**4. Solutions to early distress in concrete constructions**

It has been recognized that rebars with surface deformations corrode excessively, leading to concrete constructions with such rebars reaching states of distress early.

The obvious solution to the problem would have been to use plain round bars as in the past. But engineers, having used in design and construction rebars of medium strength and high strength steel over the decades, would not like to go back to the use of rebars of steel having yield strength of 40 to 50 percent of the yield strength of steel in today's rebars.

Two options are available.

#### **OPTION 1: WATERPROOFING TREATMENT.**

In recognition of the fact that the problem of early distress, cited in the preceding, resulted primarily from a combination of two factors:

a.extra susceptibility (compared to that of plain round bars of mild steel) of ribbed bars, high yield strength deformed bars, and ribbed CTD and TMT bars to corrosion

#### b.availability of a moist environment inside concrete

and in further recognition that the problem of early distress in reinforced concrete structures could be avoided by preventing a moist environment inside concrete, Kar [13, 16, 27, 28] developed effective, practical and durable waterproofing systems and the corresponding specifications for waterproofing treatment to virtually all types of concrete structures, the implementation of which would prevent absorption of water/moisture into concrete, as in the cases of buildings, bridges and similarly exposed structures, or would prevent migration of water/moisture through the treated surface, as it would be in the cases of basements, tunnels, etc.

**23**

*Rebars for Durable Concrete Construction: Points to Ponder*

water, carbon dioxide or aggressive chemicals."

plain round bars of mild steel.

requiring repeat treatment

proofing treatment; Kar [12, 13, 16, 27, 28].

c.there can be shortcomings in workmanship

following shortcomings:

treatment as in the case of TMT bars (**Figure 6(c)** and **(d)**).

This waterproofing system has also the capacity to prevent the ingress of CO2 and O2

The concept of making concrete structures durable through surface protection in the nature of waterproofing treatment is gradually gaining ground in the USA and in other countries, and BIS, in recognition that concrete constructions with ribbed bars, as in IS 1786 [6], required extra protection against corrosion in the rebars, made waterproofing treatments a requirement for durability. SubSection 8.2.1 of IS 456:2000 [25] partly reads: "The life of the structure can be lengthened by providing extra cover to steel, by chamfering the corners or by using circular cross-section or by using surface coatings which prevent or reduce the ingress of

It needs to be noted here that the provision of waterproofing treatments to concrete structures became essential because of the failure of the ribbed CTD and TMT bars, conforming to IS 1786 in India, ASTM A615/A615M [18] in the USA or bars conforming to similar other Standards/Specifications in other countries, to make concrete structures as durable as those used to be when the rebars had plain surfaces, and high strengths in the rebar materials were not achieved through the highly detrimental processes of cold twisting beyond yield as in the case of CTD bars (**Figure 6(b)**) or through quenching/thermal hardening/thermomechanical

Kar's [16, 27, 28] art of making reinforced concrete structures durable through the provision of waterproofing treatment on the surface of such structures is an indirect way of solving the problem that was or that is invited with the use of the potentially damaging ribbed rebars of high strength steel, that was encouraged by ASTM International, BIS, ISO and such other organizations, which recommended and permitted the use of ribbed rebars, with or without the added processes of (a) cold twisting, as in CTD bars, or (b) quenching as in TMT bars, in the false belief or hope that concrete structures, reinforced with such bars, would be at least as durable as concrete structures of earlier era, which were reinforced with

Though surface protection systems have worked pretty well, it does have the

a.this additional treatment requires additional project time and expenditure

b.the materials used, and the specifications followed, may not be appropriate

d.such external treatments may be damaged or may have limited life spans,

e.it does not solve the problem of excessive corrosion on the surface of rebars prior to concreting (**Figure 6(c)** and **(f )**), leading to reduction or total loss of bond between rusted rebars and concrete whereas the availability of competent "bond" between rebars and the surrounding concrete is a pre-requisite for

In spite of these shortcomings, it is essential that all concrete structures, reinforced with ribbed rebars of steel, as in IS 1786 [6], ASTM A305 [4] or conforming to other Standards, be provided with surface protection in the nature of water-

successful performance of reinforced concrete construction.

*DOI: http://dx.doi.org/10.5772/intechopen.95401*

into the structure.

*Design of Cities and Buildings - Sustainability and Resilience in the Built Environment*

unstoppable; Kar [1].

ductile and to protect itself; Kar [1].

with low ductility." [[10], pp. 203–204].

bars of steel to corrosion at accelerated rates.

of steel in today's rebars.

to corrosion

Two options are available.

6.the rate of corrosion increases with increasing stress levels; the rate accelerates as the stress or strain approaches yield levels, and the surface becomes unstable once at or beyond yield, whereupon the bars become incapable of being passivated and consequently the process of corrosion becomes

The CTD and TMT processes are in violation of the inherent nature of steel to be

These CTD and TMT bars of high strength steel have another shortcoming to contend with: "The effect of stresses on corrosion is reflected more distinctly in the mechanical characteristics of the reinforcement, specially of high-strength steels

On the basis of extensive work in Russia, Alekseev [10] commented on the above scenario thus: "the durability of reinforcement specimens with a stepped (deformed) profile may be roughly an order less than that of smooth specimens since the former have stress concentrators on the surface at the bases of projections, which represent sites of preferential formation of cracks." [[10], pp. 221–222].

The preceding explains the reasons behind the intrinsic susceptibility of ribbed

It is the effect of this high susceptibility of ribbed bars to corrosion that led to the observations by Papadakis et al. [19] and Swamy [20], and to the types of early

It has been recognized that rebars with surface deformations corrode excessively, leading to concrete constructions with such rebars reaching states of distress early. The obvious solution to the problem would have been to use plain round bars as in the past. But engineers, having used in design and construction rebars of medium strength and high strength steel over the decades, would not like to go back to the use of rebars of steel having yield strength of 40 to 50 percent of the yield strength

In recognition of the fact that the problem of early distress, cited in the preceding,

a.extra susceptibility (compared to that of plain round bars of mild steel) of ribbed bars, high yield strength deformed bars, and ribbed CTD and TMT bars

and in further recognition that the problem of early distress in reinforced concrete structures could be avoided by preventing a moist environment inside concrete, Kar [13, 16, 27, 28] developed effective, practical and durable waterproofing systems and the corresponding specifications for waterproofing treatment to virtually all types of concrete structures, the implementation of which would prevent absorption of water/moisture into concrete, as in the cases of buildings, bridges and similarly exposed structures, or would prevent migration of water/moisture through the treated surface, as it would be in the cases of basements, tunnels, etc.

distress in reinforced concrete constructions, as depicted in **Figures 3**–**5**.

**4. Solutions to early distress in concrete constructions**

**OPTION 1: WATERPROOFING TREATMENT.**

b.availability of a moist environment inside concrete

resulted primarily from a combination of two factors:

**22**

This waterproofing system has also the capacity to prevent the ingress of CO2 and O2 into the structure.

The concept of making concrete structures durable through surface protection in the nature of waterproofing treatment is gradually gaining ground in the USA and in other countries, and BIS, in recognition that concrete constructions with ribbed bars, as in IS 1786 [6], required extra protection against corrosion in the rebars, made waterproofing treatments a requirement for durability. SubSection 8.2.1 of IS 456:2000 [25] partly reads: "The life of the structure can be lengthened by providing extra cover to steel, by chamfering the corners or by using circular cross-section or by using surface coatings which prevent or reduce the ingress of water, carbon dioxide or aggressive chemicals."

