Subject matter

The main objectives of the TC are presented hereafter:

• Report on current strategies to mitigate alkali-aggregate reaction (AAR) induced development in affected concrete infrastructure.
• Describe and evaluate alternative/non-traditional materials and techniques to mitigate AAR-induced development in new and affected aging structures.
• Appraise the behaviour and efficiency of alternative/non-traditional materials and techniques to mitigate AAR-induced development in new and affected aging structures through accelerated test procedures in the laboratory.

Terms of reference

• The TC will run for 5 years;
• Members will be primarily from universities and research institutions, although members from industry with expertise not only in AAR, but rather in strategies to mitigate AAR in affected aging infrastructure are expected and welcome;
• The work will include an extensive literature review on current strategies used to mitigate AAR-induced development in the field along with alternative/non-traditional materials and techniques with potential to be used in new and affected structures. Moreover, accelerated test procedures will be used/adapted to assess the performance of the proposed non-traditional/alternative materials in the laboratory.
• It is worth noting that by the definition of the word “mitigation” (i.e. action of reducing the severity), this TC aims to study materials and products that will only start acting once AAR is triggered; the latter means that a) these products are not intended for AAR prevention and, b) depending on the environmental conditions, these products will act sooner or later, besides acting differently on structural members displaying distinct micro-climates or damage degrees.
• The TC work will be relevant to the entire construction sector yet especially for critical concrete infrastructure such as nuclear power plants, dams, bridges, roads, etc.

Detailed working programme

Alkali-aggregate reaction (AAR) is one of the most harmful distress mechanisms affecting the durability and serviceability of aging structures worldwide. AAR is a chemical reaction between the alkali hydroxides from the concrete pore solution and some unstable mineral phases from the aggregates used to make concrete. AAR generates a reaction product, the so-called AAR-gel that swells upon moisture uptake, leading to major cracking and reducing the mechanical properties of the affected material. AAR is normally divided into two mechanisms: alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR); ASR is by far the most frequent mechanism observed around the globe. Normally, AAR-affected structures start displaying signs of deterioration after 5 to 25 years; the latter depends upon the reactivity of the aggregates and amount of alkalis used in the concrete mixture, along with some environmental conditions (i.e. moisture, temperature, etc.) and features of the affected structure (i.e. type, confinement effect, amount of reinforcement, etc.).
Several approaches, recommendations, and test procedures, have been developed to assess the potential alkali-reactivity of concrete aggregates and the efficiency of preventive measures (e.g. control of the cement & concrete alkali content, use of supplementary cementing materials (SCMs), use of lithium based admixtures, etc.). Despite a few issues with some of these test procedures (e.g. long-term duration, alkali leaching, long-term correlation with field structures, evaluation of job concrete mixtures, etc.) and the constant need of improvement in the different standards/protocols, the majority of experts agree that, in general, it is now possible to build new concrete infrastructure with limited risk of AAR. However, there is currently no consensus about the most efficient method(s) (e.g. surface treatments for moisture control, chemical treatments, strengthening, stress relief, etc.) that should be implemented, and when, for the rehabilitation of AAR-affected concrete structures/structural members. This situation is extremely critical for some structures whose risk associated to AAR development are extremely
high since they cannot be easily replaced nor rehabilitated such as dams, nuclear power plants, tunnels, bridges, etc. Moreover, although major structural failure has not been observed in AAR-affected structures, important structural implications might take place and were already reported in literature such as major deflections of structural components, yielding of the rebars and important spalling of concrete cover.

As per the above discussion, one may distinguish AAR mitigation into two distinct, yet related streams:
Stream A:
o AAR mitigation in fresh concrete: This stream deals with the use of alternative/non-conventional materials and techniques to mitigate AAR-induced expansion and damage in new concrete. These products shall start acting after AAR is triggered (i.e. occurrence of the first deterioration signs). Some examples of products are self-healing materials, fibres, etc. Therefore, it is expected that these materials/products may be activated at distinct time periods in different locations/members of the structure as per their micro-climates and damage degrees.
Stream B:
o Mitigation in hardened concrete: This stream aims to study alternative/non-conventional materials and techniques to decrease AAR-induced expansion and deterioration rate in affected aging structures. Some of the possible examples of products are waterproofs, cracks injection, slot cuttings and lithium-based admixtures.

