Steel Reinforced Polymer Research Articles

A spike-shaped anchorage for steel reinforced polymer (SRP)-strengthened concrete structures

Steel reinforced polymer (SRP) composite has recently emerged as an effective and economical solution for strengthening of reinforced concrete (RC) structures. Premature debonding failure of unanchored SRP at low load levels generally governs the performance of RC structures strengthened with externally bonded SRP. Therefore, a novel yet simple spike-shaped anchorage system was proposed in this study to prevent the debonding failure of SRP and to improve the interfacial shear capacity. Experimental investigation through single-lap shear tests of SRP-concrete joints showed that the anchorage system changed the failure mode from composite debonding to fiber rupture. In addition, the anchorage system substantially increased the peak load and reduced the interfacial slippage of the SRP-concrete joint compared to the unanchored condition. A numerical procedure based on the finite difference method was developed to predict the full-range load response, and results matched well with the full-range experimental responses of anchored and unanchored specimens. Parametric study of the test results and numerical simulation based on finite difference method both showed that the fiber rupture failure mode could be achieved for anchors in various positions along the bonded length. The closer the anchor is to the loaded end, the less global slip was obtained when the load reached the peak value.

Short-term and long-term behavior of RC beams strengthened by galvanized steel mesh laminate

Steel reinforced polymers (SRP) are useful in a wide range of applications, including strengthening of existing reinforced concrete (RC) structures, due to their numerous advantages like high strength, low maintenance, light weight and low cost. As such, galvanized steel mesh (GSM) has emerged as an option in the strengthening of RC structures. Although numerous studies were performed on GSM strengthened RC beams, the durability of such strengthening systems is yet to be fully understood. This paper presents an experimental and numerical study to investigate the strengthening effect of GSM on RC beams as well as the time-depended behavior of RC beams with and without strengthening. Reinforced concrete beams without strengthening were used as control beams. For the strengthened beams, both high cord density galvanized steel mesh (HSM) and medium cord density galvanized steel mesh (MSM) were used after which the beams were exposed to Qatar atmospheric environment for different periods of time following 28 days of water curing. Finite element analyses using ABAQUS software were conducted to predict the mechanical behavior of the RC beams with and without strengthening. The results showed an improvement ranging from 46% ∼ 57% in the ultimate load of GSM strengthened beams, with a higher initial cracking load ranging from 3% ∼ 35% when compared to the unstrengthened beams. On the other hand, the time-dependent effect of the concrete has limited influence on the initial cracking load of both systems (before and after strengthening) while the ultimate load increased gradually with time due to the strength development of concrete. The results obtained from this study are useful in improving the design, construction, and maintenance of RC beams strengthened with GSM laminates.

Flexural behavior RC beams strengthened and repaired with SRP composite

This paper presents the results of an experimental investigation conducted to study the flexural response of reinforced concrete (RC) beams strengthened and repaired using externally bonded steel reinforced polymer (SRP) composite. SRP composite strips were bonded to the tension face of four RC beams, which were tested in four-point bending. Parameters varied were the presence/absence of U-wrap anchorages and loading rate. The difference in the loading rates employed did not appear to have a significant effect on the yield load, maximum load, or failure mode. The strengthening system without anchorage increased the yield load by 16% relative to the control beam, whereas the strengthening system with U-wrap anchorages increased the yield load by 26–34% relative to the control beam. Values of the maximum strain in the composite at the maximum load of the strengthened beams with U-wrap anchorages ranged from 0.9% to 1.2% and were more than twice as large as that of the strengthened beam without anchorage. The peak fiber stress obtained from single-lap direct-shear tests of SRP-concrete joints was similar to the maximum fiber tensile stress obtained by sectional analysis for the strengthened beam without anchorage.

Prediction of Bonding Strength of Externally Bonded SRP Composites Using Artificial Neural Networks.

