Journal of Composites for Construction

The hybrid composite beam (HCB) is an innovative idea that incorporates traditional construction materials (i.e., steel and concrete) with fiber-reinforced polymer (FRP) composites in an efficient configuration to optimize the beam constituents’ performance. The HCB is comprised of three main subcomponents: a composite shell, a compression reinforcement, and a tension reinforcement. The shell is comprised of a glass-fiber-reinforced polymer (GFRP) box. The compression reinforcement consists of self-consolidating concrete (SCC) that is pumped into a profiled conduit within the shell. The tension reinforcement consists of galvanized-steel tendons anchored at the compression reinforcement ends. The HCB is a promising technology in bridge applications because it has several advantages over the conventional structural members (e.g., a prolonged lifetime and a lighter weight). This new technology was recently used to construct three bridges in Missouri. This research study performs the first investigation and analysis for an in-service HCB bridge superstructure’s behavior subjected to thermal loads and proposes a thermal design methodology. This investigation is crucial because the thermal stresses, if not accounted for during the design, can significantly affect a bridge superstructure’s durability, which this new HCB technology strives to address. Beam elements from the longest-spanning HCB constructed bridge in Missouri were instrumented with various strain gauges and thermocouples. The constituting elements’ temperatures and the corresponding induced strains were recorded over 6 months. The proposed algorithm was used to predict the induced strains. A two-step thermostructural finite-element analysis (FEA) was performed to analyze the HCB’s thermal behavior and further evaluate the proposed algorithm’s performance. The results of this study showed that the proposed algorithm was able to predict, with acceptable accuracy, the thermal stresses and strains in a HCB bridge superstructure. Subsequently, this algorithm is recommended as a useful tool for designing and analyzing HCB bridges that are undergoing environmental thermal effects. The current study also presents recommendations for modifying the thermal gradients recommended by AASHTO for the thermal design of reinforced concrete (RC) superstructures to better suit HCB bridges. Finally, the study proposed techniques for increasing the stiffness of a HCB bridge superstructure, while at the same time minimizing the induced thermal stresses under temperature fluctuations.

The improved strength and ductility of fiber-reinforced polymer (FRP) confined concrete is a result of external confinement, which places an effective constraint on its lateral dilation. For a reliable confined concrete design, an understanding of the confinement mechanism and dilation of the concrete core, which can be achieved by studying its dilation characteristics, is crucial. In the absence of any experimental evidence, concrete behavior under the confinement of new materials remains unclear, and the direct application of existing confinement models developed for other FRPs may not be suitable. This paper discusses in detail the dilation characteristics of concrete confined by a polyethylene terephthalate (PET) FRP composite, which is a newly developed FRP with a bilinear stress-strain response, low elastic modulus, and large rupture strain (LRS). The effects of column parameters, such as the number of PET FRP layers, corner radius, and cross-section shape, are also evaluated. The experimental results show that under the soft confinement of PET FRP, the concrete core exhibited a significantly higher lateral dilation, Poisson’s ratio, dilation rate, and volumetric expansion compared to concrete confined by other FRPs. However, because of the LRS capacity, PET FRP controlled the unstable response efficiently and exhibited a remarkably long stable response, particularly in noncircular cross-sections. The results also indicate that the soft confinement of PET FRP could not curtail volumetric expansion but restrained it significantly under the effect of the considered parameters.

Confinement of concrete using glass fiber–reinforced polymer (GFRP) spirals was evaluated using small-scale concrete cylindrical specimens with a 254-mm diameter and 762-mm height under concentric axial compression. The contribution of longitudinal GFRP bars to confinement was excluded by using wood dowels as longitudinal reinforcement to maintain a constant spiral pitch. Thus, concrete confinement was provided exclusively by the GFRP spiral. An ultimate hoop strain of 1.0 to 1.5% was achieved for the GFRP spirals of well-confined small-scale concrete specimens. Expressions were developed for the confined compressive strength and ultimate axial compressive strain of concrete confined with GFRP spirals. The resulting confinement model is compared with axial column tests of reinforced concrete columns with GFRP spirals and GFRP longitudinal bars from the present study and the literature. An expression is proposed for the ultimate axial compression capacity of concrete columns reinforced with GFRP spirals and GFRP longitudinal bars.

