Alkaline Environment Of Concrete Research Articles

Study of the passivation of carbon steel in simulated concrete pore solution using scanning electrochemical microscope (SECM)

A protective passive layer is formed on the surface of embedded reinforcing carbon steel bar in the alkaline environment of concrete. Corrosion of steel in concrete environment is a function of formation, strengthen and stability of this layer. In this study, Scanning Electrochemical Microscopy (SECM) was exploited to study the kinetic and the formation of the passive layer in a simulated concrete pore solution. The specimen was cut, epoxy mounted and polished from a carbon steel reinforcing bar. Then, it was immersed in a concrete simulated pore solution containing 0.5 mM potassium ferrocyanide as a mediator for 48 h and amperometry SECM approach curves were recorded over the unbiased surface of the freshly polished surface. A platinum microelectrode tip with the diameter of 25 µm was used in the experiment. The effective heterogeneous electron rate coefficient (keff) variation for the mediator regenerating surface reaction was estimated during the time of exposure. Decreasing values of keff, throughout passivation period, indicates formation and increase in the strength of the passive layer formed on the surface of the steel in the alkaline environment in concrete pore solution.

Experimental and mechanical performance of shotcrete made with nanomaterials and fiber reinforcement

Studying the application of nanomaterial in cement matrixes is a new subject which has attracted the attention of researchers. Development of engineered cementitious composite has produced a solution to numerous shotcrete application challenges. In this study, mechanical tests were conducted to evaluate the strength of the concrete modified by fibers and nanomaterials in terms of comparison with conventional concrete and shotcrete.It is included in the application of Nano-Al2O3 and Nano-SiO2, which are some of the cement constituent. With proper homogenization, Nano-Al2O3 acts as filler for improving the pozzolanic reaction in the concrete. Nano-SiO2 particles also prevent the diffusion of external destructive factors into the concrete, thus improving the durability and stability of concrete. On the other hand, application of Alkali Resistant (AR) glass fibers improves the tensile and compressive strengths as well as concrete continuity and also resolves the changes created in mechanical properties of the fibers caused by alkaline concrete environment. Test results revealed that the maximum increase in compression and flexural strengths of modified concrete were 20.6 and 52%, respectively of its optimum compounds for 0.7% glass fiber with 1% Nano-Al2O3 and 1.5% Nano-SiO2, while in the modified shotcrete, these values were increased by 22.9 and 75%, respectively. In addition, application of the mentioned materials led to decrease of penetration depth and water absorption of the modified concrete by 46.78 and 2.5%, respectively, in relation to the conventional concrete. Also, addition of fibers and nanomaterials in shotcrete reduced the effect of cracking due to shrinkage and the modified shotcrete material rebound.

Interlaminar shear capacity of thermally damaged GFRP bars under alkaline concrete environment

The paper investigated the effect of alkaline concrete environment on thermally damaged glass fiber reinforced polymer (GFRP) reinforcing bars through an accelerated aging test in alkaline solution. In the experimental test, the GRFP bar specimens were exposed to high temperatures and then immersed in alkaline solution with a pH of 12.6 presenting the similar alkalinity of concrete. Other specimens, which were unexposed to high temperatures, were also immersed in the alkaline solution for the same period of time in order to compare the performance changes in inter-laminar shear strength and stiffness capacities (ILSS and ILSSf). From the test results, the bar specimens which were thermally damaged by high temperature showed more degradation in ILSS than the thermally undamaged ones; rapid performance degradation occurred for longer immersion periods. Analysis of scanning electron microscope images showed that the fine cracks on the surface and resin matrix damage developed during exposure to high temperature led to accelerate alkali penetration, thereby resulting in rapid performance degradation.

