Tension Face Research Articles

Enhancing the Punching Load Capacity of Reinforced Concrete Slabs Using an External Epoxy-Steel Wire Mesh Composite

The present experimental work investigates the applicability and performance of a new strengthening method for concrete slabs, intended to increase their punching resistance using combination layers of steel wire mesh with epoxy attached to the concrete slabs’ tension face. Six simply supported square reinforced concrete slab specimens were tested up to failure under a central concentrated load. The main parameters in the study are the concrete compressive strength (30 MPa and 65 MPa) and the configuration of a bundle externally fixed to the tension side of the tested slabs. The experimental results appeared to greatly enhance the performance of the specimens, as they were externally strengthenined under this new method. When compared to the control slabs, the punching load and stiffness of the strengthened slabs increased up to 28% and 21%, respectively.

Effectiveness of Various Techniques Using FRP for the Strengthening of R. C. Beams with Tension Lap Splices

This paper presents an experimental program conducted to investigate the flexural performance of RC beams with tension reinforcement lap splice strengthened using externally bonded FRP different techniques in splice region. The specimens were reinforced on the tension side with four deformed bars spliced at mid span. The tested beams are of 3200 mm total length and 250*120 mm cross section, tested in positive bending. The considered parameters were splice length, type of FRP (glass or carbon), strengthening techniques in splice region (externally confine strips around cross section, Near Surface Mounted technique "NSM" stirrups, externally bonded sheets or bars on the tension face, number of GFRP strips layers (one layer, two layers, and three layers), and shape of NSM stirrups (Box or U shape). No additional anchorage mechanism or bonding methodology was applied for the FRP strips on the concrete except the epoxy adhesive. The effect of these factors on the failure of modes, the ultimate load, the bond strength and that the ductility were investigated. The results indicated that all applied strengthening techniques were efficient in improving the bond strength of the lap splices, the ductility, and the load-deflection behavior of the tested beams, especially when strips were installed over the splice region. This study approved that NSM technique gave more prominent simplicity of employ.

Mechanics-based approach for predicting the short-term deflection of CFRP plated RC beams

Most design codes available today for predicting the deflection of adhesively plated RC beams use a full-interaction moment-curvature approach that requires the flexural rigidity to be quantified empirically. Due to their empirical nature, these design rules can only be applied within the bounds of the tests from which they were derived. Furthermore, as these design rules follow a full-interaction analysis, the slip between the reinforcement and adjacent concrete was not considered and the method does not cope with the discrete rotation of the cracks; that is, the deflection associated with crack widening was not directly considered. As an alternative, partial-interaction mechanics-based methods can be used. In this study, a mechanics-based approach for quantifying the deflection of adhesively plated RC beams was presented. The approach took into account the slip between the reinforcement and adjacent concrete, the formation and widening of flexural cracks, and the intermediate crack debonding mechanism of the externally bonded plate. The deflection from the mechanics-based approach was determined by considering the discrete rotation of individual cracks and the curvature of uncracked regions of the beam. The deflection results derived from the mechanics-based approach were compared with the experimental results of seven adhesively plated CFRP RC beams bonded to their tension face and a significant correlation between the results was observed. The mechanics-based approach does not require any components on the member level to be quantified empirically; thus, it could be useful in predicting the deflection of adhesively plated RC beams with new types of reinforcement material.

Flexural Capacity Enhancement of Timber Beams Partially Confining the Principal Compression Arch using Carbon Fiber Reinforced Polymer Composites

Timber is widely used as a structural element because of its engineering and mechanical properties. This study focuses on the flexural behavior of timber beams externally reinforced with carbon fiber reinforced polymer (CFRP) composites at the tension face and the responses of the fundamental principal compression arch because of confinement from end anchorage. Beams of three different types of timber are studied. All the beams had the same length, width, and span length and were tested under four-point loading. Different CFRP lamination techniques were adopted, with and without U-clamp confinement as end anchorage, to investigate the flexural capacity enhancement of CFRP strips as reinforcement for timber beams. The profile of the principal compression arch is estimated experimentally from fundamental flexural strain-along-depth phenomena by post-processing high definition images extracted from test videos employing digital image correlation technique (DICT) in the MATLAB R2011a framework. Similar responses were found from finite element analysis using ANSYS 11.0. Effective confinement of the principal compression arch produced significant enhancements of flexural capacities and stiffness in the strengthened timber beams.

