Strengthening Of RC Structures Research Articles

External strengthening of RC structures with buckling-restrained braced frames

External strengthening of RC structures with buckling-restrained braced frames

Analysis of Strengthening RC Beams Using Stainless Steel Continuous Reinforcement Embedded at Ends Using ANSYS

Abstract: An innovative system for the flexural strengthening of RC structures designated continuous reinforcement embedded at ends (CREatE) is presented in this research work. The main charateristics and procedures for the application of this new strengthening techniques were described. To evaluate the performance and efficiency of this technique, a set of RC T-beams was subjected to a four pointed bending test setup. Different application arrangements and different amount of reinforcement were considered, and the CREatE technique was tested under monotonic and cyclic loading histories. The tests were modeled using the nonlinear finite element method (FEM) to predict the performance of the RC T beam, which allowed analysing, in detail and with good agreement with the experiments, the influence of the CREatE technique on the (1) strains developed in the concrete, (2) cracking patterns, and (3) strains developed in the stirrups. Apart from the expected increases in the flexural stiffness and load bearing capacity of the T beam, the result showed that the use of the CREatE technique led to higher ductility indexes in the displacement compared with traditional techniques. Moreover, with the CREatE technique, premature debonding of the reinforcement material from the concrete tensioned surface commonly observed in externally bonded reinforcement (EBR) strengthening system was eliminate.

Experimental investigation on the effect of geopolymer adhesive on the bond behavior between CFRP and concretes

AbstractThe application of geopolymer adhesive to CFRP‐strengthened the RC structure is a new green reinforcement method, and the bonding behavior of geopolymer adhesive was studied in this paper. The geopolymer mechanical properties orthogonal test was conducted and the optimal proportion was obtained, which summarized the influence of slag powder content, liquid–solid ratio, and water glass modulus. A total of 15 adhesively bonded CFRP‐concrete double‐lap joints were fabricated and tested under shear loading to investigate the effect of geopolymer adhesive on the bonding behaviors. Modified bond‐slip models of the geopolymer adhesive specimens were developed and compared with the test date. The results showed that the failure models of epoxy adhesive and geopolymer adhesive are different. The bond strength, interface stiffness, and maximum shear stress of the geopolymer adhesive specimen can reach 93.6% ~ 97.6%, 95.7% ~ 132.9%, and 91% ~ 94% of the bond strength of epoxy adhesive respectively. The predicted τmax and s0 of the proposed model are in excellent agreement with the experimental data, and the maximum deviation is −9.58%. This study proposes that geopolymer could be applied as an effective adhesive in the strengthening of RC structures.

Strengthening RC Beams Using Stainless Steel Continuous Reinforcement Embedded at Ends

AbstractAn innovative system for the flexural strengthening of RC structures designated continuous reinforcement embedded at ends (CREatE) is presented in this research work. The main characteristi...

Flexural Strengthening of RC Structures with TRC—Experimental Observations, Design Approach and Application

Today, the need for structural strengthening is more important than ever. Flexural strengthening with textile reinforced concrete (TRC) is a recommendable addition to already proven methods. In order to use this strengthening method in construction practice, a design model is required. This article gives a brief overview of the basic behavior of reinforced concrete slabs strengthened with TRC in bending tests as already observed by various researchers. Based on this, a design model was developed, which is presented in the main part of the paper. In addition to the model, its assumptions and limits are discussed. The paper is supplemented by selected application examples to show the possibilities of the described strengthening method. Finally, the article will give an outlook on open questions and current research.

Durability and tensile characterization of wet lay-up flax/epoxy composites used for external strengthening of RC structures

This study presents the characterization of the mechanical and durability performance of wet lay-up flax/epoxy composites used for the external strengthening of concrete structures. The composites are manufactured using different weight bases and flax woven fabric as well as a bidirectional glass fabric for the control samples. The tomography observations reveal good impregnation of the fibres and porosity mainly located along the fibre bundles. The influence of the choice of fabric and the number of fabric layers on the properties are then examined by mechanical tensile testing. The glass and flax composites present comparable tensile stress at failure. The properties, calculated with the portion of the dry fibres only, remain constant with the number of layers. Durability tests are then performed to estimate the residual properties after two kind of ageing, i.e., accelerated and natural.

