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Several inspection methods can be used to assess the corrosion state of steel reinforcement in concrete. Especially for periodical field surveys and monitoring, non-destructive testing (NDT) methods are to be preferred as they do not cause any or very limited damage to the existing concrete. In this paper, the corrosion state of three reinforced concrete beams exposed to marine environment for 25 years was evaluated by measuring three parameters; electrochemical potential, concrete resistivity and corrosion rate. The measurements were performed with commercial devices. It was found that all devices are applicable for field inspections. Among the methods selected for the study, the electrochemical potential measured in a fine grid and analysed statistically offered the best possibility of evaluating the corrosion state; preferably in combination with selected excavations for determination of the level of corrosion. © Article authors. This is an open access article distributed under the Creative Commons Attribution- NonCommercial-NoDerivs licens. (http://creaticecommons.org/licenses/by.nc-nd/3.0/).

The research presented in this paper briefly summarises findings from a project on impact of cracks on reinforcement corrosion within the Norwegian Public Roads Administration’s R&D program “Ferry-free coastal route E39”. A main part of the project was the collection of longterm field data on the influence of cracks on chloride ingress and reinforcement corrosion. Published by Norsk Betongforening.

Cracks facilitate ingress of aggressive substances. Depending on the exposure, this might lead to early reinforcement corrosion initiation. Possible self-healing of cracks may reduce their long-term impact, and conditions for self-healing is therefore of interest for designers and owners. The present study covers self-healing in a marine exposed reinforced concrete column, which for parts of the exposure period was dynamically loaded and cathodically protected by sacrificial anodes. The results might suggest temporary use of sacrificial anodes for cathodic protection of steel reinforcement in cracked concrete in marine tidal and submerged exposure to limit early rapid chloride ingress and facilitate early self-healing by formation of magnesium and calcium precipitates in cracks. Norwegian Public Roads Administration (NPRA), DNV GL

This paper reports results from experiments aimed at better understanding the influence of fibre dosage and fibre geometry on the AC frequency needed to determine the DC resistivity of cementitious materials containing steel fibres. Impedance spectroscopy and DC galvanodynamic measurements were performed on mortar prisms with varying fibre reinforcement to determine the matrix resistivity (related to ionic current within the pore solution) and composite resistivity (accounting for both ionic current and electronic current through the fibres). The results showed that adding steel fibres did not significantly affect the DC nor the AC matrix resistivity of the mortar prisms. However, the steel fibres yielded a drastic reduction of the frequency associated to the AC matrix resistivity from ∼1 kHz in plain mortar to ∼1 Hz in steel fibre reinforced mortar. These findings revealed the need to adequately adjust the frequency in AC resistivity measurements of steel fibre reinforced cementitious materials.

Norwegian concrete structures are exposed to salt contamination from both sea water and de-icing salts. Chlorides penetrate the concrete cover and initiate corrosion of the reinforcement. The deterioration process can propagate at high rates and endanger serviceability and structural safety. Concrete resistivity is usually assumed to correlate closely with the corrosion process, and its measurement is in principle simple and costefficient, and widely recommended to support service life assessment. However, the extent to which concrete resistivity measurements can provide us with detailed information about the corrosion process needs critical examination. The main objective of this PhD project was to study the applicability of concrete resistivity measurement for predicting the rate of reinforcement corrosion induced by chlorides. A review of the literature led to the conclusion that a general correlation could be found between increasing concrete resistivity and decreasing corrosion rate, but a large scatter was observed within and between the studies reviewed. Various possible reasons for the discrepancies were identified: researchers used dissimilar procedures for determining both concrete resistivity and corrosion rate, and concrete composition was recognised as affecting the overall tendency. Interestingly, the literature contained very limited discussion on the mechanisms involved in providing the correlation assumed between concrete resistivity and corrosion rate. On the basis of the literature review, it seemed appropriate to investigate the relationship between concrete resistivity and corrosion rate further using specially designed laboratory experiments. In total, three experimental studies were conducted using mortar to investigate the subject. The first study focused on the limitations of measuring galvanic currents in macro-cells. It was confirmed that galvanic current measurements provide an indication of the actual corrosion current, and thereby rate, only if the actively corroding segment is sufficiently small. The second experimental study investigated the limitations of bulk resistivity measurements for predicting the ohmic resistance in a macro-cell between small anodes and a large cathode network. It was found that for small anodes the ohmic resistance between the anode and the cathode is not directly correlated with the bulk resistivity. Local inhomogeneities strongly influence the measurements. The ratio between bulk resistivity and the resistance between a specific anode and the cathode was not constant over time and varied in particular with changes in the moisture conditions. The third experimental study examined the influence of mortar resistivity on the ratelimiting steps of chloride-induced corrosion. It was shown that for small anodes the local conditions around the anode are more decisive for the anodic and ohmic partial processes than the bulk resistivity. The cathodic partial process was found to be practically independent of the mortar resistivity. For the setup and materials investigated, it was not possible to identify any one rate-limiting step as dominant. The controlling rate-limiting step varied for the different materials investigated. For mortar prepared with fly ash, and therefore possessing high mortar resistivity, the concept of anodic-resistance control suggested in the literature was found to fit well. Cathodic control was identified for mortars with a comparatively low bulk resistivity (prepared only with Portland cement). Furthermore, it was confirmed that the generally assumed inverse correlation between corrosion rate and concrete resistivity depends very much on the material composition. In conclusion, it was found that there can be no direct relationship between concrete resistivity and the corrosion rate. The dependency on material composition and in particular the fact that concrete resistivity is not directly correlated with the underlying partial processes that determine the corrosion rate must be considered as decisive on this question. It must therefore be assumed that the direct relationships between concrete resistivity and corrosion rate introduced in the literature, and any models and general threshold values based upon them, are not generally applicable.

