Fiber Reinforced Polymer Materials Research Articles

Quantitative evaluation of compactness of concrete-filled fiber-reinforced polymer tubes using piezoceramic transducers and time difference of arrival

Owing to its light weight and corrosion resistance, the concrete-filled fiber-reinforced polymer tube (CFFT) structure has a broad application prospect; the concrete compactness is key to the strength of CFFTs. To meet the urgent requirement of compactness monitoring of CFFTs, a quantitative method, which uses an array of four equally spaced piezoceramic patches and an ultrasonic time difference of arrival (TDOA) algorithm, is developed. Since the velocity of the ultrasonic wave propagation in fiber-reinforced polymer (FRP) material is about half of that in concrete material, the compactness condition of CFFT impacts the piezoceramic-induced wave propagation in the CFFT, and differentiates the TDOA for different receivers. An important condition is the half compactness, which can be judged by the Half Compactness Indicator (HCI) based on the TDOAs. To characterize the difference of stress wave propagation durations from the emitter to different receivers, which can be utilized to calculate the concrete infill compactness, the TDOA ratio (TDOAR) is introduced. An innovative algorithm is developed in this paper to estimate the compactness of the CFFT using HCI and TDOAR values. Analytical, numerical, and experimental studies based on a CFFT with seven different states of compactness (empty, 1/10, 1/3, 1/2, 2/3, 9/10, and full) are carried out in this research. Analyses demonstrate that there is a good agreement among the analytical, numerical, and experimental results of the proposed method, which employs a piezoceramic transducer array and the TDOAR for quantitative estimating the compactness of concrete infill in a CFFT.

Design provisions for FRCM systems bonded to concrete and masonry elements

Abstract Fibre Reinforced Cementitious Matrix (FRCM) materials are becoming an alternative strengthening technique to the well-known Fibre Reinforced Polymer (FRP) materials applied by means of epoxy resin, especially for masonry elements. Experimental tests have shown that the performance of elements strengthened with FRCM systems depends on the bond behaviour at both the reinforcement/substrate and grout/grid interface. Because of the novelty of the FRCM technique and the wide spectrum of FRCM materials (both grouts and grids) present on the market, no design indications are now available for such a strengthening technique, for both concrete and masonry elements. In this paper, a critical analysis of the available literature on bond tests on masonry and concrete elements externally bonded with FRCM materials was done, firstly. Successively, the collected results were examined in terms of failure mode and maximum strain in order to identify the main geometric and mechanical properties of the reinforcement and the substrate influencing the performance of the strengthening technique. Regression analyses were, thus, carried out on the collected experimental results, separately for concrete and masonry elements, in order to asses reliable formulations predicting the maximum strain in function of the axial stiffness of the reinforcement, the mechanical properties of the substrate, and the experimentally observed failure modes. Basing on the average provisions of the maximum strain in the FRCM reinforcement, characteristic values were also assessed according to a ‘design by testing’ procedure.

Developing polymer composite-based leaf spring systems for automotive industry

Abstract Composite-based mono-leaf spring systems were designed and manufactured to replace existing mono-leaf metal leaf spring in a light commercial vehicle. In this study, experimentally obtained mechanical properties of different fiber-reinforced polymer materials are presented first, followed by the description of the finite element analytical model created in Abaqus 6.12-1 (Dassault Systemes Simulia Corp., RI, US) using the obtained properties. The results from the finite element analysis are presented next and compared with actual size experimental tests conducted on manufactured prototypes. The results demonstrated that the reinforcement type and orientation dramatically influenced the spring rate. The prototypes showed significant weight reduction of about 80% with improved mechanical properties. The hybrid composite systems can be utilized for composite-based leaf springs with considerable mechanical performance.

The effects of composite laminate stiffness and loading on stress resultant concentration factor around a hole

An analytical tool is developed using conformal mapping and the complex variable method for determining the stress resultant concentration factor in an infinite plate with a circular or elliptical hole. This tool is validated against finite element analysis. A population space of 305 laminates is generated by stacking unidirectional carbon fibre reinforced polymer material at various orientations covering a wide spectrum of homogenised stiffness values. The analytical approach is then used to study the effect(s) of loading conditions, stacking sequences and laminate homogenised stiffness values on the stress resultant concentration factor at the edge of a circular hole.