Single diagonal precast prestressed concrete bracing for strengthening existing concrete frames

A new method for seismic retrofitting concrete structures is proposed using an X-shaped precast prestressed concrete brace. This PPC brace is made of four precast concrete parts and middle section that are assembled and added to the existing frame. This method has the following benefits: there is no need to work with wet concrete in site or anchor and bolt to the existing frame, which may lead to a fast and economic retrofitting method. The X-shaped concrete brace was made in half size and its efficiency is confirmed. That X-shaped precast prestressed concrete brace was simulated and evaluated by computerized analysis using ABAQUS FEA modeling by authors of this paper earlier, and the proposed brace showed proper results in reducing lateral drifts and that method is considered as a proper method for seismic retrofitting. Here we are suggesting a different setup for the brace, which may lead to faster and more economic strengthening of RC structures. The new setup is omitting the middle section and suggests the brace to be used as a single diagonal. A model of this proposed method is simulated in ABAQUS FEA and the results show it is proper in reducing lateral displacements.

DUCTILITY ENHANCEMENT OF RC BEAMS STRENGTHENED WITH STRAIN HARDENING CEMENTITIOUS COMPOSITES

Ultra High-Performance Strain Hardening Cementitious Composites (UHP-SHCC) is a newly developed construction material, has large advantages on large strain capacity as well as high compressive and tensile strength, which is useful for strengthening or repair concrete members. An important obstacle needs to be studied, is the strain localization occurs in the UHP-SHCC strengthening layer around the substrate concrete cracks, which severely limits the ductility of the strengthened beam. In the present study, five specimens were tested experimentally, one as a control and four strengthened with variable reinforcement ratios embedded in the strengthening layer. The recorded tests showed that it is sufficient to use 1.2% additional reinforcement ratio embedded in the strengthening layer for beams strengthened with UHP-SHCC to eliminate the observed early strain localization and to gain adequate ductility. Another important conclusion is the strengthening of RC structures using an unreinforced UHP-SHCC layer may lead to a brittle failure.

Behavior of RC beams strengthened with strain hardening cementitious composites (SHCC) subjected to monotonic and repeated loads

Ultra High Performance Strain Hardening Cementitious Composites (UHP-SHCC) is a newly developed construction material, has large advantages on large strain capacity as well as high compressive and tensile strength, which is useful for strengthening or repair concrete members. However, some important obstacles need to be studied, such as the strain localization occurs in the UHP-SHCC strengthening layer around the substrate concrete cracks, which severely limits the ductility of the strengthened beam subjected to monotonic and repeated loads. In the present study, ten specimens were tested experimentally, which divided into two major groups, each group contains five specimens, one as a control and four strengthened with constant UHP-SHCC layer thickness having variable reinforcement ratios embedded in the middle of the strengthening layer. Two types of loading were applied; Monotonic loading for the first group and short time repeated loading for the second group. The recorded tests showed that use additional reinforcement embedded in the strengthening layer for beams strengthened with UHP-SHCC become sufficient at certain limit to eliminate the observed early strain localization and to gain adequate ductility under both monotonic and repeated loading. Another important conclusion is the strengthening of RC structures using an unreinforced UHP-SHCC layer may lead to a brittle failure especially in case of repeated loading.

Effect of Elevated Temperature on Flexural Behavior of RC Beams with NSM CFRP

The strengthening of RC structures with fiber reinforced polymer (FRP) materials has demonstrated enormous potential as a repairing material. However, their thermal and mechanical behaviors after exposure to elevated temperature remain an obstacle due to FRP’s sensitivity to high temperatures as compared with order structural materials. In this paper an experimental works was conducted to study the behavior of RC beam with NSM CFRP with different adhesive materials exposed to elevated temperature of 550°C for different periods of time and cooling schemes. In this experimental study 6 beams were cast, one beam (BN) was not exposed to elevated temperature as a control beam and the remaining beams were exposed to elevated temperature. These beams were tested under four-point loading. The effect of variables was studied in terms of ultimate failure load, maximum deflection and failure mode. From the test results, it could be concluded that ready-mix mortar could be as an alternative adhesive material. The ultimate load-carrying capacity of the RC beam was reduced for longer duration of elevated temperature.

Flexural Strengthening of RC Slabs with SRP/SRG: An Experimental-Numerical Comparison

Steel reinforced polymer (SRP) and steel reinforced grout (SRG) systems have emerged as promising and cost-effective technologies for the external strengthening of RC structures. An experimental campaign was recently performed to investigate the effectiveness of SRG/SRP systems in increasing the flexural strength of RC slabs. Test results are herein used to validate a Fiber based finite element code developed with the aim of simulating the flexural behavior of SRP/SRG strengthened RC beams/slabs. The model provides an accurate prediction of the cross-sectional response by explicitly modeling each constituent, their position inside the discretized cross-section, and their nonlinear behavior. The satisfactory comparison between numerical predictions and experimental results assessed the accuracy of the developed code.

