Alkali-silica reactivity criteria for concrete aggregates

Transcription

1 Available online at Materials and Structures 38 (April 2005) Alkali-silica reactivity criteria for concrete aggregates M. Berra 1, T. Mangialardi 2 and A. E. Paolini 2 (1) CESI S.p.A., Milan, Italy (2) Facoltà di Ingegneria, Università di Roma La Sapienza, Rome, Italy Received: 17 October 2003; accepted: 13 July 2004 ABSTRACT In this study, the suitability of the threshold alkali level, TAL, the kinetic parameter, ln k, and the microstructural disorder coefficient, Cd, of the aggregates, taken as alkali-silica reactivity criteria, was assessed using different typologies of Italian natural ASR-susceptible aggregates of known field performance. The TAL, ln k, and Cd were determined using a modified version of the RILEM AAR-3 concrete prism expansion test, the ASTM C1260 mortar-bar expansion test, and the infrared spectroscopy test, respectively. It was found that the three reactivity criteria are all appropriate for discriminating between reactive and innocuous aggregates. However, the TAL proves to be a more suitable criterion for interpreting the field performance data of the aggregates investigated. There exists a linear relationship between TAL and ln k, or between TAL and Cd, which provides a rapid means of estimating the threshold alkali levels of ASR-susceptible aggregates from the results of the ultra-accelerated mortar-bar test and/or the infrared spectroscopy test. A TAL-based classification of the degree of reactivity of the aggregates, as well as some modifications of the reactivity domains established by the infrared spectroscopy method are also proposed RILEM. All rights reserved. RÉSUMÉ Dans cette étude on a vérifié la fiabilité du Niveau de Seuil des Alcalis, NSA, du paramètre cinétique, ln k, et du coefficient de désordre microstructurel, Cd, comme critères de réactivité alcalis-silice (A-S), en utilisant de différentes typologies de granulats naturels d origine italienne susceptibles de manifester cette réactivité et dont le comportement en service à long terme était bien connu. Les paramètres de réactivité NSA, ln k et Cd ont été déterminés respectivement au moyen de l essai d expansion sur des éprouvettes de béton RILEM AAR-3 dûment modifié, de l essai d expansion sur des éprouvettes de mortier ASTM C1260 et de l essai de spectrographie infrarouge. Les résultats montrent que tous les trois critères sont bien appropriés pour distinguer les granulats réactifs de ces non réactifs. Toutefois, le paramètre NSA se révèle le plus approprié pour évaluer les donnés de comportement en service des granulats examinés. Il existe une relation linéaire entre NSA et ln k, ou entre NSA et Cd, en mesure de fournir un moyen rapide pour estimer le Niveau de Seuil des Alcalis des granulats susceptibles de réaction A-S à partir des résultats de l essai ultra-accéléré sur des éprouvettes de mortier et/ou de la spectrographie infrarouge. On a aussi proposé un classement du niveau de réactivité des granulats basé sur le paramètre NSA et un changement des critères de réactivité établis par la méthode de spectrographie infrarouge. 1. INTRODUCTION The use of aggregates susceptible of alkali-silica reaction (ASR) in concrete may cause a severe degradation of the concrete structures, especially if these structures are exposed to environments (high relative humidity, deicing salts) that will greatly promote the development of deleterious expansion associated to ASR. Under established exposure conditions, the severity of degradation is strongly related to the alkali-reactivity of the aggregates used for the specific structure, the alkali content of the concrete (kg Na 2 Oeq/m 3 ), and the size of the concrete structure [1]. These parameters also greatly affect the minimum dose level of supplementary cementing material (fly ash, silica fume, natural pozzolan, blast furnace slag) needed to prevent deleterious ASR expansion (preventive measure). Thus, the preliminary knowledge of the degree of alkali-reactivity of the aggregates is essential to design concrete formulations suitable to guarantee the required service life of a specified structure. Editorial Note Università degli Studi di Roma La Sapienza (Italy) is a RILEM Titular Member. Mr. Mario Berra is a RILEM Senior Member. He participates in the work of RILEM TC 191-ARP Alkali-reactivity and prevention - Assessment, specification and diagnosis of alkali-reactivity RILEM. All rights reserved. doi: /14144