Combined Effects of Curing Temperatures and Alkaline Concrete on Tensile Properties of GFRP Bars

A significant number of studies have been conducted on the tensile properties of GFRP bars embedded in concrete under different environments. However, most of these studies have been experimentally based on the environmental immersion test after standard-curing and the lack of influence on the tensile properties of GFRP bars embedded in concrete during the curing process of concrete. This paper presents the results of the microscopic structures through scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and tensile properties of GFRP bars, which were employed to investigate the combined effects of curing temperatures and alkaline concrete on tensile properties of GFRP bars. The results showed that the higher curing temperature aggravated the influence of the alkaline concrete environment on GFRP bars but did not change the mechanisms of mechanical degradation of the GFRP bars. The influence of different curing temperatures on the tensile strength of GFRP bars was different between the bare bar and bars in concrete. Finally, the exponential correlation equation of two different test methods was established, and the attenuation ratio of the tensile strength of GFRP bars embedded in concrete under different curing temperatures was predicted by the bare test.

Development of solid state embeddable reference electrode for corrosion monitoring of steel in reinforced concrete structures

Corrosion of steel reinforcement concrete structures is monitored through the surface mounting techniques using liquid based reference electrodes. Due to the limited usage of liquid based reference electrodes solid state reference electrode are introduced recently. In the present study, we fabricated and characterized Mn3O4 based pellet electrode for corrosion assessment of steel rebar in high alkaline medium through electrochemical methods and the results are compared with conventional saturated calomel reference electrode (SCE). The results indicate that the fabricated pellet electrode exhibits better characteristics suitable for high alkaline concrete environment and differentiate the passive and active status of steel rebar.

Long term durability of pultruded polymer composite rebar in concrete environment

Pultruded glass fibre reinforced polymer (GFRP) composite rebar was immersed in alkaline concrete environment for 0, 1, 2, 3, 4, 6, 14 and 24 months at 60 °C to evaluate its durability in concrete structure. Moisture absorption of the rebar was found to be only 0.76%. It was also found that both the glass transition temperature (Tg) and the short beam shear strength were retained by about 91.5%; and the above properties were remained almost unchanged during the ageing period from 1 month to 24 months. Design tensile strength and tensile elastic modulus of the rebar were retained by 100% after 24 months exposure in concrete environment. Degradation of GFRP rebar was not evident in the FT-IR results as supported by scanning electron micrographs and energy dispersive X-ray analysis. Moisture absorption was found to be a critical factor that controlled thermal and mechanical properties of GFRP rebar.

Feasibility study on corrosion monitoring of a concrete column with central rebar using BOTDR

Abstract. Optical fiber Brillouin sensor in a coil winding setup is proposed in this paper to measure the expansion deformation of a concrete column with a central rebar subjected to accelerated corrosion. The optical sensor monitored the whole dynamic corrosion process from initial deformation to final cracking. Experimental results show that Brillouin Optical Time Domain Reflectometer (BOTDR) can accurately measure the strain values and identify the crack locations of the simulated reinforced concrete (RC) column. A theoretical model is used to calculate the RC corrosion expansive pressure and crack length. The results indicate that the measured strain and cracking history revealed the development of the steel bar corrosion inside the simulated RC column. Keywords: rebar corrosion; expansion pressure; BOTDR; optical fiber; distributed sensing monitoring 1. Introduction Steel corrosion is one of the predominant factors for the degradation of steel reinforced concrete (RC) structures especially for structures exposed to aggressive environments. Corrosion of steel rebar is considered negligible when it is fully surrounded by the alkaline environment of concrete. However, corrosion can be introduced during penetration of chloride ions and carbon dioxide. To quantify the rate of corrosion, electro-chemical methods such as corrosion potential, AC impedance, half-cell and linear polarization methods have been suggested (Andrade

Low-cost glass fiber composites with enhanced alkali resistance tailored towards concrete reinforcement

