Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency

The aim of this study was to clarify whether the C₃A content of cement had a significant effect on electrochemical chloride extraction (ECE) treatment efficiency. It is known that a higher C₃A content in a cement gives it superior chloride complexing ability resulting in the formation of an “insoluble” calcium chloro aluminate compound.ECE was applied using cylindrical specimens made from concrete containing two levels of C₃A (4.3% and 9.05%). Specimens were 5 cm in diameter and 10 cm in height. Steel was placed in the axial direction with an embedded length of 7 cm. These specimens were immersed in an NaCl solution and dried in a stream of air at 40 °C for 10 months. The corrosion was monitored by half-cell potential and polarization resistance measurements. After steel depassivation, ECE was applied for 20, 30, 40 and 50 days using a constant current density of 1 A/m² of steel. At the end of the treatment, the specimens were maintained at 20 °C and 70% RH in order to observe the evolution of the steel (electrochemical measurements).The results show that, after 30 days of treatment, the chloride content remained constant in the specimen. This was probably due to OH⁻ ion formation on the steel. The OH⁻ ions “contribute” to the current transport, decreasing the ECE efficiency. Concerning the C₃A content, ECE efficiency was slightly affected by C₃A because only a part of the bound chloride ions was released. From the point of view of corrosion, half-cell potential showed a shift in the positive direction, indicating little corrosion activity at 20 °C and 70% RH. However, polarization resistance measurements showed that 2 months post treatment corrosion rates were significant, although the corrosion rate decreased from 6 μA/cm² to 2.5 μA/cm².     

Effects of seawater on AAR expansion of concrete

Recently, AAR was identified in submerged piles of some bridges in tidal waters. Microstructural examination detected chloroaluminate salts in some cracks. To clarify whether seawater had influenced the deterioration an experimental program was planned to examine the effects of sodium chloride on AAR under various curing conditions.Concrete prisms containing either of highly-reactive, slowly-reactive or nonreactive aggregate, and either low or high alkali contents, were stored in saltwater (representing seawater) or at 100% RH, at temperatures of 38, 60 and 80 °C, for expansion measurement over 600 days, after which the temperature for those stored in saltwater was lowered to 23 °C, to check its effect on further expansion, which could be attributed to precipitation of ettringite and/or Ca-chloroaluminate.The results indicate that the type of aggregate and concrete alkali content had the greatest effect on AAR expansion. Exposure to saltwater did not have any significant effect on the AAR expansion.     

Effects of alkali addition on the mechanical properties and durability of concrete

Increasing the concrete alkali content from 0.6% to 1.25% of Na₂O_e of the cement mass by adding NaOH to the mixture water has harmful effects on most mechanical properties (compressive, splitting, direct tensile, and flexure strengths) of concrete made with a water-to-cement (w/c) ratio of 0.41 and limestone aggregates not susceptible to alkali-silica reaction (ASR), however not on the elasticity modulus measured under compression or direct tension. Shrinkage tests at 50% RH and 23 °C started after 7 days at 100% RH and 23 °C show that the low-alkali concrete shrinks more than the high-alkali one, despite similar water losses. Freeze-thaw tests performed on air-entrained concretes show that the two concretes resist well to freezing and thawing while showing similar air-void systems. When examined under the scanning electron microscope (SEM), the hydrates in the two concretes present similar microstructure; however, the high-alkali concrete shows a more reticular and porous microtexture, which could explain the reduction in strength.     

Evaluation of the short and long-term efficiencies of electro-chemical chloride extraction

The short and long-term effects of electro-chemical chloride extraction (ECE) treatment on corrosion rate of corroded reinforcement have been studied. The factors controlling ECE treatment were identified, and the alterations which occurred in the chloride profile of the cover zone due to ECE process were followed. Contaminated reinforced OPC and SRC mortar specimens made with 1 or 2% NaCl, by mass of cement, were prepared, cured and then subjected to 3 or 5% NaCl for one year. These specimens were finally treated with ECE using various impressed current densities (1 and 2 A/m²) and electrolytes (water and calcium hydroxide) for 2, 4 and 8 weeks. The state of corrosion was monitored regularly every 2 weeks from halting ECE process up to 52 weeks using zero-resistance ammeter device for measuring the corrosion current density of reinforcement. Selected samples from the cover zone of the untreated and treated specimens were finally taken to assess their chloride profiles.     