2 374 M. Berra et al. / Materials and Structures 38 (2005) At present, the most popular test methods for the assessment of the alkali-reactivity potential of ASRsusceptible aggregates appear to be the concrete prism expansion test at 38 C and 100% relative humidity (RH), currently regarded as a reference test, and the mortar-bar expansion test in a 1M NaOH solution at 80 C (ultraaccelerated mortar-bar test). The first method in various but similar versions has been standardised by the Canadian Standards Association (CSA A A) [2], the American Society for Testing and Materials (ASTM C1293) [3], and the RILEM (AAR-3) [4], as well as by the national standardisation bodies of other countries. The second method is available as CSA A A [5], ASTM C1260 [6], RILEM AAR-2 [7], and under other local designations. However, both test methods show some shortcomings. The concrete prism expansion test requires a lengthy period, up to 12 months or more, for reliable results to be obtained and it has also the disadvantage of running at a fixed alkali content of the concrete (5.25 kg Na 2 Oeq/m 3 in the ASTM and CSA versions, and 5.5 kg Na 2 Oeq/m 3 in the RILEM version). As a result, some slowly alkali-reactive siliceous aggregates may be classified as innocuous by this test method [8]. On the other hand, the ultra-accelerated mortar-bar test is considered to be a severe screening test due to the severity of the test conditions but it has the advantage of getting a quick answer (test duration = 16 days) as compared to the concrete prism test. Recently, Johnston and Fournier [9] proposed a kineticbased method for interpreting ASTM C1260 mortar-bar test results that appears capable of overcoming some of the shortcomings currently encountered using a percent expansion criterion for defining potentially alkali-reactive aggregates (overestimate of the reactivity of some aggregates or, at the other extreme, underestimate of the reactivity of certain aggregates due to insufficient expansion). Correlation of expansion data with a kinetic equation provides a kinetic parameter, ln k, assumed as a reactivity criterion, that is capable of differentiating between reactive and innocuous aggregates regardless of the expansion values obtained at established times (currently, 14 days) of immersion of mortar bars in NaOH solution. In our previous papers [8, 10, 11], it was found that the threshold alkali level (TAL) of the aggregate, defined as the minimum alkali level of the concrete enough to promote the deleterious expansivity of the specified aggregate, appears to be a good reactivity criterion for the selection of ASRsusceptible aggregates. It was also shown [8] that the TAL value for a given aggregate may be determined from the results of concrete prism expansion tests at 38 C and 100% RH, when these tests are performed at varying alkali level of the concrete. Alternatively, the TAL value could be determined using the ultra-accelerated concrete prism expansion test in alkaline solutions at 150 C, and testing concrete mixes at varying alkali content [10]. However, this test method (test duration = 22 days) requires further research work for its validation. A TALbased testing protocol for greywacke aggregates has also been proposed in the UK a few years ago [12]. Among the reactivity criteria derived from a direct analysis of ASR-susceptible aggregates, the criterion proposed by Bachiorrini [13] and referred to as the microstructural disorder coefficient, Cd, of the silica lattice of the aggregate appears to be very promising. This reactivity parameter is derived from the results of Infrared spectroscopy analysis of the aggregate. A relationship between the coefficient Cd and the alkalireactivity of the aggregates as assessed by field service records has also been reported by Bachiorrini et al. [14]. In the present study, the suitability of the threshold alkali level, TAL, the kinetic parameter, ln k, and the microstructural disorder coefficient, Cd, of the aggregates, taken as alkali-silica reactivity criteria, was assessed using different typologies of Italian natural ASR-susceptible aggregates of known field performance. The TAL, ln k, and Cd were determined using a modified version of the RILEM AAR-3 concrete prism expansion test, the ASTM C1260 mortar-bar expansion test, and the infrared spectroscopy test, respectively. A correlation between TAL and ln k or between TAL and Cd was attempted in view of a possible use of the ultra-accelerated mortar-bar test and/or the infrared spectroscopy test as a rapid means of estimating the threshold alkali levels of ASR-susceptible aggregates. A classification of the degree of reactivity of such aggregates based on their TAL values was also attempted. 