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Fiber Reinforced Polymers for Structural Strengthening

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Materials Chemistry".

Deadline for manuscript submissions: closed (31 August 2021) | Viewed by 36175

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Dr. Jose Sena-Cruz

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Department of Civil Engineering, University of Minho, Guimarães, Portual
Interests: FRP materials and structures; durability and long-term behavior; testing; finite element analysis
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The use of fiber reinforced polymers (FRP) in the construction sector mostly started in 1970–1980s, with several applications in all-composite and hybrid structures, internal reinforcement of new structures and internal/external strengthening of existing structures. Among different sectors, such aerospace, automotive, wind energy, marine and sporting goods, construction continues to be second largest market for composite materials. This achievement is intrinsically related to the research developed by different institutions and researchers worldwide. In spite of that, innovative structural solutions and unprecedented insights on the existing subjects are continually proposed for the composites construction sector. This Special Issue is mostly devoted to these subjects.

Assoc. Prof. Jose Sena-Cruz
Guest Editor

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Keywords

FRP materials and structures
Struyctural Applications
Innovative solutions
Disruptive ideas/advanced contributions on the existing knowledge
Testing
Finite Element Analysis

Published Papers (13 papers)

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Research

7 pages, 1390 KiB

Open AccessArticle

Fabricated CAD/CAM Post-Core Using Glass Fiber-Reinforced Resin Shows Innovative Potential in Restoring Pulpless Teeth

by Naoko Suzaki, Satoshi Yamaguchi, Eriko Nambu, Ryousuke Tanaka, Satoshi Imazato and Mikako Hayashi

Materials 2021, 14(20), 6199; https://doi.org/10.3390/ma14206199 - 19 Oct 2021

Cited by 9 | Viewed by 2405

Abstract

The prevention of root fractures of pulpless teeth is an important clinical issue to maintain healthy teeth through lifetime. The aim of this study was to examine a clinically effective treatment method for strengthening vulnerable pulpless teeth using CAD/CAM (computer-aided design/computer-aided manufacturing) fiber-reinforced post-core by conducting a fracture resistance test. A post-core made with a fiber-reinforced resin disk TRINIA (TR, SHOFU, Kyoto, Japan) was fabricated using a CAD/CAM system. The fiber-layer orientation of the CAD/CAM post-core was parallel to the axis of the restored tooth. A post-core using a conventional composite and a fiber post (CF) was also prepared. A fracture resistance test of teeth restored with the post-cores and zirconia crowns was conducted using a universal testing machine, and fracture patterns were identified by micro-CT observation. The fracture load of the roots restored with TR was 1555.9 ± 231.8 N, whereas that of CF was 1082.1 ± 226.7 N. The fracture load of TR was 43.8% that was significantly higher than that of CF (Student’s t-test, p < 0.05). The restored teeth with CAD/CAM resin post-core were found to be repairable even after fracture. These results suggest that the CAD/CAM indirect fiber post-core has the potential to strengthen the vulnerable pulpless teeth. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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17 pages, 6861 KiB

Open AccessArticle

Pilot Demonstration of a Strengthening Method for Steel-Bolted Connections Using Pre-Formable Carbon Fiber Cloth with VaRTM

by Takahiro Matsui, Kohei Suzuki, Sota Sato, Yuki Kubokawa, Daiki Nakamoto, Shijir Davaakhishig and Yukihiro Matsumoto

Materials 2021, 14(9), 2184; https://doi.org/10.3390/ma14092184 - 24 Apr 2021

Cited by 3 | Viewed by 2075

Abstract

In recent years, many seismic retrofitting methods have been performed to improve the structural performance and prevent the brittle failure of structural members. In the case of steel structures, slender seismic braces have been widely used for buildings, towers, and bridges. The brace connections should resist the full plastic axial tension load to ensure adequate plastic deformation performance for vibration energy absorption. However, certain connections do not satisfy these requirements. Recently, carbon fiber reinforced plastic (CFRP) has been used extensively to strengthen existing structures because of its high-strength, high elastic modulus, and light-weight characteristics. In this paper, we investigate the applicability of CFRP strengthening for brace connections and gusset plates with stepped surfaces using the vacuum-assisted resin transfer molding technique as a pilot demonstration. Stepped surfaces can be eliminated by using alternative CFRP layers to straighten the structural CFRP layers in order to effectively transfer the axial stress. Eventually, it is shown that CFRP strengthening can improve the connection strength and plastic deformation with 3% elongation, even if the CFRP is molded on the stepped surface. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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27 pages, 13133 KiB

