Expansion behaviour of glass aggregates in different testing for alkali-silica reactivity
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- Zhu, H., Chen, W., Zhou, W. et al. Mater Struct (2009) 42: 485. doi:10.1617/s11527-008-9396-4
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The potential for reaction between amorphous silica in recycled glass used as aggregate in concrete and alkalis in cement is the subject of debate in current concrete literature. Whilst the ASTM C1260 accelerated mortar bar method is conventionally used for rapid ASR assessment, there is doubt about its suitability for glass aggregates. This paper reports upon a comparison of the relative ASR reactivity of various colours of recycled glass aggregates using the ASTM C1260 and C227 test methodologies. The results show that with limited exception the ASTM C1260 method does not cause glass aggregates to react by the end of the prescribed test period. In contrast, the ASTM C227 method causes all glass aggregates to react within 2 weeks, despite the test being designed for 12 months or even longer if necessary. This paper compares and contrasts the results of the two methods over a wide range of glass aggregate and cementitious systems made with two sizes of mortar bar, draws conclusions about the reasons for the differences observed and makes remarks on the expansion behaviour of glass aggregates in cementitious systems.
KeywordsASRGlass aggregateASTM C1260ASTM C227Reaction mechanisms
Crushed waste glass was first used as a concrete aggregate in 1974  with disappointing results due to expansive alkali-silica reaction (ASR) between the amorphous silica in the glass and the alkaline cement paste. Over the past decade however, a number of major research projects in the USA and the UK [2–14] have been conducted in this area. The studies agree that all glass aggregates exhibit rapid ASR in normal OPC concrete and that pozzolanic cement replacement materials may significantly reduce, but not always mitigate, the expansive ASR reaction.
International dimensional change ASR methods
ASTM C 227: Standard test method for potential alkali reactivity of cement-aggregate combinations
High alkali cement (Na2Oeq > 0.6%) mortar bar test to determine cement-aggregate reactivity
Specimens stored in high-humidity containers at 38°C
Requires on year or even longer if necessary for completion
Excessive leaching of alkalis from specimens reported 
ASTM C 1260: Standard test method for potential alkali reactivity of aggregates (mortar-bar method)
Mortar bar test for aggregate reactivity
Bars immersed in 1N NaOH solution for 14 days at 80°C
Suitable for screening, but because of exposure severity, potentially unsuitable as absolute test. Results of concrete prism test should prevail 
ASTM C 1293: Standard test method for concrete aggregates by determination of length change of concrete due to alkali-silica reaction
Concrete prism test, regarded as best indicator of field performance, conducted at 38°C/100% RH
High-alkali cement (1.25% Na2Oeq), with a cement content of 420 kg/m3
Coarse aggregate test (non-reactive fine aggregate) or vice-versa. Requires one year for completion
Can be used to test effectiveness of suppressants over 2 years 
Widely accepted, but lengthy test method
BS 812-123: Testing aggregate—method for determination of alkali-silica reactivity—concrete prism method
Concrete prism test, generally regarded as best indicator of field performance, conducted at 38°C/100% RH
High-alkali cement (>1.0% Na2Oeq), with a cement content of 690 kg/m3
Coarse aggregate test (non-reactive fine aggregate) or vice-versa. Requires one year for completion
Length, but widely accepted in the UK
RILEM TC106-3: Detection of potential alkali-reactivity of aggregates—method for aggregate combinations using concrete prisms
Concrete prism test, conducted at 60°C/100%RH
High-alkali cement (0.9–1.2% Na2Oeq, raised to 1.25% by adding NaOH), with a cement content of 440 kg/m3
Test requires 20 weeks for completion, mainly used in mainland EU
In several major published studies [2–4, 6–13, 24–27] the ASTM C1260 test method has been used to assess the ASR reactivity of glass aggregate, mainly because it gives rapid, repeatable results on relatively small samples. The ASTM C227 test method has been used less widely [21, 27] on glass in concrete because of its disadvantages such as long curing period, less reliable for slow or late expansive aggregates.
The ASTM requirement of the mortar bar size for ASR length changing measurement is 25 × 25 × 285mm (long-thin bars) [16, 17]. However, bars of this size are long, slender and fragile. As the numbers of bars cast for testing in this study was very large with a planned test duration of up to 5 years, the main body of research was carried out using a bar size of 40 × 40 × 160 mm (short-fat bars). A subsidiary study examined the effect of bar sizes on ASR expansion using the ASTM C1260 and C227 prescribed bar size and the size adopted in the main research programme. It needs to clarify that the test criteria with the short-fat bars may be different to that of the long-thin bars. However, the recommended test criteria are adopted in this study for simplicity and comparison purpose.
