Closed-Form Model and Parametric Study on Connection of Concrete-Filled FRP Tubes to Concrete Footings by Direct Embedment

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been introduced as a new system for piles, columns, and poles. A simple moment connection based on direct embedment of the CFFT into concrete footings or pile caps, without using dowel-bar reinforcement, has been proposed by the authors. Robust analytical models to predict the critical embedment length (Xcr) were also developed and experimentally validated. In this paper, a comprehensive parametric study is carried out using the models developed earlier along with a newly developed closed-form model for the general case of axial loading, bending, and shear applied to the CFFT member. The parameters studied are the diameter (D), thickness (t), length outside the footing (L), and laminate structure of the FRP tube, as well as the tube-concrete interface bond strength (τmax?), concrete compressive strength in the CFFT (fct′) and footing (fc′), and the magnitude and eccentricity of axial compressive or tensile loads. It was shown that increasing D, L/D, τmax?, and fc′ of the footing, or the axial compression load, reduces (X/D)cr, whereas increasing t and fct′ of the CFFT, the fraction of longitudinal fibers in the tube, or the axial tension load, increases Xcr. As the axial load eccentricity increases, Xcr reduces for tension loads and increases for compression loads until both cases converge asymptotically to the same Xcr value, essentially that of pure bending.     

Bio-Based Composite Roof for Residential Construction

Bio-based composites are an innovative new class of materials that offer many advantages over traditional fiber reinforced composite materials, and may be suitable for use in light duty construction. A study was conducted to develop a bio-based composite roof structure, suitable for residential construction. First, to develop basic information about the flexural performance of the bio-composite, seven different beams were fabricated using a soybean oil-based resin and natural reinforcement (flax or recycled paper). The beams were tested in four-point bending to failure. Results showed that the beams made with recycled paper wrapped around the foam cores were the strongest and the stiffest of the beams tested. Using this composite design, a monolithic composite roof system was designed for residential construction. A scale model unit width beam from the roof was fabricated and tested. Tests included three- and four-point bending, a quasi-distributed load test, and test to failure. The model beam was shown to satisfy the design deflection criteria and had an ultimate strength 11 times greater than the design load. As this was a true scale model specimen, the results demonstrate that the prototype roof would also satisfy the design criteria for strength and stiffness, with similar factors of safety.     

Physical Properties of Glass Fiber Reinforced Polymer Rebars in Compression

Forty-five glass fiber reinforced polymer (GFRP) rebars were tested in compression to determine their ultimate strength and Young’s modulus. The rebars (or C-bars), produced by Marshall Industries Composites, Inc., had an outside diameter of 15 mm (#15 rebar), and unbraced lengths varying from 50 to 380 mm. A compression test method was developed to conduct the experiments. Three failure modes, that are directly related to the unbraced length of the rebar, are identified as crushing, buckling, and combined buckling and crushing. The crushing region represents the failure mode a GFRP rebar would experience when confined in concrete under compression. The experimental results showed that the ultimate compressive strength of the #15 GFRP rebar failing by crushing is approximately 50% of the ultimate tensile strength. Based on a very limited number of tests, in which strain readings were acceptable, Young’s modulus in compression was found to be approximately the same as in tension.     

Confinement-Shear Lattice Model for Concrete Damage in Tension and Compression: I. Theory

The mechanical behavior of the mesostructure of concrete is simulated by a three-dimensional lattice connecting the centers of aggregate particles. The model can describe not only tensile cracking and continuous fracture but also the nonlinear uniaxial, biaxial, and triaxial response in compression, including the postpeak softening and strain localization. The particle centers representing the lattice nodes are generated randomly, according to the given grain size distribution, and Delaunay triangulation is used to determine the lattice connections and their effective cross-section areas. The deformations are characterized by the displacement and rotation vectors at the centers of the particles (lattice nodes). The lattice struts connecting the particles transmit not only axial forces but also shear forces, with the shear stiffness exhibiting friction and cohesion. The connection stiffness in tension and shear also depends on the transversal confining stress. The transmission of shear forces between particles is effected without postulating any flexural resistance of the struts. The shear transmission and the confinement sensitivity of lattice connections are the most distinctive features greatly enhancing the modeling capability. The interfacial transition zone of the matrix (cement mortar or paste) is assumed to act approximately in series coupling with the bulk of the matrix. The formulation of a numerical algorithm, verification by test data, and parameter calibration are postponed for the subsequent companion paper.     