Steel bridge piles, many of which have been in service for over half a century across the United States, are now deteriorating because of corrosion. They often require retrofitting to restore their capacities to their initial design capacities. Grout-filled fiber-reinforced polymer (FRP) jackets are one promising repair alternative among others. This paper presents the findings of a full-scale experimental investigation to study the behavior of steel piles with localized simulated corrosion damage that were repaired underwater using grout-filled FRP jackets. Fourteen full-scale deteriorated piles were repaired with grout-filled FRP jackets and were tested under axial compression. The repair system was able to restore the axial capacity of the piles to the initially designed capacity. A rational approach is proposed and was implemented for the design of the tested repairs in this study. The results indicate that grout-filled FRP jackets can be effectively used to repair steel H-piles with localized corrosion and that the proposed design approach can be used to design the jackets for piles with different degrees of corrosion.

The effectiveness of fiber-reinforced polymer (FRP) based strengthening on the monotonic compression response of reinforced concrete (RC) short columns has been studied extensively. Effectiveness of FRP strengthening in improving cyclic compression response of slender RC circular and square columns is investigated. A total number of 24 slender RC columns of 1,200 mm height were cast in two groups of circular and square columns. Specimens of each group had four unstrengthened and eight strengthened columns. Two schemes were followed to strengthen the columns. In the first scheme of strengthening, one layer of carbon FRP (CFRP) sheet was attached to the columns, maintaining CFRP fibers in the transverse direction. In the other strengthening scheme, however, the first two layers of CFRP sheets were externally bonded to the column, keeping the fibers along the height of the column, and then one layer of CFRP sheet was wrapped over the longitudinal sheets with its fibers in the transverse direction. Specimens were tested under eccentric monotonic and cyclic compression loadings. The cyclic compression involved several loading-unloading compression cycles before failure. Test results showed that CFRP strengthening of slender columns is equally effective in cyclic compression as in monotonic compression. A simple formulation is also presented to derive load-moment ($P\text{-}M$) interaction diagrams for FRP-strengthened columns. The prediction complies well with the experiments. The CFRP confinement of RC columns decreases the slenderness limit of the columns and can be reasonably predicted by the recommended equation. This reduction is more in CFRP-confined circular columns than square columns.

The use of fiber-reinforced polymer (FRP) materials is becoming commonplace in various civil engineering applications due to the several advantages they offer over traditional materials. However, the limited knowledge about some aspects of the behavior of these materials, particularly when subjected to elevated temperatures, is hampering their widespread use. This paper presents experimental and analytical investigations about the shear behavior of pultruded glass fiber-reinforced polymer (GFRP) profiles at elevated temperatures. The primary objectives were twofold: to quantify the changes in the in-plane shear modulus and strength caused by temperature increase, namely when glass transition temperature ($T_g$) of the material is exceeded; and to assess the accuracy of different analytical models in simulating those changes. The experimental campaign consisted of shear tests, performed in V-notched specimens obtained from a GFRP pultruded flat plate, from room temperature ($T_{\mathrm{room}}$) up to 180°C. The results obtained show that (1) the in-plane shear strength presents significant reduction with temperature (88% at 180°C, compared to $T_{\mathrm{room}}$); (2) such reduction is considerably higher compared to that obtained from 10° off-axis tests (in particular, for $T<140°\mathrm{C}$); and (3) the shear modulus reduction with temperature is slightly lower than that experienced by the in-plane shear strength (78% at 180°C). All empirical formulations assessed in the present study were able to provide reliable estimates of the degradation with temperature of the shear strength and modulus.

