There is little knowledge about the relation between AAR-induced damage observed in structures and the expansion potential obtained with accelerated tests. In this study, aggregates used in structures damaged by AAR (alkali-aggregate-reaction) were tested with the microbar test (MBT / AFNOR XP 18-594) and the concrete performance test (CPT / AFNOR P18-454). After the tests, the samples were examined using optical and scanning electron microscopy.
The project has the following objectives:
- Definition of practical criteria to identify AAR and to assess the extent of damages on concrete structures. Development of recommendations to appropriately characterise the condition of structures damaged by AAR.
- Relation between the potential reactivity of the aggregates and the development of damage present in structures.
- Examination of the usability of residual expansion measurements for quantifying the remaining potential reactivity of concrete from structures. Recommendations for carrying out these measurements and for the analysis of the results.
The following scientific methods will be used during the project:
- Physical tests: compressive strength, modulus of elasticity, splitting tensile strength, porosity
- Chemical tests: alkali content and cement content of the concrete
- Microscopy: light and electron microscopy
- AAR test: residual source mass in accordance with LCPC 44
Five structures covering the different regions of Switzerland are selected. First, the state of damage is analysed systematically. This includes a visual assessment and a characterisation of the concrete properties in the laboratory. Furthermore, the residual expansion of concrete cores is measured. These investigations always include an area of the structure with a low and high degree of damage respectively. The effect of expansion on the concrete properties is assessed by bringing together all results. Moreover, the residual core expansion should permit to assess the remaining potential for expansion and damage present in the structure. Based on the gained findings two recommendations are prepared; one covers the steps necessary to characterise the state of damage in structures affected by AAR and the other covers the determination of residual core expansion and its analysis.
Based on the results, the significance of the microbar test has to be examined very critically. The agreement of measured expansion, reacted rock types and the composition of the reaction products between the on-site concrete and the reproduced concrete subjected to the CPT clearly indicates that the reaction mechanisms in the structure and in the concrete performance test are comparable. As such, the concrete performance test seems to be an appropriate tool to test the potential reactivity of specific concrete mixtures.
Andreas Leemann, Christine Merz: An attempt to validate the ultra-accelerated microbar and the concrete performance test with the degree of AAR-induced damage observed in concrete structures. CEMENT AND CONCRETE RESEARCH, JULY 2013