The cement industry contributes about 8% of the anthropogenic CO2 emissions, partly due to large consumption of cement and the emissions associated with the production (for Portland cement (PC) approximately 800 kg/t). One approach to lower the CO2 footprint is to substitute part of the Portland clinker by supplementary cementitious materials (SMCs), e.g. fly ash blends (PCFA). Blended cements may present advantages compared to PC. However, a main drawback is a lower carbonation resistance. This disadvantage is known and taken into account in the standards. The objective of the PhD study was to improve the understanding of carbonation-induced corrosion in reinforced concrete structures containing fly ash blends. For the tested materials and exposure conditions, the carbonation depth measured using thymolphthalein compared to carbonation depth determined using optical microscopy, calcium hydroxide to calcium carbonate profiles determined using thermogravimetric analysis, and free alkali metals (Na, K) in the pore solution determined using cold water extraction. Based on this, the pH indicator was used in the present study for detection of carbonation. Thymolphthalein solution sprayed on a freshly split surface allows to determine the spatial variation of the carbonation depth. Based on investigations of the microstructure and solid phases, carbonation up to 5% CO2 appears representative for natural carbonation. An increased degree of carbonation and an increased content of sulphate in the pore solution were observed in samples carbonated at 100% CO2. However, the impact of the exposure condition was limited compared to the changes upon carbonation (from non-carbonated to carbonated condition). Corrosion initiation is reported in the literature before the carbonation front compares to the concrete cover. We found that corrosion did not start until the concrete (mortar)-steel interface was carbonated as identified using pH indicator thymolphthalein. A relationship between the carbonated fraction of the mortar-steel interface (as identified using thymolphthalein) and the open circuit potential was observed. Carbonation causes a decrease in the hydroxyl concentration, variations into the carbon content, a reduction in the carbonate-to-bicarbonate ratio, and a release of detrimental ions such as sulphates and chlorides in the pore solution. This strongly increases the probability of corrosion compared to non-carbonated pore solution. Microcell corrosion rates of reinforcement embedded in carbonated concrete were determined at different moisture conditions. The moisture content was found to have a large impact on the microcell corrosion current. High microcell current densities were observed for wet samples, whereas low microcell current densities were found over drying. The microcell current density was slightly higher in carbonated PCFA compared to PC. When reinforcement embedded in wet carbonated concrete was electrically connected to reinforcement in wet non-carbonated concrete, a macrocell corrosion rate of the same magnitude as the microcell corrosion rate was measured. The relative contribution of the partial processes in macrocell corrosion (cathodic polarization of passive steel, difference in potential between active and passive steel, and anodic polarization of active steel) depended on the cathode-to-anode ratio of the macrocouple. Overall PCFA presented lower carbonation resistance and the embedded reinforcement corroded slightly faster than in PC. However, it appears that the corrosion rate in carbonated PC and PCFA concrete is only of importance at high moisture content. Reinforcement in carbonated PCFA concretes kept constantly at 90% RH corroded at low rate. In summary: The total (microcell and macrocell) current density should be used for service life prediction of reinforced concrete structures exposed to CO2 and periods of wetness (i.e. XC4) The possible use of PCFA concrete exposed to CO2 and periods of wetness (i.e. XC4) should be carefully considered

Investigation of the debonding length at the concrete/steel interface have been investigated based on different types of cracks in concrete. Different methods for investigation have been used in the laboratory.

Moisture transport in concrete under a temperature gradient is a topic that has challenged researchers for at least some 60 years. No conclusions are still drawn on how to describe it theoretically and how to quantify it experimentally. In this paper some of the theoretical and experimental work done is summarized. A description of the flux of moisture by the relative humidity and the water vapour content as the two transport potentials is proposed. A comparison with laboratory and field data is given and the consequences in some applications are discussed.