Effect of drying-rewetting on the alkali concentration of the concrete pore solution

Experience has shown that the pore solution alkalinity of resaturated concrete was lower than expected. Laboratory-concrete specimens were made with reactive aggregates, stored over water at 38 °C, and then broken into two pieces. The pore solution of half-specimens was extracted by high-pressure squeezing. The other half-specimens were dried at ambient air and rewetted in humid air to their initial weight. The pore solution was then extracted and compared with the composition results of the first extraction. The results obtained from this investigation confirmed that a certain part of the alkali ions in pore solution that had become fixed by drying are not subsequently extracted after rewetting. The alkali concentration [Na+K] was reduced from 34% to 61% by the drying and rewetting treatments.     

Effect of the cement chemistry and the sample size on ASR expansion of concrete exposed to salt

Mortar bars and concrete prisms made with a very alkali-silica reactive limestone were stored at 38 °C in 1 M NaOH and NaCl solutions. A high-alkali (HA) cement and a low-alkali (LA) cement were used in order to evaluate the cement chemical composition on the expansion and on the chemistry of the pore water. The mortar bars immersed in 1 M NaOH presented much more expansion than mortar bars stored at 100% RH or in 1 M NaCl. The behaviour of the concrete prisms was completely different. Low expansion was obtained for concrete prisms made with the LA cement immersed for more than 5 years in 1 M NaCl solution, while the expansion was over 0.45% for concrete prisms made with the HA cement. Chemical equilibrium between the pore waters and the immersion solution was much longer to obtain for the concrete prisms (near 3 years) than for the mortar bars (less than 3 months). The results obtained in this study show that the type of sample used (mortar bars or concrete prisms) and the cement composition strongly influence the harmful effects of ASR in concrete exposed to salt.     

The effects of potassium and rubidium hydroxide on the alkali-silica reaction

Expansion of mortar specimens prepared with an aggregate of mylonite from the Santa Rosa mylonite zone in southern California was studied to investigate the effect of different alkali ions on the alkali-silica reaction in concrete. The expansion tests indicate that mortar has a greater expansion when subjected to a sodium hydroxide bath than in a sodium-potassium-rubidium hydroxide bath. Electron probe microanalysis (EPMA) of mortar bars at early ages show that rubidium ions, used as tracer, were present throughout the sample by the third day of exposure. The analysis also shows a high concentration of rubidium in silica gel from mortar bars exposed to bath solutions containing rubidium. The results suggest that expansion of mortar bars using ASTM C 1260 does not depend on the diffusion of alkali ions. The results indicate that the expansion of alkali-silica gel depends on the type of alkali ions present. Alkali-silica gel containing rubidium shows a lower concentration of calcium, suggesting competition for the same sites.     

Penetration profile of chloride ion in cracked reinforced concrete

A detailed observation on the penetration profile of chloride ions through and around a crack in reinforced concrete structures was carried out. Electron probe microanalysis (EPMA) and colorimetric tests were conducted on cracked specimens, which were exposed to NaCl solution at a temperature of 20 °C and a humidity of 60% RH, after being conditioned in the same condition for 2 months. Research parameters included water to cement ratio (w/c), single and multicracks, exposed direction, crack width, NaCl solution concentration and cover thickness. Increasing w/c led to a higher ingression rate of Cl⁻ ions, not only from the exposed surface but also around the cracks. It was found that the penetration depth from the surface of the cracks was equal to or slightly higher than that from the exposed surface for higher w/c mixes of 0.45 and 0.65. The transportation of Cl⁻ ion was strongly influenced by the bulk movement of the solution inside the concrete.     

