FIELD OF THE INVENTION
The present invention relates generally to a ribbed Thermo Mechanically Treated (TMT) reinforcing bar deformed by hot rolling and equipped with ribs inclined with respect to the central axis. More specifically, the invention relates to an improved thermo-mechanically treated rebar with a rib configured through optimization of root radius and height of the rib to enhance fatigue-life of the bar without compromising strength/adherence of the bar with surrounding concrete.
BACKGROUND OF THE INVENTION
Rebars are used inside concrete as reinforcement for various construction applications. These steel bars are of circular cross-section having protrusions or ribs on the surface which are essential for developing a bond with the surrounding concrete. Conventionally reinforcing bars are supplied with guaranteed properties like strength, ductility, weldability and bond-strength (assessed though Pull-out test). However, reinforced concrete structures used in bridges, offshore oil-rigs, buildings & towers, chimneys, windmills, dams and machine foundations are subjected to dynamic/cyclic loading in service due to traffic-load, strong wave and wind actions, earth-quake and machine vibrations. It is necessary to consider the problem of fatigue in the above applications where rebars with superior fatigue strength are required.
It is known that the ribs/protrusions on the surface of TMT reinforcing bars (rebars). Rebars are used inside concrete as reinforcement for various construction applications. These steel bars are of circular cross-section having protrusions or ribs on the surface which are essential for developing bond with the surrounding concrete. The protrusions or ribs are of two types e.g. longitudinal and transverse. Longitudinal ribs are straight and run parallel to the axis of the bar. Longitudinal ribs are formed during the process of rolling due to filling of material in the gap between top and bottom rolls. Transverse ribs are

formed on the surface of the bar while rolling the bar using profiled top and bottom rolls. TMT reinforcing bar generally comprises of a pair of parallel longitudinal ribs and a number of equidistant transverse ribs in between the two longitudinal ribs and inclined with respect to the central axis. The geometry of transverse ribs governs the bonding or adherence of bar with surrounding concrete. The geometry of transverse ribs is controlled with precision during rolling by adjusting the roll gap and proper maintenance of the rolls. Reinforced concrete structures used in bridges, offshore oil-rigs, machine foundations etc. are subjected to dynamic/cyclic loading in service due to traffic-load, strong wave and wind actions, earth-quake and machine vibrations. In the above applications where rebars with superior fatigue strength are required.
It is known that fatigue failures initiate at the surface and the presence of stress concentration or notch on the surface expedite fatigue failure thereby reducing fatigue strength. In case of rebars, the ribs or protrusions which are essential for bonding with concrete are known to be associated with severe stress concentrations factors. There are few reports in the literature where stress concentrations close to the ribs on the surfaces of rebars have been investigated. Jhamb and MacGregor (1974) studied the stress concentration on the surface of deformed bars using two dimensional finite element analysis and concluded that the ratio of the lug base radius(r) to the lug height(h) has pronounced effect on stress concentration factor(KT). Zheng and Abel (1998) also carried out two dimensional FEM analyses to investigate stress concentrations on the rebar-surface and found that maximum stress concentration occurs at the root of the rib and base metal. They found that root radius (r/h ratio) is the prime factor in affecting the stress concentration in addition to width to height (w/ h) ratio of the transverse ribs, and the flank angle (the angle the transverse ribs form with the bar axis). However, the optimum value of r/h ratio to minimize stress concentration at the root of rib for a rebar of given diameter or for a particular rib-profile has not been demonstrated in detail in the above papers either in terms of numerical simulations or in terms of experimentations. Moreover, in the above

