• Structural Engineering and Mechanics
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• v.88 no.5
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• pp.451-462
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• 2023

Lumped Damage Mechanics (LDM) is a theory proposed in the late eighties, which assumes that structural collapse may be analyzed as a two-phase phenomenon. In the first (pre-localization) stage, energy dissipation is a continuous process and it may be modelled by means of the classic versions of the theory of plasticity or Continuum Damage Mechanics (CDM). The second, post-localization, phase can be modelled assuming that energy dissipation is lumped in zones of zero volume: inelastic hinges, hinge lines or localization surfaces. This paper proposes a new LDM formulation for cracking in concrete structures in tension. It also describes its numerical implementation in conventional finite element programs. The results of three numerical simulations of experimental tests reported in the literature are presented. They correspond to plain and fiber-reinforced concrete specimens. A fourth simulation describes also the experimental results of a new test using the digital image correlation technique. These numerical simulations are also compared with the ones obtained using conventional Cohesive Fracture Mechanics (CFM). It is then shown that LDM conserves the advantages of both, CDM and CFM, while overcoming their drawbacks.

• Smart Structures and Systems
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• v.4 no.4
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• pp.391-406
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• 2008

This study attempts to develop a real-time debonding monitoring system for carbon fiber-reinforced polymer (CFRP) strengthened structures by continuously inspecting the bonding condition between the CFRP layer and the host structure. The uniqueness of this study is in developing a new concept and theoretical framework of nondestructive testing (NDT), in which debonding is detected without relying on previously-obtained baseline data. The proposed reference-free damage diagnosis is achieved based on the concept of time reversal acoustics (TRA). In TRA, an input signal at an excitation point can be reconstructed if the response signal measured at another point is reemitted to the original excitation point after being reversed in the time domain. Examining the deviation of the reconstructed signal from the known initial input signal allows instantaneous identification of damage without requiring a baseline signal representing the undamaged state for comparison. The concept of TRA has been extended to guided wave propagations within the CFRP-strengthened reinforced concrete (RC) beams to improve the detectibility of local debonding. Monotonic and fatigue load tests of large-scale CFRP-strengthened RC beams are conducted to demonstrate the potential of the proposed reference-free debonding monitoring system. Comparisons with an electro-mechanical impedance method and an inferred imaging technique are provided as well.

• Structural Engineering and Mechanics
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• v.39 no.2
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• pp.207-221
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• 2011

This paper aims to study the behavior of short reinforced concrete columns confined with external glass Fiber Reinforced Polymers (GFRP) sheets under eccentric loads. The experimental part of the study was achieved by testing 9 specimens under eccentric compression. Three eccentricity ratios corresponding to e/t = 0, 0.10, 0.50 in one direction of the column were used. Specimens were divided into three groups. The first group was the control one without confinement. The second group was fully wrapped with GFRP laminates before loading. The third group was wrapped under loading after reaching 75% of failure loads of the control specimens. The third group was investigated in order to represent the practical case of strengthening a loaded column with FRP laminates. All specimens were loaded until failure. The results show that GFRP laminates enhances both failure load and ductility response of eccentrically loaded column. Moreover, the study also illustrates the effect of confinement on the first crack load, lateral deformation, strain in reinforcement and failure pattern. Based on the analysis of the experimental results, a simple model has been proposed to predict the improvement of load carrying capacity under different eccentricity ratios. The predicted equation takes into consideration the eccentricity to cross section depth ratio, the ultimate strength of GFRP, the thickness of wrapping laminate, and the time of wrapping (before loading and under loading). A good correlation was obtained between experimental and analytical results.