It needs to be noted here that the provision of waterproofing treatments to concrete structures became essential because of the failure of the ribbed CTD and TMT bars, conforming to IS 1786 in India, ASTM A615/A615M [18] in the USA or bars conforming to similar other Standards/Specifications in other countries, to make concrete structures as durable as those used to be when the rebars had plain surfaces, and high strengths in the rebar materials were not achieved through the highly detrimental processes of cold twisting beyond yield as in the case of CTD bars (**Figure 6(b)**) or through quenching/thermal hardening/thermomechanical treatment as in the case of TMT bars (**Figure 6(c)** and **(d)**).

Kar's [16, 27, 28] art of making reinforced concrete structures durable through the provision of waterproofing treatment on the surface of such structures is an indirect way of solving the problem that was or that is invited with the use of the potentially damaging ribbed rebars of high strength steel, that was encouraged by ASTM International, BIS, ISO and such other organizations, which recommended and permitted the use of ribbed rebars, with or without the added processes of (a) cold twisting, as in CTD bars, or (b) quenching as in TMT bars, in the false belief or hope that concrete structures, reinforced with such bars, would be at least as durable as concrete structures of earlier era, which were reinforced with plain round bars of mild steel.

Though surface protection systems have worked pretty well, it does have the following shortcomings:


In spite of these shortcomings, it is essential that all concrete structures, reinforced with ribbed rebars of steel, as in IS 1786 [6], ASTM A305 [4] or conforming to other Standards, be provided with surface protection in the nature of waterproofing treatment; Kar [12, 13, 16, 27, 28].

## **OPTION 2: PSWC-BAR AS A SOLUTION.**

A better solution to the problem of early distress in reinforced concrete constructions with conventional rebars of medium strength and high strength steel would be to use plain round bars as it used to be before the 1960s or 1970s.

That would have solved the problem of excessive corrosion in rebars, and that would have made reinforced concrete constructions as durable as such constructions used to be in the past.

But the problem is that the requirement of much longer development/anchor length might not have permitted the use of plain round bars of medium strength and high strength steel.

With the innovative concept of PSWC-BAR, Kar [14] provided a direct solution (at no added effort or cost) to the problem of early distress in concrete constructions with ribbed rebars of high strength carbon steel. PSWC-BAR was initially named as C-bar.

Kar [5] explained why PSWC-BAR is the most ideal rebar for reinforced concrete constructions.

The use of PSWC-BAR, at no added effort or cost, not only solves the problem of early distress in reinforced concrete constructions through several-fold enhancement of life span of such constructions, it also enhances several fold the ductility and energy-absorbing capacity of reinforced concrete constructions; Kar [2].

The several-fold enhancement of life span, at no added effort or cost, has the effect of lowering the life cycle cost of reinforced concrete construction to a fraction of what it is today.

The use of PSWC-BAR increases load-carrying capacities of reinforced concrete elements, and through the several-fold enhancement of life span, the use of PSWC-BAR minimizes the harmful effects of construction on the environment and the global climate through considerable lowering of the need for the manufacture of cement, steel, etc. Kar [29].

**PSWC-BAR**, characterized by its **plain surface** and **wave-type configuration** (**Figure 10**), solves the problem of early distress in reinforced concrete constructions that can result from the use of conventional ribbed bars of medium strength and high strength steel, by eliminating initiation of corrosion at the roots of ribs.

PSWC-BAR, because of the absence of ribs or any other special surface feature, if made of the same steel, will not corrode more than conventional plain round bars would do.

PSWC-BAR, because of its gentle wave-type configuration, enhances "effective bond", i.e., "engagement" between rebar and concrete; Kar [2]. Tests on beams and columns at different universities have shown that, among all types of rebars, PSWC-BAR, with its wave-type configuration, provides the best "engagement" between rebar and concrete, leading to significant enhancement of the various positive attributes of reinforced concrete; Kar [2, 17, 30, 31] and Varu [32].

While the test for loose rust and bond, or say, loss of bond, may lead to disqualification of most or all ribbed bars, conforming to IS 1786, and such other Standards, numerous tests on beams and columns have consistently shown that among rebars of steel, the use of PSWC-BAR, free from the ill effects of ribs, and if manufactured as Grade A of Hot Rolled Medium and High Tensile Structural Steel, as in IS 2062 [8], or conforming to appropriate Standards for plain round bars, can lead to the best load-carrying capacities, ductility and energy-absorbing capacity; Kar [2], indicating thereby that the "effective bond" is the best in the case of PSWC-BARs.

Besides these big fundamental differences between today's ribbed bars, as in IS 1786, and PSWC-BARs (**Figure 10**) as in IS 2062 [8], there lies the undisputedly

**25**

tional rebars.

PSWC-BARs.

*typical PSWC-BAR.*

**Figure 10.**

**Figure 11(a)** and **(b)** show clearly that:

catastrophes during earthquakes

PSWC-BARs are used.

*Rebars for Durable Concrete Construction: Points to Ponder*

stark difference between the very poor time-dependent performances (durability) of concrete structures, reinforced with ribbed bars, as in IS 1786 [6], ASTM A615/A615M [18] and such other Standards/Specifications elsewhere and the time-dependent performances of concrete structures, reinforced with hot rolled plain round bars with wave-type configuration, which are characteristic of

*PSWC-BAR of steel, characterized by plain surface and gentle wave-type configuration. (a) typical PSWC-BARs of steel, characterized by plain surface and gentle wave-type configurations. (b) schematic view of a* 

There are various other advantages of using PSWC-BAR as rebars in reinforced concrete construction. A comparison of the load–displacement plots in

a.because of several fold higher ductility and energy-absorbing capacity, the use of PSWC-BARs as rebars has the potential to prevent structural failures and

b.because of several times higher deflection (displacement) of flexural elements,

there can be visible warnings before failure, thereby saving lives.

quake resistant concrete constructions are provided in **Table 1**.

c.load-carrying capacities of reinforced concrete elements increase when

Recommended mechanical properties of PSWC-BAR for durable and earth-

Kar [5, 14–17] has written extensively on PSWC-BAR, and, encouraged by the many benefits, which the use of PSWC-BARs can provide, students at different universities have written a number of theses on the relative performances of concrete elements, reinforced with PSWC-BARs and conven-

*DOI: http://dx.doi.org/10.5772/intechopen.95401*

**Figure 10.**

*Design of Cities and Buildings - Sustainability and Resilience in the Built Environment*

A better solution to the problem of early distress in reinforced concrete constructions with conventional rebars of medium strength and high strength steel would be to use plain round bars as it used to be before the 1960s or 1970s.