External bonding of fiber reinforced composites is currently the most popular method of strengthening building structures. Debonding performance is critical to the effectiveness of such strengthening. Many models of bond prediction can be found in the literature. Most of them were developed based on laboratory research, therefore, their accuracy with less popular strengthening systems is limited. This manuscript presents the possibility of using a model based on neural networks to analyze and predict the debonding strength of steel-reinforced polymer (SRP) and steel-reinforced grout (SRG) composites to concrete. The model is built on the basis of laboratory testing of 328 samples obtained from the literature. The results are compared with a dozen of the most popular analytical methods for predicting the load capacity. The prediction accuracy in the neural network model is by far the best. The total correlation coefficient reaches a value of 0.913 while, for the best analytical method (Swiss standard SIA 166 model), it is 0.756. The sensitivity analysis confirmed the importance of the modulus of elasticity and the concrete strength for debonding. It is also interesting that the width of the element proved to be very important, which is probably related to the low variability of this parameter in the laboratory tests.

Concrete columns confined with SRP: Effect of the size, cross-sectional shape and amount of confinement

This paper presents an experimental study carried out to investigate the behavior of concentrically loaded concrete columns confined with steel-reinforced polymer (SRP) jackets. Several parameters, i.e. size of the specimen, shape of the cross-section, number of layers, and density of steel fiber sheet are investigated. Axial and hoop strain contours on the surface of the composite are obtained with digital image correlation (DIC). SRP confinement improves the load-carrying capacity and ductility of columns. The increment of peak stress and the energy absorption, with respect to the control specimens, for columns of two different sizes suggests the presence of size effect.

Bond assessment of two types of SRP strengthening systems subjected to severe conditions

Using composite materials to retrofit structures has expanded quickly the last few years due to its applicability and economic feasibility. For most of these systems including the new ones, their mechanical properties have been investigated extensively, but their durability bond performance still has not. Therefore, due to the lack of data regarding the long term performance, it is has been hard on the construction industry to implement these systems. Thus, to enrich the construction and design standards with bond durability information, an investigation was carried out at Missouri University of Science and Technology, Rolla, Missouri, USA to study the bond durability of Steel Reinforced Polymer (SRP) strengthening system. Two types of SRP systems were applied including: micro-fine galvanized (RG) and micro-fine brass coating (RNG) steel fibers where both of them were exposed to several harsh conditions. The conditions used in this study included tap water submersion for 3000 h, hot water submersion for 4000 h at 50 °C (122° F), and salt water submersion for 1500 and 4000 h. Additionally, other specimens were made and placed outside for real-time weather exposure. The real-time weather exposure included subjecting the specimens to temperature and humidity fluctuations, freeze–thaw cycles, and ultraviolet (UV) radiation from the sun. Upon completion the desired exposure duration, all the specimens were tested for bending using a three-point load fixture to evaluate the bond performance of the strengthening system. All the results were then compared with control specimens to obtain a better evaluation of their bond performance. Furthermore, tensile coupon tests were made as well to enhance the mechanical evaluation. The results showed that there was a substantial decrease in bond strength resulted from the water immersion. In addition to the water influence, temperature also negatively affected the durability of bond strength. On the other hand, salt presence did not affect the bond strength. Based on the performance of the strengthening system, an environmental reduction factor, CE, of 0.75 was proposed for wet environment and 0.65 for harsh environment.

Assessment of Existing Bond Models for Externally Bonded SRP Composites

This manuscript assessed the applicability of the existing ultimate bond formulas originally developed for externally bonded carbon fiber-based composites in the analysis of steel-reinforced polymers (SRPs). In the first part, the methods of predicting the bond capacity are reviewed, the differences are indicated, and the factors determining the bond are discussed; then, using the bond test results of over 400 samples available in the literature, the bond prediction methods are assessed by graphical comparison. The evaluation mainly concerned concrete elements and epoxy adhesives; however, to a lesser extent, a similar analysis was performed for the masonry substrate and grout matrices. The results showed the relatively good applicability of the majority of bond prediction models for the analysis of SRP composites. In most cases, the ultimate bond force was slightly underestimated, which was beneficial in the design of this type of strengthening. Larger discrepancies concerned weaker grout matrices.

