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C-Bar :  A Rebar for Durable Concrete Construction

Dr Anil K Kar Engineering Services International BC 96 Salt Lake City, Kolkata - 700064

permakar@engineerings.in

KEY WORDS

C-bar; concrete constructions;  concrete  structures; corrosion;  ductility; durable concrete constructions; durable concrete structures; durability; energy absorbing capacity; rebar; reinforced concrete; reinforcement; reinforcing bar INTRODUCTION Reinforced concrete tops the list of construction materials.

The easy availability of the component materials of concrete, easy formability and durability of reinforced concrete structures, built with concrete up to the 1950’s, all contributed towards making concrete the most preferred material of construction.

Properties of cement and reinforcing bars (rebars), the two principal components in reinforced concrete, have, however, undergone dramatic changes over the years, more particularly after the 1950’s, resulting in quicker and stronger constructions. On the flip side, though reinforced concrete structures of earlier periods proved to be durable, such structures, built during recent decades, have been characterized by early decay and distress.

This early decay and distress in concrete constructions of recent periods is a worldwide phenomenon, and the phenomenon started becoming prominent in the 1970’s.

Historically, structures started developing early signs of distress following the introduction of high strength rebars of steel in the 1960’s in western countries. The use of high strength deformed (HSD) bars started a decade later in India. The HSD rebars are distinguishable by the presence of protrusions/lugs/ribs  on their surface (Fig. 1). HSD rebars, with distinct yield points, are also referred to as high yield strength deformed (HYSD) bars.

Though in most cases of reinforced concrete structures, which suffered early distress, the cause of such distress could be related to corrosion in HSD rebars, other than work in Russia and work by the writer in India, there does not appear to have been any other effort towards finding the reasons for the extra susceptibility of HSD bars to corrosion.

The writer has done extensive investigative and theoretical work on the causes of intense susceptibility of HYSD and other forms of HSD rebars to corrosion. The work in Russia involved test observations.

The writer suggested solutions to the problem of corrosion in rebars in the form of (a) use of C-bars (described later) as rebars and/or (b) provision of surface protection systems (in the nature of waterproofing treatment) to concrete structures.

STATE OF  REINFORCED  CONCRETE

In an article in ACI Materials Journal in 1991 Papadakis, et al observed : “The last two decades have seen a disconcerting increase in examples of the unsatisfactory durability of concrete structures, specially reinforced concrete ones." 1 Of particular significance are the observations made by Swamy, who remarked that “the most direct and unquestionable evidence of the last two/three decades on the service life performance of present constructions and the resulting challenge that confronts us is the alarming and unacceptable rate at which the infrastructure systems all over the world are suffering from deterioration when exposed to real environments.”  Swamy also lamented the “unacceptably poor performance of reinforced concrete structures in spite of the tremendous advances that have been made in understanding of the science engineering and mechanics of materials and structures.” 2

It can help to recognize here that (a) 	both Papadakis and Swamy identified the decade of the 1970’s as the period when concrete structures started showing signs of distress early (b) 	their findings were primarily based on the performance of reinforced concrete structures (c ) 	the phenomenon of early distress in reinforced concrete structures was preceded by the introduction of HSD rebars in concrete construction in the 1960’s.

IDENTIFICATION OF SHORTCOMINGS  OF  HSD  REBARS AS A  PRELUDE  TO THE  DEVELOPMENT  OF C-BAR

Since corrosion in rebars is at the root of the problem, and since the problem of early distress in concrete structures is evident mostly in the case of constructions with HSD rebars (Fig. 1) than in the case of constructions with plain round bars of low strength steel (Fig. 2), it is quite possible that there is something inherent in HSD rebars, which makes such bars specially prone to early corrosion.

The vulnerability of HSD rebars, particularly of the cold twisted deformed (CTD) type, was amply demonstrated in a survey which was conducted in 1999 by the writer’s office.

