Subject matter

Nowadays, structural concretes are supposed to fulfil the required performance during relatively long service lives, around 50 years for buildings and more than 100 years for large-scale infrastructures such as sea-link projects. The knowledge on the long-term durability of structural concretes is crucial for the design and management of these infrastructures. One important source of these knowledges comes from the long-term exposure and observations of concrete durability in filed conditions, compared to the theoretical insight and the short-term laboratory tests. These data provide realistic behaviours of structural concretes under field exposure conditions, which can support the design and assessment of long-term durability of concrete infrastructures, through qualitative comparison or quantitative modelling. That is reason that the engineering community since long-time valued the return of experiences from the in-field exposure. Actually, the long-term exposure data constitute a large part of body of knowledge on the long-term behaviours of engineering materials including structural concretes.
The systematic tests of structural concretes in marine exposure conditions started as early as 1930’s, e.g. the Treat Island, US (operated by Waterways Experiment Station), and served to investigate the long-term durability of concretes in terms of the material compositions, the cracking extent (width), the mechanical states, and, most recently, the protection measures. Nowadays these field exposure stations have been set up on worldwide scale, e.g. Träslövsläge field exposure site (BMB project, Sweden), Simonstown and Granger Bay (South Africa), Punta Matamoros (Cayo Santa María, Cuba) and Zhanjiang Marine Exposure Station (China, operated by CCCC Fourth Harbor Engineering Institute, Co., Ltd.). The chloride ingress has been the central issue of the study on the long-term exposure data, the profiling method was established to deduce the apparent chloride diffusivity, together with the corresponding surface chloride content, using the analytical solution of the Fick’s law. Following this line, the most important finding is possibly the ageing effect of the apparent chloride diffusivity with time, which is reported in DuraCrete reports (1998-2001) and integrated into the fib Model Code (2006). And this model has been widely used in various design works of concrete durability. While most in field exposure stations serves as a general support for scientific research, recent engineering practice adopts the long-term exposure station, constructed on the project site along with the new infrastructure, as an information source of durability monitoring and updating to help the life cycle maintenance and management of concrete structures. In the past 20 years, most large-scale marine concrete infrastructures in China have integrated the exposure station on the project site into the maintenance planning.
Albeit these achievements, the exploitation of long-term exposure data is far from satisfactory, and the added value of these data is not yet fully generated. The first concern is that the data collection from exposure stations is rather intuitive, and a systematic format for data collection/presentation is missed. Since the setup and operation of exposure stations are both time-labour consuming and budget demanding, the standardized format of data presentation will greatly increase the (re-)usage of these exposure data and more valuable for research community. Second, it is more and more clear that the interpretation of exposure data through apparent chloride diffusivity is not enough, and the research community, with deepened knowledge on the coupling between the chlorides ingress and other processes, is ready to investigate more engineering indicators through more elaborated modelling. Lastly, a technical guideline is expected for setup and operation of exposure stations, including the exposure conditions planning, data collection, data interpretation, and the application to the service life design and management. The technical committee aims at these lacks on the study of long-term exposure data, and proposes correspondingly three works phases to address the above issues, treating respectively the correct presentation, the rational interpretation and the efficient application of long-term exposure data.

Terms of reference

- TC life is estimated as 4-5 years.
- TC members: the members have extensive geographic representation covering Asia, Europe, America and Africa, and the members come from both academic community (researchers and students) and industrial sectors (producers and contractors). Along the TC works, more new members from Chinese side, especially the industrial sectors, are expected. The total number of members is expected to be within 20 persons.
- The proposed TC works include three parts: (1) the gathering of long-term exposure data from in-field stations accessible to the TC members, distributed in four continents (Africa, Asia, America and Europe). This part of work relies on the TC member collaboration on the data-sharing level, and a detailed data-sharing policy will be set up within the group (among the academic members and industrial members); (2) the research part, exploiting the long-term data using deepened coupling models and the involved experimental works, is done mainly by the academic members and experts on the subject, in collaboration with the exposure sites, and some robin tests are expected for the similitude study for exposure-exposure and exposure-laboratory; (3) the general technical guideline for exposure stations is to be jointly finished by academic and industrial members.
- One of the direct results (deliverable) of this TC is the general technical guideline for the setup and operation of marine exposure stations. This guideline would greatly interest the operators of existed exposure stations, who are motivated to upgrade their facilities to obtain higher efficiency, and also the infrastructure owners, who rely on the performance of exposure stations to support the life cycle management and maintenance.

