Jürgen Mietz

Monitoring of concrete structures with respect to rebar corrosion

Abstract From security aspects as well as economic reasons it is desirable to localize reinforcement corrosion at an early stage and with non-destructive techniques in order to initiate the necessary rehabilitation measures in time. The potential mapping technique, i.e. measuring half-cell potentials at concrete surfaces with reference electrodes, is a helpful tool to obtain information about the actual state of rebar corrosion. The potential measurement can be modified by additionally applying a short anodic pulse by means of a small counter electrode and a galvanostat. In some cases this technique leads to noticeable improvements with respect to information about the probability of corrosion. The fundamentals of the two kinds of potential measurement techniques are explained and results obtained at different types of structures are described.

Assessment of test methods for evaluation stress corrosion cracking susceptibility of prestressing steels

Prestressing steels can be exposed in prestressed concrete structures in construction stage (ungrouted duct) to environmental conditions causing formation and growth of hydrogen induced cracks with brittle fracture of the steel. The risk of this hydrogen induced stress corrosion cracking can be minimized by appropriate treatment, but there is a requirement for construction to approve only prestressing steel having no enhanced susceptibility to stress corrosion cracking. The paper describes the historical development of the different test methods proposed in the past and gives an overview on the general requirements for a suitable test method to assess the hydrogen-induced stress corrosion susceptibility of prestressing steels. Since 1982, based on investigations on construction sites and transfer of the results into laboratory tests, a test method is available enabling realistic estimation of suitable application possibilities of prestressing steels. This so called DIBt-test is approved to distinguish between prestressing steels susceptible to hydrogen induced stress corrosion cracking and those suitable and therefore approvable steels at practical environmental conditions. Furthermore the test enables estimation of corrosion risk for newly developed prestressing steels with higher strength. Beurteilung der Prufverfahren zur Untersuchung der Spannungsrisskorrosionsempfindlichkeit von Spannstahlen Spannstahle konnen in Spannbetonbauwerken im Bauzustand (unverpresstes Hullrohr) Umgebungsbedingungen ausgesetzt sein, die zur Bildung und Ausbreitung wasserstoffinduzierter Risse mit Sprodbruch des Stahles fuhren konnen. Das Risiko dieser wasserstoffinduzierten Spannungsrisskorrosion kann einerseits durch eine sachgemase Verarbeitung der Spannstahle minimiert werden, andererseits besteht die Notwendigkeit nur solche Spannstahle fur die Baupraxis zuzulassen, die unter baupraktischen Bedingungen keine erhohte Empfindlichkeit fur Spannungsrisskorrosion besitzen. Die vorliegende Arbeit beschreibt die historische Entwicklung der verschiedenen vorgeschlagenen Prufverfahren und gibt einen Uberblick uber die allgemeinen Anforderungen an ein geeignetes Verfahren zur Beurteilung der wasserstoffinduzierten Spannungsrisskorrosionsempfindlichkeit von Spannstahlen. Seit 1982 steht, begrundet durch Untersuchungen auf Baustellen und Umsetzung der Ergebnisse in Laborversuchen, ein Prufverfahren zur Verfugung, das eine wesentlich realistischere Einschatzung der Einsatzmoglichkeiten von Spannstahlen zulasst. Dieser sog. DIBt-Test hat sich bewahrt, um unter praktischen Umgebungsbedingungen gegen H-induzierte Spannungsrisskorrosion empfindliche Spannstahle von geeigneten und damit zulassungswurdigen Stahlen zu unterscheiden. Des weiteren erlaubt die Prufung ebenfalls eine Abschatzung des Korrosionsrisikos fur neuentwickelte, hoherfeste Spannstahle.