The survey of some bridges and buildings in the public domain in Calcutta revealed that, no matter what the type of cement (ordinary Portland or slag Portland) in concrete was, the use of CTD rebars, which constituted a special form of HSD rebars, predisposed concrete structures to early decay and distress through accelerated rates of corrosion in such  rebars3-5.

The survey further demonstrated that any worsening in the atmosphere over the years could not have contributed to any selective acceleration in the process of decay and distress in reinforced concrete structures, which were constructed with CTD rebars. Similarly, it could not be suggested that, compared to the quality of workmanship in the cases of constructions with plain round bars of lower strength steel, the quality of workmanship was selectively poorer in the cases of constructions with CTD bars. There must have therefore been something inherently undesirable with CTD bars, if also not with other forms of HSD rebars.

A CTD rebar, which is a special form of HSD rebars, is distinguishable from other HSD rebars by the twisted pattern of its two longitudinal ribs. Internally, a CTD bar is characterized by its having been strained and stressed beyond yield at a cold state during the manufacturing stage itself.

The significant point to note here is that, because of design and manufacturing requirements, strains and stresses beyond yield levels remain locked in CTD bars right from the manufacturing stage.

The writer had suggested that the rate of corrosion increased with increasing stress levels and it accelerated as stresses approached yield stress levels.5

(Though the word “stress” is used repeatedly in this article, the word “strain” might be more appropriate in the context of corrosion.)

It is explained later that should strains/stresses reach or cross yield, corrosion will proceed unabated, and inside concrete such bars cannot be passivated against corrosion.

The inherent susceptibility of CTD bars (with locked in stresses and strains at and beyond yield) to corrosion can be recognized in Fig. 3.



The writer observed that though other forms of HSD bars may be superior to CTD bars, stresses (and thus strains) in the cases of such other forms of HSD rebars too would or could reach yield levels under service load conditions at least due to the combination of (a) residual stresses, which develop during the making of bars with surface protrusions (Fig. 1), (b) nominal stresses under load, coupled with their enhancement, in keeping with the phenomenon of stress concentration as a result of the presence of lugs or protrusions on the surface of HSD or high yield strength deformed (HYSD) rebars and (c) additional stresses on lugs or protrusions which may develop due to wedge action against surrounding concrete.

The writer, and the writer, together with Vij, had sought to explain that the rate of corrosion would increase with increasing strain and stress levels, particularly when such strain and stress levels would approach yield strain or stress levels, and “once strains/stresses in surface elements would cross yield levels, the naturally occurring oxide film (Fe2O3) on the surface of steel rebars would be disrupted, the intergrain regions of metal would be exposed to the corrosive environment and corrosion in the case of steel rebars would proceed at a fast pace.”3-8

The phenomenon of the rate of corrosion increasing with increasing stress levels found expression in the words of Alekseev when he stated that “experiments confirmed that stressed reinforcement is electrochemically more active than unstressed reinforcement and hence may undergo intense corrosion. Adequate data are available in the literature confirming this premise. For example, the more intense damage of more stressed sections of low-alloy steel facings on ships is quite well known. Under stresses proximate to the yield limit, the corrosion rate of structural steels increases.” 9

Since stressed metal corrodes faster than unstressed metal, and since higher stresses lead to faster rates of corrosion, HSD bars, with their surface protrusions, and as a consequence, higher strains and stresses, are bound to corrode faster than plain round bars, if both types of bars will have the same chemistry and such bars will be subjected to similar service load conditions or environments.

Rebars inside concrete, specially with OPC, could normally be expected to have protection against corrosion through passivation. The writer, and the writer, together with Vij, however, explained that as stresses and strains in surface elements of HSD rebars could reach yield levels and as the surface elements would become unstable at post-yield stages, passivation of the surface would not be possible at the unstable post-yield state. In other  words,  the  alkaline pore water, contributed by Ca(OH)2  inside concrete, would fail to passivate and protect steel against corrosion at post-yield stages, thereby making HSD rebars susceptible to early corrosion and reinforced concrete structures prone to early decay and distress.

