Quantifying Alkali-Silica Reaction in Concrete: Damage Rating IndexDan Cukierski1

1Laboratory Manager, Duratec Australia

Abstract : Damage Rating Index (DRI) is a petrographic-based technique which assesses the level of deterioration from Alkali-Silica Reaction (ASR) in concrete. Unlike standard concrete petrography (AST M C856), DRI results in a numeric value which directly correlates with the amount of damage present. The greater the DRI value, the higher the degree of ASR. These values categorize the degree of ASR from non e to very serious. This paper gives a general overview of DRI and the methodology behind the analysis . Comments are also made on the use of the technique to aid in the service life prediction of concrete structures.

Keywords: ASR, DRI, Petrography, Testing, Service Life

1. Alkali-Silica Reaction

1.1 ASR MechanismASR is an expansive chemical reaction that can occur in hardened concrete between specific forms of silica in the aggregate fraction and the alkali hydroxides (specifically sodium and potassium) in th e pore solution . For ASR to progress, a total of three factors must be present (1):

Sufficient moisture in the pore structure of the concrete (not less than 85% relative humidity) Reactive form or forms of silica in the aggregate in significant concentration High alkalinity in the pore solution adjacent to a reactive aggregate particle

W hen the above criteria are met, ASR will initiate . The product of this chemical reaction is a hydrated silica gel which can be expansive. As more gel forms, the expansive forces can exceed the tensile strength of the concrete, resulting in cracking. This reaction will continue to progress until one of the criteria above is exhausted. Note that ASR can initiate in concrete without causing significant deterioration.

1.2 ASR Diagnosis and PrognosisWhen ASR has initiated and caused deterioration, diagnosing the mechanism is crucial to extend the service life of the structure . ASR results in characteristic deposits , textures and crack patterns that can be identified by an experienced petrographer following ASTM C856 (Standard Practice for Petrographic Examination of Hardened Concrete). These diagnostic features include but are not limited to (2):

Radial cracking initiating from reactive aggregate particles ASR gel deposits in air voids and/or cracks/microcracks Cracking/microcracking that extends through aggregate particles ASR gel-soaked cement paste adjacent to reactive aggregate particles

The prognosis of the mechanism relies on the constituents of the cement paste. The constituents of importance in this case include cement type, cement conte nt, absence/presence of supplementary cementitious materials (SCMs), SCM content, portlandite content/distribution and carbonation. Based on these constituents, an experienced petrographer can comment on the likelihood of ASR to continue or if the reaction has likely reached completion. Although concrete petrography is a vital tool in identifying ASR and other deterioration mechanisms, the method itself does not quantify the level of deterioration.

2. ASR Quantification - Damage Rating Index (DRI)

2.1 DRI BackgroundDRI was developed as a method to quantify ASR-related deterioration in hardened concrete . It was first published by Dunbar and Grattan-Bellow in 1995 ( 3 ) and was a breakthrough in the remediation space. Used exclusively for dams, the practice was for DRI to be conducted on samples from different areas of the same structure. The resulting values were compared to determine which sections were in need of urgent

repair compared to other areas. This allowed for customized remediation strategies that targeted only the most problem areas , leaving the less impacted areas undisturbed which saved on costs and time . The use of DRI has grown extensively throughout North America and Europe since its inception, and the method is now utilized to assess a wide variety of structures encompassing a plethora of climates and exposure conditions.

2.2 DRI MethodologyThe original DRI method by Dunbar and Grattan-Bellow was not published as a standard test method, and still there is no published test standard for DRI. As a result of this , the original method has been amended by numerous petrographers and researchers. Though different methods for DRI exists, t he general methodology for the majority of the DRI methods is as follows:

A sample of concrete (typically a core) is saw-cut down its long axis One of the resulting core halves is lapped (semi-polished) to yield a clean, reflective surface A grid of 1 x 1 cm squares is drawn on or laid over the lapped surface (Figure 1) At least 100 of the 1 x 1 cm squares are examined

Figure 1. Lapped core half with an overlain grid of 1 x 1 cm squares.DRI analysis is carried out using a stereomicroscope at ~15x magnification. The sample is evaluated on a square-by-square basis , ensuring a thorough investigation of the entire lapped concrete sample. When a square is in the field of view down the microscope , notable features are identified and their frequency is counted . Each notable feature has a corresponding multiplier value assigned to it. The number of specific notable features counted in each square is multiplied by its corresponding multiplier value. This is done on at least 100 squares. The results are summed to give a DRI value which corresponds to a degree of ASR.