Glass fiber reinforced polymer (GFRP) composites were modified through introduction of ion-exchangers in order to enhance their longevity in the alkaline environment of concrete. Glass fiber composites offer a desired balance of performance and cost for replacement of corrosion-prone steel reinforcement in concrete; their rapid deterioration in the alkaline environment of concrete is, however, a major drawback. Ion exchangers are insoluble solids carrying cations (or anions) which can be exchanged with ions of the same sign. Cation exchangers of hydrogen form replace alkali metal cations (e.g., K+ in alkaline solutions diffusing into the polymer matrix) with H+. This exchange of cations neutralizes aggressive alkaline solutions by converting K+ OH− (and Na+ OH−, etc.) into H2O. Fine weak-acid cation exchangers, when introduced as coating on glass fiber composite bars (applied immediately after pultrusion and before curing of polymer matrix) proved to be compatible with the industrial-scale pultrusion process of composite bars without any need to modify the curing process. Thorough laboratory investigations and industrial-scale pultrusion efforts, were undertaken, which successfully demonstrated that introduction of selected ion exchangers into the polymer matrix (or a surface layer of matrix) does not interfere with the pultrusion process, and yields significant gains in the alkali resistance of glass fiber composites. The durability characteristics of glass fiber composite bars modified with weak-acid cation exchanger coating were investigated under two different accelerated aging conditions. The modified composite bars provided desirable durability attributes under these two different accelerated aging effects, including exposure to the alkaline pore solution of concrete and salt solution. The mechanical characteristics of modified glass fiber composite bars (with weak-acid cation exchanger coating) were investigated by conducting compression, flexure, and shear tests. The results indicated that the modification of pultruded bars did not alter their mechanical performance.

Mechanical properties of recycled PET fibers in concrete

Fiber-reinforced concrete represents the current tendency to apply more efficient crack-resistant concrete. For instance, polyethylene terephthalate (PET) is a polyester polymer obtained from recyclable bottles; it has been widely used to produce fibers to obtain cement-based products with improved properties. Therefore, this paper reports on an experimental study of recycled-bottle-PET fiber-reinforced concrete. Fibers with lengths of 10, 15 and 20 mm and volume fractions of 0.05, 0.18 and 0.30% related to the volume of the concrete were used. Physical and mechanical characterization of the concrete was performed, including the determination of compressive strength, flexural strength, Young’s modulus and fracture toughness as well as analysis using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Flexure and impact tests were performed after 28 and 150 days. No significant effect of the fiber addition on the compressive strength and modulus of elasticity was observed. However, the Young’s modulus was observed to decrease as the fiber volume increased. At 28 days, the concrete flexural toughness and impact resistance increased with the presence of PET fibers, except for the 0.05 vol.% sample. However, at 150 days, this improvement was no longer present due to recycled-bottle-PET fiber degradation in the alkaline concrete environment, as visualized by SEM observations. An increase in porosity also has occurred at 365 days for the fiber-reinforced concrete, as determined by MIP.

콘크리트 환경과 고온의 복합환경이 FRP 보강근의 계면전단성능에 미치는 영향

약FRP 보강근의 화재 노출시 내구성능에 대한 연구들은 고온 노출이 인장성능에 미치는 영향에 대한 분석이 대부분을 차지한다. 그러나, 신설 구조물에 삽입된 FRP 보강근은 먼저 콘크리트 내에서 습윤환경 및 알칼리에 의한 성능저하가 발생하며, 이와 같은 FRP 보강 콘크리트 구조물에 화재가 발생하면, 보강근은 고온에 노출된다. 그러므로 화재에 의해 손상 받은 FRP 보강근의 평가시에는 콘크리트 환경과 고온에 의한 영향을 동시에 고려하여야 한다. 이 연구에서는 장기간동안 용액에 노출된 FRP 보강근에 60℃, 100℃, 150℃ 및 300℃의 온도를 가하고, 계면전단강도를 측정, 비교 하였다. 실험 결과에 따르면, 환경과 고온의 복합영향이 FRP 보강근의 역학적 특성에 미치는 영향이 환경만의 영향 또는 고온만의 영향 보다 큰 것으로 나타났다.