Steel-Reinforced Polymers and Steel-Reinforced Composite Mortars for Structural Applications—An Overview

Bonding of external reinforcement is currently the simplest, fastest, and most popular method of strengthening concrete and masonry structures. Glass and carbon organic fibers are the dominant materials used, but alternatives also include high-strength steel wires. The mechanical properties of such steel are comparable to those of carbon fiber. Due to their good compatibility with mortars, steel wires are particularly well suited to the revitalization of historic buildings. The manuscript provides an overview of research and experience in the use of steel-reinforced polymers (SRPs) and steel-reinforced composite mortars (SRCMs, also called steel-reinforced grout (SRG)) for structural strengthening. The examples described are for concrete beams, slabs and columns, walls, and masonry arches. The results of laboratory tests are discussed. The summary presents the advantages and disadvantages of composites based on ultra-high-strength steels compared with more popular carbon fiber composites.

Interface bond between FRP systems and substrate: Analytical modeling

In the field of external strengthening and repairing of existing reinforced concrete structures, steel reinforced polymers (SRP) systems have emerged as a competitive alternative to the use of the more common carbon and glass fiber reinforced polymer (FRP) composites. Experimental investigations have frequently shown the potentials of these innovative composite systems in improving the performance of deficient structural members. At the same time, additional studies are needed to expand the existing knowledge and either to provide design recommendations or to develop specific guidelines. The paper fills some of the foregoing knowledge gaps by presenting an analytical investigation on the bond behaviour between SRP and concrete in which closed-form solutions are derived to predict the entire debonding propagation process. In particular, accurate and simplified local shear stress-slip (τ-s) laws are employed in the proposed modelling from which different expressions for the interfacial shear stress distribution, the axial stress profile and the concrete-SRP relative displacement (slip) are developed and commented in the paper; analytical estimates of the SRP effective bonded length are provided as well as relationships for calculating the maximum axial stress (or peak force) at SRP laminate debonding.The analytical procedure was, firstly, applied to simulate some single-lap shear tests performed in a previous experimental program with the purpose to investigate the influence on the debonding propagation process of the following main parameters: a) concrete strength, b) concrete surface finish and c) steel tape density. Then, the comparisons between the numerical simulations and the experimental results available for some bond tests have allowed for verifying the accuracy of the proposed modelling.

Adhesion between SRP and masonry: Laboratory simulations of the field moisture and salt conditions

Steel-reinforced polymer (SRP) composites are a new addition to the fiber-reinforced polymer (FRP) composite toolbox for structural strengthening. While several authors investigated the bond behavior of SRP composites applied to a masonry substrate, little is known about the role played by the presence of moisture and salt in the wall. This study aims at investigating the bond behavior of SRP-masonry joints under two conditions: i) SRP strips are applied to masonry blocks that have been previously subjected to an artificial weathering protocol consisting in salt crystallization cycles; ii) SRP strips are applied to masonry blocks, and then the SRP/masonry joints are saturated with water. Single-lap shear tests are conducted on the SRP-masonry joints that underwent those two conditions. Digital image correlation (DIC) is used to obtain the strain on the surface of the SRP strips. The distribution of salts in the masonry blocks and at the SRP-masonry interface after failure are investigated as well. Finally, the tensile strength of brick tested in both dry and saturated conditions is determined and compared.

Fractional factorial design model for seismic performance of RC bridge piers retrofitted with steel-reinforced polymer composites

This study explores the effects of key design parameters on the performance of seismically deficient rectangular cross-section reinforced concrete (RC) bridge piers strengthened with steel-reinforced polymer (SRP) composites. Nonlinear response of bridge piers was modeled using fiber-based section discretization. Three-level fractional factorial design of experiments at 5% significance level was used to capture the effects of design parameters and their interactions, including concrete compressive strength, yield strength of steel bars, geometric ratio of longitudinal bars, internal transverse reinforcement spacing, pier aspect ratio, and number of retrofitting SRP layers. A parametric study was used to examine the main influence of and interactions between these factors on the seismic performance of SRP-strengthened piers at different damage limit states, including concrete core crushing, longitudinal reinforcement yielding and buckling, and ductility performance. Results show that the lateral load-carrying capacity and ductility performance of SRP-confined RC bridge piers were significantly influenced by the pier aspect ratio, materials properties, and amount of transverse and longitudinal reinforcement. Moreover, the resistance to buckling base shear, and overall ductility increased with increasing number of SRP layers.

