Subject matter

Material: Reinforced concrete  

Structures: Performance of steel reinforced concrete structures exposed to various exposure conditions   

Phenomena or specific aspect: Moisture transport, concrete carbonation, chloride ingress, steel corrosion, durability

Level of investigation:  Literature review, general survey, theoretical study, experimental investigation

Background:

The durability of cementitious materials is closely related to their moisture conditions. Many degradation processes, including steel corrosion, frost attack, ASR, carbonation, and sulphate attack, need sufficient water to generate damage. Liquid water movement may carry aggressive agents, such as chloride and sulphate ions, into concrete. The penetration of chlorides may lead to loss of the passive layer of steel reinforcing bars and thus leads to corrosion of rebars. Carbon dioxide is able to diffuse into concrete and cause concrete carbonation if concrete is unsaturated with water. With the decrease of pH in pore solution resulting from carbonation, steel corrosion may occur.

After corrosion initiation, however, the corrosion rate is affected by many factors, such as porosity, cement type, exposure condition, and water content. Empirical experience from practice has long shown that negligible corrosion damage occurs in structures, even upon carbonation of the cover depth, in exposure conditions with limited wetting of the concrete surface[1]. In agreement with this empirical knowledge, various scientific studies found that the key parameter controlling the corrosion rate of steel in carbonated concrete is the water content at the steel surface [2]. Recent results measured by neutron imaging showed that only when water front reached the steel surface, the measured corrosion rate became significant [3]. In other words, carbonation does not necessarily lead to steel corrosion; only with the “right” water condition at the steel surface, carbonation-induced corrosion can become nonnegligible and cause damage to the structure.

This has major implications for ensuring durability while reducing the environmental footprint of the cement industry. For modern binders with low content of clinker, even though they can be carbonated faster than the conventional Portland cements, by controlling the “right” water condition at the steel surface, the risk of steel corrosion can be kept low. The consequence of this, however, is that the moisture ingress through the (carbonated) concrete cover becomes a highly important mechanism to ensure corrosion resistant structures. Methods to measure and model this process at the engineering level are thus needed.

For these reasons, accurately determining moisture conditions in the concrete is crucial for studying the durability of reinforced concrete structures. The initial source of water in concrete is from the material preparation. During the service of concrete structures, the change of relative humidity in the surrounding environment induces transport and redistribution of moisture inside concrete. Therefore, a moisture transport model is useful to simulate and determine moisture condition within concrete. Before using models for prediction, they need to be calibrated and validated by reliable experimental data. Moisture transport occurs in many complex and interrelated mechanisms. The commonly known transport mechanisms are diffusion (ordinary, Knudsen, surface, etc), capillary transport (permeation), film flow, adsorption/desorption, evaporation and condensation. The early studies simply took Darcy’s law or diffusion equations (Fick’s law) to simulate the unsaturated moisture transport and later on the Richards' equation became a general one as capillary transport and moisture diffusion can be merged in one transport equation. In the literature, some models try to consider as many transport mechanisms as possible (such as Knudsen and surface diffusion and film flow), while they generally introduced new undermined parameters or assumptions for the additional transport mechanisms. The complexity of these models becomes a major obstacle to applying them in engineering practice. In many cases, the needed input parameters are not available. Moreover, engineers may not have the expert knowledge required to handle sophisticated scientific moisture transport models.

A prevailing key question is thus the identification of moisture transport models that are able to capture the key features of moisture transport and moisture state that are directly related to steel corrosion, while keeping the degree of complexity such that needed input parameters can be measured or otherwise made available in engineering practice.   

Models require experimental data for calibration and validation. The complex models need more experimental data and thus become less feasible than the early simple models. Different models require different experimental data for calibration and validation. The previous TC 248-MMB concluded “the best method/methods (of measuring moisture) must be selected on the basis of the material, the purpose of the analysis and the resolution required”[4]. Therefore, for the purpose of investigating the risk of steel corrosion, the method of measuring moisture state can only be determined after the modelling approaches are confirmed as they must fulfil the requirement of modelling approaches to capture the key features of moisture transport and moisture state.

