ACI Structural Journal/July-August 2008 1 ACI Structural Journal, V. 105, No. 4, July-August 2008. MS No. S-2007-051.R1 received February 8, 2007, and reviewed under Institute publication policies. Copyright © 2008, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the May- June 2009 ACI Structural Journal if the discussion is received by January 1, 2009. ACI STRUCTURAL JOURNAL TECHNICAL PAPER Following previous studies, the authors describe an experimental study performed to analyze the influence of the interface treatment on the seismic behavior of columns strengthened by reinforced concrete (RC) jacketing to increase their ultimate bending moment. A numerical study, subsequently conducted to further investigate this subject, is also presented. It has been concluded that, for undamaged columns with a bending moment/shear force ratio greater than 1.0, it is not necessary to consider any type of interface treatment before casting a RC jacket with a thickness less than 17.5% of the column width to obtain a monolithic behavior of the composite element. Keywords: cyclic loading; interface; jacketing; seismic response; strengthening; surface preparation. INTRODUCTION Reinforced concrete (RC) jacketing is a strengthening technique most frequently used in seismic retrofitting. 1 It has been widely used after earthquakes in Mexico, Japan, the Balkans, and the U.S. 2-4 To optimize the structural performance of the composite element, it is essential to ensure its monolithic behavior, which implies ensuring total adherence between the original column and the added jacket. To fulfill this objective, the current practice consists of increasing the roughness of the interface surface, applying a bonding agent and, in some cases, steel connectors. Due to the reduced thickness of the jacket, a self-consolidating high-strength grout is usually adopted. In all published experimental studies on this subject, the preparation of the column surface before jacketing is always referred to. 3,5-9 Nevertheless, a quantitative analysis of its influence is never reported. The authors decided to perform experimental studies to quantify the influence of four parameters on the bond strength between concretes with different ages and different characteristics. The parameters considered were: 1) the roughness of the interface surface; 2) the use of a bonding agent; 3) the added concrete mixture; and 4) the application of steel connectors. Slant shear tests and pushoff tests were adopted to determine bond strength in shear and pulloff tests were used to assess bond strength in tension. It was concluded that: 1) between those adopted, sandblasting is the best roughness treatment 10 ; 2) the use of epoxy resins does not improve the interface strength if sandblasting is used 11 ; 3) adding a high-strength concrete (HSC) increases the inter- face strength 12 ; and 4) the use of steel connectors does not significantly increase the interface debonding stress, although, after that, the shear stress is highly dependent on the relation between the cross section area of steel connectors and the area of the interface. 13 Afterward, the authors conducted monotonic tests on seven column-footing models. 14 These columns were strengthened by RC jacketing after the interface surface had been prepared according to the conclusions drawn from the results of the experimental studies previously referred to. It was concluded that, for current undamaged columns subjected to bending moment/shear force ratios greater than 1.0 m (3.281 ft), a monolithic behavior of the composite element can be achieved, even without increasing their surface roughness or using bonding agents or applying steel connectors, before strengthening by adding an RC jacket with a thickness lesser than 17.5% of the column width. In spite of that, it should be noted that, for other conditions such as cyclic loading, RC short columns, or thicker jackets, these conclusions may not apply. The experimental study presented in this paper adds relevant information to the previous research conducted by the authors, defining the response of the considered strengthened columns to cyclic loading. In fact, this is particularly important to predict if, for seismic loading, the jacketed column still shows a monolithic behavior. The numerical simulation, conducted after the experimental study and also presented in this paper, tests the hypothesis that, for short columns, debonding of the jacket may occur. RESEARCH SIGNIFICANCE The research presented in this paper has proven that, for the conditions considered (undamaged reinforced concrete columns with a bending moment/shear force ratio greater than 1.0), it is not necessary to previously prepare the interface surface, namely, increasing its roughness, applying a bonding agent, or eventually steel connectors, before casting RC jacketing with a thickness less than 17.5% of the column width to achieve a monolithic behavior of the composite element subjected to cyclic loading. This achievement leads to: a) significant savings in expensive materials, such as epoxy-based bonding agents; b) significant savings in time-consuming operations (for example, the application of steel connectors); and c) avoidance of the use inadequate tools to increase the roughness of the column surface, such as jackhammers, that promote microcracking of the concrete substrate. EXPERIMENTAL INVESTIGATION Seven column-footing models were built using concrete with approximately 35 MPa (5076 psi) nominal compressive strength (Table 1) measured with cubic specimens at 28 days and steel with 520 MPa (75,420 psi) nominal yielding stress. Title no. 105-S45 Reinforced Concrete Jacketing—Interface Influence on Cyclic Loading Response by Eduardo N. B. S. Júlio and Fernando A. B. Branco
Reinforced Concrete Jacketing—Interface Influence on Cyclic Loading Response
May 10, 2023
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