Ueli Angst

On the Applicability of the Wenner Method for Resistivity Measurements of Concrete

Concrete resistivity measurements are favored for durability assessments due to the relatively simple measurement procedure and the documented correlations between concrete resistivity and transport properties, as well as reinforcement steel corrosion rates. For the measurement, it is common to use a four-point probe with four equally spaced electrodes (spacing distance a) and convert the measured resistance R into apparent resistivity with the so-called Wenner expression, ρapp = 2πaR. By means of experimental measurements and model simulations, the present study confirmed that the application of this formula is adequate, even if most of the assumptions underlying its derivation are violated. The reasons for this are systematically discussed. The experimental results also indicate that measurements with common four-point setups overestimate the resistivity with respect to bulk resistivity measurements. This is likely due to the presence of coarse aggregates that distort the current field at the sense electrodes.

Climbing robot for corrosion monitoring of reinforced concrete structures

This paper introduces a climbing robot for corrosion monitoring of reinforced concrete structures such as cooling towers, dams or bridges. The robot combines a vortex adhesion mechanism with a wheel electrode sensor for potential mapping of the concrete surface. A detailed description of the system is presented first. A special effort was made during the design in order to develop a lightweight device. The climbing robot is well suited for rough surfaces and can climb on vertical surfaces or move upside-down. The experiments that have been done to validate the concept are presented afterwards. They show that the climbing robot has several advantages over the traditional corrosion monitoring technique. This robot will therefore provide engineers in charge of infrastructure maintenance with the means to do their job much better than they can today. It offers them a way to circumvent all present barriers and brings a radical innovation in this area.

The size effect in corrosion greatly influences the predicted life span of concrete infrastructures

Size significantly affects the ability of reinforced concrete specimens to withstand corrosion under chloride exposure. Forecasting the life of concrete infrastructures in corrosive environments presents a long-standing and socially relevant challenge in science and engineering. Chloride-induced corrosion of reinforcing steel in concrete is the main cause for premature degradation of concrete infrastructures worldwide. Since the middle of the past century, this challenge has been tackled by using a conceptual approach relying on a threshold chloride concentration for corrosion initiation (Ccrit). All state-of-the-art models for forecasting chloride-induced steel corrosion in concrete are based on this concept. We present an experiment that shows that Ccrit depends strongly on the exposed steel surface area. The smaller the tested specimen is, the higher and the more variable Ccrit becomes. This size effect in the ability of reinforced concrete to withstand corrosion can be explained by the local conditions at the steel-concrete interface, which exhibit pronounced spatial variability. The size effect has major implications for the future use of the common concept of Ccrit. It questions the applicability of laboratory results to engineering structures and the reproducibility of typically small-scale laboratory testing. Finally, we show that the weakest link theory is suitable to transform Ccrit from small to large dimensions, which lays the basis for taking the size effect into account in the science and engineering of forecasting the durability of infrastructures.

Corrosion inhibitors for reinforced concrete

Abstract This chapter briefly presents the mechanism of reinforcement steel corrosion in concrete, focusing on chloride-induced pitting corrosion, which is generally the main deterioration mechanism in reinforced concrete infrastructures. Corrosion inhibitors have long been proposed as one of many possible corrosion mitigation strategies. This chapter presents the state-of-the-art on corrosion inhibitors used as admixtures, with particular emphasis on long-term and especially field studies. While there is a general agreement from both laboratory and field studies that mixed-in corrosion inhibitors can delay the onset of chloride-induced corrosion by a factor of two to three, there are no results clearly demonstrating that corrosion inhibitors can reduce the corrosion rate after corrosion initiation. A particular lack of knowledge concerns organic corrosion inhibitors where conclusive long-term field results are lacking.