It is widely accepted that numerous intrinsic features of concrete may change the time period required for AAR to be triggered in the field such as a) type (i.e. fine vs coarse) and nature (i.e. lithotype) of the reactive aggregate, b) water exposure and humidity, and, c) alkali loading of the concrete mixture. Furthermore, some characteristics of the structure itself may also play an important role on AAR-induced development such as geometry, confinement/reinforcement, macro/micro-climates, etc. Nevertheless, AAR often presents a slow induced development, which makes the understanding of its prognosis (i.e. induced development over time) complicated upon detection..
Several studies have been conducted over the last few decades and much has been accomplished on preventing AAR in new structures along with diagnosing (detection of cause and degree of damage) affected concrete. Yet, much less is known and have been done on the prognosis (i.e. evolution of the induced deterioration over time) and rehabilitation of affected structures. In this regard, analytical and numerical chemo-mechanical models have been established in the micro and macro scales to predict the further behaviour of affected infrastructure, which showed to be more or less accurate depending on the type of structure evaluated and approach selected. However,, very few novel research and developments have been achieved in mitigating AAR in the field. Therefore, it is extremely important to gather further information on the current knowledge (i.e. state-of-the-art) in the area along with developing alternative/non-conventional materials and solutions to lessen AAR impact in critical concrete infrastructure.

Work plan: The works to be conducted in the TC will cover the two streams aforementioned, related to AAR mitigation in fresh and hardened concrete, with the use of novel, non-traditional/alternative products. In order to cover these aspects, four different Work Package Groups (WPG) are considered as follows:
o WP1 – Strategies to mitigate AAR in affected concrete structures
- Extensive literature review on current rehabilitation strategies to mitigate AAR-affected concrete structures, considering their technical principles, difficulties and limitations to be applied to different types of structures, efficiency in reducing AAR-induced expansion and damage along with cost.;
- Discussion on the potential use of alternative/non-conventional materials and techniques to be applied in the field to mitigate (i.e. decrease the expansion rate) AAR-induced development.
o WP2 – Non-traditional mitigation materials/products in fresh and hardened concrete
- Evaluate (i.e. efficiency, availability, cost, experience in use, etc.) the use of alternative/non-conventional products to mitigate AAR in the field such as nano-particles, organic or inorganic encapsulated products, etc.;
- Summary of the most promising/established alternative materials used to mitigate AAR in new and existing structures, with careful attention to their efficiency, dosage requirement, influence on the properties of the material (compressive, tensile and flexural strength, heat of hydration, workability, porosity, setting time, etc.).
o WP3 – Accelerated test procedures to evaluate non-traditional mitigation products in fresh and hardened concrete
- Assessment of established RILEM accelerated test procedures to assess the efficiency of alternative products to mitigate ASR-induced development in the laboratory.
- Discussion and adaptation of stablished techniques to provide to assess alternative materials;
- The non-traditional mitigation materials used in the experimental tests will be proposed by WP2, in coordination with WP3.
o WP4 – Coordination between WPs to produce a STAR and papers on Mitigation of AAR
- The main objective of this TC is to produce a STAR with the current knowledge and experience on AAR mitigation.
- The WP1, 2 and 3 will conduct literature reviews and collect information that will be included in the STAR.
- To avoid overlapping or lack of information to be included in the STAR, all the information related to the literature review will be coordinated by WP4.
- The commercial information should be analysed before to include in the conclusions inSTAR. I will be only included whether considered useful and relevant information.
- The WP3 in coordination with WP2 will produce papers to be published at M&S with the experimental results obtained by the laboratory tests.

Structure and responsible of the WPs: The structure and the links between WPs is define then:
o WP1 – Strategies to mitigate AAR in affected concrete structures
- Leader: Luis Oliveira (Portugal)
o WP2 – Non-traditional mitigation products in fresh concrete mixtures:
- Leader: Medhat Shehata (Canada)
o WP3 – Test procedures and strategies to evaluate non-traditional mitigation products in fresh concrete
- Leader: Leandro Sanchez (Canada)
o WP4 – Coordination between WPs to produce a STAR and papers on Mitigation of Alkali-Silica Reaction
- Leader: Esperanza Menéndez (Spain) and Antonio Santos Silva (Portugal)

It is anticipated to have co-leaders in all WPs. Time-zones will be considered for the selection of leaders to facilitate communication. Figure 1 illustrates the organization of the WPs and the coordination between them.

Schedule: If the TC is approved by the TAC while the Sheffield meeting, the schedule of activities will be:

• December 2020: Online meeting with the leaders of the WPs to define the main lines and strategies of the WPs. Information to all the TC members.
• March 2021: Kick-off meeting in Paris, joint with the celebration of 75th Anniversary of RILEM.
• Two general meetings are anticipated per year. One presential, jointly with the RILEM Spring meeting or RILEM Week and the second online.

Finally, online meetings will also be organized with small groups (e.g. WP members) of TC-members.