elastomeric Sealants

File Opener

All images courtesy Buildings Diagnostics Inc.

by David H. Nicastro, PE, and Beth Anne Feero, EIT
Sealant is used in the exterior joints of every modern building, but usually incorrectly. Owners have high expectations for performance and durability, but premature sealant failure is common, resulting in air and water infiltration, property damage, and expensive repair work. Design details and installation practices contribute to the service life, but there is a bigger problem in the industry—many sealant products cannot resist movement and weathering, which are their core functions.

It is nearly universal for architectural specifications to demand sealant products to comply with ASTM C920, Standard Specification for Elastomeric Joint Sealants. What most specifiers do not realize is this standard requires only 250 hours of testing in an accelerated weathering machine. For most of the United States, this amount of radiation represents less than two months of outdoor sunlight exposure—and with no simultaneous extension or compression of the sealant specimens.

There is shockingly little data available to demonstrate which products can handle long-term weathering, especially in combination with cyclic movement. The authors’ testing of more than 180 sealant specimens is discussed in this article, with the hope more designers will demand this type of testing from manufacturers.

Sealant durability
Elastomeric sealant joints should be able to last for 20 or more years, but rarely do (Figure 1). For exterior cladding joints in commercial construction, high-performance sealants are needed—those that can accommodate movement of at least ±25 percent of the nominal joint width, adhere to multiple substrates, and withstand long-term weather exposure. Since ASTM C920 has such modest requirements, there are countless products that meet it, with no correlation to durability. Therefore, this manufacturing specification does not provide sufficient guidance to a designer for product selection (Figure 2).

Figure 1

Figure 1: These sealant joints had no chance for durability—the joint configurations were poorly designed, the sealant product was inferior, and it was improperly installed. Failure was inevitable.

Figure 2

Figure 2: According to the manufacturer, this sealant complies with ASTM C920, but that manufacturing specification only requires the equivalent of two months of outdoor weathering. Obviously, this product has inadequate weather resistance, which would be demonstrated by more robust testing.

 

A better predictor of durability is specifying products that have earned certificates from the Product Validation Program created by the Sealant, Waterproofing, and Restoration Institute (SWR Institute). The program validates products by independently verifying they comply with the manufacturers’ claimed movement performance in accordance with ASTM C719, Adhesion and Cohesion of Elastomeric Joint Sealants Under Cyclic Movement (Hockman Cycle), which compresses and extends sealants repeatedly at hot and cold temperatures. While this can add assurance about a sealant’s ability to accommodate joint movement as advertised, it is unfortunate the Product Validation Program does not include any weathering (Figure 3).

Figure 3

Figure 3: This sealant failed completely in adhesion, pulling away cleanly from the aluminum window frame. In the authors’ testing, products with Sealant, Waterproofing, and Restoration (SWR) Institute validation of movement capability performed better, remaining adhered during extension. Unfortunately, validation testing does not include weathering, so combined testing is still needed.

Like the three legs of a stool, durable sealant joints also require support from the project participants.

Designer
The specifier must provide proper joint dimensions and locations, and select appropriate products for the expected movement, environmental exposure, and substrates. As sealant joints will eventually fail, they should not be critical to façade performance—good designs include backup systems so air and water infiltration do not occur simply because of intermittent sealant failures. Excellent design guidance is available in ASTM C1193, Standard Guide for Use of Joint Sealants.

Manufacturer
Silicone sealants have an advantage for weather resistance because of their inorganic chemistry, but formulation is at least as important as the polymer type for achieving long-lasting sealant products. As discussed later in this article, primers are crucial for sealant durability, yet are rarely recommended. Manufacturers should be held accountable for the information they provide, and not just the sealant in the tube.

Contractor
Good design and manufacturing cannot compensate for poor sealant installation. Skilled workers are needed to install sealant meticulously in accordance with industry standards and manufacturer’s requirements, along with the project specifications.

Conventional test data
When designers select a sealant product, they typically expect it to be able to withstand weathering (e.g. ultraviolet [UV] light, rain, heat, and cold) and movement (e.g. joint expansion and contraction). These parameters are addressed by two separate test methods included in the manufacturing specification ASTM C920.

Movement ability
Movement ability is addressed by the previously mentioned ASTM C719. At least for SWR Institute-validated products, a product data sheet should be a reliable indicator of movement ability.

Weathering
For weathering, ASTM C793, Standard Test Method for Effects of Laboratory-accelerated Weathering on Elastomeric Joint Sealants, exposes sealant specimens to 250 hours of radiation exposure (i.e. fluorescent UV, xenon arc, or open-flame carbon arc) as the material is visually assessed before and after it is bent around a mandrel at a cold temperature. This is considered ‘accelerated weathering’ because the machine has more intense radiation than sunlight.

However, the acceleration factor ranges from about 4:1 to 8:1, depending on the amount of natural sunlight being compared regionally. Therefore, this test exposure is really only comparable to about two months outside. Obviously, this is inadequate to predict long-term durability.

Combined testing
Although not performed by manufacturers (or at least not yet reported), there is a test method that provides more realistic weathering: ASTM C1589, Standard Practice for Outdoor Weathering of Construction Seals and Sealants. The standard includes several alternative procedures, each also requiring extension and compression of the sealant specimens in conjunction with weather exposure (Figure 4).

Figure 4

Figure 4: The outdoor exposure site at The Durability Lab includes racks for testing sealant specimens in accordance with ASTM C1589 Procedure C. More than 180 specimens were included in the study reported here; about half were visible in this photo.

ASTM C1589’s Procedure C is a ‘user-friendly’ method developed so anyone can obtain his or her own data, combining outdoor weathering and cyclic movement. It is becoming widely adopted by consultants and contractors who wish to determine for themselves how sealant performs, rather than relying on manufacturers’ data sheets. One should make no mistake—this is a severe test that quickly weeds out inferior products, but it can take years to obtain long-term data comparing better products.

Using Procedure C, the authors tested 29 different sealant products in an outdoor exposure site in Austin, Texas, with periodic movement. This long-term study is ongoing, but it has already yielded important observations.

Product selection
Products were chosen for testing that comprise the largest market share of high-performance sealants. Most of the products are silicones and urethanes, but several hybrids were included. The selected products are all recommended for use on concrete and aluminum substrates, which are common in façades (both were included in the test specimens).

All the tested products claim to be able to pass C719 testing (as required by C920) with movement of at least ±25 percent; some have SWR Institute validation certificates for ±50 percent movement or more. Most of the sealants are one-part products (i.e. available in common caulk tubes) that cure on exposure to atmospheric moisture. A few two-part products were also included, which require mixing in a separate catalyst.

Specimens
Seven specimens were made for each of the 24 primary sealant products, and three specimens each for an additional five secondary products that were not expected to perform well (Figure 5). The color of each sealant was white to eliminate variability in weather resistance that comes from pigments. (Testing each standard color of each product would exponentially increase the cost, but would certainly be interesting.)

Figure 5

Figure 5: Each specimen consists of sealant bonded to a concrete substrate and an aluminum substrate, separated by nylon spacer blocks, and held together with a bolted clamping device. In this photo, the larger 15.9-mm (5/8-in.) spacer blocks have been inserted, extending the joint width 25 percent from the neutral casting dimension. In the spring, the specimen will be compressed and 9.5-mm (3/8-in.) spacer blocks will be inserted.

For the primary products, four specimens were primed, and three were unprimed. One of the primed specimens is a ‘file specimen’ (so named because it is kept in a file drawer and not exposed to weathering, but it is tested for movement). Those products with only three specimens were all primed.

The authors believe primer is imperative for long-term durability based on observations of previous testing—this should become more apparent as the current testing continues, with longer exposure and more movement cycles. Primers improve adhesion through a number of mechanisms, depending on the chemistry of the sealant and primer and the type of substrate. They act as surface conditioners, emulsify laitance, and can serve as an extra cleaning step, since they typically contain mostly solvent.

Primer is always required for joints that will be immersed in water, but manufacturers are reluctant to recommend primers for other installations. There is a perceived disadvantage competing with products that claim ‘primerless adhesion’ (despite the fact those products typically use primer on their SWR Institute validation testing).

The specimens were constructed as specified in ASTM C1589, which requires the ASTM C719 standard dimensions of 12.7 x 12.7 x 50.8 mm (½ x ½ x 2 in.). This square cross-section does not match the hour-glass shape that should be used in actual building joints to optimize performance; however, the square shape has been used in movement testing for decades because it provides more reproducible data. Using the same square shape will also allow the data from the current testing to be compared to manufacturers’ published data for movement ability.

The sealant was cured for at least 28 days before testing, which is longer than referenced in ASTM C1589. However, observations of previous testing indicated some products cure so slowly they should not be tested sooner.

Rack
The racks to hold the specimens during their exposure were constructed as shown in ASTM C1589, except metal bars had to be added behind the mesh to which the specimens were attached to reduce deflection (sag under the weight of the specimens was changing their angle of solar exposure). Guy wires were also added to reinforce the racks and to reduce sway from high winds.

Figure 6

Figure 6: The racks were positioned facing the equator (solar south) and at an azimuth angle of 45 degrees from horizontal.