Quantum-Chemistry Based Studying of Rebar Passivation in Alkaline Concrete Environment

Quantum-Chemistry Based Studying of Rebar Passivation in Alkaline Concrete Environment

An Investigation on Durability of Mixture of Alkali-Resistant Glass and Epoxy for Civil Engineering Application

Glass Fiber Reinforced Polymer(GFRP) rebars are easily deteriorated by concrete alkalinity. This paper focuses on the improvement of durability of newly developed GFRP rebars exposed to chemical attack in the alkaline environment of concrete. The milled E-glass fibers which are mixed in surfaace resin can increase bond strength between GFRP rebars and the concrete. In this study, the effect of milled alkali-resistant glass fibers (milled AR glass) and milled E-glass in the ribs on the durability of GFRP rebar is investigated through short span shear tests and moist absorption tests of the baare rebar. To accelerate the effect of the alkalinity, aging procedures were applied to test specimens. According to thhe test results, lower mix ratio of milled fibers in the ribs contribbuted to more durability than if higher mix ratio was used. This was due to more voids and defects in ribs with high mix ratio of milled fibers. GFRP rebars with milled AR glass fibers did not show great improvement in ISS strength compared to those with milled E-glass fibers. These results indicate that the milled glass fibers works as a reinforcing material, but optimum mix ratio to minimize causing of defects and voids in the manufacturing proccess should be determined.

Making reinforced concrete immune from chloride corrosion

Chloride-induced corrosion is a major factor affecting the durability of reinforced-concrete structures. However, a high tolerance to chloride contamination has often been achieved under laboratory conditions. A significant factor affecting the chloride threshold level for corrosion initiation in concrete is the presence of defects, such as air voids, in the cement paste at the steel–concrete interface. To explain this observation it is noted that corrosion initiation and propagation involve the local production of acid at sites where the steel passive film breaks down in the otherwise highly alkaline concrete environment. Thus, solids with pH-dependent dissolution behaviour affect the corrosion process. Solid phase corrosion inhibitors would release hydroxyl ions to prevent this local reduction in pH. By reducing the entrapped air-void content at the steel to values below 0·2% it was possible to increase the chloride threshold level from 0·2% to above 2% chloride by weight of cement as the voids no longer dilute the inhibitive solid phases. A high tolerance to chloride contamination may also be achieved by reducing the electrolyte content of voids and by using electrochemical treatments to generate a reservoir of hydroxide-containing salts at the steel–concrete interface. Chloride contamination at levels above 2% chloride by weight of cement at a reasonable steel cover depth may not be possible in concrete exposed to many environments when the concentrating effects of evaporation are minimised. Thus, a chloride threshold level of 2% may effectively render steel immune to chloride-induced corrosion.

Field study of glass-fibre-reinforced polymer durability in concrete

The Canadian Highway Bridge Design Code (CHBDC) does not permit the use of glass-fibre-reinforced polymer (GFRP) for primary reinforcement or prestressing tendons in concrete components. The restriction on the use of GFRP in concrete was based on published laboratory studies indicating that GFRP is not stable in the alkaline environment of concrete. In 2004, ISIS Canada sponsored an extensive study of the durability of GFRP in concrete by removing cores from GFRP-reinforced concrete components of five 5- to 8-year-old structures from across Canada. Three teams working independently at several Canadian universities used a variety of analytical methods to (i) investigate whether the GFRP in concrete field structures had been attacked by alkalis and (ii) compare the composition of GFRP removed from in-service structures with the composition of control specimens that were saved from the projects and not exposed to the concrete environment. The analytical results have confirmed that the GFRP in concrete did not suffer any damage during the 5–8 years of exposure. As a result of this study, the CHBDC in its forthcoming (second) edition has permitted the use of GFRP for both primary reinforcement and prestressing tendons in concrete components, provided the maximum stress level in GFRP at the serviceability limit state is kept at or below 25% of its ultimate strength. It was also found that, contrary to some claims, concrete over GFRP bars does not crack even if the depth of cover is as thin as 28 mm.Key words: alkali attack, barrier wall, crack, deck slab, depth of cover, fibre-reinforced polymer (FRP), glass-fibre-reinforced polymer (GFRP).