On the importance of execution for obtaining the designed durability of reinforced concrete structures

The design and execution of reinforced concrete structures shall together with planned maintenance lead to the intended level of safety and serviceability during the entire service life of the structure. The possible impact of execution on the performance of the finished concrete structure is not always recognized; e.g., is pre‐testing of alternative mixes often undertaken at laboratory conditions. Mix design should include an assessment of the stability and property development of the concrete at the anticipated execution conditions as well as an evaluation of the robustness of the concrete to natural variations in mix composition and execution. Through selected examples the paper addresses the importance of execution for obtaining the designed performance and durability of reinforced concrete structures.     

Modelling reinforcement corrosion – usability of a factorial approach for modelling resistivity of concrete

As part of the macro cell corrosion process of reinforcement steel, the resistivity of concrete plays a crucial role. In order to investigate the influence parameters on the resistivity of concrete, the results of a discrete quantification were presented and implemented into a factorial approach for modelling corrosion propagation. First results delivered significant deviations from values obtained by measuring. Using Gauss' method of least mean squares provided a decrease in deviations. The hereby obtained deviations were lower than the scatter of the measuring results. A usability of the proposed factorial approach could therefore be approved.     

Covercrete with hybrid functions – A novel approach to durable reinforced concrete structures

Due to the corrosion of steel in reinforced concrete structures, the concrete with low water–cement ratio (w/c), high cement content, and large cover thickness is conventionally used for prolonging the passivation period of steel. Obviously, this conventional approach to durable concrete structures is at the sacrifice of more CO₂ emission and natural resources through consuming higher amount of cement and more constituent materials, which is against sustainability. By placing an economically affordable conductive mesh made of carbon fiber or conductive polymer fiber in the near surface zone of concrete acting as anode we can build up a cathodic prevention system with intermittent low current density supplied by, e.g., the solar cells. In such a way, the aggressive negative ions such as Cl^?, , and can be stopped near the cathodic (steel) zone. Thus the reinforcement steel is prevented from corrosion even in the concrete with relatively high w/c and small cover thickness. This conductive mesh functions not only as electrode, but also as surface reinforcement to prevent concrete surface from cracking. Therefore, this new type of covercrete has hybrid functions. This paper presents the theoretical analysis of feasibility of this approach and discusses the potential durability problems and possible solutions to the potential problems.     

Analysis of the parameters affecting probabilistic predictions of initiation time for carbonation‐induced corrosion of reinforced concrete structures

Probabilistic approaches are available for service life design of reinforced concrete structures subjected to reinforcement corrosion. The International Federation for Structural Concrete (fib) has proposed a model code where design equations and related parameters, involving materials properties, exposure conditions and construction details, are reported. This paper investigates the role of design parameters in the prediction of service life by applying the fib design procedure to existing structures suffering carbonation‐induced corrosion. Results of the modelling were compared to results of the inspection and a significant difference was observed. In order to investigate the reasons for this difference, the roles of concrete cover thickness and carbonation depth were considered separately in the modelling of service life. The concrete cover thickness had a significant effect on the output of the service life modelling, which however was not sufficient to explain the discrepancy between in situ and modelling results. The modelled values of carbonation depth were also affected by errors; sensitivity analysis showed that, although some parameters had higher influence than others, no single parameter had a predominant role.     

DFG‐research‐group: Modelling reinforcement corrosion – overview of the project

A strong interest for the durability of reinforced concrete structures currently exists in industry and research [1]. Against the background of immense costs for maintaining reinforced concrete structures and repairing damage caused by corroding reinforcement steel, this interest lead to a German joint research project. The aim of this network‐based (www.bam.de/dfg537.htm) research group is, to deliver the basic knowledge of the corrosion propagation and to make a probabilistic tool available for engineers so that a complete design for durability, concerning reinforcement corrosion, will be possible.     

