**Abstract**

Reinforced concrete is the number one medium of construction. It is important to have good quality concrete and reinforcing bar (rebar). It is equally important to have competent bond between rebar and concrete. About six decades ago ribbed rebars of high strength steel started replacing plain round bars of mild steel, the use of which had made reinforced concrete constructions durable. It was overlooked that ribbed rebars of carbon steel would be highly susceptible to corrosion at accelerated rates. That would not only make reinforced concrete constructions reach states of distress early, that could also destroy or reduce bond between ribbed rebars and concrete. The continued use of ribbed rebars of high strength carbon steel demonstrates a widespread lack of understanding of the phenomenon of bond between rebars and concrete. This lack of understanding of bond has led to the introduction of epoxy coated ribbed rebars, ribbed stainless steel bars and glass fiber reinforced and granite reinforced polymer rebars, all of which permit reinforced concrete carry static loads because of engagement between such rebars and concrete. But the load-carrying capacity of reinforced concrete elements is impaired, and such elements become vulnerable to local or even total failure during vibratory loads. The use of PSWC-BAR, characterized by its plain surface and wave-type configuration, permits the use of medium strength and high strength steel. In the absence of ribs, the rate of corrosion is greatly reduced. The use of PSWC-BARs, at no added effort or cost, in lieu of conventional ribbed bars, leads to enhancement of effective bond or engagement between such rebars and concrete, thereby leading to increased load-carrying capacity, several-fold higher life span, ductility and energy-absorbing capacity, and great reduction in life cycle cost and adverse impact of construction on the environment and the global climate. In keeping with a lack of understanding of bond between rebars and concrete, there is arbitrariness in the selection of the required level of percent elongation and ductility of rebars.

**Keywords:** bond, corrosion, deformed bar, durability of concrete constructions, percent elongation, PSWC-BAR, reinforcing bar, ribbed rebar

## **1. Introduction**

Reinforced concrete is the number one medium of construction, in which reinforcing bar (rebar) is one of the two component elements; the other element being concrete.

It was in the mid-nineteenth century when builders in different countries experimented with concrete, reinforced with steel elements of different types.

Easy availability of the component materials, easy formability, rigidity, strength, safety and durability of reinforced concrete construction made more and more people interested in such constructions.

Plain round bars of mild steel became the standard rebar.

The time-dependent performance of concrete structures, reinforced with such bars, set the standards of performance in the context of durability.

Besides the external elements, e.g., water/moisture, oxygen, carbon dioxide, chlorides, sulphates, alkalis, and other deleterious materials, which can have destabilizing effects on concrete constructions, it cannot be overlooked that the intrinsic properties of the two principal constituent materials, viz., concrete and rebars, have much to do with durability of reinforced concrete; Kar [1].

Besides concrete and rebar, "bond" between concrete and rebar, though not a material by itself, and though no one buys it or pays for it like they buy or pay for concrete and rebar, is a property that is no less important than concrete and rebar are in the context of reinforced concrete construction.

Very little consideration has been given to what leads to good "bond", and what can prevent "bond" between concrete and reinforcing elements. Also important can be the selection of an appropriate percent elongation, better still, ductility, of the material of the rebar.

In the context of "bond" and its influence on the performance of reinforced concrete, Kar [2] has suggested three terms, viz., "bond", "effective bond" and "engagement". While the last two are synonymous, that cannot be said of "bond".

Kar [2] has shown that the quality of "engagement" between rebar and concrete can greatly influence the performance of reinforced concrete elements and structures.

Buoyed by the performance of reinforced concrete, with plain round bars as rebars, engineers thought of making reinforced concrete constructions more economical by using rebars of higher strength steel.

Gradually, many different types of round reinforcing bars were introduced; Abrams [3].

Forgetful of earlier unsatisfactory experiences in the nineteenth and early twentieth century with bars, having different types of protrusions on the surface, engineers decided that the use of high strength steel would be possible by increasing the bond between rebar and concrete by providing ribs on the surface of such rebars.

Plain round bars of mild steel thus gave way to rebars of high strength steel wherein the bars are characterized by the presence of ribs on the surface (**Figures 1** and **2**). Ribbed bars were introduced in the belief that ribbed surfaces would increase bond between rebars and concrete.

The provision of ribs on the surface of rebars of high strength steel was facilitated in 1947 by ASTM International [4] publishing ASTM A305, that provided Specifications on rebar deformation patterns.

Contrary to the beliefs and expectations that (a) the presence of ribs on the surface of rebars of steel would increase the "bond" between rebars and the surrounding concrete, and (b) there would be no detrimental effect of the ribs on the performance of concrete constructions, which may be reinforced with ribbed rebars, the presence of ribs on the surface of rebars may create void spaces, at isolated locations, between rebars and concrete, thereby decreasing "bond". However, the wedge action of ribs, together with the reduced "bond", may (or may not) lead to an increase in the "engagement" between rebars and concrete.