Detailed working programme

The TC work is preliminary divided into three phases (work pakages, WP): (1) the establishment of long-term exposure database for structural concrete exposed to natural marine environments, and the correct presentation of long-term exposure data; (2) the exploitation of the long-term exposure and observation data via mechanism interpretation, and the rational indicators for engineering use; (3) the application of long-term exposure/observation data and their exploitation to the service life design and management of concrete infrastructures. These three work packages are detailed in the following.

[WP-1] Long-term exposure database and the correct presentation. On the basis of the long-term exposure (field) stations accessible to the TC members, the first aim of this work phase is to establish a long-term exposure and observation database of structural concretes. Since the TC members have extensive geographic coverage, this database is supposed to bear global sense compared to the databases of same type. As the first step of this operation, the focus is put on the natural marine exposure conditions, and both the long-term data from exposed concrete specimens and extracted cores are involved. If possible, the collection of long-term data covers also the concretes with protection measures (silane impregnation and coatings), and concretes with cracks. Then, these data will be arranged following a standard procedure, including the original concrete proportioning, concrete raw materials, exposure conditions (thermal, moisture and aggressive agents), and the deterioration performance (penetration depths, profiles, visual state, etc.), from which a normalized format of exposure data is expected. The objective of this phase, apart from the exposure database, is to propose a standard format for complete description of long-term exposure data, which serves as the basis for better understanding and interpretation of the long-term durability performance of structural concretes.

[WP-2] Rational interpretation of long-term exposure/observation data. So far the most widely used parameter to represent the long-term durability performance is the apparent chloride diffusivity, and several RILEM TCs have worked on this subject. This work phase differs from the apparent chloride diffusivity study and focuses on the correct mechanism interpretation of the long-term data. Actually, in exposure conditions, several fundamental processes exist simultaneously: the ingress of marine chlorides, the ingress of other aqueous ions (sulfates), the carbonation of surface layer (in air-exposed zones), the leaching (in water immersion/splashing zones) and/or frost actions on the concrete surface (tidal zones in cold climates). Using one apparent chloride diffusivity is much too simplistic to obtain meaningful results for engineering application. Accordingly, this phase of work starts with interpretation of chloride ingress under two typical scenarios of long-term exposure: combination of surface carbonation and chloride ingress into concrete (atmospheric exposure of specimens) and the combination of ingress of chlorides and other ions (sulfates) and leaching. Together with the collected exposure data, elaborated transport-reaction modeling is to be employed to quantify, under given environmental conditions, the respective contribution of different processes to the chloride ingress/penetration. On this basis, new engineering indicators are expected to be found, and new interpretations of the global “apparent chloride diffusivity” are expected. Afterwards, the relation (correlation) between the chloride ingress in exposure conditions and laboratory accelerated tests (RCM and immersion tests) is to be investigated, in order to solve the similitude between the two processes. The quantified similitude is important for the concrete durability in quality control phase using the results from accelerated tests as control parameters. Another similitude issue involves the behaviors of same concretes exposed at different sites: the understanding of the difference will help the prediction of concrete durability performance at one site different from the exposure site, which would greatly reduce the cost of multiple exposure stations. Moreover, the dispersion of long-term exposure among specimens is subject to specific study, quantifying the statistical characteristics related to the exposure conditions and the performance. Overall, the second work phase is to address better understanding and better interpretation of long-term data, via new engineering indicators in addition to the apparent chloride diffusivity.

[WP-3] Technical requirements for long-term field exposure stations and the application. So far, the long-term exposure/field stations have been set up considering mainly the exposure conditions, and the arrangement of specimens, the interpretation afterwards and the implementation of long-term exposure data into the service life design and management of concrete infrastructures are often user-tailored, dependent on the technical resources, and far from systematic. This work phase, on the basis of the foregoing phases on the correct representation of long-term exposure data and the rational interpretation of these data, proposes technical requirements of long-term field exposure stations in views of more efficient and sustainable exploitation of long-term exposure data. These requirements reflect the state-of-the-art knowledge accumulated on the subject, and serves as the technical guideline for new exposure stations construction and upgrading of existed stations. Secondly, this phase will continue on the rational application of long-term data to the service life design and management of concrete infrastructures, which is split into two topics: how to help the durability quality control in new constructions, and how to help the durability performance updating (maintenance planning) during the service life phase. The former is to use the results from the similitude study between the long-term durability performance under exposure conditions and the latter will draw from the recent practice on several larger-scale sea-link project using updating techniques of durability processes from long-term exposure data. Overall, this phase of work will provide the basic technical requirements of long-term exposure stations, and the guideline for the integration of long-term exposure data into the service life design and management of concrete infrastructures.