Efficient Strengthening Technique to Increase the Flexural Resistance of Existing RC Slabs

Composite materials are being used with notable effectiveness to increase and upgrade the flexural load carrying capacity of reinforced concrete (RC) members. Near-surface mounted (NSM) is one of the most promising strengthening techniques, based on the use of carbon fiber-reinforced polymer (CFRP) laminates. According to NSM, the laminates are fixed with epoxy based adhesive into slits opened into the concrete cover on the tension face of the elements to strength. Laboratory tests have shown that the NSM technique is an adequate strengthening strategy to increase the flexural resistance of RC slabs. However, in RC slabs of low concrete strength, the increase of the flexural resistance that NSM can provide is limited by the maximum allowable compressive strain in the compressed part of the slab, in order to avoid concrete crushing. This restriction reduces the effectiveness of the strengthening, thus limiting the use of the NSM technique. A new thin layer of concrete bonded to the existing concrete at the compressed region is suitable to overcome this limitation. Volumetric contraction due to shrinkage and thermal effects can induce uncontrolled cracking in the concrete of this thin layer. Adding steel fibers to concrete [steel fiber-reinforced concrete (SFRC)], the postcracking residual stress can be increased in order to prevent the formation of uncontrolled crack patterns. In the present work, the combined strengthening strategy, a SFRC overlay and NSM CFRP laminates, was applied to significantly increase the flexural resistance of existing RC slabs. Experimental results of four-point bending tests, carried out in unstrengthened and strengthened concrete slab strips, are presented and analyzed.     

Mixed-Mode Fracture of Hybrid Material Bonded Interfaces under Four-Point Bending

A combined analytical and experimental approach is presented to characterize mixed-mode fracture of hybrid material bonded interfaces under four-point bending load, and closed-form solutions of compliance and energy release rate (ERR) of the mixed-mode fracture specimens are provided. The transverse shear deformations in each sublayer of bimaterial bonded beams are included by modeling the specimen as individual Timoshenko beams, and the effect of interface crack-tip deformation on the compliance and ERR are taken into account by applying the interface deformable bilayer beam theory (flexible-joint model). The higher accuracy of the present analytical solutions for both the compliance and ERR of mixed-mode fracture specimens is manifested by comparing them with the solutions predicted by the conventional beam theory (CBT) and finite-element analysis (FEA). As an application example, the fracture of wood–fiber-reinforced plastic (FRP) bonded interface is experimentally evaluated by using mixed-mode fracture specimens [i.e., four-point asymmetric end-notched flexure (4-AENF) and four-point mixed-mode bending (4-MMB)], and the corresponding values of critical ERRs are obtained. Comparisons of the compliance rate change and the resulting critical ERR based on the CBT, the present theoretical model, and FEA demonstrate that the crack-tip deformation plays an important role in accurately characterizing the mixed-mode fracture toughness of hybrid material bonded interfaces under four-point bending load.     

Flexural Performance of Concrete Beams Reinforced with FRP Grids

This paper evaluates the flexural performance of simply supported concrete beams subjected to four-point monotonic loading and reinforced with a 2D fiber-reinforced plastic (FRP) grid. The main parameter of the study is the amount of longitudinal FRP reinforcement. With respect to a balanced strain condition, three underreinforced and two overreinforced FRP designs were tested with three identical beams per design. Laboratory recorded load-deflection, failure mode, cracking behavior, and reinforcement strain data are compared with theoretical predictions calculated according to traditional steel-reinforced concrete procedures. The study concludes that, with respect to ACI 318-95, flexural capacity is accurately predicted, but shear strength is not. Deflection compatibility between test results and ACI predictions employing the Branson effective moment of inertia was dependent on the percentage of longitudinal reinforcement. In general, observed flexural stiffness was less than that predicted by Branson's equation. A moment-curvature deflection procedure employing a bilinear concrete model compared very well with measured deflections. Finally, the grid configuration provides an effective force transfer mechanism. Cracking occurred at transverse bar locations only, and FRP tensile rupture was achieved with no observed deterioration in force transfer mechanics.     