2 Experimental details
Chemical composition of waste glass
Ordinary Portland cement (OPC) of strength class 42.5N to BS EN 197–1  with sodium oxide equivalent (Na2Oeq) of 0.6–0.65% was used to make the C1260 test specimens. High alkali Portland cement (HAPC) with Na2Oeq of 1.08% was used to prepare the C227 test specimens.
ASTM grading requirement for aggregate
2.2 Mix proportions and test procedures
2.2.1 ASTM C1260 test
Mortar proportions of cement: aggregate: water of 1:2.25:0.47 were used. For each mix, three 40 × 40 × 160 mm mortar bars were cast at room temperature and cured for (24 ± 2) hours at 20°C in plastic bags to sustain high ambient RH. After demoulding, these were stored at 80°C in water for another 24 h, then transferred and immersed in 1N NaOH at 80°C until test. The length change of the prisms was measured using a length comparator specified by BS 812-123. An initial reading was taken immediately after demoulding, a zero reading after storing in distilled water at 80°C for 24 h. Subsequent readings were taken following the C1260 test procedure within 1 min of removal from the 80°C alkali storage conditions.
2.2.2 ASTM C227 test
Following C227 , mortar mix proportions of cement: aggregate: water of 1:2.25:0.47 were used. For each mix, three 40 × 40 × 160 mm mortar bars were cast at room temperature and cured for (24 ± 2) h at 20°C in plastic bags around 100% RH. After demolding, the length and initial readings of the bars were measured at 20°C. The bars were then wrapped in damp cloth, sealed with plastic wrapping and placed in sealed plastic bags (one bag per mix). The specimens were then transferred to a controlled environmental room maintained at (38 ± 2)°C and ≥98% RH. Subsequent readings were taken every fortnight for the first 26 weeks and every 13 weeks thereafter. The samples were then stored at (23 ± 2)°C for at least 16 h prior to measurement to remove the possibility of thermal expansions or contractions affecting the results.
2.3 Effect of mortar bar size on ASR expansion
In addition to the 40 × 40 × 160 mm bars, an additional series of bars of 25 × 25 × 285 mm (specified by both C1260 and C227 methods) using both methods to determine the effect of bar size on apparent ASR reactivity of flint and green mortars.
3 Results and discussion
3.1 ASTM C1260 test
Short term (up to 28 days) and long term (up to 133 days) ASTM C1260 test results are detailed below.
3.1.1 Short term results
3.1.2 Long term results
The other significant observation from these results is that the time to initiate ASR reaction and the rate of reaction thereafter varies with glass colour. Since the major chemistry of the green, blue, amber and flint glass is very similar, Table 2, this implies that the metals used to impart colour to glass may have an effect on the ASR reactivity [12, 13].
3.2 ASTM C227 test
A large discrepancy between the C227 and C1260 results, however, was observed with the control sand, which is well within C227 test limits up to a test age of 52 weeks (Fig. 3), yet appears to be “potentially reactive” when tested by C1260 (Fig. 2).
3.3 Comparison of ASTM C1260 and C227
3.4 Effect of bar size on apparent ASR expansion using the C1260 and C227 test methods
Figure 6 shows that with the C1260 method, the longer bars (280 mm) with smaller cross-section (25 × 25 mm) start expanding earlier than shorter bars (160 mm) with larger cross-section (40 × 40 mm). With the C227 method, however, this expansion trend reverses: the expansion of shorter bars with larger cross-section is higher than that of longer bars with smaller cross-section, Fig. 7. Grattan-Bellew  has shown similar results. Lu et al.  examined the effect of bar length with equal cross sectional areas (40 × 40 × 160 & 40 × 40 × 285 mm) on alkali-carbonate reaction (ACR) using a test condition similar to C1260 test and found that the expansion results due to ACR of these bars are similar. Bakker  investigated the effect of bars with different cross sectional areas and same length (20 × 20 × 160, 40 × 40 × 160 and 100 × 100 × 160 mm) on ASR expansion tested to C227 and concluded that a significant increase in expansion was found as the bar cross-sectional area increased. These previous studies consistently suggest that the cross sectional area of the test bars plays an important role in the perceived ASR expansion. No reference known to the authors has offered an explanation of why this may be, nor addressed the issues that bar size may have on the C1260 test results.
