Michael J. McCarthy

In-situ performance of CPF concrete in a coastal environment

The paper describes the work of an investigation carried out to evaluate the performance of controlled permeability formwork (CPF) in a seawall. Tests were carried out in the splash (SP) and inter-tidal (I/T) regions of the seawall at two locations (built in two phases, approximately 1 year apart), which used CPF, and on a groyne where the system was not used (reference (Ref) concrete). The tests were made both in-situ and on cores removed from the walls, in the laboratory. Properties measured included, core strength, surface hardness, capillary porosity, absorption, carbonation, chloride profiles, half-cell potentials and concrete resistivity. The results indicate that CPF gave improved performance compared to the Ref concrete for almost all properties. Comparisons with other laboratory and early site-based data indicate that the benefits observed in these for CPF concrete were obtained in the test concretes of the study. In addition, there appeared to be no depletion in CPF concrete performance due to the aggressive conditions.

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.

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.

Use of portland pfa cement in combination with superplasticizing admixtures

Abstract The results of a test programme undertaken to achieve parity between concretes made with Portland cement (PC, Class 42.5 N) to BS 12 and Portland pulverized-fuel ash cement (PPFAC) to BS 6588 at equivalent cement content, by controlling the water content of the mix and the use of a superplasticizing admixture are described. With 27% PFA as a constituent of the cement, parity at 28 days was achieved at water contents of between 145 and 160 kg/m3, depending on the cement content. A mix design chart is developed from the data to illustrate how the water and PPFAC contents of concrete can be adjusted to achieve a particular design strength. Issues relating to practical aspects of use are also considered.

Mix proportioning and engineering properties of conditioned PFA concrete.

Abstract This paper describes the mix design procedure developed for use with conditioned (moistened) pulverized-fuel ash (PFA) in structural concrete. It is shown that a given strength can be achieved with conditioned PFA concrete with some reduction in w/c ratio, compared to equivalent dry PFA mixes. The adjustment required is progressive with conditioned PFA storage period (up to 6 months), but is not influenced significantly by either dry PFA characteristics or conditioning/storage variables. For a given strength, the development of compressive strength (up to 180 days), flexural strength, modulus of elasticity ( E ), drying shrinkage ( ξ S ) and creep ( ξ C ) are essentially the same for dry and conditioned PFA mixes. The practical implications of the work are presented.

Recovery, processing, and usage of wet-stored fly ash

Abstract With changes in how electricity is generated, there may be a gradual reduction in available fly ash for use in concrete and other applications. The significant quantities of fly ash stored around power plants in stockpiles and ponds, in some cases over many years, represent a source of material that could help address this situation. Wet-stored fly ash can undergo physical and chemical changes during storage, affecting its reactivity potential and it may be prone to variability. Recovery of this is therefore likely to involve evaluation of the material properties, and development of processing strategies. This chapter examines (i) the characteristics of wet-stored fly ash, (ii) methods of sampling material, (iii) various wet processing techniques that can be used to remove or concentrate components, and (iv) use in cementitious systems. Consideration is also given to the direct use of conditioned/stockpile fly ash in concrete.

Effect of metakaolin on early strength of GGBS ternary concrete

The article reports a laboratory experimental programme that investigated effect of metakaolin on the early strength of concrete made with ternary combinations of Portland cement (CEM I) with ground granulated blast slag (GGBS) and metakaolin (MK). The various level of cement combinations (65%CEM I+30%GGBS+5%MK, 45%CEM I+45%GGBS+10%MK and 45%CEM I+40%GGBS+15%MK) was examined in comparison to CEM I and equivalent GGBS binary concretes for up to 28 days. Results show that the reduction in early strength is greater with the higher cement replacement level. However, the ternary concrete containing 15%MK has minor increase in early strength compared to those with 10%MK but a significant increase in strength is examined at later age (28 days). It is concluded that the presence of MK compensates the adverse effect of GGBS at early strength development and improves the strength at later ages.

Challenges of Concrete Construction: Volume 6, Concrete for Extreme Conditions: Proceedings of the International Conference held at the University of Dundee, Scotland, UK on 9–11 September 2002

This volume includes key papers in the following areas: Chemically and physically aggressive environments Marine and underwater concrete Temperature and humidity effects on concrete Design for accidental damage Extreme loading conditions Concrete for specialist situations