It has been found that moisture transport in cementitious materials is very different from other porous building materials; that is known as anomalous moisture transport. Moisture transport in cementitious materials can be divided into two stages, a fast and a slow stage. The fast stage is governed by capillary transport and thus may be well simulated by the traditional moisture transport models, while the slow one is beyond the ability of these models and generally happens after a certain duration of the fast stage. Considering the long-term properties and durability of concrete, the slow moisture transport stage becomes non-negligible. The commonly reported causes of the slow stage are the slow down effect of moisture transport by the small pores, the alteration of microstructure due to the change of moisture content, the redistribution of air trapped in the material, etc. Moisture transport models, including both transport stages, have been proposed in the literature [5,6]. Nevertheless, because of the lack of adequate understanding of these causes, the effect of the slow stage on concrete durability deserves more attention and further validations are still needed for these models.

Regarding steel corrosion in reinforced concrete, water transport and moisture condition at the steel-concrete interface (SCI), where corrosion initiates and corrosion products primarily precipitate, must be fully known. However, the microstructure and moisture properties of the SCI are very different from the bulk concrete [7]. Eventually, only water at the SCI will directly influence corrosion initiation and corrosion rate. Nevertheless, moisture transport in the SCI has received little attention compared with moisture transport in the concrete cover. Further considering the fact that the microstructure of cement-based materials is very sensitive to the moisture condition [8], we have new challenges to use the conventional theories and models to explain and simulate moisture transport at the SCI. Therefore, it is extremely important to re-evaluate theories and models for analysing how moisture transport and moisture condition in cementitious materials can affect the durability of concrete structures.

Based on the above review, it is clear that the study of moisture state in concrete can help predict the service life of reinforced concrete structures and would potentially promote the use of low clinker content binders. However, key questions regarding modelling approaches, experimental methods,  the long-term moisture transport, and moisture state at the SCI, still need to be fully understood and answered. The proposed TC is devoted to identifying, understanding, explaining, and solving the above mentioned problems and challenges. By doing these, the TC will provide recommendations, at the level of engineering practical, on choosing moisture transport model, selecting reliable validation experimental data and methods to obtain these data. The outcomes of this TC have a great interest to both academic research and engineering applications. 

Aims and objectives:

1. Review moisture transport models in terms of transport mechanisms and their application conditions, in particular for investigating the risk of steel corrosion (such as in carbonated, low CO2 emission concretes). Identify the key features that require experimental calibration.
2. Summarize and compare experimental methods for obtaining the calibration and validation data, including the above-mentioned anomalous moisture transport; additional tests may be needed if the required data are not available.
3. Compare simulation and prediction models based on the accuracy of validation results.
4. Review and compare simulation and experimental methods to determine moisture condition at the SCI.
5. Model suggestions for engineering practice (simplified) and academic research.

References:

[1]         U. Angst, F. Moro, M. Geiker, S. Kessler, H. Beushausen, C. Andrade, J. Lahdensivu, A. Köliö, K.I. Imamoto, S. von Greve-Dierfeld, M. Serdar, Corrosion of steel in carbonated concrete: Mechanisms, practical experience, and research priorities – A critical review by RILEM TC 281-CCC, RILEM Tech. Lett. 5 (2020) 85–100. https://doi.org/10.21809/rilemtechlett.2020.127.

[2]         M. Stefanoni, U. Angst, B. Elsener, Corrosion rate of carbon steel in carbonated concrete – A critical review, Cem. Concr. Res. 103 (2018) 35–48. https://doi.org/10.1016/j.cemconres.2017.10.007.

[3]         Z. Zhang, P. Trtik, F. Ren, T. Schmid, C. Dreimol, U. Angst, Dynamic effect of water penetration into carbonated mortar on steel corrosion: a neutron imaging, electrochemical and modeling study, Cement. Submitted (2022) 100043. https://doi.org/10.1016/J.CEMENT.2022.100043.

[4]         L.O. Nilsson, Methods of measuring moisture in building materials and structures: State-of-the-art report of the rilem technical committee 248-mmb, in: RILEM State-of-the-Art Reports, 2018: pp. 1–280. https://doi.org/10.1007/978-3-319-74231-1.

[5]         Z. Zhang, U. Angst, A Dual-Permeability Approach to Study Anomalous Moisture Transport Properties of Cement-Based Materials, Transp. Porous Media. 135 (2020) 59–78. https://doi.org/10.1007/s11242-020-01469-y.

