M.R. Jones

Development of chloride-resisting concrete using fly ash

Abstract This paper describes how low-lime fly ash can be used to develop chloride-resistant concrete by improving both its physical resistance to the ingress of chlorides and binding capacity of these ions in the cover zone. This includes optimizing the fly ash content, reducing the water/binder ratio of the concrete, processing fly ash to improve its particulate characteristics and, finally, using ternary blends with silica fume or metakaolin. This last method is shown to provide the highest degree of chloride resistance. A tentative classification of chloride-bearing environments together with recommendations for the specification of concrete for structures exposed to these environments, is proposed.

Studies using 27Al MAS NMR of AFm and AFt phases and the formation of Friedel's salt

This paper describes the application of the magic angle spinning (MAS) NMR spectroscopy to study the chemical environment of 27Al-bearing phases in Portland cement-based concrete. A specific methodology is described that allows reliable spectra to be determined for combinations of different types of cements and fillers (in this case, Portland cement, fly ash, slag, silica fume, metakaolin and limestone filler). As well as the study of ‘molecular structure’ of cement matrix, the paper reviews the mechanism of Friedels salt formation in cement systems. Mechanisms based on ion exchange of chloride for hydroxide in hydroxy-AFm and on chloride absorption on formation are discussed. Finally, the nature of the chloride/hydrate binding phenomena are described to provide a reasonable robust and fundamental picture of the role different cements can play in the provision of overall concrete durability to chloride ingress from a chemical perspective.

CONTRIBUTION OF PFA TO CONCRETE WORKABILITY AND STRENGTH DEVELOPMENT

Abstract An assessment of the nature of the components of pulverized-fuel ash (pfa) has shown a dominance of hollow, glassy spheres with a composite shell. Total of partial coating by magnetite can be present. The form, size and physical nature of the particles are shown to be the main influences affecting the workability and water demand of fresh pfa concretes. The relationship between progressive pfa particle reaction and strength gain with time has been observed for standard cured concretes up to two years old. The early pfa contribution to strength development is largely a beneficial interaction with the normal hydration of Portland cement while the pozzolanic reaction becomes significant after 28 days. The potential performance of pfa can be predicted from a consideration of its fineness, as measured by 45 μm sieve residue.

Concrete containing ternary blended binders: Resistance to chloride ingress and carbonation

This study examined the chloride and carbonation durability performance of concrete containing ternary blended binders in comparison to PC and binary PC/PFA concrete of equivalent standard 28 day cube strengths of 20, 40 and 60 N/mm2. In addition, the nature of the near surface pore structure of the concrete has been inferred from its initial surface absorption. It has been shown that the chloride resistance of all the ternary binder concrete (TBC) is significantly higher than corresponding PC and PC/PFA mixes. On the other hand, however, under worst case conditions it was found that after 30 weeks accelerated exposure, carbonation depths were generally greater in the TBC mixes. The degree to which this occurred was found to relate to the amount of PC replaced.

DETERMINATION OF TOTAL AND SOLUBLE CHLORIDES IN CONCRETE

Abstract Methods to determine chloride concentrations have been evaluated using aqueous solutions and hardened concrete samples and are shown to be sufficiently accurate for solutions. Dilute acid extractions do not completely dissociate chlorides from powder concrete samples and the strength of the acid extraction regime gives rise to different measured chloride contents. On the other hand, the water-soluble chloride content is shown to be independent of the method of extraction, providing the extraction period is equal to or greater than 24 hours. Chloride contents in concrete can be expressed as either total, acid-soluble or water-soluble proportions.

Quantifying chloride-induced corrosion from half-cell potential

Abstract The paper describes a study undertaken to examine a methodology to determine the rate and severity of chloride-induced corrosion of steel embedded in concrete using the half-cell potential test. Preliminary studies undertaken in the laboratory, using a wide range of concrete variables, indicated that an effective relationship between half-cell potential (Ecorr) and corrosion current (Icorr), from polarisation resistance measurements, did indeed exist. The study was subsequently extended to an external marine environment where sections of full-scale beam, slab and column elements were exposed to seawater attack for a period of 5 years. By instrumenting these elements prior to exposure, it has been possible to demonstrate that by establishing a local relationship between Ecorr and Icorr, the extent of corrosion damage can be reasonably estimated over the full section. The practical implications of this and a proposed test procedure are discussed.