An embedded multi‐parameter corrosion sensor for reinforced concrete structures

A prototype of an embedded corrosion sensor has been developed for assessing the corrosion status of reinforcing steel bar (rebar) in concrete. The integrated sensor unit includes an Ag/AgCl probe, a metallic oxide probe, a multi‐electrode array sensor (MAS), and a four‐pin (Wenner) array stainless steel electrode for chloride content, pH, microcell corrosion current, and localized concrete resistivity measurements, respectively. A stable solid probe was used as the reference probe in this unit to express the potentiometric measurement of chloride content and pH probes. In this study, the chloride and pH probes were calibrated in simulated pore solutions (SPSs) regarding temperature and pH fluctuations. The corrosion current results of the MAS probe in SPSs matched very well with those obtained by the linear polarization resistance technique, which was conducted on companion carbon steel specimens. A sensor prototype was embedded into a paste cylinder for long‐term performance evaluation. Up‐to‐date results show that the sensor probes exhibit excellent sensitivity and reliability through 1 year of monitoring. Continuous monitoring in the laboratory for extended periods is underway.     

Development of corrosion sensors for monitoring steel‐corroding agents in reinforced concrete structures

Corrosion sensors were devised to develop a system whereby the depth of chloride permeation from concrete surfaces can be monitored non‐destructively on a real‐time basis using such sensors embedded in cover concrete of reinforced concrete structures. The proposed corrosion sensors were subjected to accelerated corrosion in NaCl solutions, mortar specimens intrinsically containing chlorides, and mortar specimens impregnated with chloride solutions, while recording the changes in the resistance readings. The resistance of sensors increased as the degree of corrosion increased. The time to the first change in the resistance decreased and the corrosion degree and resistance increased as the chloride concentration increased and as the distance from mortar surfaces decreased. It was therefore confirmed that the corrosion sensors are capable of monitoring the depth of chloride permeation with sufficient accuracy.     

Ten‐year results of galvanic sacrificial anodes in steel reinforced concrete

Zinc sacrificial anodes have been included in patch repairs to steel reinforced concrete structural elements suffering from corrosion since the mid‐1990s. A number of these anode‐containing repairs have been monitored with time. One of the first monitored sites was of a locally repaired cross beam of a bridge structure in Leicester, UK, which has now completed 10 years since its original repair and anode installation. This paper reviews the performance of the anodes installed at the Leicester site in terms of anode current output and steel reinforcement polarisation and corrosion rate over the period. It also presents results of analysis of recovered anodes exposed for 10 years which still show electrolyte continuity, uniform consumption of the zinc and coherent encasing mortar. The knowledge gained from the 10 year results has enabled the development of new, higher current output anodes, which are now trialled in this and other sites.     

Investigation on the effectiveness of the repair method 8.3 “Corrosion protection by increasing the electrical resistivity” in chloride-containing concrete Part 3: The influence on corrosion of chloride-contaminated concretes under protective coatings

With the implementation of the repair method “increasing the electrical resistivity by coating” (MR 8.3), no direct repassivation of the reinforcing steel is initially intended. The success of the repair is rather linked to the change of the corrosion-relevant parameters over time. These include an increase in the concrete resistivity due to dehydration and gradual decrease in corrosion currents and driving voltages on the reinforcing steel. Within the scope of a research project funded by the German Research Foundation (DFG), application limits for the repair principle W-Cl could be defined. The chloride content present in the concrete at the rebar is the significant limiting factor for the application. While the corrosion activity even with moderately dehydrated specimens under diffusion-retarding coatings at chloride contents of 1 wt% Cl⁻/c is in the range of the passive current density, this cannot be generally determined for chloride contents of 2 wt% Cl⁻/c. The type of coating has a decisive influence on the dehydration of the concrete. For example, less dense concretes under a permeable coating (acrylic dispersion/OS 4) can dry out to such an extent that the passive current density is reached. With semipermeable coatings and the presence of high chloride contents of 2 wt% Cl⁻/c, the repair principle W-Cl does not lead to success according to the available test results.     

Reinforced concrete structures – shall concrete remain the dominating means of corrosion prevention?

