Electrical resistivity measurements in steel fibre reinforced cementitious materials

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

The steel–concrete interface

Although the steel–concrete interface (SCI) is widely recognized to influence the durability of reinforced concrete, a systematic overview and detailed documentation of the various aspects of the SCI are lacking. In this paper, we compiled a comprehensive list of possible local characteristics at the SCI and reviewed available information regarding their properties as well as their occurrence in engineering structures and in the laboratory. Given the complexity of the SCI, we suggested a systematic approach to describe it in terms of local characteristics and their physical and chemical properties. It was found that the SCI exhibits significant spatial inhomogeneity along and around as well as perpendicular to the reinforcing steel. The SCI can differ strongly between different engineering structures and also between different members within a structure; particular differences are expected between structures built before and after the 1970/1980s. A single SCI representing all on-site conditions does not exist. Additionally, SCIs in common laboratory-made specimens exhibit significant differences compared to engineering structures. Thus, results from laboratory studies and from practical experience should be applied to engineering structures with caution. Finally, recommendations for further research are made

The role of concrete resistivity in chloride-induced macro-cell corrosion of reinforcement

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

Self-healing and Chloride Ingress in Cracked Cathodically Protected Concrete Exposed to Marine Environment for 33 Years

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 GLpublishedVersio

Prisons and imprisonment in Scotland

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

25 years of field exposure of pre-cracked concrete beams; combined impact of spacers and cracks on reinforcement corrosion

The state of corrosion of three pre-cracked concrete beams was investigated after 25 years of marine exposure (atmospheric, tidal and submerged). The influence of inadequate plastic spacers was found to overrule the possible impact of bending cracks on reinforcement corrosion. Increased chloride ingress was detected in cracked areas, but very little corrosion was observed where cracks reached the reinforcement. Also, high chloride content was measured in uncracked parts of the beams, but no or very limited corrosion was observed outside spacers and a few cracks. It is hypothesised that corrosion, initiated early at weak spots (here at plastic spacers) can protect the remaining reinforcement from corrosionpublishedVersio

Durability aspects of cracks in concrete: field observations

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

Durability aspects of cracks in concrete: field observations

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