Corrosion Risk and Humidity Sensors for Durability Assessment of Reinforced Concrete Structures

Transcription

1 Corrosion Risk and Humidity Sensors for Durability Assessment of Reinforced Concrete Structures Ralph Bäßler *1, Jürgen Mietz 1, Michael Raupach 2, Oskar Klinghoffer 3 1 Federal Institute for Materials Research and Testing VII 4.2, D Berlin / Germany 2 Institute for Building Materials, ibac, RWTH-Aachen, D Aachen / Germany 3 Force Institute, DK-2605 Brøndby / Denmark ABSTRACT Corrosion initiation and propagation of reinforcement can be monitored by means of different sensors embedded in concrete. Different sensor types for monitoring of time to corrosion initiation, as well as for new structures as for post-mounting in existing structures, were developed and tested in several materials and at different environmental conditions. Key parameters were concrete composition, chloride content, humidity, temperature. Sensor readings were compared to - the response of sensors embedded in concrete during casting, a type, well known and already proved by practical use and - other characterization methods of concrete properties, such as water and chloride content and the corrosion state of the metal by visual inspection. Results show a good correlation of the longtime behavior of corrosion potential, current and resistance obtained from sensors installed in existing specimens and pre-embedded sensors. Ingress of chloride concentration and change of humidity inside the concrete could be detected and enabled an estimation of the possible corrosion behavior. These results combined with other results obtained during an EU research project will contribute to improve evaluation procedures for determination of corrosion risk on concrete structures. End-users become able to optimize their maintenance management systems and can therefore reduce costs and traffic impairments. Keywords: Corrosion Risk, Humidity, Chloride Induced Corrosion, Sensors, Reinforced and Prestressed Concrete Structures * Correspondence: ralph.baessler@bam.de; Telephone: ; Fax: EUROCORR 00, August 2000

2 1 INTRODUCTION The major part of the European infrastructure has reached an age where capital costs have decreased. But inspection and maintenance costs have grown such extensively, that they constitute the major part of the current costs. During a Brite/Euram Project several European partners develop and produce an integrated monitoring system. So the inspection and maintenance costs and the traffic impairments can be reduced. Additionally the operator of the structures will be able to take protective actions before damaging processes start. One major part of this project is the determination of the corrosion state at the rebars in new and existing structures depending on the deterioration of the concrete. Different types of new sensors were developed and are now being tested under laboratory conditions. 2 EXPERIMENTAL SETUP Two types of sensor systems were evaluated. Corrosion risk sensors for determination of time to corrosion initiation caused by critical chloride ingress in different depths. Humidity sensors for detection of water content, another well known basic factor for corrosion initiation. Results of all sensor systems for use in existing structures were evaluated at laboratory conditions by comparison with results obtained from systems already installed in the specimens respectively received by analyzing the material itself and will be used as a base for future onsite tests

3 2.1 Corrosion Risk Sensor Different specimens varying in concrete composition were exposed: - constantly at room temperature and laboratory atmosphere - cyclically at 20 C and 40 C at a relative humidity of 70 % A part of the specimens was cyclically ponded with a 2 %-NaCl-solution to simulate chloride intrusion. Figure 1: Locations of the Sensor Systems on the Specimen [1] Two different types of corrosion risk sensor systems were evaluated. At every sensor potential and current versus corresponding cathode was measured regularly. At the Expansion-Ring-Electrode R values were obtained from rings in different depths tested versus an external cathode bar (V) [2]. Additionally resistance between adjacent rings were recorded to receive information about sensor behavior. Figure 2: Expansion Ring Anode and External Cathode At the Corrosion-Risk-Nail-System T current and potential of small conic metal bars in different depths (A) sealed with silicon were measured versus a Ti-mesh (B) embedded in mortar. For confidential reasons the potential of the cathode was compared to a MnO 2 -electrode (C) [3]. Figure 3: Corrosion Risk Nail System All results of these systems were compared to - 3 -

4 values obtained by an Anode-Ladder-System Q installed before casting the concrete [4]. 2.2 Humidity Sensor Figure 4: Specimen for Humidity Tests Figure 5: Multi Ring Electrode System Specimens (figure 4) varying in concrete composition with and without chloride deterioration were completely humidified and constantly exposed: - at room temperature, laboratory atmosphere (i.e. slowly drying out from 100 % RH at the beginning of exposure) and - at 20 C and 100 % relative humidity Two different types of humidity sensor systems installable in existing structures were evaluated. Comparing investigations were carried out at specimens described in the former chapter. At the Multiring-Electrode S, embedded in mortar, the electrical AC-resistance between adjacent rings in different depths was recorded (figure 5). Resistance follows indirect proportional the humidity of the material and can be used for humidity profiles. At the Capacitance Probe U (figure 4), installed in a pre-drilled hole, frequency was measured. Frequency is proportional to the capacitance of the concrete and thereby the humidity. This sensor system allows determination of water content course in-between surface and installation depth over a long time period. All results were compared to humidity profiles obtained by determination of water content in segments at different depths of concrete cores W