2. MATERIALS AND METHODS Thirteen natural ASR-susceptible aggregates, designated by letters A to O, were tested in this study. All aggregates came from Italian quarries and were available both as coarse and as fine aggregate. As evidenced by the RILEM AAR-1 petrographical examination [15], aggregates A to G were composed primarily of carbonate rocks impure with argillaceous and fossiliferous components, and accompanied by varying amounts (not greater than 10% by mass) of fine-grained flint and calcite, and minor quantities of chalcedony, quartzites, gneisses, strained mono- and polycrystalline quartz. Aggregates H to N essentially consisted of quartzitic rocks containing strained quartz at varying undulatory extinction angles, accompanied by minor amounts of limestone, flint, and metamorphic rocks. Aggregate O was predominantly a rhyolitic rock consisting of quartz, moderately to highly-altered feldspar, chlorite, and biotite. Strained quartz, micro-crystalline quartz and fragments of calcite and limestone were also present. According to the classification of the aggregates given in the RILEM AAR-1 test method, all of the aggregates tested were designated Class II-S (aggregates containing particulate constituents judged to be potentially alkali-silica reactive). However, on the basis of past field experiences, aggregate O could also be designated Class III-S (very likely to be alkalisilica reactive), whilst aggregates A, B, M, and N could be assigned to Class I (very unlikely to be alkali-reactive). A low-alkali Portland cement (CEM I 42.5; Na 2 Oeq = 0.59%; MgO =1.65%; Blaine specific surface area =400 m 2 /kg) and a high-alkali Portland cement (CEM I 52.5; Na 2 Oeq = 1.15%; MgO =1.30%; Blaine specific surface area =500 m 2 /kg) were used for the concrete prism and mortar-bar expansion tests, respectively. The microstructural disorder coefficient, Cd, was determined on samples of test aggregate, preliminarily ground to particle sizes less than 40 m. KBr pellets containing 2 wt.% of ground aggregate were made and analysed using an

3 infrared spectrophotometer. From the IR spectra the value of Cd (cm -1 ) was calculated as Cd = /A b (1) where is the broadening estimated at 1/3 of the height of 1 band of SiO 4 groups (taken as the analytical band), that absorbs in the region of wave numbers from 830 to 700 cm -1, and A b is the relative optical density. The value of A b was calculated as A b = log [(P+Z)/Z] (2) M. Berra et al. / Materials and Structures 38 (2005) where P is the height of 1 band and Z is the difference between the minimum transmittance of 1 and 3 bands, the latter absorbing at wave numbers of about 1100 cm -1 and being taken as zero value of transmittance. Further details on this test method are available in the original paper of Bachiorrini [13]. The coefficient of variation for Cd measurements on five replicate specimens was always less than 5%. Mortar samples for expansion tests were prepared using the test aggregate, high-alkali cement, and deionised water as mixing water. According to the ASTM C1260 test procedure, the coarse fraction of the quarried aggregate was first crushed to the fine aggregates sizes and then combined with the fine aggregate. The grading of the test aggregate ( mm), as well as the mix proportioning (aggregate to cement weight ratio = 2.25; water to cement weight ratio = 0.47) and the curing of the prismatic specimens (25 x 25x 285 mm) in 1M NaOH solution at 80 C were made in accordance with the ASTM C1260 test procedure. The coefficient of variation for expansion measurements on three replicate specimens was always less than 5% when the average percent expansion (E%) exceeded 0.10%. Concrete mixes for expansion tests were prepared using both the fine and coarse test aggregate, low-alkali Portland cement, and deionised water as mixing water. The grading of the combined aggregate (0-20 mm) and the concrete mix proportions (water to cement weight ratio = 0.455; coarse aggregate: fine aggregate: cement = 2.83: 1.22: 1 by mass; cement content = 440 kg /m 3 ) were the same as those specified in the RILEM AAR-3 concrete prism test. The only modification of this test consisted of varying the alkali content of the concrete mix through appropriate additions of reagent-grade NaOH pellets to the mixing water in order to achieve different alkali levels over the range from 2.6 (no added alkali) to 10.0 kg Na 2 Oeq/m 3. The preparation of concrete prisms (75 x 75 x 250 mm) as well as their storage at 38 C and 100% RH, and expansion measurements were made in accordance with the RILEM AAR-3 test procedure. The coefficient of variation for expansion measurements on three replicate specimens was less than 7% when the average percent expansion exceeded 0.05 %. 3. RESULTS AND DISCUSSION 3.1 Mortar-bar expansion tests Figs. 1a and 1b show the ASTM C1260 test results up to 56 days of immersion of mortar bars in 1 M NaOH solution at 80 C. Fig. 1 - Results of ASTM C1260 mortar-bar tests. Table 1 compares the alkali-reactivity status of the test aggregates (NR: non-reactive; R: reactive), as assessed by field service records, with the diagnoses of reactivity obtained when the results of the ASTM C1260 test were interpreted on the basis of the expansion level of mortar bars after 14 days of immersion in NaOH solution (E 14 %), by using the tentative criteria suggested by the ASTM C1260 (E 14 <0.10%: innocuous; 0.10% E %: not conclusive results; E 14 >0.20%: potentially deleterious) [6] or by the RILEM TC 191-ARP Outline Guide (E 14 <0.10%: non-expansive; 0.10% E %: potentially reactive; E 14 >0.20%: expansive) [16]. Also reported in this Table are the diagnoses of reactivity obtained using the expansion

