Civil Engineering Association

State-of-the art report: Key parameters influencing the alkali aggregate reaction

PREFACE

This report is one of a series produced as output from PARTNER, a project funded by the European Community under the “Competitive and Sustainable Growth“ programme.

The overall objective of this project is to provide the basis for a unified test procedure for evaluating the alkali reactivity of aggregates across the different European economic and geological regions. It will enable CEN TC-154, Aggregates, to fulfil the requirements of the Aggregates Mandate, M125, which identifies durability against alkalis as a necessary performance characteristic in the specification of aggregates for concrete (EN 12620) to meet the Essential Requirements of the CPD for Strength and Safety. The project will achieve this by:

• Evaluating the tests developed by RILEM, and some regional tests, for their suitability for use with the wide variety of aggregate and geological types found across Europe.
• Calibrating the results of these accelerated tests against behaviour in concrete in real structures and in field sites.
• Producing an “atlas” of the geology and petrography of European aggregates.
• Educating European petrographers and testing organisations in the effective use of these methods.
• Making recommendations, based on the above work, to CEN for suitable CEN methods of test and specifications to ensure durability against alkalis.

The project has 24 Partners from 14 countries, covering most of Europe, from Iceland to Greece.


SUMMARY

As part of the PARTNER project, a State-of-the-art study regarding Alkali Aggregate Reactions (AAR) has been carried out. This topic has been studied for at least 60 years, and many proceedings, books and articles have been written.. Therefore, the purpose of this report is not to write another educational book regarding AAR. The task is, however, to give an updated description of the mechanisms of AAR that can influence the results from the different test methods used in the PARTNER project. Thus, this report mainly focuses on the different key parameters influencing AAR. Evaluation of any structural effects (i.e. effects depending on structure type, dimensions, reinforcement, etc.) is not included in the report.

The ultimate challenge when testing for AAR in a laboratory is to provide quick, reliable results regarding the reactivity of certain types of aggregate, or even more important assessment of specific concrete job mixes (i.e. performance testing). The results are required to mirror the durability behaviour in real structures designed for life time for up to 100 years.

As discussed in the report, many parameters will influence the alkali aggregate reactivity. Some of the parameters will only influence the reactivity in the laboratory, while others will have an overall contribution, both in the laboratory and in real structures. The following key parameters are discussed in the report in relation to AAR:

• Temperature
• Humidity, moisture and degree of saturation
• Content of alkalis
• Role of calcium hydroxide (CH)
• Types and content of reactive rock types
• Aggregate particle size / grading
• Size of test prisms
• Air entrainment, paste porosity and water/cement ratio
• Storage conditions - leaching

The authors have not made any attempt of ranking the influence of the different parameters, because the influence of any changes in a parameter may vary a lot dependent of the situation, both when performing a laboratory test and in a real concrete structure. However, the experience has shown that
in particular any variations in the humidity and/or the alkali content (due to leaching) in the test specimens lead to incorrect results. It is also very important to bear in mind the influence of the different parameters when performing a performance test that should reflect how a given concrete mix will behave in a real concrete structure for a long service life.

Annex A includes a description of the existing national standards and demands for the European countries.

The work was initiated in the beginning of the PARTNER project. However, results from recent research within this project are not included in this report.