Within the framework of the research contract AGB 2001/471 the situation in respect to the damages on concrete structures due to the alkali aggregate reaction (AAR) in Switzerland has been investigated by the applicant [Merz 2006].
In connection with the “execution of measures” and the “examination” of structures (condition survey, condition evaluation, recommendation of remedial measures) the following research topics have been identified:
a) Evaluate the existing remedial measures and test their efficiency
b) Develop new repair techniques and test their efficiency
c) Monitoring of the development of the condition.
In respect to the AAR damages on walls of all types (e.g. anchored and non anchored retaining walls, side walls of bridges abutments, entrance areas of tunnels and underpasses, freestanding walls) it has been pointed out in [Merz 2006], that in Switzerland walls are the most frequently damaged structures/components with a portion of 44%.
The rehabilitation of walls represents a special challenge, since they are often soaked by water coming from the (not accessible) backside and because it is difficult to stop this harmful and unwanted water ingress by simple means. In contrast to this, other components and structures (e.g. bridges, columns) might easier be repaired with conventional techniques.
The scope of this project is to investigate and to assess the possible techniques for the rehabilitation of AAR damaged retaining walls. The results should allow elaborating recommendations for the repair of existing retaining walls damaged by AAR.
In the context of this project, in 2009 the ASR damaged retaining wall S0751 on the Simplon
Pass road (Canton of Valais) has been evaluated and its condition examined in detail as the
main object of the investigations. Subsequently, one of two test areas was treated with a water-
repellent impregnation. Thus, the simplest rehabilitation method should be simulated. The
other test area was not treated for comparison purpose. For the monitoring, the following
measurements were performed:
- water absorption on-site and on cores in the laboratory (before and after the impregnation
- and in 2012)
- crack width index (2009, 2012)
- air and concrete temperature (continuous measurements)
- superficial concrete moisture (periodic measurements)
- length change of deformeter measurement segments (periodic measurements)
- electrical resistivity of concrete with resistance sensors (measure for the concrete humidity)
- (continuous measurements)
- length change with vertical and horizontal extensometers as well as with bore hole extensometers.
As a secondary object, the retaining wall S0701 was selected. This wall shows significantly
larger ASR damages than the object S0751. On this wall, the concrete humidity and length
changes with the deformeter method were determined periodically. Further, in 2009 and
2012, the crack width indices were determined as well. The retaining wall S0701 was built
with a very similar concrete as the object S0751. The heavier ASR damage is almost certainly
due to the fact that the downhill retaining wall S0701 is much wetter than the uphill and
slightly recessed retaining wall S0751. It was also suspected that the external de-icing salt/alkalis immersion has accelerated the damage development.
Following steps will be provided during the project
- Evaluation of two to three practically applicable repair techniques
- Application of the most promising measures within the framework of pilot projects (field test) on selected retaining walls with different levels of damages
- Verification of the effect and success, respectively, of the repair measures with an appropriate monitoring of the structure.
To characterize the state of AAR-damaged buildings measurements are carried out on site and laboratory tests.
For the repair of damaged buildings AAR there are basically a whole series of measures. The most appropriate method to be applied, tested and judged in the context of pilot projects (field tests).
For the monitoring of AAR-damaged buildings, in particular for the temporal evolution of strain and crack width, suitable sensors should be evaluated. For continuous data acquisition to the system of the Swiss Society for Corrosion Protection, has that developed for the detection of corrosion technical parameters are adapted and used
The investigations realised on the two retaining walls lead to the main findings summarized below:
- Air and concrete temperature show the usual seasonal fluctuations. The concrete is usually warmer than the air.
- Water absorption and concrete humidity are crucial parameters for the development of ASR damage. They allow to explain the very different ASR damage intensities between the down- and uphill walls. In summer of 2012 the test area 1 (with water-repellent treatment) had about a 1 to 1.5 mass% lower water content than the untreated test area 2
- For the determination of the water absorption of concrete on-site, mainly for concrete without water-repellent impregnation, the Karsten tubes and the double-chamber measuring cell are not suitable or only under certain conditions. The effect of the impregnation can be better monitored and assessed with a periodic determination of the water absorption coefficient on cores in the laboratory or with continuous measurements of the electrical resistivity of concrete with built-in resistance sensors.
- Periodic deformeter measurements, periodic determinations of the crack width indices or periodic geodetic measurements are only of limited suitability for monitoring the length change and ASR-related expansion of structural elements. The major difficulties are the inadequate temperature compensation and the insufficient precision. The use of these methods requires that a) the expansion rate is rather high or the measurements are realised for a very long period of time and b) the measurements are always taken at the same period of the year. In Switzerland the early morning hours in late summer are suitable.