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A special issue of Polymers (ISSN 2073-4360).

Deadline for manuscript submissions: closed (15 December 2017) | Viewed by 67853

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Prof. Dr. Alper Ilki

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Structural and Earthquake Engineering Laboratory, Civil Engineering Faculty, Istanbul Technical University, 34469 Maslak Istanbul, Turkey
Interests: structural engineering; reinforced concrete structures; masonry structures; seismic retrofit with advanced materials; seismic performance assessment
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Prof. Dr. Masoud Motavalli

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Structural Engineering Laboratory, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
Interests: application of advanced materials (such as fiber-reinforced polymer composites and shape memory alloys in civil engineering); structural rehabilitation and repair; seismic retrofitting; large and full scale laboratory and field experiments
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Special Issue Information

Dear Colleagues,

We are happy to announce the third collaboration between the Smart Monitoring, Assessment and Rehabilitation of Structures (SMAR) conference and Polymers. This Special Issue is for extended versions of the best papers from the SMAR 2017 conference (http://www.smar2017.org/) in the field of rehabilitation of civil structures using fiber-reinforced polymer (FRP) composites. The topics include, but are not limited to:

External strengthening of concrete, timber, and steel structures using FRP composites
Strengthening of masonry and historic structures using FRP composites
Confinement of concrete columns using FRP composites
Near surface mounting reinforcement using FRP composites
Seismic Retrofitting using FRP composites
Durability issues of FRP strengthened structures as related to harsh environments
Fire protection systems for FRP strengthened structures
Practical applications and case studies.

Prof. Dr. Alper Ilki
Prof. Dr. Masoud Motavalli
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Polymers is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

FRP composites
external strengthening
structural rehabilitation
durability
fire protection systems
NSMR using FRP
seismic retrofitting

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Published Papers (11 papers)

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Research

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17 pages, 3207 KiB

Open AccessArticle

Behaviour of Prestressed CFRP Anchorages during and after Freeze-Thaw Cycle Exposure

by Yunus Emre Harmanci, Julien Michels and Eleni Chatzi

Polymers 2018, 10(6), 565; https://doi.org/10.3390/polym10060565 - 23 May 2018

Cited by 16 | Viewed by 3738

Abstract

The long-term performance of externally-bonded reinforcements (EBR) on reinforced concrete (RC) structures highly depends on the behavior of constituent materials and their interfaces to various environmental loads, such as temperature and humidity exposure. Although significant efforts have been devoted to understanding the effect of such conditions on the anchorage resistance of unstressed EBR, with or without sustained loading, the effect of a released prestressing has not been thoroughly investigated. For this purpose, a series of experiments has been carried out herein, with concrete blocks strengthened with carbon fiber-reinforced polymer (CFRP) strips, both unstressed, as well as prestressed using the gradient anchorage. The gradient anchorage is a non-mechanical technique to anchor prestressed CFRP by exploiting the accelerated curing property of epoxy under higher temperatures and segment-wise prestress-force releasing. Subsequently, strengthened blocks are transferred into a chamber for exposure in dry freeze-thaw cycles (FTC). Following FTC exposure, the blocks are tested in a conventional lap-shear test setup to determine their residual anchorage resistance and then compared with reference specimens. Blocks were monitored during FTC by conventional and Fabry–Pérot-based fiber optic strain (FOS) sensors and a 3D-digital image correlation (3D-DIC) system during gradient application and lap-shear testing. Results indicate a reduction of residual anchorage resistance, stiffness and deformation capacity of the system after FTC and a change in the failure mode from concrete substrate to epoxy-concrete interface failure. It was further observed that all of these properties experienced a more significant reduction for prestressed specimens. These findings are presented with a complementary finite element model to shed more light onto the durability of such systems. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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20 pages, 35123 KiB