No thought was spared as to the likely consequences the use of bars, with surface deformations or ribs, could have on the long term performance, or even on the immediate performance and load-carrying capacities of reinforced concrete constructions; Kar [1, 2, 5].

**15**

bars (**Figure 2**).

**Figure 1.**

**Figure 2.**

*decade of the 1960's.*

or TMT type or not.

*Rebars for Durable Concrete Construction: Points to Ponder*

Engineers and manufacturers of rebars blindly followed the lead of ASTM International. The Bureau of Indian Standards (BIS) in India published the Standard IS 1786 on High Strength Deformed Steel bars and Wires for Concrete

*Typical high strength TMT rebars with surface deformations, which replaced plain round bars starting the* 

*Typical cold twisted deformed (CTD) rebar, with lugs and protrusions on the surface and stresses beyond yield* 

*on the entire body, which replaced plain round bars starting the decade of the 1960's.*

Though plain round bars, as in IS 432 (Part I) [7], and Grade A bars in IS 2062 [8] were available, gradually plain round bars gave way to ribbed bars where the strength of steel in rebars was increased artificially by twisting the bars beyond

With time, manufacturers of rebars in India and elsewhere adopted the technique of increasing strength through the centuries-old practice of quenching, couched in diplomatic language as thermomechanical treatment, giving rise to TMT

During the last sixty years or so, almost all reinforced concrete constructions worldwide have been with ribbed rebars of high strength steel, whether of the CTD

The time-dependent performances of concrete structures (**Figures 3**–**5**), reinforced with these later day rebars, failed to match the time-dependent performance of concrete structures, which were reinforced with plain round bars of mild steel. The relatively poor performance of concrete structures since the introduction of high strength rebars, with surface deformations, has caused worldwide concern.

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

Reinforcement --- Specification [6].

yield at a cold state, giving rise to CTD bars (**Figure 1**).

*Rebars for Durable Concrete Construction: Points to Ponder DOI: http://dx.doi.org/10.5772/intechopen.95401*

#### **Figure 1.**

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

Plain round bars of mild steel became the standard rebar.

much to do with durability of reinforced concrete; Kar [1].

are in the context of reinforced concrete construction.

economical by using rebars of higher strength steel.

bars, set the standards of performance in the context of durability.

people interested in such constructions.

material of the rebar.

structures.

Abrams [3].

between rebars and concrete.

constructions; Kar [1, 2, 5].

Specifications on rebar deformation patterns.

Easy availability of the component materials, easy formability, rigidity, strength,

The time-dependent performance of concrete structures, reinforced with such

Besides the external elements, e.g., water/moisture, oxygen, carbon dioxide, chlorides, sulphates, alkalis, and other deleterious materials, which can have destabilizing effects on concrete constructions, it cannot be overlooked that the intrinsic properties of the two principal constituent materials, viz., concrete and rebars, have

Besides concrete and rebar, "bond" between concrete and rebar, though not a material by itself, and though no one buys it or pays for it like they buy or pay for concrete and rebar, is a property that is no less important than concrete and rebar

Very little consideration has been given to what leads to good "bond", and what can prevent "bond" between concrete and reinforcing elements. Also important can be the selection of an appropriate percent elongation, better still, ductility, of the

In the context of "bond" and its influence on the performance of reinforced concrete, Kar [2] has suggested three terms, viz., "bond", "effective bond" and "engagement". While the last two are synonymous, that cannot be said of "bond". Kar [2] has shown that the quality of "engagement" between rebar and concrete can greatly influence the performance of reinforced concrete elements and

Buoyed by the performance of reinforced concrete, with plain round bars as rebars, engineers thought of making reinforced concrete constructions more

Gradually, many different types of round reinforcing bars were introduced;

the bars are characterized by the presence of ribs on the surface (**Figures 1** and **2**). Ribbed bars were introduced in the belief that ribbed surfaces would increase bond

The provision of ribs on the surface of rebars of high strength steel was facilitated in 1947 by ASTM International [4] publishing ASTM A305, that provided

Contrary to the beliefs and expectations that (a) the presence of ribs on the surface of rebars of steel would increase the "bond" between rebars and the surrounding concrete, and (b) there would be no detrimental effect of the ribs on the performance of concrete constructions, which may be reinforced with ribbed rebars, the presence of ribs on the surface of rebars may create void spaces, at isolated locations, between rebars and concrete, thereby decreasing "bond". However, the wedge action of ribs, together with the reduced "bond", may (or may not) lead

No thought was spared as to the likely consequences the use of bars, with surface deformations or ribs, could have on the long term performance, or even on the immediate performance and load-carrying capacities of reinforced concrete

to an increase in the "engagement" between rebars and concrete.