• Advances in concrete construction
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• v.13 no.1
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• pp.35-43
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• 2022

The experimental investigations into hooked-end round steel fibers (HSF) effect on the age-dependent strengths of high volume fly ash (HVFA) concrete is studied. The concrete was prepared with class F fly ash used as partial cement replacement varied from 0% to 70% on an equal weight basis. Two percentages of HSF (i.e., 0.5% and 1.5% by volume fraction) of 50 mm length were added in plain, and 50% fly ash concrete mixes. The compressive and flexural tensile strength was determined at 7, 28, 56, and 90 days. The strength results of fly ash concrete mixes with and without steel fibers were compared with the plain concrete strength. The test results indicated that the strength of fly ash concrete is comparable with the plain concrete strength and further increases with an increase in the percentage of steel fibers. The maximum flexure strength of HVFA concrete is found with 0.5% steel fibers. It is concluded that the HVFA concrete with steel fibers of 50 mm length can effectively be used in concrete construction. The analytical models are proposed to predict the age-dependent compressive and flexural tensile strength of HVFA concrete with and without HSF. The compressive and tensile strength of HVFA concrete with HSF can be predicted using these models when the 28-day compressive strength of plain concrete is known. The present study will be helpful in the design and construction of reinforced and pre-stressed concrete structures made with HVFA and HSF.

• Steel and Composite Structures
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• v.19 no.5
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• pp.1077-1094
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• 2015

This paper presents an experimental study on the effectiveness of simultaneous application of carbon fiber-reinforced polymer (CFRP) and steel jacket in strengthening slender reinforced concrete (RC) column. The columns were 200 mm square cross section with lengths ranging from 1600 to 3000 mm. Ten columns were tested under axial load. The effects of the strengthening technique, slenderness ratio, cross-section area of steel angle and CFRP layer number were examined in terms of axial load-axial strain curve, CFRP strain, steel strip strain and steel angle strain. The experiments indicate that strengthening RC columns with combined CFRP and steel jacket is effective in enhancing the load capacity, ductility and energy dissipation capacity of RC column. Based on the existing models for RC columns strengthened with CFRP and with steel jacket, a design formula considering a slenderness reduction factor is proposed to predict the load capacity of the RC columns strengthened with combined CFRP and steel jacket. The predictions agree well with the experimental results.

• Structural Engineering and Mechanics
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• v.54 no.3
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• pp.521-543
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• 2015

Depending on the damage type as well as the level of damage observed after the earthquake, certain measures should be taken for the damaged buildings. In this study, structural repairing of two different types of damaged RC beam-column assembly by carbon fiber-reinforced polymer sheets is investigated in detail as a member repairing technique. Two types of 1:1 scale test specimens, which represent the exterior RC beam-column connection taken from inflection points of the frame, are utilized. The first specimen is designed according to the current Turkish Earthquake Code, whereas the second one represents a deficient RC beam-column assembly. Both of the specimens were subjected to cyclic quasistatic loading in the laboratory and different levels of structural damage were observed. The first specimen displayed a ductile response with the damage concentrated in the beam. However, in the second specimen, the beam-column joint was severely damaged while the rest of the members did not attain their capacities. Depending on the damage type of the specimens, the damaged members were repaired by CFRP wrapping with different configurations. After testing the repaired specimens, it is found that former capacities of the damaged members were mostly recovered by the application of CFRPs on the damaged members.

• Structural Engineering and Mechanics
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• v.61 no.1
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• pp.1-13
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• 2017

The effectiveness of the repair scheme for the damaged captive-columns with CFRPs (Carbon Fiber Reinforced Polymer) was investigated in terms of response quantities such as strength, ductility, dissipated energy and stiffness degradation. Two 1/3 scale, one-story one-bay RC (Reinforced Concrete) frames were designed to represent the substandard RC buildings in Turkish building stock. The first one, which is the reference specimen, is the bare frame without infill wall. Partial infill wall with opening was constructed between the columns of the second frame and this caused captive column defect. Severe damage was observed with the concentration of shear cracks in the second specimen columns. Then, the damaged members were repaired by CFRP wrapping and retested. For the three test series, similar reversed cyclic lateral displacement under combined effect of axial load was applied to the top of the columns. Overall response of the bare frame was dominated by flexural cracks. Brittle type of shear failure in the column top ends was observed in the specimen with partial infill wall. It was observed that former capacity of damaged members of the second frame was recovered by the applied repair scheme. Moreover, ultimate displacement capacity of the damaged frame was improved considerably by CFRP wrapping.