That would have solved the problem of excessive corrosion in rebars, and that would have made reinforced concrete constructions as durable as such construc-

But the problem is that the requirement of much longer development/anchor length might not have permitted the use of plain round bars of medium strength

With the innovative concept of PSWC-BAR, Kar [14] provided a direct solution (at no added effort or cost) to the problem of early distress in concrete constructions with ribbed rebars of high strength carbon steel. PSWC-BAR was initially

The use of PSWC-BAR, at no added effort or cost, not only solves the problem of early distress in reinforced concrete constructions through several-fold enhancement of life span of such constructions, it also enhances several fold the ductility and energy-absorbing capacity of reinforced concrete constructions; Kar [2]. The several-fold enhancement of life span, at no added effort or cost, has the effect of lowering the life cycle cost of reinforced concrete construction to a fraction

The use of PSWC-BAR increases load-carrying capacities of reinforced concrete elements, and through the several-fold enhancement of life span, the use of PSWC-BAR minimizes the harmful effects of construction on the environment and the global climate through considerable lowering of the need for the manufacture of

**PSWC-BAR**, characterized by its **plain surface** and **wave-type configuration** (**Figure 10**), solves the problem of early distress in reinforced concrete constructions that can result from the use of conventional ribbed bars of medium strength and high strength steel, by eliminating initiation of corrosion at the roots of ribs. PSWC-BAR, because of the absence of ribs or any other special surface feature, if made of the same steel, will not corrode more than conventional plain round bars

PSWC-BAR, because of its gentle wave-type configuration, enhances "effective

bond", i.e., "engagement" between rebar and concrete; Kar [2]. Tests on beams and columns at different universities have shown that, among all types of rebars, PSWC-BAR, with its wave-type configuration, provides the best "engagement" between rebar and concrete, leading to significant enhancement of the various positive attributes of reinforced concrete; Kar [2, 17, 30, 31] and Varu [32].

While the test for loose rust and bond, or say, loss of bond, may lead to disqualification of most or all ribbed bars, conforming to IS 1786, and such other Standards, numerous tests on beams and columns have consistently shown that among rebars of steel, the use of PSWC-BAR, free from the ill effects of ribs, and if manufactured as Grade A of Hot Rolled Medium and High Tensile Structural Steel, as in IS 2062 [8], or conforming to appropriate Standards for plain round bars, can lead to the best load-carrying capacities, ductility and energy-absorbing capacity; Kar [2], indicating thereby that the "effective bond" is the best in the case

Besides these big fundamental differences between today's ribbed bars, as in IS 1786, and PSWC-BARs (**Figure 10**) as in IS 2062 [8], there lies the undisputedly

Kar [5] explained why PSWC-BAR is the most ideal rebar for reinforced

**OPTION 2: PSWC-BAR AS A SOLUTION.**

tions used to be in the past.

and high strength steel.

concrete constructions.

named as C-bar.

of what it is today.

would do.

cement, steel, etc. Kar [29].

**24**

of PSWC-BARs.

*PSWC-BAR of steel, characterized by plain surface and gentle wave-type configuration. (a) typical PSWC-BARs of steel, characterized by plain surface and gentle wave-type configurations. (b) schematic view of a typical PSWC-BAR.*

stark difference between the very poor time-dependent performances (durability) of concrete structures, reinforced with ribbed bars, as in IS 1786 [6], ASTM A615/A615M [18] and such other Standards/Specifications elsewhere and the time-dependent performances of concrete structures, reinforced with hot rolled plain round bars with wave-type configuration, which are characteristic of PSWC-BARs.

There are various other advantages of using PSWC-BAR as rebars in reinforced concrete construction. A comparison of the load–displacement plots in **Figure 11(a)** and **(b)** show clearly that:


Recommended mechanical properties of PSWC-BAR for durable and earthquake resistant concrete constructions are provided in **Table 1**.

Kar [5, 14–17] has written extensively on PSWC-BAR, and, encouraged by the many benefits, which the use of PSWC-BARs can provide, students at different universities have written a number of theses on the relative performances of concrete elements, reinforced with PSWC-BARs and conventional rebars.

#### **Figure 11.**

*Ductile response of beam reinforced with PSWC –BAR. (a) Load–displacement plot of beam with conventional rebars showing failure as the stress in rebars reached the yield stress level. (b) Load–displacement plot of beam with PSWC-BARS showing failure as the stress in rebars went past yield and approached the ultimate. Note: The two plots in (a) and (b) are drawn to different scales.*


*Note: 1) Y/Y specified ratio refers to ratio of actual yield strength to specified yield stress of the test piece. 2) TS/Y specified ratio refers to ratio of tensile strength to specified yield stress of the test piece. Additional Note: 1) The steel shall be suitable for welding processes.*

#### **Table 1.**

*Mechanical properties of steel in PSWC-BARs.*

### **5. Bond in reinforced concrete**

Bond between rebars and their surrounding concrete is of utmost importance in the context of reinforced concrete.

**27**

concrete.

*Rebars for Durable Concrete Construction: Points to Ponder*

Standards may be specifically prepared for PSWC-BARs.

or similarly between concrete and a stainless steel bar.

or stainless steel bars and the surrounding concrete.

eighty percent of that of uncoated bars.

wrong. There are various reasons for it.

Any reduction in bond, below a certain level, will lead to a reduction, or in extreme cases, a total loss of load-carrying capacities of the constructed structures, as it happened during the Bhuj earthquake on 26 January 2001 when three buildings, reinforced with epoxy coated bars, collapsed 300 kilometers away in Ahmedabad,

In the case of plain rebars of mild steel or carbon steel, when free from the damaging effects of the ribs as well as the CTD and TMT processes, there will be chemical bond between the mortar in concrete and the hard adherent products of very limited corrosion on the steel material, as in the cases of plain round bars of mild steel or, better still, PSWC-BARs, conforming to plain round rods of Grade A of structural steel in the Indian Standard IS 2062 [8], in which case the rods are given the wavetype configuration (**Figure 10**) at the end of the rolling mill process; Kar [14]. Similarly, PSWC-BARs can be made to conform to provisions in existing Standards/Specifications for plain round bars in other countries. Alternatively,

The chemical bond between the mortar in concrete and the hard and adherent products of corrosion on the surface of PSWC-BARs develops shear capacity at the interface of concrete and the rebar for the transfer of forces, through shear, from

In the context of reinforced concrete, this is the "bond" engineers have been

This should suggest that, technically speaking, there can be no "bond" between concrete and a painted surface, like the surface of an epoxy coated bar (**Figure 8**),

The same situation can develop if there will be loose rust on the surface of rebars as in the case of ribbed CTD or TMT bars (**Figure 6(f )**), as in IS 1786, which are the

**Figure 6(g)** shows that the loss of bond rendered the reinforcement, that was provided for load-carrying requirements, insufficient even as minor temperature reinforcements, and thereby led to the development of through-the-thickness shrinkage cracks in the shear walls even though it was a well-engineered project, except that, as per conventional practices in India, ribbed bars, as in IS 1786 [6], are used without the required scrutiny for "bond", that is set in SubSection 5.6.1 of IS

This is what happened in the case of the ribbed TMT bars in **Figure 6(f )** even when the bars were manufactured by a leading manufacturer of rebars and other

It is recognized that manufacturers/sellers of epoxy coated and stainless steel bars may not agree to the suggestion that there is no "bond" between epoxy coated

The observations by engineers may be right, but their claims on "bond" are

There is generally no "bond" between concrete and epoxy coated or stainless

Any resistance to pull-out forces in the case of epoxy coated ribbed bars or ribbed stainless steel bars is essentially due to the wedge action of ribs embedded in

In the absence of any reliable test method to measure "bond" or bond strength in the cases of ribbed bars, engineers too tend to agree with manufacturers and sellers of epoxy coated and stainless steel bars, and they might even suggest that their tests have shown that the bond strength of epoxy coated bars is sixty percent or even

*DOI: http://dx.doi.org/10.5772/intechopen.95401*

India (**Figure 7**).

concrete to rebars.

most widely used rebars in India.

products of steel in India. There is more to "bond".

steel bars (**Figures 7**–**9**).

familiar with.

456 [25].

