by Katie Daniel | February 3, 2016 4:25 pm
by Del Williams
A recent 60 Minutes episode, “Falling Apart,” noted America’s roads, bridges, airports, and rail lines are outdated, neglected, and need to be fixed. The segment goes on to state, “nearly 70,000 bridges in America—one out of every nine—is considered to be structurally deficient.” Indeed, according to the American Society of Civil Engineers (ASCE), “32 percent of the major roads in America are now in poor condition and in need of major repairs.”
The majority of America’s infrastructure has outlived its intended service life, and a failure of traditional waterproofing coatings has led to significant corrosion of structural elements, including corroded rebar and crumbling concrete. New high-performance waterproofing alternatives are replacing more traditional techniques, such as sheet goods and short-term liquid sealants, to help civil and structural engineers cost-effectively protect and maintain critical infrastructure. Examples of projects that can be protected
by these technologies include:
Spray-applied waterproofing products—comprising spray-on materials such as polyureas, polyurethanes, and poly methyl methacrylates (MMAs)—are seamless, rugged, quick-curing, impervious to water, able to bridge cracks, and capable of lasting decades without extensive maintenance.
“Spray-applied waterproofing now accounts for over 50 percent of the membranes applied to our bridges,” said Alexander Bardow, Massachusetts’ Department of Transportation (MassDOT) state bridge engineer.
“What drove us to spray-applied waterproofing is its enhanced durability, bonding to concrete, and crack bridging ability,” said Bardow, who oversees and helps to prioritize work on 5000 bridges that receive federal funds for MassDOT. “If cracks form due to deck deterioration or traffic loading, then water gets into those cracks and the concrete matrix. The [underlying] membrane must be pliable enough to accommodate these cracks without failing.”
Drawbacks of traditional techniques
In past decades, MassDOT utilized mopped emulsion reinforced with fiberglass fabric for bridge decks, then later turned to sheet goods. While traditional methods of waterproofing such as these, or non-elastomeric coatings like epoxies and paints, have long been used, they can have drawbacks, and must be continually inspected and maintained over time.
Reinforced emulsion was not a good deck waterproofing system because it debonds and allows water to seep underneath and into the deck. After failure with reinforced emulsion, prefabricated sheet-applied membranes became the focus. These are easy to apply, but more durability was necessary.
Non-elastomeric coatings such as epoxies and paints do not have the elasticity to bridge cracks, and tend to be ‘moisture-resistant’ rather than truly ‘waterproof.’
Spray-applied waterproofing membranes, on the other hand, display better performance and have measurable performance characteristics that can be counted on for extended deck life.
“We can expect spray-applied waterproofing membranes to last about 30 to 50 years,” Bardow adds. “We would still repave and resurface the asphalt overlay, but would not need to do anything to maintain the underlying waterproof membrane during that time.”
A newer generation of elastomeric coating, spray-applied waterproofing membranes are resolving the longevity, reliability, and quick application issues of older techniques.
Since crack-bridging is a critical element of these newer technology membrane systems, ASTM has developed its ASTM C1305, Standard Test Method for Crack Bridging Ability of Liquid-applied Waterproofing Membrane. For liquid membranes with an independent wearing course, the test consists of applying the membrane system across two concrete masonry units (CMUs) with matching faces. The test sample is then brought down to –26 C (–15 F) for 24 hours to stabilize it at this temperature. The testing requires the two blocks be pulled apart at a rate of 3 mm (1/8 in.) per hour to a maximum opening of 3 mm, then closed to a zero gap at the same speed. The test fixture must maintain the sample at the –26 C temperature through the 10 cycles of testing required. Some manufacturers have conducted this test for many additional cycles to further assess their product.
The American Railway Engineering and Maintenance-of-way Association (AREMA) also includes spray-applied membranes in its guidelines, and some products exceed these parameters. The spray-applied waterproofing membrane previously cited, for instance, exceeds AREMA Chapter 8, Part 29 requirements for waterproofing membranes, and is designed to last the lifetime of the structure. It is impervious to de-icing chemicals, water, ballast, stray current, and other factors that contribute to accelerated deterioration and wear of elevated rail/road structures and tunnels.
Some notable physical properties of a spray-applied elastomer waterproofing system include:
When bridges are traditionally sealed, the decks need to be resealed almost every five years. The introduction of spray-applied waterproofing materials, on the other hand, can last five decades or more. This saves time and money because it does not require the repeated removal of asphalt overlay, the resealing of the deck, and repaving of the surface—not to mention saving the required lane closures. Perhaps most importantly, it would stop rebar corrosion and concrete spalling in decks to keep infrastructure safely operating.
