International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
|Start > Contributions >Lectures > Monitoring:|
Monitoring of concrete structures on ASR:
Jaap Bakker & Frits Postema
|Fig 1: Extreme internal ASR cracking in a vertical cross section of the bridgedeck.|
Monitoring of structural safety is only possible if a parameter can be found that has a fixed relation with this structural tensile strength. Unfortunately no non-destructive alternative was found. Therefore, direct monitoring of structural safety was no option.
Monitoring of failure indicators
Several options where considered. Crack measurement would not provide a adequate indicator, since no direct relation can be derived between cracking and tensile strength [2,3]. Deformation measurement would not give a useful result either, since failure occurs brittle, not necessarily preceded by large deformations. Alternatively the use of acoustic emission was studied. An ASR affected concrete beam, derived from a demolished bridge deck, was loaded up to failure, while simultaneously the cracking intensity in the concrete was followed (figure 2). Even though the cracking intensity increased considerably before failure, no reliable criterion was found for practical use in traffic.
|Fig 2: Testing of the structural capacity of beams derived from a concrete plate bridge deck.Simultaniously cracking intensity was measured using AE.|
Monitoring of damage development
ASR will lead to damage in case of sufficient moisture in the concrete. Therefore, renovation was aimed at drying the bridges. Moisture ingress was minimised, using bituminous material and hydrophobic agents. All bridges have had a safety assessment based on destructive testing. Therefore, the initial safety is known. It is assumed that expansion (resulting in a loss of structural safety) should stop when the concrete dries out. Therefore, monitoring of expansion and moisture are good indicators for ASR activity. Expansion and moisture in concrete is correlated to temperature. For this reason all three aspects are monitored in time simultaneously.
Expansion in the bridge deck in vertical direction
Vibrating wire sensors (figure 3) where placed vertically in the bridge deck (imbedded in a low shrinkage injection grout) to measure expansion. This type of sensors consist of a thin wire, anchored at both ends, and an electric coil. An electric field sets the wire into vibration. As the concrete expands, the strain in the wire will increase, resulting in a higher vibrating frequency.
|Fig 3 & 4: Two vibrating wire sensors are connected to a steel rod and placed in a hole in the concrete and grouted in. Results after two years of monitoring show expansion at the moist locations (B en D). Location A1, A2 and C2 follow the temperature curve (not in this figure). The measured expansion at C1 is caused by bad injection of the hole the sensor was placed in.|
After two years of monitoring, a clear difference showed between the two bridges. In one bridge deck, the expansion clearly followed the concrete temperature. No signs of ASR expansion where measured. The other bridge showed a clear ongoing expansion at several (but not all) measurement locations (figure 4). Those locations showing ongoing expansion all correlated with those location with high moisture content, except for one (C1). This sensor was removed for investigation. The drilling hole showed to be badly injected. This explained the unexpected result. One of the other expansion locations was removed. Cracks continued from the old concrete into the fresh mortar and reaction products where found in the cracks, proving actual concrete expansion. It should be noted that the cracking of the injection mortar might lead to overestimation the actual expansion.
Monitoring of concrete resistance, using multiring electrodes (MRE)
The concrete electrical resistance is related to the moisture content in the concrete. An increase in concrete resistance can be used as an indicator for the drying of the concrete. The MRE measures the concrete resistance at a frequency of 108 Hz., between 14 stainless steel rings. This results in a moisture profile over the concrete depth. The sensor (figure 5) is placed in a hole in the bridge deck, which was injected with a cement based low shrinkage grout.
Unfortunately, the results of two year monitoring where not consistent. MRE's are frequently used in new concrete successfully. In this case however, the use of grout might have influenced the measurements due to shrinkage effects or incomplete injection. Nevertheless, those places which where considered to be moist and which showed expansion, had a higher average resistance than those at the other locations in the bridge deck.
Monitoring of moisture and temperature using combined relative humidity - temperature sensor
The sensor (figure 6) measures the temperature and the relative humidity in a small air chamber at the top of the sensor. Like the other sensors, they where placed in a hole in the bridge deck and grouted in.
Fig 5: Multiring electrodes.
Fig 6: Relative humidity and temperature sensors.
In the two year of monitoring some of the sensors showed unrealistic high results. This is explained by condensation effect in the moist chamber. It is assumed that the sensor will not remain stable if this happens. Nevertheless, the measurement results corresponded reasonably with the other measurements, locating the most humid places.