4 376 M. Berra et al. / Materials and Structures 38 (2005) Table 1 - Field performance data and diagnoses of reactivity of the aggregates tested with the ultra-accelerated mortar-bar method Aggregate Field performance Ultra-accelerated mortar-bar test Interpretative guidance ASTM RILEM Hooton and Rogers A NR*** NR NR NR B NR*** NR NR NR C R** PR R PR D R* PR R PR E R* PR R PR F R* PR R PR G R* PR R PR H R* PR R PR I R* PR R PR L R** PR R PR M NR*** Not PR NR conclusive N NR*** Not PR NR conclusive O R*** PR R PR NR: non-reactive; R: reactive; PR: potentially reactive * In some cases; ** In most cases; *** In all cases limits (E% = 0.15%, 0.33%, and 0.48% at 14, 28, and 56 days, respectively) proposed by Hooton and Rogers [17] for immersion times of mortar bars in NaOH solution up to 56 days (dashed curves in Figs. 1a and 1b). This approach to interpreting the results of Figs 1a and 1b was validated by the fact that the high-alkali Portland cement used in the present study for the ultra-accelerated mortar-bar test also conformed to the requirements (alkali content, Blaine specific surface area) specified in the RILEM version of the mortar-bar test (RILEM AAR-2). Thus, apart from the difference in the way of testing the coarse and fine fraction of quarried aggregate (in the RILEM AAR-2 test, the two fractions of quarried aggregate shall be tested separately), the ASTM and RILEM versions of the mortar-bar test only differed somewhat in the grading of the test aggregate. In order to unify the terminology in the presentation (Table 1) and discussion of the reactivity diagnoses obtained with the various interpretative criteria, the term non-reactive was used in place of innocuous or non-expansive, and the term reactive in place of deleterious or expansive. The data in Table 1 revealed that there were discrepancies between the various reactivity diagnoses obtained with the mortar-bar test, especially in the case of aggregates M and N yielding expansion levels at 14 days in the intermediate range 0.10% to 0.20%. In particular, these two aggregates were classed as non-reactive according to the criteria suggested by Hooton and Rogers but as potentially reactive by the RILEM criteria. Moreover, the reactivity diagnoses obtained for a significant number of test aggregates appeared to be not completely satisfactory when compared to their field performance. As shown in Table 1, aggregates M and N have shown innocuous behaviour in all field cases investigated. Nevertheless, these two aggregates could even be classed as potentially reactive with the mortar-bar test (RILEM criteria). Aggregates D to I have shown deleterious field performance only in some particular circumstances (concrete structures of small section exposed to severe environments, relatively high alkali content of the concrete mix). Nevertheless, these aggregates were always classed as potentially reactive (ASTM and Hooton and Rogers criteria) or as reactive (RILEM criteria). On the other hand, for aggregates showing field service records of nonreactivity (aggregates A and B) or reactivity (aggregates C, L and O) in most or all of the cases investigated, there was always a good correspondence between their field performance and the reactivity diagnoses obtained with the mortar-bar test. Such a comparison confirmed the difficulty and severity of the ultra-accelerated mortar-bar test in discriminating between innocuous and reactive aggregates when the results of this test were interpreted using a percent expansion criterion. As anticipated, the expansion data up to 14 days of immersion in NaOH solution (Figs. 1a and 1b) were also correlated with the kinetic equation developed by Johnston and Fournier [9] in order to determine the reactivity parameter, ln k, for each of the aggregates tested. This kinetic equation is E t (%) = 1 + E 3 (%) exp [-k (t 3) M ] (3) where E t (%) is the percent expansion at the time t (days) of immersion of mortar bars in NaOH solution, E 3 (%) is the percent expansion after 3 days of immersion in NaOH solution, k is a rate constant which combines the effect of nucleation, muldimensional growth, geometry of reaction products and diffusion, and M is an exponential term related to the form and growth of the reaction products. Solving the logarithmic form of Equation (3) by a leastsquares fit yields the reactivity parameter, ln k, as the intercept of the regression line. Further details on the development of Equation (3) are available in the original paper of Johnston & Fournier [9]. These Authors also proposed a tentative value of -6.0 for ln k that appears to be able to discriminate between reactive and innocuous aggregates regardless of the 14-day percent expansion value obtained in the ultra-accelerated mortarbar test. Values of ln k > -6.0 were associated with reactive Table 2 - Values of ln k, Cd and TAL for the aggregates investigated Aggregate ln k R 2 Cd (cm -1 ) TAL (kg Na 2 Oeq/m 3 ) A B C D E F G H I L M N O