This bond, when adequately developed, permits composite response of reinforced concrete through effective transfer of forces between concrete and rebar. *Design of Cities and Buildings - Sustainability and Resilience in the Built Environment*

*Ductile response of beam reinforced with PSWC –BAR. (a) Load–displacement plot of beam with* 

*Note: 1) Y/Y specified ratio refers to ratio of actual yield strength to specified yield stress of the test piece. 2) TS/Y specified ratio refers to ratio of tensile strength to specified yield stress of the test piece.*

*ultimate. Note: The two plots in (a) and (b) are drawn to different scales.*

gauge length 5.65√ A, where A is the cross-sectional area of the test piece

*Additional Note: 1) The steel shall be suitable for welding processes.*

*conventional rebars showing failure as the stress in rebars reached the yield stress level. (b) Load–displacement plot of beam with PSWC-BARS showing failure as the stress in rebars went past yield and approached the* 

**Sl. No. (1) Property (2) Fe 415 (3) Fe 500 (4) Fe 550 (5)** i) yield stress*, Min*, N/mm2 415.0 500.0 550.0 ii) yield stress*, Max*, N/mm2 500.0 600.0 660.0 iii) Y/Yspecified ratio1 1.02–1.2 1.02–1.2 1.02–1.2 iv) TS/ Yspecified ratio2 ≥ 1.15 - ≤ 1.40 ≥ 1.15 - ≤ 1.40 ≥ 1.15 - ≤ 1.40

20.0 16.0 12.0

**26**

**Table 1.**

**Figure 11.**

**5. Bond in reinforced concrete**

*Mechanical properties of steel in PSWC-BARs.*

v) Elongation, percent, *Min*. on

the context of reinforced concrete.

Bond between rebars and their surrounding concrete is of utmost importance in

This bond, when adequately developed, permits composite response of reinforced concrete through effective transfer of forces between concrete and rebar. Any reduction in bond, below a certain level, will lead to a reduction, or in extreme cases, a total loss of load-carrying capacities of the constructed structures, as it happened during the Bhuj earthquake on 26 January 2001 when three buildings, reinforced with epoxy coated bars, collapsed 300 kilometers away in Ahmedabad, India (**Figure 7**).

In the case of plain rebars of mild steel or carbon steel, when free from the damaging effects of the ribs as well as the CTD and TMT processes, there will be chemical bond between the mortar in concrete and the hard adherent products of very limited corrosion on the steel material, as in the cases of plain round bars of mild steel or, better still, PSWC-BARs, conforming to plain round rods of Grade A of structural steel in the Indian Standard IS 2062 [8], in which case the rods are given the wavetype configuration (**Figure 10**) at the end of the rolling mill process; Kar [14].

Similarly, PSWC-BARs can be made to conform to provisions in existing Standards/Specifications for plain round bars in other countries. Alternatively, Standards may be specifically prepared for PSWC-BARs.

The chemical bond between the mortar in concrete and the hard and adherent products of corrosion on the surface of PSWC-BARs develops shear capacity at the interface of concrete and the rebar for the transfer of forces, through shear, from concrete to rebars.

In the context of reinforced concrete, this is the "bond" engineers have been familiar with.

This should suggest that, technically speaking, there can be no "bond" between concrete and a painted surface, like the surface of an epoxy coated bar (**Figure 8**), or similarly between concrete and a stainless steel bar.

The same situation can develop if there will be loose rust on the surface of rebars as in the case of ribbed CTD or TMT bars (**Figure 6(f )**), as in IS 1786, which are the most widely used rebars in India.

**Figure 6(g)** shows that the loss of bond rendered the reinforcement, that was provided for load-carrying requirements, insufficient even as minor temperature reinforcements, and thereby led to the development of through-the-thickness shrinkage cracks in the shear walls even though it was a well-engineered project, except that, as per conventional practices in India, ribbed bars, as in IS 1786 [6], are used without the required scrutiny for "bond", that is set in SubSection 5.6.1 of IS 456 [25].

This is what happened in the case of the ribbed TMT bars in **Figure 6(f )** even when the bars were manufactured by a leading manufacturer of rebars and other products of steel in India.

There is more to "bond".

It is recognized that manufacturers/sellers of epoxy coated and stainless steel bars may not agree to the suggestion that there is no "bond" between epoxy coated or stainless steel bars and the surrounding concrete.

In the absence of any reliable test method to measure "bond" or bond strength in the cases of ribbed bars, engineers too tend to agree with manufacturers and sellers of epoxy coated and stainless steel bars, and they might even suggest that their tests have shown that the bond strength of epoxy coated bars is sixty percent or even eighty percent of that of uncoated bars.

The observations by engineers may be right, but their claims on "bond" are wrong. There are various reasons for it.

There is generally no "bond" between concrete and epoxy coated or stainless steel bars (**Figures 7**–**9**).

Any resistance to pull-out forces in the case of epoxy coated ribbed bars or ribbed stainless steel bars is essentially due to the wedge action of ribs embedded in concrete.

In the present context of bond, the epoxy coating on fusion bonded epoxy coated bars, as in IS 13620 [24], ASTM A775 [21], ASTM A934/A934M [33], ASTM A1055 [23] and similar Standards/Specifications on epoxy coated bars in other countries can be thought of as "coats of paints" as noted in SubSection 5.6.1 of IS 456 [25].

Recognizing that coats of paints, like loose rust, oil, etc. could destroy or at least reduce "bond", IS 456, the basic reinforced concrete code of practice in India, has put words of caution in SubSection 5.6.1 of its Section **5.6 Reinforcement** thus: 5.6.1 "All reinforcement shall be free from loose mill scales, loose rust and coats of paints, oil, mud or any other substances which may destroy or reduce bond. Sand blasting or other treatment is recommended to clean reinforcement."

In construction with fusion bonded epoxy coated rebars in India or elsewhere, no sand blasting or other treatment is provided so as to meet the requirements set in IS 456 or in any other document, and so as to ensure that there would be competent and adequate bond between such bars and the surrounding concrete.

It is possible that in recognition of this reality, IS 456 in its Section **5.6 Reinforcement** did not consider epoxy coated bars, as in IS 13620 [24], or stainless steel bars, as in IS 16651 [26], for possible use as rebars in reinforced concrete construction.

Though IS 456, the basic Indian Standard for reinforced concrete construction, does not approve of the use of epoxy coated bars as in IS 13620 [24] and stainless steel bars as in IS 16651 [26], such bars, which do not bond with concrete, with attended shortcomings in the performance of concrete constructions, do find use in reinforced concrete constructions in India and elsewhere.

In a series of tests by Varu [32] on thirtythree reinforced concrete columns at Nirma University in Ahmedabad, India, nine columns were reinforced with epoxy coated bars; of which three columns were with epoxy coated plain round bars, three columns were with epoxy-coated ribbed TMT bars of the type in IS 1786 [6], and three columns were with epoxy coated PSWC-BARs.

There is no suggestion that PSWC-BARs and conventional plain round bars may ever be given epoxy coating for protection. But in the test program these bars too were given epoxy coating just to have a more comprehensive understanding of the influence of surface coating (see SubSection 5.6.1 of IS 456 [25]) on load-carrying capacities and "bond" or "engagement".

The full details will be found in the thesis by Varu [32]. The observations can also be found in a few articles; Kar [2], and Kar, Dave and Varu [30].

Among other observations, it was observed:


In the absence of any bond, the use of epoxy coated and stainless steel bars will lead to under-performance of reinforced concrete elements; Kar et al. [30] and Kar [2], and the use of such bars can lead to unacceptable consequences during vibratory loads (**Figure 10**), specially during earthquake events (**Figure 8**), as it happened when several multi-storey buildings in Ahmedabad collapsed on 26 January 2001 during the earthquake at Bhuj 300 km away.