Open AccessArticle

Influence of Temperature on the Mechanical Performance of Unidirectional Carbon Fiber Reinforced Polymer Straps

by Danijela Stankovic, Luke A. Bisby and Giovanni P. Terrasi

Materials 2021, 14(8), 1903; https://doi.org/10.3390/ma14081903 - 11 Apr 2021

Cited by 9 | Viewed by 1889

Abstract

The performance of pretensioned, laminated, unidirectional (UD), carbon fiber reinforced polymer (CFRP) straps, that can potentially be used for example as bridge deck suspender cables or prestressed shear reinforcements for reinforced concrete slabs and beams, was investigated at elevated temperatures. This paper aims to elucidate the effects of elevated temperature specifically on the tensile performance of pretensioned, pin-loaded straps. Two types of tests are presented: (1) steady state thermal and (2) transient state thermal. Eight steady-state target temperatures in the range of 24 °C to 600 °C were chosen, based on results from dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). Transient state thermal tests were performed at three sustained tensile load levels, namely 10, 15, and 20 kN, corresponding to 25%, 37%, and 50% of the ultimate tensile strength of the pin-loaded straps at ambient temperature. In general, the straps were able to retain about 50% of their ambient temperature ultimate tensile strength (UTS) at 365 °C. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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21 pages, 5063 KiB

Open AccessArticle

Failure Mechanisms of GFRP Scarf Joints under Tensile Load

by Carineh Ghafafian, Bartosz Popiela and Volker Trappe

Materials 2021, 14(7), 1806; https://doi.org/10.3390/ma14071806 - 06 Apr 2021

Cited by 6 | Viewed by 2570

Abstract

A potential repair alternative to restoring the mechanical properties of lightweight fiber-reinforced polymer (FRP) structures is to locally patch these areas with scarf joints. The effects of such repair methods on the structural integrity, however, are still largely unknown. In this paper, the mechanical property restoration, failure mechanism, and influence of fiber orientation mismatch between parent and repair materials of 1:50 scarf joints are studied on monolithic glass fiber-reinforced polymer (GFRP) specimens under tensile load. Two different parent orientations of [−45/+45]_2S and [0/90]_2S are exemplarily examined, and control specimens are taken as a baseline for the tensile strength and stiffness property recovery assessment. Using a layer-wise stress analysis with finite element simulations conducted with ANSYS Composite PrepPost to support the experimental investigation, the fiber orientation with respect to load direction is shown to affect the critical regions and thereby failure mechanism of the scarf joint specimens. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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25 pages, 5418 KiB

Open AccessArticle

Durability of Epoxy Adhesives and Carbon Fibre Reinforced Polymer Laminates Used in Strengthening Systems: Accelerated Ageing versus Natural Ageing

by Ricardo Cruz, Luís Correia, Aloys Dushimimana, Susana Cabral-Fonseca and José Sena-Cruz

Materials 2021, 14(6), 1533; https://doi.org/10.3390/ma14061533 - 21 Mar 2021

Cited by 19 | Viewed by 2880

Abstract

This work addresses the durability of structural epoxy adhesives and carbon fibre reinforced polymer (CFRP) laminates typically used in strengthening of existing reinforced concrete structures exposed to natural ageing. The experimental program included four natural (real) outdoor environments inducing ageing mainly caused by carbonation, freeze-thaw attack, elevated temperatures, and airborne chlorides from seawater. Moreover, a control (reference) environment (20 °C of temperature and 55% of relative humidity) and an environment involving water immersion of the materials under controlled temperature (20 °C of temperature) were also included in this investigation. The characterization involved the assessment of the physical, chemical and mechanical properties along a study period of up to two years. Furthermore, comparisons between the natural ageing tests developed in the scope of the present work and accelerated ageing tests existing in the literature were performed. Regarding to the epoxy adhesives, an increase in the glass transition temperature with the time was observed, while the tensile properties decreased, regardless of the outdoor environment. The CFRP laminates were marginally affected by the studied environments. Despite the remarkable dispersion of the results observed in the accelerated ageing tests for the period investigated, this testing protocol yielded higher mechanical degradation than under natural ageing. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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17 pages, 2384 KiB