Flexural behaviors and capacity prediction on damaged reinforcement concrete (RC) bridge deck strengthened by ultra-high performance concrete (UHPC) layer

Abstract Based on strengthening a damaged bridge deck in the field, this paper conducted flexural experiments on a scaled damaged bridge deck strengthened by reinforced ultrahigh performance concrete (UHPC) layer with high temperature steam and normal temperature curing; this paper investigates cracking capacity, ultimate capacity, deformation features and failure mode of RC-UHPC composites under positive bending moment and negative bending moment, respectively. Experimental results indicate that cracking and ultimate capacity of composites increased by about 2.5 and 2 times respectively with UHPC located at tension face comparing with intact RC slab; while for UHPC at compression face, no change in cracking capacity was observed and ultimate capacity was 30% higher than that of intact RC slab. Meanwhile, stiffness also obviously increased with tensile stress in steel reinforcement in RC slabs, decreasing after strengthening for all strengthened slabs; and propagation of cracks in RC slabs was restrained and delayed due to ultrahigh ductility and behavior of strain hardening of UHPC. Based on experimental results and failure mode, analytical equations for cracking and ultimate flexure capacity of UHPC-RC slabs were constructed, and applicability of equations was validated by some experimental results in this paper and other literature with good accuracy by predicted equations. Finally, the research achievements of this paper were effectively applied on the reinforcement design of damaged deck of main girder in a cable-stayed bridge.

Nonlinear analysis of hollow slender reinforced concrete columns under eccentric loading

In this paper the behavior of slender reinforced concrete columns with longitudinal hole under axial compression load and uniaxial bending was investigated. The specimens of the research includes the analysis of eight slender columns of dimensions (150 150 1300 mm). the nonlinear finite element analysis has been carried out using ANSYS computer program (version 16.1) to simulate the behavior of reinforced concrete slender columns with longitudinal hole. Two parameters are considered (longitudinal steel ratio of the column longitudinal bars and grade of longitudinal steel reinforcement(fy)). The results showed that when the steel ratio is increased in hollow reinforced concrete slender columns, the ultimate load is appreciable increased. Four different values of steel ratios (1.6%,2.3%,3.2% and 4.2%) were used in the numerical analysis, the ultimate load is increased by(0.00%,7.14%, 17.59% and 35.51%) respectively. Also, the increase in the steel ratio resulted in decreasing the tensile steel stresses specially at the tension face of the slender column which finally leads to increase the load carrying capacity. The finite element solution revealed that the effect of using different values of yield stress was insignificant at early stages of loading and almost all the numerical columns had the same values of deflections and stresses at the same load level up to a tensile stress value in steel about 300 MPa. At stages close to ultimate load, deflections and tensile stress values were different for the considered numerical columns. , it was noted that, the selected four grades of steel (350, 450, 550 and 650) MPa led to increase the ultimate load by about (0.00%,19.98%, 31.97%, 41.12%) respectively.

Numerical Analysis for U‐Shaped Thin‐Walled Structure Reinforced Timber Beam Based on Thin‐Layer Beam Theory

This paper presents a theoretical model, taking into account the shear deformation subjected to the influence of U‐shaped member by geometric parameters as flange height based on thin‐layer beam theory, to analyze the structural bending behavior of U‐shaped member reinforced timber composite beams, and the feasible design forms of U‐section have been pointed out. The algorithm for this composite beam is the most practical and effective method to meet the accurate solution. The formulas for the common forms of U‐section are presented. It aims to develop a rational engineering approach. The proposed model has been validated by comparing the results obtained in the present analysis with experimental results and finite element analysis. Furthermore, the results suggested that the value of flange height can be one‐fifth the beam height based on the present analysis by comparison of two types of beams. And it is shown that the model provided here correlates consistently and satisfactorily with a wide range of timber beams reinforced by a thin‐walled structure such as steel or aluminum alloy sheet bonded to their tension faces.

Strengthening of capacity deficient RC beams - An experimental approach

Any revision of seismic codes usually demands a higher capacity from structural members, making existing structures unsafe particularly from strength considerations. Retrofitting of capacity deficient members is very suitable for tackling such situations. This paper presents an experimental study on different retrofitting measures adopted for strengthening a series of reinforced concrete (RC) beams. Four identical RC beam specimens were casted, out of which three specimens were strengthened by different schemes (viz., bolted hot rolled flat, bolted cold-formed steel channel, and carbon fibre reinforced polymer (CFRP) laminate, respectively) on their tension face and tested under four-point monotonic loading. This study focuses on the investigation of the flexural behaviour of these retrofitted beams, observed in terms of strength and stiffness. It was concluded that all retrofitting measures improved the structural performance of these beams. However, the cost involved with each strengthening mode was proportional to the improvement in the performance achieved.