The DRI method used determine s what features are to be counted and what the corresponding multipliers are. For the sake of this paper, I have used the criteria given by Shrimer, 2019 (4). The notable features and corresponding DRI multipliers are presented in Table 1 and the DRI value and degree of ASR are presented in Table 2.

Table 1. ASR Features and DRI Multipliers (4)

Notable Feature DRI Multiplier Figure

Crack in Aggregate 0.25 N/ACrack in Aggregate with ASR Gel 2 1 and 2

Debonded Aggregate 3 N/AReaction Rim 0.5 1

ASR Gel in Air Void 2 1Crack in Matrix 0.5 N/A

Crack in Matrix with ASR Gel 4 2Corroded Aggregate 3 N/A

Table 2. Classification of Degree of ASR (4)

DRI Degree of ASR

0-40 Negligible40-125 Minor

125-300 Moderate300-500 Significant500-650 Serious

>650 Very Serious

Figure 1. Stereomicroscope image highlighted selected notable features.

ASR Gel in Air Void

Crack in Aggregatewith ASR Gel

Reaction Rim

Figure 2. Stereomicroscope image highlighted selected notable features.

2.3 DRI in PracticeWhen conducting a durability assessment or residual life analysis, understanding which concrete elements a re the most deteriorat ed is crucial. Standard laboratory testing for th is type of investigation include s compressive strength, chloride content and carbonation. A test method like DRI (and ASTM C856) would be a valuable addition to this testing suite.

DRI is most effective when carried out over several concrete elements in the same structur e. The m ix design, exposure conditions and age all have an impact on ASR which will affect the resulting DRI . Ensuring that various concretes in the structure have been samples and analysed is critical to obtaining a representative dataset . The resulting DRI from the different concrete elements can then be compared to assess which concretes and which exposure conditions have resulted in the highest degree of ASR . It has been proposed that DRI be carried out as a routine investigation of certain structures to track the progression of ASR over time (5) . While ASTM C856 is used to comment on future ASR potential , regular DRI analysis of the same structure can give quantitative data for the progression of ASR . This would allow for ASR-related deterioration to be modelled in a similar fashion to carbonation or chloride ingress . Using this approach, DRI is a valued methodology for predicting service life of structures that are impacted by ASR.

3. Ongoing Work in DRI

Crack in Aggregatewith ASR Gel

Crack in Matrix with ASR Gel

3.1 DRI ReproducibilityVilleneuve et al (6) conducted a study in 20 08 with 20 petrographers from North America and Europe. The goal of the study was to determine the variability of DRI between different petrographers and to develop methods where the DRI was more reproducible. In the study , petrographers analysed the same sample using the original DRI method published by Dunbar and Grattan-Bellow in 1995. The resulting DRI were used to calculate a coefficient of variation (C of V). The higher the C of V, the more widespread the results. The study used four concrete samples, and all four were analysed by 9 of the 20 petrographers. The C of V between the 9 petrographers using the original DRI method was as low as 22% for one sample was as high as 54% for another . After this finding a new DRI method was proposed (Table 3), and the same samples were tested using the proposed method. The C of V using the proposed method was as low as 23% for one sample and as high as 35% for another . An important note here is the sample with a 54% C of V is the same sample that had a C of V of 35% using the modified method, a significant reduction in variability . This is an important step in working towards standardization of a DRI method.

Table 3. ASR Features and DRI Multipliers – Proposed Method (6).