Examples of CTD and HYSD bars undergoing corrosion (more particularly at roots of lugs) early, even before the use of such bars in construction, can be found in photographs of rebars in Figs. 3 to 5.





It has already been stressed that stresses and strains on the surface of ribbed bars are most likely to reach or cross yield levels, and the consequences can be grave. In this context, the explanations and observations of Alekseev can be recalled thus : “During elastic deformation of steel, the area of anodic sections arising as a result of the mechanical damage of oxide film is not much. The high current density arising on the microanodes as a result of the work of local micropores of the type film-pore gives rise to their anodic polarisation and the return of steel to the passive state. During ductile deformation, however, the area of anode sections increases sharply and the current density generated at the microanodes is inadequate for their passivation and the steel remains in an active state.” 9

In the above, ductile deformation refers to post-yield deformation.

If it will not be possible to passivate steel at a post-yield state, it should not be difficult to visualize that rebars in conditions, as in Figs. 3-5, will also not be passivated inside concrete, thereby leading to distress in reinforced concrete structures fairly early in life.

Furthermore, the ribs on the surface have a greater potential for causing separations between HSD rebars and the surrounding concrete, which in itself encourages corrosion.3-8

Mohammed, et al observed thus : ”Due to the formation of gaps, the bottom part of horizontal steel, shows significant macrocell and microcell corrosion………. Deformed bars corroded more than plain bars.” 10

The different Figures, observations and explanations clearly demonstrate that it is an inherent characteristic of HSD rebars that such bars, with surface lugs or protrusions and the attendant surface strains and stresses close to or beyond yield levels, are most likely to suffer early corrosion, resulting in early distress in concrete structures. It was succinctly put forward by Alekseev thus : “In accelerated tests, 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.” 9

It is thus no wonder that compared to reinforced concrete structures, built with plain round bars of earlier periods, more recently constructed reinforced concrete structures, built with HSD rebars, have performed poorly in terms of durability.

The causes behind the propensity for early corrosion in the case of HSD rebars have been identified. The problem can be magnified where HSD rebars may be quenched with water as a part of the TMT or thermo-mechanical treatment (i.e., thermal hardening) process with a view to improving certain properties of rebars. This improvement, particularly higher strength, is achieved through the TMT route by sacrificing ductility and resistance to corrosion. This happens because water, under pressure, can interfere with the formation of the protective film of Fe2O3 when the HSD bar passes through the quenching box. Moreover, the achievement of a satisfactory ratio of strength, ductility and corrosion resistance of the metal can be sensitive to not only the carbon content in the steel metal, but also to the quenching temperature and pressure.

The rebars in Fig. 5 are TMT bars of the HYSD type. The extent of corrosion has rendered the rebars unfit for construction. But the construction goes on, and the consequences cannot be desirable.

THE CONCEPT  OF  C-BAR

Recognizing that (a) corrosion in rebars is the predominant causal factor for early distress in reinforced concrete structures, (b) the process of providing ribs on the surface of rebars causes the build-up of strains/stresses, (c) transportation and handling can damage the ribs, (d) nicks at locations of damage can be the starting points of corrosion, (e) the localized presence of ribs leads to enhancement of nominal stresses in a phenomenon known as stress concentration,             (f) there is added stress due to wedge action of ribs against concrete, (g) all of these can make the total stress and strain approach or cross yield stress and strain levels, (h) the rate of corrosion accelerates as stresses/strains approach yield levels, and (i) corrosion becomes uncontrollable as stresses and strains cross yield levels, the writer11-13 proposed that rebars be in the form of     C-bars, characterized by their plain surface (i.e. devoid of any ribs or surface protrusions) and deformed (sinusoidal) configurations along the length (Fig. 6).

Technical details  on C-bar can be found in (Refs. 11-13). The articles give the background information behind the innovative idea of C-bars and these provide the description of the physical form of C-bar and its manufacturing process.

The writer suggested that three to seven mm offset (excursion of the axis of the bar from its original straight line orientation) in the deformation pattern and pitch lengths (distance between two successive peaks on the same side of the original axis) of fifteen to thirty times the diameter of the bar would be practical (Fig. 7).