Mechanical Stabilization of Cemented Soil–Fly Ash Mixtures with Recycled Plastic Strips

An experimental investigation was undertaken to evaluate the mechanical behavior of a soil–cement–fly ash composite, reinforced with recycled plastic strips (high-density polyethylene) that were obtained from postconsumer milk and water containers. The primary motivation for the study was to investigate the innovative reuse of several candidate waste materials in geotechnical and pavement applications. The specific objectives of the research were: (1) to evaluate the compressive, split tensile, and flexural strength characteristics of the material, and (2) to determine the effectiveness of recycled plastic strips in enhancing the toughness characteristics of the composite. Since cement-stabilized materials are weak in tension, the main focus of the experimental program was to conduct a series of specially instrumented split tensile and flexural tests on mixes containing various amounts of cement, fly ash, and plastic strips. For a meaningful comparison of test results, all specimens were prepared at a constant dry density. The standard ASTM C496 procedure for split tensile test was slightly modified by attaching two horizontal linear variable differential transformers (LVDTs) to measure the diametral deformation of the specimen due to compressive loading in an orthogonal direction. This modification enabled the evaluation of the postpeak toughness behavior of the composite. For some specimens, a strain gauge was attached to the middle of the face perpendicular to the loading plane in order to correlate the results with the one found using the LVDTs. All tests were performed with a 90 kN universal testing machine with deformation control. Experimental data show that the soil–cement matrix stabilized with 4% to 10% by weight of fly ash and reinforced with 0.25% to 0.5% (by weight) plastic strips (having lengths of 19 mm or 38 mm) can achieve a maximum compressive strength of 7000 kPa, a split tensile strength of 1000 kPa, and a flexural strength of 1200 kPa. These ranges in strength values are suitable for a high-quality stabilized base course for a highway pavement. To quantify the reinforcing effects in the postpeak region, a dimensionless toughness index is proposed. It is found that the use of fiber reinforcement significantly increases the postpeak load carrying capacity of the mix and thus the fracture energy. It is concluded that the lean cementitious mix containing recycled materials offer a lot of promise as an alternative material for civil engineering construction.     

Effect of hydrogen on four-point bend tests of U-notched AISI 1090 steel

U-notched, four-point bend tests of 1090 steel were performed in the, presence or absence of precharged hydrogen. Failure occurred in a mode II manner under mixed mode I–II loading, with cracking preceded by plastic instability in the form of surface rumpling and shear localization. The critical strain for surface instability was less in compression than in tension and was reduced by hydrogen, markedly in the tension case and weakly in the compression case. The results are discussed in terms of models for plastic instability and for hydrogen degradation.     

Effect of hydrogen on four-point bend

U-notched, four-point bend tests of 1090 steel were performed in the presence or absence of precharged hydrogen. Failure occurred in a mode II manner under mixed mode I–II loading, with cracking preceded by plastic instability in the form of surface rumpling and shear localization. The critical strain for surface instability was less in compression than in tension and was reduced by hydrogen, markedly in the tension case and weakly in the compression case. The results are discussed in terms of models for plastic instability and for hydrogen degradation.     

Computational Methodology for Modeling Fracture in Fiber-Reinforced Polymer Composites

A framework is presented for analyzing the inelastic behavior and fracture of polymer matrix composites. Physics-based viscoplastic constitutive equations are used and supplemented with a matrix cracking model and an energy-based debonding model. The capabilities of the framework are illustrated by finite-element solutions of boundary-value problems under plane strain conditions using the unit-cell concept. In the application, focus is centered on the effect of manufacturing induced voids on local modes of fracture under both tension and compression.     

Damage Assessment and Ductility Evaluation of Post Tensioned Beams with Hybrid FRP Tendons

The study presented in this article concentrated on investigating the ductility and characterization of damage in concrete beams post tensioned with hybrid carbon-glass fiber-reinforced polymer (HFRP) composites. The investigation included an approach for design of flexural members with HFRP tendons and characterization of damage, load deformation response, ultimate strength, and failure modes. Direct tensile tests of hybrid FRP rods in a previous study had indicated elastoplastic response, enhanced ductility, and increased strain capacity. In this context, the current study focused on design and fabrication of post tensioned beams using glass or steel rebars for partial prestressing. All the beams were tested in flexure under four-point bending configuration. Results of the study are presented in terms of ductility index and enhanced load-deflection response in comparison with the conventional FRP materials. Damage characterization involved evaluating the specific features of the acoustic emissions for detecting the elastoplastic transition in the hybrid tendons. The method involved use of a high-resolution fiber-optic interferometer for detection and separation of acoustic emissions. By using the time domain response, it was possible to spatially localize the damage at various stages of the loading. Spectral energy of the acoustic emissions facilitated separation of carbon and glass fiber fractures.     

