Houssam Toutanji

Stress-Strain Characteristics of Concrete Columns Externally Confined with Advanced Fiber Composite Sheets

This paper presents the results of experimental and analytical studies on the performance of concrete columns externally wrapped wtih carbon and glass fiber reinforced polymer composite (FRPC) sheets. Concrete columns were wrapped with three different types of FRPCs: two carbon and one glass. Confined and unconfined specimens were loaded in uniaxial compression. Axial load and axial and lateral strains were obtained to evaluate stress-strain behavior, ultimate strength, stiffness, and ductility of the wrapped specimens. Results show that external confinement of concrete by FRPC sheets can significantly enhance the strength, ductility, and energy absorption capacity of the concrete specimens. An analytical model to predict the entire stress-strain relationship of concrete specimens wrapped with FRPC sheets was developed. The proposed model consists of two distinct parts. In the first part, the behavior is similar to that of plain concrete, since lateral expansion of the confined concrete is insignificant. The second region, which is mainly dependent on the stiffness of the FRPC, is recognized in which the FRP wrap is fully activated. Comparison between the experimental and analytical results indicates that the model provides satisfactory predictions of the stress-strain response. Results from a series of experimental tests on concrete confined with FRP sheets available in the literature are also compared favorably with results obtained by the model.

BEHAVIOR OF CONCRETE COLUMNS CONFINED WITH FIBER REINFORCED POLYMER TUBES

New types of structural columns are being developed for new construction. They are made of concrete-encased fiber reinforced polymer (FRP) tubes. The concrete-filled FRP tubes are cast in place. The tube acts as formwork, protective jacket, confinement, and shear and flexural reinforcement. It can also be used to complement or replace conventional steel reinforcement of the column. This paper presents the results of experimental and analytical studies of the performance of concrete columns conbined with carbon and glass FRP composite tubes. Concrete-filled FRP tubes are instrumented and tested under uniaxial compressive load. Test variables include type of fiber, thickness of tube, and concrete compressive strength. Results show that external confinement of concrete by FRP tubes can significantly enhance the strength, ductility, and energy absorption capacity of concrete. Equations to predict the compressive strength and failure strain, as well as the entire stress-strain curve of concrete-filled FRP tubes were developed. A comparison between the experimental results and those of analytical results indicate that the proposed model provides satisfactory predictions of ultimate compressive strength, failure strain, and stress-strain response. The study shows that the available models generally overestimate the strength of concrete confined by FRP tubes, resulting in unsafe design.

The use of rubber tire particles in concrete to replace mineral aggregates

Abstract The effect of the replacement of mineral coarse aggregate by rubber tire aggregate is investigated in this paper. Four different volume contents of rubber tire chips were used: 25, 50, 75 and 100%. The incorporation of these rubber tire chips in concrete exhibited a reduction in compressive and flexural strengths, the reduction in compressive strength was approximately twice the reduction of the flexural strength. The specimens which contained rubber tire aggregate exhibited ductile failure and underwent significant displacement before fracture. The toughness of flexural specimens was evaluated for plain and rubber tire concrete specimens. The test revealed that high toughness was displayed by specimens containing rubber tire chips as compared to control specimens.

Durability characteristics of concrete beams externally bonded with FRP composite sheets

Abstract The strengthening of concrete structures in situ with externally bonded fiber reinforced plastic (FRP) composite sheets is increasingly being used for repair and rehabilitation of existing structures. This paper provides information in the area of long-term durability of concrete beams externally bonded with FRP sheets. It was intended to study the effect of harsh environmental conditions such as wet/dry cycling using salt water on the performance of FRP-bonded concrete beams and on the interfacial bond between the fiber and the concrete. Concrete beams were strengthened with four different types of FRP sheet: two carbon and two glass. Three different types of two-part epoxy were used. Test variables included (1) the type of fiber, (2) the type of epoxy system, and (3) the environmental exposure condition. The specimens were conditioned in two different environments: (a) room temperature (+20 °C), and (b) 300 wet/dry cycles (salt water was used for the wet cycles and hot air at 35 °C and 90% humidity for the dry). At the end of each exposure, load-deflection curves of the specimens were obtained in order to evaluate their maximum capacity, stiffness, and ductility. The performance of the wet/dry exposed specimens was compared with those kept at room temperature. Results showed that specimens subjected to wet/ dry environmental conditions and those kept at room temperature exhibited significant improvement in flexural strength when FRP sheets were bonded to the tension face of the concrete beams. However, the specimens subjected to wet/dry conditions showed less improvement than those kept at room temperature. None of the specimens failed due to FRP rupture but rather due to the debonding between the FRP sheet and the concrete interface. The selection of epoxy was shown to be very important for using the FRP strengthening technique, especially in a marine environment.