Investigating Some Parameters Affecting Flexural Behaviour of Reinforced Concrete Beams Strengthened with CFRP Laminate

In this paper, the influence of some important parameters affecting flexural behaviour of reinforced concrete beams strengthened with one layer of CFRP laminate has been studied. For this purpose, six reinforced concrete beams were cast and tested in the laboratory. Based on the obtained data, when CFRP laminate is applied to the tension face, too close to the steel rebar, the flexural strength of the strengthened beam is reduced. In general, the performance of the beam strengthened with one wide CFRP strip is better than that strengthened with two equivalent narrow strips. Ultimate load capacity of each strengthened beam was calculated based on the method given by the ACI 440.2R and compared with the test one. It is concluded that to avoid the steel rebar-CFRP laminate interaction effect, the CFRP laminate depth to the effective depth ratio (df / d) should not be smaller than about 1.17.

An experimental study on flexural strengthening of RC beams using CFRP sheets

The use of carbon fiber reinforced polymer (CFRP) sheets is becoming a widely accepted solution for strengthening and repairing rein-forced concrete (RC) structures. To date, the behavior of RC beams, strengthened with 60˚ and 45˚ inclined CFRP sheets, has not clearly explained. An experimental program is proposed in this paper to investigate the flexural behavior of RC beams strengthened with CFRP sheets. CFRP sheets were epoxy bonded to the tension face to enhance the flexural strength of beams inducing different orientation angles of 0˚, 45˚, 60˚ and 90˚ with the beam longitudinal axis. The study shows that strengthening RC beams with CFRP sheets is highly influenced by the orientation angle of the sheets. The orientation angle plays a key role in changing the crack pattern and hence the failure mode. The influence of CFRP sheets was adequate on increasing the flexural strength of RC beams but the ductility of the beams was reduced. The best performance was obtained when strengthening RC beam obliquely using 45˚ inclined CFRP sheets where the specimen experienced additional deflection and strength of 56% and 12% respectively and the reduction in its ductility was the least. It is recom-mended to strengthen RC beams, which are weak in flexure, using 45˚ inclined CFRP sheets.

Application of a self-compacting ultra-high-performance fibre-reinforced concrete to retrofit RC beams subjected to repeated loading

The aim of this paper is to describe the performance of reinforced concrete (RC) beams retrofitted with a self-compacting ultra-high-performance fibre-reinforced concrete (UHPFRC) under three-point bend cyclic loading. It is found that retrofitting the RC beams with a thin UHPFRC strip on the tension face increases their endurance limit under a non-zero mean stress cyclic loading from approximately 40% to approximately 60% of their static three-point flexural strength. Moreover, the retrofitted beams behave as a composite structure, with no delamination of the retrofit strip being observed in any of the fatigue tests.

Numerical simulation of FRP-strengthened RC slabs anchored with FRP anchors

Reinforced concrete (RC) members such as beams and slabs can be strengthened in flexure by bonding fibre-reinforced polymer (FRP) composite plates to their tension face. Experimental studies have shown that anchorage of the FRP plates with anchors made from FRP (i.e. FRP anchors) can delay and even eliminate premature debonding failure. This paper reports on the finite element (FE) analysis of a series of FRP-strengthened RC test slabs anchored with FRP anchors. Prior to introducing the FE modelling strategy, a semi-empirical load-slip model is proposed for modelling the FRP anchors. The load-slip model, which is calibrated from FRP-to-concrete joint tests, is novel and is also essential for the analysis of the slabs. The numerically simulated results of load, deflection and strain for the test slabs are shown to be in good agreement with test measurements. The results help provide needed insight to the influence of FRP anchors upon FRP-strengthened members.

Strengthening of RC Beams Using Externally Bonded Fibre Reinforced Polymer Composites

An experimental study is presented on the behaviour of reinforced concrete (RC) beams retrofitted externally with glass fibre reinforced polymer (GFRP) fabrics. Out of ten beams, one beam without GFRP and nine beams wrapped in various lay-up patterns with one, two, three and four layers of GFRP fabrics have been tested for flexure under two-point loading. Loads corresponding to the first crack/delamination and ultimate failure of the beams have been recorded and types of failure have also been observed. Load versus deflection graphs have been plotted at salient locations of beams. Thereafter, a critical discussion has been made with respect to increase in the flexural strength of retrofitted beams as compared to the control beam in order to explore the optimal use of GFRP fabrics for strengthening the RC beams. In addition, a design proposal has been developed in extension to IS: 456-2000 to predict the ultimate design strength of RC beams strengthened with fibre reinforced polymer (FRP) fabric sheets. Thereafter, the values of the experimental flexural strength of the strengthened RC beam have been compared with the corresponding values of design flexural strength computed from present formulation and ACI: 440-2R-08 for its validation. The results obtained from the present experimental study show that flexural strength of the strengthened RC beams increases with increase in number of layers for all lay-up patterns. However, the beams wrapped with two layers GFRP fabrics in the tension face and half of the both sides below neutral axis show superior performance with respect to flexural strength, ductility and economy.