literature the magnitude of stress gradient close to the ribs has not been dealt with. The two dimensional FEM analyses are not suitable for rebars where the configuration is such that three dimensional FEM analysis should be carried out for more accurate prediction. The effect of above changes in the r/h, w/h or angle of ribs on bond stress with concrete has not been examined at all.
There are a number of patents on modification of rib design on rebar surface to improve bond strength with concrete e.g. GB899839 (AMAGASAKI IRON & STEEL MFG COM, 1962), GB1044172 (TOR ISTEG STEEL CORP, 1966), GB1146651 (FERROTEST G M B H, 1969), GB1444076 (1976), EP1253259 (Schöck Entwicklungsgesellschaft mbH, 2002), IN2012KO01149A (SAIL, 2012). Patents which deal with modification of rib design to improve fatigue strength of rebars are limited in number. Japanese patent JP51129823 deals with improvement of fatigue strength of ribbed reinforcing steel bar by subjecting it to shot peening. US4811541 deals with hot rolled steel rebar where helically arranged ribs are introduced to improve fatigue strength. However, no direct evidence of improvement of fatigue strength is included in the detailed description of this patent. In another prior art, a German Patent DE3340887 (1985), ribbed reinforcing bars, intended to be used in reinforced concrete under fatigue loading, with good fatigue strength values have been dealt with. In this patent, it has been specified that to increase the fatigue strength values, the geometrical parameters of the ribbed reinforcing bar must be chosen such that a minimum characteristic value S in dependence on the material quality of the steel used is exceeded. S is a measure of stress concentration and is a function of nominal diameter, rib height, rib root radius, rib width, rib spacing etc. However, in the above Patent, it is not clearly demonstrated as to how this value of S was arrived at. No detailed scientific experimental work or numerical simulation steps were furnished in the detailed description or summary of the patent. Moreover, this invention does not cover the effect of modification of rib profile on bond strength with concrete.

Limitations of prior art:
1. Although there are few technical disclosure available in the non-patent literature which deal with modification of rib design to improve fatigue strength of rebar, the optimum value of r/h ratio to minimize stress concentration at the root of rib for a rebar of given diameter or for a particular rib-profile has not been demonstrated therein. Moreover, in the above literatures, the magnitude of stress gradient close to the ribs has not been dealt with at all. The literature is also silent on the effect of modified rib design on bond strength with concrete.
2. The number of patents on modification of rib design to enhance fatigue strength are limited. Most of the patents deal with modified rib design to improve bond strengths with concrete.
3. In the limited number of patents, direct evidences/demonstrations of improvement in fatigue life are not generally available, for example, US4811541.
4. In German Patent DE3340887, no detailed scientific experimental work or numerical simulation steps to arrive at the modified rib design were disclosed. Moreover, this invention does not cover the effect of modification of rib design on bond strength with the concrete.
The present invention is aimed at addressing the above limitations of the prior art.
OBJECTS OF THE INVENTION
It is therefore an objection of the present invention is to propose an improved thermo-mechanically treated rebar with a rib configured through optimization of

root radius and height of the rib to enhance fatigue-life of the bar without compromising strength/adherence of the bar with surrounding concrete.
Another object of the invention is to propose an improved thermo-mechanically treated rebar with a rib configured through optimization of root radius and height of the rib to enhance fatigue-life of the bar without compromising strength/adherence of the bar with surrounding concrete, which can be subjected to dynamic/cyclic loading in service e.g. bridges, offshore oil-rigs, buildings and towers, chimneys, windmills, dams and machine foundations.
A still another object of the invention is to propose an improved thermo-mechanically treated rebar with a rib configured through optimization of root radius and height of the rib to enhance fatigue-life of the bar without compromising strength/adherence of the bar with surrounding concrete, which minimizes the stress concentrations and stress gradient at the rib to prevent fatigue failure.
A further object of the invention is to propose an improved thermo-mechanically treated rebar with a rib configured through optimization of root radius and height of the rib to enhance fatigue-life of the bar without compromising strength/adherence of the bar with surrounding concrete, in which the optimized rib-design retains optimum level of bond-strength/adherence with the surrounding concrete through stress transfer between the concrete and rebar through an interfacial bond.
A still further object of the invention is to propose an improved thermo-mechanically treated rebar with a rib configured through optimization of root radius and height of the rib to enhance fatigue-life of the bar without compromising strength/adherence of the bar with surrounding concrete, which is enabled to validate that the optimized geometry of the improved rebars with ribs does not lead to reduction of the bond strength of the rebar.

SUMMARY OF THE INVENTION
Accordingly, there is possible an improved thermo-mechanically treated rebar with a rib configured through optimization of root radius and height of the rib to enhance fatigue-life of the bar without compromising strength/adherence of the bar with surrounding concrete. Further, the process of manufacturing the improved reinforcing bars with optimized rib design to achieve higher fatigue strength and optimum bond strength comprises the following steps: continuous casting of billets from liquid steel of desired chemical composition; hot rolling of the billet by profiled rolls into bars of circular cross section having protrusions/ribs on the surface; and quenching the re-bars with water spraying and then cooling in air to obtain desired microstructure in the cross section.
The present invention and its objects and advantages are substantiated with the help of accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS AND TABLES
Figure 1 The rib profile of the reinforcing bar (selected for
experimental work and numerical simulation in this invention)
shown schematically.
Figure 2 Representative microstructure of the rebar selected for
experimental work and numerical simulation in this invention
Figure 3 Variation of maximum principal stress with r/h ratio of
transverse rib in (a) 8 mm and (b) 16 mm diameter
reinforcing bar.
Figure 4 Distribution of maximum principal stress close to the
transverse rib in 16 mm rebar with (a) r/h ratio 0.0 (σmax = 973
MPa) and (b) r/h ratio 1.25 (σmax = 703 MPa).