Investigations on hydrogen-induced embrittlement of quenched and tempered prestressing steels

In Germany in recent years failures of more than 30-year-old prestressed concrete structures have been observed. In all these cases a quenched and tempered steel type, strength class St. 1420/1570 was used. In order to assess the corrosion risk, i.e. crack initiation and propagation, under depassivating conditions lifetime tests in environments relevant for building practice were carried out using different prestressing steels of this quenched and tempered type of material. From the results it can be concluded that the specific susceptibility of certain prestressing steels is the determining factor with respect to the occurrence of cracks rather than environmental factors. Untersuchungen zum wasserstoffinduzierten Sprodbruch verguteter Spannstahle In den letzten Jahren ist es verschiedentlich zu Schadensfallen von Spannbetonbauteilen gekommen, bei denen verguteter Spannstahl der Festigkeitsklasse St 1420/1570 eingesetzt war. Durch systematische Untersuchungen an verschiedenen Typen dieser Stahlsorte sollte geklart werden, ob unter depassivierenden Bedingungen Riseinleitung und -ausbreitung unter praxisnahen Verhaltnissen zu erwarten ist. Die durchgefuhrten Untersuchungen lassen folgern, das die spezifische Anfalligkeit des Spannstahls fur das Auftreten von Rissen einen wesentlich hoheren Einflus besitzt als die elektrolytseitigen Parameter, da sich die verschiedenen Stahle auch bei vergleichbaren Korrosionsangriffen unterschiedlich verhalten.

Langzeiterfahrungen mit einem kathodischen Korrosionsschutzsystem für Stahl in Beton

In 1986 in Germany the first cathodic protection system for reinforced concrete structures was put into service. Installation, commissioning and the regular monitoring were scientifically supported. Due to necessary traffic related reconstruction measures the system was switch off after 15 years of service. Investigations carried out during that period should verify laboratory test results at a real structure and furthermore yield information on the long-term behaviour concerning durability, temperature dependence, potential distribution, chloride migration etc. With increasing time of service the protection criteria could only be met by increasing the transformer rectifier voltage (the system worked in the constant voltage mode). Extensive investigations of the anode material (conductive polymer wire with a copper core) after 15 years of service showed that irreversible changes substantially decreased the polymers conductivity limiting the functionality. Today, these kind of anodes are not used any more for the application in cathodic protection systems of reinforced concrete structures.

Detection of reinforcement corrosion in reinforced concrete structures by potential mapping: Theory and practice

Electrochemical potential mapping according to guideline B3 of DGZfP (German Society for Nondestructive Testing) is a recognized technique for the localization of corroding reinforcing steels. In reinforced concrete structures the measured potentials are not necessarily directly linked to the corrosion likelihood of the reinforcing steel. The measured values may be significantly affected, different from, e.g., stress measurement, by different influences on the potential formation at the phase boundary metal/concrete itself as well as the acquisition procedure. Due to the complexity of influencing factors there is a risk that the results are misinterpreted. Therefore, in a training concept firstly the theoretical basics of the test method should be imparted. Then, frequently occurring practical situations of various influencing factors will be made accessible to the participants by a model object specially designed for this purpose. The aim is to impart profound knowledge concerning the characteristics of potential mapping for detecting corrosion of reinforcing steel in order to apply this technique in practice as reliable and economical test method.

Untersuchungen zur Spannungsrißkorrosionsgefährdung von Spannstählen (Stäben) bei erhöhter Vorspannung

The application of prestressing methods using bars lead to additional bending stresses or internal stresses when plastic deformation takes place at curvatures. These stresses are superimposed by the well-planned prestressing. The probability for hydrogen-induced stress corrosion cracking in corrosive media (especially water in the metal sheath) is considerably increased in the presence of internal stresses and if the prestressing level is enlarged. The permissible prestressing levels of the EU guidelines are higher than the values fixed in DIN 4227 with the same permissible radii of curvature. In order to find out if there is an increased risk of stress corrosion cracking, exposure tests were performed with prestressing bars grade St 900/1030 and St 1080/1230, respectively, in an electrolyte simulating metal sheath conditions. All specimens withstood the 2000 h lasting exposure test without fracture which means that the risk of hydrogen-induced brittle fracture could be excluded under practical conditions provided that the current state of the art has been considered. Tensile tests after the exposure revealed a reduction of strain values due to the exposure. Fracture analysis by means of SEM did not show any signs of embrittlement for the materials tested.