**2. Observations following**

#### **2.1 The use of ribbed bars**

Following the use of ribbed bars of high strength steel, the world has seen a significant fall in the long term performance of reinforced concrete constructions. Sights of decay and distress in concrete constructions, reinforced with ribbed rebars of steel, became inescapable (**Figures 3**–**5**) within years of their construction.

A 1999 survey of bridges and buildings of reinforced concrete construction in the public domain in and around Kolkata, India revealed that while none of the structures, built since the 1940s with plain round bars of mild steel, showed any sign of distress, all the structures built with ribbed bars (**Figure 1**) in the 1970s and 1980s were showing signs of distress; Kar [11].

In a 1991 article in ACI Materials Journal, American Concrete Institute, Papadakis, Vayenas and Fardis [19] wrote: "The last two decades have seen a disconcerting increase in examples of the unsatisfactory durability of concrete structures, specially reinforced concrete ones."

Sixteen years later in 2007, Swamy [20] from UK was more forthright in his expression when he wrote in the Indian Concrete Journal: "The most direct and unquestionable evidence of the last two/three decades on the service life performance of our constructions and the resulting challenge that confronts us is the alarming and unacceptable rate at which our infrastructure systems all over the world are suffering from deterioration when exposed to real environments."

An analysis of the observations by Papadakis et al. [19], by Swamy [20] and by others leads to the recognition that the relatively poor performance of reinforced concrete constructions followed the start of use of ribbed rebars of high strength steel.

**Figure 5** shows typical conditions of concrete columns, reinforced with ribbed rebars (**Figure 2**), ten years after the construction of a building in Kolkata. All the columns at the ground level of the building suffered a similar fate.

The findings of the 1999 survey as well as the structures in **Figures 3**–**5** show clearly that compared to concrete structures, reinforced with plain round bars of mild steel, concrete structures, reinforced with ribbed bars of medium strength and high strength steel, reach states of distress much earlier.

**19**

**Figure 7.**

*Rebars for Durable Concrete Construction: Points to Ponder*

c.the presence of ribs on the surface of today's rebars.

This excessive corrosion in ribbed rebars of carbon steel suggests that the susceptibility of ribbed rebars to corrosion at accelerated rates is an intrinsic nature

However, there had been hesitation by engineers in recognizing that today's ribbed bars were highly susceptible to corrosion at accelerated rates, and this exces-

a.the damages caused to the ribs at the time of provision of ribs on the surface

b.the damages caused to the ribs at the time of transportation and handling of

The hesitation to recognize ribs as a principal cause of excessive corrosion in rebars led not only to the continued condemnation of all new reinforced concrete constructions to early decay, distress and failure, but also to ASTM International, BIS and such other organizations publishing multiple Specifications/Standards on rebars as imagined solutions to the problem of early distress in reinforced concrete constructions, e.g., ASTM International publishing A775 [21] for epoxy coated ribbed bars, and on its failure to solve the problem, ASTM International A955/A955M for Deformed and Plain Stainless Steel Bars [22], and when that did not work, ASTM International published A1055 [23] for zinc (first coat) and epoxy (2nd coat), which too has serious limitations, as epoxy coating prevents the all-important "bond" with

The lack of "bond" can have serious consequences: (a) cracks in structures (**Figure 6(g)**), (b) lowered load carrying capacities (Kar [2]), and (c) chunks of

Like epoxy coated bars, stainless steel bars too fail to solve the problem, as ribbed bars of stainless steel too may corrode under conditions of exposure of concrete structures to chlorides, and additionally such bars may not bond or may

Failing to recognize that the problem of early distress in today's reinforced concrete constructions is due to the use of ribbed rebars of steel as in the Indian Standard IS 1786 [6], BIS published the Indian Standard IS 13620 [24] for Fusion

Just as BIS failed to recognize that the problem of early distress in reinforced concrete constructions started with the use of ribbed bars as in IS 1786, BIS also

*Concrete easily separates from epoxy coated rebars under vibratory loading conditions whereas all structures* 

*are required to resist vibratory loads due to earthquakes; separation led to failure of buildings.*

concrete falling (**Figure 9**) or even structures collapsing (**Figure 7**).

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

sive corrosion in today's rebars is due to.

of ribbed rebars of carbon steel.

rebars

concrete (**Figures 7** and **8**).

not bond well with concrete.

Bonded Epoxy Coated Reinforcing bars.

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

in ASTM A615/A615M [18].

certain cases.

stressed concrete.

as influenced by rebars.

**2. Observations following**

1980s were showing signs of distress; Kar [11].

specially reinforced concrete ones."

