Search Content

This research report looks at the chemical composition of reinforcing bars, and the sulfur content in particular, and their influences on the corrosion resistance of rebar. The research supports the original hypothesis--which suggests that the reduction in sulfur inclusions would benefit corrosion resistance. The reduction could result in significant savings that would more than offset the higher initial costs for these bars. To test the hypothesis, the study examined the corrosion resistance of four kinds of steel reinforcing bars; ordinary, low sulfur, copper and tungsten, and nickel. As in other series in the past, this research indicates conflicting results for different measurement techniques used to quantify corrosion rates. In addition, the mechanism that results in low sulfur bars showing a three-fold increase in corrosion life are not clear and need more study. The report recommends a long-term follow-up study on the use of both small cube and slab specimens in the laboratory, as well as full-scale specimens in the field

Grout materials and grouting practices used in post-tensioned (PT) bridge construction prior to 2003 frequently resulted in the formation of air- or water-filled pockets; termed grout voids; inside PT tendons. Grout voids have been identified as conditions that can promote a corrosive environment within PT tendons. This report details the second phase of a two-phase project that was commissioned by MnDOT to identify best practices for the investigation of PT tendon conditions in bridge structures constructed prior to 2003. This report presents information and recommendations that will assist MnDOT in selecting and developing a project approach; work plan; and budget for future inspection and repair contracts for their pre-2003 era PT bridge inventory.

Steel corrosion on bridges and ancillary structures due to environmental effects and deicing chemicals is a serious problem for Minnesota's infrastructure. The ability to detect; locate; and measure corrosion is an important aspect of structure inspection. Accurate thickness measurements and corrosion mapping are essential for determining load capacity of structural members on bridges and ancillary structures. The Minnesota Department of Transportation purchased an OmniScan Phased Array Corrosion Mapping System. Unlike conventional ultrasonic equipment; this system provides detailed three-dimensional images of structural members including the remaining section of members that exhibit corrosion. This gives engineers better tools to visualize and evaluate the condition of bridges than was previously possible. With the future purchase of additional transducers; the OmniScan can also be used for enhanced inspection of welds and bridge pins.

One of the most serious and costly problems presently facing highway agencies in many parts of the country is premature deterioration of concrete bridge decks. The major cause has now been linked to de-icing chemical "chloride induced corrosion of the reinforcement. In some cases, bridge decks designed to last 40 years have required major repairs after only 10 years of service due to corrosion related "chuckholing" or spalling. The Minnesota Department of Transportation has become increasingly aware of this problem during the last six years and has initiated a comprehensive program to resolve it. The two basic approaches being used to try and solve the problem are: 1. Keep the salt and moisture out of the decks with protective overlays and membranes, modified concretes or sealers. 2. Protect the steel from corrosion once the concrete's protective nature has been destroyed by chloride contamination, using epoxy coated and galvanized rebars. Bridge decks were constructed or reconstructed using protective membranes, modified concretes and coated bars. Testing and evaluation of field performance consists of visual observations, delamination detection, chloride content determinations, checking depth of concrete cover over rebars and electrical potential measurement

The main purpose of this investigation was to conduct an in-depth study to determine the level of corrosion protection offered by epoxy-coated bars in four bridge decks in the Minneapolis/St. Paul, MN metropolitan area. The bridges studied were built between 1973 and 1978 and all decks had a top mat built with epoxy-coated bars. The bottom mat was epoxy-coated in only one deck whereas black steel was used for the bottom mat in the other bridges. These bridges had been assessed in 1996 and thus, the present study is a follow up investigation to obtain data and assess the field performance of epoxy-coated bars over a period of approximately 30 years. The investigation included field inspection and surveys of the decks, as well as laboratory tests of concrete core and bar samples. After 30 years of service, the overall condition of the epoxy-coated bars is good to very good, with no or modest levels of corrosion activity. In only one bridge, corrosion activity appears to be moderate to severe. The majority of corroded bars were found near joints or at crack locations. The amount of delamination in all decks is very low.

