Michael D.A. Thomas

PERFORMANCE OF PFA CONCRETE IN A MARINE ENVIRONMENT - 10-YEAR RESULTS

Abstract Steel reinforced 100×100×300 mm concrete prisms, with nominal strength grades C25, C35 and C45 and different pfa levels (0–50%), were exposed to various curing treatments during the first 28 days prior to exposure in the tidal zone of the BRE marine exposure site. Chloride concentration profiles and rebar weight losses had previously been measured on specimens exposed for 1, 2 and 4 years and this paper reports the results of similar measurements after 10 years exposure. Chloride profiles were also measured for specimens after 1 and 28 days immersion in seawater under laboratory conditions. Pfa concretes showed substantially increased resistance to the penetration of chlorides compared with control Portland cement (PC) concrete specimens. The improved resistance of the pfa concrete to the penetration of chlorides resulted in reduced corrosion of steel bars imbedded in the concrete. Threshold chloride levels for corrosion, estimated from relationships between steel weight loss and chloride content at the location of the steel, were found to decrease with increasing pfa content. Chloride concentration profiles after 28 days of immersion in seawater showed that considerable chloride penetration occurred during this period due to sorption (capillary suction) of the seawater into the unsaturated specimens. This results in a significant error in diffusion coefficients calculated from the concentration profile using the standard solution to Fick’s second law. The error may be substantial for pfa concretes where chlorides penetrating due to sorption immediately after exposure may outweigh subsequent diffusion during continued seawater exposure. The performance of the concretes, particularly the PC concretes, in this programme is considered in the light of current and new British Standard recommendations for concrete exposed to marine tidal conditions. The adequacy of these recommendations is, however, difficult to assess because the highest concrete quality tested fell a little short of the minimum quality required in the recommendations and cover to reinforcement was also less than required. Nevertheless, the superior performance of concretes containing 30% or more pfa was clearly demonstrated.

THE USE OF BULK DIFFUSION TESTS TO ESTABLISH TIME-DEPENDENT CONCRETE CHLORIDE DIFFUSION COEFFICIENTS

The proper determination of chloride diffusion values, including how they change with time, is important for service life modelling. Currently, there are two major approaches for using chloride diffusion coefficients to predict the service life of structures. The average diffusion coefficient or the instantaneous diffusion coefficient can be used. Using instantaneous diffusion coefficients is a more flexible technique, but requires a more advanced evaluation of bulk diffusion test results to establish material parameters. This paper describes an analytical procedure for determining the instantaneous chloride diffusion value as a function of time for a concrete using data from bulk diffusion tests. The importance of interpreting the data correctly is illustrated with simulated bulk diffusion test data, generated using a finite-difference model for diffusion. In addition, the application of this procedure for evaluating diffusion values is illustrated with experimental data.

The effect of pozzolans and slag on the expansion of mortars cured at elevated temperature: Part I: Expansive behaviour

The expansive behaviour of heat-cured mortars containing pozzolans and slag was investigated. In most cases, the addition of any amount of these materials to the mixture typically reduced the long-term expansion, slowed the rate of expansion, and delayed the onset of expansion. However, the efficacy of a particular pozzolan or slag in controlling expansion may depend on its Al2O3 content. Metakaolin, which contains a high amount of reactive Al2O3, was the most effective at controlling expansion at relatively low cement replacement levels. Slag and fly ash, which are also sources of Al2O3, were also effective at suppressing expansion at higher replacement levels. Silica fume was less effective at controlling expansion at conventional replacement levels, and even at higher replacement levels expansion may only be delayed.

A NOVEL METHOD FOR DESCRIBING CHLORIDE ION TRANSPORT DUE TO AN ELECTRICAL GRADIENT IN CONCRETE: PART 1. THEORETICAL DESCRIPTION

Abstract This paper is the first of two reporting the development of a novel method for describing the transport of chloride ions in concrete under an electrical gradient. In this part, the theory is described and developed. In the second paper, experimental evidence to support the proposed theory is provided. This new theory is designed to correct some deficiencies of the current state of the art. Namely, it considers the effect of concrete being a porous media rather than considering concrete as an aqueous solution as is done with the standard laws; for example, the Nernst–Planck equation.

Apparent Diffusivity Model for Concrete Containing Supplementary Cementitious Materials

Concrete’s resistance to chloride diffusion is one of the primary factors governing the concrete structure service life and life-cycle costs. This paper presents a new model developed for estimating the apparent concrete diffusivity based on the mixture proportions, cementitious materials used, and concrete age. The model includes the effects of supplementary cementitious material types commonly found in other service life models such as fly ash, ground-granulated blast-furnace slag, and silica fume. Also included are ultra-fine fly ash and metakaolin, which were not available in previous service life models. For validation of the model, chloride profiles have been measured on concrete blocks exposed daily to seawater for 25 years at the Treat Island, ME concrete exposure site. Concrete mixtures tested as part of the validation dataset contained up to 80% ground-granulated blast-furnace slag, 25% fly ash, or 20% silica fume, and were compared against the predicted values and are presented in this paper.

