Assessing Steel Corrosion Risk in Innovative Cement Concretes

Summary

Corrosion of steel embedded in concrete infrastructure components remains a persistent challenge, costing the US economy billions of USD annually. As shown in Figure 1 below, the direct and indirect cost of corrosion in the US is approximately 6 % of the US GDP [1-3]. Existing standards addressing the degradation of reinforced concrete infrastructure are limited in their ability to make meaningful predictions of remaining service life because they do not adequately capture the complexities of steel corrosion. Corrosion is controlled by a combination of environmental factors acting as boundary conditions, the transport of these factors through the concrete, and the chemistry of the binder used to create the concrete. The increased variety of cements available in the marketplace and increasing demands from environmental factors have combined to increase the variability of corrosion in reinforced concrete structures, requiring a new methodology to measure corrosion to estimate the service life of new and existing structures.

Description

Figure 1: Direct and indirect corrosion costs worldwide and in the United States. Data sourced from [1-3]. Credit: NIST.

Credit: NIST

Objective
Develop new measurement standards to assess the corrosion of steel embedded in new and existing infrastructural assets.

Technical Idea
The Assessing Steel Risk in Innovative Cement Concretes project will combine characterization of concrete pore solution chemistry, electrochemical impedance spectroscopy, and models of species transport to build high-fidelity modeling tools to assess the risk and progression of steel corrosion in concretes made with innovative cement binders.

Innovative concrete materials, such as portland limestone cements, are increasingly used to create reinforced concrete infrastructure. Although corrosion in reinforcement steel has been studied for decades, the research has focused (almost entirely) on the specifics of sodium chloride-induced corrosion in ordinary portland cement concrete. The present set of heuristics (diffusion coefficient, depth of cover, chloride per mass of cement) that guide current building codes, such as American Concrete Institute (ACI) ACI CODE-318-19(22), has been adequate for structures comprised of portland cement-based concrete. New binder chemistries are being introduced to the construction market. These innovative concrete materials have increased the complexity of corrosion in two ways: changing environmental exposure and changing concreting materials. New materials have a lower pH than OPC concretes with chemistries that may alter the initiation and progression of steel corrosion. Both factors suggest that existing heuristics will be inadequate to assess susceptibility to corrosion accurately. As a result, a more in-depth study of corrosion mechanisms and moisture transport in concrete is needed.

This project will investigate the corrosion mechanism of steel in concrete by combining measurements of the concrete pore solution chemistry with electrochemical impedance spectroscopy. These results will be used to develop models of the corrosion rate that incorporate environmental factors relevant to the corrosion mechanism of interest as input. The output of this model can inform structural engineers designing new structures about the hazard associated with corrosion. It will inform the infrastructure stakeholders about the remaining service life of a structure. Developing this model requires a new approach to measuring corrosion and assessing the durability of concrete materials. Developing these measurements for modeling will enable new standards for durability assessment that can be incorporated into existing building codes.

Research Plan
The plan is divided between developing new metrology for corrosion in innovative concrete, developing plans for characterizing the chemistry of innovative concrete binders, and developing models to assess the service life of new and existing structures. This project will work toward these goals through three thrusts: metrology for corrosion of steel in concrete, implementing knowledge gained by developing forward-looking building standards, and engaging with stakeholders to implement recommendations and changes to building codes and standards.

Metrology for Corrosion of Steel in Concrete
The metrology for corrosion of concrete steel reinforcement would comprise activities for developing new measurement standards needed to provide inputs to numerical models and assess existing structures. These activities would include:

Laboratory studies of corrosion in low-cement binder concreting materials. This work would cover concrete mixes using supplementary cementitious materials and many concrete mixes made from binders other than OPC. The objective is to identify the physico-chemical properties of the concrete that lead to the initiation of corrosion in concrete materials.

Laboratory studies of moisture transport and storage in concretes. The objective would be identifying the relevant transport mechanisms to develop a model. Subsequent efforts would focus on validating the model.

