BEHAVIOUR OF FERROCEMENT ENCASED CONCRETE COLUMNS

Abstract

1. Introduction: This thesis is a theoretical and experimental investigation on the behavior of ferrocement encased concrete columns. The main thrust is on assessing the increase in strength and ductility and stress - strain behaviour of ferrocement encased concrete columns due to passive confinement of concrete by ferrocement mesh wires. The analysis and experimental findings have established a basis for the design of such columns. Retrofitting of columns or repair of damaged/distressed columns may be carried out by means of ferrocement encasement. Hollow ferrocement casings are sometimes used as 'lost' formwork in partially prefabricated construction. In case of insitu construction, wire encasement may be provided around the vertical reinforcement and ties before concreting. In all cases, the wire mesh encasement resists the lateral expansion of the column core caused by the vertical compression. This introduces lateral confinement, and structural properties of concrete are modified. Confinement of concrete may be classified as passive or active. Passive confining pressures depend on longitudinal compression and develop due to the restraining influence of circular or square ties or spiral reinforcement, or even due to the encasement by pipes or external hoops. On the other hand active confinement pressures are independant of longitudinal compression, and are applied by external hydraulic pressure or mechanical means such as prestressing wires etc. 2. General Review: A number of investigations have been carried out on concrete passively confined by hoops and ties or spiral reinforcement. Some studies have even investigated the role of longitudinal reinforcement in providing confinement. Influence of low and high strain rates have been studied. Medium and high grade as well as lightweight concretes have been investigated. Confinement of concrete by steel pipes or plastic pipes is reported in literature. Other studies have utilized active confining pressures. The findings have confirmed that both strength and ductility increase with an increase in confining pressures. d Most of these investigations either pertain to columns or to studying stress-strain behaviour of concrete under multi-axial states of stress. The latter information is required for non-linear stress analysis of (ii) some concrete structures by numerical methods, such as the finite element method. The increase in ductility of columns, due to confinement, is considered in earthquake resistant design. However, the increased strength is generally not given importance in design. In case of spirally reinforced columns some codes permit a five percent increase in strength of columns due to confinement of the core. Part of the reason, for not considering a larger strength increase is because the cover concrete spalls off before the strength increase can occur. Also in case of circular or square ties, the sections midway between the ties are ineffectively confined. However, in ferrocement the cover is only a few millimetres thick, the ratio of core area to the cover area is generally large, and so spalling is delayed. The mesh wires are closely spaced and there are several layers of mesh. Hence concrete is effectively confined. Thus confinement by ferrocement can offer certain advantages over confinement by ties, hoops and spiral reinforcement. 3. Scope of Investigation: At the start of this work, there was only one report in literature on confinement by ferrocement, in which circular ferrocement pipes were filled with concrete, and tested under axial and eccentric loading. The findings were positive for ductility, but reflected only a small increase in strength. This was possibly due to the low ratio of core to casing area and also because the cover for mesh may have been large. Thus it was doubly necessary to conduct this *- investigation which comprises a theoretical and an experimental study. The main parameters covered are: « quantity and type of mesh and yield stress of wires. * grade of core concrete and mortar. * circular and square sections. * axial and eccentric loads. * effect of slenderness ratio (ie inelastic buckling). * plain or reinforced cores. * sequence of construction (core first or casing first or both together) * repair of distressed columns. v 4. Theoretical Investigation: Experimental and theoretical studies on confinement in concrete columns provide the basis for confinement by (iii) ferrocement. Studies on steel or plastic pipes filled with concrete also have relevance. The detailed behaviour of ferrocement encased short columns has been theoretically examined with respect to the main parameters, with the emphasis being on increase in strength and ductility, and the stress-strain curve of confined concrete. Effect of quantity of reinforcement, grade of concrete and mortar and ratio of core diameter to column diameter has been investigated. Several formulae have been proposed for axially loaded short columns to determine the peak load and peak strain. A simple equation has been adopted to predict the stress-strain curve for concrete confined by ferrocement. This has been used in the analysis for eccentric loads and for inelastic buckling. Derivations and analysis were carried out for eccentrically loaded sections, for square and circular shapes with plain or reinforced concrete cores. A computer analysis, using numerical integration, was carried out to obtain load-moment interaction diagrams. Effect of slenderness ratio I.e. inelastic buckling has been investigated by Engesser's formula using the actual stress-strain curves of confined and unconfined concrete.The Perry-Robertson formula and some other formulae have also been tried. Effect of eccentricity on buckling has also been studied analytically. Variation of confinement effectiveness with slenderness ratio has been examined. 5. Experimental Investigation: This was planned so as to investigate the major parameters and to substantiate the theoretical findings.Since the parameters are many a large number of specimens around 300)were tested. These covered short, square and circular specimens loaded axially as well as eccentrically. In all eighteen long columns, with pinned end conditions, were tested for buckling. These columns were of circular cross-section and had three different slenderness ratios. One set of columns was also provided with vertical reinforcement and ties. Some short specimens were also tested for cyclic loading and for reversal of moments. All testing was for low strain rates—both high strain rates and vibratory loading were excluded, and can be the focus of attention of a future study. Some tested specimens were also encased in ferrocement, and retested to study the effectiveness for repair. (iv) Suitable equipment and procedures had to be devised for casting and testing. Observations were generally for failure loads, strains, first appearance of cracks and failure mode. Electrical resistance strain guages, mechananical extensometers and compressometers and dial guages were used for strain and deformation measurements. The crack widths were measured by an illuminated microscope. In general there was reasonable corelation with theory. 6. Conclusions: Only general conclusions are given here. Ferrocement encasement was effective in confinement of concrete in short columns for axial and eccentric loads A considerable strength and ductility increase was achieved. For square sections, the experimental strength and ductility increase were somewhat lower than in circular sections. In the majority of tests the experimental failure loads and corresponding strains and also the stress-strain curves tallied with the theoretical values. Thus, the formulae proposd for square and circular sections can be used to determine the strength of short encased columns with plain as well as reinforced cores. Analytical load-moment interaction diagrams of eccentrically loaded confined sections were similar in nature to those of unconfined columns, and clearly showed that there is a considerable increase in strength. The analysis also shows that there is an increase in rotation capacity. Tests on eccentrically loaded short columns verified the analysis procedure developed here. Due to buckling the confinement effectiveness reduces with an increase in slenderness ratio. There is a limiting value of slenderness ratio beyond which both confined and unconfined columns have the same buckling load. However, the majority of building columns can be designed as short columns. Even in such columns use of ferrocement mesh encasement in the end zones would help in developing plastic hinges, which reduce the probability of collapse of the structure during an earthquake. The experimental test results verified the inelastic buckling analysis. The advantages of ferrocement encased concrete columns for repair/retrofitting, prefabrication and for insitu construction have (v) been clearly established by this study.The test results, proposed formulae and the graphs Included in the thesis can be used to predict the strength, ductility and toughness of such columns for the practical ranges of most of the parameters studied. Some design guidellness have been proposed. With due consideration these could possibly result in code provisions. 7. Further Research & Applications: The areas for further reasearch have been identified in the thesis in detail. The main thrust of work can be on vibratory loads, loading at high strain rates, creep studies and analysis and testing of columns with rectangular sections. There appears to be considerable advantage in extending the application of precast ferocement casings to bridge piers. Erection of formwork in mid-stream can be hazardous and time consuming. In some rivers there is flooding for several months of the year, when construction may come to a standstill. Ferrocement casings could be manufactured near the river and floated to the proper position on barges and erected to serve as lost formwork. If properly developped, this technique could help achieve economy and saving in construction time, and find applications to offshore structures.