Mechanical properties of perforated and partially demineralized bone grafts

Changes in flexural rigidity and compression strength of 18 sheep tibias were investigated after laser perforation and partial demineralization. Test bones were divided into three groups: Group 1, no treatment; Group 2, laser hole grid; and Group 3, laser hole grid and partial demineralization. Starting in the anterior direction at the tibial tuberosity, the flexural rigidity was determined using a nondestructive four-point bending test. The elliptical distribution of the flexural rigidity before and after a specific treatment was compared. After the bending test, a cylindrical center section of each test bone was loaded axially to failure to determine subsequent changes in compression strength. Results showed that perforation alone produced minimal reduction of rigidity and insignificant changes in compression strength. However, additional partial demineralization resulted in larger reductions. In compression testing, perforated and partially demineralized bone specimen showed marked decrease of the ultimate failure stress. The observed increase in failure strain appeared to be related to compression of the laser holes. The findings of this study suggest that partial demineralization and perforation can be applied to diaphyseal bone grafts and that their decreased mechanical properties are a function of the bone volume reductions produced by both processes.     

Behavior of Composite Unreinforced Masonry–Fiber-Reinforced Polymer Wall Assemblages Under In-Plane Loading

An experimental investigation was conducted to study the in-plane behavior of face shell mortar bedded unreinforced masonry (URM) wall assemblages retrofitted with fiber-reinforced polymer (FRP) laminates. Forty-two URM assemblages were tested under different stress conditions present in masonry shear and infill walls. Tests included prisms loaded in compression with different bed joint orientation (on/off-axis compression), diagonal tension specimens, and specimens loaded under joint shear. The behavior of each specimen type is discussed with emphasis on modes of failure, strength and deformation characteristics. Results showed that the application of FRP laminates on URM has a great influence on strength, postpeak behavior, as well as altering failure modes and maintaining the specimen integrity. The retrofitted specimens reached compressive strength of 1.62–5.64 times that of their unretrofitted counterparts, depending on the bed joint orientation, and joint shear strength increased by eightfold.     

Mechanism of Bond Behavior of Concrete Beams Strengthened with Near-Surface-Mounted CFRP Rods

Bond tests were conducted on concrete beams strengthened with near-surface-mounted (NSM) nonprestressed and prestressed carbon fiber-reinforced polymer (CFRP) rods under static loading. In the NSM technique, the CFRP rods are placed inside precut grooves and bonded to the concrete with epoxy adhesive. Six concrete beams were tested. The test variables included presence of internal tension steel reinforcement (unreinforced and reinforced), use of NSM CFRP strengthening (nonprestressed and prestressed), and type of CFRP rod (spirally wound and sand blasted). The beams were tested statically in four-point bending. Based on the test results, the transfer length for the prestressed CFRP rod in epoxied groove was 150 and 210 mm for the sand blasted and spirally wound rods, respectively. The main failure mode was debonding between the CFRP rod and the epoxy that starts at sections close to the midspan then, as the load increases, it propagates toward the supports. At failure, the beams strengthened with a given rod type showed the same CFRP strain at sections close to the support (29% of ultimate strain for spirally wound bars and 39% of ultimate strain for sand blasted bars). A cracked section analysis was carried out and compared well with the measured results.     

Effect of loading condition and stress state on damage evolution of silicon particles in an Al-Si-Mg-Base cast alloy

Damage evolution of Si particles in a Sr modified cast A356(T6) Al alloy is quantitatively characterized as a function of strain under tension, compression, and torsion. The fraction of damaged Si particles, their size distributions, and orientation distribution of particle cracks are measured by image analysis and stereological techniques. Silicon particle cracking and debonding are the predominant damage modes. Particle debonding is observed only under externally applied tensile loads, whereas particle cracking is observed under all loading conditions. The relative contributions of Si particle debonding and fracture to the total damage strongly depend on stress state and temperature. For all loading conditions and stress states studied, the average size of damaged Si particles is considerably larger than the bulk average size. The rate of damage accumulation is different for different loading conditions. At a given strain level, Si particle damage is lowest under compression and highest under torsion. The anisotropy of the damage is highly dependent on the deformation path and stress state. Under uniaxial tension, the cracks in the broken Si particles are mostly perpendicular to the loading direction, whereas in the compression test specimens they are parallel to the loading direction. The Si particle cracks in the torsion and notch-tension test specimens do not exhibit preferred orientations. The quantitative microstructural data are used to test damage evolution models.     