Experimental and analytical investigation on the bond of SRP systems to concrete

Steel Reinforced Polymer (SRP) systems have recently emerged as attractive solutions for external strengthening and repairing of existing structures. Experimental studies have frequently shown the potentials of these innovative composite systems in improving the performance of concrete and masonry structures. However, additional studies are needed to expand the existing knowledge and either to provide design recommendations or to develop specific guidelines. The paper fills some of the foregoing knowledge gaps by discussing the experimental results of 130 direct single-lap shear tests performed to investigate the bond behavior between the SRP reinforcement and the concrete substrate; different concrete surface finishes were investigated, i.e., sandblasted, bush hammered and grinded which are among the surface treatments mostly used in practice. Comparisons between the maximum load resisted by the SRP-concrete joints and the theoretical value calculated using some relationships available for similar FRP-concrete assemblages are discussed.Then, the experimental results relative to SRP-concrete joints with sandblasted finish were used to: a) calibrate the parameters of some bond-slip interface models available in the literature for FRP-concrete interface, and b) compare the obtained relationships with the interface models proposed in a previous study for SRP-concrete joints with bush-hammered and grinded concrete surface finish only. The bond-slip models calibrated for the three considered concrete surfaces were finally used to develop theoretical relationships between the maximum bond stress in the steel strip at debonding and the concrete strength. Comparisons with the experimental data were also performed.

Analysis of Watt’s linkage under dynamic loading

From past few years, there has been a rising trend of using composite materials for suspension system of vehicles as it reduces overall weight to a great extent. Also, they are unaffected by chemicals and corrosion. Watt’s linkage is an assembly of a bell crank and two horizontal rods of equal length. It is a mechanical linkage in which the bell crank is pivoted to axle at its centre traces vertical line, allowing axle to move vertically. The assembly is connected to the chassis of the vehicle with help of the horizontal rods, which constraints the lateral movement of body of vehicle and locates the axle under the vehicle. Low alloy steels are commonly used material for manufacturing the linkages in suspension system. In this paper, the Watt’s linkage components are designed and assembled in PTC Creo 3.0 and the finite element analysis of Watt’s linkage for two materials, namely, AISI 4142 steel and Carbon Fibre Reinforced Polymer (CFRP) is carried out. The results for total deformation and equivalent stress are produced using ANSYS Workbench 2019 R2 for dynamic loading. The maximum total deformation for 4142 steel is 0.31891 mm and for CFRP is 2.6298 mm.

A nonlinear semi-analytical model for predicting debonding of FRP laminates from RC beams subjected to uniform or concentrated load

A semi-analytical model is developed to determine in FRP retrofitted reinforced concrete (RC) beams the interfacial shear and peeling stresses, the FRP laminate and the RC section strain and stresses at all loading stages up to failure. The FRP is assumed to be externally bonded to the beam but can undergo slip and relative vertical displacement at its interface with the concrete. The model is developed by satisfying the requirements of equilibrium and strain compatibility while concurrently allowing for interfacial deformations. FRP is treated as a linear elastic, steel as elasto-plastic strain hardening and concrete as fully nonlinear material in compression and tension, including tension stiffening. The governing equations are formulated as two second order differential equations with their dependent variables being the strain in the FRP and the relative normal displacement of the interface. The equations are solved for discrete states (uncracked, cracked, yielded) experienced by the RC section and their associated level of interfacial slip. The model results are compared with available experimental results for several beams retrofitted with carbon FRP or steel reinforced polymer (SRP) laminates subjected to either four point bending or simulated uniform load, with satisfactory agreement between them.

