Performance of Concentrically Loaded RC Wall-like Columns Upgraded with Innovative Hybrid NSM/CFRP System

Abstract

In RC (reinforced concrete) frame structures, wall-like columns are laid within the space occupied by masonry walls to maximize usable space and thus minimize the column projections into the usable area. These columns often require strengthening owing to various reasons, including increasing the number of stories, changes in building usage, and others. The use of a hybrid system comprising NSM (near-surface mounted) steel rebars combined with CFRP (carbon-fiber reinforced polymer) laminates may be considered a sound technique for strengthening such wall-like building columns. The prime aim of this study is to devise an efficient scheme using a hybrid NSM/CFRP system to strengthen existing RC wall-like columns. Six half-scale RC wall-like columns were prepared and tested under monotonic concentric axial compression. Two columns were unstrengthened to serve as control specimens (CW1 and CW2), and four specimens were strengthened using four different schemes (SW1, SW2, SW3, and SW4). As favored by architects, all strengthening schemes were designed so that the dimensions of the column cross-section were not increased. The effects of strengthening schemes on the enhancement of axial capacity, energy dissipated, and stiffness were evaluated to find the most efficient scheme. Among the four studied schemes, using vertical continuous NSM rebars in combination with the wrapping of the three CFRP layers onto the exterior column surface (in specimen SW2) was the most efficient as it enhanced the ultimate load capacity by 80%. Three-dimensional FE (finite element) analysis was also conducted to predict the response of test specimens. The test results matched well with the FE outputs, which justified the accuracy of the used constitutive models for concrete, steel rebars, and CFRP sheets.

Behavior of FRP-Retrofitted Wall-like RC Columns after Preloading to Simulate In-Service Conditions

“…It should be noted that the experimental campaign in this study is an extension of the previous authors' research covered in Refs. [24,25]. The test matrix used in this study is shown in Table 1.…”

“…The design of the control and strengthened specimens are detailed in Refs. [24,25]. As clarified in Table 1, the test matrix included six casted columns having eight concentric compression tests.…”

“…As clarified in Table 1, the test matrix included six casted columns having eight concentric compression tests. Out of the six casted columns, two specimens (having four tests) were cast in this study, whereas the other four specimens were cast and tested by the authors in previous studies [24,25]. In the designation of columns, the symbols "CON" and "ST" refer to control and strengthened specimens, respectively.…”

“…The majority of research in the field of FRP-strengthened RC rectangular columns subjected to concentric loading has focused on sections with depth-to-width ratios of up to 2. There have been only a limited number of studies [14][15][16][17][18][19][20][21][22][23][24][25] that specifically addressed the retrofitting of wall-like columns with the help of FRP composites. Tan [14] conducted experiments to investigate FRP-enhanced columns with an aspect ratio of 3.65.…”

“…The study explored different strengthening methods, encompassing anchored and unanchored CFRP wrapping, both without and with the modification of sections. Their findings led to the conclusion that the efficiency of CFRP confinement was nearly doubled when anchors were appropriately distributed Elsanadedy et al [24] conducted a study to come up with an efficient scheme, without section modification, for strengthening existing RC wall-like columns employing a hybrid of NSM and CFRP wrapping systems. Six half-scale specimens were subjected to concentric compression testing.…”