Influence of stress restraint on the expansive behaviour of concrete affected by alkali-silica reaction

The primary objective of this study was to ascertain whether the Threshold Alkali Level (TAL) of the concrete aggregates may be taken as a suitable reactivity parameter for the selection of aggregates susceptible of alkali-silica reaction (ASR), even when ASR expansion in concrete develops under restrained conditions. Concrete mixes made with different alkali contents and two natural siliceous aggregates with very different TALs were tested for their expansivity at 38 °C and 100% RH under unrestrained and restrained conditions. Four compressive stress levels over the range from 0.17 to 3.50 N/mm² were applied by using a new appositely designed experimental equipment. The lowest stress (0.17 N/mm²) was selected in order to estimate the expansive pressure developed by the ASR gel under “free” expansion conditions. It was found that, even under restrained conditions, the threshold alkali level proves to be a suitable reactivity parameter for designing concrete mixes that are not susceptible of deleterious ASR expansion. An empirical relationship between expansive pressure, concrete alkali content and aggregate TAL was developed in view of its possible use for ASR diagnosis and/or safety evaluation of concrete structures.     

Alkali-silica reaction, pessimum effects and pozzolanic effect

The pessimum proportion and pessimum size effects for alkali-silica reaction-induced deterioration of concrete (ASR) and the pozzolanic effect of fine siliceous admixtures in concrete have been explained based on the proposed ASR model [T. Ichikawa, M. Miura, Modified model of alkali-silica reaction, Cem. Concr. Res. 37 (2007) 1291-1297.]. The attack of alkali hydroxide to aggregate particles composed of ASR-reactive minerals generates the layer of hydrated mature alkali silicate and the layer of less hydrated immature alkali silicate under the mature layer. The mature alkali silicate preferentially reacts with Ca²⁺ ions to convert to fragmental calcium alkali silicate, because the reaction accompanies a significant volume contraction. The immature alkali silicate gradually reacts with Ca²⁺ ions to cover the surface of the reactive minerals with tight layers of calcium alkali silicate called reaction rims. The reaction rims allow the penetration of alkaline solution but prevents the leakage of viscous alkali silicate generated afterward, so that the alkali silicate is accumulated inside the rims to give an expansive pressure enough for cracking the aggregate and the surrounding concrete. Due to the absorption of Ca²⁺ ions by mature alkali silicate, too much increase of the proportion of reactive aggregate causes the deficiency of Ca²⁺ ions for the formation of reaction rims, so that the ASR expansion decreases after passing the pessimum proportion. Very fine reactive aggregate and admixtures with the grain size less than ~ 50 µm preferentially react with alkali hydroxide to convert to mature alkali silicate without leaving any reactive minerals. Homogeneous mixing of the sufficient amount of very fine siliceous admixtures in concrete therefore inhibits the ASR by absorbing Ca²⁺ ions for the rim formation. The resultant fragmental calcium silicate fills the pores in concrete to increase the strength and the durability of the concrete. The admixtures thus act as pozzolanic materials.     

Surface potential distribution over a zinc/steel galvanic couple corroding under thin layer of electrolyte

In this study, surface potential and surface pH changes over a zinc/steel galvanic couple corroding in artificial seawater (ASW) at 60 and 90% RH have been investigated. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test. The potential difference over the zinc and steel surface in 90% RH was very low (less than 200 mV) showing that whole steel surface was under galvanic protection. On the other hand, in 60% RH, after several days of corrosion the potential difference between the zinc coating and the steel surface was very high (more than 500 mV) and hence the galvanic protection was limited to interface region. The X-ray analysis of the sample corroded in 60% RH has shown that the zinc corrosion products were deposited on the steel surface near the interface, the same region has shown a low pH compared to than in other part of the steel surface. This led to conclude that with the progress of corrosion, the coating surface of zinc coated steel acidifies by the hydrolysis reaction of the dissolved zinc ions, and the iron surface showed the alkalinity by the oxygen reduction reaction. Moreover, the parts of the steel surface covered with zinc corrosion products had developed relatively less noble potential than other parts indicating that zinc corrosion products took a role to protect the base steel against corrosion. It was assumed that this behavior was related to a combination of the water absorbing capability of zinc corrosion products and adsorption of zinc ion on the steel surface due to low pH.     

Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions

Electrochemical chloride extraction (ECE) is used for the rehabilitation of chloride-contaminated concrete. Anions such as chloride and hydroxide are pushed away from the cathode (reinforcement), and cations such as sodium, potassium and calcium are attracted to the cathode. During ECE an increase of the concrete resistance can be observed. The results of a petrography study on ECE-treated concrete are presented in this paper. It also investigates the influence of pore size on ion migration using a concrete substitute model with known pore size. Findings showed that the pore size and pore size distribution of concrete are altered due to ECE. It is therefore suggested that concrete acts as active migration medium in the migration process by releasing ions into the pore solution. Moreover, small pores hinder the migration of ions, which may partially be responsible for changes in concrete resistance.     

Studies on expansion properties in mortar containing waste glass and fibers

The utilization of waste glass in concrete can cause cracking and weakening due to expansion by alkali-silica reaction (ASR). In this study, ASR expansion and properties of strength were analyzed in terms of waste glass content, glass color (brown, green), fibers (steel fiber, polypropylene fiber) and fiber content, in anticipation of reducing ASR expansion.Results showed that green waste glass was more usable than brown because its expansion was less than that of brown glass. Using the accelerated ASTM C 1260 test of waste glass, no pessimum content was found. Furthermore, when fibers and waste glass were combined, there was an effect on the reduction of expansion and strength loss due to ASR between the alkali in the cement paste and the silica in the waste glass. In particular, adding 1.5 vol.% of steel fiber to concrete containing 20% waste glass reduced the expansion ratio by 40% and increased flexural strength by up to 110%, a vast improvement when compared with using only waste glass (80 °C H₂O curing) by itself.     

Laboratory and field investigations of the influence of sodium chloride on alkali-silica reactivity

Concrete cylinders, 255 mm in diameter, were made with high- and low-alkali cements, a highly alkali-silica-reactive coarse aggregate, and subjected to various conditions at 38 °C: (1) immersion in 3% NaCl solution; (2) immersion in 6% NaCl solution; (3) humid air at 100% RH, and (4) 14-day cycles including 12 days in humid air, 2 days of drying, and 3 h in 6% NaCl solution. After 1 year, a number of cylinders were drilled to obtain dry powder samples from different depths, which were analyzed for total and soluble chloride and for soluble sodium and potassium. Concrete cores were also taken in a number of parapets and abutments, either exposed to deicing salts or not, on which chemical analyses were also performed on slices taken at different depths from the exposed surface. The results obtained suggest that making concrete with a low-alkali content is an effective way to prevent expansion due to alkali-silica reaction even for concretes exposed to seawater or deicing salts; this is attributed to the fact that the OH⁻ ion concentration in the pore solution, and then the pH, is decreased in the near-surface layers of concrete exposed to sodium chloride, which does not penetrate at depth in concrete.     

Effects of material and environmental parameters on chloride penetration profiles in concrete structures

On the basis of the transport mechanism of chloride ion, a prediction model of chloride penetration into concrete structures has been developed. The model includes the diffusion of chloride and its dependences on temperature, age, relative humidity, chloride binding and chloride convection by moisture transport. The experimental program has been set up to verify the model developed in the present study. Several series of concrete specimens were immersed in 3.5% chloride solutions for 15 weeks, and the chloride profiles of the specimens were measured and compared to the predicted chloride profiles. In addition, field measurements have been also conducted. From 10-year-old bridge piers, the chloride profiles in concrete under tidal zone were measured and compared with the predicted chloride profiles. The effects of chloride binding, relative humidity, temperature, exposure condition, and age-dependence on the chloride penetration in concrete were clarified from the present analyses. It was found from the present study that all these variables affect greatly the chloride penetration profiles in concrete. The comparison of the laboratory and field test data with the present theory confirms that the proposed model can be realistically used to predict the penetration of chloride ions into concrete structures under sea environments. Further, these results may be efficiently used for the realistic assessment and design for durability of concrete structures.     