Performance of FRP confined and unconfined geopolymer concrete exposed to sulfate attacks

In this study, the effects of magnesium sulfate on the mechanical performance and the durability of confined and unconfined geopolymer concrete (GPC) specimens were investigated. The carbon and basalt fiber reinforced polymer (FRP) fabrics with 1-layer and 3-layers were used to evaluate the performances of the specimens under static and cyclic loading in the ambient and magnesium sulfate environments. In addition, the use of FRP materials as a rehabilitation technique was also studied. For the geopolymerization process of GPC specimens, the alkaline activator has selected a mixture of sodium silicate solution (Na2SiO3) and sodium hydroxide solution (NaOH) with a ratio (Na2SiO3/NaOH) of 2.5. In addition to GPC specimens, an ordinary concrete (NC) specimens were also produced as a reference specimens and some of the GPC and NC specimens were immersed in 5% magnesium sulfate solutions. The mechanical performance and the durability of the specimens were evaluated by visual appearance, weight change, static and cyclic loading, and failure modes of the specimens under magnesium sulfate and ambient environments. In addition, the microscopic changes of the specimens due to sulfate attack were also assessed by scanning electron microscopy (SEM) to understand the macroscale behavior of the specimens. Results indicated that geopolymer specimens produced with nano-silica and fly ash showed superior performance than the NC specimens in the sulfate environment. In addition, confined specimens with FRP fabrics significantly improved the compressive strength, ductility and durability resistance of the specimens and the improvement was found higher with the increased number of FRP layers. Specimens wrapped with carbon FRP fabrics showed better mechanical performance and durability properties than the specimens wrapped with basalt FRP fabrics. Both FRP materials can be used as a rehabilitation material in the sulfate environment.

Experimental Investigation of Punching Shear on FRP Strengthened Slab

Punching in slabs is generally seen by the application of concentrated loads or to the presence of columns. One of the major phenomenon related to flat slabs is its punching shear capacity at slab column connection. Provided that bending capacity is installed, punching shear failure is due to the development of a truncated cone-shaped surface at the slab-column connection. Frequently, there is the need to strengthen existing flat slabs against punching shear failure. However many research works have been done on punching shear. Moreover, we want to know the behavior of RC flat slab under FRP material against punching shear. The effect of FRP bars against punching shear is checked.The objective of the current study was to explain the feasibility of RC flat slab to examine the application of steel rods, FRP rebar on the improving of punching shear. Extensive applications of the fiber-reinforced polymer (FRP) as new construction materials have been recently accomplished. FRP materials are lightweight, high strength, and no-corrosive materials. By virtue of these advantages, there is a wide range of recent, current, and potential applications of these materials that cover both new and existing structures. Among different types of FRP materials, a fiber-reinforced polymer (FRP) is used extensively in the structural engineering field. This study was carried out to examine the viability of using FRP bars for the punching shear strengthening of the slab.

Development of Innovative HFRP Bars

The main factors determining the choice of fiber-reinforced polymer (FRP) materials are the intended use of the designed structure and the environmental conditions in which it will be located. Currently, the FRP-based materials have a variety of applications in the construction industry, from the secondary structural elements of buildings, to a complicated designs, where the only FRPs were used. The advances in FRP technology have spurred interest in introducing innovative hybrid fiber-reinforced polymer (HFRP), which potentially can be used as reinforcing/enhancing material. This paper describes the investigation on newly-developed hybrid fiber-reinforced polymer HFRP bars, which were created by modification of basalt fiber-reinforced polymer BFRP bars in terms of physical substituting of the certain amount of basalt fibers by the part of carbon fibers. Modification is aimed at achieving of better properties in obtained material and simultaneously ensuring cost-effectiveness concept. The investigation includes the preparation and numerical considerations on HFRP bars as well as first attempts of experimental structural testing of innovative HFRP bars.

Behaviour of strengthened timber beams using near surface mounted Basalt Fibre Reinforced Polymer (BFRP) rebars

Reinforcement of structural timber members with fibre reinforced polymer (FRP) rods offers merits over that of the conventional steel type. In recent times, near surface mounted (NSM) FRP reinforcement with timber has emerged as a promising alternative for reinforcing timber structures in both flexural and shear loading configurations. Previous investigations have shown that NSM FRP reinforcement technique has higher bond performance than externally bonded equivalents because it (NSM FRP technique) is able to utilise the full capacity of the FRP materials. In spite of these merits, the investigations and the use of this innovative technique are limited. In this paper, an experiment was conducted to investigate the bond characteristics and performance of NSM basalt FRP reinforcement with solid timber structures. In order to predict the performance of the reinforced beam structures, unreinforced control timber members of the same timber characteristics were tested. The results showed that the average bond capacity of the NSM FRP reinforced members was 16% higher than the corresponding unreinforced beams.