The racks were positioned facing the equator (solar south) and at an azimuth angle of 45 degrees from horizontal (Figure 6). The latitude at the exposure site is 30 degrees north, so a lower rack angle could have been used to increase the radiation by 33 percent for faster weathering; however, 45 degrees is specified in the standard for consistency. A weather station is installed at the site to record the actual solar radiation experienced by the specimens.

Cycle and measurements
The specimens are being tested at ±25 percent movement—a reasonable amount of movement for a specifier to expect a high-performance sealant to achieve. All specimens (except the file specimens) were installed on the racks shortly after the spring equinox at a compressed width of 9.5 mm (3/8 in.), which represents 25 percent compression from their fabricated neutral width. They were extended to 15.9 mm (5/8 in.) after the fall equinox (six months later), which represents 25 percent extension from their neutral width.

Those specimens surviving the first testing were placed back onto the rack in the extended position, and will be moved again into compression when the spring equinox arrives. This simulates the typical annual cycle of building joints closing in the summer as the substrates heat up, and opening in colder winter weather (Figure 7).

Observations of previous testing indicate compression is more damaging than extension for most products. Therefore, starting the testing in compression was harsh, but reasonable to eliminate inferior products quickly. Almost a third of the specimens failed during the first extension—including some products claiming ±50 percent movement ability—as shown in Figure 8.

FIgure 7_replacement

Figure 7: The annual cycle consists of compressing the specimens on the spring equinox and extending them on the fall equinox. This simulates the typical annual cycle of building joints closing in the summer as the substrates heat up, and opening in the colder winter weather.

Figure 8

Figure 8: Hand-crank vises were used to extend the specimens. Calipers were used to measure the width of the sealant after removing the clamping device to determine the amount of compression set after six months of being compressed. There was no relationship between the amount of compression set and subsequent failure during extension.

 

 

 

 

 

 

 

 

 

Most products take a ‘compression set’ after being compressed on the rack for six months—in fact, many stayed near their fully compressed width of 9.5 mm after being removed from their specimen holders (Figure 9). However, the amount of compression set did not correlate with failure, demonstrating the mechanism that causes failure during the compression cycle is more complex.

It is likely compression causes stress along the bonding surface, which later becomes visible as adhesion failure once the specimen is pulled into extension. This can also occur in building joints, where adhesion damage during an early compression cycle appears later to be a tension failure, with a gap between the failed sealant and the joint substrate (Figure 10).

Figure 9

Figure 9: All seven specimens of this sealant product failed during the first extension, including the file specimen that was not exposed to weathering. Other products, such as those shown in Figure 5, exhibited no distress–this demonstrates the test method’s usefulness in identifying high-performance sealants.

Figure 10

Figure 10: Notice the similarity of this failure pattern on a real building joint to the specimen shown in Figure 9. Although the sealant appears to be adhesively failing in tension, the failure may have been induced during its first cycle of compression.

 

 

 

 

 

 

 

 

 

 

Testing methodology
On removal from the outdoor exposure rack and its individual metal clamping device, each specimen was extended with a hand-crank vise. The vise jaws were fitted with C-shaped clamps to fit around the specimens. The specimens were slowly extended to their final state of 15.9 mm (5/8 in.), or less if failure occurred.

A specimen was considered to ‘pass’ when there was no visible distress during the extension. ‘Distress’ was recorded when there was visible adhesion loss or tearing at any point during the testing, but the specimen was still able to be placed back outside for further weathering. ‘Fail’ was defined when the sealant substantially separated from one of the substrates. Three weeks after extending, the sealants were observed on the racks to obtain the final data reported here; as expected, additional failures had occurred since the initial observations during extension.

Results
After the first round of testing, only 33 of the 184 specimens had no observable distress—however, some of that distress is minor and may not lead to failure. More importantly, 29 percent of the specimens failed already. Figure 11 shows the percentage of failures for each sealant type, and primed versus unprimed behavior (except the hybrid data is combined because there were only a total of 11 specimens).

Figure 11

Specimen description Number of specimens Number of failures Failure
Silicone Primed 51 8 16 percent
Unprimed 45 15 33 percent
File specimens 15 1 7 percent
Total 111 24 22 percent
Urethane Primed 30 9 30 percent
Unprimed 24 13 54 percent
File specimens 8 2 25 percent
Total 62 24 39 percent
Hybrid Total 11 6 55 percent
SWRI-validated products 117 20 17 percent
Non-validated products 42 23 55 percent
All specimens 184 54 29 percent

In general, the silicone sealants behaved significantly better than the urethanes. Both behaved better than the hybrids, but there were too few hybrid products to be certain of this trend. Primer improved the performance of every sealant product tested but one. In fact, unprimed specimens were twice as likely to fail as primed specimens—this trend is expected to become more pronounced as the surviving specimens continue to weather. The SWR Institute validation was also a remarkable predictor of success; non-validated products were more than three times as likely to fail the first extension as the validated products.

Conclusion
Sealant durability depends on manufacturing, design, and construction. Premature failure can result from any of these, so a designer may not be able to control all the variables. However, specifiers can adopt strategies to optimize service life:

  1. Specify sealant products that have been validated by the SWR Institute.
  2. Where possible, use silicone sealant to take advantage of its inorganic polymer that does not deteriorate in sunlight (there are some cases where other chemistry is desirable, which is beyond the scope of this article).
  3. Use primer on all substrates. Ask the manufacturer’s technical representative to advise on which primers to use and how to apply them, but insist priming be incorporated in the project testing, contractor’s ‘buy-out’ process, and final construction.
  4. Design joints so sealants are strained less than half their rated movement (e.g. use larger joint widths).

The research presented here shows simply specifying sealant products must comply with ASTM C920 is not sufficient. All the tested products claim to meet C920, but there were vast differences in behavior. Most importantly, C920 requires so little weathering it is misleading for specifiers to rely on it. More robust testing is needed to determine a sealant’s ability to perform with both weathering and movement—this is the purpose of ASTM C1589, as used in the testing reported here.

David H. Nicastro, PE, is the founder of Building Diagnostics Inc., specializing in the investigation of problems with existing buildings, designing remedies for those problems, and resolving disputes which arise from them. He is a licensed professional engineer, and leads the research being performed at The Durability Lab, a testing center at the University of Texas at Austin. He can be reached by e-mail at dnicastro@buildingdx.com.

Beth Anne Feero, EIT, is a graduate student who is studying architectural engineering at the University of Texas at Austin. She serves as the graduate research assistant for The Durability Lab, which researches and tests the durability of building components, identifying factors causing premature failure. She can be reached via e-mail at bfeero@buildingdx.com.

Durability of Water-resistive Barriers

wrb_file opener

Images courtesy Building Diagnostics

by Beth Anne Feero, EIT and David H. Nicastro, PE
Many new water-resistive barrier (WRB) products are being introduced, including liquid-applied membranes. These new products join traditional wraps, self-adhered membranes, felts, and building paper, making for a crowded marketplace. A WRB will be concealed behind cladding, where it cannot be inspected, maintained, or replaced, so it must last for the design life of the building. However, will the new products be durable?

WRBs are required by building codes, and are installed on nearly every building, so it is surprising the industry lacks standardized tests for many fundamental properties of WRB products. To evaluate their performance, 17 WRB products (new and old) were tested in various common construction details, along with accessory products (flashings, tapes, and sealants). This long-term study is ongoing, but has already yielded important observations.

Product selection
As its name implies, the purpose of a WRB is to stop liquid water from penetrating through a wall section. However, more products are sold as air barriers, with a corollary benefit of serving as a WRB—and, in some cases, as a vapor barrier. The products included in this study were confirmed with the manufacturers to function as WRBs, but their marketing literature does not always make that clear. As this industry is rapidly evolving, there is some confusion as to the terminology and functions of these products (Figure 1).

wrb_Fig 1 - rolling

Figure 1: This photo shows a water-resistive barrier (WRB) being applied—
or is it an air barrier? Or vapor barrier? Or weather-resistive barrier? Similar products are marketed differently and can have different capabilities, so the functions must be verified with the manufacturer. Images courtesy Building Diagnostics                                                                        

The 2012 International Building Code (IBC) states that in typical sheathing applications, “a minimum of one layer of No. 15 asphalt felt, complying with ASTM D226, Standard Specification for Asphalt-saturated Organic Felt Used in Roofing and Waterproofing, for Type 1 felt or other approved materials, shall be attached to the studs or sheathing.” A similar statement is found in the International Residential Code (IRC). For stucco and masonry veneer applications, both codes require wood-based sheathing “include a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D paper.”

Many buildings are still being constructed with felt or building paper serving as the WRB. However, the industry is shifting toward products that can also serve as an air barrier—sometimes called ‘weather-resistive barriers.’ For their WRB function, these products have to be approved by building officials based on test reports demonstrating they perform as well as paper or felt. This does not seem like a very high bar, but it could be if durability were considered instead of just initial test results (Figure 2).

wrb_Fig 2 - WRB behind masonry

Figure 2: Once the masonry installation is complete, this WRB will be hidden, so it will need to last for the life of the building. The product was just introduced, but only long-term testing can verify design life.

Even if they provide only marginal waterproofing, paper and felt have been in service for decades, so their long-term behavior is well-established. By comparison, a few of the new WRBs have suffered various forms of product failure soon after application, resulting in manufacturers revising the formulations or application instructions (Figure 3).

wrb_Fig 3 - split joint

Figure 3: A WRB split at the sheathing joints before cladding was installed, which points to a concern about the robustness of thin barrier products. The manufacturer of this then-new product helped resolve the problem, and updated its recommendations for how to treat sheathing joints.

