Michael D A Thomas

Modelling chloride diffusion in concrete: Effect of fly ash and slag

Data from long-term field and laboratory studies of concrete exposed to chloride environments were analyzed using a chloride transport model developed at the University of Toronto. The results show that the incorporation of fly ash and slag may have little impact on transport properties determined at early ages (e.g., 28 days), but can lead to order of magnitude improvements in the long term. This means that the rate of chloride penetration during the first 6 months or so of exposure is similar for concretes with and without these materials. However, after a few years of exposure, chloride ingress slows to a much-decreased rate in fly ash and slag concretes, leading to dramatic increases in the predicted service life. Predictive models and laboratory test methods for determining chloride ingress should take account of the time-dependent nature of the transport processes in concrete, especially when supplementary cementing materials, such as fly ash or slag, are used.

Chloride thresholds in marine concrete

This paper reports results from an ongoing study of the performance of fly ash concrete in marine exposure. Reinforced concrete specimens exposed to tidal conditions were retrieved at ages ranging from 1 to 4 years. Steel reinforcement mass losses are compared with chloride contents at the location of the bar for concrete specimens of various strength grades and with a range of fly ash levels. The maximum level of chloride that could be tolerated without significant mass loss due to corrosion was found to vary with fly ash content. This threshold chloride level decreased with increasing fly ash content; values obtained were 0.70%, 0.65%, 0.50% and 0.20% acid-soluble chloride (by mass of cementitious material) for concrete with 0%, 15%, 30% and 50% ash, respectively. Despite the lower threshold values, fly ash concrete was found to provide better protection to the steel under these conditions, due to its increased resistance to chloride ion penetration.

The effect of fly ash composition on the expansion of concrete due to alkali-silica reaction

Abstract This paper presents the results from expansion tests on concrete prisms and mortar bars containing reactive aggregate and different types and levels of fly ash. Eighteen fly ashes representing those commercially available in North America were tested. The results show that the bulk chemical composition of the fly ash provides a reasonable indication of its performance in physical expansion tests but cannot be used to accurately predict the degree of expansion or the minimum safe level of fly ash required to suppress expansion to an acceptable limit. Generally, for a given fly ash replacement level (RL), the expansion increases as the calcium or alkali content of the ash increases or its silica content decreases. A corollary to this is that the minimum level of fly ash required to limit the expansion to an acceptable level increases as the calcium or alkali content of the ash increases or its silica content decreases. Most of the variation in fly ash performance can be explained on the basis of pore solution composition; those ashes effective in reducing the alkalinity of the pore solution extracted from cement paste samples were also efficient in controlling expansion. The data from this study provide further support for the use of the accelerated mortar bar test as a means for evaluating the efficacy of pozzolans in controlling expansion due to alkali–silica reaction (ASR).

Use of ternary cementitious systems containing silica fume and fly ash in concrete

This paper reports the results from laboratory studies on the durability of concrete that contains ternary blends of portland cement, silica fume, and a wide range of fly ashes. Previous work has shown that high CaO fly ashes are generally less effective in controlling alkali silica reactivity (ASR) and sulfate attack compared with Class F or low lime fly ashes. Indeed, in this study it was shown that replacement levels of up to 60% were required to control expansion due to ASR with some fly ashes. However, combinations of relatively small levels of silica fume (e.g., 3 to 6%) and moderate levels of high CaO fly ash (20 to 30%) were very effective in reducing expansion due to ASR and also produced a high level of sulphate resistance. Concretes made with these proportions generally show excellent fresh and hardened properties since the combination of silica fume and fly ash is somewhat synergistic. For instance, fly ash appears to compensate for some of the workability problems often associated with the use of higher levels of silica fume, whereas the silica fume appears to compensate for the relatively low early strength of fly ash concrete. Diffusion testing indicates that concrete produced with ternary cementitious blends has a very high resistance to the penetration of chloride ions. Furthermore, these data indicate that the diffusivity of the concrete that contains ternary blends continues to decrease with age. The reductions are very significant and have a considerable effect on the predicted service life of reinforced concrete elements exposed to chloride environments.

INCREASING CONCRETE DURABILITY WITH HIGH-REACTIVITY METAKAOLIN

Abstract High-reactivity metakaolin (HRM) is a manufactured pozzolan produced by thermal processing of purified kaolinitic clay. Field performance and laboratory research of concrete containing HRM have demonstrated its value for bridge decks, bridge deck overlays, industrial flooring, high-strength concrete and masonry products. This paper discusses laboratory evaluations to assess the long-term performance of concrete containing HRM produced in North America for resistance to chloride penetration and reduction in expansion due to alkali-silica reactivity. Bulk diffusion testing indicated that HRM substantially reduced chloride ion penetration in concrete with w/cm of 0.30 or 0.40. Reductions in diffusion coefficients compared to control specimens were of the order of 50% and 60% for concrete with 8% and 12% HRM, respectively. Also, the performance of the concrete containing 8% or 12% cement replacement with HRM showed improved performance versus merely reducing the w/c from 0.4 to 0.3. Such reductions can be expected to have a substantial impact on the service life of reinforced concrete in chloride environments. Expansion tests on concrete prisms containing reactive aggregates showed that 15% HRM can prevent deleterious expansion due to alkali-silica reactivity (ASR). The mechanism of control is likely linked to the substantial reduction in pore solution alkalinity seen in pastes containing 20% HRM in comparison to the control specimen which contained no supplementary cementing materials. However, the reduction was not large enough to depassivate steel reinforcement.