**29**

example.

*Rebars for Durable Concrete Construction: Points to Ponder*

The failures occurred due to separation between epoxy coated rebars and the

This should suggest that all concrete structures which were constructed with

a.the margin of safety in structures with epoxy coated ribbed bars is less than what it may be thought to be as per conventional design; Kar [2] suggested modification to current design practices by considering the "effective bond" or "engagement" instead of assuming that there is competent "bond" between

b.all concrete structures, reinforced with epoxy coated bars, remain specially vulnerable against vibratory loads, including earthquakes, as evidenced in the failure of structures in Ahmedabad during the Bhuj earthquake of 26 January

In the cases of rebars, with ribs on the surface, where a certain amount of resistance to slippage is available, it is partly due to "bond" and partly due to the interlocking of the ribs with the surrounding concrete. From an engineering point of view, this resistance to slippage may preferably be referred to as "effective bond"

Thus, though in the context of reinforced concrete, engineers have traditionally used only one term, i.e., "bond", and though in the context of reinforced concrete, where the rebar is a conventional plain bar of mild steel or carbon steel (**Figure 6(a)**), the use of the term "bond" may not create any confusion, the terms "effective bond" and "engagement" may be the more appropriate terms in the case of ribbed bars (**Figures 2** and **6(b)** and **(c)**) and PSWC-BARs (**Figure 10**), ribbed stainless steel bars, ribbed epoxy coated bars, polymer coated glass fiber reinforced bars, etc. In the case of a PSWC-BAR, devoid of ribs or any other surface feature, there will be the "bond" on the entire surface, and in addition, the wave pattern along the length of the bar will provide physical resistance to slippage. The sum total of the "bond" and the "physical resistance" in the case of a PSWC-BAR can be termed as

Tests on numerous reinforced concrete beams and columns, with reinforcing bars of different types, at different universities have consistently shown that "the effective bond" or "engagement" is the highest in the case of PSWC-BARs, leading to the highest load-carrying capacities as well as several hundred percent higher ductilities and energy-absorbing capacities compared to the cases of conventional

In the context of reinforced concrete, there should thus be a recognition of

For similar reasons, the use of the term "engagement" will hopefully avoid a false belief that there is bond between stainless steel bars and the surrounding concrete, and it will hopefully avoid the type of collapses of reinforced concrete bridges and buildings that Ahmedabad was witness to during the earthquake of 26 January

There are instances where chunks of concrete fell down from bridge decks which were constructed with ribbed TMT bars as in IS 1786 [6]. **Figure 9** shows one such

It should help put a stop to the use of not only the conventional epoxy coated bars, as in IS 13620 [24], but also to bars where the top coat is with epoxy as in

"effective bond" or "engagement", and a clear understanding of "bond".

fusion bonded epoxy coated rebars, are suspect. In other words,

These should be proof enough that any claim of 60–80 percent "bond" between

*DOI: http://dx.doi.org/10.5772/intechopen.95401*

epoxy coated bars and concrete is wrong.

epoxy coated rebars and concrete.

or "engagement", instead of "bond".

"effective bond" or "engagement".

bars without the wave-type configuration; Kar [2].

2001, 300 kilometers away at Bhuj (**Figure 7**).

2001.

surrounding concrete (**Figure 7**).

*Design of Cities and Buildings - Sustainability and Resilience in the Built Environment*

or other treatment is recommended to clean reinforcement."

reinforced concrete constructions in India and elsewhere.

three columns were with epoxy coated PSWC-BARs.

Among other observations, it was observed:

2001 during the earthquake at Bhuj 300 km away.

capacities and "bond" or "engagement".

and adequate bond between such bars and the surrounding concrete.

It is possible that in recognition of this reality, IS 456 in its Section **5.6 Reinforcement** did not consider epoxy coated bars, as in IS 13620 [24], or stainless steel bars, as in IS 16651 [26], for possible use as rebars in reinforced concrete

In the present context of bond, the epoxy coating on fusion bonded epoxy coated bars, as in IS 13620 [24], ASTM A775 [21], ASTM A934/A934M [33], ASTM A1055 [23] and similar Standards/Specifications on epoxy coated bars in other countries can be thought of as "coats of paints" as noted in SubSection 5.6.1 of IS 456 [25].

Recognizing that coats of paints, like loose rust, oil, etc. could destroy or at least reduce "bond", IS 456, the basic reinforced concrete code of practice in India, has put words of caution in SubSection 5.6.1 of its Section **5.6 Reinforcement** thus: 5.6.1 "All reinforcement shall be free from loose mill scales, loose rust and coats of paints, oil, mud or any other substances which may destroy or reduce bond. Sand blasting

In construction with fusion bonded epoxy coated rebars in India or elsewhere, no sand blasting or other treatment is provided so as to meet the requirements set in IS 456 or in any other document, and so as to ensure that there would be competent

Though IS 456, the basic Indian Standard for reinforced concrete construction, does not approve of the use of epoxy coated bars as in IS 13620 [24] and stainless steel bars as in IS 16651 [26], such bars, which do not bond with concrete, with attended shortcomings in the performance of concrete constructions, do find use in

In a series of tests by Varu [32] on thirtythree reinforced concrete columns at Nirma University in Ahmedabad, India, nine columns were reinforced with epoxy coated bars; of which three columns were with epoxy coated plain round bars, three columns were with epoxy-coated ribbed TMT bars of the type in IS 1786 [6], and

There is no suggestion that PSWC-BARs and conventional plain round bars may ever be given epoxy coating for protection. But in the test program these bars too were given epoxy coating just to have a more comprehensive understanding of the influence of surface coating (see SubSection 5.6.1 of IS 456 [25]) on load-carrying

The full details will be found in the thesis by Varu [32]. The observations can

a.unlike in the cases of the twentyfour columns with uncoated rebars of different types, there were clear indications at the failure region of all the nine columns with epoxy coated rebars that there was no bond of concrete/concrete mortar

b.the epoxy coated bars led to failure of the columns at loads which were less than the loads at which the other similarly constructed, but with uncoated bars of same/similar manufacture had failed. It appeared that the coated bars did

In the absence of any bond, the use of epoxy coated and stainless steel bars will lead to under-performance of reinforced concrete elements; Kar et al. [30] and Kar [2], and the use of such bars can lead to unacceptable consequences during vibratory loads (**Figure 10**), specially during earthquake events (**Figure 8**), as it happened when several multi-storey buildings in Ahmedabad collapsed on 26 January

also be found in a few articles; Kar [2], and Kar, Dave and Varu [30].

with the epoxy coated bars. A typical case is seen in **Figure 8**.

not participate in sharing loads on the columns; Kar et al. [30].

**28**

construction.

The failures occurred due to separation between epoxy coated rebars and the surrounding concrete (**Figure 7**).

These should be proof enough that any claim of 60–80 percent "bond" between epoxy coated bars and concrete is wrong.

This should suggest that all concrete structures which were constructed with fusion bonded epoxy coated rebars, are suspect. In other words,


In the cases of rebars, with ribs on the surface, where a certain amount of resistance to slippage is available, it is partly due to "bond" and partly due to the interlocking of the ribs with the surrounding concrete. From an engineering point of view, this resistance to slippage may preferably be referred to as "effective bond" or "engagement", instead of "bond".