Synergistic effect on the degradation rate of pultruded glass fiber-reinforced polymer bridge panel after 20 years of exploitation

Glass fiber-reinforced polymer pultruded profiles have great potential in the construction industry, presenting certain advantages when compared with traditional materials, including the potentially improved durability under fluctuating levels of environmental factors. The paper presents analysis of glass fiber-reinforced polymer composite, acquired from cable-stayed Fiberline Bridge exploited for 20 years in the fjord area of Kolding, Denmark. Fragment of composite material used for Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis tests was therefore subjected to natural aging as a result of temperature amplitudes, permanent solar radiation as well as aggressive impact of sea salt contained in the moisture in the air around the coastal area. Complex comparative analysis presented in this paper, and based on Fourier transform infrared, thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis tests, pertained to both unspool composite glass fiber-reinforced polymer material (composite 1) and the one after 20 years of natural aging (composite 2). Dynamic mechanical analysis was allowed to detect thermal effects based on the changes in the modulus or damping behavior. The differential scanning calorimetry experiments were performed on the glass fiber-reinforced polymer material in order to determine the mass variation and the energy changes suffered by the materials, as a function of temperature and time.

Thermal cycling of composite laminates made of out-of-autoclave materials

Abstract Carbon fiber-reinforced polymer material has been widely used in space/aerospace industries for manufacturing of spacecraft structures, satellite panels and antennas. In space, composites can be subjected to periodic thermal cycling (TC) in which temperature ranges from −196°C to 180°C, depending on the operational condition. The effect of TC on the properties of flat laminates made of unidirectional (UD) and fabric out-of-autoclave (OOA) material will be presented. Flat laminates were made using the recommended cure cycle by the material supplier. Then, the samples were cut and subjected to the thermal cycle. To do so, the samples were dipped into liquid nitrogen (−196°C) and then transferred to the oven (140°C). After different numbers of cycles (30, 60, 100, 150 and 200), the cross section of the specimens was examined under a microscope for microcrack detection. Other mechanical and physical properties including interlaminar shear strength, storage modulus and coefficient of thermal expansion (CTE) were measured. It was identified that TC can affect the examined properties by two competing factors: (i) more cross-linking in the polymer chain due to post-curing at high temperature during TC and (ii) microcrack formation due to the induced thermal stresses as a result of the matrix/fiber CTE mismatch. It was found that TC can cause microcrack formation and propagation around the voids in the laminate and affect its properties. Depending on the size and shape of the void, microcracks can form at different stages of TC.

Fatigue Damage and Degradation Model for Carbon Fibre Reinforced Polymer Materials

AbstractA major issue concerning Carbon Fibre Reinforced Polymer (CFRP) materials under cyclic loading is that the materials often undergo material degradation starting from the initial stage of the loading. For a laminate that consists of different fibre orientation plies, this becomes crucial as it causes redistribution of stresses and strains during the lifetime of the component. Understanding the importance of a reliable fatigue damage and degradation model in the development of reliability assessment of CFRP materials, the present studies contribute to this by considering the effect on a single‐ply material level, exclusively under fatigue loading. The model is restricted to be applied to stiff and brittle materials with characteristic properties similar to those of carbon‐epoxy composites. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of curing conditions on mode-II debonding between FRP and concrete: A prediction model

The rehabilitation and strengthening of concrete structures using Fiber-Reinforced Polymer (FRP) materials have been widely investigated. As a priority issue, however, the effect of curing conditions on the bonding behavior between FRP and concrete structures is still elusive. This study aims at developing a prediction model to accurately capture the mode-II interfacial debonding between FRP strips and concrete under different curing conditions. Single shear debonding experiments were conducted on FRP-concrete samples with respect to different curing time t and temperatures T. The J-integral formulation and constrained least square minimization are carried out to calibrate the parameters, i.e., the maximum slip s- and stretch factor n. The prediction model is developed based on the cohesive model and Arrhenius relationship. The experimental data are then analyzed using the proposed model to predict the debonding between FRP and concrete, i.e., the interfacial shear stress-slip relationship. A Finite Element (FE) model is developed to validate the theoretical predictions. Satisfactory agreements are obtained. The prediction model can be used to accurately capture the bonding performance of FRP-concrete structures.

