Overview
Underground structures are central to the transport network and energy production from hydropower. Through the interaction of concrete sulfate-mountain water can cause damage by sulfate attack with a corresponding reduction of the lifetime of underground buildings. Investigations of damage mechanisms in the interaction between concrete and underground water in various Swiss tunnel have shown that a sulfate attack was present in all cases studied. As a preventive measure effective concrete concepts must be used (possibly in combination with waterproofing systems). The basis for this is a reliable and meaningful test of sulphate resistance of concrete. It has been shown that since 2003, enshrined in the SIA 262/1 test method has certain problems. These include unexplained outliers, large variations in collaborative trials and have little to no knowledge of the mechanisms occurring during the test (especially with various additives). This optimised concrete concepts with a large uncertainty about durability are afflicted because of test results. According costly consequential damage cannot be ruled out. Targeted improvements of test sequences and evaluation, which are supported on a matrix of sulphate resistance tests on concrete on the one hand and microstructural studies in combination with X-ray diffraction and thermodynamic modeling on the other hand, are accuracy and reliability of the test results will be improved.
The suitability of concrete concepts against a sulfate attack must be guaranteed. Developing of these concrete concepts requires meaningful test methods. The extensive experience of the year 2003 regarding sulphate resistance showed that there is a need to increase knowledge about the decisive variables in this process. Therefore, the presented project includes the following targets:
- Reducing existing uncertainties
- Testing influences and recognise influences of material compositions
- Detecting the chemical and physical processes during the test
- Testing and edjusting instructions and evaluation, if necessary
- Assessing of the test results for verification
Using microstructural and chemical analyses, the damage occurring during the test mechanisms are analysed and provided in connection to the measurement results. Using thermodynamic modeling, the test are made and then adjusted by varying different parameters (concrete composition, cement chemistry, sulfate solution, temperature), the test matrix is theoretically expanded. This allows a more accurate estimation of the possible in-situ processes.
Adjustment test sequence
In defined concrete mixtures the following parameters are varied:
- Concentration of sulfate solution
- Duration of storage in the sulfate solution (number of cycles)
- Temperature of the sulfate solution
- Drying temperature
The porosity and permeability of the concrete is weighted indirectly in the current format of the test on the sulfate uptake. However, direct comparisons with the effective porosity and permeability are not yet available. Therefore, concrete mixtures with different w / c ratio determines not only the sulfate resistance, but also porosity and permeability (mercury intrusion and oxygen diffusion).
When evaluating the effective aluminate, the porosity and the permeability are considered and weighted. A comparison with the standard referred evaluated measurements shows whether it can be improved by this measure.
Damage mechanisms
In selected concrete mixtures, the microstructure of the test specimens at various stages of examination is analysed by X-ray diffraction and electron microscope. This investigation can on the one hand provide information about chemical zonings within the test specimen, including the composition of the reaction products and, secondly, document the evolution of the damage. Thus, the damage mechanisms in the examination can be derived. Importantly, it is necessary to clarify whether the expansion comes only through the formation of ettringite or possibly also by a crystallisation pressure of sodium sulfate or gypsum into existence. It can be deduced whether the occurring during the test reaction products correspond to the experience in situ. This can be used on the vast experience of the participating institutions with sulfate damage to tunnel concrete.
Additionally, the frequently occurring question of the influence of air-entraining agent in the concrete and a possible alkali-aggregate reaction (AAR) during the test by means of corresponding concrete mixtures and subsequ
Funding
Results
Under this project, SIA 262/1 appendix D was revised in the parts regarding process and evaluation of the sulfate resistance test. As part of this revision, the determination of the measurement uncertainty of the revised test is crucial. Until its implementation, recommendations can be provided to builders, entrepreneurs and testing laboratories.