Open AccessArticle

New Model for Analytical Predictions on the Bending Capacity of Concrete Elements Reinforced with FRP Bars

by Kostiantyn Protchenko, Przemysław Leśniak, Elżbieta Szmigiera and Marek Urbański

Materials 2021, 14(3), 693; https://doi.org/10.3390/ma14030693 - 02 Feb 2021

Cited by 7 | Viewed by 2518

Abstract

Many studies on Fibre-Reinforced Polymers Reinforced Concrete (FRP-RC) beams tested in flexure have been performed by various researchers around the world. This work presents the results of statistical and mathematical analyses based on experimental data; 102 samples were collected and supplemented from 16 different scientific papers. The load capacity of the beams determined on the basis of the tests was compared with the load capacity calculated on the basis of the recommendations of ACI 440.1R-15. The results obtained from experimental studies showed that for 91.4% of the samples, the underestimation of the load capacity on average was equal to 15.2% of theoretical, and for 33.3% of the beams, the load capacity was overestimated by 26.7%. The paper proposes a new empirical coefficient incorporating material parameters to be implemented into ACI 440.1R-15 flexural design approach in order to improve the accuracy of this model in scope of the nominal flexural strength capacity of FRP-reinforced beams estimation. Modifications to flexural design of FRP-RC beams with the use of ACI 440.1R-15 design code were proposed. As a result, the reliability of the analytical model is increased; therefore, the new model guarantees higher safety and cost efficiency of designed concrete structures reinforced with FRP bars. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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19 pages, 6761 KiB

Open AccessArticle

Evaluation of FRP Bars under Compression and Their Performance in RC Columns

by Laith AlNajmi and Farid Abed

Materials 2020, 13(20), 4541; https://doi.org/10.3390/ma13204541 - 13 Oct 2020

Cited by 40 | Viewed by 2972

Abstract

The behavior of fiber-reinforced polymer (FRP) bars under compression is not fully understood yet due to the limited research in this area. However, the long-term durability, weathering resistance, and exceptional mechanical properties of FRP bars justify the need for their use in compression members. The main objectives of this study are to evaluate the mechanical properties of glass FRP (GFRP) and basalt FRP (BFRP) bars under compression and examine their performances as main longitudinal reinforcements in reinforced concrete (RC) columns. In the first part of this research, a series of static compression tests were conducted on GFRP and BFRP bars of different diameters. The second part of this research numerically investigated the behavior of FRP-RC columns under concentric and eccentric loading using the mechanical properties of the FRP bars obtained experimentally. Nonlinear finite element models were developed to simulate the compressive behavior of the concrete columns reinforced with GFRP and BFRP bars. The FE models were verified with the experimental results conducted previously. The verified FE models are then utilized to conduct a parametric analysis considering two different column geometries and cross-sections, five reinforcement ratios, two concrete compressive strengths, three types of ties materials, and several loading eccentricities to develop a set of interaction diagrams that may provide valuable data for design purposes. The results indicated that the FRP bars could have a significant contribution to the overall capacity of FRP-RC columns by up to 35% of the total force at failure, depending on the reinforcement ratio. The performance of both the GFRP- and BFRP-RC columns was almost similar in terms of capacity, deflection, and bar strength contribution. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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34 pages, 14168 KiB