5 aggregates whilst values of ln k < -6.0 were indicative of innocuous aggregates. Table 2 gives the values of ln k for each of the aggregates tested in the present study, together with the values of the coefficient of determination, R 2. As shown by the high values of R 2, all sets of data successfully fit to equation (3). Using a value of 6.0 for ln k, it was found that aggregates A, B, M and N were classifiable as non-reactive, while the other nine aggregates were classed as reactive (Table 2). This classification of the aggregates was consistent with their field performance (Table 1). Moreover, the different values of ln k obtained for the reactive aggregates agreed well with their field service records. For example, the values of ln k for aggregates O (-2.1) and C (-2.8) were higher than those of aggregates D to L (values of ln k in the range -3.7 to -5.0) and this was consistent with the greater reactivity exhibited by the first two aggregates in the field performance. Such a comparison proved the suitability of the kinetic parameter, ln k, as a reactivity criterion for assessing the alkalireactivity potential of ASR-susceptible aggregates, when a value of 6.0 for ln k was taken for discriminating between reactive and innocuous aggregates. 3.2 Infrared spectroscopy tests Table 2 gives the values of the disorder coefficient, Cd, for all of the aggregates tested in the present study. According to the judgement criteria established by the infrared spectroscopy method (Cd 120: non-reactive; 120 < Cd 200: slowly reactive; 200 < Cd 300: rapidly reactive; Cd> 300: pozzolanic behaviour) [14], aggregates A, B, M and N were classed as non-reactive while the other nine aggregates, including aggregate O (the most reactive) were classified as slowly reactive. With regard to aggregate O, its Cd value (187 cm -1 ) was, however, near the lower limit of the reactivity domain corresponding to rapidly reactive aggregates. This classification of the tested aggregates was consistent with their field performance (Table 1), thus demonstrating the suitability of the disorder coefficient, Cd, as a reactivity criterion for the assessment of the alkali-reactivity potential of ASR-susceptible aggregates. M. Berra et al. / Materials and Structures 38 (2005) Concrete prism expansion tests Figs 2a and 2b show the 1-year percent expansions of the concrete prisms incorporating each of the tested aggregates plotted as a function of concrete alkali content. In these figures, the alkali content of 5.5 kg Na 2 Oeq/m 3 was denoted by a dashed vertical line, bearing in mind that this is the value of alkali content of the standard concrete mix [4]. These data showed the great influence of the alkali content of the concrete in promoting the expansivity of the prisms incorporating each of the aggregates tested. Although criteria for the interpretation of the results of the RILEM AAR-3 concrete prism test have not yet been finally agreed, some tentative criteria have been recently suggested by the RILEM TC 191-ARP Outline Guide [16]. According to this interpretative guidance, expansion levels of concrete prisms of less than 0.05% after 1 year of testing are likely to indicate non-expansive materials, whilst results exceeding Fig. 2 - Effect of concrete alkali content on the 1-year expansion of concrete prisms incorporating each of the aggregates investigated. 0.10% indicate expansive materials. For all practical purposes, in the absence of additional local experience, aggregates yielding AAR-3 results in the intermediate range 0.05% to 0.10% will need to be regarded as being potentially alkali-reactive. From Figs. 2a and 2b it can be seen that, in the case of a concrete alkali content of 5.5 kg Na 2 Oeq/m 3, the use of the percent expansion limits of 0.05% and 0.10% would classify aggregates C, L and O as potentially alkali-reactive (the 1-year expansion levels being always in the intermediate range 0.05% to 0.10%), whilst the other ten

6 378 M. Berra et al. / Materials and Structures 38 (2005) aggregates would be classed as non-expansive. For aggregates D to I, this classification was in contrast with their field service records (Table 1) and the reactivity diagnoses obtained with the infrared spectroscopy test and the ultra-accelerated mortar-bar test using ln k as a reactivity criterion (Table 2). These discrepancies confirmed the difficulty of the standard concrete prism test in assessing the alkalireactivity potential of slowly reactive siliceous aggregates. Indeed, in the Annex of the RILEM AAR-3 test method it is recognised that some reactive aggregates could be identified as only marginally reactive with the standard concrete mix. For such aggregates, a test concrete with higher alkali content is suggested. Using the data in Figs. 2a and 2b and the percent expansion limit of 0.05% (dashed horizontal line), it was possible to determine the threshold alkali level, TAL, for each aggregate as the abscissa of the point of intersection between the