Notable Feature DRI Multiplier

Closed/Tight Cracks in CoarseAggregate Particle 0.25

Opened Cracks or Network Cracksin Coarse Aggregate Particle 2

Cracks or Network Cracks withReaction Product in Coarse

Aggregate Particle2

Coarse Aggregate Debonded 3Disaggregated/Corroded

Aggregate Particle 2

Cracks in Cement Paste 3Cracks with Rection Product in

Cement Paste 3

3.2 DRI For Other MechanismsSanchez et al (7) conducted a study of laboratory made concrete and used DRI to assess for deterioration from mechanisms that cause internal swelling reactions (ISR). Laboratory specimens made with reactive aggregates were subjected to environments to initiate ASR, delayed ettringite formation (DEF) and /or deterioration from cyclic freezing and thawing (FT) . The length of the laboratory samples was recorded before and after exposure to determine the level of expansion that had occurred. DRI was conducted on the samples after exposure. It should be noted that the criteria listed in Table 3 were used in this study.

ASR, DEF and F T all have unique crack patterns, textures and characteristics which can be identified by ASTM C856. However, as noted above, the method cannot quantify deterioration from these methods. The results of the study by Sanchez et al show that DRI can be used to not only quantify deterioration by ISR but the number of features observed can be used to distinguish which ISR is occurring . This is a critical study whereby DRI has been shown to assess concrete for mechanisms other than DRI. Given that there is no standard test method, DRI could even be customizable for different deterioration mechanisms.

4. ConclusionsCurrent DRI methods are suitable to evaluate hardened concrete to quantify the degree of ASR in existing structure s . Methods proposed by Shrimer and Villen e uve , though different, are both suitable to determine the extent of deterioration. Existing structures can be analysed by the same method and same operator over their service life to model deterioration due to ASR, aiding in assessing the residual life of the structure.

The next step with DRI is to develop a method that is reproduceable between operators and have an international test standard published. More case studies need to be completed and collaboration is key in attaining this milestone.

4. AcknowledgementI would like to acknowledge Duratec and the Technical Team for giving me the opportunity to contribute to Concrete 202 1 and their commitment to offering petrographic services to the industry . Thank you to our clients and the concrete community for th eir support and trust, we couldn’t do it without you . Sarah and Theo , thank you for putting up with the incessant talk about petrography, concrete and aggregate.

5. References

1. Poole, A ., and Sims, I., “Concrete Petrography – A Handbook of Investigative Techniques ” , CRC Press, 2016.

2. Cukierski, D., “Utilizing Petrography to Identify Deterioration Mechanisms in Hardene d Concrete”, Proceedings of the Concrete Institute of Australia’s Biennial Conference , 8-11 September 2019, Sydney, Australia.

3. Dunbar, P. and Grattan-Bellew, P., “Results of Damage Rating Evaluation of Condition of Concrete from a Number of Structures Affected by AAR,” Proceedings of the CANMET/ACI Workshop on Alkali-Aggregate Reactions in Concrete, Dartmouth, NS, 1995, pp. 257-265.

4. Shrimer, F., “Use of the Damage Rating Index as Input for Service Life Prediction in Alkali-Silica Reaction Affected Concrete,” Advances in Cement Analysis and Concrete Petrography,ASTM STP1613, D. Cong and D. Broton, Eds., ASTM International, West Conshohocken, PA,2019, pp. 89–104.

5. Shrimer, F., “The Damage Rating Index: Assessing the Severity of Alkali-Aggregate Reaction in Concrete Dams,” Proceedings of the USSD Annual Conference, 2015, Louisville, USA.

6. Villeneuve, V., Fournier, B. et al., “Determination of The Damage in Concrete Affected by ASR – The Damage Rating Index (DRI)”, Proceedings of the 14 th International Conference on Alkali- Aggregate Reaction (ICAAR), 2012, Austin, USA.

7. Sanchez, L., Drimalas, t. et al., “Assessing Condition of Concrete Affe c ted by Internal Swelling Reactions (ISR) through the Damage Rating Index (DRI)”, Cement 1-2, 2020.