Open AccessArticle

Experimental Study on Concrete under Combined FRP–Steel Confinement

by Stefan Kaeseberg, Dennis Messerer and Klaus Holschemacher

Materials 2020, 13(20), 4467; https://doi.org/10.3390/ma13204467 - 09 Oct 2020

Cited by 18 | Viewed by 2059

Abstract

The confinement of reinforced concrete (RC) compression members by fiber-reinforced polymers (FRPs) is an effective measure for the strengthening and retrofitting of existing structures. Thus far, extensive research on the stress–strain behavior and ultimate limit state design of FRP-confined concrete has been conducted, leading to various design models. However, these models are significantly different when compared to one another. In particular, the use of certain empirical efficiency and reduction factors results in various predictions of load-bearing behavior. Furthermore, most experimental programs solely focus on plain concrete specimens or demonstrate insufficient variation in the material properties. Therefore, this paper presents a comprehensive experimental study on plain and reinforced FRP-confined concrete, limited to circular cross sections. The program included 63 carbon FRP (CFRP)-confined plain and 60 CFRP-confined RC specimens with a variation in the geometries and in the applied materials. The analysis showed a significant influence of the compressive strength of the confined concrete on the confinement efficiency in the design methodology, as well as the importance of the proper determination of individual reduction values for different FRP composites. Finally, applicable experimental test results from the literature were included, enabling the development of a modified stress–strain and ultimate condition design model. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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17 pages, 4985 KiB

Open AccessArticle

Compressive Strength of Modified FRP Hybrid Bars

by Marek Urbański

Materials 2020, 13(8), 1898; https://doi.org/10.3390/ma13081898 - 17 Apr 2020

Cited by 11 | Viewed by 2971

Abstract

A new type of HFRP hybrid bars (hybrid fiber reinforced polymer) was introduced to increase the rigidity of FRP reinforcement, which was a basic drawback of the FRP bars used so far. Compared to the BFRP (basalt fiber reinforced polymer) bars, modification has been introduced in HFRP bars consisting of swapping basalt fibers with carbon fibers. One of the most important mechanical properties of FRP bars is compressive strength, which determines the scope of reinforcement in compressed reinforced concrete elements (e.g., column). The compression properties of FRP bars are currently ignored in the standards (ACI, CSA). The article presents compression properties for HFRP bars based on the developed compression test method. Thirty HFRP bars were tested for comparison with previously tested BFRP bars. All bars had a nominal diameter of 8 mm and their nonanchored (free) length varied from 50 to 220 mm. Test results showed that the ultimate compressive strength of nonbuckled HFRP bars as a result of axial compression is about 46% of the ultimate strength. In addition, the modulus of elasticity under compression does not change significantly compared to the modulus of elasticity under tension. A linear correlation of buckling load strength was proposed depending on the free length of HFRP bars. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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15 pages, 10241 KiB

Open AccessArticle

Post-Fire Characteristics of Concrete Beams Reinforced with Hybrid FRP Bars

by Kostiantyn Protchenko and Elżbieta Szmigiera

Materials 2020, 13(5), 1248; https://doi.org/10.3390/ma13051248 - 10 Mar 2020

Cited by 19 | Viewed by 3051

Abstract

One of the main concerns of experimental and numerical investigations regarding the behavior of fiber-reinforced polymer reinforced concrete (FRP-RC) members is their fire resistance to elevated temperatures and structural performance at and after fire exposure. However, the data currently available on the behavior of fiber-reinforced polymer (FRP) reinforced members related to elevated temperatures are scarce, specifically relating to the strength capacity of beams after being subjected to elevated temperatures. This paper investigates the residual strength capacity of beams strengthened internally with various (FRP) reinforcement types after being subjected to high temperatures, reflecting the conditions of a fire. The testing was made for concrete beams reinforced with three different types of FRP bars: (i) basalt-FRP (BFRP), (ii) hybrid FRP with carbon and basalt fibers (HFRP) and (iii) nano-hybrid FRP (nHFRP), with modification of the epoxy matrix of the rebar. Tested beams were first loaded at 50% of their ultimate strength capacity, then unloaded before being heated in a furnace and allowed to cool, and finally reloaded flexurally until failure. The results show an atypical behavior observed for HFRP bars and nHFRP bars reinforced beams, where after a certain temperature threshold the deflection began to decrease. The authors suggest that this phenomenon is connected with the thermal expansion coefficient of the carbon fibers present in HFRP and nHFRP bars and therefore creep can appear in those fibers, which causes an effect of “prestressing” of the beams. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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43 pages, 24031 KiB

