Bond mechanism of FRP rebars to concrete

The bond mechanism of Fiber Reinforced Polymer (FRP) rebar to concrete was studied. Five different types of 12.7 and 12.0-mm rebar subjected to different surface treatments were tested, and the bond mechanism was compared with that of untreated FRP rods and ordinary deformed steel. High bond values were obtained for rods exhibiting a stiff deformed surface, on which large deformations were molded by resin, and for rods with a rough surface whereby the roughness has resulted either from winding a helical fiber together with embedded sand particles or from using excess polymer. The bond values recorded were equivalent to or larger than those of ordinary deformed steel. Low bond strength was obtained both for rods with a thick polymeric layer of low mechanical for rods with a thick polymeric layer of low mechanical properties and for rods with smooth surfaces. Different pre-peak and post-peak behavior was observed for the various rods when the entire set of P-s (Pullout load vs. slip) curves were compared. Brittle behavior was apparent wherever the external layer of the rod exhibited large deformations formed in a stiff matrix. Where the surface was rough, more ductile behavior was detected. The wedging of particles into the surface can alter the load-slip behavior, from one of slip-weakening to one of slip-hardening.     

Bond behaviour of flat stainless steel rebars in concrete

In recent years there has been an increasing interest in applying stainless steel (SS) reinforcement in concrete structures to avoid corrosion derived durability problems. Concrete-steel bond behaviour and applicability of existing standards have not yet been extensively studied with respect to SS. Moreover, an interest in applying flat rebars as reinforcement elements has emerged. The idea is based on the optimization of the rebar surface in contact with the concrete as well as on the reinforcement optimization for shallow slabs. To advance in the evaluation of the bond between concrete and flat SS strips and to compare with the behaviour of standard carbon steel (CS) round reinforcement 72 bond tests have been conducted. The test program consists of pull-out tests to centrally reinforced concrete specimens. Traditional concrete and self compacting concrete are used for embedding. For a better analysis and understanding of the failure aspect of the specimens, fluorescent epoxy injection has been conducted. Results are compared to the CEB-FIP 1990 bond model. The authors conclude that, the use of SS instead of CS is not considerably influencing the bond capacity of the reinforcement when ribbed samples are used while parameters as reinforcement shape or rib pattern are more deterministic. An adaptation of the CEB-FIP 1990 bond model is proposed for flat reinforcements tested in this work.     

Performance of hybrid rebars as longitudinal reinforcement in normal strength concrete

Although steel is most commonly used as a reinforcing material in concrete due to its competitive cost and favorable mechanical properties, the problem of corrosion of steel rebars leads to a reduction in life span of the structure and adds to maintenance costs. Many techniques have been developed in recent past to reduce corrosion (galvanizing, epoxy coating, etc.) but none of the solutions seem to be viable as an adequate solution to the corrosion problem. Apart from the use of fiber reinforced polymer (FRP) rebars, hybrid rebars consisting of both FRP and steel are also being tried to overcome the problem of steel corrosion. This paper evaluates the performance of hybrid rebars as longitudinal reinforcement in normal strength concrete beams. Hybrid rebars used in this study essentially consist of glass fiber reinforced polymer (GFRP) strands of 2 mm diameter wound helically on a mild steel core of 6 mm diameter. GFRP stirrups have been used as shear reinforcement. An attempt has been made to evaluate the flexural and shear performance of beams having hybrid rebars in normal strength concrete with and without polypropylene fibers added to the concrete matrix.     

碳纤维增强复合材料筋混凝土梁非线性力学性能

为了研究碳纤维增强复合材料(CFRP)筋混凝土梁的非线性力学性能,基于非线性理论推导了CFRP筋梁的有限元分析模型：对4个预应力CFRP筋混凝土梁进行了非线性全过程分析,考察了预应力CFRP筋、GFRP筋和普通钢筋的应力发展规律。与试验资料对比可知,计算结果与试验数据吻合良好,说明采用弥散裂缝模式、Owen屈服准则和Hinton压碎准则能较好地描述混凝土开裂、屈服和压碎特性,同时也说明了CFRP筋及其力学效应用组合单元模拟的有效性以及本文中研制程序的正确性。CFRP筋具有高强度性能,梁试件破坏时CFRP筋均未失效;与受拉区配筋为钢筋相比,GFRP筋在全过程中处于弹性阶段。     

Efficiency of fly ash in concrete

Earlier efforts towards an understanding of the efficiency of fly ash in concrete has led to the introduction of rational methods. Based on the results available on some of the more recent pulverised fuel ashes, the authors evaluated the efficiency of fly ash in concrete over a wide range of percentage replacements (15–75%). It was clearly shown that the overall efficiency of fly ash cannot be adequately predicted using a single efficiency factor at all percentages of replacements. The overall efficiency factor (k) has been evaluated at all percentages of replacements considering the general efficiency factor (k_e) and the percentage efficiency factor (k_p). This study resulted in a quantitative assessment of the behaviour of fly ash in concrete, especially for the 28 day compressive strength at different percentages of replacement.     

