ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/395356
FHWA Post-Tensioning Grouts: Challenges and a Path Forward by Reggie Holt, Federal Highway Administration Two recent issues with post-tensioning (PT) grout have generated concern over the long-term performance of PT tendons. These two independent issues occurred close to the same time, which has led to confusion by many in the bridge community. This article will provide background information on these two issues, as well as measures developed to address each issue. ELEVATED CHLORIDES In 2010, the discovery of PT grout with elevated chloride levels in a straddle bent cap in Texas triggered a follow-up investigation by the grout manufacturer, Sika. Their investigation revealed that its SikaGrout 300PT product was sometimes produced at its Marion, Ohio, plant with levels of chloride compounds that exceeded the American Association of State Highway and Transportation (AASHTO) and the Post-Tensioning Institute (PTI) specified limit of 0.08% by weight of cement. Approximately 24 million pounds of SikaGrout 300PT were produced at this plant from 2001 to the time production ceased in 2010. At that time, approximately 16 million pounds were used in bridge applications in the United States. The investigation revealed that cement provided by a third-party supplier, and used in the SikaGrout 300PT production, was the source of the chlorides. Chloride levels ranged from well above to well below the specified limit. The governing ASTM specification for portland cement, ASTM C150, does not include limits on chloride. In addition, the AASHTO and PTI specifications only specified chloride thresholds for PT grouts, not for constituent materials of those grouts. Consequently, this issue went undetected for many years with very limited chloride level data being captured for PT grouts. The PTI M55.1-12 Specification for Grouting of Post-Tensioned Structures has since added requirements for frequent chloride testing during production. Subsequent to the identification of this issue, Sika and Federal Highway Administration (FHWA) each started independent efforts to collect chloride data, identify end users, and perform research to determine the long-term performance of PT strand exposed to high levels of chloride. Results from these efforts were used to develop an FHWA technical advisory (TA) on assessing and managing the long-term performance of post-tensioned bridges having tendons installed with grout containing elevated levels of chloride (www.fhwa.dot.gov/bridge/ t514033.pdf). This TA, released in November 2013, provides guidance on identifying SikaGrout 300PT end-users, assessing PT tendon service life risks, as well as recommended management practices. SEGREGATED PT GROUT A second issue deals with segregation of prepackaged, thixotropic PT grouts during or immediately after the grouting of PT ducts. PT grouts commonly used in the United States have sporadically been observed to exhibit this condition. When it forms, the grout matrix is typically separated into three common layers, with a top layer of soft (putty-like) grout, a middle layer of friable grout, and a lower layer of hard grout. The upper soft layer in segregated grout is highly corrosive and is therefore of great concern. Segregated grout formations are typically isolated to tendon high point and anchorage locations with volumes generally varying from minimal to filling the upper third of the of the PT duct along a limited length within the tendon or anchorage.