Open AccessArticle

Comparative Analysis of Existing RC Columns Jacketed with CFRP or FRCC

by Marta Del Zoppo, Marco Di Ludovico, Alberto Balsamo and Andrea Prota

Polymers 2018, 10(4), 361; https://doi.org/10.3390/polym10040361 - 24 Mar 2018

Cited by 45 | Viewed by 5815

Abstract

Reinforced concrete (RC) columns typical of existing structures often exhibit premature failures during seismic events (i.e., longitudinal bars buckling and shear interaction mechanisms) due to the poor quality concrete and the absence of proper seismic details in the potential plastic hinge region. The Fiber Reinforced Polymers (FRP) externally bonded reinforcement is known to be a valid technique to improve the shear capacity or the ductility of existing RC columns. However, few experimental tests have proven its effectiveness in the case of columns affected by shear interaction mechanisms. In this work, the behavior of existing RC columns with border line behavior between flexure and shear have been investigated in the case of poor quality concrete and light FRP strengthening with local jacketing and medium quality concrete and strong FRP strengthening with local jacketing, in order to highlight the effect of concrete strength on the effectiveness of the retrofit intervention. As an alternative to FRP jacketing; the effectiveness of the Fiber Reinforced Cementitious Composite (FRCC) jacketing for the seismic strengthening of columns with highly deteriorated concrete cover or columns already damaged by an earthquake is also evaluated. Six full-scale RC columns have been tested under cyclic loading: one was used as a control specimen; four were strengthened in the potential plastic hinge region with carbon FRP (CFRP); and one was fully jacketed with FRCC. The comparison between poor and medium quality concrete columns showed that the CFRP local jacketing is more effective in the case of poor quality concrete. The FRCC jacketing appears to be a sound repair strategy and a suitable alternative to the FRP jacketing in case of poor quality; however, more experimental research is needed for improving this retrofit technique. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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23 pages, 39807 KiB

Open AccessArticle

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

by Arkadiusz Kwiecień, Piotr Krajewski, Łukasz Hojdys, Marcin Tekieli and Marek Słoński

Polymers 2018, 10(4), 356; https://doi.org/10.3390/polym10040356 - 22 Mar 2018

Cited by 22 | Viewed by 5476

Abstract

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. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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13 pages, 5333 KiB

Open AccessArticle

Prestressed Unbonded Reinforcement System with Multiple CFRP Plates for Fatigue Strengthening of Steel Members

by Ardalan Hosseini, Elyas Ghafoori, Masoud Motavalli, Alain Nussbaumer, Xiao-Ling Zhao and Roland Koller

Polymers 2018, 10(3), 264; https://doi.org/10.3390/polym10030264 - 4 Mar 2018

Cited by 33 | Viewed by 10244

Abstract

Carbon fiber reinforced polymer (CFRP) composites have exhibited a great potential for strengthening of steel structures. In the current study, an innovative prestressed unbonded reinforcement (PUR) system is introduced for fatigue strengthening of existing steel members. The system relies on a pair of mechanical clamps; each holds multiple CFRP plates and anchors their prestressing forces to the steel substrate via friction. A finite element model was established to optimize the design of the required mechanical components of the system. A set of static and fatigue tests was conducted on the developed mechanical clamps as the key elements of the proposed PUR system. The performance of the PUR system was then evaluated using a set of fatigue tests on two precracked steel plate specimens, one without any strengthening system and the other one strengthened with the proposed PUR system. In the latter specimen, the CFRP plates were prestressed up to about 800 MPa (approximately 30% of the CFRP tensile strength), which resulted in a complete fatigue crack arrest in the precracked steel plate. Furthermore, neither slippage of the mechanical clamps nor any prestress loss in the CFRP plates was observed after 7.5 million fatigue cycles. Based on the promising experimental results, obtained from the sets of fatigue tests performed in the current study, it can be concluded that the proposed PUR system can be considered as an efficient alternative to the conventional bonded reinforcement solutions for fatigue strengthening of damaged steel members. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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14 pages, 19483 KiB

Open AccessArticle

Fatigue and Durability of Laminated Carbon Fibre Reinforced Polymer Straps for Bridge Suspenders

by Fabio Baschnagel, Rea Härdi, Zafiris Triantafyllidis, Urs Meier and Giovanni Pietro Terrasi

Polymers 2018, 10(2), 169; https://doi.org/10.3390/polym10020169 - 10 Feb 2018

Cited by 20 | Viewed by 6428

Abstract

Steel cables and suspenders in bridges are at high risk of corrosion-fatigue and in some cases of fretting-fatigue in their anchorages. These factors greatly limit the service stresses of a specific cable system and involve expensive protection measures. In order to investigate the above limitations, the fretting fatigue behaviour of pin-loaded carbon fibre reinforced polymer (CFRP) straps was studied as models for corrosion-resistant bridge suspenders. Two types of straps were tested: small model straps with a sacrificial CFRP ply and large full-scale straps. In a first phase, five fully laminated and carbon pin-loaded CFRP model straps were subjected to an ultimate tensile strength test. Thereafter, and in order to assess their durability, 20 model straps were subjected to a fretting fatigue test, which was successfully passed by 4 straps. An S-N curve was generated for a load ratio of 0.1 and a frequency of 10 Hz. In a second phase, one full-scale strap was tested for its ultimate tensile strength and two full-scale straps were fatigue-tested. The influence of fretting fatigue loading on the residual mechanical properties of the straps was also assessed, and although fretting fatigue represented an important limitation for laminated CFRP straps, it could be shown that the investigated CFRP tension members can compete with the well-established steel suspenders. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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14 pages, 6226 KiB