The C-bars in Fig. 6 have 4.5 mm offset and 30d pitch length, where d is the diameter of the bar.

C-bar is not restricted to steel of any particular chemistry. It can be hot (preferable) or cold formed and there is no restriction to subjecting C-bar to the TMT process (if durability and performance under earthquake environments will be considerations, thermo-mechanical treatment should preferably be avoided.)

TEST FINDINGS

The test findings, reported herein, are based on tests with C-bars having 5 mm offset and         30d pitch length.

The article by Kar and Haji Sheik,14 based on work at IIT Kharagpur and at B. S. Abdur Rahman University, Chennai, shows that

a)	The deformed shape of C-bars, when provided to act as reinforcing bars in beams, does not cause any spalling of concrete. b)	The use of C-bars improves true bond (as differentiated from resistance to slippage) between C-bar and the surrounding concrete, leading to several-fold increases in ductility and energy absorbing capacity of flexural elements.

Unpublished data on column tests at Nirma University, Ahmedabad show that

a)	There is no buckling of C-bars, when used (even without lateral ties) as rebars in columns. b)	Unlike in the case of conventional rebars, there may not be any failure of columns, that could be precipitated by failure of bond between C-bars and the surrounding concrete. c)	While failure of bond between conventional bars and surrounding concrete may lead to premature failure of columns, the superior bond between C-bars and surrounding concrete permits loading till full compressive strength of concrete is reached. d)	There is room for a rethink on the performance and design of reinforced concrete columns under load.

Unpublished analytical (linear finite element analysis) work at Indian Institute of Technology Kanpur as well as at B. S. Abdur Rahman University shows that compared to the case of flexural and compression elements with conventional rebars, concrete stresses in the immediate vicinity of  C-bars are more uniform and the peaks are lower for the same superimposed load, particularly in the case of columns, thereby enhancing the safety margin (alternatively stated, increasing the load-carrying capacity) of concrete elements which may be reinforced with C-bars.

Additional experimental work for different types of performance at B. S. Abdur Rahman University has shown the superiority of C-bars over plain round bars in all respects when used as rebars. Because of high susceptibility to corrosion, ribbed bars were not considered in the tests.

BENEFITS OF  USING  C-BAR

Given the fact that the world-wide occurrence of signs of distress in concrete structures fairly early in life started showing up following the start of use of ribbed bars, and given the fact that the localized formation and presence of ribs invite corrosion, the absence of ribs or lugs or protrusions on the surface of C-bars can greatly help in limiting the rate of corrosion in such bars, and therewith delay   the   development   or  onset  of  conditions  of  distress  in  reinforced  concrete structures.

Since the use of C-bars, in lieu of conventional rebars with surface deformations, will minimize corrosion, strength and ductility of structures will be preserved over longer periods of time, and thus, should structures be subjected to occasional overloads, e.g. earthquakes, concrete structures, reinforced with C-bars, will have better chances of survivability.

Also, given the preliminary findings that the use of C-bars, as rebars, very significantly improves the composite behaviour of reinforced concrete, together with several-fold enhancement in the energy-absorbing capacity (very important for survivability in earthquake situations) of concrete flexural elements and a very significant increase in the load carrying capacity of concrete columns, C-bar appears to have the potential of becoming the standard rebar for the design and construction of reinforced concrete structures, specially when it is recognized that all of the improvements (reduced rebar corrosion and longer life span of concrete structures, higher strength for columns, greater resistance to earthquakes, etc.) can be achieved without any extra processing and without having to spend anything beyond what it would cost to manufacture any other rebar of the same chemistry.

All of these imply

i)	less demand (over a period of time) on environmental resources, which provide the basic construction materials, e.g., aggregates, ores for rebars, water, materials for formwork ii)	reduced environmental pollution and degradation, whether under, on or above ground

CONCLUDING REMARKS

C-bar, with its characteristic plain surface and sinusoidal configuration and with its excellent capabilities for enhancing the life span and energy absorbing capacity of reinforced concrete structures, thereby saving lives during earthquakes, and lessening the impact on the environment, all at no added cost, has the potential to be the standard rebar for the construction of reinforced concrete structures.