Four times a year measurements where taken. Together with some additional measurements (crack mapping, deformations), all data was gathered in an Excel-sheet. This method of data acquisition showed to be very time consuming, and therefore costly in term.
Because not all measurements where taken simultaneously (for instance the deformations and the concrete temperature), they where sometimes hard to relate. Furthermore, the frequency of measurement was not high enough for sufficient accuracy to distinguish small ASR expansion from temperature effects at all cases. The risk of human error is significant, if large amounts of data are copied into a spread sheet. This may lead to wrong conclusions.
Based on the pilot significant changes where made in the monitoring plan. Different sensors where chosen, and the data acquisition was fully automated. Since no adequate explanation was found why one of the bridges in the pilot showed expansion and the other didn't, the decision was made to monitor all other bridges. Besides the primary goal (expansion monitoring), the monitoring project has an educational effect. The large scale monitoring will provide information about the effectiveness of the renovation strategy. This knowledge can be used for other renovation projects.
Since significant changes where made in the monitoring system, an extended pilot was set up for two more bridges. After one year the results where evaluated, some small adjustments where made. Subsequently the other 14 bridges where instrumented. From the first of February 2003 on, the monitoring system is fully operational.
The data acquisition system
Each bridge is instrumented with a data logger and a GSM communication line. This provides the opportunity to measure in short intervals. In this manner, delayed effects of temperature and moisture fluctuation on expansion can be taken into account. Once a day the data logger sends all measurements to a central database. Each data logger has sufficient backup capacity, in case of disturbances of data communication. The central database is accommodated at the office of the monitoring device provider. In this manner, monitoring expertise and data management remains in one hand. The central database is backed up periodically, to avoid data loss in case of a calamity. All bridges are provided with solar cells and batteries for energy supply.
The user interface
One of the biggest challenges of a large monitoring project is the handling of large amounts of data. Each hour, a total of around two hundred measurements are taken. Therefore, much attention was given on the presentation of the results.
|Fig 7: Screens for bridge selection and selection of monitoring data per bridge.||Fig 8: Screens for bridge selection and selection of monitoring data per bridge.|
The monitoring results can be accessed through an internet site. By means of this site data can be presented graphically. Trends can be watched over a longer period based on day averages. For analysis purposes measurements can also be presented per hour for a short time span. Moisture fluctuations, temperature and expansions can be related. For detail analysis purposes an export function is available, transposing a data selection to an Excel file.
For each structure, the internet site provides some extra information. Each bridge can be selected through a graphical selection screen, showing the highway 59(figure 7). Detailed information about the structure, the damage history and the monitoring system is presented. Monitoring results can be selected in a standard analysis graph, or studied in detail for a optional period, per sensor or per sensor type (averages). The selection screen is presented in figure 8.
Monitoring of local expansion in vertical direction and temperature
The vibrating wire sensor is adapted for this application. The sensor is extended to a measuring length of approximately the bridge deck thickness (figure 9). An hydraulic anchor secures the sensor in the centre of a borehole in the concrete. Unlike the pilot-sensor, the borehole is not grouted, but stays open. This way the grout does not influence the measurement, and sensors can be replaced or maintained if needed. The first type of anchor used proved to be unsuitable. The synthetic core deformed much more due to temperature changes than the concrete. Due to this, a shrinkage was measured at rising temperature. Therefore, the synthetic anchor core was replaced by a steel core.
|Fig 9 & 10: Vibrating wire sensors with hydraulic anchors, , specially designed to be placed in a borehole in the old concrete. The graph shows the relation between the average expansion (8 sensors) and average temperature (8 sensors)over three months. In case of ASR-expansion, the temperature and expansion lines will recede.|
Temperature is measured in the sensor simultaneously with expansion. The new sensors are temperature compensated, thus the actual concrete expansion is measured (figure 10).
Monitoring of total expansion of the bridge deck
At each side of the bridge, joint meters are placed between the abutment and the bridge deck. The total expansion of the bridge deck is the sum of both deformations at the joints. The abutments are considered to be stable. This is checked if needed. As figure 11 shows the expansions are primary temperature related. It is expected that ASR-expectation will primary occur in the unrestrained direction (vertically). Nevertheless, the measurement resolution is high enough to measure small expansions (0,01 mm/m average ASR expansion in a year in a 50 m. bridge deck will result in 0,5 mm. expansion).