The acknowledged serious deterioration of reinforced concrete structures due to chloride induced corrosion has been the main fuel for research and development of very dense and impermeable concrete, so‐called high performance concrete (HPC). This development has dominated concrete research up through the 80'ies and 90'ies. The results have technically been successful. However, the practical use of such concretes on site have often posed serious difficulties, resulting in at times very low performance concrete structures although HPC was specified. The discrepancy between concrete quality reached in the laboratory, what is being specified in the design and what can realistically be achieved on site is seldom in balance. Alternative means of more or less reliable means of corrosion prevention, often based on organic materials, have during recent years been developed to protect our inorganic concrete and reinforcement. However, a highly reliable means of corrosion prevention has been the introduction of stainless steel reinforcement, which is available with dimensions and strengths directly interchangeable with ordinary carbon steel reinforcement. It has been proven that stainless steel and carbon steel can be in metallic contact when cast into concrete, without causing galvanic corrosion. This seems, for the present, to be like an unexpectedly simple and highly reliable solution to the corrosion problems. As exemplified, this technology is rapidly gaining momentum in highly corrosive environments – and concretes being much more robust to execution can now take over from HPC.     

Prediction of corrosion rate in reinforced concrete structures – a critical review and preliminary results

This paper presents a critical review of some of the available corrosion rate prediction models focusing mainly on chloride‐induced corrosion. In addition to the proposals made for the improvement of these models, preliminary results of an ongoing study aimed at developing a chloride‐induced corrosion rate prediction model using both natural and accelerated corrosion results are presented. Even though a reasonable correlation between natural and accelerated i_corr is presented here, more data are still required to ascertain the relationship.     

Full surface inspection methods regarding reinforcement corrosion of concrete structures

For reinforced concrete structures a localisation of all significant critical areas can only be done by a full surface inspection. The economic advantages are obvious: uncritical areas have not to be repaired expensively 1 . The first step of the assessment should always be a visual inspection 2 . The range of deterioration causes can be limited and the degree of deterioration may be estimated roughly. The inspection program can be adjusted to the requirements. By means of a full surface potential mapping areas with a high risk for chloride induced reinforcement corrosion can be localised, although no deteriorations are visually detectable at the concrete surface. In combination with concrete cover depth and resistivity measurements areas with corrosion promoting exposure conditions can be localised even if the reinforcement is not yet depassivated. The following publication gives an overview about the essential full surface investigation methods to localise critical areas regarding corrosion of steel in concrete. The selection of methods is based on the inspection procedure given in 2 .     

Development of a calculation procedure for the statistical interpretation of the results of potential mapping performed on reinforced concrete structures

Potential mapping is a widely applied and accepted non-destructive measuring technique to assess the condition of concrete structures with respect to corrosion of the embedded reinforcement steel. Most often a single reference electrode is used to perform point measurements according to a predefined grid. At present there is much debate on the appropriate potential level to allow for a distinction between passive and actively corroding steel. In the RILEM recommendation for half-cell potential measurements, a statistical analysis of the results of potential mapping is advocated in order to arrive at an improved quantification of this potential region pertaining to the structure under investigation. However, the underlying calculation procedure has not been elaborated and consequently there is no practical and unambiguous guidance for the consulting engineer on how to perform such a statistical analysis. In this paper, the theory for the statistical analysis is presented based on the assumption that the potential distribution for both actively corroding and passive steel can be adequately described by a normal probability density distribution. This theory is applied to a number of situations to exemplify the procedure by clear graphs. The potential criteria derived will demonstrate a significantly smaller range as compared to the criteria presented in ASTM C 876. Moreover, with this statistical analysis any desired probability level of detection of corroding reinforcement steel can be chosen.     

Investigations of an ethanolamine‐based corrosion inhibitor system for surface treatment of reinforced concrete

Laboratory investigations were performed to assess the efficacy of a proprietary ethanolamine‐based corrosion inhibitor system when applied to the surface of reinforced concrete specimens that were chloride‐contaminated to varying extents in the presence or absence of carbonation. The corrosion responses of embedded steel bars at various depths of cover were monitored electrochemically during a controlled programme of cyclic wetting and drying undertaken for several months prior to the inhibitor treatment and for approximately eighteen months thereafter. Gravimetric measurements of the quantities and distribution of corrosion on the steel were also made on completion of the exposure tests. Analysis of aqueous extracts from treated concrete revealed that the ethanolamine component of the inhibitor system penetrated to depths of more than 15 mm within the concrete. It was found that, for inhibitor‐treated specimens, there was some reduction in the corrosion rate of pre‐corroding steel at low cover depths in non‐carbonated concrete with modest levels of chloride contamination. At higher levels of chloride contamination and in carbonated specimens, however, the ethanolamine‐based inhibitor was apparently ineffective under the conditions investigated.