5 3 MEASUREMENTS AND EVALUATION All measurements were carried out weekly using hand held instruments on specimens made from concrete type OPC 35 F / CEM 32,5. Room temperature shifted between 20 C and 24 C and humidity change was in the range from 45 to 60 %. 3.1 Corrosion Risk Sensor At specimens without chloride deterioration no response of all sensors could be observed. As expected, no critical conditions exist in any depth. So discussion focuses on results obtained on specimens deteriorated with chloride. In figure 6 and 7 current values versus respective cathode measured on a concrete specimen exposed at room temperature are shown. The specimen was pre-covered with a chloride deteriorated 2 cm concrete layer and cyclically ponded with 2 %-NaCl-solution Expansion-Ring-Anode 1000 i versus cathode [µa] ,1 corrosion criterion 5 mm 15 mm 25 mm 35 mm 45 mm time [d] Figure 6: Current readings at different depths of the Expansion-Ring-Electrode on specimen ponded with 2 %-NaCl-solution exposed at room temperature At the Expansion-Ring-Anode the first two rings (5 and 15 mm) responded within 20 days by current increase caused by the already critical chloride concentration due to the deteriorated layer in the upper 2 cm. They remained depassive within the test period (figure 6). The cyclic behavior of the current at the upper rings is caused by ponding cycles. Current at the third and fourth ring (25 and 35 mm) started to increase after 50 days. This is in accordance to the response of the Anode-Ladder-System, where the 30 mm were passed at day

6 Major fluctuations while sensors were covered by solution mislead to the assumption of a higher corrosion stage. Stopping the solution coverage (ponding) interpretable results, showing activity within the upper 4 cm, were achieved. Critical chloride values are expected within this depth and will be evaluated in future work Corrosion Risk Nail System 1000 i versus cathode [µa] ,1 corrosion criterion 31 mm 38 mm 46 mm 53 mm time [d] Figure 7: Current readings at different depths of the Corrosion Risk Nail System on specimen ponded with 2 %-NaCl-solution exposed at room temperature The Corrosion-Risk-Nail-System responded (figure 7) as expected from the Expansion-Ring- Electrode results. Electrodes showed activity according to their depth. The two upper ones remain depassive during the test period. The very early current increase of the 31 mm-nail before day 50, as obtained at both other systems, could be caused by chloride, which was washed out of the already existing chloride layer during the nail installation and sealing procedure. Also at the Corrosion-Risk-Nail-System the same influence of ponding solution on the current values as observed as described at the Expansion-Ring-Electrode. Stopping the ponding causes that chloride did not ingress anymore and the corrosion current decreases. Differences in the current values are caused by different sizes of the anode area and were relativized by the corrosion (active/passive) criterion to compare all results achieved from the different electrode systems. 3.2 Humidity Sensor At all specimens no influence of chloride deterioration on the effectiveness of the sensors could be detected, so discussion focuses on general behavior of the sensors

7 3.2.1 Core segments In figure 8 the humidity profile obtained on blocks exposed at laboratory atmosphere from the real water content in the specimen are shown. Cores were cut in 15 mm segments. Water content was calculated by the following formula: m water content = m as received saturated - m - m dry dry 100% By that calculation the relation of actual water content relative to the maximum capability of water assimilation is considered. Water Cont. rel. to max. Capacity [%] max. capacity time [d] Figure 8: Humidity profile of concrete specimen during exposure at room temperature Starting at values of approximately 90 % at the beginning of exposure, decrease of water content confirms the drying out process. Deeper zones dry more slowly. This is reflected by slighter steepness of the decreasing water content. On specimen exposed at 100 % RH, where no drying out is expected, the water content relative to the maximum water capacity remains stable at around 85 % Multiring-Electrode At completely humidified specimens exposed in a climate chamber (20 C, 100 % RH) constant resistance values of 1,8 kω at all rings during the whole time period show no sign for drying out. This correct sensor response was confirmed by results on the core segments, already described