Fiber Reinforced Polymer as External Reinforcement for Single Cast-in Anchors in Plastic Hinge Zones

This paper presents a test of cast-in anchors in the plastic hinge zone of a reinforced concrete column. Design codes, such as ACI 318-11, do not allow concrete anchors in concrete that could be substantially damaged during an earthquake unless special reinforcement is provided. Steel reinforcement has been proven effective in protecting core concrete; however, it does not protect cover concrete, which is critical to the shear behavior of anchors. Fiber reinforced polymer (FRP) composite material was used in this study to protect cover concrete around the test anchors. The column specimens were subjected to quasi-static cyclic loading while the single anchors were loaded in cyclic shear. Compared with the anchors in unprotected concrete, the FRP reinforced anchors developed a higher shear capacity at a much smaller displacement. This indicates that FRP fabrics can be effective as external reinforcement for concrete anchors. Meanwhile, a systematic study is needed before practical applications.

Shear strengthening of corroded reinforced concrete columns using pet fiber based composties

Polyethylene terephthalate (PET) fiber-reinforced polymer (FRP) composites are characterized by a large rupture strain (LRS) (usually larger than 5%) but a low elastic modulus compared to conventional FRP materials made of carbon, glass, and aramid fibers. Previous studies have proved that the use of PET FRP sheets as jacketing materials for plain concrete or reinforced concrete (RC) columns could efficiently enhance their ductility. This paper presents a combined experimental and analytical study on the shear strengthening of corroded RC columns with stirrups having different levels of corrosion before strengthening. The shear resistance contributions from the substrate column and PET FRP sheets at various volume ratios were carefully examined and their interaction mechanisms were discussed. Design equations following a conventional truss analogy were then proposed to predict the shear strength of PET FRP-wrapped columns considering the effect of stirrup corrosion, and the predictions were verified against the test results.

Influence of mortar joints on the behaviour of FRP materials bonded to different masonry substrates

The use of epoxy bonded fibre reinforced polymer (FRP) materials as strengthening technique for existing masonry structures is becoming in the last years more and more widespread, but some topics, as the effect of the mortar joints on the bond performance at the reinforcement/masonry substrate interface, still represents an open question.This paper is focused on studying in detail the effect of the mortar joints on the bond performance of FRP materials made of different type of fibres (carbon, glass, basalt, steel) externally bonded by means of epoxy resin to three different masonry substrates, very common in Italy: tuff stones, Lecce stones, and clay bricks. Firstly, the experimental results of bond tests carried out by three laboratories on three different masonry substrates were discussed in detail in order to highlight the effect of the mortar joints as the masonry typology changes. Successively, statistical analysis were performed on a more extended database of bond tests coming from literature and carried out on similar masonry substrates. The analysis were aimed to define design formulations for calculating the debonding load of FRP materials bonded to different masonry substrates with and without mortar joints.

Experimental and numerical analysis of step drill bit performance when drilling woven CFRPs

This paper focuses on the influence of the step drill bit geometry on the damage induced during drilling Carbon Fiber Reinforced Polymer materials (CFRPs). Step geometry designed with the aim of avoiding composite damage in CFRPs drilling, is compared to conventional twist configuration. Despite the reduction of thrust force and torque observed when using the step drill, the delamination was only reduced at low feed rates. A numerical model developed for the step geometry was validated with experimental data demonstrating its ability to predict thrust force and delamination for different values of feed rate and cutting speed. Numerical model allowed the development of a parametrical study. Finally, using a response surface methodology a mechanistic model and surface diagrams have been presented in order to help in the selection of optimum variables minimizing drilling induced damage.