3.5.1 Effect of test type (C1260 vs. C227)
In the C227 test, the alkali level of the cement paste is high and the reaction takes place throughout the specimen from the test initiation. However, some leaching of alkali into the wet cloth may occur and can be confirmed by simple pH testing. If the surface alkali levels reduce significantly due to the combination of consumption by reaction and leaching, it could be expected that the rate of ASR expansion might be affected. In this study, the expansion rates of the relatively faster-reacting blue and flint glasses started to reduce after 39 weeks, whilst for the relatively slower-reacting amber and green glasses, this did not occur until after 52 weeks, Fig. 3.
3.5.2 Effect of bar size
The above discussion can also explain the opposite effects of bar size on the C1260 and C227 tests. With C1260, the smaller the bar cross-sectional area is, the more quickly OH- ions can completely penetrate the pore system, initiate reaction and thus yield the higher expansions shown in Fig. 6. The opposite is true for C227, because of the consumption of alkalis due to ASR reaction and the leaching of alkalis out of the bars. Moreover, when the cement alkali level reduces beyond a certain point, the reaction rate slows and finally stops. Figure 7 shows that the rate of expansion of flint and green glass aggregate with the smaller cross sections decreases to zero after 8 and 26 weeks respectively, whilst the expansions are still increasing up to 52 weeks for larger cross section bars made with the same materials.
3.5.3 Time-lag of ASR expansion in the ASTM C1260 test
The notable time-lag in the C1260 expansion curves (Figs. 2 and 4) are consistent with the fact that the C1260 test yields results in direct relation to the rate of penetration of alkali from the 1N NaOH immersion solution. An additional concern that may apply is high degree of variability in the diffusion coefficients of cement pastes, particularly when pozzolanic materials are used as the pore refinement of these may be significantly accelerated by high temperatures .
According to C1260 (Fig. 9a), the apparent effect of CPFA is to totally mitigate the ASR expansion of highly reactive flint glass aggregate up to a test age of 56 days (4 times longer than the recommended 14-day test duration in C1260). However the expansion results of the BS 812-123 test on the same flint glass aggregate, Fig. 9b, shows that the CPFA improves ASR resistance but nevertheless fails the BS 812-123 test. It is outside the scope of this paper to discuss the relative effectiveness of pozzolanic materials for mitigating ASR reactivity of glass aggregate but the different apparent reactivity with the ASTM C1260 and BS 812-123 tests suggests that the pozzolanic reaction of this material is stimulated by the high temperature (80°C) and 1 N NaOH solution used in the C1260 test . Thus there may be a propensity for the C1260 test to yield false negatives and caution is recommended with this test, particularly when highly reactive aggregates, such as glass, are used in concrete.
- (i)Relative reactivity of glass aggregates
All four colours of glass aggregates assessed in the research were very alkali-silica reactive when tested using ASTM C1260 (with extended test duration) and ASTM C227 (within normal test duration). However the reaction rate varies and appears to be ranked as follows: blue > flint > amber > green.
- (ii)Comparison of C1260 and C227 methods
The C1260 test method did not indicate that the glass aggregates under test were ASR-reactive (except for the blue glass), despite them appearing to be reactive when tested using the C227 method.
The bar size used in the reported tests has a significant and opposite effect on the perceived ASR potential of glass aggregates tested by the two methods. With the C1260 method, where the aggressive alkali permeates into the sample, the smaller the cross-sectional area used, the faster the reaction. With the C227 method where alkali may leach out of the specimen, the opposite is true.
- (iii)Appropriate AST testing of glass aggregate
The C1260 test conditions (80°C and 1N NaOH solution) are likely to promote highly accelerated reaction of any pozzolanic material used to make the test specimens. In the case of glass aggregate, the grading is such that some 40% of the total aggregate under test may be pozzolanic. Thus it is inevitable that the C1260 test method will yield unreliable false negative results when used to test glass aggregates. It is therefore suggested that the particle size of glass aggregates should be 1.18 mm when C1260 test method is used.
The C227 test method, due to the aggregate grading, may also be partially susceptible to the failings of the C1260 test method, albeit to a less degree because of the lower temperature (38°C).
Where possible, concrete prism test methods such as BS 812-123 and ASTM C1293 conducted at 38°C/100% RH should be used in the first instance .
The authors are grateful to The Waste & Resources Action Programme for partially funding this research work and to Mr. Kieran Nash for his extensive technical support on this wide research study.