8 100 Resistance [kω] 10 1 sensor depth [mm] 5-7, , , , , , time [d] Figure 9: Resistance readings at Multiring-Electrode during exposure at room temperature The drying out process of the specimen exposed at room temperature and humidity is reflected by increase of resistance at every ring of the Multiring-Electrode (figure 9). Starting at 1,8 kω (completely humidified) the measured resistance increases continuously. After approximately one month concrete in the depth of the first ring reached a value where further drying processes cannot be quantified anymore. Deeper located rings show also a more slowly drying than the rings closer to the surface. These results correspond to values obtained by the segmented cores (figure 8), where a stronger decrease of the water content in the outer segments was observed. For comparison of the water content results with sensor values the m segment humidity = as received m dry - m dry 100% was used. A logarithmic correlation between water content and resistance could be observed and can be used for water content calculation from the resistance values

9 3.2.3 Capacitance Probe Sensor in 3 cm frequency time [d] Figure 10: Frequencies at the capacitance-probe installed in 30 mm depth during exposure at room temperature An increase of frequency by time (figure 10) at the Capacitance Probes reflects the drying out process of the outer 3 cm at the specimen. It has a similar shape as resistance measured at the corresponding rings of the Multiring-Electrode. So both sensor types reflect the drying out process in the same way. Comparing determined water content (by segment humidity) with the frequency values a linear dependency within an error range of ± 0,5 mass% (mainly caused by the kind of determination of the segment humidity) was observed. So knowing the frequency the concrete humidity can be estimated. 4 DISCUSSION Presented results of our tests show, that the real conditions at the specimen can be reflected by tested sensor systems. Despite of variations in installation procedures and different methods they all seems practicable for detection of corrosion risk respectively humidity in concrete specimens. All corrosion risk systems enable the user to determine the depth of critical condition in the concrete. Installed above the reinforcement, the systems can be used to estimate, when reinforcement will start to corrode. Humidity as a basic parameter for corrosion risk can be detected by both tested humidity sensors. By Multiring-Electrode detailed humidity profiles can be obtained. So the intrusion of humidity can be estimated. Capacitance Probe gives an average of humidity between sur

10 face and installation depth. Installed above the first reinforcement layer and calibrated correctly, it enables detection of humidity front while passing its depth. 5 CONCLUSIONS Corrosion risk and humidity sensors were tested in several materials and at different environmental conditions. Key parameters were concrete material, chloride content, humidity, temperature. Special attention was paid on the behavior of sensors to be installed in existing structures. Various installation procedures were tested. The responses of these sensors were compared to responses of sensors embedded in the concrete during casting, a type, well known and already proved by practical use and other methods for characterization of the concrete properties (water content). Additional comparison of obtained result to the real corrosion state of the metal (by visual inspection, weight loss measurements) is planned within the next tests. Tested sensors reflect the concrete behavior like pre-embedded sensors. Therefore they all are practicable for use in existing structures to determine the critical depth for reinforcement corrosion with one exemption. They shall not be used under water. The combination of presented results with others, received during this project, shall help to find the right integrated system suitable for end-users like road authorities and bridge owners. 6 OUTLOOK Described experiments will continue. During this project it is planned to perform additional investigations regarding to the influence of the chloride concentration variation, concrete humidity, temperatures in the laboratory and further tests of sensor performance on real structures. Results will be available within the next years and will be presented in future papers. These results combined with other results obtained during this BRITE/EURAM-project will contribute to develop an integrated corrosion monitoring system so that end-users become able to optimize their maintenance management systems and therefore costs and traffic impairments can be reduced

11 ACKNOWLEDGEMENTS This work is funded by the European Community as a BRITE/EURAM-project, contract number BRPR-CT Authors gratefully thank for the support. Furthermore contribution of all partners within this project, as in addition to the authors there are Autostrade, Danish Road Institute, OSMOS-Dehacom, DLR, Rambøll and S+R Sensortec, are deeply acknowledged. REFERENCES [1] Bäßler, R., Mietz, J., Raupach, M., Klinghoffer, O.; Corrosion Monitoring Sensors for Durability Assessment of Concrete Structures, Proceedings SPIEs 7th International Symposium on Smart Structures and Materials, Newport Beach,article (2000) [2] Schießl, P.; Raupach, M.: A New Sensor System for Monitoring the Corrosion Risk in Existing Structures. In: Second International Conference on Concrete Under Severe Conditions - Environment and Loading (CONSEC 98), Tromsö, Norway, , [3] Klinghoffer, O.: In-Situ Monitoring of Reinforcement Corrosion by Means of Electrochemical Methods; Nordic Concrete Research, No. 16, 1/95, Oslo, (1995) [4] Schießl, P.; Raupach, M.: Monitoring System for the Corrosion Risk for Steel in Concrete. In: Concrete International (1992), No. 7,