**2.1 The use of ribbed bars**

construction.

strength steel.

on ribbed rebars of high strength steel. The most commonly used rebars are covered

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

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 pre-

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

The focus here is on rebars and durability of reinforced concrete constructions,

Following the use of ribbed bars of high strength steel, the world has seen a significant fall in the long term performance of reinforced concrete constructions. Sights of decay and distress in concrete constructions, reinforced with ribbed rebars of steel, became inescapable (**Figures 3**–**5**) within years of their

A 1999 survey of bridges and buildings of reinforced concrete construction in the public domain in and around Kolkata, India revealed that while none of the structures, built since the 1940s with plain round bars of mild steel, showed any sign of distress, all the structures built with ribbed bars (**Figure 1**) in the 1970s and

In a 1991 article in ACI Materials Journal, American Concrete Institute, Papadakis, Vayenas and Fardis [19] wrote: "The last two decades have seen a disconcerting increase in examples of the unsatisfactory durability of concrete structures,

Sixteen years later in 2007, Swamy [20] from UK was more forthright in his expression when he wrote in the Indian Concrete Journal: "The most direct and unquestionable evidence of the last two/three decades on the service life performance of our constructions and the resulting challenge that confronts us is the alarming and unacceptable rate at which our infrastructure systems all over the world are suffering from deterioration when exposed to real environments." An analysis of the observations by Papadakis et al. [19], by Swamy [20] and by others leads to the recognition that the relatively poor performance of reinforced concrete constructions followed the start of use of ribbed rebars of high

**Figure 5** shows typical conditions of concrete columns, reinforced with ribbed rebars (**Figure 2**), ten years after the construction of a building in Kolkata. All the

The findings of the 1999 survey as well as the structures in **Figures 3**–**5** show clearly that compared to concrete structures, reinforced with plain round bars of mild steel, concrete structures, reinforced with ribbed bars of medium strength and

columns at the ground level of the building suffered a similar fate.

high strength steel, reach states of distress much earlier.

conditions of distress in prestressed concrete constructions.

**18**

This excessive corrosion in ribbed rebars of carbon steel suggests that the susceptibility of ribbed rebars to corrosion at accelerated rates is an intrinsic nature of ribbed rebars of carbon steel.

However, there had been hesitation by engineers in recognizing that today's ribbed bars were highly susceptible to corrosion at accelerated rates, and this excessive corrosion in today's rebars is due to.


The hesitation to recognize ribs as a principal cause of excessive corrosion in rebars led not only to the continued condemnation of all new reinforced concrete constructions to early decay, distress and failure, but also to ASTM International, BIS and such other organizations publishing multiple Specifications/Standards on rebars as imagined solutions to the problem of early distress in reinforced concrete constructions, e.g., ASTM International publishing A775 [21] for epoxy coated ribbed bars, and on its failure to solve the problem, ASTM International A955/A955M for Deformed and Plain Stainless Steel Bars [22], and when that did not work, ASTM International published A1055 [23] for zinc (first coat) and epoxy (2nd coat), which too has serious limitations, as epoxy coating prevents the all-important "bond" with concrete (**Figures 7** and **8**).

The lack of "bond" can have serious consequences: (a) cracks in structures (**Figure 6(g)**), (b) lowered load carrying capacities (Kar [2]), and (c) chunks of concrete falling (**Figure 9**) or even structures collapsing (**Figure 7**).

Like epoxy coated bars, stainless steel bars too fail to solve the problem, as ribbed bars of stainless steel too may corrode under conditions of exposure of concrete structures to chlorides, and additionally such bars may not bond or may not bond well with concrete.

Failing to recognize that the problem of early distress in today's reinforced concrete constructions is due to the use of ribbed rebars of steel as in the Indian Standard IS 1786 [6], BIS published the Indian Standard IS 13620 [24] for Fusion Bonded Epoxy Coated Reinforcing bars.

Just as BIS failed to recognize that the problem of early distress in reinforced concrete constructions started with the use of ribbed bars as in IS 1786, BIS also

#### **Figure 7.**

*Concrete easily separates from epoxy coated rebars under vibratory loading conditions whereas all structures are required to resist vibratory loads due to earthquakes; separation led to failure of buildings.*

#### **Figure 8.**

*The bond between epoxy coated rebar and concrete will be negligible, as seen in a column; the ribs on the surface of rebars engage the concrete up to a limit and that too when the loading is monotonous; absence of bond led to lower load-carrying capacity.*

#### **Figure 9.**

*A view of the deck of the Jogeswari flyover in Mumbai seven years after construction; concrete separated from rebars with poor bond qualities.*

failed to recognize that, as cautioned in SubSection 5.6.1 of its Standard IS 456 [25], epoxy coated bars would not bond with the surrounding concrete, whereas the availability of the required "bond" is an essential requirement for reinforced concrete.