Shear Span–Depth Ratio Effect on Behavior of RC Beam Shear Strengthened with Full-Wrapping FRP Strip

AbstractShear span-to-effective depth ratio (av/d) is known to affect the shear behavior of RC members. However, this effect has not been appropriately addressed in existing design guidelines for the fiber-reinforced polymer (FRP) strengthening of RC structures. This paper focuses on the effect of av/d on the behaviors of RC beams shear strengthened with full-wrapping FRP strips. A total of six strengthened beams and six normal beams are tested, with av/d ratios ranging from 1.0 to 3.5. The experimental results indicate that the FRP shear contribution increases initially with the av/d ratio, but decreases when the ratio is beyond 2.5. The FRP strain along the critical shear crack distributes in different manners for different av/d ratios. The authors’ experimental results and the test results from the literature are compared with predictions from available design guidelines. Although some recent design guidelines always provide a safe value, it can be overconservative. Conversely, other recent design guid...

Effect of GM patterns on ductility and debonding control of FRP sheets in RC strengthened beams

Strengthening and retrofitting of old structures under current use, are important issues that have been considerably studied in the last few decades. There are many aims for structural strengthening; among them, resisting higher design loads, compensating strength loss due to deterioration, correcting design or construction deficiencies and increasing ductility are the most important. Fiber reinforced polymers (FRPs) have been recognized as one of the best materials used for strengthening of RC structures; especially in the form of externally bonded reinforcement (EBR). However, a major problem in its application is debonding of FRP from concrete surface. Surface preparation is a conventional method to overwhelm debonding recommended by many researches and by ACI 440-2R code. Nevertheless, this method is only partially beneficial and is unable to prevent debonding and yet, debonding causes premature failure in FRP strengthened RC members. Grooving method (GM) is a recently proposed alternative of surface preparation that can improve the behavior of strengthened members and control debonding phenomenon; however, the method must be optimized. In this paper the effect of different longitudinal grooving patterns is investigated and three criteria of ultimate load capacity, ductility and failure mechanism are studied for the strengthened beams. Furthermore, the effect of different thicknesses of CFRP sheet, i.e., thicknesses of 0.17, 0.30 and 0.43mm are also investigated. Grooves are applied on critical areas called high-stressed zones and are omitted from zones with low state of stresses. The results showed that the specimens strengthened by grooves at the end of the plate performed better regarding the strength and ductility compared to other specimens; where the fibers ruptured in all selected thicknesses.

Transfer zone of prestressed CFRP reinforcement applied according to NSM technique for strengthening of RC structures

This study presents an experimental program to assess the tensile strain distribution along prestressed carbon fiber reinforced polymer (CFRP) reinforcement flexurally applied on the tensile surface of RC beams according to near surface mounted (NSM) technique. Moreover, the current study aims to propose an analytical formulation, with a design framework, for the prediction of distribution of CFRP tensile strain and bond shear stress and, additionally, the prestress transfer length. After demonstration the good predictive performance of the proposed analytical approach, parametric studies were carried out to analytically evaluate the influence of the main material properties, and CFRP and groove cross section on the distribution of the CFRP tensile strain and bond shear stress, and on the prestress transfer length. The proposed analytical approach can also predict the evolution of the prestress transfer length during the curing time of the adhesive by considering the variation of its elasticity modulus during this period.

Reliability Analysis of RC Columns and Frame with FRP Strengthening Subjected to Explosive Loads

Some structures, both military and civilian, might experience explosive loads during their service life. Owing to high uncertainties in blast load predictions and structural parameters, accurate assessment of the performances of structures under explosion loads is a challenging task. For example, a number of experimental studies using fiber-reinforced plastic (FRP) strengthening of RC structures have been reported in the literature. Most of these studies demonstrate that FRP strengthening is effective in increasing the blast load–carrying capacities of RC structures. However, significant variations in the effectiveness of FRP strengthening have also been observed owing to the large uncertainties in blast loading, RC and FRP material properties, and workmanship in preparing the test specimens and conducting experimental tests. Very few studies that take into consideration these uncertainties in analyzing the effectiveness of FRP strengthening of RC structures on blast loading resistance can be found in the literature. This study performs a reliability analysis to assess the performance of RC columns with or without FRP strengthening in resisting blast loads. Statistical variations of blast loading predictions derived in a previous study are adopted in this study. To define structural performance, pressure-impulse (P-I) curves with a damage criterion on the basis of axial load-carrying capacity that were developed in previous studies for RC columns without strengthening or with FRP strip, FRP wrap, or both FRP strip and wrap strengthening are used. Considering the uncertainties in blast loading predictions and RC column and FRP material properties and dimensions, limit-state functions corresponding to different damage levels of RC columns with or without FRP strengthening are formulated. The statistical variations of blast loading, RC column dimension, longitudinal and transverse reinforcement ratio, and concrete, steel, and FRP material strength and FRP thickness are considered. The failure probabilities of RC columns corresponding to different damage levels with or without FRP strengthening are calculated. The effectiveness of FRP strengthening on the RC column’s blast loading resistance capacities is discussed. The importance of considering the random fluctuations on blast loading and RC column parameters in assessing the blast loading effect on RC columns is demonstrated. A structural system reliability analysis is also carried out to examine the probability of structural collapse as a result of blast loading applied to the front of an example two-span multistory RC frame. The results obtained in this study demonstrate the effectiveness of FRP strengthening on structure protection and can also be used to assess RC structure performance under blast loadings.

