Effect of eccentric compression loading on the strains of FRCM confined concrete columns
Introduction
In the past years, many papers have been published to demonstrate that the load carrying capacity and ductility of compressed concrete elements can be effectively increased through the use of various types of FRP composite strengthenings. The majority of these studies are concentrated on the behaviour of FRP confined concrete columns under axial load, e.g. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The load carrying capacity of the FRP confined RC columns mainly depends on the type of fibres, kind of the matrix, the number of strengthening layers, the shape of the cross section, the concrete strength to compression, the direction and orientation of fibres, the presence of a longitudinal steel reinforcement, as well as on the temperature at which the element operates. The impact of the eccentric and of the column slenderness on the effectiveness of FRP strengthenings does not remain insignificant. The description of the impact of these parameters appears in a significantly lower number of papers, e.g. [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. For the strengthenings of compressed elements under the FRCM system, there is no sufficient knowledge on this subject matter, except for a few papers [26], [27], [28], [29], [30], [31], [32], [33]. The experimental results of bond tests on FRCM materials for the external strengthening of RC elements have been presented in the papers [34], [35], [36]. Presented below is the subject matter of several publications related to eccentrically compressed FRP confined concrete columns.
In their paper, Hadi and Le [12] studied the impact of the orientation of CFRP (Carbon Fibre Reinforced Polymers) fibres on the behaviour of axially and eccentrically compressed hollow core columns. The testing was performed in four groups of elements that were subjected to axial compression and that on the eccentrics: 25 and 50 mm. The elements from the first group remained without strengthening, as reference elements. The elements from the second group were wrapped in three layers of a CFRP sheet with a horizontal direction of fibres. In the third group one longitudinal and two horizontal sheet layers were used. The elements from the last group were wrapped in two layers of the sheet oriented at ±45° and in one horizontal sheet layer. The authors noticed that for all the three types of strengthening the efficiency of hollow core square columns increased. The improvement in ductility was more noticeable than the increment of strength for all the types of strengthening. The increase in strength was marginal. Columns that were wrapped solely horizontally turned out to have the highest ductility. This is to confirm the previous conclusions made by Kusumawardaningsih and Hadi [13] from the testing in which the authors analysed the efficiency of the CFRP strengthening of hollow core square columns.
El Maaddawy et al. [14] conducted an interesting testing of the impact of the cross section shape on the effectiveness of the strengthening of reinforced concrete (RC) members confined with CFRP. Round, square and rectangular elements, with two different proportions, were subjected to axial and eccentric compression with the indicator: e/h = 0.46 and e/h = 0.60. As expected, the authors noticed that the load carrying capacity and the ductility of axially loaded elements depends on the shape of the cross-section. The increase in load carrying capacity and axial deformation is higher than that for rectangular elements. The CFRP strengthening also improves the efficiency of eccentrically compressed elements, but yet to a considerably lower degree. The authors noticed that the shape of the cross-section had a low impact on the ductility of the eccentrically loaded members. Rectangular sections had a lower ductility as compared to round and square sections.
The majority of the testing on FRP confined concrete elements was performed on thickset elements to a low scale, without the impact of slenderness and the effects of additional bending. However, in the practice, it is the bending moment M and its interaction with compression force P that is a decisive factor for the load carrying capacity of eccentrically compressed columns. Bisby and Ranger [15] compared the efficiency of unconfined RC elements and FRP confined RC elements under axial, eccentric load and those subjected to bending. The aim of the testing was to describe axial and hoop strains and to create P–M interaction diagrams for FRP confined RC columns. The authors noticed that too low a number of horizontal measuring points on the FRP jacket does not provide a correct description of destructive strains. Similar conclusions were set forth in a later paper by Wu and Jiang [16] that a tear of the FRP jacket follows in a random section in which hoop strains are not measured. On the grounds of the analysis of the ratio of axial strains to hoop strains, Bisby and Ranger found that eccentrically compressed elements are capable to transfer considerably higher compression strains on the side of force action than axially compressed elements.
Rahai and Akbarpour were dealing with biaxial, eccentric compression [17]. They made columns, to a natural scale, which they strengthened with 1–4 CFRP sheet layers in a layout 0° (crosswise), 90° (lengthwise) and at ±45° (at an angle to the column’s longitudinal axis). All the columns were subjected to a bidirectional eccentric compression. The strengthened elements showed an increase in the moment capacity and ductility, and a higher degree of energy dispersion at destruction. The rise in the moment capacity was due to the presence of the longitudinal CFRP strengthening. This strengthening produced a rise in the longitudinal stiffness of columns and a reduction in their curvature.
The analysis of the impact of slenderness on the behaviour of FRP confined concrete elements can be found, among others, in papers [18], [19]. El-Hacha and Abdelrahman [18] conducted the testing on slender columns made from concrete, wrapped in a SFRP (Steel Fibre Reinforced Polymers) steel mesh. In each of the three groups of columns, with a H/D (height to diameter) ratio equal to 2, 3 and 4, each three columns were strengthened while the next three columns were left as reference columns. All the elements were subjected to compression up to destruction. The testing demonstrated the effectiveness of the strengthening method through the wrapping in the SFRP steel mesh. The authors recorded a rise in load carrying capacity, an improvement in the deformability and ductility of strengthened columns as compared to the reference elements. The increase in the elements’ slenderness resulted in the decrease of these parameters. This indicates that the slenderness of columns affects the effectiveness of the strengthening. Similar conclusions were reached by Soliman [19], who tested slender concrete columns confined in plastic tubes. The results of his testing show that the slenderness of concrete columns confined in plastic tubes decreases with the rise in slenderness.
In my own paper [32], the initial results of the testing of reinforced concrete columns strengthened with FRCM (Fibre Reinforced Cementitious Matrix) composites subjected to eccentric compression have been presented. The measurements of horizontal displacements of columns show the capacities of those elements to considerable plastic deformations. In that paper, the results of the testing of limit load carrying capacities have been analysed. This paper has been dedicated to the analysis of the state of strain and the description of deformations of FRCM confined concrete columns subjected to eccentric compression.
Section snippets
Experimental research
15 slender reinforced concrete columns were made and tested, out of which 12 were strengthened with the FRCM mesh, and 3 were left as reference columns without strengthening. The columns were divided into three groups depending on the eccentric value – 0 mm (concentric load), h/12 and h/6. In each of the groups, there were four strengthened columns and one column without strengthening. In each group, two columns were wrapped in one or two layers of the FRCM mesh, whereas the next two columns
Experimental results and discussions
All the elements were being loaded until failure. During the loading, the value of the force N and that of longitudinal (vertical) strains εv and transverse (horizontal) strains εh were recorded. In Table 3 the values measured have been listed, corresponding to the highest load Nu. The values of strains εv from the strain gauges located in the compression plane were specified. The values on the more compressed side (on the side of the action of force) were designated as εv2 and those on the
Conclusion
The destruction of the eccentrically compressed FRCM confined concrete columns is signalised by a slow loss of adhesion of the PBO mesh on the final overlap length. The loss in the continuity of the FCRM strengthening leads to the crushing of strenuous concrete. The transverse strain of the BPO mesh at the destruction of the strengthening do not reach the value obtained when testing the composite specimens for tension [38].
The limit values of strains due to the compression of elements wrapped
Acknowledgments
Author greatly acknowledge the VISBUD-Projekt Sp. z o.o. (<http://www.visbud-projekt.pl>) for providing the strengthening material used in the experimental investigation.
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