Effects of confinement level, cross-section shape and corner radius on the cyclic behavior of CFRCM confined concrete columns
Introduction
The use of Fiber Reinforced Polymer (FRP) composites as wraps or jackets for upgrading existing reinforced concrete columns has become increasingly popular in recent years. This jacketing technique has been developing rapidly as it makes it possible to obtain a large increment of the deformation capacity of the concrete in critical regions [1], where large plastic deformations are expected, increase in concrete strength and reduction in loss of stiffness and strength due to cyclic action, protection and prevention of corrosion, with minimal change in the weight and geometry of the structure.
However, some drawbacks occur when organic resins are used to bind the fibers: for example, inapplicability on wet surfaces, lack of vapor permeability, high costs of epoxy resin and of specialized workers for application, inapplicability at temperatures of less than 10 °C or more than 30 °C, sensitivity to elevated or low temperature. Epoxy resin degrades quickly when temperature grows, releasing toxic fumes; fiber embedded in matrix resin system undergoes liquid transition at very high temperatures, resulting in a weak bond between concrete and external reinforcement.
To solve these drawbacks, inorganic matrix systems have been introduced as a replacement for the organic matrix systems [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Unfortunately, as a consequence of the granularity of the mortar, penetration and impregnation of fiber sheets is very difficult to achieve since mortars cannot wet individual fibers. However, the use of fiber textile, in conjunction with the development of new materials for the fiber [8], [9], [11] are able to ensure an adequate bond between textile and matrix.
Despite a lower tight interaction, the inorganic matrix has a number of advantages, such as full compatibility with the concrete substrate, the same resistance to fire, applicability on wet surfaces and low costs of application (easy application even on rough or irregular surfaces) and does not requires qualified workmen, and is a sustainable and reversible system.
During the past decade, extensive research has been conducted on the behavior of FRP confined concrete columns, but fewer studies have been concerned with the use of the inorganic matrix composite system as a confinement technique.
Researches on the use of textile for reinforcement of concrete element started in the 1980s [12], while in the late 1990s fervent researches began on the use of fiber embedded in inorganic matrix for design and strengthening of concrete elements [2], [13]. The first studies regarding the use of carbon fiber sheet embedded in inorganic matrix for confinement of concrete element reported in the international literature is that of Wu and Teng [3], where small cylinders were tested, and the inorganic matrix was found to ensure the same behavior, in terms of strength and deformability increment, as the traditional epoxy resin. Triantafillou et al. [4] investigated the use of Textile Reinforced Matrix (TRM) jacketing as an alternative to FRP jacketing for confinement of concrete elements. They used carbon fiber textile, and investigated the influence on confinement efficiency of the use of inorganic mortar versus epoxy resin and of the strength of the inorganic mortar, and the role of the number of textile layers. The experimental test proved that TRM jacketing was effective in providing a strength and deformability increment, with reduced efficiency with respect to FRP jacketing of about 80% for strength and 50% for ultimate strain, with the exception of square columns, where almost the same efficiency was found. Both the strength of the mortar and the number of textile layers were found to have great influence on the performance of the confining system. Moreover, it was found that failure of TRM was less abrupt compared with that of its resin impregnated counterparts, due to slowly progressing fracture of individual fiber bundles. In a subsequent paper, Bournas et al. [5] investigated the effectiveness of the same confining system in providing strength and deformability increases to reinforced concrete columns prone to buckling of longitudinal reinforcement, due to poor detailing of the stirrups. They found TRM confining jackets approximately 10% less effective than FRP ones when small-scale specimens subjected to an axial load were considered, and more effective as the number of textile layers grows, while no reduction in effectiveness was found in tests on nearly full-scale columns under cyclic flexure. Ombres [6] evaluated the effectiveness of Carbon Fiber Reinforced Cement Mortar (CFRCM) in confining columns with circular, square and rectangular cross-sections under axial compressive loads with variation in the amount of fiber reinforcement and at temperatures ranging from 25 °C to 90 °C. They found that the constitutive behavior of the confined concrete changes with variation in the number of confining layers of CFRCM, ranging from strain softening behavior for a low amount of fiber layers to strain hardening for heavily confined specimens; the increase in the temperature produced a reduction in the strength of the confined elements, while the influence on the ultimate strain value was negligible. De Caso y Basolo et al. [7] explored different types of grouts as inorganic matrices along different types of fiber architecture (unidirectional and bidirectional meshes of low-density and high-density glass fiber sheet) and three types of grout, namely acrylic-modified Portland cement based