Contact and Noncontact Ultrasonic Nondestructive Test in Reinforced Concrete Beam

Contact‐type ultrasonic test is commonly used in construction industry where gel‐couplant is applied to the material being tested and the transducers to assure that wave propagation will travel through without any air gaps. However, this method has disadvantages, since surface preparation is considered prior to testing. Another method of testing without the worry of air gaps that causes scattering of waves before it reaches the medium is the use of the noncontact ultrasonic test. In particular, the air‐coupled ultrasonic test is done in this paper for reinforced concrete beams. Sixteen plain concrete cube specimens under the compression test and six reinforced concrete beam specimens under the four‐point bending test are made with water‐cement ratio of 40% and 60%. The plain concrete cubes are investigated to establish the relationship of the contact ultrasonic test and load. Added parameters are considered to investigate the sensitivity of the contact and noncontact ultrasonic test in reinforced concrete beams. These are ultrasonic wave path and the neutral axis index. It shows that the higher water‐cement ratio produces good sensitivity in the noncontact ultrasonic test, since it produces more cracks on the tension face. Lower water‐cement ratio gives good sensitivity with load for the contact ultrasonic test, since it has its ultrasonic wave path passing through the concrete experiencing compression. In addition, the neutral axis index for a member subjected to bending is an important factor in assessing the sensitivity of both contact and noncontact ultrasonic test.

13.10: Angles connected by one leg in tension

ABSTRACTThe paper deals with bolted connections of angles connected by one leg subjected to tension force. These connections are characterized by the eccentricity of the tension force and are dealt with in the EN 1993‐1‐8 standard that provides the design resistance for block tearing, for bolt bearing and for reduced effective net cross‐section that is based on β factors. A closer look on the design resistance of the reduced effective net cross‐section shows that the calculation of β may lead to inconsistencies. An addition of a bolt may lead to decrease of the net cross‐section resistance. Moreover, the formulas for the calculation of the reduced net cross‐section resistances are only a different presentation of the formulas for the block tearing resistance. Analysis of 631 test results from literature shows that the calculation of the block tearing resistance is sufficient and that equations 3.12 and 3.13 of EN 1993‐1‐8 are redundant. The study will also show that a constant distribution of stress on the tension face may be assumed in the formula for the block tearing resistance of the angles connected by one leg in tension (eq. 3.9, EN 1993‐1‐8), similarly to the provisions in American standard AISC 360–05. A modified bolt bearing strength function is also presented and statistically analysed on 631 test results. It is simpler and less conservative than the one in EN 1993‐1‐8.

Study of flexural response of engineered cementitious composite faced masonry structures

This study aims to develop and demonstrate the effectiveness of precast engineered cementitious composite (ECC) plates for strengthening of masonry beams by bonding them on tension face as well as both on tension and compression faces like sandwich beam. Two types of bonding materials have been used, i.e., epoxy and cement mortar for bonding the ECC plates with masonry beam. Experimental flexural response has been predicted for tension strengthened as well as sandwich beams. Detailed parametric study has been performed using ABAQUS commercial software package. The parametric study incorporates the effect of various parameters such as ECC thickness in compression and tension, epoxy and cement mortar as bonding agents, and bonding agent thickness on the flexural response of strengthened beams. All beams were tested under four-point loading systems. The numerical results obtained using ABAQUS are validated with experimental results. The present study results reveal that the application of precast ECC increases the strength and deformability of masonry beams and hence demonstrate its effectiveness as strengthening element for masonry structures. Furthermore, recommendations regarding optimum thickness of ECC strips/plates and bonding agents have been made for external strengthening of masonry beams with precast ECC strips.

Retrofitting of Reinforced Concrete Beams using Reactive Powder Concrete (RPC)

Strengthening of existing damaged structures is one of the leading studies in civil engineering. The purpose of retrofitting is to structurally treat the member with an aim to restore the structure to its original strength. The focus of this project is to study the behaviour of damaged Reinforced Concrete beam retrofitted with Reactive Powder Concrete (RPC) Overlay. Reinforced concrete beams of length 1200 mm, width 100 mm and depth 200 mm were casted with M30 grade of concrete in the laboratory and cured for 28 days. One beam is taken as control and are tested under two point loading to find out ultimate load. Remaining beams are subjected to 90 % ultimate load of control beams. The partially damaged beams are retrofitted with Reactive Powder Concrete Overlay at the full tension face of the beam and side overlay depends upon the respectable retrofitting techniques with 10 mm and 20 mm thick layer to find optimum. Materials like steel fibres are added to enhance the ductility by eliminating coarse particle for homogeneity of the structure. Finally, the modes of failure for retrofitted beams are analysed experimentally under two point loading & compared the results with Control beam.