A NOVEL METHOD FOR DESCRIBING CHLORIDE ION TRANSPORT DUE TO AN ELECTRICAL GRADIENT IN CONCRETE: PART 2. EXPERIMENTAL STUDY

Abstract This paper is the second of a set of two that develops a novel method for describing the transport of chloride ions in concrete under an electrical gradient. In Part 1, the development of a method to predict the movement of chloride ions under an electrical gradient is outlined. This new theory is designed to correct some deficiencies of the current state of the art. Namely, it considers the effect of concrete being a porous media rather than considering concrete as an aqueous solution as is done with the standard laws, e.g., the Nernst–Planck equation. In this part, experimental evidence is presented to support the theory.

Characterization of Fly Ashes for Sulfate Resistance

The sulfate resistance of mixtures containing six fly ashes was studied with reference to their chemical and mineralogical compositions. Quantitative analysis of the mineralogical composition of fly ashes was carried out through X-ray diffraction (XRD) using the Rietveld method of analysis. Bulk and glass composition of the fly ash particles was also determined by using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDXA). The sulfate resistance of fly ash mortars was evaluated using the ASTM C1012/C1012M test. The performance of each fly ash in the sulfate resistance test was related to the chemical and mineralogical composition of the fly ash and its position on the ternary (CaO-SiO2-Al2O3) diagram. The study showed that the nature of glass is an important factor in determining the sulfate resistance of fly ash mixtures in a sulfate environment.

USE OF A LITHIUM-BEARING ADMIXTURE TO SUPPRESS EXPANSION IN CONCRETE DUE TO ALKALI-SILICA REACTION

The findings of a laboratory study on the effects of a lithium-based admixture on alkali-silica reaction are presented. The admixture essentially represents an intermediate stage in the lithium extraction process of the ore spodumene. Spodumene ore is normally crushed and fired in a kiln to “untie” the lithium prior to its extraction. The material at this stage is known as decrepitated spodumene (DS). DS is basically an amorphous alumino-silicate that contains significant quantities of “available” lithium. As such, DS has the potential to act both as a pozzolanic mineral admixture as well as a chemical suppressant of alkali-silica reactivity. Laboratory testing included pore solution studies of pastes and mortars containing DS and expansion studies of mortar bars containing Pyrex glass (ASTM C311) and concrete prisms containing reactive aggregates from three different sources in North America. The results show that significant quantities of lithium are released by the DS into the pore solution of pastes and mortars over time. After 1 year, the Li+ concentration of pastes with 15 to 20 percent DS (mass replacement of high-alkali cement) was approaching 0.3 moles/liter. The efficacy of DS in controlling expansion in concrete containing reactive aggregates was found to depend on the nature of the reactive aggregate. Higher quantities of DS were required with more reactive aggregate. This is thought to be due to the rapid expansion observed for concretes containing such aggregates. The expansion occurs before significant lithium is released from the DS, and thus higher levels of DS are required. It is possible that the efficiency of DS with such aggregates may be improved by increasing the fineness or modifying the manufacturing process.

Lessons Learned from the Treat Island Marine Exposure Site

The marine exposure site on Treat Island near Eastport, Maine, was built more than 75 years ago and during this period a wide range of concrete types have been placed on the site. Treat Island represents a very severe exposure condition with the highest tides in the world, salinity typical of the Atlantic Ocean and approximately 100 freeze-thaw cycles per annum. The various research programs that have used this facility have investigated the effects of numerous parameters including fibre-reinforcement, polymer-impregnation, supplementary cementing materials, sulfur concrete, high-alumina cement, ettringite-based rapid-set binders, w/cm and strength, ultra-high-performance concrete, corrosion-resistant reinforcement, impact of load and cracking, “mechanical air-entrainment”, and use of corrosion-inhibiting admixtures. Performance has been evaluated in a number of ways including visual assessment, pulse velocity, dynamic modulus, chloride profiling, and electro-chemical corrosion monitoring. The paper presents an overview of “lessons learned” with detailed information on factors affecting the rate of chloride ingress.

Lowering the Carbon Footprint of Concrete by Reducing Clinker Content of Cement

Significant efforts have been made to reduce carbon dioxide (CO2) emissions associated with the manufacture of portland cement, primarily by making the process more energy efficient and increasing the use of alternative fuels. Further reductions in CO2 can be achieved by lowering the clinker component of the cement because the pyroprocessing used to manufacture clinker produces approximately 1 tonne of CO2 for every tonne of clinker. Traditionally reductions in the clinker content of cement have been achieved by producing blended cement consisting of portland cement combined with a supplementary cementing material (SCM). In Canada, it is now permitted to intergrind up to 15% limestone with cement clinker to produce portland limestone cement or blended portland limestone cement. Recent trials were conducted at the Brookfield cement plant in Nova Scotia to evaluate the performance of a blended cement containing 15% ground, granulated blast furnace slag (an SCM) with that of a blended portland limestone cement containing the same amount of slag plus 12% interground limestone. Performance was evaluated by the construction of a section of concrete pavement using concrete mixtures produced with the two cements and various amounts of fly ash (another SCM). A wide range of laboratory tests were performed on the concrete specimens cast on site during the placement of the concrete pavement. The results indicated that the cements were of equivalent performance.