Testing and modeling of seismically retrofitted RC columns with high axial load and shear demand

Abstract

here are many substandard building stock built in many parts of the world. Consequently, these buildings and structures face many catastrophic problems in seismically active regions such as Turkey, Greece, parts of Italy, etc.. The amount of these buildings plays a direct role in the amount of life loss following a devastating earthquake. Turkey is an active seismic country in which many substandard structures were built over the course of time. In previous years, many people were killed by occurrence of even some moderate (in rural areas) to major earthquakes. Had buildings constructed properly according to the authorized regulations and design codes, they would not have been collapsed. One of the most important deficiencies of such substandard buildings is lower concrete compressive strength than considered in the original design, causing the columns to be subjected to high axial loads approximately close to their axial load capacity. Poor curing conditions, high water-to-cement ratio, using inappropriate aggregate class, concrete segregation, poor vibration, etc. are some of the major reasons for poor quality and low compressive strength concrete. The second important issue is the light transverse reinforcement resulting in poor ductility due to poor confinement of concrete, and high shear demand leading to shear failure. Such columns with low concrete quality as well as largely spaced stirrups will be vulnerable in terms of axial failure and partial or full structural collapse. These undesired brittle failure modes would be amplified and worsened when columns bear high axial loads with respect to their axial load capacity and possess relatively high shear demand, simultaneously. The other issues are the utilization of plain reinforcements with improper detailing such as 90 degree-hook, insufficient lap splices, and deficiency of many seismic details which are mandatory nowadays in seismic design codes. As a result of the abovementioned poor concrete quality and improper reinforcement detailing, ductility of such deficient structures is not sufficient, and they are not able to satisfy the specified rotational or displacement limits predefined by design codes for related target performance levels. Thus, displacement demand is not being satisfied and the lateral load capacity of the structure decreases suddenly during a possible ground motion. Retrofitting, when properly implemented, is one of the promised methods to improve the seismic behavior of deficient structures. One of the most well-known methods to retrofit substandard RC buildings is wrapping columns with Carbon Fiber Reinforced Polymer (CFRP) for ductility improvement. Many researchers investigated the efficiency of this material in behavior of structural elements. While CFRP is widely recognized for its effectiveness in retrofitting RC members, there are certain drawbacks associated with its utilization. One key limitation is its susceptibility to high temperatures and fire. CFRP materials can degrade and lose their strength when exposed to elevated temperatures. Additionally, the adhesion between CFRP and concrete interface can be affected by factors such as moisture and long-term environmental exposure, leading to a reduced effectiveness over time. Furthermore, the cost of CFRP materials and the specialized installation techniques required can be relatively high (when compared to conventional jacketing methods), potentially posing financial challenges for retrofit projects. Finally, the aesthetic impact of CFRP application on the appearance of structures might not always align with the original architectural design, making it necessary to carefully consider the visual aspects of retrofitting. Also, damage evaluation and assessment of members would not be convenient after the occurrence of an earthquake by investigation of the crack propagation and crack width. Nowadays, a new retrofitting method, namely fiber-reinforced cementitious mortar (FRCM) jacketing, is being implemented by the scientific community. In retrofitting with FRP jackets, epoxy resin (organic resins) is used as the matrix to bind and impregnate fabrics. However, in retrofitting with FRCM, mortar (inorganic matrix) is used as the binder. Then, any mesh types or grids like carbon, glass, etc are buried inside the matrix. In this thesis, a novel retrofitting technique is proposed to dispose of the abovementioned drawbacks. Specifically designed substandard reinforced concrete (RC) columns were retrofitted by means of a novel composite material glass fiber reinforced mortar (GFRM) with and without basalt mesh. In the framework of this thesis, there were in total 8 RC specimens; four of them had a cross-sectional aspect ratio of 1.0 (square), where the other four used a cross-sectional aspect ratio of 2.0 (rectangular). Each group consisted of one substandard specimen, one 1975-Turkish-SeismicDesign-Code-compliant specimen, one GFRM retrofitted of substandard specimen, and one basalt-buried-GFRM retrofitted of substandard specimen. All columns were tested under constant axial load and reversed cyclic lateral displacement excursions. Crack propagation, failure mode, strain profiles of longitudinal bars, lateral loaddisplacement hysteretic curves, lateral load-displacement backbone envelope curves, energy dissipation capacity, ductility, stiffness, residual displacement, etc. were evaluated in the experimental program. Afterward, according to the observed behavior and captured experimental data, a numerical model developed in OpenSees is proposed. The proposed model successfully reproduced the load-displacement hysteretic curves of all specimens considering the buckling of the longitudinal rebars, extra rotation through moment rotation spring as a result of the longitudinal rebar slip due to strain penetration effects in the column-foundation interface. Also, for most of the specimens the stress-strain behavior in local level was in agreement with the experimental data. After verification of the proposed numerical model with the experimental data, a thorough parametric study is conducted to contain a vast majority of substandard buildings' properties. In this parametric work, different concrete compressive strengths, volumetric transverse reinforcement ratios, longitudinal reinforcement area to gross cross-section ratios, and axial load levels representing old/aged and new buildings are considered. In conclusion, retrofitting substandard reinforced concrete columns with novel GFRM composite material stands as a comprehensive strategy. It elevates structural performance, bolsters seismic resilience, and improves displacement ductility and energy dissipation capability. Also, in comparison to CFRP its cost-efficiency, and convenient applicability are crystal clear.