Athanasios I. Karabinis

Concrete confined by FRP material: a plasticity approach

Carbon fiber-reinforced polymer (FRP) material has proved to be more efficient than other composites when applied to concrete columns as an external reinforcement. Because of its enhanced durability characteristics compared to glass or aramid, and its relatively high E-modulus, carbon FRP shows a higher confining performance. The behavior of 22 cylindrical 200×320 mm specimens (height to diameter ratio 1.6) that are externally wrapped by carbon FRP sheets in low volumetric ratios (0.23–0.7%) is presented. The specimens are subjected to axial monotonic load until failure occurs. Carbon FRP confinement, even in low volumetric ratios, seems to considerably increase the strength and especially the ductility of concrete. A constitutive model based on plasticity theory is applied. The model proposed and applied to steel-confined concrete in this paper is modified and calibrated so as to incorporate the dilation characteristics of the FRP-confined concrete. The model provides the stress–strain curves of axially loaded circular columns that are confined by FRP reinforcement. It can be used in FRP tube-encased concrete as well as in FRP sheet-wrapped concrete. Satisfactory correlation of experimental and analytical results is observed.

Design-Oriented Strength Model for FRP-Confined Concrete Members

AbstractThis study concerns assessing 20 existing models for predicting the compressive strength of concrete uniformly confined externally with composite materials. An extended database of 471 experiments on cylindrical concrete columns is utilized for the comparisons. The research classifies the experimental data in three distinguished subcategories according to the information available for the mechanical properties of the FRPs to investigate their effect on the divergences of the models. Apart from the use of the tensile strength obtained from coupon tests, the use of properties from the manufacturer data on FRP fibers can lead to minimum error of predicted strength of concrete externally wrapped or encased in FRP tubes. The study results in the proposal of an upgraded empirical model that indirectly encompasses the unique deformational characteristics of different Young’s moduli of FRP sheets and tubes. The model utilizes the strong linear dependence of the product of the varying effective strain at f...

Axially Loaded Reinforced Concrete Columns with a Square Section Partially Confined by Light GFRP Straps

The study presents the experimental behavior of reinforced concrete columns strengthened externally by transverse glass fiber-reinforced polymer (GFRP) sheets. The columns had a square section and very low concrete strength. The internal steel reinforcement comprised sparse stirrups and longitudinal bars with two different qualities, S220 or B500C. The specimens were lightly confined by full wraps or partial GFRP straps. The straps were placed in between the existing steel stirrups. Three different widths of GFRP straps were examined. A series of plain concrete columns with identical GFRP strengthening was designed in order to assess the partial wrapping effects and compare them with reinforced concrete columns. The columns were subjected to axial compressive monotonic load up to failure. The achieved strength and ductility levels of the strengthened columns suggest that partial wrapping can efficiently upgrade the mechanical behavior of columns with a square section. Fully or partially GFRP wrapped columns with S220 slender bars require relatively higher GFRP strengthening than their counterparts with B500C columns to achieve similar ultimate strains.

A computational approach for the seismic damage response under multiple earthquakes excitations of adjacent RC structures strengthened by ties

Civil Engineering systems of adjacent reinforced concrete (RC) structures, which have been environmentally degraded, are considered in order to be seismically strengthened by cable-elements (ties). A numerical approach for estimating the effects of pounding (seismic interaction) on the response of such adjacent structures under multiple earthquakes excitation is presented. For the system of the governing partial differential equations (PDE) a double discretization, in space by the Finite Element Method and in time by a direct incremental approach, is used. The unilateral behaviors of both, the cable-elements and the interfaces contact-constraints, are strictly taken into account and result to inequality constitutive conditions. So, in each time-step, a non-convex linear complementarity problem is solved. The decision for the optimal cable-strengthening scheme is obtained on the basis of computed damage indices. It is found that pounding and cable strengthening have significant effects on the earthquake response and, hence, on the seismic upgrading of existing adjacent RC structures.

EXPERIMENTAL AND NUMERICAL RESPONSE OF CFRP-STRENGTHENED CONCRETE BEAMS WITH LOW OR MEDIUM STEEL CORROSION

The present study deals with the validation of a numerical model that can approximate the flexural behavior of corroded reinforced concrete beams patch repaired and strengthened with carbon fiber reinforced polymer (CFRP). Six reinforced concrete beams were corroded under an accelerated corrosion technique to low and medium corrosion levels respectively. Two beams were used as control, while the remaining four were patch repaired and strengthened with externally bonded reinforcement (EBR) and near surface mounted reinforcement (NSM) with equivalent axial rigidity. After repairing the concrete substrate and subsequent bonding of the CFRP reinforcement, the beams were tested to failure under fourpoint loading. The developed 3D finite element models take into account the different mechanical properties and detailing of corroded bars and of the mortar patch as well as of the existing concrete – mortar patch interface and of the NSM or EBR FRP – mortar patch interface. The FE analyses may reproduce the observed modes of failure including crushing of the concrete after steel yielding and debonding of CFRP. The analytical load-deflection curves compare well with the experimental flexural behavior of the corroded RC beams patch repaired and strengthened with EBR and NSM CFRP laminates. 343 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 343-352

Interface Capacity of Repaired Concrete Columns Strengthened with RC Jackets

Abstract The study describes the retrofit of repaired elements by reinforced concrete (RC) jacketing conducted to quantify the influence of initial construction deficiencies and of different type of anchors to the ability of the interface to transfer loads. Sixteen specimens (section scale 1:2) were designed with variables the initial deficiencies and the confinement ratio. The results indicate that: a) the maximum resistance load and dissipated energy of initially damaged specimens are decreased; b) surpassing a specific amount of damage, columns even suitably repaired present lower strain capacity, c) welded bars lead to buckling of longitudinal bars.

PROPOSED MODEL OF PREDICTING THE REDUCED YIELD AXIAL LOAD OF REINFORCED CONCRETE COLUMNS DUE TO CASTING DEFICIENCY EFFECT

Abstract The study deals with the investigation of the effect of casting deficiencies- both experimentally and analytically on axial yield load or reinforced concrete columns. It includes 6 specimens of square section (150x150x500 mm) of 24.37 MPa nominal concrete strength with 4 longitudinal steel bars of 8 mm (500 MPa nominal strength) with confinement ratio ωc=0.15. Through casting procedure the necessary provisions defined by International Standards were not applied strictly in order to create construction deficiencies. These deficiencies are quantified geometrically without the use of expensive and expertise non-destructive methods and their effect on the axial load capacity of the concrete columns is calibrated trough a novel and simplified prediction model extracted by an experimental and analytical investigation that included 6 specimens. It is concluded that: a) even with suitable repair, load reduction up to 22% is the outcome of the initial construction damage presence, b) the lower dispersion is noted for the section damage index proposed, c) extended damage alters the failure mode to brittle accompanied with longitudinal bars buckling, d) the proposed model presents more than satisfying results to the load capacity prediction of repaired columns.