dashed horizontal line and the expansion curve corresponding to the aggregate considered. The values of TAL for all of the aggregates tested are reported in Table 2. These values of TAL may explain the different field performance of the aggregates investigated. In particular, aggregate O had a TAL value (3.6 kg Na 2 Oeq/m 3 ) that is commonly lower than the normal alkali content (as arising from the mix design) of the concrete formulations used in the building industry. This may explain why this aggregate has always shown deleterious behaviour in service. On the other hand, it is unusual to find concrete structures with alkali contents of 8.0 kg Na 2 Oeq/m 3 and more. This may explain why aggregates A, B, M and N have always shown innocuous behaviour in the field. For the other tested aggregates (C - L) having TAL values over the range from 4.4 to 7.2 kg Na 2 Oeq/m 3, their field performance is strongly related not only to the initial alkali content of the concrete but also to the sizes of the concrete structures and the severity of the environmental exposure conditions. In this regard, it must be considered that wetting/drying cycles, humidity gradients, freezing/thawing cycles, and electric currents may cause migration and concentration of alkalies in concrete [18-21]. Enhancement of the alkali concentration in concrete may also arise when concrete structures are exposed to external sources of potential alkalies (for example, deicing salts). This is particularly true in the case of concrete members of small section. As a result, alkali-silica reactions may be accelerated and localised deleterious reactions may be initiated even if the initial alkali content of the concrete is below the TAL of the aggregate used for the particular concrete structure. These considerations could explain why aggregates C (TAL=4.4 kg Na 2 Oeq/m 3 ) and L (TAL=5.5 kg Na 2 Oeq/m 3 ) were found to be reactive in most of the field cases investigated, whilst aggregates D to I (TAL values in the range 6.0 to 7.2 kg Na 2 Oeq/m 3 ) have shown deleterious reaction in the field only in a few cases. Thus, with respect to the other two reactivity criteria investigated (ln k, Cd), the threshold alkali level appeared to be a more appropriate criterion for predicting the field performance of concrete structures incorporating ASRsusceptible aggregates under different exposure conditions. Fig. 3 - Relationship between the threshold alkali level, TAL, and the kinetic parameter, ln k, of the aggregates. It follows that the determination of the TAL associated with a preliminary petrographical examination of the aggregates should represent two basic steps of any test methodology aimed at selecting the ASR-susceptible aggregates and the preventive measures to be used in relation to the exposure conditions and the sizes of the concrete structures. In this regard, the data of the present study provided further validation of the test methodology proposed in our recent paper [11] for qualifying siliceous aggregates against alkali-reaction in concrete. 3.4 Relationship between TAL and ln k Fig. 3 shows the TAL values plotted against ln k for all of the aggregates investigated. In this figure, the value of ln k= -6.0 is shown by a dashed vertical line differentiating between reactive and innocuous aggregates. A linear relationship was obtained and this relationship can be expressed as TAL = ln k (4) with a determination coefficient, R 2, of Therefore, the use of Equation (4) could represent a rapid way of estimating the threshold alkali levels of ASRsusceptible aggregates from the results of the ultraaccelerated mortar-bar test, when these results are interpreted using ln k as a reactivity criterion. According to Equation (4), a value of ln k = -6.0 yields a TAL value of 8.0 kg Na 2 Oeq/m 3. This means that aggregates with TAL values of less than 8.0 kg Na 2 Oeq/m 3 would be classed as reactive when tested with the ultra-accelerated mortar-bar method (ln k as a reactivity criterion). 3.5 Relationship between TAL and Cd As shown in Fig. 4, plotting TAL against Cd for all of the tested aggregates resulted in a linear relationship which can be expressed as

7 M. Berra et al. / Materials and Structures 38 (2005) Fig. 4 - Relationship between the threshold alkali level, TAL, and the microstructural disorder coefficient, Cd, of the aggregates. TAL = Cd (5) with a determination coefficient, R 2, of According to Equation (5) and the reactivity criteria established by the infrared spectroscopy method (dashed vertical lines in Fig. 4), innocuous aggregates (Cd 120) would be characterised by TAL values of not less than 7.4 kg Na 2 Oeq/m 3. This TAL value did not substantially differ from that (8.0 kg Na 2 Oeq/m 3 ) obtained using Equation (4). Moreover, slowly reactive aggregates (120 < Cd 200) would have TAL values in the range from 2.8 to 7.4 kg Na 2 Oeq/m 3, whilst rapidly reactive aggregates (200< Cd 300) would be characterised by TAL values of less than 2.8 