R.D. Hooton

Influence of Silica Fume Replacement of Cement on Physical Properties and Resistance to Sulfate Attack, Freezing and Thawing, and Alkali-Silica Reactivity

The results are presented of a series of tests of the physical properties and durability of pastes, mortars, and concretes made with silica fume replacements for part of the portland cement: silica fume reduced the permeability of cement pastes and concretes; silica fume reduced the size of pores and total porosities of both cement and concretes; silica fume reduced the leachable calcium hydroxide contents of cement pastes; silica fume reduced the level of alkalies leached from the pastes; silica fume improved the resistance of ASTM C 441 Pyrex mortar bars to deleterious expansion; silica fume provided resistance superior to that of a sulfate resistant portland cement in ASTM C 1012 tests for sulfate resistance; and silica fume provided resistance to freezing and thawing to a series of high-strength, non-air-entrained concretes.

A study of the effect of chloride binding on service life predictions

One of the major causes of deterioration of reinforced concrete structures is chloride-induced corrosion of the reinforcing steel. The magnitude of the damage is especially large in structures exposed to marine environments and de-icing salts. The capacity of the concrete cementitious system to bind chloride ions has an important effect on the rate of chloride ionic transport in concrete and on the corrosion initiation of the steel reinforcement. This paper reviews mathematical models used in the literature to describe chloride binding in concrete. The impact of the different binding relations on the time-dependent chloride penetration profiles is investigated by solving the chloride mass conservation statement with a finite-difference approach. Results are presented for a concrete structure subjected to two different exposure conditions: submerged in seawater and exposed to de-icing salts. The implication of accounting for chloride binding in service life estimations is discussed.

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.

An overview and sensitivity study of a multimechanistic chloride transport model

Service life prediction models have been developed in order to ensure adequate durability of reinforced concrete structures in chloride environments. Many of the models currently available are overly simplistic, assuming chloride ingress occurs solely by diffusion and that boundary conditions and concrete properties (i.e., diffusivity) remain constant with time. This paper describes a recently developed model that considers multimechanistic transport, chemical binding, and the time-dependent nature of concrete properties. The results of a sensitivity analysis carried out on eight of the input parameters in the model are also reviewed.

Effects of cyclic chloride exposure on penetration of concrete cover

Concretes are in a state of flux between saturated and partially saturated conditions as they undergo continuous cycles of wetting and drying. In saturated concrete, dissolved ions enter through diffusion, whereas in partially saturated concrete, ion-containing fluids are absorbed by capillary suction and concentrated by evaporation of water. The primary focus of this study was to examine the effects of cyclic wetting and drying with sodium chloride solution on chloride ingress into concrete. Chloride profiles of samples exposed to various lengths and numbers of cycles were determined for three mixtures of concrete: two containing slag and/or silica fume with a 0.4 w/cm (water to cementing materials ratio) and one with a 0.3 w/cm. It was found that longer drying times increase the rate of chloride ingress. A good relationship exists between the depth of chloride penetration and the square root of the number of cycles.

CORROSION EFFECTS ON BOND STRENGTH IN REINFORCED CONCRETE

Corrosion damage of reinforced concrete (RC) structures is a serious problem responsible for billions of dollars in repairs of highway structures every year. Corrosion of reinforcing steel in RC affects structural performance in two different ways: by loss of steel section and through deterioration of steel-concrete bond. This experiment investigated the effects of corrosion products on bond strength. To test the strength of corroded reinforcing bar anchorages and to quantify the effect of deterioration of the bar surface on development length, RC slabs were cast with the ends of the reinforcing bars anchored in the concrete for a known length. Bond breakers were used over the center portion of the bars to control the force demand input to the test anchorage. This measure also served to protect those regions from corrosion. The anchorage zones were corroded to various degrees by applying electrical voltage to the bar ends. Specimens were tested in flexure to assess strength and mode of failure. Results were summarized by means of simple parametric design expressions that relate bond strength to steel area loss.

Long-term testing of the chloride-penetration resistance of concrete containing high-reactivity metakaolin

This paper presents the long-term results of a study investigating the chloride penetration resistance of concrete containing high-reactivity metakaolin (HRM). The metakaolin evaluated in this study is a highly processed kaolinite clay that has been heat-treated under controlled conditions to produce high pozzolanic activity. Six concrete mixtures were cast with 0%, 8%, or 12% by mass replacement of portland cement with HRM at water-to-cementitious materials ratios (w/cm) of 0.30 or 0.40. The early age test results are summarized and include (1) strength, (2) 28-day and 90-day bulk diffusion, (3) rapid chloride permeability, and (4) resistivity. The long-term tests performed include bulk diffusion testing at 140 days, 1 year, and 3 years and chloride migration testing. It was found that strength increased at all ages with decreasing w/cm and increasing content of HRM. The results from all of the early-age experimental work showed that higher metakaolin content and lower w/cm decreased diffusion, permeability, and conductivity and increased resistivity. Resistance to chloride migration increased with increasing metakaolin content and decreasing w/cm. The long-term bulk diffusion testing showed a continued improvement in chloride resistance for all of the mixtures.

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.

Water-permeability measurement of high performance concrete using a high-pressure triaxial cell

Abstract Water permeability of concrete is used to indicate its durability. Accurate and reproducible measurement of water permeability is difficult and becomes more difficult as the quality of concrete increases. When high-performance concrete (HPC) is tested, these concerns become more pronounced. HPC is used widely to improve the durability and performance of structures but there are few test procedures able to evaluate its permeability-related properties. In this study the water permeabilities of concretes including HPC were measured using a highpressure triaxial cell with a sensitive and automated measurement capability. Special analysis procedures were developed to obtain useful data from the extremely low volume of water being measured. This method was able to measure a wide range of permeability values from 10 −12 m/s to 10 −16 m/s, with reproducible measurements on replicates.

Influence of voltage on chloride diffusion coefficients from chloride migration tests

A chloride migration test method is described and test results are given for concrete subjected to a range of potential gradients. A method for setting the potential across the sample length using reference electrodes and Luggin capillaries is described. Apparent and effective diffusion coefficients were calculated from chloride break-through time and steady state chloride flux respectively at each potential. Accounting for polarization and iR drop losses is significant particularly at lower applied potentials.