Thus, though in the context of reinforced concrete, engineers have traditionally used only one term, i.e., "bond", and though in the context of reinforced concrete, where the rebar is a conventional plain bar of mild steel or carbon steel (**Figure 6(a)**), the use of the term "bond" may not create any confusion, the terms "effective bond" and "engagement" may be the more appropriate terms in the case of ribbed bars (**Figures 2** and **6(b)** and **(c)**) and PSWC-BARs (**Figure 10**), ribbed stainless steel bars, ribbed epoxy coated bars, polymer coated glass fiber reinforced bars, etc.

In the case of a PSWC-BAR, devoid of ribs or any other surface feature, there will be the "bond" on the entire surface, and in addition, the wave pattern along the length of the bar will provide physical resistance to slippage. The sum total of the "bond" and the "physical resistance" in the case of a PSWC-BAR can be termed as "effective bond" or "engagement".

Tests on numerous reinforced concrete beams and columns, with reinforcing bars of different types, at different universities have consistently shown that "the effective bond" or "engagement" is the highest in the case of PSWC-BARs, leading to the highest load-carrying capacities as well as several hundred percent higher ductilities and energy-absorbing capacities compared to the cases of conventional bars without the wave-type configuration; Kar [2].

In the context of reinforced concrete, there should thus be a recognition of "effective bond" or "engagement", and a clear understanding of "bond".

For similar reasons, the use of the term "engagement" will hopefully avoid a false belief that there is bond between stainless steel bars and the surrounding concrete, and it will hopefully avoid the type of collapses of reinforced concrete bridges and buildings that Ahmedabad was witness to during the earthquake of 26 January 2001, 300 kilometers away at Bhuj (**Figure 7**).

There are instances where chunks of concrete fell down from bridge decks which were constructed with ribbed TMT bars as in IS 1786 [6]. **Figure 9** shows one such example.

It should help put a stop to the use of not only the conventional epoxy coated bars, as in IS 13620 [24], but also to bars where the top coat is with epoxy as in

ASTM A1055 [23], and also to stainless steel bars as in ASTM A955/A955M [22] and IS 16651 [26], as, unlike in the cases of low carbon steel bars, stainless steel bars will not develop a thin layer of strong adherent rust on their surface for bonding with mortar in concrete.

Also, these bars stand in the way of composite response of concrete and the embedded bars, because of which even the capacity to carry static loads would be less than those which would have been arrived at on the basis of conventional design practices; Kar et al. [30] and Kar [2].

In the context of bond, besides the information provided hereinabove, Kar [14] had suggested that in the case of ribbed bars, coarse aggregates could in places rest on/against neighboring ribs (**Figure 12**), thereby blocking mortar from bonding with rebars, and also preventing passivation of rebars at such isolated locations. The void spaces aid the cause of corrosion.

In their tests, Mohammed, et al. [34] too observed void spaces beneath ribbed bars, resulting in higher rates of corrosion in ribbed bars than in the case of plain bars.

Whether in India or abroad, it has been the practice to assume that the use of ribbed bars provides the required bond between such bars and the surrounding concrete.

Though the presence of ribs on the surface of bars decreases the "bond", when compared to the cases of plain bars, the presence of ribs on the surface of bars may in some cases increase the "engagement".

**Figure 6(g)** presents a case where the absence of "bond" led to a decrease in the "engagement" between rebar and the surrounding concrete.

To start with, ribs were provided on the surface of rebars of high strength steel with an intent to increase bond between such rebars and concrete. This act boomeranged as it led to an acceleration in the rate of decay in reinforced concrete constructions.

The high strength in steel was/is gained in some cases either through the twisting of the bars beyond yield at a cold state or through quenching. The provision and the presence of the ribs, coupled with the twisting beyond yield or the quenching, lead to corrosion at unacceptably accelerated rates on the surface of the rebars; Alekseev [9, 10], and Kar [1, 5, 11–17] (**Figure 6(b)** and **(f )**), resulting in reduction or total destruction of the "bond" (**Figure 6(g)**). While the immediate effect of the

#### **Figure 12.**

*Barrier effect of ribs, lugs and protrusions on the surface of ribbed rebars of steel preventing cement mortar from coming in contact with rebar.*

**31**

*Rebars for Durable Concrete Construction: Points to Ponder*

destruction of "bond" is visible in **Figure 6(g)**, the long term effects are visible in

which have been set in SubSection 5.6.1 for reinforcement in IS 456 [25].

lower the load-carrying capacities of such constructions.

the varied problems of all other bars of high strength steel.

any other substance which may destroy or reduce bond.

as well as to the environment and the global climate.

light of the requirements in SubSection 5.6.1 of IS 456.

of columns, were reasonably reinforced, developed through-the-thickness shrinkage cracks, about a metre apart as excessive loose rust on ribbed TMT bars (**Figure 6(f )**), prevented/destroyed "bond" between concrete and the highly

Besides questionable "bond", the ribbed CTD and TMT bars, as in IS 1786 [6], meant for use as rebars in reinforced concrete construction, may not be permitted to be used as rebars, as because, such bars, with high susceptibility to corrosion at accelerated rates, will in many or most cases, fail the qualification test for rebars

An example will be found in **Figure 6(g)** where it is seen that in the construction of six 48–52 storeyed buildings at a site, the shear walls, which in the absence

Visits to construction sites revealed that easily visible through-the-thickness shrinkage cracks in new constructions were very common. This lack of "bond" can

The bars, conforming to IS 1786, were thus unfit for construction, at least in the

In the face of all the problems of insufficient "bond" in the case of ribbed rebars of high strength steel, epoxy coated ribbed bars, ribbed bars of stainless steel, and unacceptably high rate of corrosion in rebars, conforming to IS 1786, PSWC-BAR of medium tensile and high tensile steel (**Table 1**), conforming to IS 2062 [8], or to any other appropriate Standard/Specification for plain round bars of carbon steel of high strength steel, stands out as the only bar of high strength steel that is free from

PSWC-BAR, endowed with the property of best "engagement", i. e., "effective bond" with concrete, also stands out as the only bar, the use of which, besides several-fold enhancement of life span, increases, by several hundred percent ductility and energy absorbing capacity of reinforced concrete construction (**Figure 11**)

It is apparent that there has not been a clear understanding of the phenomenon of "bond" between rebar and concrete, what creates this "bond", what can affect the development of "bond", and what are its roles in the performance of reinforced

It is because of this lack of understanding of "bond" and its significance that made manufacturers and sellers of rebars, designers of reinforced concrete structures, construction engineers, and officials of BIS and such other organizations, who put the stamp of approval on ribbed rebars, overlook all these years the reality, the cautions in text books and Standards which read something like: all reinforcement shall be free from loose mill scales, loose rust and coats of paints, oil, mud or

It is this total failure to recognize the many significances of "bond" in the realm of reinforced concrete that facilitated the unchecked use of ribbed bars in reinforced concrete construction all these years, and in the process caused very significant losses to property owners, and great harm to the national wealth of countries,

The facts, that (a) ribbed bars, conforming to IS 1786 and to Standards/ Specifications on ribbed bars in other countries, are highly prone to the development of loose rust on the surface of such rebars, (**Figure 6(f )**), (b) this rust can "destroy or reduce bond" between concrete and rebars (**Figure 6(g)**), (c) without competent bond between rebar and concrete there cannot be reinforced concrete in its true sense, and (d) the loose rust will prevent any possible passivation of rebars by the alkaline pore water in concrete, and thus stand in the way of protection of

*DOI: http://dx.doi.org/10.5772/intechopen.95401*

**Figures 3**–**6(h)** and **(i)**.

rusted fresh rebars.

and Kar [2].

concrete.

