Estimating the alkali contribution from fly ash to expansion due to alkali—aggregate reaction in concrete

Abstract

This paper reports the findings from an investigation to determine the ‘effective’ alkali contribution from jly ash to the expansion of concrete containing natural reactive UK aggregates. Concrete prism expansion tests were carried out using flint/chert sand (from three different sources), a crushed siltstone and a crushed siliceous limestone; results are also reported for a gre-vwacke aggregate. Fly ash from three commercial sources was used. The results demonstrate that the ‘efective’ alkali contribution from the fly ash, estimated from expansion results, varies depending on the level of replacement and nature of the reactive aggregate. With moderately reactive aggregates, such as flint, 25% fly ash was found to be effective in preventing cracking, regardless of the OPC alkali content. However, with more reactive aggregates (i.e. aggregates that react at lower alkali levels), Fly ash concrete expanded at lower OPC alkali contents than control specimens, indicating an ‘efective’ alkali contribution from the fly ash. Higher levels of fly ash are required to prevent cracking with these aggregates. Effective alkali ‘contributions’ (from fly ash) determined for a particular aggregate are not applicable to concrete containing other reactive aggregates, and specifications need to be cognizant of the need for higher ash replacement levels with more reactive aggregates.

Alkali–silica reaction (ASR) expansion: Pessimum effect versus scale effect

“…In the previous model, a threshold of alkali concentration of 0.625 mol/l of Na + was considered, under which ASR did not occur [34]. It was based on experimental data showing that, with an alkali content lower than 3 kg per m 3 of concrete, no ASR-expansion was observed [49][50][51][52]. In tests performed in non-saturated conditions (in air with RH above 95%), the concentration of alkali played an important role in the attack of the silica.…”

“…The result of this assumption is shown in Figure 7: if no threshold is taken into account, the expansions determined by the modelling at 50 days are of the same order as the expansions obtained for the measurements. The apparent threshold effect observed in experimentations [49][50][51][52] can be explained by the alkali fixation in C-S-H. This fixation consumed a part of alkali which are no more available in the pore solution and reduced the attack kinetics and consequently the ASR-expansion kinetics.…”

Effects of aggregate size and alkali content on ASR expansion

“…Usually, high alkali content implies deeper aggregate attack, and thus a larger volume of gel per m 3 of reactive aggregate. However, many papers [24][25][26][27][28][29][30] ( ) The results of the calculations of the parameters are given in Table 2 Table 2). The larger the cement paste restraint is, the larger the volume of ASR-gel must be to obtain significant expansion; and if there is a lot of gel, lc has to be large to prevent expansion for the small reactive particles.…”

Coupled effects of aggregate size and alkali content on ASR expansion

“…In this study, a decrease of about 70 to 80% of the expansion was obtained when the alkali content of mortar mixtures was reduced from the higher (13.4 kg/m 3 ) to the lower value (6.2 kg/m 3 ). Many authors propose a threshold effect of alkali, generally between 3 and 5 kg of alkali per m 3 of concrete [20][21][22][23][24], below which ASR expansion is small. Fig.…”