Similarly, as ASTM International published Specifications on epoxy coated bars and stainless steel bars, without a recognition or understanding of the basic cause(s) of early distress in reinforced concrete constructions of recent decades, and the significance of "bond" between rebars and the surrounding concrete, BIS in India followed suit by publishing the Indian Standard IS 16651:2017 on High Strength Deformed Stainless Steel bars and Wires for Concrete Reinforcement Specification [26].

The story is the same in many other countries. It is recognized here that:

a.corrosion in rebars is greatly influenced by the intrinsic nature of the particular rebars; e.g., stainless steel bars will not generally corrode whereas mild steel and medium tensile steel bars will corrode, and high tensile strength steel bars with higher carbon contents will corrode more and at faster rates

**21**

*Rebars for Durable Concrete Construction: Points to Ponder*

**3. Intrinsic susceptibility of ribbed bars to corrosion**

a.the provision and presence of ribs invite corrosion

b.the surface conditions/features on the rebar influence the rate of corrosion; the provision and the presence of ribs, as in bars conforming to IS 1786 [6] and ASTM A615/A615M [18] lead to acceleration in the rate of corrosion; Alekseev

c.the manufacturing process influences the rate of corrosion; by stretching/stressing the bars beyond yield, the CTD process leads to corrosion at accelerated rates; the TMT process too hastens corrosion due to stresses from quenching effort; Alekseev

It has been recognized earlier that the problem of early distress in reinforced concrete structures started showing up following the start of use of steel reinforcing

**Figure 6(c)** shows corrosion all over the surface of relatively fresh ribbed

The four bottom bars in **Figure 6(b)** show the start of corrosion preferentially at the ribs of untwisted ribbed bars while the four top bars show corrosion all over the surface of the ribbed bars as a consequence of stressing the bars

b.high stresses, specially stresses beyond yield, lead to corrosion at accelerated

It cannot be overlooked that the ribs were provided out of a perceived necessity of improved "bond" between rebar and concrete when the rebars were upgraded from low-carbon to medium carbon or high carbon steel for higher strength. The truth is that the presence of ribs on the surface of rebars decreases "bond" between rebars and concrete. But the ribs may provide greater resistance to longitudinal movement of the bars relative to the surrounding concrete. Also, as found in the preceding, the ribs encourage corrosion in rebars; Alekseev [9] and Kar [1, 5].

Whether of the CTD or TMT type, or not, the reasons for ribbed bars of carbon

1. residual stresses develop at the bases of ribs during the manufacturing stage

2. cracks or surface damages, which trigger corrosion, may develop at the ribs at

3.nominal stresses in ribbed rebars under load are enhanced in keeping with the phenomenon of stress concentration due to the presence of ribs or cracks

4. additional stresses develop in ribs in a loaded structure due to the wedge action

5.the sum-total of stresses and strains in Items 1 to 4 approach or reach yield

steel being intrinsically susceptible to corrosion at accelerated rates are:

the time of manufacture, during transportation and handling

of such ribs against surrounding concrete

stress or strain levels

**Figure 6(d)** shows the start of corrosion at the ribs of TMT bars.

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

[9, 10], and Kar [1, 5, 12]

[9, 10], and Kar [1, 5, 11, 12].

bars with ribs on the surface.

TMT bars.

beyond yield.

rates.

These show that:

*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*

failed to recognize that, as cautioned in SubSection 5.6.1 of its Standard IS 456 [25], epoxy coated bars would not bond with the surrounding concrete, whereas the availability of the required "bond" is an essential requirement for reinforced

*A view of the deck of the Jogeswari flyover in Mumbai seven years after construction; concrete separated from* 

*The bond between epoxy coated rebar and concrete will be negligible, as seen in a column; the ribs on the surface of rebars engage the concrete up to a limit and that too when the loading is monotonous; absence of* 

Similarly, as ASTM International published Specifications on epoxy coated bars and stainless steel bars, without a recognition or understanding of the basic cause(s) of early distress in reinforced concrete constructions of recent decades, and the significance of "bond" between rebars and the surrounding concrete, BIS in India followed suit by publishing the Indian Standard IS 16651:2017 on High Strength Deformed Stainless Steel bars and Wires for Concrete Reinforcement

a.corrosion in rebars is greatly influenced by the intrinsic nature of the particular rebars; e.g., stainless steel bars will not generally corrode whereas mild steel and medium tensile steel bars will corrode, and high tensile strength steel bars

with higher carbon contents will corrode more and at faster rates

**20**

concrete.

**Figure 9.**

*rebars with poor bond qualities.*

**Figure 8.**

*bond led to lower load-carrying capacity.*

Specification [26].

It is recognized here that:

The story is the same in many other countries.