Experimental Protocol to Determine the Chloride Threshold Value for Corrosion in Samples Taken from Reinforced Concrete Structures

The aging of reinforced concrete infrastructure in developed countries imposes an urgent need for methods to reliably assess the condition of these structures. Corrosion of the embedded reinforcing steel is the most frequent cause for degradation. While it is well known that the ability of a structure to withstand corrosion depends strongly on factors such as the materials used or the age, it is common practice to rely on threshold values stipulated in standards or textbooks. These threshold values for corrosion initiation (Ccrit) are independent of the actual properties of a certain structure, which clearly limits the accuracy of condition assessments and service life predictions. The practice of using tabulated values can be traced to the lack of reliable methods to determine Ccrit on-site and in the laboratory. Here, an experimental protocol to determine Ccrit for individual engineering structures or structural members is presented. A number of reinforced concrete samples are taken from structures and laboratory corrosion testing is performed. The main advantage of this method is that it ensures real conditions concerning parameters that are well known to greatly influence Ccrit, such as the steel-concrete interface, which cannot be representatively mimicked in laboratory-produced samples. At the same time, the accelerated corrosion test in the laboratory permits the reliable determination of Ccrit prior to corrosion initiation on the tested structure; this is a major advantage over all common condition assessment methods that only permit estimating the conditions for corrosion after initiation, i.e., when the structure is already damaged. The protocol yields the statistical distribution of Ccrit for the tested structure. This serves as a basis for probabilistic prediction models for the remaining time to corrosion, which is needed for maintenance planning. This method can potentially be used in material testing of civil infrastructures, similar to established methods used for mechanical testing.

Electrochemistry and capillary condensation theory reveal the mechanism of corrosion in dense porous media

Corrosion in carbonated concrete is an example of corrosion in dense porous media of tremendous socio-economic and scientific relevance. The widespread research endeavors to develop novel, environmentally friendly cements raise questions regarding their ability to protect the embedded steel from corrosion. Here, we propose a fundamentally new approach to explain the scientific mechanism of corrosion kinetics in dense porous media. The main strength of our model lies in its simplicity and in combining the capillary condensation theory with electrochemistry. This reveals that capillary condensation in the pore structure defines the electrochemically active steel surface, whose variability upon changes in exposure relative humidity is accountable for the wide variability in measured corrosion rates. We performed experiments that quantify this effect and find good agreement with the theory. Our findings are essential to devise predictive models for the corrosion performance, needed to guarantee the safety and sustainability of traditional and future cements.

Corrosion Challenges and Opportunities in Digital Fabrication of Reinforced Concrete

This contribution addresses corrosion of steel in digitally fabricated concrete. In recent times the concrete processing for digital fabrication applications has been greatly advancing, rising the interest of research institutions, industrial partners, governments and public media. Nevertheless, for a broad large scale application, not just the technological feasibility, but also the long term durability needs to be ensured. This contribution presents a general overview of recently developed digital fabrication technologies and assesses them from the point of view of reinforcement corrosion risks. Experimental results are presented and a number of potential durability issues specific to digital fabrication are raised. On the other hand, we highlight opportunities for making more corrosion-resistant concrete structures by taking advantage of digital fabrication technology.

Development of a novel methodology to assess the corrosion threshold in concrete based on simultaneous monitoring of pH and free chloride concentration

Both the free chloride concentration and the pH of the concrete pore solution are highly relevant parameters that control corrosion of the reinforcing steel. In this paper, we present a method to continuously monitor these two parameters in-situ. The approach is based on a recently developed electrode system that consists of several different potentiometric sensors as well as a data interpretation procedure. Instrumented mortar specimens containing different amounts of admixed chlorides were exposed to accelerated carbonation, and changes in free chloride concentration and pH were monitored simultaneously over time. The results revealed the stepwise decrease in pH as well as corresponding increases in free chlorides, resulting from the release of bound chlorides. For a pH drop of about 1 unit (from pH 13.5 down to pH 12.5), the free chloride concentration increased up to 1.5-fold. We continuously quantified the ratio Cl−/OH− that increased steeply with time, and was found to exceed a critical corrosion threshold long before carbonation can be detected with traditional indicator spray testing, even at admixed chloride contents in the order of allowable limits. These results can strongly influence the decision-making in engineering practice and it is expected to significantly improve condition assessments of reinforced concrete structures.

Impact of IR Drops on the −850 mVCSE Cathodic Protection Criterion for Coated Steel Pipes in Soil

AbstractBy means of numerical modeling and experimental measurements, this work addresses the longstanding controversy of whether or not IR (ohmic) drops need to be considered in on-potential catho...