DETAILED DESCRIPTION OF THE INVENTION
Reinforced concrete structures used in bridges, offshore oil-rigs, buildings & towers, chimneys, windmills, dams and machine foundations are subjected to dynamic/cyclic loading in service due to traffic-load, strong wave and wind actions, earth-quake and machine vibrations. In the construction of the above, rebars of superior fatigue strength are required. The present invention revolves around development of a reinforcing bar of higher fatigue strength in addition to optimum bond strength for the above applications.
It is known that fatigue failures initiate at the surface and the presence of stress concentration or notch on the surface expedite fatigue failure thereby reducing fatigue strength. In case of rebars, the ribs or protrusions on the surface which are essential for developing bonding with concrete are known to be associated with severe stress concentrations factors. The essence of this invention is to reduce the stress-concentrations at the ribs by optimizing the rib configuration. The stated modifications reduce their potential of initiating fatigue failures thereby improving the fatigue life of rebars. The parameters which affect stress concentrations at the rib are:
• Radius of transverse rib root (r) or r/h ratio
• Decreasing the width of transverse rib to height (w/h) ratio
• Reducing the flank angle
• Increase the spacing between the transverse ribs
However, the present inventors noted that modification of the r/h ratio is by far the most effective technique to reduce stress concentrations at the rib because it requires the least changes to the existing manufacturing procedure of the rebars. In view of this we have focused on this method. The goal is to reduce stress concentrations and achieve the target gain in fatigue life through this approach. The primary aim is to find the fatigue life of a rebar by continuously modifying its

geometry and investigate the effect of the modification in rib geometry on the bond strength.
With a view to attain the objectives of the invention, the process includes: -
• providing an accurate finite element model of an actual rebar with specified diameter and rib geometry, and with material properties as obtained from test data. The schematic drawing of the rebar to depict rib profile is shown in Figure 1. The chemical composition and tensile properties of the rebar which are used in the experimental work are shown in Table 1. Typical microstructure is shown in Figure 2.
• simulating a single loading cycle of the rebar by applying prescribed displacement boundary conditions to the rebar.
• monitoring the stress concentrations that arise in the rebar during a single loading cycle.
• Performing further simulations on several possible modified rebar geometries to decide on an optimum rebar geometry that results in an acceptable reduction in stress concentrations at critical locations in the rebar.
• adapting empirical procedures from notch fatigue studies (which use the stress concentrations at a notch to predict the fatigue life of a notched specimen) that would allow stress concentrations obtained from finite element simulation of a single loading cycle on a rebar to predict the fatigue life of the rebar.

• using each of the empirical procedures, adapted from the notch fatigue studies, to predict the S-N curve of a rebar whose actual S-N curve is known experimentally.
• determining the empirical procedure which can predict the actual S-N curve most accurately.
• using the best procedure determined above, to predict the fatigue life of
the rebar following modifications (that reduce stress concentrations) to its
geometry.
Since the intersection of the transverse rib with the base metal can be thought of as a notch, the methodology adopted is based on concepts from notch fatigue. The goal in notch fatigue is to predict the fatigue behavior of a component based on observed stress concentrations. The elastic stress concentration factor Kt is a key parameter. It is defined in the following manner:

However the fatigue strength of a component is not dependent entirely on Kt. Experimental results indicate that the fatigue life of the material depends on the volume of material that is subjected to the critical stress as well as the magnitude of the critical stress itself. An additional factor, called the fatigue notch factor, Kf. is therefore often used in empirical studies which combine the effect of Kt as well as the volume effect. Kf. is defined as the ratio of the fatigue strength in a smooth bar to the fatigue strength in a notched bar.