*Design of Cities and Buildings - Sustainability and Resilience in the Built Environment*

mortar in concrete.

plain bars.

concrete.

constructions.

practices; Kar et al. [30] and Kar [2].

void spaces aid the cause of corrosion.

in some cases increase the "engagement".

"engagement" between rebar and the surrounding concrete.

ASTM A1055 [23], and also to stainless steel bars as in ASTM A955/A955M [22] and IS 16651 [26], as, unlike in the cases of low carbon steel bars, stainless steel bars will not develop a thin layer of strong adherent rust on their surface for bonding with

Also, these bars stand in the way of composite response of concrete and the embedded bars, because of which even the capacity to carry static loads would be less than those which would have been arrived at on the basis of conventional design

In the context of bond, besides the information provided hereinabove, Kar [14] had suggested that in the case of ribbed bars, coarse aggregates could in places rest on/against neighboring ribs (**Figure 12**), thereby blocking mortar from bonding with rebars, and also preventing passivation of rebars at such isolated locations. The

In their tests, Mohammed, et al. [34] too observed void spaces beneath ribbed

Whether in India or abroad, it has been the practice to assume that the use of ribbed bars provides the required bond between such bars and the surrounding

Though the presence of ribs on the surface of bars decreases the "bond", when compared to the cases of plain bars, the presence of ribs on the surface of bars may

**Figure 6(g)** presents a case where the absence of "bond" led to a decrease in the

The high strength in steel was/is gained in some cases either through the twisting of the bars beyond yield at a cold state or through quenching. The provision and the presence of the ribs, coupled with the twisting beyond yield or the quenching, lead to corrosion at unacceptably accelerated rates on the surface of the rebars; Alekseev [9, 10], and Kar [1, 5, 11–17] (**Figure 6(b)** and **(f )**), resulting in reduction or total destruction of the "bond" (**Figure 6(g)**). While the immediate effect of the

*Barrier effect of ribs, lugs and protrusions on the surface of ribbed rebars of steel preventing cement mortar* 

To start with, ribs were provided on the surface of rebars of high strength steel with an intent to increase bond between such rebars and concrete. This act boomeranged as it led to an acceleration in the rate of decay in reinforced concrete

bars, resulting in higher rates of corrosion in ribbed bars than in the case of

**30**

**Figure 12.**

*from coming in contact with rebar.*

destruction of "bond" is visible in **Figure 6(g)**, the long term effects are visible in **Figures 3**–**6(h)** and **(i)**.

Besides questionable "bond", the ribbed CTD and TMT bars, as in IS 1786 [6], meant for use as rebars in reinforced concrete construction, may not be permitted to be used as rebars, as because, such bars, with high susceptibility to corrosion at accelerated rates, will in many or most cases, fail the qualification test for rebars which have been set in SubSection 5.6.1 for reinforcement in IS 456 [25].

An example will be found in **Figure 6(g)** where it is seen that in the construction of six 48–52 storeyed buildings at a site, the shear walls, which in the absence of columns, were reasonably reinforced, developed through-the-thickness shrinkage cracks, about a metre apart as excessive loose rust on ribbed TMT bars (**Figure 6(f )**), prevented/destroyed "bond" between concrete and the highly rusted fresh rebars.

Visits to construction sites revealed that easily visible through-the-thickness shrinkage cracks in new constructions were very common. This lack of "bond" can lower the load-carrying capacities of such constructions.

The bars, conforming to IS 1786, were thus unfit for construction, at least in the light of the requirements in SubSection 5.6.1 of IS 456.

In the face of all the problems of insufficient "bond" in the case of ribbed rebars of high strength steel, epoxy coated ribbed bars, ribbed bars of stainless steel, and unacceptably high rate of corrosion in rebars, conforming to IS 1786, PSWC-BAR of medium tensile and high tensile steel (**Table 1**), conforming to IS 2062 [8], or to any other appropriate Standard/Specification for plain round bars of carbon steel of high strength steel, stands out as the only bar of high strength steel that is free from the varied problems of all other bars of high strength steel.

PSWC-BAR, endowed with the property of best "engagement", i. e., "effective bond" with concrete, also stands out as the only bar, the use of which, besides several-fold enhancement of life span, increases, by several hundred percent ductility and energy absorbing capacity of reinforced concrete construction (**Figure 11**) and Kar [2].

It is apparent that there has not been a clear understanding of the phenomenon of "bond" between rebar and concrete, what creates this "bond", what can affect the development of "bond", and what are its roles in the performance of reinforced concrete.

It is because of this lack of understanding of "bond" and its significance that made manufacturers and sellers of rebars, designers of reinforced concrete structures, construction engineers, and officials of BIS and such other organizations, who put the stamp of approval on ribbed rebars, overlook all these years the reality, the cautions in text books and Standards which read something like: all reinforcement shall be free from loose mill scales, loose rust and coats of paints, oil, mud or any other substance which may destroy or reduce bond.

It is this total failure to recognize the many significances of "bond" in the realm of reinforced concrete that facilitated the unchecked use of ribbed bars in reinforced concrete construction all these years, and in the process caused very significant losses to property owners, and great harm to the national wealth of countries, as well as to the environment and the global climate.

The facts, that (a) ribbed bars, conforming to IS 1786 and to Standards/ Specifications on ribbed bars in other countries, are highly prone to the development of loose rust on the surface of such rebars, (**Figure 6(f )**), (b) this rust can "destroy or reduce bond" between concrete and rebars (**Figure 6(g)**), (c) without competent bond between rebar and concrete there cannot be reinforced concrete in its true sense, and (d) the loose rust will prevent any possible passivation of rebars by the alkaline pore water in concrete, and thus stand in the way of protection of

rebars against corrosion unless concrete constructions will be given surface protection in the nature of waterproofing treatment, have not sunk into the minds of all those who should have known, are obvious from the continued poor performance of the structures in **Figures 3**–**6(h)** and **(i)**, and uncounted other structures which have been and are being constructed with ribbed bars.

Kar [2] has shown that besides success and failure, and besides the issue of durability, the "effective bond" or "engagement" between rebars and the surrounding concrete may influence the load-carrying capacity, ductility and energy-absorbing capacity of reinforced concrete elements.

## **6. Percent elongation of rebar**

Percent elongation is an important measure of ductility of rebars, that can influence the performance of the rebar and in turn the performance of concrete elements under load as well as under exposure to the environment; Kar [14]. The percent elongation is of course a very important property that may greatly influence the survivality of reinforced concrete constructions during earthquake events.

In recognition of the fact that the changing material compositions and manufacturing processes, as well as the increasing yield strengths of rebar materials during recent decades, are generally associated with decreasing percent elongation, the Specifications of ASTM International and the Standards of BIS allow/permit the use of rebars with smaller percent elongation properties with increasing yield strength of the rebar material.

It is recognized here that there are certain differences between the gage/gauge lengths in the ASTM and BIS test specimens. However, these differences do not substantially affect the following observations on percent elongation.

ASTM A615/A615M [18] of 12 Jan, 2016 has set the minimum percent elongation of rebars for Grades 75, 80 and 100, i.e., yield strengths of 520 MPa, 550 MPa and 690 MPa, to 7 percent for rebars having diameters up to 25 mm, and an even lower 6 percent for rebars having diameters greater than 25 mm, whereas for Grade 40 (280 MPa) and Grade 60 (420 MPa) bars, ASTM sets the minimum percent elongation at 12 and 9, respectively.

