Structural performance of RC columns retrofitted with steel-reinforced grout jackets under combined axial and lateral loading
Introduction
Most reinforced concrete (RC) buildings in southern Europe were built in the first half of the 20th century to carry only gravity loads, by implementing the allowable stress design philosophy which did not allow any control of the failure mode and the corresponding deformation capacity of the individual members [1]. Νon-uniform distribution of stiffness and/or mass along the height of the building, poor material quality, and insufficient reinforcement detailing, are some of the main deficiencies that substantially increase the vulnerability of the existing building inventory when exposed to natural hazards, such as earthquakes.
Inadequate confinement has proven detrimental for the integrity of old-type buildings under seismic excitations (Fig. 1). Cyclic inelastic deformation reversals have a severe impact on both strength and deformation capacity of structural members due to the degradation of mechanisms such as concrete in tension (diagonal tension failure) and steel in compression (longitudinal steel buckling) [2]. Typical RC column construction detailing practice till the early 1980s comprised longitudinal reinforcement with bar diameter ranging from 12 mm to 20 mm and open (i.e. anchored with 90° hooks in the ends) stirrups of 6 mm (rarely 8 mm) diameter placed at distances from 200 mm to 600 mm (Fig. 1). Due to the large unsupported length of the longitudinal bars, premature buckling is the anticipated mode of failure for these elements, limiting their compression strain capacity.
In case of old type detailing columns, the bar slenderness ratio s/Db (s is the stirrup spacing and Db is the bar diameter of the longitudinal reinforcement), which reflects the stability of compression reinforcing bars supported laterally by stirrups, is generally between 10 and 50 [1]. Based on the work of previous researchers [3], [4], [5], [6], for s/Db<6 and bars with significant strain hardening, axial load-carrying capacity is greatly enhanced beyond the yielding load, thus inelastic buckling of reinforcing steel bars is expected to occur. For s/Db>10, the compression reinforcing bars may undergo elastic buckling prior to yielding [7], [8].
In columns with sparse confinement reinforcement, sideways buckling is the usual form of compression reinforcement failure due to lateral shear distortion of the member in the plastic hinge regions (Fig. 1). An effective way to mitigate buckling of steel reinforcement in the regions of high compression strain demands is by wrapping the column ends externally. The provided confinement allows the concrete in compression to considerably increase its strain capacity. If the strain capacity of the confined concrete is higher than the critical strain at the onset of reinforcement buckling, redistribution between the compressed bars at incipient buckling and the encased concrete is possible, thereby postponing buckling to occur at a higher strain level [9], [10].
Various jacketing techniques have been developed to provide external confinement to substandard RC columns, including Fiber Reinforced Polymer (FRP) [e.g. [11], [12], [13], [14], [15]] and Textile Reinforced Mortar (TRM) jacketing [e.g. [16], [17], [18]], Post-Tensioned Metal Straps (PTMS) [e.g. 19], ferrocement jacketing [e.g. 20], welded wire mesh jacketing [e.g. 21], shape memory spirals [e.g. 22], fiber reinforced concrete jacketing [e.g. 23] and ultra high performance-fiber reinforced cementitious composites (UHP-FRCC) jacketing [e.g. [24], [25]]. The Steel-Reinforced Grout (SRG) jacketing was used in a pilot study in 2007 by Thermou and Pantazopoulou [26] for the seismic retrofitting of three pre-damaged 1:2 scaled columns with poor detailing. Two of the columns had failed in shear and the third one in a shear/buckling mixed mode of failure. The single-layered SRG jackets with density 1.85 cords/cm substantially modified the behavior of the retrofitted specimens by altering the modes of failure observed in the pre-damage state. The retrofitted specimens increased both their strength and deformation capacity. In a more recent study [27], SRG jacketing was applied to 1:2 scaled lightly reinforced columns which were susceptible to rebar buckling failure with the compression reinforcing bars losing their stability prior or close to yielding. Single-layered SRG jackets with textile density of 1 and 2 cords/cm managed to increase the compressive strain ductility by 100 % and thus delaying bar buckling and allowing the columns to improve their strength and strain capacity.
For the first time, this paper aims to study the effectiveness of SRG jacketing in delaying bar buckling of full-scale columns representative of the old construction practice when subjected to combined axial loading and cyclic lateral displacement reversals, simulating seismic loading. Three alternative SRG jacketing schemes were applied to three full-scale cantilever columns, whereas the fourth column served as the control specimen. Parameters of study were the density of the textile (1.57 and 4.72 cords/cm) and the number of layers (1 and 2). The test results demonstrated the efficiency of SRG jacketing at preventing the brittle failure mode and substantially improving the structural performance of old-type RC columns. Code formulations, which rely on the design philosophy of FRP design, were used to assess the strength and deformation capacity of the SRG jacketed columns.
Section snippets
Test specimens and parameters of investigation
Four identical reinforced concrete (RC) columns representative of the pre-1970s old-type detailing in southern Europe were tested under reversed cyclic loading simulating earthquake effects. The cantilever columns, constructed at full-scale, were typical building columns extending from column mid-height between floors to the beam-column connection. The columns were designed by following the provisions of the first Greek seismic code introduced in 1959 [28] and were all susceptible to
Damage assessment - failure mechanisms
In this section, the damage evolution of the control and SRG jacketed columns with increasing drift is discussed with the help of the Digital Image Correlation (DIC) technique.
Control column: Upon the application of lateral load and at drift level equal to 0.25 %, the first crack formed at the control column – footing connection. More flexural cracks distributed at length equal to 60 cm from the column-footing connection (8-10 cm distance between the cracks, Fig. 7) appeared as the drift
Assessing the performance of SRG jacketed columns using code formulations
The design philosophy presented in Chapter 8 of fib Bulletin 90 [8] for seismic retrofitting of existing substandard concrete structures using FRPs is implemented for the case of SRG jacketed columns tested herein to assess their strength and deformation capacity. The addition of SRG jackets to deficient RC columns aims to remove brittle failure modes, so that the flexural capacity may be fully developed and sustained up to a certain level of displacement ductility. For existing RC structures, μ
Conclusions
An experimental investigation was carried out to study the efficiency of SRG jackets in modifying the response of substandard, full scale cantilever RC columns subjected to combined axial loading and cyclic lateral displacement reversals. Four column specimens were constructed following the construction practice of the 1970 s in southern Europe with a bar slenderness ratio of s/Db = 15.6. All the columns were designed to be susceptible to sideways bar buckling, a typical mode of failure in
CRediT authorship contribution statement
G.E. Thermou: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft, Writing – review & editing. V.K. Papanikolaou: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft, Writing – review & editing. I. Hajirasouliha: Conceptualization, Validation, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The experimental program was conducted in the Laboratory of Reinforced Concrete and Masonry Structures, School of Civil Engineering, Aristotle University of Thessaloniki. Special thanks are attributed to Kerakoll S.p.A. for providing the Ultra High Tensile Strength Steel (UHTSS) textiles and mortar. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 700863.
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