kg Na 2 Oeq/m 3. Using Equation (5) and letting TAL = 0 (the TAL of highly reactive aggregates is very low but it is certainly greater than zero), it was calculated that the Cd value discriminating between rapidly reactive aggregates and pozzolanic materials is about 250 cm -1 against 300 cm -1 as proposed by Bachiorrini et al. [14]. This calculated value of Cd was thought to be more likely, taking in mind that siliceous aggregates such as opal and fused quartz, known for their very high ASR expansivity, are characterised by Cd values of 236 cm -1 and 245 cm -1, respectively [22]. Moreover, on the basis of the results of the RILEM AAR-3 concrete prism test (Figs 2a and 2b; concrete alkali content = 5.5 kg Na 2 Oeq/m 3 ) it appeared to be more appropriate to subdivide the wide TAL range corresponding to slowly reactive aggregates (2.8 to 7.4 kg Na 2 Oeq/m 3 ) into the following two reactivity domains: 2.8 TAL 5.5: moderately reactive and 5.5 <TAL<7.4: slowly reactive. According to equation (5), a TAL value of 5.5 kg Na 2 Oeq/m 3 yields a Cd value of about 155 cm -1. Thus, the new classification of the degree of reactivity of the aggregates based on the Cd values would be as follows: Cd 120: nonreactive; 120<Cd<155: slowly reactive; 155 Cd 200: moderately reactive; 200 <Cd 250: rapidly reactive; Cd>250: pozzolanic behaviour. The above TAL-based classification of the aggregates also suggested that limiting the initial alkali content of the concrete mix to < 2.8 kg Na 2 Oeq/m 3 could represent an effective preventive measure for minimising the risk of deleterious ASR expansion in concrete structures incorporating moderately or slowly reactive aggregates, provided that there are no internal and/or external factors leading to a localised significant increase of the concrete alkali content during the service life of the structures. In the case of concrete structures incorporating moderately reactive aggregates under severe exposure conditions, or in the case of structures incorporating rapidly reactive aggregates, a further reduction of the initial alkali content of the concrete mix and/or the use of appropriate amounts of supplementary cementing materials will be required to counteract the development of deleterious ASR expansion. However, such a design approach could be too conservative for a great number of natural ASR-susceptible aggregates, as was the case of the aggregates tested in this study. This shortcoming may be overcome if the selection of the aggregates, as well as the level of the preventive measures are established on the basis of both the TAL of the aggregate and the highest concrete alkali content expected in the field, the latter being estimated from past field experiences. 4. CONCLUSIONS The three alkali-silica reactivity criteria investigated, namely, the threshold alkali level, TAL, derived from the results of a modified version of the RILEM AAR-3 concrete prism test, the kinetic parameter, ln k, derived from a correlation of the results of the ASTM C1260 ultra-accelerated mortar-bar test with a kinetic-based model, and the disorder coefficient, Cd, of the aggregate, derived from the infrared spectroscopy analysis, prove to be all appropriate for discriminating between reactive and innocuous aggregates, at least with regard to the typologies of Italian natural ASRsusceptible aggregates examined in this study. In contrast, some difficulties in the assessment of the alkalireactivity potential of such aggregates are encountered using the RILEM AAR-3 concrete prism test (concrete alkali content = 5.5 kg Na 2 Oeq/m 3 ) or the ASTM C1260 ultra-accelerated mortar-bar test, when the results of the latter test are interpreted using a percent expansion criterion. With respect to the other two reactivity criteria investigated (Cd, ln k), the use of the TAL proves to be more appropriate for interpreting the field performance data of the tested aggregates, as well as for selecting the aggregates and the preventive measures to be used in relation to the exposure conditions and the sizes of the concrete structures. There exists a linear relationship between TAL and ln k or between TAL and Cd, which provides a rapid way of estimating the threshold alkali levels of ASR-susceptible aggregates from the results of the ultra-accelerated mortar-bar test and/or the infrared spectroscopy test. On the basis of the relationship between TAL and Cd, and the results of the RILEM AAR-3 concrete prism test, the following TAL-based classification of the degree of reactivity of ASR-susceptible aggregates is proposed:

8 380 M. Berra et al. / Materials and Structures 38 (2005) TAL<2.8 kg Na 2 Oeq/m 3 : rapidly reactive; 2.8 TAL 5.5: moderately reactive; 5.5 <TAL<7.4: slowly reactive; TAL 7.4: non-reactive. Accordingly, some modifications of the reactivity domains originally established by the infrared spectroscopy method are suggested leading to the following Cd-based classification of the degree of reactivity of ASR-susceptible aggregates: Cd 120: non-reactive; 120<Cd<155: slowly reactive; 155 Cd 200: moderately reactive; 200 <Cd 250: rapidly reactive; Cd>250: pozzolanic behaviour. ACKNOWLEDGEMENTS This work was carried out within the framework of the National Research Funding Program for Electric System (MICA DM 26 Jan. 2000). REFERENCES [1] CSA A A, Standard Practice to Identify Degree of Alkali-Reactivity of Aggregates and to Identify Measures to Avoid Deleterious Expansion in Concrete (Canadian Standards Association, Rexdale, 2000) [2] CSA A A, Potential Expansivity of Aggregates (Procedure for Length Change Due to AAR in Concrete Prisms) (Canadian Standards Association, Rexdale, 1994) [3] ASTM C , Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction (American Society for Testing and Materials, West Conshohocken, 2003) CD-ROM, pp. 6. [4] RILEM Recommended Test Method AAR-3 (formerly B-TC 106-3), Detection of Potential Alkali-Reactivity of Aggregates - Method for Aggregate Combinations using Concrete Prisms, Mater. Struct. 33 (229) (2000) [5] CSA A A, Test Method for Detection of Alkali-Silica Reactive Aggregate by Accelerated Expansion of Mortar Bars (Canadian Standards Association, Rexdale, 1994) [6] ASTM C , Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method) (American Society for Testing and Materials, West Conshohocken, 2003) CD-ROM, pp.5. [7] RILEM Recommended Test Method AAR-2 (formerly A-TC 106-2), Detection of Potential Alkali Reactivity of Aggregates The Ultra-Accelerated Mortar-Bar Test, Mater. Struct. 33 (229) (2000) [8] Berra, M., Mangialardi, T. and Paolini, A.E., A new approach for assessing the potential alkali-expansivity of slowly reactive siliceous aggregates, Adv. Cem. Res. 11 (3) (1999) [9] Johnston, D. and Fournier, B., A kinetic-based method for interpreting accelerated mortar bar test (ASTM C1260) data, in Alkali-Aggregate Reaction, Proceedings of the 11 th International Conference, Québec, Canada, 2000 (Centre de Recherche Interuniversitaire sur le Béton (CRIB), Laval and Sherbrooke Universities, 2000) [10] Berra, M., Mangialardi, T. and Paolini, A.E., Rapid evaluation of the threshold alkali level for alkali-reactive siliceous aggregates in concrete, Cem. Concr. Composites 21 (1999) [11] Berra, M., Mangialardi, T. and Paolini, A.E., Une nouvelle méthodologie de qualification des granulats siliceux vis-à-vis de l alcali-réaction dans le béton, Mater. Struct. 36 (259) (2003) [12] British Cement Association, Testing Protocol for Greywacke Aggregates - BSI/517/1/20 ad hoc Group on ASR (BCA, Crowthorne, Berkshire, 1999) 1-8. [13] Bachiorrini, A., A method to test the alkali reactivity of siliceous aggregates: infrared spectroscopy, in Concrete Durability, Proceedings of the K. and B. Mather International Conference, Atlanta, USA, 1987 (ACI-SP 100, American Concrete Institute, Detroit, 1987) [14] Bachiorrini, A., Baronio, G., Berra, M., Delmastro, A., Montanaro, L. and Negro, A., Infrared spectroscopy in the evaluation of aggregates in ASR deteriorated concretes from many parts of the world: comparison with other methods, in Concrete Alkali-Aggregate Reaction, Proceedings of the 7 th International Conference, Ottawa, Canada, 1986 (Noyes Publications, New Jersey, 1987) [15] RILEM Recommended Test Method AAR-1, Detection of Potential Alkali-Reactivity of Aggregates - Petrographic Method, Mater. Struct. 36 (261) (2003) [16] RILEM TC191-ARP, RILEM Recommended Test Method AAR-0: Detection of Alkali-Reactivity Potential in Concrete Outline Guide to the Use of RILEM Methods in Assessments of Aggregates for Potential Alkali-Reactivity, Mater. Struct. 36 (261) (2003) [17] Hooton, R.D. and Rogers, C.A., Development of the NBRI rapid mortar bar test leading to its use in North America, in Alkali-Aggregate Reaction in Concrete, Proceedings of the 9 th International Conference, London, UK, 1992 (The Concrete Society, Wexham, 1992) [18] Moore, A.E., Effect of electric current on alkali-silica reaction, in Effects of Alkalies in Cement and Concrete, Proceedings of the 4 th International Conference, West Lafayette, USA, 1978 (School of Civil Engineering, Purdue University, West Lafayette, 1978) [19] Nixon, P.J., Collins, R.J. and Rayment, P.L., The concentration of alkalies by moisture migration in concrete-a factor influencing alkali-aggregate reaction, Cem. Concr. Res. 9 (4) (1979) [20] Natesaiyer, K. and Hover, K.C., Investigation of electrical effects on alkali-aggregate reaction, in Concrete Alkali- Aggregate Reaction, Proceedings of the 7 th International Conference, Ottawa, Canada, 1986 (Noyes Publications, New Jersey, 1987) [21] Xu, Z. and Hooton, R.D., Migration of alkali ions in mortar due to several mechanisms, Cem. Concr. Res. 23 (4) (1993) [22] Berra, M., Mangialardi, T., Paolini, A.E. and Turriziani, R., Critical evaluation of accelerated test methods for detecting the alkali-reactivity of aggregates, Adv. Cem. Res. 4 (1) (1991/92)

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