219-ACS : Alkali Aggregate Reaction in concrete structures: performance testing and appraisal
Technical Committee 219-ACS
Deputy Chair: Dr. Ian SIMS
Activity ending in: 2014
The purpose of this technical committee is to enable engineers to design concrete structures which will not be susceptible to alkali reactions and, in the case of structures that are affected by such reactions, to enable the engineer better to appraise and manage or repair the structure. The programme will include:
(i) developing test methodologies for the reliable accelerated performance testing for susceptibility to alkali reactions of particular concrete mixes (including those containing recycled aggregates) for use in concrete structures,
ii) reviewing computer models of the reaction, in order to prepare practical guidance for their effective and dependable application,
(iii) producing guidance on the appraisal, management and repair of concrete structures that are affected by alkali reactions, and
(iv) to assist in ensuring a long service life of concrete structures it will develop a test methodology for determining the long term contribution of alkalis in certain aggregates to the alkali reaction in the concrete.
Although tests for the reactivity of aggregates have been developed and published by previous RILEM committees (see 5) these do not directly assess the susceptibility to alkali reactions of the particular concrete mixes to be used in a project. In particular, they do not take into account the mitigating effect of supplementary materials such as fly ash or slag, or the specification of low alkali levels in the mix. By using such mitigation measures, a much wider selection of aggregates can be used safely. However, at present there are no reliable performance tests by which the actual concrete mix to be used can be tested. Although such performance tests have been proposed, they have not been validated against actual performance in structures or field tests. In the international network of experts on Alkali Reactions there are now sufficient data to enable such a validation to be undertaken. Therefore this work will propose a performance test or tests and validate its accuracy using this international body of knowledge. This will involve the collection and marshalling of the international data and regional experience, plus undertaking laboratory trials of the proposed performance tests to compare with the field data. In this work it is also intended to assess the effectiveness of such performance tests in testing concrete mixes containing recycled aggregates , including those from structures damaged by ASR. The previously developed AAR tests for aggregates will also be reconsidered, with respect to their effectiveness with recycled aggregates.
Over several decades, many research teams have sought to develop computer models that could be used for predicting the risk and potential effects of AAR in particular circumstances, but only in recent times have the levels of understanding and complex data processing become sufficient for such an approach to be considered practicable and potentially reliable. The proposed committee will review the most advanced models and any trials carried out to calibrate or prove those models, in order to identify candidates for further development. It is intended that guidance will be prepared, to enable practising engineers confidently to use AAR modelling for dependable predictive purposes.
Previous RILEM committees (see 5) have developed guidance on the diagnosis of alkali reactions in concrete structures. The proposed work will go beyond this to develop guidance for the engineer on how to evaluate the present and eventual severity of the alkali reaction, to appraise the effect on the structural capacity and its likely effect on service life and how best to manage and/or repair the structure to achieve the best possible remaining service life.
In general the aggregate is not considered to contribute alkali to the reaction. However, in some structures it has been confirmed that certain types of aggregate have contributed alkalis and this has led to long term deterioration, particularly in massive structures like dams, despite the use of a concrete mix which was low in alkalis. Some sorts of aggregate could potentially contribute very large quantities of reactive alkali to the concrete mix. There is no agreed methodology for assessing the alkalis in aggregates but recent reports to previous RILEM committees (see 5) have shown that there is now a sufficient understanding of the reactions involved for such a test methodology to be developed. It is proposed that such a methodology will be developed and assessed against a selection of vulnerable aggregates in an international inter-laboratory trial.
Terms of reference
To enable the design of structures that are not susceptible to alkali reactions and to maximise the service life of structures that are affected by such reactions, including the:
i) development of and validation of a performance test for concrete mixes resistant to alkali reactions
ii) development of guidance on the practicable use of predictive computer models
iii) development of guidance on the appraisal of structures affected by alkali reactions and their effective management
iv) development and validation of a test methodology for the contribution of alkalis in aggregates to alkali reactions in concrete.
It is estimated that the work of the committee will take 5 years. This is because the alkali reactions are slow, even in the laboratory. The different work items will proceed in tandem.
