1 Department of Civil Engineering, Technical University of Denmark2 Section for Construction Materials, Department of Civil Engineering, Technical University of Denmark3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark4 Materials and Surface Engineering, Department of Mechanical Engineering, Technical University of Denmark5 Department of Mechanical Engineering, Technical University of Denmark
Kort dansk abstract Korrosion af armering pga. indtrængende klorider er den væsentligste årsag til begrænset holdbarhed af armerede betonkonstruktioner i marint miljø og konstruktioner udsat for tøsalt i vinterperioderne. Til undersøgelse af armeringens tilstand anvendes så vidt muligt metoder, der ikke påvirker konstruktionen. Elektrokemiske potentialemålinger til vurdering af korrosionsrisiko og mere avancerede metoder til bestemmelse af korrosionshastigheden. Der findes forskellige kommercielle instrumenter til bestemmelse af armeringsståls korrosionshastighed; men både laboratorieundersøgelser og målinger på konstruktioner har vist, at instrumenterne ikke giver sammenlignelige resultater. Med det formål at skabe baggrund for videreudvikling af et af de kommercielle instrumenter er centrale elementer af både instrumentopbygning og måleprocedure undersøgt i dette ph.d. projekt. For at forbedre mulighederne for vurdering af måledata er langtidseffekten af temperatur og fugtighed på korrosionshastigheden bestemt. Der er udviklet en metode til bestemmelse af strømfordelingen mellem instrumenternes modeelektroder og armeringen. Metoden gør brug af en segmenteret armeringsstang indstøbt i beton. Metodens anvendelighed er demonstreret ved undersøgelse af to kommercielt tilgængelige korrosionshastighedsmåleinstrumenter: GECOR 6 og GalvaPulse. Den observerede manglende mulighed for at lokalisere aktive områder og overestimering af korrosionshastigheden af passiv armering skyldes, at modelektrodens strøm ikke kan afgrænses; mens underestimering af korrosionshastigheden af et relativt lille område skyldes manglende mulighed for bestemmelse af anodens areal. Målingerne foretages ved at påtrykke en strømpuls og følge ændringen af det elektrokemiske potentiale. Den målte korrosionshastighed er afhængig af polarisationstiden og for passiv armering også strømpulsens størrelse. Baseret på observationerne er der givet anbefalinger for polarisationstider og -strømme ved bestemmelse af armeringsstål korrosionshastighed. Undersøgelse af korrosionshastigheden efter to år indikerer, at temperaturen ikke kun påvirker korrosionshastigheden, men også de dannede produkter. Abstract Condition assessment of reinforced concrete structures may be facilitated by non-destructive techniques. Since the publication of the first version of the ASTM C876 standard in 1977 the use of half-cell potential mapping has been widely accepted as a non-destructive ”state of the art” technique for detection of corrosion in concrete structures. And, over the last decade, the trend in corrosion monitoring has moved towards quantitative non-destructive monitoring of the corrosion rate of the steel reinforcement. A few corrosion rate measurement instruments have been developed and are commercially available. The main features of these instruments are the combined use of an electrochemical technique for determining the corrosion rate and a so-called ”confinement technique”, which in principle controls the polarised surface area of the reinforcement, i.e. the measurement area. Both on-site investigations and laboratory studies have shown that varying corrosion rates are obtained when the various commercially available instruments are used. And in the published studies, conflicting explanations are given illustrating the need for further clarification. Only by examining the effect of the confinement techniques and the electrochemical techniques separately the variations in measured corrosion rates can be explained. Such work was conducted in the present project. A method for quantitative assessment of current confinement techniques is presented in the thesis. The method comprises monitoring of the operation of the corrosion rate instrument and the distribution of current between the electrode assembly on the concrete surface and a segmented reinforcement bar embedded in concrete. The applicability of the method was demonstrated for two commercially available corrosion rate instruments, the GECOR 6 and GalvaPulse instruments, which are based on different confinement techniques as well as different electrochemical techniques. The variations in measured corrosion rates were explained, and the instruments’ performance evaluated. On passive reinforcement neither of the instruments were able to effectively confine (or compensate for) the lateral spread-out of the counter-electrode current. As a result both instruments overestimated the corrosion rate of the passive steel. For reinforcement with one or several actively corroding areas on an otherwise passive reinforcement bar, it was found that neither of the instruments could locate the corroding areas. This was due to the lateral current flow from the electrode assembly on the concrete surface to the actively corroding areas on the reinforcement bar independent of the position of the elecvii trode assembly. In the presence of a single small corroding area with a high corrosion rate both instruments underestimated the actual corrosion rate. The underestimation was due to a combination of the constant confinement length, here much larger than the active area, and the obtained confinement. For unconfined measurements it was found that a distinction between passive and actively corroding steel with a low corrosion rate or small corroding areas is almost impossible. As was the case with the confined corrosion rate measurements, it was found that actively corroding areas could not be located. The conclusions regarding current confinement are based on investigations on concrete slabs with cover thickness of 30 and 75 mm representing most chloride exposed structures. However, the concrete had a relatively high w/c-ratio (0.5) and therefore a relatively low electrical resistivity, facilitating the distribution of current and thus proving a conservative assessment of the efficiency of the confinement techniques. For modern concretes with lower w/c-ratios and supplementary cementitious materials, which have higher resistivity, improved efficiency of the current confinement techniques may be expected. In addition to the effect of the confinement techniques, the effect of the polarisation time and current on the measured polarisation resistance and thus the corrosion current density were investigated. The two electrochemical techniques used in the GECOR 6 and the GalvaPulse instruments were considered in the study: the galvanostatic linear polarisation resistance technique and the galvanostatic potential transient technique, respectively. Measurements were performed on 45 concrete specimens each with 10 steel bars prepared from concrete with and without admixed chloride to obtain passive and actively corroding steel bars. Varying corrosion rates were obtained by exposing the 45 specimens to 15 different climates, being a combination of five temperatures (1 to 35 ◦C) and three relative humidities (75 to 96 %RH). On passive reinforcement the measured polarisation resistance - and hence corrosion rate - was for both galvanostatic techniques found to be highly affected by the polarisation time and current. No plateau at either short or long polarisation times (10 to 165 seconds) or low or high currents (0.25 to 100 μA) was identified. Nevertheless, it was found that a qualitative estimate clearly showing the passive state of steel reinforcement can be obtained with either technique even though stationary conditions are not achieved and the obtained potential response is outside the linear current-potential range around the free corrosion potential. On actively corroding reinforcement a large effect of the polarisation time but only a minor effect of the polarisation current on the measured polarisation resistance were found for both galvanostatic techniques. Also, it was found that the effect of the polarisation time is practically independent of the corrosion rate. For both galvanostatic techniques guidelines were given for polarisation times and currents for non-destructive corrosion rate measurements on reinforcement steel in concrete. Finally, a study on the effect of temperature and relative humidity on the corrosion rate of steel in concrete was conducted. Contrary to published short-term corrosion studies the Arrhenius equation was found inadequate for describing the temperature dependency of the corrosion rate in this study where measurements were made after approximately two years of constant exposure.