Opportunities and Risks Arising from the Properties of FRP Materials Used for Structural Strengthening

Modern composite materials, made on the basis of non-metallic continuous fibers are increasingly used in civil engineering, in particular to strengthen building structures. In strengthening of RC structures the utilization of externally bonded FRP composites is only up to 35% because of the pilling-off failure mechanism. This problem can be solved by the use of pre-tensioned composite laminates. Due to more complex behavior, the strengthening of structures using prestressing technology needs a careful design approach and a full understanding of the behavior of both the materials and elements. The advantages and risks of the presented technology, which may determinate the success of the entire project, are highlighted in the article.

Analysis of FRP/Concrete Interface Creep Using Fractional Derivative Rheological Model

FRPhas been widely used in the strengtheningof RC structures because of its low density, high strength and anti-corrosion. Long term performance ofFRP/concrete interface is one of the keys for its successful engineering application, and however it has notbeen well studied. In this article a fractional derivative rheological model was proposed which is able to characterize the complicated creepdeformation of epoxy FRP/concreteinterface layer with simple expression. Numerical study revealed thatinterfacial creep might lead to notable stress redistribution and largeinterfacial deformation has possibility to cause weakening of FRPstrengthening.

Non-compression X-bracing system using CF anchors for seismic strengthening of RC structures

Cross-bracing (X-bracing) is one of the most popular methods of seismic retrofitting. Research results indicate that X-bracing significantly increases structural stiffness and enhances the strength of the structures. However, researchers have also noted that conventional steel X-bracing methods can involve brittle failure at the connection between the brace and the building, as well as buckling failure of the braces. The current research thus investigated the structural properties of a new type of non-compression carbon fibre (CF) X-bracing system with the goal of overcoming the brittle and buckling failures that can occur with conventional steel X-bracing methods. The proposed non-compression X-bracing system uses CF bracing and anchors to replace the conventional steel bracing and bolt connection. This paper reports on the seismic resistance of a reinforced concrete frame strengthened using CF X-bracing. Cyclic loading tests were carried out, and the structural stiffness and ductility were investigated along with the hysteresis of the lateral load–drift relations. CF is less rigid than the conventional materials used for seismic retrofitting, resulting in some significant advantages: the strength of the structure increased markedly with the use of CF X-bracing and no buckling failure of the bracing was observed.

Efficiency analysis of strengthening of RC structures with prestressed CFRP composites

The subject of the paper is the analysis of the efficiency of strengthening of RC structures for flexure with use of prestressed CFRP composites. Basic methods of application of the strengthening are described along with the issues connected with the prestressing and anchorage of the CFRP materials. The authors prepared a database of the results of experimental tests based on the published research, which was the basis for the analysis of failure modes of the strengthened members and its efficiency depending on the characteristics of members. The aim of the paper was to define the influence of steel reinforcement ratio, composite reinforcement ratio and initial CFRP prestressing strain on the efficiency of the applied strengthening in terms of load capacity and serviceability conditions.

A State-of-the-Art Review on Debonding Failures of FRP Laminates Externally Adhered to Concrete

There is significant concern in the engineering community regarding the safety and effectiveness of fiber-reinforced polymer (FRP) strengthening of RC structures because of the potential for brittle debonding failures. In this paper, previous research programs conducted by other researchers were reviewed in terms of the debonding failure of FRP laminates externally attached to concrete. This review article also discusses the influences on bond strength and failure modes as well as the existing experimental research and developed equations. Based on the review, several important conclusions were re-emphasized, including the finding that the bond transfer strength is proportional to the concrete compressive strength; that there is a certain bond development length that has to be exceeded; and that thinner adhesive layers in fact lower the chances of a concrete-adhesive interface failure. It is also found that there exist uncertainty and inaccuracy in the available models when compared with the experimental data and inconsistency among the models. This demonstrates the need for continuing research and compilation of data on the topic of FRP’s bond strength.