Subject matter

Materials: corrugated steel (rebars) and high strength steel (cold drawn steels). Carbon Steel and Stainless Steel.

Structures: reinforced and prestressed structures (tendons).

Phenomena: corrosion; stress corrosion cracking; hydrogen embrittlement; fracture mechanics; durability; service life.

Level of investigation: literature survey and theoretical study.

Approach: the TC will take into account the effect of the media, the material properties, like hydrogen diffusion and trapping, and the mechanical properties, like the fracture toughness. This TC is related to:

• 046-FST: Fatigue of steel temperature effects
• 055-CCA: Corrosion of galvanized steel tap-water piping
• 060-CSC: Corrosion of steel in concrete
• 080-TMS: Tension modulus of prestressing steel stands
• 083-CUS: Fundamental mechanical properties of metals
• 128-MRC: Materials requirements for repair of corrosion damaged reinforced concrete structures
• 141-SSM: Stainless steel as building material
• 147-FMB: Fracture mechanics applications to anchorage and bond, chaired by Lennart ELFGREN
• 154-EMC: Electrochemical techniques for measuring metallic corrosion, chaired by Carmen ANDRADE
• 178-TMC: Testing and modelling chloride penetration in concrete, chaired by Carmen ANDRADE

INTRODUCTION

It is a well-known fact that corrosion is the main cause of structure degradation. A particular case of corrosion of the steel embedded in concrete is the Environment Assisted Cracking (EAC), which can appear in prestressed structures. The Environment Assisted Cracking takes into account two main phenomena: i) Stress Corrosion Cracking (SCC), and ii) Hydrogen Embrittlement (HE), that can appear together and reduces substantially the structure safety. Stress corrosion cracking is usually defined as cracking owing to a process involving joint corrosion and straining of a metal because of residual or applied stress (Galvele 1987; Galvele 1993; Newman 2008; Sanchez et al. 2007a; Sanchez et al. 2007b; Turnbull 2001). The stresses required are tensile and essentially static. So, stress corrosion cracking is driven by the local environment, mechanics, and materials characteristics, and the chemical, electrochemical, and transport processes. From a scientific point of view, the different mechanisms involved on Stress Corrosion Cracking have not been satisfactorily explained. Numerous mechanisms have been proposed to understand the brittle failure of metals under stress, but only some of them, four specially, are considered to be relevant: i) Mechanism of Anodic Dissolution (Parkins 1980; Parkins et al. 1982), ii) Film-induced Cleavage Model (Sieradzki and Newman 1987), iii) Surface Mobility SCC Mechanism (Galvele 1986; Galvele 1987; Galvele 1993), iv) Environmentally Enhanced Plasticity (Magnin 1996; Magnin et al. 1996). Coherent with the lack of agreement in the type of mechanism that operates, there is no general agreement on testing methods for the study of the Stress Corrosion Cracking. On the other hand, Hydrogen Embrittlement is believed to be one of the main reasons for cracking of steel structures under stress (Elices et al. 2008; Nürnberger 2002; Oriani 1972; Oriani and Josephic 1977). Hydrogen Embrittlement can occur even during the Stress Corrosion Cracking process. Hydrogen is generated on the steel surface in the cathodic corrosion reaction and is incorporated to the iron lattice. High strength steels often include a ferritic core made of alpha-iron (body-centered-cubic lattice, bcc). To control and prevent the cracking of steel it is necessary to understand the chemical and physical properties of hydrogen inside bcc iron. There are some experimental studies about the hydrogen transport, diffusivity and trapping (Enos and Scully 2002; Li et al. 2004; Thomas et al. 2002; Thomas et al. 2003) which define the crack propagation rate according to electrochemical and mechanical conditions. From the theoretical point of view, there are several studies at different scales: abinitio calculations (Castedo et al. 2011; Sanchez et al. 2013; Sanchez et al. 2008; Sanchez et al. 2010), molecular dynamics calculations (Song and Curtin 2011; Song and Curtin 2013; Song and Curtin 2014) and finite element calculations (Caspersen et al. 2004; Lew et al. 2006; Serebrinsky et al. 2004).

AIMS AND SCOPE

The main objective is to produce a state of the art report on the following aspects of stress corrosion cracking of concrete-reinforcing steels:

i. Stress Corrosion Cracking mechanisms of construction steel.

ii. Hydrogen Embrittlement mechanisms applied to construction steel.

iii. Existing methods to study the Stress Corrosion Cracking of construction steel.

iv. Existing methods to study the Hydrogen Embrittlement of construction steel.

v. Durability design and failure analysis of concrete-reinforcing steels.

Terms of reference

TIME NECESSARY AND PROVISIONAL TIME SCHEDULE

The estimated duration of the TC work is 4 years. The start of the TC activities is possible from September 2015 onwards. More details on the time schedule and working programme are given in “Methodology” below and in section 4.

MEMBERSHIP

The proposed TC will bring together experts from the field of durability, corrosion, engineers, mechanical properties, fracture mechanics and transport processes. Members will be recruited from academia and from the industry (consultant engineers, testing institutes like OCAS, ACERINOX, SHELL, etc.).

METHODOLOGY

The committee will prepare a state of the art report (STAR). The STAR will be based on both documented results from the literature and theoretical, conceptual reasoning within the committee. In addition to bibliographical research, the committee work will consist in exchange of experience and results, including unpublished results. The STAR will include the main SCC and HE tests applied to high strength steels. At the same time, it will show the main theories and tools to estimate the durability and mechanical behaviour for concrete reinforcement. In addition to common committee meetings (at least one per year), it is planned to organize two workshops with invited presentations on selected topics in order to stimulate and focus the discussion.

Detailed working programme

The main milestones are:

• Month 1: The TC work will be started with a 1st workshop. It will propose to create two task/groups to address the state of the art for the SCC and HE.
• Month 12: State of the art of SCC and HE of high strength construction steels: main mechanisms and estimation of crack propagation rate.
• Month 24: Review of methodology and results of SCC and HE applied to high strength construction steels.
• Month 36: Compare experimental results and theoretical calculations based in previous mechanisms.
• Month 48: Final report and workshop/meeting.