Fatigue Modeling of Concrete-Filled Fiber-Reinforced Polymer Tubes

Field applications and laboratory research have shown the feasibility of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) in bridges. Yet, their widespread applications require developing appropriate design and analysis tools for different types of loading, particularly fatigue loads. An analytical tool is developed to trace the response of CFFTs under fatigue loading. The FRP material models are calibrated against fatigue and creep coupon tests. Material models are cast into a fiber element analysis, with an algorithm to simulate strain profile, moment-curvature and residual bending strength at any given time or after any number of fatigue cycles in a single or multiple stages of loading. Comparisons with available test data show good agreement with model predictions. A detailed parametric study shows that fatigue response of CFFT beams can improve by either increasing the reinforcement index or the effective modulus of FRP tube in the longitudinal direction. Higher load ranges may drastically reduce fatigue life. Therefore, it is important to limit the load level on CFFTs for a reliable and predictable member performance. The study also recommends reducing fiber orientation in angle plies with respect to the axis of the beam to improve fatigue performance of the CFFT member.     

Normal- and High-Strength Concrete Circular Elements Wrapped with FRP Composites

Reinforced concrete columns usually have a minimum amount of transverse steel reinforcement this transverse reinforcement can have non negligible effects on the response of columns retrofitted with fiber-reinforced polymers (FRP). This paper presents a test program that was designed to study the behavior of small- and large-scale normal- and high-strength concrete circular columns confined with transverse steel reinforcement, FRP, and both transverse steel reinforcement and FRP under concentric loading. The effect of the main variables—such as the unconfined concrete strength, the volumetric ratio, the type and the yield strength of the transverse steel reinforcement, the concrete cover, and the number of FRP layers—are studied in this research program. The test results show that the enhancement of the confined concrete strength and strain is more pronounced in specimens with normal-strength concrete. It is also shown that the rupture of the FRP in the specimens with higher volumetric transverse steel reinforcement ratios corresponds to larger axial compressive strength and strain and that the postpeak behavior of these specimens is more ductile.     

Fatigue Life Assessment and Static Testing of Structural GFRP Tubes Based on Coupon Tests

Fiber-reinforced polymer (FRP) hollow tubes are used in structural applications, such as utility poles and pipelines. Concrete-filled FRP tubes (CFFTs) are also used as piles and bridge piers. Applications such as poles and marine piles are typically governed by cyclic bending. In this paper, the fatigue behavior of glass-FRP filament-wound tubes is studied using coupons cut from the tubes. Several coupon configurations were first examined in 24 tension and five compression monotonic loading tests. Fatigue tests were then conducted on 81 coupons to examine several parameters; namely, loading frequency as well as maximum-to-ultimate (σmax/σult) and minimum-to-maximum (σmin/σmax) stress ratios, including tension tension and tension compression, to simulate reversed bending. The study demonstrated the sensitivity of test results and failure mode to coupon configuration. The presence of compression loads reduced fatigue life, while increasing load frequency increased fatigue life. Stiffness degradation behavior was also established. To achieve at least one million cycles, it is recommended to limit (σmax/σult) to 0.25. Models were used to simulate stiffness degradation and fatigue life curve of the tube. Fatigue life predictions of large CFFT beams showed good correlation with experimental results.     

Microplane Model M5 with Kinematic and Static Constraints for Concrete Fracture and Anelasticity. I: Theory

Presented is a new microplane model for concrete, labeled M5, which improves the representation of tensile cohesive fracture by eliminating spurious excessive lateral strains and stress locking for far postpeak tensile strains. To achieve improvement, a kinematically constrained microplane system simulating hardening nonlinear behavior (nearly identical to previous Model M4 stripped of tensile softening) is coupled in series with a statically constrained microplane system simulating solely the cohesive tensile fracture. This coupling is made possible by developing a new iterative algorithm and by proving the conditions of its convergence. The special aspect of this algorithm (contrasting with the classical return mapping algorithm for hardening plasticity) is that the cohesive softening stiffness matrix (which is not positive definite) is used as the predictor and the hardening stiffness matrix as the corrector. The softening cohesive stiffness for fracturing is related to the fracture energy of concrete and the effective crack spacing. The postpeak softening slopes on the microplanes can be adjusted according to the element size in the sense of the crack band model. Finally, an incremental thermodynamic potential for the coupling of statically and kinematically constrained microplane systems is formulated. The data fitting and experimental calibration for tensile strain softening are relegated to a subsequent paper in this issue, while all the nonlinear triaxial response in compression remains the same as for Model M4.