For this study, products were selected based on criteria that would be considered by designers and contractors for a multi-family building—one of the largest segments of the construction industry. (Most of these products would also be appropriate for large commercial projects, as well.) Research narrowed the products to those believed likely to continue to be sold in their present formulation—since some test results will not be attainable for several years, it would be pointless to invest in long-term testing of products that seemed questionable during the research phase. Additionally, products with a short allowable exposure time before cladding were eliminated because newer materials are being introduced with longer exposure time. As research indicates other viable products, they will be tested in the future.

Interpreting manufacturer’s instructions
Field investigations suggest most failures are caused by ‘what is on the outside of the can’—in other words, even the best products fail prematurely when the installation instructions are not clear (or followed). Sufficient information from the manufacturer is needed for correct use, including which accessory products are required and appropriate design details for typical penetrations, edges, and openings. Providing explicit, concise, and comprehensive instructions for the designer and installer is essential, but rare.

Before the test specimens were constructed, extensive time was spent researching the manufacturers’ publications. Some of the conditions being tested were not found in the instructions and details. For other products, there were multiple accessory choices that were not clearly defined, leading to possible wrong selections. For most of the products studied, it was necessary to contact the manufacturer to confirm the selections; in some cases, the technical support representatives contradicted the published information (and each other when both local and home-office personnel were contacted). While it is always a good design practice to involve the manufacturer, it would be better for the industry if these common details were clearly conveyed.

Another problem is some of the instructions provided are unrealistic. For example, several of the selected products require special detailing around brick ties, which is not economically viable (Figure 4). Voiding a manufacturer’s warranty is less of a concern than the potential for failures in systems that cannot self-seal such fastener penetrations.

wrb_Fig 4 - tie

Figure 4: This type of fussy detailing around brick ties is required by several WRB manufacturers, but it is not realistic. The cost, sequence of construction, and scheduling burdens would cause contractors to choose another product—or worse, not waterproof these frequent penetrations through the WRB as specified.

Ease of installation
Installers want WRB products that are easy and fast to install—to be economical, they want to avoid mixing multiple components, long drying times, multiple accessories, or fussy sequencing requirements. Keeping systems to the fewest products (such as a primary membrane for the field of the wall and a single accessory for detailing) reduces installation time and the possibility of errors.

The test products were installed by a specialty contractor, whose observations and comments indicated there was a wide range of difficulty in applying the products.

UV exposure rating
There are several new WRB products that are not sensitive to ultraviolet (UV) light, which would therefore make them good candidates to be used behind rainscreen cladding. These sophisticated, engineered cladding systems feature pressure-equalized compartments behind a veneer with permanently open joints, so it is important sunlight not damage the membrane through the joints.

For more common cladding systems, it is important to pay attention to the manufacturer’s published maximum exposure time for WRBs before they are covered with cladding. Interestingly, they each publish a single time limit (typically one to 12 months), but obviously the actual exposure during that period would vary depending on the climate and season. Therefore, the risk of prolonged exposure is uncertain.

Overall, the mockup specimens appear to be performing well, but some damage was already observed before reaching the claimed exposure limit. Most of the test specimens were first exposed in the winter; it can be expected the damage due to UV exposure would be worse if the testing started in the summer.

Contractors want longer exposure time, and the newest products being introduced claim to have this capability. This trend may tempt manufacturers into a marketing ‘arms race’ of sorts. During this research, some manufacturers changed their exposure rating without changing the formulation. The testing described below was designed to evaluate both the current exposure ratings and any increased timeframes claimed in the future. Although longer exposure time benefits the contractor, it is not clear benefit accrues to the owner.

Fire resistance
Performing fire testing is beyond the study’s scope, but the manufacturers’ literature typically addresses this requirement. With recent changes in code requirements (and more expected with the next IBC), water-resistive barriers are now being scrutinized for fire resistance in wall assemblies.

WRBs that are combustible and placed on a building over 12.2 m (40 ft) tall having a Type I−IV construction must be tested in an assembly that passes National Fire Protection Agency (NFPA) 285, Standard Method of Test for the Evaluation of Flammability Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components. A recent trend appears to be WRB manufacturers partnering with various other component manufacturers (including insulation, masonry anchors, and cladding) to create complete wall assemblies meeting this requirement.

Mockup test design
In the first phase of this study, WRB products were tested as part of a complete system. The research outlined above guided the design of mockup test specimens that represent most of the difficult details encountered on a typical project. Each specimen consists of a 0.9 x 0.6-m (3 x 2-ft) simulated wood stud wall segment with the WRB placed on plywood sheathing (except one product with an integral sheathing).

Each specimen contained multiple details a WRB system must be able to handle, as shown in Figure 5:

  • 
sheathing joint at the center (3.2-mm [1⁄8-in.] gap);
  • 
outside corner at the bottom and left edge;
  • window jamb flange at the right edge;
  • octagonal electrical junction box penetration;
  • large-diameter penetration (e.g. dryer vent);
  • small-diameter penetration (e.g. plumbing pipe); and
  • masonry veneer anchor (i.e. brick tie).
wrb_Fig 6 - Specimen Accessory Layout

Figure 5: The design of the mockup test specimens included many common details to explore the capability of each WRB system to seal them, as well as the manufacturer’s instructions. Accessory products were applied in strict accordance with the WRB manufacturers’ requirements. Image courtesy Classic Constructors LP

The details were sealed as required by each manufacturer—not based on what is commonly done in the field (Figure 6). While some of those requirements seem burdensome, the initial testing is intended to evaluate the system as recommended by the manufacturer.

The specimens were hung on metal racks facing solar south to be subjected to high solar radiation (in addition to wind and rain). They were placed vertically to simulate typical wall construction. Laying them back at an angle would have increased solar radiation to significantly accelerate the weathering, but it would eliminate another significant factor in performance: gravity. WRB products are often observed in the field to sag after exposure, which tugs on the flashings (Figure 7).

wrb_Fig 7 - detailing

Figure 6: As its name implies, the purpose of a water-resistive barrier is to stop liquid water from penetrating through a wall section. However, more products are sold as air barriers, with a corollary benefit of serving as a WRB. Image courtesy Classic Contractors LP

Once the specimens are exposed for their documented UV exposure rating, they are partially covered with a cement board to represent siding. This cladding is attached with screws, which tests additional fastener penetrations while also allowing the siding to be removed for observations. Only the top half of each specimen is covered; the bottom remains exposed to observe how the products deteriorate after the specified exposure time.

Clear rigid plastic was installed over the back of the specimens to prevent direct water entry, while allowing monitoring of any moisture penetration or damage from the front (product side). A metal coping protects the top of each specimen, but it is not sealed to the cladding—water can enter above the cladding so the WRB will be wetted by rain even after the cladding is applied (the essence of a WRB).

wrb_Fig 8 - sag REPLACEMENT PHOTO

Figure 7: The flashing sagged at the bottom of this wall. Gravity is a factor in WRB system durability, so long-term test specimens at The Durability Lab are therefore vertically oriented. Photo courtesy Buildings Diagnostics

Besides the large mockup specimens, additional small ones are being tested for material properties, including adhesion, that are beyond this article’s scope. Other small specimens are used for the nail sealability testing described below.

Conclusion
Improving durability requires addressing long-term behavior, which is typically related to weathering and chemical formulation. These aspects of a product’s ‘design life’ will be evaluated during the long-term testing described above; a future article will summarize those findings after sufficient exposure. Some premature failure has already been observed, which indicates the need to be vigilant in product selection regarding claimed exposure ratings.

The research and mockup testing also were designed to evaluate a product’s ‘service life,’ which is often governed by the original installation. The results so far indicate the industry needs better information, instructions, and details from manufacturers. To be economical and durable, an ideal WRB system would include only a few required products that are easy to install, cure rapidly, adhere well, seal around fastener holes, and withstand UV exposure for an extended time. None of the products tested so far met all of these criteria.

The industry also needs standard test methods and product specifications that are applicable to WRBs (and air barriers). Some of these are in development by professional societies, but those will take time to develop. Meanwhile, specifiers should be aware the tests cited by manufacturers may not be applicable to the performance criteria needed for a WRB. Most notably, a product’s claimed ability to seal around fasteners should be scrutinized—this study revealed the test methods and published information are generally unreliable regarding this fundamental property of WRBs.

Note: The authors gratefully acknowledge the continuing support and leadership of David W. Fowler, PhD, PE—the faculty advisor for the research being performed at The Durability Lab, a testing center at The University of Texas at Austin.

 Nail Sealability
wrb_Fig 5 - nail test

ASTM nail sealability testing for roofing products is often cited by water-resistant barrier (WRB) manufacturers. This severe test consists of checking for leaks after driving two nails through a waterproofing product applied to plywood, filling a cylinder with 125 mm (5 in.) of water, and chilling the entire specimen for three days. (An environmental chamber at The Durability Lab is visible in the background.) Image courtesy Building Diagnostics

One of the most important—and least understood—durability characteristics of a waterproofing material is its ability to seal around fastener penetrations. Since there is no industry standard for water-resistive barriers (WRBs), this property is commonly measured by ‘borrowing’ a roofing material specification: ASTM D1970, Standard Specification for Self-adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection. Many WRB manufacturers publish that their products pass this test; others are silent about nail sealability.