Open AccessArticle

Prediction of Deflection of Reinforced Concrete Beams Strengthened with Fiber Reinforced Polymer

by Mykolas Daugevičius, Juozas Valivonis and Tomas Skuturna

Materials 2019, 12(9), 1367; https://doi.org/10.3390/ma12091367 - 26 Apr 2019

Cited by 8 | Viewed by 3387

Abstract

The article analyses the calculation of the deflection of reinforced concrete beams strengthened with fiber reinforced polymer. This paper specifically focuses on estimating deflection when the yielding of reinforcement is reached. The article proposes a simple method for calculating deflection that was compared with the experimentally predicted deflection. The carried out comparison has showed that the proposed method is suitable not only for the strengthened beams but also for the reinforced concrete beams with a varying reinforcement ratio. The suggested calculation method is based on the effective moment of inertia, such as the one introduced in the ACI Committee 318 Building Code Requirement for Structural Concrete (ACI318). The development of deflection was divided into three stages, and equations for the effective moment of inertia were proposed considering separate stages. In addition, the put forward equations were modified attaching additional relative coefficients evaluating a change in the depth of the neutral axis. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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26 pages, 12606 KiB

Open AccessFeature PaperArticle

Unified Theory for Flexural Strengthening of Masonry with Composites

by Giancarlo Ramaglia, Francesco Fabbrocino, Gian Piero Lignola and Andrea Prota

Materials 2019, 12(4), 680; https://doi.org/10.3390/ma12040680 - 25 Feb 2019

Cited by 14 | Viewed by 3138

Abstract

Recent calamitous events have shown the fragility of the existing masonry buildings. Many of them are heritage structures, such as churches and monumental buildings. Therefore, optimized strengthening strategies are necessary. Experimental studies performed on masonry elements strengthened with composite systems have shown the performance of these materials. However, further development is necessary to optimize the intervention strategies. In fact, due to the lack of general validity models, the design is usually based on prescriptive approaches according to manufacturers’ broad instructions, often producing systems with low efficiency and overestimations of the amount of reinforcement. In this paper a generalized approach is proposed to assess the flexural behavior of masonry sections strengthened with composites. The proposed theory has allowed performance of a sensitivity analysis assessing the impact both of the mechanical parameters of masonry and of the strengthening system. In particular, the impact of several constitutive relationships of composites (linear, bilinear, or trilinear) have been evaluated in terms of ultimate behavior of the strengthened masonry. For strengthening systems more compatible with the masonry substrate, the form of the stress–strain relationship becomes a key aspect. For such cases, the modeling of the reinforcement plays a fundamental role and the form of the relationship is strongly correlated to the type of reinforcement selected, e.g., organic versus inorganic matrix. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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20 pages, 4815 KiB

Open AccessFeature PaperArticle

Influence of Surface Preparation Method on the Bond Behavior of Externally Bonded CFRP Reinforcements in Concrete

by Sérgio Soares, José Sena-Cruz, José Ricardo Cruz and Pedro Fernandes

Materials 2019, 12(3), 414; https://doi.org/10.3390/ma12030414 - 29 Jan 2019

Cited by 17 | Viewed by 3118

Abstract

In last decades significant investigation has been carried out in order to predict the bond strength of externally bonded reinforcement (EBR) systems with carbon fiber reinforced polymer (CFRP) materials in concrete and, as consequence of that, many analytical expressions can be found in the literature, including in standards. However, these expressions do not account for the influence of several parameters on bond behavior such as the type of surface preparation which is a mandatory and critical task in the strengthening application. The present work gives contributions to reduce this lack of knowledge. For this purpose, an experimental program composed of single-lap shear tests was carried out, the main parameters studied being: (i) the type of concrete surface preparation (i.e., grinding and sandblasting) and (ii) the bond length. Prior to the application of the EBR CFRP system, the roughness level provided by the different methods of surface preparation was characterized by a laser sensor. Test results revealed that sandblasting concrete surface preparation yielded higher values, in terms of maximum shear force and fracture energy. Finally, existing expressions in standards were upgraded in order to account for the concrete surface roughness level in the estimation of the bond strength. Full article

(This article belongs to the Special Issue Fiber Reinforced Polymers for Structural Strengthening)

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