Typically, predicts a notched fatigue strength considerably
higher than what would be obtained from estimates of Kt alone. Fatigue Strength is defined as the maximum stress range (S) that may be repeated without causing failure for a specified number of loading cycles (N). A stress based formulation was developed that could be used to predict the fatigue life of a model on the basis of results obtained from the application of a single cycle of load on the model. All of the methods attempt to define a fatigue-effective stress that is a function of the amplitude of the applied stress (Sa) and a
material auxiliary parameter, α:

The methods differ in the form of the function f: each method postulates a different form of the function f. The fundamental assumption, shared by all three methods, is that the fatigue-effective stress is a material property: for a particular material it only depends on the number of loading cycles N. In the highly stressed volume approach, the fatigue-effective stress is defined as:

In the above, V0 is the volume of Bar 0, V90, the volume of a notched bar where the stresses are in excess of 90% of the peak stress, In the Stress Gradient approach the fatigue-effective stress is obtained as a function of the peak stress , the normalized stress gradient at the location of
the peak stress, χ, and the “notch support factor” The normalized gradient at
the location of the peak stress χ is given by:


Where is the rate of variation of stress with radial distance along the most
highly stressed section of the bar, while rmax is the radial location in the section .The functional form for the fatigue-effective stress is then given by :


In the Notch Sensitivity approach, the fatigue-effective stress is defined in terms of the notch sensitivity q, the peak stress , the notch radius and the
size parameter

If the above exercise is carried out for different values of N, it is possible to obtain the material parameter α as a function of N. The resulting α’s ensure best fit between the fatigue-effective stresses of reference and notched bars for all values of N. Once the dependence of α on N is known, the fatigue life of notched bars made of the same material, but for which S-N curves are not available, can be predicted.
In the following discussion the reference bars are described as bars of “type” 0 while notched-bars whose S-N curves are known are described as bars of “type” 1. Notched bars whose S-N curves are not known, and whose fatigue lives are to be predicted, are described as bars of “type” 2.
Three dimensional finite element models for study of stress concentrations due to high cycle fatigue present challenges in terms of computational cost and time as a large number of elements are required to accurately discretize such a model. To solve the given problem efficiently, a procedure combining finite element

analysis with empirical relations adapted from notch fatigue was developed to predict the fatigue life for the modified rebar. This formulation can predict the result of high cycle fatigue test if the results of a single cycle of axial loading of the model are available. Thus computation time was reduced considerably.
Considerable amount of time in the present project was spent in order to make the models efficient in terms of computation time and cost. Depending on the criticality of a particular region in determining the stress concentrations, different levels of mesh refinement were used. Using these “optimally” meshed finite element models, three alternative approaches were tested to predict the fatigue life of Bar 2. The predicted fatigue life for Bar 2, compared to that for the unmodified bar, Bar1, was found to be significantly higher, for all three approaches.
In order to find the r/h ratio which leads to the largest reduction in the maximum principal stress at the root of a transverse rib, different fillet geometry corresponding to a particular r/h ratio were considered. A single cycle of load was applied with maximum stress of 250 MPa. From Figure 3 it is seen that the maximum principal stress decreases as r/h ratio increases upto a value of 1 and 1.25 for 8 mm and 16 mm diameter rebar respectively, after which it increases again. Based on the above result subsequent testing with modified rebars was done on a 16mm rebar with r/h ratio of 1.25. The effect of adding fillets is clear from contour plots of the principal stress. The distribution of principal stress is shown in Figure 4 for the rebar with r/h = 0.0 and 1.25 respectively. It is seen that the maximum principal stress reduces substantially, from 973 MPa to 767 MPa when r/h ratio is 1.25. This is further borne out by Figure 5 which shows the variation in principal stress along a radial path from the location of amax to the center of the most-stressed cross-section of the rebar. It is evident that the stress gradient is less in rebar having r/h ratio is 1.25. The effect of reduced stress concentration and stress gradient at the rib root leads to enhancement of fatigue-