The membership of the committee will be a mix of materials scientists with expertise in alkali reactions and engineers with knowledge and experience of managing AAR affected structures. The members will be recruited from scientists and engineers known to the proposed chairman and secretary through their international contacts, long involvement in the International Alkali aggregate (ICAAR) conferences (the secretary is a member of the International Organising committee of these) and work on previous RILEM committees. Emphasis will be placed on RILEM members who can contribute data and knowledge, particularly on field experience and engineering management.
Given the international nature of the membership, communication by e-mail and use of the RILEM website will be central to the work of the committee but at least one meeting will be held each year with additional meetings of key project leaders. Detailed minutes of the meetings will be posted on the RILEM website.
The work will include both data collection and inter-laboratory testing and annual reports of progress will be submitted to RILEM.
Detailed working programme
i) Performance Test:
· Collect data on field tests and performance of structures where concrete mixes and materials are accurately known.
· Evaluate different types of accelerated performance tests that could be used.
· Validate preferred test(s) by international inter-laboratory trial using the performance tests to test concrete mixes whose long term behaviour is reliably known from the field data.
· Assess effectiveness of the performance tests for mixes containing recycled aggregates including aggregates from structures damaged by ASR.
· Take the opportunity to consider the effectiveness of the existing RILEM tests for AAR on recycled aggregates.
ii) Prediction Using Computer Models:
· Collate and review the AAR computer models devised and published over the previous 10-year period.
· Establish the principles and parameters that are evidently required for an effective model.
· Critically consider the laboratory and field tests and trials that have been undertaken in an endeavour to calibrate and validate these models.
· Identify a model (or optional models) that appears to be potentially effective and would warrant further development.
· Prepare practical guidance on predictive AAR modelling, for the use of engineers engaged in actual construction.
iii) Guidance on appraisal and management; headings for proposed document(s):
· effects of AAR on material and structural properties
· from diagnosis to appraisal of residual structural capacity
· aid to management of ASR affected structures
· treatment and repair
iv) Alkalis in aggregates
· formulation of test methods based on previous RILEM report
· identification and collection of suspect and reference aggregates
· international inter-laboratory trial of methods using these aggregates
v)Publication and dissemination
· publication of output as RILEM recommendations in Materials and Structures
· amalgamation into RILEM book
· promoting the work of the committee at key International conference such as the ICAAR series.
This work will extend and build on the work of RILEM TC106 and RILEM TC 191-ARP. These committees have been largely concerned with the development of tests for aggregate reactivity although they have also developed an international specification for the avoidance of ASR damage and guidance on the diagnosis of ASR in structures. In their work they have built up a body of expertise and contacts which will be of great assistance to the proposed new committee.
There will also be close ties with the ongoing series of international AAR conferences (ICAAR), which have previously requested RILEM TC 191-ARP to develop an international specification.
There will be close ties also with International standardisation bodies such as ASTM and CEN. CEN are currently evaluating the RILEM aggregate tests as the basis for CEN methods of test.
The direct benefits of the work of the TC will be new internationally agreed RILEM guidance on the appraisal, management and repair of concrete structures which are affected by alkali reactions and new RILEM test methodologies for the project-specific performance testing of concrete mixes for use in concrete structures, to ensure non-susceptibility to alkali reactions and for determining the long term contribution of alkalis in certain aggregates to the alkali reaction in the concrete.
As has been the case with the output from earlier RILEM committees on this subject, it is possible, and even likely, that these methods and guidance will be adopted by international Standards bodies such as CEN.
The intention will be to initially publish these test methods and guidance as RILEM Recommendations in Materials and Structures but then to draw them together into a RILEM book or books.
No specific symposia, training courses or exhibitions are planned, but, if required or approved by RILEM, it would be possible to arrange a meeting to publicise and/or explain the RILEM recommendations.
Group of users
The groups targeted by these Recommendations will be practising engineers, specifiers and testing laboratories.
Specific use of the results
The specific uses of these results will be in managing structures affected by alkali reactions, also in specifying, designing and proving concrete mixes for new structures on a project-specific basis.
The economic impact will be large. Deterioration of concrete structures because of alkali reactions is an increasing problem worldwide. It particularly tends adversely to affect large infrastructure structures such as bridges and dams. Better management of the structures affected by the reaction will enable them to achieve better service life and to continue fulfilling their economic function. Equally, better specification of concrete to avoid the reaction will have major economic benefits. It will allow the use of a wider range of aggregates and by avoiding over-specification will enable the use of more economic mix designs for concrete.