Similarly, IS 1786 [6], through its Amendment No. 03, dated 19-09-2017, has set the minimum percent elongation at 10.0, 10.0 and 10.0 for rebars of yield strengths 600 MPa, 650 MPa and 700 MPa, whereas it has set allowable percent elongations at 14.5 to 18.0 for different varieties of 415 MPa bars, and 12.0 to 16.0 for different types of 500 MPa bars.

Several questions arise, viz.,


**33**

*Rebars for Durable Concrete Construction: Points to Ponder*

strengths of 520 MPa, 550 MPa and 690 MPa?

[18] has set the same elongation at 7 percent or 6 percent for steel having yield

d.If 6 percent elongation is considered acceptable for 690 MPa steel, then why should such a low percent elongation be not acceptable in the cases of rebars

e.how is it that, when the achievable (with reasonable effort) percent elongation gets smaller and smaller with increasing yield strength, IS 1786 [6] has set the same figure of 10 percent for rebars having yield strengths of 600 MPa,

f. how is it that when ASTM A615/A615 M [18] finds it difficult to achieve percent elongation greater than 6 for 600 MPa hot rolled bars, IS 1786 finds a 10 percent elongation achievable for 700 MPa TMT bars, when it is known that, compared to hot rolled processes, as in the USA, the TMT process, as in India, leads to hardening and lowering of ductility and percent elongation properties?

There needs to be a clear understanding of the significance of percent elongation and or ductility of rebars in the context of performance of reinforced concrete

It may be desirable to set, irrespective of the yield strength of steel, a single value, below which the percent elongation or ductility will not be acceptable in the

set for the required percent elongation or ductility of rebars.

of 550 MPa, preferably to 500 MPa; Kar [5].

In view of the fact that virtually all structures in India and in many other countries are required to be earthquake resistant, a reasonably high value may have to be

In this conflicting scenario, with a view to minimizing the rate of corrosion and also to improve ductility and energy absorbing capacity, PSWC-BAR, conforming to IS 2062, and possessing the property of improving "effective bond" over and above the normally available "bond", with a minimum percent elongation of 16, is recommended as the rebar of choice. The yield stress will be limited to a maximum

Greater details on the development and mechanical properties of PSWC-BAR, together with design aid, so as to take advantage of the power of PSWC-BAR to enhance load-carrying capacity, as well as ductility and energy-absorbing capacities of reinforced concrete elements, are provided in the article: The Search for an Ideal

A better measure of the mechanical property of a rebar, and that of the performance of a concrete flexural element, reinforced with such a bar, would have been

Assuming that the percent elongation will be at least large enough to ensure that the specified yield strength and the specified ultimate strength of the bar will be achieved, the only other useful information that a percent elongation may provide is

That should suggest that vaguely specified percent elongation is an unnecessary

In contrast, while tests for yield and ultimate strengths (stresses) will ensure the said strengths (stresses), the information on ductility and the shape of the

Rebar for Durable Concrete Construction Leads to PSWC-BAR; Kar [5].

the ductility ratio rather than the arbitrarily selected percent elongation.

a vague understanding that the bar may not break during necessary bending.

specification when separate tests for bending of bars are specified.

*DOI: http://dx.doi.org/10.5772/intechopen.95401*

with steel of lower yield strengths?

650 MPa and 700 MPa?

elements.

cases of rebars of steel.

**7. Ductility ratio**

*Design of Cities and Buildings - Sustainability and Resilience in the Built Environment*

have been and are being constructed with ribbed bars.

capacity of reinforced concrete elements.

**6. Percent elongation of rebar**

of the rebar material.

tion at 12 and 9, respectively.

Several questions arise, viz.,

types of 500 MPa bars.

rebars against corrosion unless concrete constructions will be given surface protection in the nature of waterproofing treatment, have not sunk into the minds of all those who should have known, are obvious from the continued poor performance of the structures in **Figures 3**–**6(h)** and **(i)**, and uncounted other structures which

Kar [2] has shown that besides success and failure, and besides the issue of durability, the "effective bond" or "engagement" between rebars and the surrounding concrete may influence the load-carrying capacity, ductility and energy-absorbing

Percent elongation is an important measure of ductility of rebars, that can influence the performance of the rebar and in turn the performance of concrete elements under load as well as under exposure to the environment; Kar [14]. The percent elongation is of course a very important property that may greatly influence the survivality of reinforced concrete constructions during earthquake events.

In recognition of the fact that the changing material compositions and manufacturing processes, as well as the increasing yield strengths of rebar materials during recent decades, are generally associated with decreasing percent elongation, the Specifications of ASTM International and the Standards of BIS allow/permit the use of rebars with smaller percent elongation properties with increasing yield strength

It is recognized here that there are certain differences between the gage/gauge lengths in the ASTM and BIS test specimens. However, these differences do not

ASTM A615/A615M [18] of 12 Jan, 2016 has set the minimum percent elongation of rebars for Grades 75, 80 and 100, i.e., yield strengths of 520 MPa, 550 MPa and 690 MPa, to 7 percent for rebars having diameters up to 25 mm, and an even lower 6 percent for rebars having diameters greater than 25 mm, whereas for Grade 40 (280 MPa) and Grade 60 (420 MPa) bars, ASTM sets the minimum percent elonga-

Similarly, IS 1786 [6], through its Amendment No. 03, dated 19-09-2017, has set the minimum percent elongation at 10.0, 10.0 and 10.0 for rebars of yield strengths 600 MPa, 650 MPa and 700 MPa, whereas it has set allowable percent elongations at 14.5 to 18.0 for different varieties of 415 MPa bars, and 12.0 to 16.0 for different

a.if once it is recognized that the percent elongation of the steel material for rebars is an important and thus an inviolable property, that is to be set for acceptability of rebars, then why smaller percent elongation properties (as 6 in ASTM A615/A615M [18] and 10 in IS 1786 [6]) be considered permissible for higher yield strength materials, but not for smaller yield strength materials?

b.or, are the percent elongation properties, set in the Specifications/Standards violable, and the set properties merely represent values which certain manu-

c.how is it that when the achievable (with reasonable effort) percent elongation gets smaller and smaller with increasing yield strength, ASTM A615/A615M

facturers can achieve in the cases of bars they make?

substantially affect the following observations on percent elongation.

**32**

[18] has set the same elongation at 7 percent or 6 percent for steel having yield strengths of 520 MPa, 550 MPa and 690 MPa?


There needs to be a clear understanding of the significance of percent elongation and or ductility of rebars in the context of performance of reinforced concrete elements.

It may be desirable to set, irrespective of the yield strength of steel, a single value, below which the percent elongation or ductility will not be acceptable in the cases of rebars of steel.

In view of the fact that virtually all structures in India and in many other countries are required to be earthquake resistant, a reasonably high value may have to be set for the required percent elongation or ductility of rebars.

In this conflicting scenario, with a view to minimizing the rate of corrosion and also to improve ductility and energy absorbing capacity, PSWC-BAR, conforming to IS 2062, and possessing the property of improving "effective bond" over and above the normally available "bond", with a minimum percent elongation of 16, is recommended as the rebar of choice. The yield stress will be limited to a maximum of 550 MPa, preferably to 500 MPa; Kar [5].

Greater details on the development and mechanical properties of PSWC-BAR, together with design aid, so as to take advantage of the power of PSWC-BAR to enhance load-carrying capacity, as well as ductility and energy-absorbing capacities of reinforced concrete elements, are provided in the article: The Search for an Ideal Rebar for Durable Concrete Construction Leads to PSWC-BAR; Kar [5].