Forgetful of earlier unsatisfactory experiences in the nineteenth and early twentieth century with bars, having different types of protrusions on the surface, engineers decided that the use of high strength steel would be possible by increasing the bond between rebar and concrete by providing ribs on the surface of such rebars. Plain round bars of mild steel thus gave way to rebars of high strength steel wherein

safety and durability of reinforced concrete construction made more and more

**14**

*Typical cold twisted deformed (CTD) rebar, with lugs and protrusions on the surface and stresses beyond yield on the entire body, which replaced plain round bars starting the decade of the 1960's.*


#### **Figure 2.**

*Typical high strength TMT rebars with surface deformations, which replaced plain round bars starting the decade of the 1960's.*

Engineers and manufacturers of rebars blindly followed the lead of ASTM International. The Bureau of Indian Standards (BIS) in India published the Standard IS 1786 on High Strength Deformed Steel bars and Wires for Concrete Reinforcement --- Specification [6].

Though plain round bars, as in IS 432 (Part I) [7], and Grade A bars in IS 2062 [8] were available, gradually plain round bars gave way to ribbed bars where the strength of steel in rebars was increased artificially by twisting the bars beyond yield at a cold state, giving rise to CTD bars (**Figure 1**).

With time, manufacturers of rebars in India and elsewhere adopted the technique of increasing strength through the centuries-old practice of quenching, couched in diplomatic language as thermomechanical treatment, giving rise to TMT bars (**Figure 2**).

During the last sixty years or so, almost all reinforced concrete constructions worldwide have been with ribbed rebars of high strength steel, whether of the CTD or TMT type or not.

The time-dependent performances of concrete structures (**Figures 3**–**5**), reinforced with these later day rebars, failed to match the time-dependent performance of concrete structures, which were reinforced with plain round bars of mild steel.

The relatively poor performance of concrete structures since the introduction of high strength rebars, with surface deformations, has caused worldwide concern.

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

**Figure 3.** *Distress in staging of overhead water reservoir due to corrosion in rebars.*

#### **Figure 4.**

*Abandoned hospital building a decade after construction in the new township of Salt Lake City, Kolkata.*

#### **Figure 5.**

*Typical distress in ground level columns caused by rust in ribbed TMT bars in a 10 year old building in Kolkata.*

**17**

**Figure 6.**

*Rebars for Durable Concrete Construction: Points to Ponder*

There had to be reasons, and the reasons were not unknown; Alekseev [9, 10], and Kar [1, 5, 11–17]. But engineers and manufacturers of rebars paid no heed.

*A collection of plain bars free from rust and ribbed CTD and TMT bars with various stages of corrosion.*

The rebars, with surface deformations, are today covered by the Indian Standard IS 1786 [6] for high strength deformed steel bars. The Standard covers both CTD and TMT bars. ASTM International in the USA published quite a few Specifications

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

#### *Rebars for Durable Concrete Construction: Points to Ponder DOI: http://dx.doi.org/10.5772/intechopen.95401*

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

*Distress in staging of overhead water reservoir due to corrosion in rebars.*

*Typical distress in ground level columns caused by rust in ribbed TMT bars in a 10 year old building in* 

*Abandoned hospital building a decade after construction in the new township of Salt Lake City, Kolkata.*

**16**

**Figure 5.**

**Figure 3.**

**Figure 4.**

*Kolkata.*

There had to be reasons, and the reasons were not unknown; Alekseev [9, 10], and Kar [1, 5, 11–17]. But engineers and manufacturers of rebars paid no heed.

The rebars, with surface deformations, are today covered by the Indian Standard IS 1786 [6] for high strength deformed steel bars. The Standard covers both CTD and TMT bars. ASTM International in the USA published quite a few Specifications

#### **Figure 6.**

*A collection of plain bars free from rust and ribbed CTD and TMT bars with various stages of corrosion.*

on ribbed rebars of high strength steel. The most commonly used rebars are covered in ASTM A615/A615M [18].

In terms of durability, the structures may be adversely affected because of the inability of concrete to stand up to the external elements, e.g., chlorides, sulphastes, etc. or even to water as its presence may permit alkali-silica reaction in concrete in certain cases.

Most often, the durability of concrete constructions is adversely affected by corrosion in the steel rebars in the case of reinforced concrete (**Figures 3–6(h)** and **(i)**), and by corrosion in the wires and strands of steel in the case of prestressed concrete.

Though less frequent, corrosion in ribbed rebars (**Figure 2**), used as secondary reinforcement in prestressed concrete constructions, can trigger unacceptable conditions of distress in prestressed concrete constructions.

The focus here is on rebars and durability of reinforced concrete constructions, as influenced by rebars.