Experimental bond behavior of Steel Reinforced Grout systems for strengthening concrete elements

Composite materials consisting of high tensile strength steel cords embedded in a cementitious matrix (Steel Reinforced Grout, SRG) are becoming an attractive solution for strengthening existing structures thanks to the lower cost than “traditional” FRP systems, the isotropy of the steel and the ability of the mortar to resist fire. With respect to Steel Reinforced polymer (SRP) systems – whose mechanical behavior is predominantly dependent on the strength of the substrate to which the steel fibers are epoxy bonded – the mechanical properties of SRG systems are mainly dependent on the cement-based matrix which is sensitive to curing the installation conditions and methods.The experimental results presented and discussed in this paper contribute to expanding the existing knowledge on the bond behavior between the SRG and concrete substrate. To this purpose, a number of SRG strips were bonded to concrete blocks by an inorganic matrix and the lap joint was subsequently subjected to direct shear tests performed in displacement control.Since mortars are very sensitive to environmental conditions and humidity as previously mentioned, two different curing conditions were used. In the first case – termed “curing a” – specimens were cured at room temperature for 28 days and wet cloths were placed on top of the composite surface each day for two weeks. In the second case – termed “curing b” – specimens were just cured at room temperature for 28 days.In addition to the curing condition, the following study parameters were considered: (a) the concrete surface roughness in the bonded region, (b) the density of the dry steel fabric, (c) the bonded interface length, and (d) the concrete strength. Relatively to the latter parameter, concrete prisms were broadly divided into two strength groups: Normal Strength Concrete (NSC) having an average concrete cylindrical strength between 13 and 25 MPa and High Strength Concrete (HSC) having an average strength equal to 40 MPa.Experimental evidence underlines that SRG systems mainly exhibited a debonding failure at steel fibers-matrix interface (due to the sliding phenomena) irrespective of the concrete strength and surface finish. Furthermore, like the SRP systems, the low tape density showed better performances with respect to the higher densities considered in this paper, both in terms of the efficiency factor (i.e., the ratio between the tensile stress experienced by SRG in each test and the average tensile ultimate strength of dry steel fabric) and of the maximum force transferred by the system.

Bond evaluation of SRP strengthening system exposed to several harsh environmental conditions

Bridge retrofitting is a billion-dollar industry. Thus, finding effective and economically feasible solutions to strengthen bridges that deteriorate to extend service life until replacement is feasible is a critical global need. There are several types of fiber reinforced polymers (FRP) based repair systems available now commercially to strengthen bridge members including systems that utilize carbon, glass, and now even steel fibers. The latter is a system most often referred to as steel reinforced polymer (SRP) and is considered fairly new to the bridge industry; yet a very promising system due to its close-to-carbon fiber strength, and close-to-glass fiber price. One current research gap for SRP has been the performance of this system relative to exposure to environmental conditioning and its overall durability resilience. In this work, the SRP strengthening system used both micro-fine galvanized and micro-fine brass coated steel fibers to externally reinforce concrete prisms using the recently published American Concrete Institute (ACI) 440.9R-15 approach to evaluate FRP bond behavior. Companion dog-bone style coupon specimens were also prepared using both SRP systems to examine tensile mechanical properties of the SRP system alone. Both types of specimens were exposed to several types of harsh environments including cycles of freeze-thaw, varying temperature, and cycles of humidity. The study investigated accelerated environmental exposure in an environmental chamber as well as direct real-time weather exposure. The environmental chamber involved subjecting the specimens to freeze-thaw cycles, cycles of varying temperature, and humidity cycles. For the real-time weather exposure, the specimens were exposed to daily weather for 12 months in Rolla, Missouri, USA where the city is moderately exposed to freeze-thaw, high-range of temperature fluctuation, and wide ranging humidity levels. The results of the flexural tests conducted indicated that the bond strength between the SRP and concrete interface was reduced by harsh environment. All conditioned specimens had a mode II or mode III bond failure, while the control specimens failed in mode I. Real-time weather specimens failed at lower deformation levels with less concrete-covered substrate than those conditioned in the environmental chamber. An ACI 440 style environmental reduction factor, CE, of 0.75 is suggested for wet-environment and 0.65 for highly aggressive environment based on the work undertaken in the overall study.