Spray-applied elastomer waterproofing material not only provides a continuous, seamless waterproofing membrane—even on irregular surfaces—but it also has a coefficient of linear thermal expansion similar to steel and concrete. These properties, along with its engineered durability, allow the material to be used as a composite within asphalt layers.
It can be sprayed horizontally, vertically, and overhead at any thickness, as well as applied at specified thickness in one continuous application. Spray-applied waterproofing can also be applied robotically at greater speeds and consistencies than other materials.
It is possible to consistently spray waterproofing material over 929 m2 (10,000 sf) of deck per day with 2032 to 2159 µm (80 to 85 mils) on any surface when using a robot. This material is also desirable for its crack bridging capability and fast setting and curing time. It passes ASTM C836-00, Standard Specification for High Solids Content, Cold Liquid-applied Elastomeric Waterproofing Membrane for Use with Separate Wearing Course, for 3-mm crack-bridging at 80 mils, can be applied in temperatures as low as –28 C (–20 F) without affecting setup or curing, and can set in 10 seconds.
For the Metra Bridge project (Orland Park, Illinois), Pine Waterproofing and Sealant installed nearly 650 m2 (7000 sf) of spray-applied waterproofing material. When measured, the bond strength readings were more than 6894 kPa (1000 psi) at 80 mils.
In another concrete application with a very irregular surface at the Chicago Department of Transportation (CDOT) the bond strength measured 4136 to 5515 kPa (600 to 800 psi) at 80 mils thick. The project involved applying 6038 m2 (65,000 sf) of material and the bridge needed to be re-opened as soon as possible. The contractor was able to install over 2787 m2 (30,000 sf) of 80 mils spray applied waterproofing material and 1016 µm (40 mils) of topcoat with aggregate within seven days. The job was completed within four weeks.
As part of a comprehensive quality control/quality assurance (QC/QA) program, daily readings—such as environmental conditions and bond strength—should be taken and recorded for these waterproofing projects.
In critical, high-traffic highway and rail bridge projects, speed of application, setting, and return
to service is a particular advantage of spray-applied waterproofing material because they minimize downtime. Some products can even accept ballast and asphalt overlays one hour after application.
To further illustrate the emphasis on importance of speed, many in the industry are turning to accelerated bridge construction techniques in addition to fast-cure waterproofing. This process involves using prefabricated bridge elements and systems such as composite decks manufactured offsite, which are then installed with closure pours and then appropriately waterproofed. This expedites the project, improves quality, reduces traffic impact, and lowers total lifecycle costs.
“When we did the Metra Fast 14 project, we had to complete a bridge project in essentially one weekend,” explains MassDOT’s Bardow. “Within that time, we had to demolish the existing bridge, set prefabricated bridge units for the new one, do all the closure pours, have the concrete set, then come back to spray-apply waterproofing membrane. The contractor accomplished this, and the speed of spray-applied membrane installation was an important factor in meeting the accelerated deadline.”
Bardow says a spray-applied membrane is standard for accelerated bridge construction for MassDOT projects.
“With the large number of closure pours we do, we were also concerned about effectively waterproofing them so water intrusion does not become a problem over time,” he explains.
While new spray-applied waterproofing technology has enabled more durable, cost-effective concrete and steel infrastructure protection, it has also opened the door to complementary technologies. For instance, to more thoroughly waterproof the entire bridge deck structure, a highly flexible expansion joint has been developed using the same coating chemistry as the waterproofing membrane. When used in conjunction with spray-applied waterproofing, these expansion joints form a high-strength chemical bond providing a single, monolithic layer of waterproofing across both the joints and bridge deck.
Demanding rail bridge applications requiring additional protection against ballast impact can use ballast mats to provide seamless, high-build protection of the waterproofing system with a secondary protection course. These mats must pass the Ballast Impact Test (loading 9.2 to 45 Kips [9200 to 45,000 lb] two million cycles). Some can do this with no damage and be open to ballast loading in one hour.
To provide DOTs with a wide-open window of time to get the asphalt wear course down, a slow-setting, 100 percent solids, high-strength elastomeric coating can also be used on projects requiring an aggregated topcoat—allowing live traffic to drive on the waterproofing membrane for up to two weeks. The slower-setting resin allows aggregate to be broadcast into it before initially set. The topcoat can be applied in temperatures as cold as –28 C without affect on setup time or curing when applied at its specified thickness in one continuous application.
New spray-applied waterproofing is designed to reliably span decades without ongoing maintenance. Civil and structural engineers have a valuable tool to help cost-effectively maintain critical infrastructure while minimizing downtime.
Del Williams is a technical writer based in Torrance, California. He writes about health, business, technology, and educational issues, and has a degree in English from C.S.U. Dominguez Hills. He can be reached at email@example.com.
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