A few high-profile events, including the demolition of a Seattle high-rise apartment building in 2011, have brought public attention to the possibility of tendon corrosion in unbonded post-tensioned (PT) concrete. Tendon corrosion is attributed to water infiltration at the anchorages due to poor concreting and finishing of the tendon stressing pockets, suggesting that an adjustment in construction practices and detailing might reduce the potential for such corrosion.
Also, a change in construction practices may counter the negative perceptions generated by the publicity surrounding tendon corrosion. To address these issues, a few years ago the Post-Tensioning Institute (PTI) began to examine ways to achieve wider use of known strategies for preventing moisture infiltration.
“Actual structural problems are less of an obstacle to using PT than image is,” says Ted Neff, executive director of the PTI. “Nevertheless, improving confidence in the system is important.”
“Clients ask about long-term durability and there has been a perception that unbonded PT is not as safe or durable as other systems,” says Cary Kopczynski, P.E., S.E., and CEO of Cary Kopczynski & Co., Inc., Bellevue, Wash., a structural engineering firm with extensive experience in PT structures. “There is an echo effect to failures. Things that happened in the 80s are still occasionally brought up now, 30 years later.”
PT construction and codes
Conventional unbonded PT construction involves laying a network of sheathed cables with ductile iron anchors at each end into the pour area. The anchor assembly is intended to transfer the stressing force to the concrete, and is comprised of a bearing plate and wedges. After the concrete has cured, the end of the PT cable is pulled with a hydraulic jack, stretching it several inches. As the jack is released, the wedges are pulled into the anchor wedge cavity, holding the cable in place. The cable “tail” is then cut close to the anchor and the stressing pocket is filled with non-shrink grout. The grout is intended to protect the tendon against corrosion, but if not done properly, water can wick into the system. Anchorages tend to be more vulnerable to corrosion, which is why most codes and specifications require using an encapsulated anchorage system on buildings located in aggressive environments. (An encapsulated tendon is completely enclosed in a watertight covering from end to-end, including anchorages, sheathing, PT coating, sleeves and an encapsulation cap over the strand tail at each end.)
In 2012, the PTI issued an addendum to its PTI M10.2-00 Specification for Unbonded Single Strand Tendons, 2nd Edition, expanding the encapsulation requirement to apply to all PT buildings. In addition to the PTI specification, a similar change has been proposed to the American Concrete Institute’s equivalent specification, ACI 423.7 Specification for Unbonded Single-Strand Tendon Materials. A 2014 update to ACI 423.7 that requires using encapsulated tendons on all buildings is in the final stages of approval.
ACI 318, Building Code Requirements for Structural Concrete, references ACI 423.7. An updated version of ACI 318, known as ACI 318-14, will be released later this year. ACI 318 is directly referenced by the International Building Code (IBC) and it is currently planned that ACI 318-14 become part of IBC’s 2015 Code. As the new standards are approved and put into effect they will eventually be adopted by local building codes as well.
These combined updates represent a major step forward in imposing the requirements of full strand and anchor encapsulation for all unbonded PT concrete building applications, which should improve both the performance and perception of PT concrete. Randall Poston, P.E. and principal at WDP & Associates, a consulting engineering firm that investigates PT problems, stated that encapsulation on the McGuire building in Seattle would have prevented corrosion.
A changing adoption rate
In early 2014, members from various segments of the PT industry reported seeing differing levels of encapsulation currently, but were nearly unanimous in their belief that adoption would be near 100% in a few years.
Today, the use of strand and anchor encapsulation in elevated commercial structures is largely regional. Florida has a high rate of encapsulation due to the corrosive environment. Other zones with high moisture, like the northern and mid-Atlantic states, also see a fairly high rate of encapsulation. In dry areas, such as Arizona, encapsulation is rare. In exposed structures, such as parking garages, encapsulation has been widespread, as high as 90%. “Sometimes, though, corrosive environments don’t tell the whole story,” says Neff. “Construction quality is one of the biggest factors influencing the performance of tendons. Furthermore, sometimes there are unanticipated project conditions in which the tendons have ongoing exposure to water.”
“Moisture can enter cavities even during construction,” says Don Kline, P.E. and owner of Kline Engineering, who does work across the northeastern U.S. and mid-Atlantic.
The consensus among interviewees was that issuing the PTI addendum did begin to move the needle toward using encapsulation in a broader range of building types. Some engineers are now voluntarily adopting the PTI standard. “The PTI is a leader,” explains Russell Price, P.E. and executive vice president of Suncoast Post Tension Ltd., a national supplier of post-tensioning materials. “The addendum alerted engineers to the change in process.”
The slow spread of awareness among structural engineers has been part of the holdup in implementing encapsulation, says James Cagley, P.E., S.E. and principal of Cagley and Associates, a consulting engineering firm that specializes in a wide range of building types. “Actually, only a small percentage of the industry fully understands PT. Many firms outsource their PT. But recently I was involved in a large job that required encapsulation. Probably as few as five years ago, on a huge job like this, it would not have been done. But now a number of engineers are learning the importance.”