The bond between carbon-fiber-reinforced polymers (CFRPs) and concrete plays a key role in externally bonded CFRP-strengthening technology, but it is susceptible to harsh environments. In the present study, the effects of freeze-thaw cycles ($-20°\mathrm{C}$ for 10 h and 30°C for 10 h) in water and 90% relative humidity (RH) on the behavior of the bond between CFRP and concrete were investigated using a single-lap shear test. The freeze-thaw process deteriorated the fracture energy, maximum bond stress, initial stiffness, and load capacity of the CFRP–concrete bond. The effects of water immersion were much more severe than those of exposure to 90% RH. In the case of freeze-thaw cycles under 90% RH conditions, the debonding mode shifted from concrete cohesive failure to adhesive/concrete interfacial debonding. In the case of freeze-thaw cycles with water immersion, the debonding mode (concrete cohesive failure) did not vary owing to the severe degradation in the concrete substrate. A model describing the degradation in the fracture energy of the CFRP–concrete bond as a function of the number of freeze-thaw cycles was developed based on parameter analysis, experimental testing, and the results sourced from literature. An environmental coefficient is proposed to account for the degradation in the CFRP–concrete bond due to freeze-thawing. Using the temperature data and a particular number of freeze-thaw cycles, the service life of the CFRP–concrete bonds in actual applications can be predicted.

This study investigated the behavior of bolted shear connections for fiber-reinforced polymer (FRP)–concrete hybrid beams. Fourteen push-out tests were conducted with varying parameters of bolt strength, bolt diameter ($d_{\mathrm{bolt}}$), bolt-embedding length in concrete ($h_{\mathrm{bolt}}$), FRP shear strength ($S_{FRP}$), and the use of ultra-high performance concrete (UHPC). Tests showed that a larger $h_{\mathrm{bolt}}$ leads to smaller slip but has only a slight effect on capacity; specimens with a larger $d_{\mathrm{bolt}}$ have greater capacity and smaller slip. Specimens with ordinary-strength bolts (OSBs) failed at bolt shank shear, whereas specimens with high-strength bolts (HSB) failed at FRP flange shear-out associated with a significant improvement in capacity compared with those with OSBs. The use of UHPC substantially reduced slip, but did not improve bolt capacity because the two failure modes do not occur in concrete. The prediction equations for capacity associated with these two failure modes were derived by modifying the existing equations. A salient finding in the load-slip curves of HSB specimens was three loading stages with different slopes: compacting, hardening, and softening. This was different from observations in previous studies on head studs in steel-concrete composite beams and FRP bolts/dowels in FRP-concrete hybrid beams. The load-slip curves of specimens with HSB were fitted by a trilinear model to facilitate design and analysis. Finally, design recommendations for bolted connections were formulated after parametric analysis.

The use of externally bonded fiber-reinforced polymer (FRP) systems in the strengthening of reinforced concrete (RC) members has become widely accepted in recent years. A significant concern with this technique is the fire resistance of the strengthened member, for which a systematic design procedure that can be easily implemented by practicing engineers is not yet available. This paper for the first time presents such a procedure for the fire resistance design of FRP-strengthened RC beams. The proposed procedure distinguishes three levels (Level I, Level II, and Level III) of fire insulation design to satisfy the specified fire resistance rating. In Level-I design, no fire insulation is provided and the FRP system is completely ignored; the RC beam itself is expected to survive the required fire resistance period. At the other extreme is Level-III design, in which the FRP system and the original RC beam need to be so insulated that they both remain effective during the required fire resistance period. Between the two extremes is Level-II design, in which a moderate level of fire insulation is provided to protect the RC beam rather than the FRP system. Level-I and Level-III design can be realized using appropriate methods proposed by the authors in previous studies. For Level-II design, a simple design method based on the so-called 500°C isotherm method is presented and assessed using numerical data generated by finite-element (FE) analyses. Although the present paper is concerned only with FRP-strengthened RC beams governed by flexural failure, the general framework presented can be readily extended to other FRP-strengthened RC components as well as FRP-strengthened RC structural systems.