Open AccessArticle

Preloading Effect on Strengthening Efficiency of RC Beams Strengthened with Non- and Pretensioned NSM Strips

by Renata Kotynia and Marta Przygocka

Polymers 2018, 10(2), 145; https://doi.org/10.3390/polym10020145 - 3 Feb 2018

Cited by 3 | Viewed by 3678

Abstract

The near surface mounted (NSM) technique has been shown to be one of the most promising methods for upgrading reinforced concrete (RC) structures. Many tests carried out on RC members strengthened in flexure with NSM fiber-reinforced polymer (FRP) systems have demonstrated greater strengthening efficiency than the use of externally-bonded (EB) FRP laminates. Strengthening with simultaneous pretensioning of the FRP results in improvements in the serviceability limit state (SLS) conditions, including the increased cracking moment and decreased deflections. The objective of the reported experimental program, which consisted of two series of RC beams strengthened in flexure with NSM CFRP strips, was to investigate the influence of a number of parameters on the strengthening efficiency. The test program focused on an analysis of the effects of preloading on the strengthening efficiency which has been investigated very rarely despite being one of the most important parameters to be taken into account in strengthening design. Two preloading levels were considered: the beam self-weight only, which corresponded to stresses on the internal longitudinal reinforcement of 25% and 14% of the yield stress (depending on a steel reinforcement ratio), and the self-weight with the additional superimposed load, corresponding to 60% of the yield strength of the unstrengthened beam and a deflection equal to the allowable deflection at the SLS. The influence of the longitudinal steel reinforcement ratio was also considered in this study. To reflect the variability seen in existing structures, test specimens were varied by using different steel bar diameters. Finally, the impact of the composite reinforcement ratio and the number of pretensioned FRP strips was considered. Specimens were divided into two series based on their strengthening configuration: series “A” were strengthened with one pretensioned and two non-pretensioned carbon FRP (CFRP) strips, while series “B” were strengthened with two pretensioned strips. Experimental tests illustrated promising results at ultimate and serviceability limit state conditions. A significant gain of the load bearing capacity, in the range between 56% and 135% compared to the unstrengthened beams, was obtained. Tensile rupture of the NSM CFRP strips was achieved, confirming full utilization of the material. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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14 pages, 1910 KiB

Open AccessArticle

Assessment of Axial Behavior of Circular HPFRCC Members Externally Confined with FRP Sheets

by Ugur Demir, Yusuf Sahinkaya, Medine Ispir and Alper Ilki

Polymers 2018, 10(2), 138; https://doi.org/10.3390/polym10020138 - 31 Jan 2018

Cited by 3 | Viewed by 4396

Abstract

The aim of this paper is to identify the axial behavior characteristics of FRP (fiber reinforced polymer) confined circular HPFRCC (high performance fiber reinforced cementitious composite) members under compression. The test program comprised of 24 circular specimens with an average compressive strength of 102.7 MPa, including 21 carbon FRP (CFRP) confined (2, 4, 6, 8 and 10 layers) and three unconfined specimens. Transverse confinement generated by external FRP sheets resulted with a remarkable enhancement in axial strength and deformability, which is extremely important to resist seismic actions. The higher was the thickness of FRP confinement, the larger was the ultimate strain (ε_cu) and peak compressive strength (f′_cc) of externally confined HPFRCC. When compared to FRP confined conventional concrete, different axial and lateral deformation characteristics were seen in FRP jacketed HPFRCC members. Higher strength and steel fiber presence in HPFRCC limited the lateral deformations which resulted with reduced strain efficiency with respect to conventional concrete. After presenting the experimental work, performance and accuracy of several available models proposed for predicting the axial behavior of FRP jacketed concrete were evaluated in a comparative manner. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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17 pages, 15625 KiB

Open AccessArticle

EBR Strengthening Technique for Concrete, Long-Term Behaviour and Historical Survey

by Christoph Czaderski and Urs Meier

Polymers 2018, 10(1), 77; https://doi.org/10.3390/polym10010077 - 17 Jan 2018

Cited by 19 | Viewed by 6130

Abstract

Epoxy bonded steel plates (externally bonded reinforcemen: EBR) for the strengthening of concrete structures were introduced to the construction industry in the late 1960s, and the use of fibre reinforced polymers (FRPs) was introduced in the 1990s, which means that these techniques have already been used in construction for 50 and 25 years, respectively. In the first part of the paper, a historical survey of the development and introduction of these strengthening techniques into the construction industry are presented. The monitoring of such applications in construction is very important and gives more confidence to this strengthening technique. Therefore, in the second part of the paper, two long-term monitoring campaigns over an extraordinarily long duration will be presented. Firstly, a 47-year monitoring campaign on a concrete beam with an epoxy bonded steel plate and, secondly, a 20-year monitoring campaign on a road bridge with epoxy bonded CFRP (carbon fibre reinforced polymers) strips are described. The paper is an expanded version of the paper presented at the SMAR2017 Conference. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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14 pages, 5150 KiB