ASTM D1970 was changed in August 2014 to reference a procedure in ASTM D7349, Standard Test Method for Determining the Capability of Roofing and Waterproofing Materials to Seal Around Fasteners. Although better than the previous method, this is still a ‘borrowed’ roofing standard that requires modification for testing WRBs.

The revised test method, which is still severe, requires placing the membrane onto a piece of 12-mm (15⁄32-in.) thick plywood, 300 x 300 mm (12 x 12 in.). Two roofing nails are driven into the center of the board until they are flush with the membrane. Under the old method, the nails would then be tapped back off of the board 6 mm (1⁄4 in.), but now they remain flush.

After cutting its bottom out, a 4-L (1-gal) paint can is placed atop the membrane over the nails and sealed with silicone. The can is filled with water to a height of 125 mm (5 in.). 
An additional can is placed underneath to catch any leaking water, and this entire specimen is put in a temperature controlled chamber at 4 ± 2 C (39.2 F ± 3.6 F) for three days.

After testing, the specimen is removed to observe any water penetration underneath the membrane, on the plywood, on the shanks of the nails, and in the can beneath the plywood. If moisture is documented in any of these locations, then the test is considered a failure.

Fluid-applied membranes cannot explicitly follow this procedure because it requires the membrane to be removed from the plywood substrate for observation. Additionally, the test method assumes an intervening material (typically a shingle for roof product testing) is placed between the nail and the specimen. It is common to modify the test method to suit the product being tested, but it is difficult to find out what modifications manufacturers made.

The new procedure in ASTM D1970 now requires all completed tests to be reported as a ‘pass’ or ‘fail.’ This seemingly straightforward requirement addressed a significant problem: some manufacturers were performing tests until reaching two passing results, and only reporting those two results. Specifiers should confirm with the manufacturers whether products claiming to “pass D1970” included failed tests and what deviations were made from the latest published test method.

The standard does not explicitly define ‘self-sealing,’ so it is useful to compare similar terms. One manufacturer defines self-sealing as “Capable of sealing itself, as or after being pierced.” This is the closest term for the property needed in a WRB—the ability to self-seal around a fastener penetration without an additional application procedure. That same company defines a ‘self-healing’ material as one with “the structural incorporated ability to repair damage caused by mechanical usage over time.” This is not a common feature of WRB products, but the term is often erroneously used as a synonym for self-sealing.

Another manufacturer refers to a similar term, ‘self-gasketing,’ as “the membrane’s ability to be cut by the threads of a self-drilling screw, then seal under compression (i.e. the screw head compresses the membrane as it is seated providing a positive seal).” This term represents an evolving trend in the industry to downplay intrinsic self-sealing capability in favor of extrinsic gasketing provided by another material compressed over the penetration. Some published requirements for products to achieve self-gasketing would not be possible in an actual wall assembly, so specifiers should be aware of the changing terminology and its implications.

Clearly, these definitions express different characteristics of a membrane’s nail sealability either with the nail, without the nail, or based on the compression of the fastener, respectively. Such a distinction implies whether a secondary sealant, protective coating, or intervening material is needed to seal around fasteners.

ASTM D1970, together with its reference to D7349, provides a direct measurement of self-sealing capability. It is a severe test, it was developed for roofing products, and some modifications are needed to test WRBs. These factors may explain why the standard is not cited by all manufacturers; however, self-sealing is crucial, and should be evaluated for any WRB.

ASTM nail sealability testing for roofing products is often cited by water-resistant barrier (WRB) manufacturers. This severe test consists of checking for leaks after driving two nails through a waterproofing product applied to plywood, filling a cylinder with 125 mm (5 in.) of water, and chilling the entire specimen for three days. (An environmental chamber at The Durability Lab is visible in the background.)

Beth Anne Feero, EIT, is a graduate student studying architectural engineering at the University of Texas at Austin. She serves as the graduate research assistant for The Durability Lab, which researches and tests the durability of building components, identifying factors causing premature failure. She can be reached via e-mail at bfeero@buildingdx.com.

David H. Nicastro, PE, is the founder of Building Diagnostics Inc., specializing in the investigation of problems with existing buildings, designing remedies for those problems, and resolving disputes which arise from them. He is a licensed professional engineer, and leads the research being performed at The Durability Lab—a testing center established by Building Diagnostics at the University of Texas at Austin (UT). He can be reached by e-mail at dnicastro@buildingdx.com.

Bearing Pad Durability in Precast Concrete Garages

garage_File Opener

All images courtesy Building Diagnostics Inc.

by Doris Eichburg, Beth Anne Feero, EIT, and David Nicastro, PE
Bearing pads are used widely in precast concrete parking garages. They function as buffers between the separate concrete members to prevent damage and facilitate movement (much like cartilage between bones at joints). Bearing pads should last for the design life of the structure, but need to be replaced when they are not sufficiently durable. The ‘surgery’ to replace the pads properly is a complex operation, with few published guidelines.

In typical precast concrete garage construction, a bearing pad is installed beneath the end of each double-tee beam stem and under other beams and panels. The pads accommodate expansion, contraction, and rotation at the bearing area, preventing spalling and cracking of the supporting concrete members. Traffic in garages also induces movement, noise, and vibration which need to be absorbed or transferred by the bearing pads. (Similar bearing pads are used in bridges and other types of structures, but they are beyond the scope of this article.)

When correctly designed and installed, good-quality bearing pads are expected to be permanent components—that is, their component design life should be equal to that of the structure. Unfortunately, the actual service life of bearing pads may be shorter than their design life, requiring replacement when they prematurely fail.

Failure mechanisms
The authors have investigated the following failure mechanisms of bearing pads that can necessitate early replacement.

Slippage
Slippage or ‘walking’ is a phenomenon where a bearing pad moves out from its original location. When a bearing pad is loose-laid and not secured to the support below (e.g. with epoxy), it is held in place only by the concrete member’s weight. Bearing pads may walk due to incremental one-way movement because expansion and contraction forces are not exactly equal, particularly when the bearing substrate is not parallel to the double-tee stem ends. Bearing pads may also walk out due to sun camber on the top level, which reduces loading on them (Figure 1).

Figure 1

garage_Fig 1 - Slipping

This bearing pad has “walked” nearly completely off the support. When a bearing pad is missing or damaged, the double-tee stem end or supporting concrete can crack or spall.

Other factors contributing to slippage include excessive shear load due to expansion or contraction of the concrete members and rotation of the double-tee beams from cars driving in a relatively empty (and unloaded) garage. The bearing pad’s elastic memory, an inherent tendency to recover to its uncompressed state, may also cause the pad to walk out.

Crushing and tearing
Bearing pads may crush or tear over time due to excessive loading or inadequate pad size/material. Bearing pads compress slightly under load, which is to be expected. However, if the compressive load significantly exceeds the pad’s capacity, the pad will first spread (increase surface area) and then tear. Non-uniform loading of the bearing pad due to camber, misalignment of the concrete members, or improper installation of the bearing pad can cause a concentrated point load that may also cause crushing of the pad (Figure 2).

Figure 2

garage_Fig 2 - Torn

The bearing pad above has crushed, spread (increased in surface area), and started to tear.

Cracking
Cracking or crazing of the bearing pads may be caused by extreme heat, ultraviolet (UV) light, and ozone. Heat and UV light are not typically significant issues, since most bearing pads are not directly exposed to them. However, ozone is a powerful oxidant that occurs naturally in the atmosphere, and in greater concentrations in polluted areas (where parking garages are also found).

Natural rubber and most synthetic bearing pads are degraded by ozone as it breaks the carbon-carbon double bonds on their surface. Degradation due to environmental attack can be mitigated by including protective additives during the manufacturing process of the bearing pads.

Bearing pad types
American Association of State Highway and Transportation Officials (AASHTO) has standards for bearing pads in bridge applications, but there are no industry standards for the common types of bearing pads used in the construction industry.

Natural rubber
Natural rubber bearing pads were the first type used. As the name implies, the main ingredient is latex extracted from rubber trees. The rubber used in today’s bearing pads is typically vulcanized (a chemical process forming crosslinks in the polymer chains of the rubber) to improve tensile strength, elasticity, and durability. One of the material’s biggest attributes is its ability to remain flexible and less stiff in colder temperatures.

Neoprene
Neoprene (or polychloroprene) bearing pads are synthetic rubber. They resist ozone attack better than natural rubber. Other benefits of neoprene bearing pads include high tensile strength, good weatherability, and resistance to heat and flame. However, neoprene bearing pads do not have the ability to remain as flexible as natural rubber in colder climates.

ROF
Random-oriented fiber (ROF) products are the most widely used bearing pads in parking garages. They are engineered to be strong, with a high compressive strength. ROF products are made of rubber elastomer with synthetic fibers added for strength, and vulcanized to form the final composition.

Bearing pad replacement procedure
Although it is commonly implemented, there is little published guidance and no industry standards regarding how to replace bearing pads in precast concrete parking garages. The general process requires the double-tee beam stems to be lifted to replace the bearing pads, and then be lowered back down. For efficiency, it is recommended the bearing pads under both stems of a double-tee beam end are replaced, even if one of the bearing pads is still in good condition, since both stems have to be lifted together.