Fig 11: Relation average temperature (8 sensors) and total bridge expansion (sum of the deformations at the joints) over three months time. In case of ASR-expansion, these temperature and expansion lines will recede. This is not (yet) observed.
Moisture is measured using the so-called TRIME-sensors  (figure 12). The principle is based on the TDR-principle (Time-Domain Reflectometry). An antenna is placed in a borehole. A set of electric pulses is sent through the antenna. The reflection time is dependant on the concrete impedance. Latter is mainly influenced by the (free) moisture content in the concrete. Free moisture in concrete is the driving force for ASR-expansion, and therefore a good parameter. A big advantage of the TRIME-sensor above the sensors used in the pilot is, that it measures moisture in the actual damaged concrete. No injection mortar is needed.
|Fig 12:||Fig 13:|
|Fig 12 & 13: TRIME sensor and measurement results over 14 months: The average moisture content (8 sensors) is related to the average temperature (8 sensors). A variation in free water in the concrete can be observed due to temperature changes and seasonal changes in relative humidity of the air.|
The TRIME sensor was designed for moisture measurement in soil. Interpretation of the measurement to a moisture content is only possible if one has representative reference material. This is not possible in damaged concrete. The material is heterogeneous, and moisture will travel through local cracks. Therefore, the measured moisture contents can only be used as an indication. Furthermore, the moisture measurements are temperature related, since the amount of free water in concrete varies with the temperature. Moreover, the measurement may be influenced by reinforcement steel close to the sensor.
The primary goal of the moisture measurement is to determine trends in time. For this moisture en temperature must be related. Trends can only be observed by graphical analysis, since individual measurements can be misleading (figure 13). Seasonal changes in relative humidity in the air will cause moisture fluctuation within the concrete. Numeric modelling showed this influence is significant.
After one year of monitoring in two of the structures, no drying of the concrete is measured yet. It should be noted though, that the renovation was finished about one year before the first monitoring.
Probability of detection of ASR expansion
ASR expansion may occur locally and may vary in expansion rate within the concrete structure. This was proven in the pilot. A local measurement on expansion may not be representative for the whole structure. On the other hand, if ASR only occurs very locally, then it may not show on the overall expansion measurement of the bridge deck. Therefore, the assessment of the structure is based on three safety nets (figure 14):
|Fig 14:||Fig 15:|
|Fig 14 & 15: Three "safety nets" for timely ASR detection;The decission processis descri bed for different inspection results.|
Based on inspection results actions to be taken are considered on yearly basis (figure 15)
The highway 59 monitoring project on ASR is rightfully considered to be the first large scale smart structure project in The Netherlands. Much will be learned about ASR-damage behaviour on the long term in bridges which have been sealed for moisture. Already, the monitoring project has provided a lot of information on how to monitor moisture and expansion in existing structures. Due to the use of a fully automated data acquisition and data communication, one can gain much extra information. This shows to be very useful. Individual measurements are often hard to relate to each other, for instance due to phase differences. Comparing day averages evens out these kind of problems.
In smart structures projects, a primary factor of success the data presentation. Modern web applications provide new possibilities.
So far, no clear ASR-expansion is measured during the first months of automated monitoring, nor has there been any prove that the concrete actually dries due to the renovation measures. It should be noted tough, that there has been more than one year between the end of the renovation and initiation of the monitoring. During this period the concrete might have dried and expansions might have stopped. Furthermore, the ASR-expansions might still be to small during these first months to detect.
Monitoring can be used to gain certainty about the safety of the structure in term. Unfortunately, in this case, due to insufficient experience in monitoring of ASR, the uncertainty shifts from uncertainty in the safety to uncertainty in the measurements. All that's measured might be wrong or not representative. Therefore, in this project, monitoring was set up at more than only one parameter (so measurements can be related), at more than one bridge. So far, the measurement results seem promising.
Thanks go out to Harry van Gaanderen who brought in many proposals which are involved in the chosen ultimately monitoring system. Furthermore, Robert van der Veen from Koenders Instruments has put a lot of personal effort in this project. The cooperation has been very good so far. Next Huibert Borsje (TNO) has been responsible for technical support during the project, which has been very useful.