Numerical modeling of the rebar/concrete interface: case of the flat steel rebars

This paper presents the methodology used to identify the mechanical behaviour of a steel-concrete interface in the case of a particular steel reinforcement (flat steel). The methodology consists in simulating the statistical mechanical behaviour of reinforced concrete tie-beams, subjected to tension, using a probabilistic discrete approach for the mechanical behaviour of the concrete under axial tension and a deterministic model for the steel-concrete interface. The model proposed for the interface is in the frame of damage mechanics taking into account physical phenomena related to the interface (cohesion and slip). The tie-beams are reinforced by a flat steel rebar with a rectangular cross section of 25 × 3.5 mm. Results of this numerical simulation have been compared to some experimental tests results. These comparisons are performed in terms of global responses (load-displacement curves) and of local responses (crack openings, number of cracks and cracks’ spacing).     

Bond over direct support of deformed rebars in normal and high strength concrete with and without fibers

This paper describes results of the second stage of a research on the bonding of deformed bars in normal and high strength concrete with and without fibers. The current study focused on the effect of confinement conditions, which are present over a direct support of a reinforced concrete beam, and compares these results with data obtained in the first stage of the research. The experimental work consisted of pullout tests, which were based on the standard RILEM flexural bond test (??beam test??) of deformed steel bars with diameters of 12 and 20?mm. Results revealed an increase in the normalized bond strength due to support conditions. An unstable post-peak slip of the tested rebar was observed when located over the support, whereas bars bonded near mid-span exhibited controlled bond?Cslip throughout the test. Up to peak load, however, bond?Cslip relations exhibited higher energy absorption over the support than near mid-span. Inclusion of fibers had only a minor effect on bond strength but had a significant influence on pre-peak energy absorption. A previously developed model of bond strength shows good agreement with current results when applying an increase factor due to the support effect.     

Effect of zinc phosphate chemical conversion coating on corrosion behaviour of mild steel in alkaline medium: protection of rebars in reinforced concrete

Abstract

We outline the ability of zinc phosphate coatings, obtained by chemical conversion, to protect mild steel rebars against localized corrosion, generated by chloride ions in alkaline media. The corrosion resistance of coated steel, in comparison with uncoated rebars and coated and uncoated steel rebars embedded in mortar, were evaluated by open-circuit potential, potentiodynamic polarization, cronoamperometry and electrochemical impedance spectroscopy. The coated surfaces were characterized by x-ray diffraction and scanning electron microscopy. First, coated mild steel rebars were studied in an alkaline solution with and without chloride simulating a concrete pore solution. The results showed that the slow dissolution of the coating generates hydroxyapatite Ca₁₀(PO₄)₆(OH)₂. After a long immersion, the coating became dense and provided an effective corrosion resistance compared with the mild steel rebar. Secondly, the coated and uncoated steel rebars embedded in mortar and immersed in chloride solution showed no corrosion or deterioration of the coated steel. Corrosion rate is considerably lowered by this phosphate coating.     

Effect of zinc phosphate chemical conversion coating on corrosion behaviour of mild steel in alkaline medium: protection of rebars in reinforced concrete

We outline the ability of zinc phosphate coatings, obtained by chemical conversion, to protect mild steel rebars against localized corrosion, generated by chloride ions in alkaline media. The corrosion resistance of coated steel, in comparison with uncoated rebars and coated and uncoated steel rebars embedded in mortar, were evaluated by open-circuit potential, potentiodynamic polarization, cronoamperometry and electrochemical impedance spectroscopy. The coated surfaces were characterized by x-ray diffraction and scanning electron microscopy. First, coated mild steel rebars were studied in an alkaline solution with and without chloride simulating a concrete pore solution. The results showed that the slow dissolution of the coating generates hydroxyapatite Ca₁₀(PO₄)₆(OH)₂. After a long immersion, the coating became dense and provided an effective corrosion resistance compared with the mild steel rebar. Secondly, the coated and uncoated steel rebars embedded in mortar and immersed in chloride solution showed no corrosion or deterioration of the coated steel. Corrosion rate is considerably lowered by this phosphate coating.     

Acoustic emission applied to study crack propagation in concrete

This study presents a use of the acoustic emission examination technique to determine the dimensions of the microcracked zone leading a macrocrack propagating in concrete. A critical analysis of the locating method is followed by a presentation of its application to conventional concrete, steel fibre concrete, and high-strength concrete.     

