Alkali-aggregate reactions in concrete : study of the relationship between aggregate petrographic properties versus expansion tests

Abstract

Alkali-Aggregate Reactions (AAR) is a deterioration mechanism in concrete that affects numerous structures worldwide. The most widespread type of AAR is the Alkali-Silica Reactions (ASR), a chemical reaction between silica sensu lato in the aggregates and the alkali hydroxides in the pore solution of concrete. Test methods to assess the potential alkali-reactivity of the aggregates have been under development for decades. The petrographic method shall always be the first step, followed by expansion tests (mortar bars and/or concrete prisms). The petrographic method has proven to be very effective, reliable, and time efficient when performed by experienced petrographers. However, some challenges in its application have been reported on a global scale for specific rock types. This thesis provides suggestions of test methods to be used as supplement to the petrographic method (RILEM-AAR-1, 2003) in order to overcome some of those challenges.

The mineral content of a variety of European aggregate samples was studied applying geology knowledge and techniques. Special focus was given to the characterization of the silica minerals within the aggregates. These results were critically reviewed against expansion results and experience in structures to ascertain the aggregate potential alkali-reactivity. As a result, a better understanding of some characteristics of the silica minerals that influence the potential alkali-reactivity of the aggregates was achieved. These finding were then used to develop methods able to quantify specific properties of the silica minerals that influence the aggregates reactivity under ASR environment. The developed methods are adapted to the characteristics of the aggregates: normally reactive or slowly reactive. The proposed methods are intended to overcome some of the limitations of the traditional petrographic method that have been reported in the literature with specific rock types. Therefore, their utilization may strengthen the petrographic method and improve its value as a tool to assess the potential reactivity of aggregates for concrete.

For slowly reactive aggregates, an image analysis petrographic method for quartz grain size and grain shape characterization was proposed. This method has proven to be more time efficient than the traditional point-counting method, while the results can be more accurate and precise. Not only a much larger number of points can be analysed, typical stereological problems such as the overestimation of the small grains produced by a two-dimensional representation of a rock (thin-section) can be easily and efficiently overcome. The correlation trends found between the grain size descriptors of quartz and expansion results confirm that the reactivity of slowly reactive aggregates is related to the total grain boundary area of quartz, which is strongly influenced by sub-granulation. This method has also great potential to be used in thin-section from concrete structures in the assessment of the structure deterioration. Some inconsistencies between grain size descriptors and expansion data were found for samples with high degree of strain.

Electron backscatter diffraction (EBSD) analysis was applied to characterize the grain boundaries of quartz and investigate its influence on the reactivity of the slowly reactive aggregates for concrete. The initial results suggest that high angle boundaries increase quartz solubility, whereas low angle boundaries seem to have a lesser effect. This method bears a great potential to improve the understanding of the influence of strain in the potential alkali-reactivity of aggregates for concrete, especially when it comes to determining the role of different geometry and origin. The findings in this field can help to overcome the limitation found in the image analysis petrography method discussed above.

For normally reactive aggregates, the use of polished sections instead of the traditional powdered specimens to perform quantitative modal analysis by x-ray diffraction (XRD) was proposed. For fine-grained rock types without preferential orientation this alternative sample preparation has proven to be as accurate and precise as the traditional powdered specimens, while it offers several advantages in concrete petrography. By using polished section to investigate the mineral content by XRD in a number of normally reactive aggregates, it was possible to show that different polymorphs and species of silica have different impacts in the reactivity of normally reactive aggregates.