Diffusion of chlorides into concrete influence of PFA quality

Abstract The influence of pfa quality, specifically alumina and glass contents and fineness, together with the effect of replacement ratio and content on the diffusion of chlorides in concrete have been investigated. It is shown that it is not the pfa quality per se that effects D but the quantity of pfa in the concrete. The use of ashes of different qualities to produce a chloride-proof concrete is suggested.

PFA concrete: Exposure temperature effects on chloride diffusion

Abstract The results are discussed of a study into the effect of exposure temperature on the coefficient of chloride diffusion (D) of concrete, with PFA contents varying from 0 to 50%, strength from 20 to 60 N/mm2 and initial curing being in either water 20°C or in air 20°C/55% RH. The effect of PFA in reducing D is shown to increase with exposure temperature. This effect is greater than the effect of design strength and is more marked where the initial curing has been poor. The work shows that the diffusion test conditions must be chosen to reflect the exposure environment of the concrete structure, as it is possible to greatly over or underestimate the durability of concrete using the standard temperature of 20°C.

Binder content influences on chloride ingress in concrete

Abstract The reported study looked at the effect of reducing free water contents, and thereby binder contents, on the ingress of chloride in concrete. Concretes with equal water binder ratio (and design strength), but with water contents reduced by up to 30 litres/m3, were tested for chloride diffusion (D) and penetration. The quality of the microstructure was inferred from initial surface absorption tests (ISAT). The results show no practical difference in chloride durability between the corresponding concretes, and that reducing the binder content, (providing that the water binder ratio is maintained) is not likely to be detrimental. However, the results reported underline the importance of binder type, in this case PFA. Implications of the results are discussed and, in light of the findings, whether specifications which demand minimum cement Contents are justified.

Benchmarking PFA grouts for magnesium sulfate bearing exposures

This paper describes a study carried out to examine the sulfate resistance of a wide range of pulverized-fuel ash (PFA) grouts. A number of material variables were considered reflecting those used in practice, including PFA content, and cement and PFA material characteristics (between sources). Several exposure environment conditions were also tested. It was found that under Class 5 sulfate exposure conditions, as classified in BRE Digest 363 (upper limit of Class XA3 to prEN 206), expansion was reduced with increasing PFA content. For grouts with PFA replacements in excess of 75% by weight, virtually no expansion was measurable over the test period. PC and PFA from different sources had only a minor effect on performance. However, the combination of PFA with sulfate resisting cement was found to reduce expansions by about 50% in comparison to corresponding PC/PFA grouts at PFA levels up to 75%. In very aggressive exposures (twice Class 5), it was found that only the very high PFA content grouts (>90% PFA) exhibited no visible damage after 12 months exposure. The influence of the additional presence of magnesium chloride appeared to modify the nature of damage occurring, but tended to inhibit it. The practical implications of the study are considered and means of specifying and categorising PFA grouts for sulfate resistance tentatively proposed.RésuméCet article décrit une étude de la résistance aux sulfates d’une vaste gamme de coulis incorporant des cendres volantes. Un certain nombre de variables ont été étudiées, pour prendre en compte celles rencontrées dans la pratique, telles que la teneur en cendres volantes et en ciment, ainsi que les caractéristiques des cendres volantes (selon leurs origines). Plusieurs conditions d’exposition ont également été prises en considération. On a trouvé que sous une exposition au sulfate de Classe 5 (selon la classification du BRE Digest 363: limite supérieure de la Classe XA3 au prEN 206), l’expansion s’est réduite en fonction de l’accroissement de la teneur en cendres volantes. Pour des coulis incorporant des remplacements de cendres volantes supérieures à 75% du poids, presque aucune expansion n’a été mesurée au cours de l’essai. Les origines différentes des ciments Portland et des cendres volantes n’ont eu qu’un effet mineur sur la performance. Cependant, la combinaison de cendres volantes avec un ciment résistant aux sulfates a réduit l’expansion d’environ 50% par comparaison avec des coulis correspondants contenant des niveaux de cendres volantes jusqu’à 75%. Dans des expositions très agressives (deux fois la Classe 5), seuls les coulis ayant une teneur en cendres volantes très élevée (>90%) n’ont montré aucun endommagement après 12 mois d’exposition. L’influence de la présence supplémentaire du chlorure de magnésium semble modifier la nature de l’endommagement, mais tendrait à l’inhiber. On discute des implications pratiques de cette étude et on propose de façon préliminaire des moyens de spécifier et de catégoriser les coulis aux cendres volantes.