Open AccessArticle

Assessing the Contribution of the CFRP Strip of Bearing the Applied Load Using Near-Surface Mounted Strengthening Technique with Innovative High-Strength Self-Compacting Cementitious Adhesive (IHSSC-CA)

by Alyaa Mohammed, Nihad T. K. Al-Saadi and Riadh Al-Mahaidi

Polymers 2018, 10(1), 66; https://doi.org/10.3390/polym10010066 - 11 Jan 2018

Cited by 8 | Viewed by 4046

Abstract

Efficient transfer of load between concrete substrate and fibre reinforced polymer (FRP) by the bonding agent is the key factor in any FRP strengthening system. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) was recently developed by the authors and has been used in a number of studies. Graphene oxide and cementitious materials are used to synthesise the new adhesive. The successful implementation of IHSSC-CA significantly increases carbon FRP (CFRP) strip utilization and the load-bearing capacity of the near-surface mounted (NSM) CFRP strengthening system. A number of tests were used to inspect the interfacial zone in the bonding area of NSM CFRP strips, including physical examination, pore structure analysis, and three-dimensional laser profilometery analysis. It was deduced from the physical inspection of NSM CFRP specimens made with IHSSC-CA that a smooth surface for load transfer was found in the CFRP strip without stress concentrations in some local regions. A smooth surface of the adhesive layer is very important for preventing localized brittle failure in the concrete. The pore structure analysis also confirmed that IHSSC-CA has better composite action between NSM CFRP strips and concrete substrate than other adhesives, resulting in the NSM CFRP specimens made with IHSSC-CA sustaining a greater load. Finally, the results of three-dimensional laser profilometery revealed a greater degree of roughness and less deformation on the surface of the CFRP strip when IHSSC-CA was used compared to other adhesives. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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22 pages, 278 KiB

Open AccessReview

Internal and External Reinforcement of Concrete Members by Use of Shape Memory Alloy and Fiber Reinforced Polymers under Cyclic Loading—A Review

by Azadeh Parvin and Janet Raad

Polymers 2018, 10(4), 376; https://doi.org/10.3390/polym10040376 - 28 Mar 2018

Cited by 14 | Viewed by 5887

Abstract

This paper presents a review of recent studies on reinforced concrete (RC) structural components, such as beam-column joints (BCJs). These members are internally or externally reinforced with corrosion free shape memory alloy (SMA), fiber reinforced polymers (FRP), or a combination of the two materials. Bonded FRP sheets or near surface mounted (NSM) FRP bars are used in external strengthening cases. The use of FRP and SMA materials in RC structures can offer great potential benefits including lifetime cost saving, durability, safety, and post-earthquake serviceability for RC structures. Although FRP materials are well known for their corrosion resistance, high strength-to-weight ratios, ease of application, and constructability; SMA materials as reinforcement allow the structures to regain their original shape after the termination of the load without any permanent large residual deformation. In summary, the presented literature review provides an insight into the ongoing research on the use of these materials for retrofitting or strengthening of RC structural components and the trends for future research in this area. The cost and durability are also discussed. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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15 pages, 3173 KiB

Open AccessReview

Durability Issues and Challenges for Material Advancements in FRP Employed in the Construction Industry

by Mariaenrica Frigione and Mariateresa Lettieri

Polymers 2018, 10(3), 247; https://doi.org/10.3390/polym10030247 - 28 Feb 2018

Cited by 87 | Viewed by 10983

Abstract

The use of fiber reinforced polymer (FRP) composites for the rehabilitation of buildings or other infrastructure is increasingly becoming an effective and popular solution, being able to overcome some of the drawbacks experienced with traditional interventions and/or traditional materials. The knowledge of long-term performance and of durability behavior of FRP, in terms of their degradation/aging causes and mechanisms taking place in common as well as in harsh environmental conditions, still represents a critical issue for a safe and advantageous implementation of such advanced materials. The research of new and better performing materials in such fields is somewhat limited by practical and economical constrains and, as a matter of fact, is confined to an academic argument. Full article

(This article belongs to the Special Issue Selected Papers from "SMAR 2017")

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