Studies On C-Bar as Reinforcement For Columns

Abstract

Majority of reinforcement steel that we use in practice has surface having lugs or protrusions. They are used to increase the bond capacity between the rebar and the surrounding concrete. In order to achieve increased bond strength and to stop rein- forcement slippage of HYSD steel bars, protrusions were introduced on the surface of the straight Mild steel bars. These straight bars with deformed axis is known as C-bar. The use of C-bars is characterized by their plain surface and a gently undu- lating wave-type configuration. A comprehensive experimental program with 33 numbers of columns, involving C- bars, plain bars and ribbed bars with various other types of uncoated and coated bars, has been undertaken in present study. Breakup of these configurations is as follows: Three columns each (i.e. Total nine columns) have been cast having normal plain bar, C-bar & HYSD as reinforcement (termed as controlled columns). Three columns each have been cast using Epoxy coated plain bar, ribbed bar and C-bar as reinforcement. Three columns each have been cast using Galvanizing coating on plain bar and ribbed bar, respectively. Further Nine columns have been cast using C-bar with 4mm offset, 5mm offset and 6mm offset without ties, respectively. All columns are of 200mm x 200mm size & 1200mm in height. Columns are designed using IS: 456:2000 and SP-16 provisions. Concrete grade is kept as M25 & reinforcement steel provided is 1.13% for all columns. For M25 grade concrete, Self-Compacting Concrete (SCC) with OPC, Fly-ash (25 % replacement), only 10mm aggregates, river sand, potable water, and super plasticizer is used. Trials have been carried out to finalize the mix proportion for Self Compacting Concrete (SCC). Tests adopted to verify SCC are marsh cone test, ow table test & V-funnel test. Compressive strength of cubes are evaluated for 7, 14, 21 and 28 days. 28 days normal curing with gunny bags and 23 days of normal air curing is provided. Tension test has been done on HYSD bars, Plain bars and C-bars to compare their tensile strength and to evaluate stress v/s strain relationship. It has been found from tension test that C-bar gives more yield strength than the plain bar but the yield strength is lower as compared to ribbed bars. In terms of elongation, C-bar gives maximum results when compared to plain bar and ribbed bar. Epoxy coating and Galvanizing has been done on the reinforcement bars. Fusion Bond Epoxy coating technique has been adopted for Epoxy coating of ribbed bars, plain bars and C-bars. Hot dip Galvanizing technique has been adopted for plain bars and ribbed bars. Axial Load on the columns at specific load intervals and corresponding to every load, displacement and lateral strains have been measured. Comparison of Ultimate failure load, maximum displacement, lateral strain and axial strain evaluated at dif- ferent positions for all categories of columns is presented in tabular as well as in graphical form. These parameters are very essential to understand the behavior of all the columns. Columns provided with C-bars show increase in the axial load carrying capacity, axial deformation & lateral strain and bond strength. C-bars can be used as a reinforcement for columns with ties as well as without ties. In column the use of C-bars is found to be more effective since the load carrying capacity is increased up to 15%. In case of stress-strain relationship for columns, columns with plain bar has more lateral strain while columns with C-bar (with ties) gives more axial stress. Based on results of experiments it has been observed that control columns fails when the ultimate compressive strength is gradually increased, which causes splitting of concrete in between the stirrups. While evaluating axial load carrying capacity it is observed that, strain, axial deformation & failure modes of the column reinforced as C-bar with 4mm offset (with ties) gives superior results. The experimental evaluation of C-bar with different offsets and without ties shows that the axial capacity increases with increase in the offset of C-bar. The failure for Columns provided with C-bars but without ties shows non- ductile failure compared to ductile failure of column with C-bar with ties. Finally it can be summarized that C-bars, compared to plain round bars and ribbed bars, gives better axial capacity, deformation characteristics. C-bar gives effective result of ultimate axial load due to intact of bond strength between concrete and steel. So C-bar can help to reduce the problem of early distress in reinforced concrete structures.