Flexural Behavior of Concrete Beams Reinforced with Glass Fiber-Reinforced Polymer (GFRP) Bars

Concrete members reinforced with glass fiber-reinforced polymer (GFRP) bars exhibit large deflections and crack widths compared with concrete members reinforced with steel. This is due to the low modulus of elasticity of GFRP. Current design methods for predicting deflections at service load and crack widths developed for concrete structures reinforced with steel may not be used for concrete structures reinforced with GFRP. This paper presents methods for predicting deflections and crack widths in beams reinforced with GFRP. To use the effective moment of inertia for concrete beams reinforced with GFRP bars, the effect of the FRP reinforcement ratios and elastic modulus of FRP were incorporated in the exponent of Bransons equation. In addition, an equation based on flexural stiffness of GFRP reinforced concrete was developed to predict deflections. Based on this investigation and past studies, a theoretical correlation for predicting crack width was proposed. Six concrete beams reinforced with different GFRP reinforcement ratios were tested. Their measured deflections and crack widths were compared with the proposed models. The experimental results compared favorably with those predicted by the models.

Axial load behavior of large-scale columns confined with fiber-reinforced polymer composites

Confinement of concrete is an efficient technique used to increase the load-carrying capacity and ductility of concrete columns and is of interest for upgrading columns, piers, and chimneys. This article reports on an experimental and analytical study of axially-loaded large-scale columns confined with fiber-reinforced polymer (FRP) wrapping reinforcement; this work updates previous studies on smaller-scale columns. The effective circumferential FRP failure strain and the effect of increasing confining action were investigated. The authors compared the different existing compressive strength models to their study results. The authors then present a revision of an existing model developed previously by the second author. This revised model addresses the effective FRP failure strain that is attributed to localized stress concentrations near failure due to nonhomogenous deformations of the damaged concrete. The authors note that the wrapping configuration of the confinement has a considerable influence on the effectiveness of the FRP wrapping. The authors conclude that although the available models were developed based on small-size cylinders, four models seem to predict the ultimate strength of large-scale columns fairly accurately (Miyauchi et al, Saafi et al, Samaan et al, and Toutanji Revised).

The effect of surface preparation on the bond interface between FRP sheets and concrete members

Abstract Fiber-reinforced polymer (FRP) composite wraps have been established as an effective method for rehabilitation and strengthening of concrete structures. They are being increasingly used as an alternative to steel for reinforcing and strengthening of concrete structures. This paper presents the experimental and analytical results of the influence of concrete surface treatment and the type of FRP sheets on the bonding strength of concrete-FRP sheet. The FRP sheets were bonded to concrete beams in two opposite sides using an epoxy resin. Variables included the type of fiber (C1, C5, and GE) and the surface treatment (water jet and sanding). With changing the surface treatment of concrete surface preparation and the type of fiber sheets, the bonding strength, bonding load–strain response and failure modes were investigated. The concrete specimens with surface roughened with water jet showed much better bonding strength than those roughened with an ordinary sander. Equations for predicting the bond load failure of concrete specimens externally bonded with carbon and glass fiber sheets compared well with those of experimental.

The influence of silica fume on the compressive strength of cement paste and mortar

Abstract The compressive strengths of silica fume cement paste and mortar were evaluated at various water-cementitious ratios. Five different water-cementitious ratios were used including, 0.22, 0.25, 0.28, 0.31, and 0.34 and two contents of silica fume, 16% and 25% by weight of cement. Superplasiticizer content was adjusted for each mix to ensure that no segregation would occur. The results show that the increase in compressive strength of mortar containing silica fume, as a partial replacement for cement, greatly contributes to strengthening the bond between the cement paste and aggregate. Partial replacement of cement by silica fume and the addition of superplasticizer increases the strength of mortar but has no influence on the strength of cement paste. Results were verified by statistical analysis using hypothesis testing at a 95% confidence level. It was also demonstrated that superplasticizer in combination with silica fume plays a more effective role in mortar mixes than in paste mixes. This can be attributed to a more efficient utilization of superplasticizer in the mortar mixes due to the better dispersion of the silica fume particles. The paper also reviews some of the available literature on the influence of silica fume on cementitious composites and unsettled questions associated with this topic.

Chloride permeability and impact resistance of polypropylene-fiber-reinforced silica fume concrete

Permeability and impact resistance of polypropylene-fiber-reinforced concrete mixtures containing silica fume with variable design proportions were studied. Two fiber lengths were considered, 12.5 and 19 mm, and four fiber volume fractions, 0, 0.1%, 0.3%, and 0.5%. Silica fume was used as a replacement by weight of cement. Two silica fume contents were used, 5% and 10%. Results showed that the incorporation of polypropylene fibers increased the permeability of conventional concrete. The addition of silica fume improved fiber dispersion in the cementitious matrix, causing a significant reduction in the permeability of the polypropylene fiber reinforced concrete. Moreover, the addition of silica fume was noted to significantly enhance the polypropylene fiber effectiveness in improving the impact resistance of concrete.

PROPERTIES OF POLYPROPYLENE FIBER REINFORCED SILICA FUME EXPANSIVE-CEMENT CONCRETE

Abstract This paper reports on a comprehensive study on the mechanical properties of expansive-cement concrete containing silica fume and polypropylene fibers. Properties studied include those of the fresh mix properties, length change, rapid chloride permeability, compressive strength, flexural behavior, and bond of hardened concrete. Silica fume content used was 5 and 10% and fiber volume fraction was 0.10, 0.30, and 0.50%. Results show that the use of 5% silica fume combined with 0.30% fiber volume fraction results in optimum mixture design for repair applications from the standpoints of workability, bond, strength, length change and permeability.