VICKSBURG, Miss. - Researchers from the U.S. Army Engineer Research and Development Center (ERDC) know a thing or two about aging and ailing infrastructure. Currently one of the largest issues facing the U.S. Army Corps of Engineers (USACE), ERDC is leading the charge to bypass traditional repair methods and pursue the use of novel materials in hopes of finding a permanent solution for reconstruction and repair efforts rather than a temporary fix; however, before this step can be reached, the cause of the problem must be identified – that’s where forensic investigation comes in.
“We are often asked to figure out what is going on with a structure and determine why it’s deteriorating,” said ERDC Research Civil Engineer Dr. Robert Moser. “This can mean that we head to the field or that the customer sends samples for various types of analysis, including optical and electron microscopy, chemical analysis, accelerated durability testing, mineralogical analysis, mechanical testing and other types of characterization. Based on the analysis performed, the cause is identified, potential issues are noted and repair concepts are developed.”
While there are a variety of issues affecting USACE infrastructure, problems related to corrosion and environmentally-assisted cracking of metallic components seem to be the most prevalent. Occurrences can be found in hydraulic steel structures, embedded reinforcing steel bars and prestressing tendons in concrete, among other components. Another critical issue is alkali-aggregate reactions in concrete. The processes, which include alkali silica reactions and/or alkali carbonate reactions, can cause extensive damage such as cracking, moving of concrete elements, and operational issues in structures as joints close and mechanical systems with tight tolerances become misaligned.
Though they may be the most prevalent, these problem causers are just the tip of the iceberg when it comes to deterioration issues ERDC has investigated. From freeze-thaw action, sulfate attack and erosion to fatigue, embrittlement and exposure to moisture, the root of the problem can be difficult to ascertain. According to Moser, this means the investigation process is often uniquely developed for each issue, material and structure.
“Our solutions and characterization techniques are typically customized for each project based on discussions with the customer and our best professional judgement,” said Moser. “The goal is to suppress the source of the deterioration and bring the serviceability of the structure back up to an acceptable level. Once we determine the problem, we bring in structural engineers to assess the level of deterioration and consequences that come along with that.”
“Some customers come to us specifically for our characterization capabilities and have their own in-house engineers who can develop repair concepts based on our input,” continued Moser. “In other cases, we recommend solutions to the problems.”
ERDC employs a tiered approach to forensic investigation, beginning with rapid-screening studies to identify deterioration mechanisms and pinpoint appropriate research techniques. More in-depth characterization and testing are then performed in pursuit of definitive answers. The organization’s unique capabilities allow research to be performed, from large-scale testing all the way down to the nano level, providing an unparalleled ability to determine what is occurring in a variety of materials. Changes in chemistry, mineralogy, electrochemical conditions and mechanical properties can easily be detected.
Recent projects include work for USACE and additional agencies, both internal and external to the Department of Defense, related to alkali-aggregate reaction damage in concrete. The issue has been found in lock and dam structures, airfields and bridges, among other locations. The work specifically focused on characterizing damage, mineralogy of aggregates that are reacting in concrete and predicting future damage.
Work has also been ongoing with the USACE Jacksonville District regarding geochemical dissolution issues with locally-available limestone aggregates in Southeast coastal regions. Because the stability of the aggregate depends on water quality, premature deterioration in lock and dam control structures is becoming a severe problem in south Florida in select regions with low carbonate alkalinity. ERDC assessed water quality, aggregate mineralogy and characterization of aggregate surfaces that have been exposed to aggressive waters. The district is performing geochemical dissolution modeling based on results, and ERDC researchers have developed a calibrated model for predicting this type of deterioration, a useful tool that is helping to determine which materials will perform best in different regions based on local water quality.
“Maintenance and repair operations are often conducted without an understanding of the underlying problems that are causing the distress, which causes the same problems to occur over and over,” said Moser. “By determining what those underlying issues are, we can help identify a better solution that will prevent or minimize distress and reduce the overall lifecycle maintenance and repair costs associated with the structure.
“ At ERDC, we’re playing on the same team as our USACE districts and want to do everything we can to determine the right answers, in a timely manner, and as cost-effectively as possible.”