The authors’ firm has designed and monitored numerous bearing pad replacement projects using the general procedures outlined in this article. As every garage is unique, the actual scope of work should be designed for each project by a licensed engineer. Caution is paramount because structural damage can occur when the lifting operation is done incorrectly. All personnel should be trained to stop immediately when structural damage to any concrete member occurs, and to secure the garage until the engineer inspects the conditions and provides further guidance.

Pre- and post-lifting inspections
The topping slab, the double-tee beam stems to be lifted, and the adjacent double-tee beam stems must be closely inspected for concrete cracking and spalling before commencing lifting operations and again after completion. This helps identify new distress caused by lifting operations.

The documentation should include dimensions and locations of distress on drawings, as well as markings on the concrete. If all bearing pads are not scheduled to be replaced at the same time, then the pads should also be inspected to document crushed, torn, or ‘walked’ pads. It is important to note sometimes the bearing pads appear to be in serviceable condition, but they are actually torn or crushed under the stems. The authors also recommend measuring the bearing length and comparing to the design capacity (Figure 3).

Figure 3

garage_Fig 3 - Crushed

This bearing pad appeared to be in serviceable condition when observed from the side and front during a garage survey. However, after the double-tee stem end was lifted to replace the bearing pads, it was discovered that the pad was crushed toward the back of the support beam. The support substrate was slightly sloped inward, which had caused an uneven force distribution on the bearing pad.

Pre-lifting repairs
Cracks identified during the initial inspection requiring structural repairs (such as epoxy injection) should be addressed before lifting operations.

Preparation
Where necessary to lift double-tee stems, welded shear connections should be temporarily saw-cut at the ends of the double-tee flanges from the abutting components. After lifting, the connections can be re-welded. Any connections that are cut should be reviewed prior in order to ensure the stability of the tee and attached components are not compromised.

Shoring
Shoring should be installed for the lifting device to distribute the double-tee weight and lifting equipment loads down to the slab-on-grade or to multiple lower floors in tall garages. If the garage has any single-
ledger beams supporting double-tee beams, one must verify they are secured against rotation. For safety, it is important to ensure the shoring is secured and does not move during lifting, especially on ramps (Figure 4).

Figure 4

garage_Fig 4 - Shoring

The contractor is measuring the distance from the wall to the center of the shoring to ensure it lines up with the shoring installed at the parking level below – an important safety check.

Jacking
It is important to ensure the double-tee beam being lifted carries no load other than self-weight. Using a hydraulic jack, both stems of a single double-tee unit should be slowly, uniformly, and simultaneously lifted, with a differential deflection maintained at no greater than 3.2 mm (1⁄8 in.). The lifting should be just enough to remove the bearing pads—any more could lead to cracking due to torsion.

The jacking pressure is not usually measured due to the difficulty in accounting for friction, resistance, and dead loads during the process. Sometimes it is necessary to lift adjacent double-tee beams at the same time to ensure the structural stability or integrity of adjacent members (e.g. stairs tied in to the sides of the double-tee stems) are not compromised (Figure 5).

Figure 5

garage_Fig 5 - Beam Jacking

Both double-tee stem ends are simultaneously lifted with a hydraulic jack to allow for bearing pad replacement.

Bearing pad replacement
After removing the existing bearing pad, all debris should be cleaned from the surfaces. The new bearing pad should be installed as far back as possible under the stem, with the long dimension centered on the tee stem. The bearing pad should not be extended beyond the edge of the supporting member. The pad can be adhered with epoxy if slipping is a concern. When the substrate is not level, shims can be used beneath the bearing pad to reduce excessive non-uniform loading that can cause premature failure—thereby defeating the project’s entire purpose (Figure 6).

Figure 6

garage_Fig 6 - New Bearing Pad

The new bearing pad is centered under the double-tee stem end.

Beam lowering
The double-tee beam stems should be lowered in the reverse order of lifting to ensure the differential deflection between any two stems does not exceed 3.2 mm (1⁄8 in).

Final inspection
The engineer should be informed of any new cracks or ones that have increased in length or width. Any spalling or cracking resulting from the lifting operations must be repaired, and any shear connector plates that were cut or broken during the work should be re-welded. After the shoring and lifting devices are removed, the garage can be returned to service.

Conclusion
Test results from several manufacturers indicate bearing pads should last the structure’s lifetime when correctly designed and installed. However, several factors contribute to their premature degradation (including inferior products, environmental attack, movement from traffic, non-uniform loading, incorrect placement, and incorrect size for application), which necessitates early replacement. Industry standards should be developed for the manufacturing of bearing pads so specifiers would have confidence in selecting durable products.

As there is a significant chance of damage from the lifting operations, bearing pad replacement should only be undertaken after evaluating the relative risk of leaving the failed pads unrepaired. For those cases where an engineer recommends proceeding, standardization of the procedures (along the lines presented in this article) would ensure durability of the replaced bearing pads and minimize the risk of structural damage to the garage.

Note: The authors gratefully acknowledge the continuing support and leadership of David W. Fowler, PhD, PE—the faculty advisor for the research being performed at The Durability Lab, a testing center at The University of Texas at Austin.

Doris Eichburg is a principal with Building Diagnostics, specializing in the investigation of problems with existing buildings, designing remedies for those problems, and resolving disputes arising from them. She participates in the research being performed at The Durability Lab—a testing center established by Building Diagnostics at The University of Texas at Austin (UT). Eichburg can be reached by e-mail at deichburg@buildingdx.com.

Beth Anne Feero, EIT, is a graduate student studying architectural engineering at UT. She serves as the graduate research assistant for The Durability Lab, which researches and tests the durability of building components, identifying factors causing premature failure. She can be contacted at bfeero@buildingdx.com.

David H. Nicastro, PE, is the founder of Building Diagnostics. He is a licensed professional engineer, and leads the research being performed at The Durability Lab. He can be reached by e-mail at dnicastro@buildingdx.com.

Minimizing Environmental Impacts with Clay Roofing

clay_Monarch-Custom Blend

Photo courtesy US Tile by Boral

by Kayla Kratz
Clay roof tiles offer numerous benefits to commercial and residential projects. Inherently, the material is associated with minimal environmental impact in its sourcing, extraction, as well as manufacture.

As environmental awareness in building design and construction has increased, the greatest focus has been on sustainable building materials, both in manufacture and performance. Many tile-makers employ modern extraction and manufacturing methods that have enhanced the responsible production of roofing products. Specifically, some have even achieved Cradle to Cradle certification—one of the most stringent sustainability standards.

A third-party, multi-attribute, ‘eco-label’ administered by the Cradle to Cradle Products Innovation Institute, the program covers a broad range of products and takes a comprehensive approach to the assessment of both a product and the practices employed in its creations. The rating considers the material’s healthiness and its constituents, recyclability, or biodegradability, energy consumption, and carbon footprint associated with its manufacture, water stewardship, and social fairness. While most clay tile exhibit high recyclability and reusability, only a few manufacturing plants have earned the Cradle to Cradle certification.

 CRADLE TO CRADLE QUALITY STANDARDS
Cradle to Cradle certification was created and is administered by the Cradle to Cradle Products Innovation Institute (known formerly as McDonough Braungart Design Chemistry). The designation focuses largely on the use of safe materials that may be disassembled after use as a product and recycled as technical nutrients or composted as biological nutrients. All certified products meet a minimum requirement in five categories.

Material Health
Material Health examines the formulation of each product. The manufacturer’s entire supply chain is assessed to identify every chemical in the product above 0.01 per cent (or 100 ppm). These are then evaluated against 24 hazard endpoints for human and environmental health.

Material Reutilization
The Material Reutilization category is an assessment taking into account both the amount of recycled material and/or rapidly renewable content used to make the product. Its inherent recyclability or biodegradability at the end of its service life is also evaluated.

Renewable Energy and Carbon Management
In this category, a measurement of energy needs (i.e. electricity and heat used, as well as on-site sources of greenhouse gas [GHG] emissions) in the final product manufacturing stage is conducted.

Water Stewardship
The Water Stewardship category assessments are highly attuned to geographic region and production process, encouraging manufacturers to address both local watershed needs and production issues affecting water.

Social Fairness
Social Fairness assesses whether business ethics move beyond the confines of the corporate office and permeate the supply chain.

Cradle to Cradle emphasizes the product’s impact on the natural environment throughout all life stages, including a mandate that the product does not become waste, but is reformed and reused without harming the environment. Demonstrating the Cradle to Cradle ideal, clay tile is 100 percent recyclable and often reusable.

In addition to the certification, clay tile’s performance properties make it a comfortable and economical roof to live under.

Additional benefits of the material include:

  • 
high performance in helping protect the structure from the elements;
  • 
reduction in attic temperature;
  • 
low maintenance and easy roof repair; and
  • 
durability and longevity.

Many of these qualities also translate into positive environmental performance. Modern production methods can complement the inherently sustainable performance with ‘green’ manufacturing techniques.

clay_2 pc Mission

Traditional Mission-style clay tile uses a pairing of two tiles to form a rain-catching and channeling system. Image courtesy US Tile by Boral

The lifecycle of clay tile
The complete sustainability picture can best be understood by looking at the lifecycle of clay tile, which can be broken into four core categories:

  • 
pre-manufacture—including raw materials and sourcing;
  • 
manufacturing—during which the materials are transformed into the building product;
  • 
service life; and
  • 
end-of-life—when the product is removed from the structure.

