INVESTIGATIONS ON CONFINEMENT OF FIBRE REINFORCED HIGH STRENGTH CONCRETE COLUMNS

Abstract

The need for ductile behavior of various structural components during any major earthquake has been acutely felt and demonstrated amply during several seismic events. Although it is preferable to dissipate seismic energy by post-elastic deformations in beams, column hinging cannot be avoided entirely in most buildings during severe earthquakes. To achieve sufficient ductility in columns, their potential plastic hinge regions should be reinforced with appropriately designed and detailed longitudinal and lateral confining steel. It is well known that the ductile behavior of conventional strength concrete sections can be attained by suitably confining the concrete. However, the strength of concrete used in the construction industry has increased gradually over the years and concretes ofstrength up to 100 MPa have been used in the various structural applications. Although high strength concrete offers advantages in terms ofperformance and economy ofconstruction, the brittle behavior ofthe material remains a major drawback for seismic applications. Therefore, concrete confinement becomes a critical issue for high strength concrete (HSC) columns in seismicallyactive regions. It is now well established that a higher degree of confinement is required in columns with higher concrete strength than in columns with lower concrete strength to achieve similar advantages. The placing of such high amount ofconfining steel in the critical portions of HSC columns shall not only be an uneconomical proposition but also make these potential hinge regions too much congested. The previous experimental studies on high strength concrete columns under concentric axial loading have shown that the concrete cover spalls prior to attaining the theoretical capacity ofcolumns. The examination ofthe mechanics and mode of spalling of the cover inHSC columns made it clear that its unsatisfactory performance primarily stemmed from the poor tensile strength ofconcrete, resulting in its premature failure. These findings point to the need for concrete materials with better tensile properties. Hence, as a solution to these problems a concept of using a combination of suitable discrete and randomly distributed fibers with a nominal amount of lateral steel has been proposed in the literature. Areview of the existing literature, however, shows that the performance of 11 high strength concrete columns under the combined confining actions of transverse steel and discrete fibers is not well documented. Thereported works in this area, which are very few in number, are limited interms ofthe range ofparameters related to lateral confinement and fibres. The theoretical analysis and design of steel fibre reinforced high strength concrete columns would need the analytical stress-strain relationships for the confined core concrete and the unconfined cover concrete. However, in the existing literature, the published data on the stress-strain modeling of unconfined and confined steel fibre high strength concrete are scarce. The current confinement provisions of the Indian Standard Code IS-13920:1993 for the ductile detailing of reinforced concrete structures are intended for only conventional strength concrete. The mere extension of these design recommendations to the case of HSC columns is questionable. The design provisions of the Code for the confinement steel do not contain adequate quantitative relationships between the design parameters and the column performance. In addition, no consideration is given to the level of axial load in the column which has been found to significantly affect its deformability. No guidelines are available for the design of confinement reinforcement of steel fibre reinforced concrete columns. Therefore, the present investigation was planned to fully establish the structural response of confined steel fibre reinforced high strength concrete columns. The experimental investigations involved; (a) casting and testing of unconfined steel fibre reinforced high strength concrete specimens (81 cylinders of 100 mm x 200 mm size) and (b) confined non fibre and steel fibre reinforced high strength concrete columns (72 circular columns of size 150 mm x 600 mm and 72 square columns of size 150mm x 150 mm x 600 mm) under monotonically increasing axial compression. Two types of crimped-type flat steel fibres viz. 25 x 2.0 x 0.6-mm and 50 x 2.0 x 0.6-mm were employed. While the test variables of the study on unconfined concrete included aspect ratio and volume fraction of crimped steel fibres and compressive strength of concrete, the scope of the experimental investigations on confined concrete covered the effect of volumetric ratio, spacing and yield strength of transverse reinforcement, longitudinal reinforcement ratio, lateral steel configuration, shape of the cross section and concrete compressive strength in addition to the aspect ratio and volume fraction fibres. The effect of mixed aspect ratio of fibres on the stressiii strain behavior of unconfined and confined high