A factorial approach to the study of alkali-silica reaction in concrete
This thesis describes a research project which investigates certain aspects of the alkali-silica reaction in concrete. A factorial approach was adopted for the experimental stages of the investigation. Such an approach is based on proven statistical techniques and has the advantage of allowing any interaction between the experimental factors to be studied in addition to studying the main effect of each separate factor. The experimental work was carried out in three stages. The first and second stages comprised physical tests which involved the monitoring of expansion in the test specimens. The third stage involved chemical analysis to determine the alkali content of the test specimens. The main parameters which were included in the investigation were: (a) the alkali content of the Portland cement (b) the use of pulverised fuel ash as a cement replacement material (c) the amount of reactive aggregate (d) the free water content of the mix In addition, the use of pulverised fuel ash as an admixture and the different effects produced by the two most commonly used reactive aggregates in laboratory tests (namely Pyrex glass and Beltane opal) were studied. In general the experimental work was carried out using standard test methods as specified by the British Standards Institution and the American Society for Testing and Materials. In particular the "mortar bar method", as described 1n A.S.T.M. e227, was used extensively. Where necessary the standard test methods were adapted to suit the experimental nature of the investigation. It is shown that the main effects of the experimental factors are all highly significant and thus have a considerable contribution towards the expansion of the mortar bar specimens. Moreover, the level of interaction between certain of the factors is also shown to be significant. This indicates that, in some cases, the effects of the individual factors are not independent. The experimental results also show the different responses produced by the two reactive aggregate materials. With respect to the use of pulverised fuel ash, certain time dependent trends are discussed which may suggest some long term instability in the beneficial effect which pulverised fuel ash can achieve by reducing expansions. The factorial approach assisted in highlighting this time dependent effect in the experimental data. The chemical analysis results have shown that pulverised fuel ash tends to increase the alkali content of the mortar bars. This is discussed with respect to the mode of operation of pulverised fuel ash in inhibiting expansion due to alkali-silica reaction, which is considered to be one of chemical reaction and not a simple dilution effect. An explanation of the contribution of pulverised fuel ash and the differences noted between its use with Pyrex glass and Beltane opal is hypothesised in terms of the relative affinity of each of these three materials for reaction with the hydroxyl ion present in the pore fluids.