The lifecycle of building products is likened to that of people, where there is a clear beginning (i.e. cradle), and a clear end (i.e. grave). Clay tile differs from many other traditional roofing materials, such as asphalt shingles, in its demonstrated minimal impact on the natural environment through all life stages, and its recyclability at the end of the roof’s life (i.e. taken back to the cradle and employed on another roof or in a completely alternate use.)

Aesthetic styles
The various profiles of tile—flat, barrel, Roman pans—complement various architectural styles, and a wide range of colors is available. Clay colors may be fade-resistant, created by a chemical restructuring of the clay during firing. If kept clean, clay roofs may undergo little visual change over a period of decades.

 The Color of Clay
The hues most widely associated with clay tile—a palette of yellows, reds, and browns—do not necessarily reflect the clay found in the ground. Those finish colors are a product of the firing process. Heat effects both a structural change in the clay—vitrification, which makes it hard and strong—and chemical changes that alter its color. Dark gray clay can become bright red with the right amount of firing.The clay’s chemistry can be altered during firing to produce special colors. A blast of oxygen at the right moment, for example, can produce dark lines or flares. Tile makers employ such treatments to produce controlled variegation and make more visually interesting tile surfaces. Both the base color and these secondary colors are permanent parts of the clay. They are not dyes or stains, and they are resistant to fading.

Even the emblematic barrel (or Mission-style) tile can be used for various looks. Installation techniques including blended colors, staggering, boosting, and serpentine patterns can be used to create visual interest, and allow a roof to make a significant contribution to the building’s architectural statement.

Modern tile configurations include variations on the tradition to meet different construction needs. Lightweight tiles are available as an alternative for re-roofing projects. A barrel-tile appearance can be accomplished either with traditional barrels or with S-shaped tiles that simplify installation and have lower labor costs.

Responsible raw materials
Sustainability in tile-making begins with the first phase of the tile’s lifecycle—the raw materials. Clay tile is made of naturally occurring geologic materials, such as clay and water. Some tile-makers use a high percentage of recycled material, or the by-products of mining processes. Utilizing this post-industrial waste clay repurposes the raw material for use in new clay tile. Tile may be up to 60 percent post-industrial recycled, and may help contribute to Leadership in Energy and Environmental Design (LEED) points for recycled content in the Materials and Resources (MR) category.

For example, when clay is locally sourced and extracted close to manufacturing plants, it dramatically reduces the environmental impacts of transportation. If the manufacturer is also within 804 km (500 mi) of the project, tile may qualify as a LEED locally sourced material.

clay_SHM6233-CR2-US Tile Corona-crop-sharp

Tile may be made from several different clays. The raw material is dried, ground, and selected for color, strength, and other properties. Then the clay is blended into a highly consistent material. Photos © Steven H. Miller. Photo courtesy US Tile by Boral

Green manufacturing
The second phase of the lifecycle—manufacturing—involves forming wet clay, drying it, and heating it to high temperature. First, the raw clay is dried and ground. It is then passed through sieves to achieve the correct particle sizing. Finally, different clays are selected for their strength or color properties, and mixed to achieve a desired blend.

The mixed dry clay is hydrated to a density and viscosity much like natural clay, free of debris, consistent in texture, blended for a specific fired color, and reasonably homogenous properties that cannot be achieved on a large scale with materials in their natural state.

The manufacturing line presses the clay through an extruding machine, forming a continuous ribbon of the desired profile, cuts it into precisely sized units, and stamps any special features into the shape, a continuous process on a long production line. The pieces are dried in a special drying chamber and are then ready to fire.

The firing, or kilning, process has been thoroughly reinvented for energy efficiency. Contemporary production can involve a roller hearth kiln—a long conveyor path that runs through a series of burners—and can fire tile in as little as 90 minutes. Since kilning is the major energy-consumer in the manufacturing process, this technique dramatically reduces the tiles’ embodied energy. It also lowers the carbon footprint, especially when the kiln is fueled by natural gas. Moreover, excess heat recaptured for use in drying the tile for firing, is another energy-saving strategy called co-generation.

This type of kiln can precisely control the application of heat to achieve consistent tile color. It can also apply special treatments to create secondary color patterns and variegation in a way that combines a visually interesting piece-to-piece randomness, yet provides an overall consistency of color distribution on an installed roof.

Responsible tile manufacturing includes waste reduction. Green waste (i.e. pre-fire clay waste and manufacturing rejects; unfired clay has traditionally been referred to as ‘green’ clay), and a selection of unused fired product can be collected and re-introduced into the manufacturing process. Production use water should be captured and recycled back into the mix, thus never leaving the facility or polluting the environment.

Safety and best practices should be employed at the facility and throughout the manufacturing process to serve the surrounding community. An air filtration system should also be employed to vacuum all pre-extrusion clay dust by-product occurring during manufacture. This captured dust may be re-introduced back into the grinding process, minimizing waste and limiting dust for safety. Additionally, clay stockpiles should be kept properly moist, and the plant regularly swept, to limit materials getting into storm water runoff.

clay_SHM6275-CR2-US Tile Corona-corr-crop

Wet (or ‘green’) clay is shaped into a continuous extruder, trimmed, and detailed before drying. Trimmed excess – seen on the conveyor in the background – is collected and recycled.

Sustainability benefits
Manufactured tile enters the third phase of its lifecycle—service life—when it is installed on the roof. A 2007 National Association of Homebuilders (NAHB) study, “Life Expectancy of Home Components,” indicates clay tile outperforms alternatives such as asphalt shingles for residential projects, making it one of the longest-lasting roofing materials, with an average 75-year life span. Similar performance could be expected on commercial roofs. This potentially reduces the number of replacements needed during the service life of a building, eliminating additional material consumption, landfill disposal, and other impacts associated with roofing manufacture, transportation, and installation.

An installed clay tile roof exhibits several characteristics contributing to energy efficiency of the building and comfort for its occupants. These effects are especially pronounced in hot weather, and clay tile has long been a favorite in hot climates across the Sun Belt.

The function of a roof is to protect the interior from wind, rain, and snow, but also from the sun. Clay tile achieves this goal by reducing the amount of solar radiant heat that impacts the roof, shedding some of that heat back away from the building, and reducing the flow of heat from the tile to the roof deck and the building interior.

The first line of defense is reflectance—the ability to ‘turn away’ solar radiant heat shining on the surface. Especially light and terra cotta colors, clay tile exhibits excellent thermal reflectance.

The second defense is thermal emittance—the ability of the surface to give off heat rather than contain it. Heat that is absorbed, rather than reflected, at the tile surface readily radiates back out of the surface; only a small percentage penetrates to the interior of the tile. Since heat is solely being applied to the top surface, it emits back from that surface, away from the building. Clay typically scores in the range of 0.85 on an emissivity scale of 0.0 to 1.0, showcasing it as a highly emissive material.

Solar reflectance and thermal emittance figures can be combined to create a measurement—solar reflectance index (SRI)—which allows different roof systems to be compared. High SRI equates to less heat transfer. The “Heat Island Project” supported by Lawrence Berkley National Laboratory (LBNL) compiled cool roof data into a database and found that gray asphalt—one of the lighter colors of roofing shingles—has an approximate SRI of 22. Whereas, red clay tiles, for example, are considerably cooler with an approximate SRI of 36.

clay_Kiln fire tile-crop

Dried tiles entering the kiln are a slightly greenish-gray color before firing.

SRI is used to determine which products qualify as ‘cool roof’ materials. The Cool Roof Rating Council (CRRC) assigns ratings on a product-by-product basis, allowing architects and designers to easily compare them. The specific SRI necessary for a cool roof varies depending on its slope and the geographic region in which the building is located.

Clay tile can contribute to cooler attic temperatures in the summer and acts as an insulator in the winter keeping the attic warmer. In 2010, the U.S. Department of Energy (DOE) estimated annual cool roof savings on air-conditioning at $0.012/m2 ($0.13/sf/year) of roof area. They estimated the penalty in heating energy at only $0.0027 m2/year ($0.03/sf/year).

Heat that penetrates the first two lines of defense can build up in the roof material. Clay tile has a third line of defense—high thermal mass, absorbing considerable heat before beginning to transfer it. This has the effect of smoothing out the interior impact of exterior temperature changes.

As daytime temperature rises and some solar heat penetrates past the tile surface, the clay ‘fills up’ with heat before it transfers heat to the interior. The thermal mass effect is most pronounced in locations where there is a wide differential between daytime and nighttime temperatures. It may delay heat flow through the envelope by as much as 10 to 12 hours, and peak heat transference may not be reached until late in the day, when exterior temperatures are already dropping and starting to cool down the roof. The interior experiences less temperature shift, and therefore may use less energy for air-conditioning. At night, absorbed heat is slowly released, which may help the building to maintain warmth.

The engineering of clay tile—its shape and the structure of its installation—provides a fourth layer of defense. There is space under the tile to allow airflow between the tile and the roof deck, improving the roof’s insulating qualities. Airflow allows some heat being radiated from the bottom of the tile to be removed before it is absorbed by the remainder of the building. Heated air is channeled upward along the deck surface and released through the ridge.