Full-range load-deflection response of FRP-strengthened RC flexural members anchored with FRP anchors

The flexural resistance of reinforced concrete (RC) members such as beams and slabs can be enhanced by bonding fibre-reinforced polymer (FRP) composite plates to their tension face. The effectiveness of the strengthening can be compromised though by premature debonding failure of the FRP which can occur at strains significantly lower than the strain capacity of the FRP. Anchorage of the FRP strengthening can, however, increase its usable strain. A convenient means with which to represent the structural behaviour of such a strengthened and anchored member then is via its load-deflection response. The task of how to model this response therefore arises. The authors, as well as other research groups, have derived closed-form solutions which describe the complete load-deflection response of FRP-strengthened RC flexural members of which pre-defined moment-curvature relationships are observed. This paper reports an extension to such existing theory in order to incorporate the anchorage effect. The theory is finally calibrated with test results of FRP-strengthened RC slabs which have been anchored with FRP anchors. The paper sets a framework for future work centred on quantification of the FRP anchor effect.

Flexural behavior of RC beams strengthened with steel-FRCM composite

This paper presents the results of an experimental investigation of the flexural response of reinforced concrete (RC) beams strengthened using externally bonded steel fiber reinforced cementitious matrix (steel-FRCM) composite. Steel-FRCM composite strips were bonded to the tension face of four RC beams, which were tested in four-point bending. Parameters varied were the presence/absence of the external (coating) layer of the matrix, presence/absence of U-wrap anchorages, and loading rate. Results are compared with those from single-lap direct-shear tests conducted on the same composite. The direct-shear tests showed that debonding of steel-FRCM joints is characterized by fiber slippage and fracture of the matrix layer at the internal matrix layer-fiber interface. In the beam tests, the strengthening system increased the yield load by 15–21% relative to the unstrengthened beam. The ratio of the load at which debonding occurred to the load at yielding ranged from 1.11 to 1.19 for each strengthened beam. The load rates employed and the presence of the external matrix layer did not appear to significantly affect the failure mode or the load and midspan displacement at debonding. The presence of the U-wraps helped restrain the peel-off of the composite observed in strengthened beams without the U-wrap, however, they did not restrain the fiber slippage at the ends of the composite, which inhibited composite action. Average values of the maximum fiber strain at composite debonding determined using strain profiles from strain gages, an approximate method, and moment-curvature analysis were 0.54%, 0.73%, and 0.83%, respectively.

Cyclic Behavior of Lap Splices Strengthened with Ultrahigh Performance Fiber-Reinforced Concrete

The cyclic behavior of six full-scale reinforced concrete (RC) beams with a deficient lap splice strengthened with ultra-high-performance fiber-reinforced concrete (UHPFRC) is experimentally investigated. The experimental program is based on the findings of previous test series carried out in the same research program that demonstrated the ability of UHPFRC to eliminate bond failure in deficient lap splices of beams and wall-type bridge columns. The objective of this experimental work is to determine the efficiency of this strengthening technique on wide flexural elements (beams, slabs, walls, or wall columns) subjected to reverse cyclic loading. Specimen reinforcement consists of two pairs of deformed bars spliced at midspan on both tension and compression faces. The strengthening technique consists of replacing normal concrete around lapped bars in the splice region by UHPFRC, which allows for keeping the original member geometry. One type of fiber, three fiber contents, two bar diameters, and two bar arrangements are considered. For isolating the UHPFRC contribution, the splice regions are free of any confinement. The beam specimens are tested at four points, bending with a constant-moment region along the splice length. The result indicates that UHPFRC with a fiber content of 2 or 3% can significantly increase the bond strength of splice bars without confinement. The levels of ductility reached for the highest fiber content meet the requirements for high ductility demand, such as in seismic design. The results demonstrate that an appropriate casting method combined with a self-compacting UHPFRC with an appropriate fiber content ensure the efficiency of the strengthening technique for providing for the continuity of lapped bars and for enabling a high ductility capacity under monotonic or cyclic loading. The results also confirm the applicability of the method for strengthening lap-spliced regions of wide elements—such as slabs, shear walls, and wall-bridge piers—without having to provide any confinement.