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.
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.
|Place of Publication||Kgs. Lyngby, Denmark|
|Publisher||Technical University of Denmark|
|Number of pages||287|
|Publication status||Published - Oct 2009|