life in Bar 2 (with modified rib design) in comparison to Bar 1 as shown in Figure 6 as also in Table 2.
Changes in rebar geometry to increase fatigue strength cannot be acceptable if they lead to significant reduction in bond strength. The objective of this study is to demonstrate that change in rib design does not lead to any significant reduction in stress transfer between concrete and the rebar through the interfacial bond. The above objective was achieved in the following manner:
Laboratory pullout tests were performed on the original rebar (Bar 1) to determine its bond strength according to IS: 2770 (Part I) – 1967 at Indian Institute of Technology, Kharagpur. Typical test set ups are shown in Figure 7. A displacement controlled pull-out experiment was performed. The applied loading consisted of an axial displacement imposed at the exposed end of the rebar, applied in the direction of the pull out. Subsequently numerical simulations of pull out tests were performed on the same rebar. Pullout test models, with concrete properties as obtained from uniaxial compression and split cylinder tests (Figure 8 and Figure 9), were set up. The interface between concrete and steel was assigned cohesive properties. The parameters of the bond slip model for the rebar were calibrated to ensure close match between the numerical and experimentally obtained pull out test results as shown in Figure 10. The calibrated numerical model was then used to simulate pull out tests for the rebar with modified geometry (Bar 2 with r/h ratio of 1.25). The bond strength obtained from the numerical simulation for Bar 2 was compared to that obtained for Bar 1 and were found to be comparable as shown in Figure 11.
The average bond stress is a combination of resistance against pullout because of friction between the rebar surface and the concrete, as well as the interfacial bearing pressure developed due to the normal contact between the rebar ribs and the concrete wedges that impede the axial motion of the rebar. The major portion of the resistance is because of the bearing, as may be seen by comparing the frictional resistance against pullout to the total resistance against

pullout as in Figure 12. In the modified rebar, Bar 2, as the height of the rib is same as in Bar 1, it may be reasonably expected that the bearing resistance to pull out will be about the same as in case of Bar 1. Hence to obtain a clear understanding of the effect of the modified rib design on pullout resistance, the frictional force between concrete and rib was compared for Bar 2 and Bar 1 as shown in Figure 12(b). The effect of modification of rib geometry on frictional behavior is found to be negligible.

WE CLAIM
1. An improved thermo-mechanically treated rebar with a rib configured through optimization of root radius and height of the rib to enhance fatigue-life of the bar without compromising strength/adherence of the bar with surrounding concrete, comprising at least two parallel longitudinal ribs along the diameter of the bar and a plurality of parallel and equidistant transverse ribs placed between the longitudinal ribs and inclined to the axis of the bar and to the longitudinal rib of the bar.
2. The improved reinforcing bar as claimed in claim 1, wherein the included angle or angle of transverse rib varies between 35 degrees to 48 degrees depending upon the diameter of the bar.
3. The improved reinforcing bar as claimed in claim 1, wherein transverse round curve surface of the TMT rebar is trapezoidal in shape.
4. The improved reinforcing bar as claimed in claim 1, wherein the height of the ribs, the width of the ribs in the middle and the pitch or spacing between two successive ribs, is defined corresponding to the nominal diameter of the TMT rebar.
5. The improved reinforcing bar as claimed in claim 1, wherein the ratio of r/h (where r is radius at root of rib and h is height of rib) is maintained at a constant value so that stress concentration and stress gradient at root of rib is minimized.
6. The improved reinforcing bar as claimed in claim 1, wherein the most preferred optimum values of r/h ratio for 8 mm and 16 mm diameter bar are 1.0 and 1.25 respectively.

7. The improved reinforcing bar as claimed in claim 1, wherein the fatigue strength increases 2 to 3 times compared to the prior art bar.
8. The improved reinforcing bar as claimed in claim 1, wherein, the optimum distribution of rib mass on surface of the TMT rebars ensures excellent adherence with concrete and provide similar level of bond strength when compared to the prior art rebar of un-improved rib design. For example, the bond stress for Bar 1 and Bar 2 for slip of 0.25 mm is 10.5 and 10.1 MPa respectively. Similarly, the bond stress for Bar 1 and Bar 2 for slip of 0.50 mm is 14.4 and 13.8 MPa respectively
9. The improved reinforcing bar as claimed in claim 8, wherein the bond stress of the improved and prior art rebar with ribs for a slip of 0.25mm is 14.4 MPa and 13.8 MPa respectively, and where the said values of bond stress was obtained with the help of experimental Pull out tests carried out in accordance to IS: 2770 and also numerical simulation thereof.
10. A Process of manufacturing improved surface rib pattern profile thermo mechanically treated (TMT) bar with high bond strength as claimed in claim 1, comprising the steps of: Continuous casting of billets from liquid steel of desired chemical composition; hot rolling of billet with the aid of profiled rolls into bars of circular cross section having protrusions/ribs on the surface; and quenching with water spraying and then cooling in air to obtain desired microstructure in the cross section.