The purpose of this project was to identify efficient and cost-effective methodologies for maintenance painting of steel bridges for MnDOT bridge crews to extend the service life of the coating system by at least five years before complete coating rehabilitation would be warranted. Five generic coating systems, chosen for their compatibility with existing MnDOT coating systems and minimal requirements for surface preparation and application, were applied over minimally prepared surfaces on two St. Paul bridges. Coating performance was evaluated annually over a three-year period through visual field observation following the MnDOT Steel Bridge Coating Condition Assessment Photographic Field Guide and MnDOT Bridge and Structure Inspection Program Manual. Adhesion testing was performed in accordance with ASTM D3359 in conjunction with the third-year evaluation. The results were photographed and documented in a matrix; identifying key performance characteristics. The predominant failure affecting two of the coating systems was delamination due to application over an anti-graffiti finish coat. Otherwise, each system performed to a standard aligning with the pre-established project goal for expanded serviceability of five years. Because developing a Bridge Maintenance Coating Program requires critical timing in the inspection process to identify existing coating condition; a Bridge Coating Repair Reference Table was developed to assist MnDOT crews with determining the appropriate preventive maintenance painting strategy based on condition and existing coating system. This work also determined that surface conditions demonstrating pitting with rusting; or surfaces with an anti-graffiti coating should not be addressed through maintenance painting but instead should be considered for coating removal and replacement.

Reinforced concrete bridge substructures in Minnesota and other northern climates possess an elevated risk for chloride induced corrosion damage; which can reduce capacity and shorten service life. Research was performed in 1997 to investigate new strategies for corrosion mitigation; including electrochemical chloride extraction (ECE) and installation of fiber reinforced polymer (FRP) wrap; which were applied on several corrosion-damaged piers on a 30-year-old bridge in Minneapolis. This report presents the results of follow-up research performed to assess the condition of the treated piers after 20 additional years of service; in order to understand the long-term effectiveness of the strategies implemented. The combination of ECE treatment and FRP wrap installation was found to be very effective; with no concrete distress or probable corrosion activity identified in the treated elements. Poor or mixed performance was observed with all other strategies; including both ECE treatment followed by application of a penetrating sealer and FRP wrap installation that was not accompanied with ECE. In addition; significant chloride contamination occurred in all of the subject piers within the 20 years since the initial study; indicating that neither FRP wrap nor concrete sealers prevented the ingress of new chlorides in the manner in which these systems were installed in the initial study. The findings indicated that performing ECE treatment; or installing FRP wraps; did not alone eliminate the risk of future corrosion activity. The most effective corrosion mitigation strategy to extend the service life of reinforced concrete bridge substructures was to minimize water and chloride exposure.

This study was concerned primarily with a laboratory evaluation of the effectiveness of several chemical inhibitors in reducing corrosion of autobody steel by deicing salts. Under the controlled exposure conditions imposed in the laboratory tests, the treated salts appear to be quite effective in reducing corrosion. It was therefore deemed necessary to further evaluate the effectiveness of anticorrosive salt under natural conditions of exposure more typical of that experienced by vehicles in normal use. A separate field evaluation was conducted beginning in the fall of 1965 and continuing through the 1966-67 winter. The results of this study, in comparison with the results of the laboratory study indicate a much reduced effectiveness. Because of the difference in the findings of the two studies, a summary of the final report of the field evaluation is given in Appendix C of this publication.

The strength and durability of reinforced concrete (RC) bridges are adversely affected by the deterioration of their structural members. When investigating bridges in need of maintenance and repair, the deterioration due to the corrosion of steel rebars is commonly found to be a primary source of structural damage and degradation. To ensure the safety and performance of RC bridges while reducing their direct and indirect costs, an accurate estimate of the extent of reinforcement section loss has central importance for a wide spectrum of engineers and decision-making authorities. This research project investigated the steps required to achieve such rebar section loss estimates. To achieve this purpose, field assessments of rebar section loss were correlated with available predictive models and later calibrated to condition-specific field data. The outcome, which has been delivered in the form of steel reinforcement section loss guidance tables, directly contributes to understanding variability in rebar section loss when making loss predictions for use in structural evaluation. This facilitates planning preventive and/or corrective actions tailored to the condition state of deteriorating bridge elements.

This Technical Summary pertains to report 2022-32, Steel Reinforcement Section Loss Guidance Tables.