Rare-earth modified chromium-aluminide coatings applied by pack cementation method on low-alloy steels

In metallurgical practice the presence of a minor amount of certain elements can induce significant changes in a material's physical, mechanical and chemical properties. In this paper we present the nature, morphology and properties of protective coatings containing Cr, Al and minor amounts of a rare earth element, applied on 2.25 Cr-1 Mo low-alloy steel. The coating technology, pack cementation, was used to achieve the simultaneous deposition of the coating elements in a one step process. Cyclic oxidation, conducted in air at 800 °C, induced thermal stresses on the alloy samples, promoting the tendency of the oxide scales formed to spall. The rare earth modified coatings, under these conditions showed the beneficial effects of the rare earth addition. These coatings, upon exposure, developed an adherent oxide scale which had better properties, and retarded the alloy's degradation. This lead to a significant reduction in the overall oxidation rate.     

Effectiveness of surface coatings to protect reinforced concrete in marine environments

Coatings can contribute to extending service life of concrete structures exposed to marine environents by reducing the rate of chloride penetration. In the present paper, the effect of coatings on the rate of chloride uptake was studied by measuring their concentration profiles at different, times, extending up to 24 months. Using a solution of the so-called Fick’s second law of diffusion, least-square fitting leads to the value of chloride concentration under the coating (C _o ) which allowed us to compare the effectiveness of different coatings. Dependeng on the formulation, some of the studied acrylic coatings could reduce the value ofC _o by more than 80% in comparison to the unpainted concrete. Water permeability coefficients were measured for the same coatings, according to EN 1062-3. The ranking of effectiveness to stop chlorides is the same as the water imperviousness, but the minimum requrement proposed by prEN 1504-2 (0.1 kg.m^?2h^?0.5) did not prove to be enough for an efficient protection against chloride permeation.     

Characterization and properties of PVD coatings applied to extrusion dies

The paper presents the research results on the microstructure, mechanical and tribological properties of coatings deposited by cathodic arc evaporation methods on the X40CrMoV5-1 hot work tool steel substrates as well as the possibility to employ the finite element method for evaluation of stresses in the investigated coatings. The tests were carried out on CrAlSiN and CrN coatings. The coatings demonstrated a dense cross-sectional morphology as well as good adhesion to the substrate. The critical load L_C2 lies within the range 54-55 N, depending on the coating type. The investigated coatings have the potential to increase the service life of an extrusion die by orders of magnitude as compared with uncoated steel.     

Simulation study on the use of strippable coatings for radiocesium decontamination of concrete

The contamination and decontamination of concrete by a soluble contaminant (radiocesium) was modeled using the finite element method. The decontamination process relied on the application of a hyper-accumulating strippable polymer (HASP?) [corrected] to sequester contaminant that was transported to the concrete surface. The model accounted for the transport of cesium by diffusion, capillary pressure-driven convection, and equilibrium adsorption of cesium within the concrete substrate and HASP? [corrected] coating. The influence of HASP? [corrected] properties (porosity, thickness, and cesium distribution coefficient), and a wide range of operation variables (HASP? [corrected] contact time, delay time until HASP? [corrected] decontamination begins, influence of evaporative boundary conditions) on final decontamination efficacy were explored. Transient saturation (moisture content) and cesium concentration profiles were used to understand key factors in the decontamination process, and water wicking experiments were performed to validate the capillary convection model. The results showed that prompt HASP? [corrected] application after the initial contamination event is critical for high decontamination efficacies. A 30-day HASP? [corrected] treatment removed about 90% of the contaminant when HASP? [corrected] was applied within 30min of the initial contamination event, whereas the decontamination process removed less than half as much cesium if months were allowed to elapse months prior to decontamination. Multiple applications of fresh HASP? [corrected] were also shown to improve decontamination performance under certain circumstances.     

Comparative evaluation method of polymer surface treatments applied on high performance concrete

An investigation on the protection characteristics of three different polymer surface treatments applied on concrete was carried out by means of a large number of tests. Two of the polymers used, i.e., silane and siloxane, are commercial products, while the third, a fluorinated polymer, is still being studied. These treatments were applied on high performance concretes (0.33 water/cement ratio) with two different curing times. Their protection efficiencies were evaluated by testing water absorption (by immersion and by capillarity) and evaporation, chlorides penetration and artificial ageing; two other tests were carried out to verify concrete surface colour changes caused by applied treatments and by external exposure. To evaluate the overall behaviour of each product, the results obtained were translated into a parametric scale and reported using comparative graphics.     

Formation and properties of diffusion coatings applied to steels by boronizing in melts

We propose the optimum modes for the formation of diffusion boride coatings on steels of various classes from liquid-metal melts. The microstructure and physical properties of boronized steels 20, 30, 45, R18, DI22, and U10 are analyzed. The advantages of boronizing from lithium-based melts, which enables one to combine the process of deposition with heat treatment, over the familiar methods of creation of a coating from powder mixtures are established. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 2, pp. 86–91, March–April, 1999.