Skip to main content

Passive layER FailuRe Mechanisms for Steel Embedded in Alkali-Activated Slag Materials

Periodic Reporting for period 1 - PERFoRM (Passive layER FailuRe Mechanisms for Steel Embedded in Alkali-Activated Slag Materials)

Reporting period: 2019-06-03 to 2021-06-02

Corrosion of steel reinforcement in concrete is a degradation mechanism representing a major worldwide infrastructure challenge. Globally, €2.2 trillion (~3.4% of the global GDP) is spent each year to prevent, mitigate, and repair civil infrastructure damage due to corrosion of steel reinforcement. This societal impact presents an urgent need to understand and monitor corrosion phenomena to improve resiliency against in-service environments. Additionally, with the advent of global climate change challenges, the decarbonization of the built environment with durable low-carbon cementitious materials is imperative. The primary aim of the Passive layER FailuRe Mechanisms (PERFoRM) project is to understand how in-service environmental conditions (e.g. CO2 and/or chloride) influence the stability and failure (i.e. corrosion) of the passive layer on steel rebar embedded within concretes based on innovative low-carbon cementitious materials (i.e. near-neutral salt activated slag materials). Investigations on the passive layer formation, stability, and breakdown are lacking due to the short timescales of their formation as well as the pore solution chemistry dependence on their physical and electrochemical properties. Thus, the PERFoRM project takes one first step to understand the processes affecting the formation, development, and breakdown of the passive layer in order to engineer reinforced low-carbon cementitious materials with improved corrosion durability.

The key objectives of PERFoRM are:
1. Developing novel durability wireless embedded (D-WirE) sensors for in-situ monitoring of concretes
2. Ground-breaking monitoring of chemical and physical changes induced by carbonation/chloride penetration in NnSAS
3. Advanced characterisation of passive layer growth, breakdown, and failure rates of steel rebars embedded in NnSAS
4. Creation of service-life models by incorporation of in-situ carbonation and chloride penetration regression models
Summary of Work Performed:
> Electrochemical characterization of time-dependent stability of passive films in alkaline simulated concrete pore solutions.
> Analysis of novel structural health monitoring and fiber-optic with Bragg grating sensor networks for physical and chemical data acquisition
> Data evaluation, analysis, and reporting of long-term electrochemical corrosion in reinforced partially-carbonated slag-metakaolin concrete materials specimens.
> Thermodynamic and electrolyte simulations of pore solutions of alkali and near-neutral salt activated slag materials to elucidate the passive layer formation, establishment, and maturity mechanisms underpinning their physical and electrochemical characteristics.
> Support the establishment and operation of UKCRIC National Centre for Infrastructure Materials micro-scale computerized tomography facility.
> Development of open-source mico-computerized tomography datasets of near-neutral salt-activated slag materials.
> Investigation on the pore structure networks (i.e. tortuosity, Cl diffusion) of sodium carbonate and sodium sulfate-activated slag materials.
> Written report on the use of wireless sensor technology and processing challenges for the future application of embedded sensors in cementitious materials.
> Contribution to the European Federation of Corrosion and RILEM TC CAM on chloride diffusion in alkali-activated materials.
> Organization of research colloquium co-sponsored by the University of Leeds Institute for Fluid Dynamics, Institute of Functional Surfaces, and the UK Collaboratorium for Research in Infrastructure and Cities.

Summary of main results and dissemination achieved:
> New understanding of the effect of the ionic conductivity of highly reductive pore solutions of slag-based AACs with novel relationships to steel metallurgical variables affecting the establishment, stability, and maturity of passive layers.
> The first reported long-term studies (335 days) on the CO2-induced corrosion performance of reinforced alkali-activated slag/metakaolin blended concretes.
> Successful production of long-timescale aged near-neutral salt activated slag materials (540 days) critical to inform service life models and performance-based materials designs.
> Development of a detailed set of research and operation procedures for the non-destructive research of novel cementitious materials utilizing micro-computerized tomography equipment.
> Dissemination of research in Science (Impact Factor = 41.845) as a perspective article on the role of sensors in the future of smart cities and cementitious materials.
> Development of micro-computerized tomographies of near-neutral salt-activated slag materials to investigate the mass transport and pore structure network of burgeoning low-CO2 cementitious materials.
Beyond the state-of-the-art, this project achieved detailed micro-computerized tomography databases of near-neutral salt-activated slag materials that have been made available for continued global research on these low-CO2 cementitious materials. These databases depict for the first time the tortuous and complex porous structure of NnSAS materials. Furthermore, long-timescale analysis of the corrosion durability in alkali-activated slag-metakaolin blended concrete materials has been investigated. This investigation provides important results on the long-term analysis, prediction, and estimation of corrosion rates. Another important output of this project was the development of ionic conductivity simulations utilizing advanced thermodynamic modeling of alkali- and salt-activated slag materials. This simulation tool was able to investigate the time-based relationship between the chemistry of the pore solution of alkali- and salt-activated slag materials and the passive layer formation mechanisms in embedded reinforcement. The results provided are important as the electrical conductivity of pore solutions describes the ionic composition of pore solutions and the mass transport possible for passivation reactions.

Overall, the databases and simulation tools developed in the project will provide societal impact by providing a pathway for the decarbonization of urban infrastructure with improved durability against corrosion damage. The results from the PERFoRM project achieve this by elucidating for the first time the relationship between the pore solution chemistry and the formation, establishment, and maturity processes of the passive layer in low-carbon cementitious materials. In-depth data analysis results from the project also benefit infrastructure practitioners as well as durability engineers with updated information (e.g. Tafel constants) to better predict the corrosion rates of embedded reinforcement in low-carbon cementitious materials (i.e. alkali-activated slag materials). Consequently, a complete understanding of passive layer formation mechanisms paired with improved information to predict corrosion rates in low-carbon cementitious materials is important to improve the durability of these materials and, hence, enable their in-service application. Thus, by doing so, the decarbonization of urban architecture can be achieved with improved resilience against corrosion-induced damage.
Micro-CT scan of working electrode (rebar) for embedding in NnSAS materials
Electrical conductivity of pore solutions of (A) NaOH- and (B) Na2SiO3-activated slag materials.
Corrosion potential (Ecorr) of carbon-steel as a function of time (hours).