In order that this potential can be fully exploited, it will be in order to conduct tests with larger span (say in excess of three metres) beams and taller (say at least three metres) columns such that adverse effects, if any, of using larger diameter C-bars can be identified.

REFERENCES

1.	Papadakis, V. G., Vayenas, C. G., and Fardis, M. N., “Physical and Chemical Characteristics Affecting the Durability of Concrete,” ACI Materials Journal, American Concrete Institute, March - April, 1991. 2.	Swamy, R. N., “Infrastructure Regeneration : the Challenge of Climate Change and Sustainability ─ Design for Strength or Durability,” The Indian Concrete Journal, The ACC Ltd., Vol. 81, No. 7, July 2007, Mumbai. 3.	Kar, A. K., “Deformed Reinforcing Bars and Early Distress in Concrete Structures,” Highway Research Bulletin, No. 65, Indian Roads Congress, December 2001, New Delhi, pp. 103-114. 4.	Kar, A. K., “Deformed Rebars in Concrete Construction,” New Building Materials & Construction World, Vol. 12, Issue 6, December 2006, New Delhi, pp. 82,83,86,88,90, 92,94,96,98,100 and 101, www.nbmcw.com 5.	Kar, A. K., “Concrete Structures ─ the pH Potential of Cement and Deformed Reinforcing Bars,” Journal of the Institution of Engineers (India), Civil Engineering Division, Vol. 82, June 2001, Calcutta, pp. 1-13. 6.	Kar, A. K., “Waterproofing  of  structures  :  Challenges  and  solutions,” New Building Materials & Construction World, Vol. 11, Issue 10, April, 2006, New Delhi, pp. 110-128, www.nbmcw.com 7.	Kar, A. K., and Vij, S. K., “Waterproofing of Structures for Durability”, New Building Materials & Construction World; Vol. 15, Issue-10, April 2010, New Delhi;. pp. 118-132, 172, www.nbmcw.com 8.	Kar, A. K., and Vij, S. K., “Enhancing the Life Span of Concrete Bridges,” New Building Materials & Construction World, Vol. 15, Issue 6, December 2009, New Delhi, pp. 114-156, www.nbmcw.com 9.	Alekseev, S. N., “Corrosion of Steel Reinforcement,” Chapter 7 in Moskvin, V. (edited by), translated from the original by V. Kolykhmatov, “Concrete and Reinforced Concrete Deterioration and Protection,” 1990, English translation, Oxford & IBH Publishing Co. Pvt. Ltd. New Delhi, 1993, original Mir Publishers, Moscow 1990. 10.	Mohammed, T.U., Otssuki, N., and Hisada, M., “Corrosion of Steel Bars with respect to orientation,” ACI Materials Journal, American Concrete Institute, March-April, 1999. 11.	Kar, A. K., “Improved Rebar for Durable Concrete Constructions”, New Building Materials & Construction World; New Delhi; Vol. 16, Issue-1, July 2010, pp. 180,182,184,186,188,190, 194,196,198-199, www.nbmcw.com 12.	Kar, A. K., “Rebar for Durable Bridge and Other Concrete Constructions”, Indian Highways; Vol. 39, No. 3; The Indian Roads Congress (IRC), New Delhi; March 2011, pp. 59-65. 13.	Kar, A. K., “A Rebar for Durable Concrete Construction”, The Masterbuilder; Vol. 13, No. 7, Chennai; July 2011, pp. 224-226,228-230,232-234,236, www.masterbuilder.co.in 14. Kar, A. K., and Haji Sheik Mohammed, M. S., “Performance of Concrete Flexural Elements Reinforced with C-Bars”, The Masterbuilder; Vol. 14, No. 7, Chennai; July 2012, pp.194-196,198-200, www.masterbuilder.co.in