Plain and threaded bearing strengths for the design of bolted connections with pultruded FRP material

Presented are results from testing 28 batches of 5 or 10 nominally identical specimens to characterise the laterally unrestrained pin-bearing strength when bolting is with and without thread. For the test series flange material is taken from a 254x254x9.53 mm Pultex® SuperStructural 1525 series shape. Strengths are measured with the Fibre Reinforced Polymer (FRP) material oriented at either 0° or 90° to the direction of pultrusion. Four steel bolt sizes of M10, M12, M16 and M20 are used, and when threaded there are different standard teeth (pitch) geometries. To remove this variable in a comparison with plain pin strengths a unique test series of 12 batches was carried out with three non-standard thread profiles. The effect on pin-bearing strength of having a threaded bolt is evaluated using mean and characteristic strengths, the latter determined in accordance with EN 1990. A key finding is that the proposed reduction factor of 0.6 in a forthcoming American LRFD standard to calculate a thread characteristic strength from the plain value is acceptable. Other findings are important to the determination of pin-bearing strength, and to us having knowledge and understanding to prepare a universal design procedure for resistances in bolted connections when the mode of failure is bearing.

Carbon fiber–reinforced polymer rod panels for strengthening concrete bridges

The practice of using fiber-reinforced polymer laminates and fabric to repair and strengthen concrete structures is well established. What limits the application of fiber-reinforced polymer materials, especially in flexural strengthening, is the equipment and man power needed for continuous application when retrofits take place over waterways or multilane roadways. A carbon fiber–reinforced polymer rod panel system consisting of 1220-mm (48-in) panels made continuous through a finger joint/splice was developed to overcome these limitations. The system’s performance hinges on whether forces can be transferred from one panel to another. This study investigated the bond characteristics of carbon fiber–reinforced polymer rods, as well as the flexural behavior of concrete members strengthened with carbon fiber–reinforced polymer rod panels, to improve knowledge of the finger joint’s behavior. Bond tests were conducted using double-lap shear specimens on individual rods with both steel and concrete substrates. Further bond tests were performed on small carbon fiber–reinforced polymer rod panels on steel substrate. Flexural tests were carried out under four-point bending on small-scale reinforced concrete beams that were strengthened using continuous carbon fiber–reinforced polymer rod panels and carbon fiber–reinforced polymer rod panels spliced using a finger joint. Case studies of four field applications are presented to provide a better understanding of the system. The new carbon fiber–reinforced polymer rod panels effectively reduce labor and equipment costs for work conducted on bridges with limited access, as they enabled the performance of repair/retrofit operations by a small crew working out of a single work platform.

Numerical Study on Deflection Behaviour of Concrete Beams Reinforced with GFRP Bars

Fiber-Reinforced Polymer (FRP) bars are gaining popularity as sustainable alternatives to conventional reinforcing steel bars in reinforced concrete applications. The production of FRP bars has lower environmental impact compared to steel reinforcing bars. In addition, the non-corroding FRP materials can potentially decrease the cost or need for maintenance of reinforced concrete structural elements, especially in harsh environmental conditions that can impact both concrete and reinforcement. FRP bars offer additional favourable properties including high tensile strength and low unit weight. However, the mechanical properties of FRP bars can lead to large crack widths and deflections. The objective of this study is to investigate the deflection behaviour of concrete beams reinforced with Glass FRP (GFRP) bars as a longitudinal main reinforcement. Six concrete beams reinforced with GFRP bars were modelled using the finite element computer program ANSYS. The main variable considered in the study is the reinforcement ratio. The deflection equations in current North American codes including ACI 440.1R-06, ACI 440.1R-15 and CSA S806-12 are used to compute deflections, and these are compared to numerical results. It was concluded in this paper that deflections predicted by ACI 440.1R-06 equations are lower than the numerical analysis results while ACI 440.1R-15 is in agreement with numerical analysis with tendency to be conservative. The values of deflections estimated by CSA S806-12 formulas are consistent with results of numerical analysis.

Vibration Analysis of a Composite Concrete/GFRP Slab Induced by Human Activities

Fiber-reinforced polymer (FRP) materials have been introduced recently in the construction of new structural systems, particularly in footbridge systems. Innovative systems that combine concrete with FRP materials lead to lighter and more slender structures as compared to conventional reinforced concrete structures, which can bring about vibration problems. In this work, a vibration analysis of a composite slab subjected to human activities is performed, both experimentally and numerically. The slab is composed of a concrete top laid on glass fiber-reinforced polymer (GFRP) I-section pultruded profiles. In the experimental analysis, two prototypes of 0.80 m width and 4.00 m span, representing a slab strip, were subjected to walking and jumping by several volunteers. In the numerical analysis, the slab was modeled by finite elements under dynamic loadings that simulate walking and jumping. Both the experimental and numerical results have indicated that the dynamic behavior under human activities of the composite slab must be considered in the design.