New Models Estimate Bridge Steel Rebar Corrosion
Researchers recently examined bridges in Minnesota as part of a project to develop guidance and methods to estimate rebar reinforcement section loss for bridge maintenance.
Conducted by a team from Iowa State University’s Institute for Transportation for the Minnesota Department of Transportation, the new prediction tool is anticipated to help accurately plan for appropriate repairs or replacement projects.
“We’ve developed and calibrated a set of models with Minnesota-specific conditions for more accurate estimates of reinforcement section loss. Such estimates are instrumental in ensuring the safety and performance of bridge structures in service,” said Behrouz Shafei, associate professor, Iowa State University.
About the Research
According to MnDOT, while bridges are generally designed for a minimum lifespan of 75 years, damage due to corrosion can occur after just a few years of exposure to conditions such as fluctuations in temperature and moisture or exposure to deicing salts.
As a result, corrosion of rebar creates rust and rebar cross-section loss, weakening the steel content. This rusting process can also cause expansion that results in concrete cracking and potential spalling.
MnDOT reports that while, conventionally, bridge inspectors visually asses bridges to look for cracks or use hammers to find delaminated concrete and visible rebar, there is no industry standard to measure the section loss of the reinforcing steel rebar and visual judgement can be inaccurate.
The Department then began to seek to develop guidance on accurately estimating rebar cross-section loss to inform repair strategies and efficiently plan preventive and corrective bridge maintenance. Better tailoring repair strategies based on these tools would be anticipated to reduce material and labor costs, as well as minimize bridge closures and traffic disruptions.
With a goal of creating guidance and aids to enhance visual and other nondestructive methods of estimating section loss, these models would be developed specifically for standardized two-inch concrete cover over reinforcement with 4,000 psi strength concrete.
For the project, researchers reportedly reviewed available literature on steel reinforcement corrosion and identified mathematical equations that would best correlate the progression of reinforcement corrosion to concrete cracking. They also examined destructive and nondestructive methods to evaluate deterioration.
They then created two formulations to best envelope rebar corrosion:
A concrete repair project on piers of a bridge near Minneapolis provided an opportunity to document observed conditions before the repair and discover actual rebar section loss during the concrete repair. Before performing concrete repairs, researchers reportedly conducted extensive photo documentation and mapping of delaminated concrete.
A concrete repair project on piers of a bridge near Minneapolis provided an opportunity to document observed conditions before the repair and discover actual rebar section loss during the concrete repair.
Then, during the repairs, researchers collected data that included photo documentation and visual assessments of rebar section loss. Additionally, they extracted select steel reinforcement samples that were taken to the laboratory for more exact measurements and testing.
The rust was removed from these samples to measure the remaining steel, using 3D scanning to plot the cross-sectional areas of the bars for calculating section loss. The team also performed mechanical tests to determine strength and the failure point of the corroded rebars.
Using these results, researchers then recalibrated predictive models for two types of rebar section loss: concrete that has maintained its overall integrity but has visible cracks in its surface (Situation 1), and concrete that has partially or completely delaminated to expose embedded steel rebars (Situation 2).
Researchers found that visual assessment was not sufficient accurate, but the 3D scanning method accurately estimated section loss. The results from 3D scanning were comparable to visual assessments when the section loss was relatively large (above 75%) but not for smaller losses. Mass measurements were consistent with 3D scanning.
According to the release, combing these results with tensile tests, the study concluded that the visual-only assessment of section loss can be conservative for very low actual losses in steel reinforcement but unconservative for larger section losses. This could result in underestimating or overestimating structural capacity, leading to costly repairs or reducing safety.
In the cracked concrete model, section loss (including upper and lower bounds) is based on the crack width from a photograph or field measurement. For the delaminated concrete model, the section loss is based on the age of the steel reinforcement.
Because the models and guidance tables have large ranges, MnDOT anticipates that the accuracy can be improved with data from additional bridges. This would not only improve the number of samples in the data set, but also represent a greater diversity of ages, locations and exposure conditions.
“The upper bound of section loss estimated by the model for cracked concrete will be helpful to avoid being overly conservative by closing a bridge when we don’t have to. We will, however, need more bridge rebar samples to refine the model for delaminated concrete,” said Paul Pilarski, bridge construction and scoping engineer, MnDOT Bridge Office.
MnDOT reportedly plans to collect additional samples during major bridge repairs.
The full research report, “Steel Reinforcement Section Loss Guidance Tables,” can be read here.
Other MnDOT Research
In October, researchers and MnDOT evaluated fiber-reinforced concrete (FRC) using performing engineered mix (PEM) design methods to help bridge decks and pavements better withstand the state’s weather.
According to the research, PEM design uses new measurement technologies to identify the parameters a mixture should have to maximize strength and durability, compared to traditional concrete methods that do not represent field conditions and are not specified to FRC.
The goal of the study, completed in June, was to identify PEM target specifications for FRC mixtures to provide for durable and long-lasting pavement. The principal investigator for the project was Manik Barman, associate professor for the University of Minnesota Duluth Department of Civil Engineering.
Researchers first performed a literature review on the PEM design procedure and key engineer parameters, fiber types and FRC properties. Then, the team designed and tested 57 concrete mixes with PEM design methods, varying with fibers and aggregate type, workability and air voids.
Two synthetic fibers commonly used in pavement, as well as three classes of coarse aggregates, were chosen by researchers based on MnDOT’s standard construction specifications. Additionally, three target plastic air contents (4%, 6% and 8%) were tested, and the mixes were kept within acceptable workability ranges.
Based on test results, researchers were then able to develop draft specifications for the recommended parameter ranges for FRC mixtures using these test results.
They added that the work supports MnDOT in creating specifications for fresh concrete properties of fiber-containing concrete into its Standard Specification for Construction. Additionally, the testing suggests that FRC mixes developed with the PEM design procedure can be durable and long-lasting for use in pavement and bridge decks.
The team reports that additional investigations into field mixes and testing, especially for an appropriate range for the V-Kelly index for FRC, may be needed.