Laboratory studies of the mechanical properties of concrete. The objective would be to identify and measure the relevant material properties that control fracture initiation and propagation in concrete to develop a crack propagation model resulting from the expansive pressures generated by corrosion.

Laboratory studies, modeling, and assessment activities require measurements with known precision and accuracy. In many cases, the measurand of interest is not directly observable and must be inferred from a calibration. Developing new, more accurate standardized test methods will be required to assess the remaining service life of the structure and provide accurate inputs to numerical models. Throughout this research plan, deficiencies in existing test methods will be identified. Assessing existing structures and providing inputs to numerical models constitutes a new objective that may require modification to existing standards to achieve. When this is the case, new test methods will be developed with these new objectives in mind, with the intention of developing standard test methods.

The data collected in laboratory studies will be used to develop a computer model for steel corrosion in concrete that incorporates the effect of chloride transport and moisture transport. The model would accept boundary conditions assessed from the structure’s environment and initial conditions from the concrete materials and predict the onset of corrosion and development stress at the steel-concrete interface and the onset of cracking, but could/should also include models that either predict or incorporate field testing (e.g., corrosion current, corrosion potential)

Forward-Looking Building Standards
A challenge for forward-looking building codes is to incorporate the effects of corrosion at the steel-concrete interface, which occur at small length scales, into a complete structural analysis of the structure. One approach to this challenge is through the use of fragility curves. Fragility curves describe the probability of a structure's failure (by some definition) given the (probabilistic) intensity of applied loads. These probability curves are developed using a combination of component-level structural testing and finite element modeling of full-scale structures using a probabilistic model of the applied loads. Fragility curves are developed and applied, assuming the structure's properties remain constant. Environmental exposure can result in the degradation of the materials and components, weakening the structure and increasing the likelihood of failure in a given loading scenario. There is a need to improve the development of fragility curves to incorporate the effects of environmental aging on reinforced concrete structures.

Developing a durability model can connect the corrosion phenomenon to building-scale performance. Laboratory studies of steel corrosion in concrete will be coupled with models of the transport of environmental species through concrete to predict the level and morphology of corrosion at the steel-concrete interface and damage to the concrete resulting from the pressure exerted by corrosion. The goal would be a high-fidelity model that captures the complex physicochemical mechanisms of corrosion at the steel-concrete interface and can be used to develop accelerated corrosion tests of reinforced concrete components. The results of these structural tests could be used to create full-scale models of structural systems that could be used to develop fragility curves.

Stakeholder Engagement
This work aims to develop standards to assess service life and develop test methods to conduct condition assessments of existing structures. This thrust will focus on engaging stakeholders to learn which approaches to service life and durability modeling might be most beneficial to the community. We will engage with the community early on in this project through a workshop. This project will seek to understand how stakeholders might use service life models, identify material-based metrology gaps, and identify gaps in knowledge needed to provide a suitable metric of the structure’s service life. The primary method of stakeholder engagement will be through a workshop held by NIST and the American Society of Civil Engineers (ASCE). The output of this workshop will be a white paper highlighting the gaps in metrology needed for accurate service life predictions and a roadmap to implementing NIST research into standards documents.

Three documents describe approaches to service life modeling used in practice. They are American Concrete Institute (ACI) CODE-365 Performing a Service Life Evaluation - Code and Commentary, fib Bulletin 34 Model Code for Service Life Design, and Unified Facilities Guide Specifications Section 03 31 20 Marine Concrete with Service Life Modeling. These code documents describe approaches to assessing the service life of structures affected by Cl transport and subsequent corrosion of embedded steel. This project will engage members of ACI 365 to improve code language for service life evaluation.

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References

[1] Angst, U. M. (2018). Challenges and opportunities in the corrosion of steel in concrete. Materials and Structures, 51(1), 4.

[2] Koch, et al. (2016). International Measures of Prevention, Application, and Economics of Corrosion Technology. NACE International, Texas, US.

[3] Koch, et al. (2002). Corrosion cost and preventive strategies in the United States (No. FHWA-RD-01-156,, R315-01). US. Federal Highway Administration.