• Earthquakes and Structures
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• v.24 no.2
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• pp.97-109
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• 2023

This experimental study investigates the effectiveness of applying carbon fiber reinforced polymer (CFRP) jackets for the retrofit of short reinforced concrete (RC) columns with inadequate transverse reinforcement and stirrup spacing to longitudinal rebar diameter equal to 12. RC columns scaled at 1/3, with round and square section, were subjected to axial compression up to failure. A damage scale is introduced for the assessment of the damage severity, which focusses on the extent of buckling of the longitudinal rebars. The damaged specimens were subsequently repaired with unidirectional CFRP jackets without any treatment of the buckled reinforcing bars and were finally re-tested to failure. Test results indicate that CFRP jackets may be effectively applied to rehabilitate RC columns (a) with inadequate transverse reinforcement constructed according to older practices so as to meet modern code requirements, and (b) with moderately buckled bars without the need of previously repairing the reinforcement bars, an application technique which may considerably facilitate the retrofit of earthquake damaged RC columns. Factors for the estimation of the reduced mechanical properties of the repaired specimens compared to the respective values for intact CFRP-jacketed specimens, in relation to the level of damage prior to retrofit, are proposed both for the compressive strength and the average modulus of elasticity. It was determined that the compressive strength of the retrofitted CFRP-jacketed columns is reduced by 90% to 65%, while the average modulus of elasticity is lower by 60% to 25% in respect to similar undamaged columns jacketed with the same layers of CFRP.

• Advances in concrete construction
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• v.8 no.4
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• pp.311-320
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• 2019

In this study, the effect of cellulose nanocrystals (CNCs) on the fire resistance properties of fiber-reinforced cement composites was investigated. The main variables were CNCs content (0.4, 0.8 and 1.2vol.% compared with cement), steel fiber ratio, and exposure temperature (100, 200, 400, 600 and 800℃). The fire resistance properties, i.e., residual compressive strength, flexural strength, and porosity, were evaluated in relation with the exposure temperature of the specimens. The CNCs suspensions were prepared to composited dispersion method of magnetic stirring and ultra-sonication. CNCs are effective for increasing the compressive strength at high temperatures but CNCs do not seem to have a significant effect on flexural reinforcement. Porosity test result showed CNCs reduce the non-hydration area inside the cement and promote hydration.

• Structural Engineering and Mechanics
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• v.82 no.3
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• pp.313-323
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• 2022

Steel laminated elastomeric bearings are commonly used in bridge structures to control displacements and rotations and transfer forces from the superstructure to the substructure. Proper knowledge of design, fabrication and erection procedures is important to ensure stability and adequate structural performance during the lifetime of the bridge. Difference in elevations sometimes leads to large size gaps between the bearing and the girder which makes the grout thickness that is commonly used for leveling deviate beyond standards. This paper investigates the structural response of High Strength Fiber Reinforced Cementitious (HSFRC) thin plinths that are used to close gaps between bearing pads and precast girders. An experimental program was developed for this purpose where HSFRC plinths of different size were cast and tested under vertical loads that simulate bridge loading in service. The structural performance of the plinths was closely monitored during testing, mainly crack propagation, vertical reaction and displacement. Analytically, the HSFRC plinth was analyzed using the beam on elastic foundation theory as the supporting elastomeric bearing pads are highly compressible. Closed form solutions were derived for induced displacement and forces and comparisons were made between analytical and experimental results. Finally, recommendations were made to facilitate the practical use of HSFRC plinths in bridge construction based on its enhanced load carrying capacity in shear and flexure.