Adhesion between SRP and Masonry: Influence of Moist Condition of Specimens and Presence of Salts in the Substrate

In recent years, steel reinforced polymer (SRP) composites have emerged as a new technology for structural strengthening, and several researches have validated the effectiveness of SRP for masonry strengthening. Research has been carried out to study the bond behavior of SRP composites applied to a masonry substrate. However, how the moist and salt on masonry surface will affect bond, which is the weak link in real strengthening applications, is little known yet. This study aims at investigating the bond behavior of SRP composites applied to moist and salt-laden masonry blocks that were subjected to an artificial weathering protocol. Single-lap shear tests were conducted to determine the bond behavior, while ion chromatography provided the salts distribution of weathered specimens to interpret some results of the shear tests.

Modeling SRP-concrete interfacial bond behavior and strength

Relatively recently a steel fabric/laminate has been proposed for externally strengthening concrete structures, using a polymeric resin. The steel fiber-polymer composite system is termed Steel Reinforced Polymer (SRP). To determine the ultimate load capacity of an SRP retrofitted concrete structure, one must accurately predict the SRP-concrete interface debonding load, which requires a robust local bond-slip model. Many design guidelines recommend mode II interfacial fracture energy limit as the failure criterion for FRP-concrete interface. For SRP strengthened members, a suitable constitutive law and failure criterion have not been established yet. Consequently, in this study the applicability of five existing bond-slip interface models for FRP-concrete interface to SRP-concrete interface is examined. The models’ parameters are calibrated for SRP-concrete interface using an experimental database by the present authors and compared with the values suggested by the original authors for FRP-concrete interface. The database involves results of tests on concrete prisms bonded to SRP strips. The experimental interfacial bond-slip relationship for the former interface is observed to have a more precipitous descent after the peak stress than predicted by the existing models; consequently, a new model is proposed here to capture this phenomenon. All the models are calibrated using a classical technique which minimizes the difference between the measured and computed interfacial shear stress values at different slip levels. The results indicate that all the models predict relatively well the slope of the ascending branch of the shear stress-slip curve, but they give substantially different values for the maximum shear stress attainable and noticeably different descending branch profiles. Among these, overall, the proposed model is in relatively better agreement with the experimental results.

Structural performance of severely damaged reinforced concrete beams after SRP repair

To experimentally examine the ability of the steel reinforced polymer (SRP) in restoring the moment capacity compromised by damage in the main steel reinforcement, six full-scale reinforced concrete (RC) beams were designed to simulate impact damage from over height vehicle collision. The simulation was represented by concrete beams reinforced with discontinuous reinforcement (splice in maximum moment region) and tested until failure due to splice. The damaged concrete was repaired, and the SRP system (longitudinal soffit laminates and transverse U-wraps) was applied to restore the original moment capacity. All beams were 10 ft (3.0 m) in length, 18 in. (457 mm) in depth, and 12 in. (305 mm) in width. Different repairing configurations were investigated. The studied variables were the provided laminate area and the amount and distribution of U-wraps. The ultimate load capacity, deflection, and mode of failure were recorded during testing. The test results were compared to beam results with continuous reinforcement. It was concluded that the repairing beams with the SRP system can restore the damaged beams to a capacity similar to that of a non-damaged reinforced concrete (RC) beam with continuous reinforcement.

Flexible Adhesive in Composite-to-Brick Strengthening-Experimental and Numerical Study.

This paper investigates composite-to-brick strengthening systems with flexible adhesive made of polyurethane (Carbon Fibre Reinforced Polyurethane (CFRPU) and Steel Reinforced Polyurethane (SRPU)) and epoxy resin (Carbon Fibre Reinforced Polymer (CFRP) and Steel Reinforced Polymer (SRP). The specimens were tested in a single lap shear test (SLST). LVDT displacement transducers (LVDT – Linear Variable Differential Transformer) and digital image correlation method (DIC) based measurement systems were used to measure displacements and strains. The obtained results were applied in a numerical analysis of the 3D model of the SLST specimen, with flexible adhesives modeled as a hyper-elastic model. The DIC and LVDT based systems demonstrated a good correlation. Experimental and numerical analysis confirmed that composite-to-brick strengthening systems with flexible adhesives are more effective on brittle substrates than stiff ones, as they are able to reduce stress concentrations and more evenly distribute stress along the entire bonded length, thus having a higher load carrying capacity.