matrix, hydraulic cement based matrix with high water retention, and a single-component magnesium-phosphate-based matrix. They found that the use of low-density glass fiber allowed for more effective fiber impregnation, and was the most effective reinforcement with both types of acrylic and hydraulic cementitious matrix, with higher levels of ultimate strength reached with the hydraulic base grout and similar levels of ultimate strain. Colajanni et al. [8], [9] tested small circular and square cross-section specimens made of low strength concrete subjected to monotonic uniaxial compression to investigate the efficiency of a Polypara-phenylene-benzo-bisthiazole (PBO) Fiber Reinforced Cementitious Mortars (FRCM) system. The experimental results showed that confinement with PBO fiber produced a noticeable increment in strength and ductility, though the low mechanical ratios of fiber considered were not always able to ensure hardening behavior up to rupture. Trapko [11] investigated the efficiency of PBO fiber bidirectional warped mesh, with four times as many fibers in the direction of the warp as in the direction of the weft, embedded in an inorganic matrix in confining concrete cylinders. He investigated the feasibility and the performance of multilayer jacketing, and the influence of external temperature, by comparison with CFRP embedded in epoxy resin, also testing specimens exposed to temperatures ranging from 30 °C to 180 °C. He stressed that FRCM could give an even better strengthening effect than CFRP, since the failure modes are quite different. CFRP specimens failed by sharp rupture of the jacketing, and the higher the number of sheet layers, the more the concrete was crushed. The damage pattern in PBO confined elements was completely different, having initiated at the external pleat, which was 70 mm for each specimen, and the damage propagated slowly. With an increase in the number of FRCM layers the performance of the confining system increased, though the author concluded that there is a limit thickness of composite jacket beyond which composite strengthening ceases to have positive effects; moreover, for FRCM heat treatment had little effect on intensity of strengthening.
It has to be emphasized that in the literature there is still a lack of knowledge on many aspects of FRCM confined concrete that have been clarified for FRP jacketing, such as the role of the roundness of the corner, as well as the effect of cyclic load in deteriorating the strength, deformability and toughness provided by the confining system.
In this context, this paper reports the main results of an experimental research aiming to investigate the behavior of medium-size low-strength concrete columns wrapped with Carbon Fiber Reinforced Cementitious Matrix (CFRCM) under monotonic and cyclic compressive axial loads. Thirty columns with circular, square and rectangular cross-sections were tested under monotonic and cyclic axial loads to investigate the effect of the confinement level, the cross-section shape and the corner radius on the stiffness, strength, and ductility of CFRCM confined concrete columns under cyclic loads.
Section snippets
Plain concrete
The columns were cast with a concrete typical of buildings that require structural retrofitting (low-strength concrete). Portland cement (ASTM International Type I) was used. The cement:sand:gravel proportions in the concrete mixture were roughly 1:1.9:2.33 by weight and the water/cement ratio was 0.35. The maximum size of the coarse aggregate was 10 mm. In order to characterize the mechanical behavior of the concrete, six cylindrical specimens measuring 150 × 300 mm were prepared and tested in
Preparation of test specimens
In addition to investigating the effects of cyclic load, this experimental program included studies on the following column parameters: shape of cross-section; percentage of confinement fibers (confinement volumetric ratio); corner radius in the prismatic sections. All specimens were axially unreinforced since jacketing–reinforcement interaction was outside the scope of the present study.
To examine the aforementioned tests variables, the following were cast: 10 specimens with circular
Control specimens (unconfined columns)
The monotonic and cyclic stress–strain curves of the control specimens (without wrapping) are shown in Fig. 4. The dashed line curves represent axial strain evaluated as the average value deduced from the electronic gauges, and the solid line curves the values derived by means of the displacements recorded by the LVDT transducer.
The figure plots typical stress–strain curves for unconfined concrete specimens under axial compression characterized by an ascending branch up to the peak and a
Conclusions
Experimental tests were performed to investigate the effects of confinement level, cross-section shape and corner radius on the monotonic and cyclic behavior of concrete columns wrapped with Carbon Fiber Reinforced Cementitious Matrix (CFRCM). The results prove that:
- (a)
The stress–strain curve of CFRCM confined concrete columns under monotonic loading is always the same as the envelope curve of the cyclic stress–strain curve in the pre-peak branch; in the post-peak range when softening behavior is
Acknowledgements
This work has been carried out within the 2010–2013 Research Project “DPC-ReLUIS (Dipartimento Protezione Civile – Rete dei Laboratori Universitari di Ingegneria Sismica)”, AT 1, Task 1.1.2. The related financial support was greatly appreciated. The Author thanks Dott. G. Mantegazza, Technical Director of Ruredil S.P.A., for providing the materials for the experimental program.
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