strength concrete was also studied by blending the short and long fibres into the concrete mix. Theoretical investigations involved development of the stress-strain models for the unconfined and the confined steel fibre reinforced high strength concrete on the basis of the test data obtained. In addition, a comparative study of the existing confinement models of non-fibrous high strength concrete was also undertaken in order to choose the best one in terms of the ability to cover wide concrete strength ranges, low and high lateral steel grades, all the possible arrangements and quantities of transverse reinforcement and the various possible cross sectional shapes. A critical examination of the validity of applying the IS-13920:1993 Code provisions for confinement reinforcement to the case of HSC columns applied with varying levels of axial load has been carried out. To this end, a detailed moment curvature analysis of the Code designed column sections was carried out. An attempt has been made to propose new refined equations for the design of confinement for the potential hinge regions of non-fibre and steel fibre reinforced HSC columns. A finite element study was also undertaken to numerically simulate the behavior of confined concrete and thereby to compare the test behavior with the analytically predicted response. For all the unconfined and confined specimens in general, the addition of fibres increased the strength and ductility of fibrous high strength concrete vis-a-vis plain high strength concrete, with the enhancements in ductility being considerably more prominent than in strength. The gain in the compressive strength of high strength concrete was observed to increase with an increase in the fibre volume fraction at a constant aspect ratio whereas it decreased for a constant volume fraction with an increase in the aspect ratio of fibres. The deformability and toughness of HSC, though increased with increasing gross fibre content, the improvements were observed to be more pronounced in the case of longer fibres as compared to the short fibres. Based upon the test results of unconfined and confined concrete specimens having mixed aspect ratios of fibres (Short fibres and Long fibres), it can be said that if the fibres of two different lengths i.e. short and long are blended together at some given total fibre content, a better overall performance in terms of crack arresting capability, strength of the composite and post-peak toughness can be achieved as compared to the case of a single fibre aspect ratio. IV Anincrease in the compressive strength of concrete seemed to have an adverse effect on the deformability of unconfined and confined steel fibre reinforced high strength concrete. The results show that the addition of steel fibres to the confined high strength concrete columns prevents their early cover spalling and hence the reduction in the carrying capacity of the columns. It has been concluded thatthe fibre additives shall be most effectively utilized if the lateral confinement level is relatively low. An increase in the volumetric ratio or decrease in the spacing of transverse reinforcement resulted in an appreciable increase in the deformability of confined steel fibre reinforced concrete. A marginal gain in the strength and a nominal increase in the toughness of confined fibrous HSC were observed due to an increase in the yield strength of lateral steel and longitudinal steel ratio and an improvement in the configuration of transverse reinforcement. The circular spiral confinement was found to be considerably more effective than a square tie confinement in confining steel fibre reinforced high strength concrete. For the range of the values considered in the study, the effect of all the above mentioned parameters of confinement on the strength and ductility ofconfined fibrous high strength concrete have been quantified. The stress-strain models for the unconfined and confined steel fibre reinforced HSC have been developed. The proposed analytical models provide a good correlation between the predicted and the experimental results. The comparative study of confinement models for non-fibrous high strength concrete recommends the Legeron and Paultre (2003) model as the best one to predict the stress-strain relationship of confined non-fibre HSC. A critical evaluation of IS-13920:1993 confinement provisions for reinforced concrete columns indicate that the design equations of the Code are inadequate to provide satisfactory ductility for high strength concrete columns especially at high axial load levels. A new set of equations have been proposed in the present study to design the confining steel of non-fibre and steel fibre reinforced high strength concrete columns, which take into account the effects of high concrete strengths and axial load levels also. The confinement reinforcement design has been made performance based by relating the required quantity of reinforcement with the ductility demand. The finite element study successfully captured the experimentally observed structural behavior of the confined concrete columns.