[6]         F. Ren, C. Zhou, Q. Zeng, Z. Zhang, U. Angst, W. Wang, Quantifying the anomalous water absorption behavior of cement mortar in view of its physical sensitivity to water, Cem. Concr. Res. 143 (2021) 106395. https://doi.org/10.1016/j.cemconres.2021.106395.

[7]         U.M. Angst, M.R. Geiker, A. Michel, C. Gehlen, H. Wong, O.B. Isgor, B. Elsener, C.M. Hansson, R. François, K. Hornbostel, R. Polder, M.C. Alonso, M. Sanchez, M.J.J. Correia, M. Criado, A. Sagues, N. Buenfeld, A. Sagüés, N. Buenfeld, A. Sagues, N. Buenfeld, A. Sagüés, N. Buenfeld, The steel--concrete interface, Mater. Struct. 50 (2017) 143. https://doi.org/10.1617/s11527-017-1010-1.

[8]         C. Zhou, X. Zhang, Z. Wang, Z. Yang, Water sensitivity of cement-based materials, J. Am. Ceram. Soc. 104 (2021) 4279–4296. https://doi.org/10.1111/JACE.17918.

Terms of reference

The estimated duration of the TC work is 4 years. The start of the TC activities is possible from spring 2023 onwards. Round Robin tests are not intended to be part of the TC. However, depending on the work progress and agreement of TC members in the course of the work, round Robin tests may be anticipated. We notice that the durations of unsaturated moisture transport experiments are not short, so we expect that the TC will take longer than expected if the round Robin tests will be required.

Members will be recruited from the RILEM community and beyond, both from academia and industry (civil engineering), based on their experience in moisture transport and durability of concrete structures. Young RILEM members currently working on these topics are welcome. The TC is expected to attract researchers and engineers world widely to promote an intercontinental collaboration, including Europe, Australia, Asia, and the Americas.

The data collection work will take much time in this TC. Data in the scientific literature are mainly from laboratory experiments. We are interested in data from field survey and on-site measurements on real concrete structures, which are important supplementary to the laboratory experiments. However, these data are generally included in project and institutional reports or other formats, not included in databases of the major publishers, which thus need much time to mine. Therefore, a TC with members from different continents and countries will be a great help for data collection. 

Detailed working programme

Based on the aims and objectives, the TC will be divided into the following work packages (WP):

• WP1: Review of moisture transport mechanisms and prediction models. (years 1, 2, 3, and 4)

The aim of this WP is to find appropriate moisture transport models for cement-based materials. Available models in the literature will be collected and reviewed in terms of their application conditions, such as low RH, high RH, drying-wetting conditions, and both carbonated and non-carbonated concrete. These models will be compared in respect to the key features that are important for investigating the risk of steel corrosion. For instance, for corrosion the relative humidity or degree of saturation are of secondary importance, while the amount and distribution of water at the steel-concrete interface may be more important.

• WP2: Experimental methods and data evaluation (years 2, 3 and 4)

The aim is to compile a data base about data on moisture states and related exposure conditions as well as concrete properties. This data base will be made accessible to the research community for the purpose of validation and calibration of current and future moisture models. Ideally the database should cover cases from the “application conditions” mentioned in WP 1. Members in this WP will focus on collecting data from scientific publications, project and institutional reports, and other sources based on the moisture transport models and their key features concluded in WP1. Data are expected from both laboratories and field, including detailed information such as concrete properties, building history, environmental conditions (RH, temperature), etc. Meanwhile, the experimental methods to obtain these data will be reviewed and compared in terms of their practicality. If the desired data are not available based on the evaluation results of experimental methods, round Robin tests will be carried out to benchmark different techniques of measuring moisture transport and moisture content. This WP will be carried out following the pace of WP1.

• WP3: Anomalous moisture transport: models and validation (years 2, 3 and 4)

Models that can consider the anomalous moisture transport will be evaluated, depending on whether enough data will be collected. Especially, WP3 should answer the questions, such as how anomalous moisture transport will affect the long-term durability of concrete structures. This WP will run parallelly with WP1 and WP2.

• WP4: Moisture transport at SCI: models and validation data (years 2, 3 and 4)

This WP firstly needs to identify the differences between the concrete cover and the SCI, in terms of microstructure, chemical composition, and moisture transport properties. A key task of WP4 is to review the existing experimental data and methods to measure and determine moisture conditions at the SCI and eventually to propose the most reliable experimental methods. This WP needs results from WP1 and WP2 as support.