These four lines of defense, taken together, may dramatically reduce the need for air-conditioning, reducing the load on the structure’s HVAC systems and lowering building operating costs.

clay_Faux Mission Installation

The shape of Mission-style tile allows for airflow beneath the tile, channeling the heat away from the building interior and out through the ridge of the roof. Photo courtesy US Tile by Boral

Cradle to cradle
The fourth phase in the lifecycle of clay roof tile begins after it is removed from the roof. In many products, this would be the ‘grave’ phase of the cradle-to-grave cycle, when many roofing materials enter the landfill.

Due to clay’s durability and longevity, it may outlast the building on which it is installed. Clay tile is 100 percent recyclable. It can be crushed and re-introduced into tile production, or put to other uses such as baseball fields.

Unlike most other building materials, however, clay tile, if properly salvaged, can be reused as roof tile. This is perhaps the most pure version of the cradle to cradle lifecycle where products are repurposed for new use.

Conclusion
The movement toward energy efficiency and lower carbon emissions has made the inherent properties of clay tile roofs more beneficial than ever. By sourcing raw materials responsibly and using modern tile making processes, manufacturers have made products compliant with contemporary sustainability values. Further, the aesthetics of clay tile continue to offer the designer appealing architectural options.

Kayla Kratz is the product manager for Boral Roofing LLC, a subsidiary of Boral USA and a provider of roofing solutions for architects, commercial, and residential builders. She is responsible for Boral Roofing’s product portfolio including new product development, strategic marketing, and product launch implementation throughout the United States. Kratz earned her bachelor’s of science in business administration from Southern Nazarene University. She can be contacted by e-mail at kayla.kratz@boral.com.

Specifying More Resilient Buildings

safer_Comcast Center Inner Core

A high-performance self-consolidating concrete mix containing 40 per cent slag cement helps give the Comcast Center’s massive inner core its needed strength. Photo courtesy Thornton Tomasetti

by Andrew Pinneke, PE, LEED AP
Resisting natural disasters and reducing environmental impacts are major challenges in the United States. During an average year, there are 10 tropical storms (six of which become hurricanes) and more than 1200 tornadoes touching down.

In South Florida, Hurricane Andrew left a wake of destruction in 1992 that totaled more than $25 billion in property damage and resulted in 44 fatalities. Along the Gulf Coast, Hurricane Katrina caused widespread devastation in 2005, resulting in at least 1833 fatalities and $108 billion in property damage. In 2012, Hurricane Sandy affected the entire eastern seaboard and caused $65 billion in damage. In densely populated New York City alone, this superstorm took the lives of 53 residents, destroyed thousands of buildings, and caused $19 billion in damages and lost economic activity.

Almost every state has been affected by extreme windstorms (Figure 1). Each year, tornadoes with gusts as high as 320 km/h (200 mph) in the Midwest and lower Great Plains result in more than 100 fatalities and 1500 injuries. In 2013, a tornado cut a swath 3.2 km (2 mi) wide and 19 km (12 mi) long through Oklahoma City, causing 26 fatalities, 400 injuries, and $2 billion in property damage.

Few regions of the country escape the wrath of Mother Nature. Flooding, which accounts for more than 75 percent of federally declared disaster areas, is the most prevalent disaster event in the United States, and earthquakes pose serious risks not just in California, but also many Midwestern and Eastern areas. About 5000 seismic events occur each year, with approximately 400 capable of causing damage to building interiors and 20 able to cause structural damage. For example, the 1994 Northridge earthquake in California caused 57 deaths, over $20 billion of damage, and destroyed or damaged 90,000 homes, offices, and public buildings.

Figure 1

safer_Wind Zones in US_HROver the past decade, the frequency and overall economic damage inflicted by destructive wildfires also has increased in more than three-fourths of the United States. In 2012, some 38 catastrophic wildfires produced $1.1 billion in economic losses according to estimates in a January 2013 report by Munich Re. The wildfire problems are not limited to California and the Southwest, as there has been a recent trend toward larger and more destructive wildfires in the Southeast and Midwest.

Championing resilience
Standards for construction and code-related enforcement vary widely across the country. Some states have adopted building codes applicable to virtually every type of structure, while others employ lesser degrees of regulation.

The International Code Council (ICC) has developed the most widely adopted set of codes to unify the nation’s building regulatory systems. Based on thoroughly tested scientific and engineering principles, these model codes provide standards used in the design, build, and compliance process to construct safe, sustainable, secure, and resilient structures.

The Federal Emergency Management Agency (FEMA) and National Oceanic and Atmospheric Administration (NOAA) provide additional guidance on how to design buildings to lessen the impact of natural disasters. This guidance to designers includes their support of the ICC standards, which are equivalent to the National Earthquake Hazard Reduction Program (NEHRP) for New Buildings and reflect the current state-of-the-art engineering requirements for wind, such as those found in American Society of Civil Engineers (ASCE) 7, Minimum Design Loads for Buildings and Other Structures.

The Fortified for Safer Business program of the Insurance Institute for Business and Home Safety (IBHS) is another valuable resource. This ‘code-plus’ program offers design criteria and construction techniques that greatly increase a new commercial building’s durability and resilience to natural and manmade hazards.

In recognition of the increasingly important need for enhanced resiliency of buildings, the National Building Museum in Washington, D.C., is hosting a Designing for Disaster exhibition, which runs through August 2. This major exhibition showcases innovative research, cutting-edge materials and technologies, and disaster-resistant designs for creating safer, more durable and disaster-resilient communities. When the exhibition opened in May 2014, a group of 20 prominent industry organizations, led by the National Institute of Building Sciences (NIBS) and the American Institute of Architects (AIA), issued a joint statement pledging to research design and construction best practices, educate their memberships, and advocate for governmental policy changes.

safer_Iowa Tornado Shelter_HR

Constructed in accordance with Federal Emergency Management Agency (FEMA) criteria, the unique design of the concrete tornado shelter at the Iowa State Fairgrounds is a prototype for other shelters across the state. Photo courtesy Tom Hurd, Spatial Designs Architects

Concrete’s role in resilient construction
In areas susceptible to natural disasters, architects must design durable, high-performance buildings with materials that not only offer resistance, but also continue to function after a catastrophic event. High-performance concrete (HPC) structures are especially suited to provide protection against natural hazards and help ensure critical services—like hospitals, evacuation shelters, and emergency operations centers—can remain in operation even under the harshest of environments.

Concrete’s inherent strength and stiffness provide a primary advantage, which can be enhanced through building design, mix formulation, and reinforcement to withstand the forces of extreme winds and flying debris. Concrete not only provides the resilience needed to protect against tornadoes and hurricanes, but its structures are also resistant to flood damage, earthquakes, wind-driven rain, corrosion, decay, insect infestation, and mold and mildew formation. The slow rate of heat transfer and inherent fire resistance enable it to tolerate flames, and slow their spread. Concrete buildings also have excellent aesthetic versatility, providing an almost endless array of colors and textures to help minimize the cost of building repairs following a disaster.

All these attributes are built into a concrete structure, so if disaster strikes, restoration is usually a matter of replacing contents and some finish materials. This is a much less daunting and expensive process than complete rebuild or build out of interiors.

In Iowa, where the annual State Fair draws thousands of visitors during the heart of tornado season, the State Fair Board called for the construction of a 483-m2 (5200-sf) shelter to help protect campers in the event of a major storm. The Iowa State Fairgrounds tornado shelter features a unique curved design to provide superior wind resistance. The roof and the curved walls of the structure are constructed of 305-mm (12-in.) thick precast concrete panels that use a special framework to retain their shape, and the interior partition walls are constructed of fully reinforced concrete masonry units (CMUs). The building’s exterior concrete canopy is mounted atop concrete piers to provide additional weather protection. The canopy is designed to withstand 400-km/h (250-mph) winds and to prevent them from becoming a debris hazard themselves during a high wind event.

Concrete is also taking center stage in construction projects in the Gulf Coast region, which is still recovering from the devastation caused by Hurricane Katrina in 2005. At the time of the storm, Alabama, Louisiana, and Mississippi did not have statewide building codes for non-state-owned buildings. As a result, Hurricane Katrina’s storm surge, high winds, floodborne debris and long-duration flooding exceeded flood depths and loads used in building design, causing massive and widespread structural failure.

safer_New Orleans Surge Barrier

Construction of the 3,2-km (2-mi) long (IHNC) barrier that protects New Orleans from flooding relied on self-consolidating concrete for monolithic pours above and below the waterline and cast-in-place piles. Photo courtesy David Spielman/GPA

In the wake of Katrina, levees for Greater New Orleans were brought up to modern building code standards, and the U.S. Army Corps of Engineers (USACE) constructed the Inner Harbor Navigation Canal (IHNC) Surge Barrier to improve the resiliency of Gulf Coast communities. The largest of its kind in the world, the barrier incorporates more than 53,520 m3 (70,000 cy) of a self-consolidating concrete mix that achieved strengths greater than 27,580 kPa (4000 psi) within 48 hours. The backbone of the main barrier consists of 1271 concrete vertical piles, each measuring 1676 mm (66 in.) in diameter and 44 m (144 ft) in length and weighing 96 tons.

Another focus of reconstruction efforts is the need for critical and essential facilities to remain functional during a catastrophic event. When completed, the new University Medical Center (UMC) in downtown New Orleans will be the only Level One trauma center in southeast Louisiana, so keeping it functional during future disasters will be essential. The UMC project, scheduled to be competed this year, is using fly ash and slag cement in specialized concrete mix designs for added performance, and to help it meet flood-resistant construction standards.