Role of fibers in controlling unrestrained expansion and arresting cracking in Portland cement concrete undergoing alkali-silica reaction

An experimental study was undertaken to investigate the role of polypropylene or brass-coated steel fibers in controlling unrestrained expansions and delaying and arresting cracking in Portland cement concrete due to alkali-silica reaction. Portland cement concrete and fiber-reinforced concrete (FRC) mixtures were prepared at a w/c ratio of 0.40 using modified Type I cement, reactive fine particles, and coarse limestone aggregates. Prism (5×5×30 cm) and plate (13.5×13.5×3 cm) specimens were prepared and cured for 7 or 28 days before exposure to a special treatment to accelerate ASR. Expansion, time of cracking, and ultrasonic pulse velocity were determined over a treatment period of 65 days using prism specimens. Ultimate cracking pattern and extent were determined after a treatment period of 85 days using plate specimens. The results showed that while fibers did not contribute significantly to controlling pre-cracking and post-cracking expansions, they played a significant role in delaying cracks formation and limiting their extent. Considering its lower cost and content, the performance of polypropylene fibers was superior to that of brass-coated steel ones. The potential of brass-coated fibers in arresting ASR cracking was significantly affected by age of concrete when subjected to treatment.     

新疆大石峡水利枢纽混凝土碱-硅酸反应抑制研究

新疆大石峡水利枢纽混凝土选用的骨料存在碱活性,可能导致工程产生碱骨料破坏问题,本文研究了当地粉煤灰和矿粉对碱-硅酸反应(ASR)的影响,并采用XRD和SEM-EDS测试分析了水化产物和界面过渡区的形态。结果表明：粉煤灰掺量≥20%(质量分数)或矿粉掺量≥40%(质量分数)都能显著抑制碱-硅酸反应;在纯水泥样品界面过渡区可以观察到无定形相,在含有粉煤灰或矿粉的样品中未观察到无定形相,这意味着碱-硅酸反应发生在纯水泥样品中;添加粉煤灰或矿粉降低了界面过渡区物相的Ca/Si摩尔比,抑制了碱-硅酸反应。     

钢筋混凝土电化学除氯技术研究现状

电化学除氯(ECE)技术通过在外加阳极和钢筋阴极之间施加电压来排除氯离子使钢筋重新恢复钝化,从而达到保护钢筋混凝土的目的。为了进一步了解电化学除氯技术的发展,本文综述了近年来国内外钢筋混凝土电化学除氯技术的研究进展,阐述了电化学除氯原理,总结了电化学除氯的影响因素,系统地介绍了电化学除氯对钢筋混凝土粘结力、宏观性能和微观结构的影响,分析了目前电化学除氯存在的问题并提出了未来的研究方向,以期电化学除氯技术在提高滨海混凝土结构耐久性方面发挥更好地作用。     

Effect of steel microfibers on corrosion of steel reinforcing bars

Steel microfiber reinforcement was previously found to be successful in mitigating alkali silica reaction in concrete, an expansive phenomenon. The use of steel microfibers to mitigate rebar corrosion, another expansive reaction, was investigated. Mortar specimens with and without steel microfiber reinforcement were exposed to a corrosive environment. All specimens were prepared with water/cement ratios of both 0.40 and 0.55, cured for 28 days, and then submerged in aerated 3.5% NaCl solution. The corrosion behavior of the specimens was monitored via electrochemical measurements. Three types of electrochemical tests were performed: corrosion potential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy. Chloride concentration measurements and microscopic analysis were performed as well. The polarization curves, Tafel, and polarization resistance measurements indicate that the steel rebar in the microfiber-reinforced mortars are more resistant to corrosion than the rebar in the control mortars, despite higher chloride concentrations. Furthermore, the steel microfiber-reinforced cement based materials have a lower electrolytic resistance. This is not indicative of a higher corrosion rate, which would be the case if it had been observed in standard mortar specimens.     

The retardation of reinforcing steel corrosion by alkyl-alkoxyl silane

The effectiveness of an alkyl-alkoxy silane treatment of concrete on reducing the rate of corrosion of reinforcing steel in the presence of chloride ions was investigated. First, the penetrating characteristics of the alkyl-alkoxy silane into dried concrete were determined. Then, the corrosion currents of 90 corrosion specimens, some of which were dried and silane-impregnated, were measured using an impressed current, accelerated corrosion technique. The penetration of alkyl-alkoxy silane into dried concrete followed an approximate linear square root time relationship. Comparisons of corrosion currents showed that alkyl-alkoxy silane showed that allowing time after treatment for the They also showed that allowing time after treatment for the ethanol carrier to evaporate and exposing the treated surface to water for a short period of time were beneficial.