Ultra-accelerated Test Research Articles

Use of an ultra-accelerated concrete prism expansion test for alkali-silica reactivity assessment

The suitability of an ultra-accelerated concrete prism expansion test (testing concrete mixes in alkaline solutions at 150°C) for the assessment of alkali-silica reactivity (ASR) was investigated using concrete mixes made with different alkali levels and different typologies of Italian natural aggregates of known field performance. The results of previous concrete prism expansion tests at 38°C and 100% RH were used for comparative purposes. It was found that the ultra-accelerated test is appropriate for discriminating between innocuous and potentially alkali-reactive concrete mixes and may also be used as a rapid method for evaluating the threshold alkali levels of ASR-susceptible aggregates. The results of the ultra-accelerated test may be interpreted on the basis of a percent expansion criterion (0·12% at 21 days) or through the use of a reactivity parameter, ln k, the latter being determined from the correlation of the test results with a kinetic-based method. A value of –3·4 for ln k appears to be a suitable reactivity criterion for interpreting the results of the ultra-accelerated concrete prism test.

Use of an ultra-accelerated concrete prism expansion test for alkali?silica reactivity assessment

The suitability of an ultra-accelerated concrete prism expansion test (testing concrete mixes in alkaline solutions at 150°C) for the assessment of alkali-silica reactivity (ASR) was investigated using concrete mixes made with different alkali levels and different typologies of Italian natural aggregates of known field performance. The results of previous concrete prism expansion tests at 38°C and 100% RH were used for comparative purposes. It was found that the ultra-accelerated test is appropriate for discriminating between innocuous and potentially alkali-reactive concrete mixes and may also be used as a rapid method for evaluating the threshold alkali levels of ASR-susceptible aggregates. The results of the ultra-accelerated test may be interpreted on the basis of a percent expansion criterion (0·12% at 21 days) or through the use of a reactivity parameter, ln k, the latter being determined from the correlation of the test results with a kinetic-based method. A value of –3·4 for ln k appears to be a suitable reactivity criterion for interpreting the results of the ultra-accelerated concrete prism test.

Use of lithium compounds to prevent expansive alkali—silica reactivity in concrete

The effectiveness of Li2CO3 and LiNO3 in preventing expansive alkali–silica reactivity in concrete mixes made with two natural siliceous aggregates and different alkali contents was investigated using both the expansion test at 38°C and 100% RH and the ultra-accelerated expansion test in alkaline solutions at 150°C. LiNO3 proves to be always effective while Li2CO3 is effective only when the difference between the concrete alkali content (Lac) and the threshold alkali level of the aggregate (TAL) does not exceed 1.7 kg Na2Oeq/m3 and the concrete mixes are stored at 38°C and 100% RH. The ultra-accelerated test methods proves to be appropriate for determining the effective dosages of LiNO3 in concrete mixes made with different alkali contents. There exists a linear relationship between the effective dosage of LiNO3 in terms of Li2O/Na2Oeq molar ratio and the difference Lac-TAL, indicating a remarkable increase of Li2O/Na2Oeq with Lac.

Rapid evaluation of the threshold alkali level for alkali-reactive siliceous aggregates in concrete

A modified version of the ultra-accelerated concrete prism expansion test in alkaline solutions at 150°C (testing concrete mixes at varying alkali content of concrete) was investigated in order to ascertain the reliability of this test method for evaluating the threshold alkali levels (TAL) of alkali-reactive siliceous aggregates in concrete. Five natural sands of known field performance were tested and their TAL values determined by the ultra-accelerated test were compared with those obtained from concrete prism expansion tests at 38°C and 100% RH. A correlation between the threshold alkali level of the sands and their microstructural disorder coefficient, Cd, as measured by infrared spectroscopy, was also attempted. The modified ultra-accelerated concrete prism test proved to be a rapid and reliable method for evaluating the threshold alkali level of reactive siliceous aggregates in concrete, provided that an expansion limit of 0.11% at 3 weeks was taken as the reactivity judgement criterion for this test. The threshold alkali level proved to be an appropriate reactivity parameter for the selection of alkali-reactive siliceous aggregates. Also, the existence of a straight-line relationship between TAL and Cd suggested the possibility of developing a new test methodology for estimating TAL, based on Cd measurements by infrared spectroscopy.

A critical review of ultra-accelerated tests for alkali-silica reactivity

A large number of ultra-accelerated test procedures, for determining the potential alkali reactivity of aggregates, have been developed, particularly in the past 15 years. An ultra-accelerated test method is defined as one which yields results within a few days or, at most, a few weeks. A number of ultra-accelerated test methods have been adopted as ‘standard tests’, but few have been adequately evaluated. The rapid globalization of the construction industry will require the harmonization of National Standard Test Methods. The major requirement of ultra-accelerated test methods is that they should correctly predict the potential reactivity of aggregates in greater than 95% of the cases. Due to the complexity and variability in the composition and grain size of aggregates, it is improbable that a single test method will be developed which would be appropriate for evaluating all types of aggregates. Another major requirement for ultra-accelerated test methods is that the interlaboratory coefficient of variation should be low, preferably less than 12%. At present, only the NBRI accelerated mortar bar method has been subject to adequate inter-laboratory evaluation. However, a more limited inter-laboratory investigation showed that the autoclave mortar bar test also shows considerable potential, as a satisfactory ultra-accelerated test method. Further refinement of the NBRI and autoclave methods is required to improve their performance with a wide variety of aggregates.