The effect of metakaolin on alkali–silica reaction in concrete

This article reports on a study to evaluate the efficacy of high-reactivity metakaolin (HRM) in controlling expansion due to alkali–silica reaction (ASR). The expansion of concretes and mortars containing 0–20% HRM as a partial replacement for OPC was studied. Concrete prisms were prepared according to the CAN/CSA A23.2-14A concrete prism method with two alkali–silica reactive aggregates: a siliceous limestone (Spratt) and a greywacke-argillite gravel (Sudbury). The amount of HRM required to control the expansion to <0.04% at 2 years was found to be between 10% and 15% depending on the aggregate. A modified version of the accelerated mortar bar method (CAN/CSA A23.2-25A) was also conducted with HRM and both reactive aggregates. n nIn addition to expansion testing, the chemistry of expressed pore solutions from cement pastes containing 0%, 10%, and 20% metakaolin was determined over a 2-year period. Incorporation of 20% metakaolin was found to significantly reduce the long-term OH−, Na+, and K+ ion concentrations in pore solutions. The reduction in the pH and the alkalinity of pore solutions correlate well with the observed reduction in expansion of the concrete prisms and mortar bars.

The effects of fly ash composition on the chemistry of pore solution in hydrated cement pastes

This paper reports the findings of an investigation to determine the influence of fly ash composition on the evolution of the pore solution chemistry in Portland cement/fly ash systems. Twelve fly ashes, selected to represent the wide range of composition of North American ashes, were used in the study. In addition to pore solution expression and analysis, inner hydration products were analyzed using energy-dispersive X-ray analysis. The study shows that the alkalinity of pore solution increases as the calcium and alkali content of the fly ash increase, and decreases as the silica content of the ash increases. However, there is no consistent trend between the composition of the inner calcium-silicate hydrate and fly ash composition.

DURABILITY OF TERNARY BLEND CONCRETE WITH SILICA FUME AND BLAST-FURNACE SLAG: LABORATORY AND OUTDOOR EXPOSURE SITE STUDIES

In September 1998, a site investigation assessing the durability performance of ternary blend concretes was initiated by an academic-government-industry consortium. Different concretes were cast in the field to assess durability performance in an outdoor exposure setting as well as with standard lab tests. Resistances to the following deterioration mechanisms were assessed: alkali-silica reactivity, chloride ion ingress, and deicer salt scaling. Compressive strength was also measured at various ages. This paper describes this project in detail and presents field observations and lab findings up to 2 years later.

Use of ternary blends containing silica fume and fly ash to suppress expansion due to alkali–silica reaction in concrete

Abstract This paper investigates the effects of cementitious systems containing Portland cement (PC), silica fume (SF) and fly ash (FA) on the expansion due to alkali–silica reaction (ASR). Concrete prisms were prepared and tested in accordance with the Canadian Standards Association (CSA A23.2-14A). Paste samples were cast using the same or similar cementitious materials and proportions that were used in the concrete prism test. Pore solution chemistry and portlandite content of the paste samples are reported. It was found that practical levels of SF with low-, moderate- or high-calcium FA are effective in maintaining the expansion below 0.04% after 2 years. Pore solution chemistry shows that while pastes containing SF yield pore solutions of increasing alkalinity at ages beyond 28 days, pastes containing ternary blends maintain the low alkalinity of the pore solution throughout the testing period (3 years).

Microstructural Studies of Alkali-Silica Reaction in Fly Ash Concrete Immersed in Alkaline Solutions

This article presents expansion and microstructural data for a series of concrete mixes containing reactive flint aggregate, with a range of fly ash levels, exposed to various alkaline salt solutions. This study was undertaken to determine whether fly ash has any influence on alkali-aggregate reaction beyond changes in pore solution chemistry; in these tests the external source of alkalis should neutralize pore solution effects. Fly ash was found to be effective in reducing expansion even after extended periods (44 months) of exposure in 1N NaOH at 80°C, notwithstanding the presence of abundant reactive silica and an inexhaustible supply of alkali hydroxides. Higher levels of ash (40%) prevent damaging expansion and cracking in this environment despite considerable evidence of reaction. In some cases, flint grains had been completely removed by dissolution. The addition of Ca(OH)2 at the mixing stage was found to increase the expansion of all the concretes; the effect on concrete with 40% ash was most marked, the expansion increasing by nearly 20 times. The most noticeable difference between deteriorated control specimens (no ash) and concrete with 40% ash was the formation of a calcium-alkali-silica rim on certain flint grains in concrete without ash. Such particles were invariably sites of expansive reaction with cracks emanating from them. The absence of such a feature in concrete with 40% ash is probably linked to the reduction in Ca(OH)2 at the cement-aggregate interface. It is possible that the formation of this reaction rim produces expansive forces itself or acts as a semi-permeable membrane preventing diffusion of alkali silicate solution from the reaction site, thereby leading to osmotic pressure generation. Regardless of the actual mechanism, the presence of Ca(OH)2 appears to be critical for the development of expansion due to alkali-silica reaction. It was observed that the alkalis of the reaction product were distributed in bands. In the Portland cement concrete specimens, the distribution of the gel consisted of a high calcium reaction rim at the aggregate-cement interface with a sodium-rich silica gel adjacent to it, followed by a potassium-rich silica gel. The potassium-rich silica gel appears to have a crystalline, needle-like structure, whereas the sodium-rich silica gel is amorphous. In fly ash concrete specimens in which the formation of calcium-rich reaction rim was prevented, it was observed that the sodium-rich gel had diffused into the surrounding cement matrix, and the potassium-rich gel had remained within the original aggregate boundary.