This 213,700-m2 (2.3 million-sf) project is one of the largest healthcare campuses under construction in the country. It includes 120,775 m2 (1.3 million sf) of concrete decking delivered at a weekly rate of 5575 m2 (60,000 sf). The high-performance concrete building envelope for the seven-story, 52,025-m2 (560,000-sf) Inpatient Tower, just one component of the 15-ha (38-acre) campus, has been designed 
to endure hurricane-force winds up to 240 km/h (150 mph), yielding a facility that will enhance public safety in the event of natural disasters.

HPC for stronger, more durable buildings
The specification of high-performance concrete (HPC) continues to grow as superior structural resiliency and durability performance become increasingly important. HPC mixtures incorporate supplementary cementitious materials (SCMs)—such as slag cement, fly ash, and silica fume—as separate components or combined in blended cement. The greatest physical benefits imparted by SCMs and blended cements (ASTM C595, Standard Specification for Blended Hydraulic Cements, and/or ASTM C1157, Standard Performance Specification for Hydraulic Cement) can be seen in the properties concrete exhibits after hardening.

Strength
Concrete gains strength at a decreasing rate over time, so varying the concrete mixture can significantly alter the rate and/or ultimate strength gain as defined by ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. Slag cement and fly ash typically lower early strengths (one to 14 days) but can significantly improve long-term strength development (28 days and beyond), depending on the proportions and materials used (Figure 2). For example, Class F fly ashes tend to have a slow strength gain curve contributing mainly to the strength beyond 28 days, whereas silica fume contributes primarily to the three to 28 day strengths. Both compressive and flexural strengths can increase markedly at 28 days and beyond with the addition of most SCMs.

Figure 2

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Test results comparing lower early strengths and higher later strengths of concrete with supplementary cementitious materials (SCMs). Image courtesy Lafarge

Permeability
Slag cement, fly ash, and silica fume all can significantly reduce the permeability of concrete to the ingress of chlorides, sulfates, and other aggressive agents present in rain, groundwater, and seawater. Silica fume has a profound effect on permeability, exhibiting as much as a five-fold reduction in permeability when using only eight percent silica fume.

Alkali-silica reaction
Most SCMs can effectively prevent excessive expansion and cracking of concrete due to alkali-silica reaction (ASR). The amount of slag cement required depends on the nature of the slag cement, the reactivity of the aggregate, and the alkali loading of the concrete. In most cases, 50 percent slag cement is sufficient with highly reactive aggregates. The amount of fly ash required typically is in the range of 15 to 55 percent, depending on the chemical composition of the ash, reactivity of the aggregate, and the alkali loading of the concrete.

Generally, Class F ashes are much more effective in controlling expansion due to ASR than Class C ashes. Silica fume can control ASR, but the amount required generally results in poor constructability. Consequently, blends of slag cement and silica fume, as well as blends of fly ash and silica fume, are often used as an alternative to straight silica fume replacement because they can be used to achieve a synergistic effect in mitigating expansion due to ASR, while producing a workable concrete.

Sulfate attack
Concrete containing SCMs generally offer superior resistance to sulfate attack as they lower the permeability, restricting the ingress of sulfate-bearing ions. In numerous cases, they additionally reduce the compounds that can react with sulfates to form deleterious compounds. Typically, slag cement, silica fume, and Class F fly ashes are effective in improving sulfate resistance. The effectiveness of Class C fly ashes depends on the ash chemistry and the replacement level.

Thermal stress
If the temperature differential between the concrete’s surface and interior is too high, the result can be cracking and loss of structural integrity. Employing high replacement levels of slag cement and/or fly ash in properly proportioned mixes can reduce the peak temperatures, as well as the rate of heat generation. Reducing the heat of hydration of the mix can moderate the development of thermal stresses within the concrete and prevent cracking.

safer_Perez Art Museum Miami

Due to its unique combination of strength, durability, aesthetics and ductility, this proprietary ultra-high-performance concrete (UHPC) product was used to construct 100 long-span, hurricane-resistant mullions for the eloquent and resilient Pérez Art Museum in Miami. Photo courtesy Herzog & de Meuron

Soaring to new heights in resiliency
A major advantage of concrete construction for high-rise buildings is the material’s inherent properties 
of heaviness and mass, which create lateral stiffness, or resistance to horizontal movement. Occupants of concrete towers are less able to perceive building motion than occupants of comparable tall buildings with non-concrete structural systems. High-strength concrete also provides the most economical way to carry a vertical load to the building foundation.

By employing high-strength concrete, the column size is reduced. At the same time, the amount of vertical reinforcement can be reduced. The net result is the least expensive column is achieved with the smallest size column, the lowest amount of reinforcement and the highest readily available concrete strength. As a result, concrete has become the material of choice for many tall, slim towers.

Two recently-constructed, high-profile skyscrapers feature concrete designed to offer the utmost safety and resilience. Philadelphia’s Comcast Center, the tallest building in the city at 297 m (975 ft), required 38,230 m3 (50,000 cy) of concrete containing slag cement for the high-performance project, of which 27,525 m3 (36,000 cy) was specified at 68,950 kPa (10,000 psi) for the central inner core. The building’s thick exterior core walls—1370 mm (54 in.) up to the 20th floor—further minimize deflection due to wind forces, and all its elevators, sprinklers, communications systems, and stairwells are encased within the concrete core.

However, it is Manhattan’s One World Trade Center that is setting a new standard for stronger, safer urban landscapes. Rising a symbolic 1776 ft (i.e. 541 m), this landmark skyscraper is the tallest in the Western Hemisphere. It has a massive cast-in place, reinforced concrete inner core that runs the full height of the tower—an extra-strong backbone that provides support for gravitational loads as well as resistance to wind and seismic forces. The concrete core walls are 1 m (3 ft) thick or more above ground and up to twice that below grade. Higher up, the concrete core walls slim down to 0.6 m (2 ft) thick.

The 152,910 m3 (200,000 cy) of concrete used in the tower’s superstructure—with a strength that has never been used on such a scale in building construction—was custom-designed to ensure high levels of durability, as well as control the heat of hydration during the mass concrete pours to minimize cracking. Supporting columns on the first 40 floors were made from 82,740 to 96,525 kPa (12,000 to 14,000-psi) self-consolidating concrete and the upper floors with 59,295 to 68,950-kPa (8600 to 10,000-psi) mix designs.

To meet the compressive strength requirements, the design and engineering team relied on a highly specialized concrete mix that included fly ash, silica fume, and slag cement. High-strength concrete was the ideal material for meeting the high-priority safety requirements for One World Trade Center because elevators, stair enclosures, and other supporting members relied on to resist wind, seismic, and other impact forces are designed with an extra measure of durability and resilience.

Innovative new UHPC materials
The concrete industry continues to explore new ways of making buildings stronger, safer, and, as demonstrated by one of its newest products, more aesthetically pleasing. Ultra-high-performance concrete (UHPC) is blended with high carbon metallic or polyvinyl alcohol (PVA) fibers, and has a unique combination of properties including strength, ductility, durability, and aesthetic design flexibility.

The Perez Art Museum in Miami is situated on Biscayne Bay, where frequent tropical storms and exposure to the salt and sea air can cause serious problems for buildings. UHPC was used for the building’s approximately 100 long-span, precast vertical mullions to blend with its cast-in-place concrete elements and support the large curtain wall glazing that surrounds the building. Due to its exceptional strength, the UHPC made it possible to create thin, sinuous mullions up to 5 m (16 ft) tall, allowing unobstructed views over the museum’s veranda while meeting the area’s hurricane resistance standards and offering increased resistance to corrosion from the sea air.

Conclusion
Although the frequency of natural disasters has not increased in the last 40 years, their safety risks and economic costs are rising dramatically due to increased urbanization and population concentration along the coasts and flood-prone areas. More than 50 percent of the U.S. population and $10.64 trillion 
of insured property is located in areas vulnerable to hurricane destruction, and nearly 134 million people will be living in hurricane-prone states by 2020.

It is clear stronger, safer concrete buildings will play an important role in protecting these growing communities from the humanitarian and economic costs of major natural disasters. In addition to satisfying minimum life safety provisions, enhancing the resilience of buildings through mandatory requirements should be a priority for every jurisdiction, especially communities in disaster-prone communities.

Relevant ASTM Standards
• 
ASTM C989, Standard Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars;
• 
ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete;
• 
ASTM C1240, Standard Specification for Silica Fume Used 
in Cementitious Mixtures;
• 
ASTM C150, Standard Specification for Portland Cement;
• 
ASTM C595, Standard Specification for Blended Hydraulic Cements; and
• 
ASTM C1157, Standard Performance Specification for Hydraulic Cement.

Andrew Pinneke, PE, LEED AP, is a construction specialist at Lafarge, consulting on a wide range of sustainable construction and building performance issues while coordinating sustainable construction and concrete technology transfer efforts. He worked as a structural engineer for almost a decade before joining Lafarge. He sits on the National Ready Mixed Concrete Association (NRMCA) Sustainability Committee, and several American Concrete Institute (ACI) committees, along with the ACI Foundation’s Strategic Development Council (SDC). Pinneke can be contacted via e-mail at andrew.pinneke@lafarge.com.