Concrete does have its weakness in resisting tensile forces, flexural tension, shear and torque. Strengthening elements, viz., steel reinforcing bars (rebars), prestressing elements, fibres, etc. are used to overcome the weakness.
A survey1-4 by Engineering Services International on concrete bridges and buildings in Kolkata has clearly revealed that the widespread existence of good old structures and neighboring new structures in distressed conditions cannot justify any suggestion that the greater corrosiveness of the environment of recent times is primarily responsible for early distress in recently constructed concrete structures.
Furthermore, the widespread and universal distribution of distressed concrete structures of recent vintage discounts poor workmanship as being the main cause of early distress in such structures.
It was borne out in the survey of public domain structures in Calcutta1-4 that high strength rebars, with surface deformations and limited ductility, could be largely to blame for the early decay and distress in reinforced concrete structures of recent decades. These high strength rebars were introduced about four decades ago. Today virtually all reinforced concrete construction is with such rebars.
Over the years, starting about four decades ago, the composition of cement has changed and the period of wet curing of concrete has also gone down from 28 days to 3-14 days or even less. These two features of materials and construction practices also had deleterious effects on the durability of concrete structures of recent construction5,6.
Today’s concrete structures are thus inherently condemned to be much less durable (Figure. 1) than concrete structures of the past. Like the recently constructed Jogeshwari flyover in Mumabi (Figure. 2), the flyover at Thane also had to be shut down for a week for repairs.
High Strength Rebars with Surface DeformationsAs stated, it was found in a survey that the use of high strength rebars with surface deformations, as opposed to the use of plain round bars of mild steel, decisively condemned reinforced concrete structures to early decay and distress.
The principal reasons for such early corrosion in high strength rebars with surface deformations are (a) lack of ductility, (b) stress concentration effect due to the presence of lugs and ribs, on the surface, leading to local stresses becoming much higher (evenbeyond yield) than the average stress, (c) stress corrosion, in which stresses beyond yield lead to early corrosion, and (d) greater microcell and macrocell formation in the case of rebars with surface deformations than in the case of plain round bars inside concrete1- 5,7. There are other reasons too1-5, e.g., cold twisting beyond yield in the case of cold twisted rebars (e.g., Torsteel) with surface deformations, other manufacturing stresses, damages to surface lugs and ribs during transportation, etc.
Confirmation of early distress in high strength rebars with surface deformations can be found in the words of Alekseev, et al8, who has reported that “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.”
Figures 3-5 vividly show the effects of stress concentration and stress corrosion in rebars even before concreting.
In a nutshell, high strength steel rebars with surface deformations are highly susceptible to early corrosion and the use of such rebars predisposes concrete structures to early decay and distress. The resulting state of today’s concrete structures is aptly recorded in Technical Circular 1/99 of the Central Public Works Department (CPWD), Government of India9, wherein it is stated that while structure as old as 50 years provide adequate service, the recent constructions show signs of distress within a couple of years of their completion.
Cement—OPC SPC and PPC
Thus, with a gradual shift towards a greater use of blended cement (PSC and PPC) in the construction of bridges, roadways and other structures, there will be more frequent instances of early distress in concrete structures, as, with the increasing use of blended cement, the advancement of the carbonation front into concrete structures will be deeper and faster, and as soon as the carbonation front will reach within a millimeter or so of the rebar or prestresseing element, such elements will be de passivated and corrosion will start in the steel elements5,10 if a moist environment will be available around such steel elements.
Thus, with a greater shift towards the use of PSC and PPC in the construction of concrete bridges, roadways and other structures, coupled with the use of high strength rebars with surface deformations, instead of plain round bars of mild steel, more of concrete structures will reach states of distress early.
Changes in OPCIn order to gain high early strength, OPC particles were made smaller and smaller over the years, contributing to high heat of hydration on concreting. Though the higher heat of hydration (due to greater specific surface of cement) has helped to partly mitigate the ill effects of inadequate curing, the development of harmful thermal stresses and cracks in concrete structures, specially when concreting is done in hot climates, has been detrimental to the life of concrete structures, as the cracks, if left unattended, permit easier ingress of water and harmful gaseous elements.
It is not just that the particle sizes of cement have become much smaller over the years during the last four decades, the C3S/C2S ratio in cement has increased very significantly, thereby increasing the heat of hydration, thermal stresses and thermal cracking in concrete.
The changes mean that the long term performance of today’s concrete structures, constructed with OPC, cannot match the long term performance of concrete structures, which were similarly built with OPC concrete a few decades ago.
Water Soluble AlkalisCements in India these days are generally found to have alkalis to the extent of 1.0 percent to 2.4 percent for the equivalent Na2O [where % Na2Oeq = % Na2O + 0.658 (% K2O)] when various codes limit the level of sodium equivalent to 0.60 to 0.80%. Among the many harmful effects of the presence of water soluble alkalis in cement or concrete, alkali-aggregate (alkalisilica) reaction can take place when the % Na2Oeq exceeds 0.60.
The excessive content of water soluble alkalis decreases the workability of cement-sand mortar and concrete, thereby adversely affecting the macro and microstructure of concrete and the bond with the rebar. The excessive content of water soluble alkalis increases the heat of hydration and it makes the concrete surface highly absorbent to the detriment of concrete structures as any absorbed water is transmitted to the interior of the structure. A moist environment is thus available in regions where reinforcing bar sand prestressing elements are located. The moist environment is ideal for the process of corrosion of rebars and prestressing elements. It is also a prerequisite for the destructive alkali-silica reaction in concrete. The excessive content of water soluble alkalis thus not only provide the alkalis for reaction with reactive silica in aggregates but also the moist environment, a requisite for the alkali-silica reaction.
Inadequate CuringThe changes in construction practices for concrete bridges, roadways and other structures over the years include the lowering of the period of curing, say from 28 days to 7-14 days at the most, even though (a) it is required and recognized that structures with OPC should be cured for longer periods of time if full advantage is to be taken of the formation of Ca(OH)2 crystals in concrete, and (b) PSC and PPC concrete must be cured longer than concrete with OPC to match, as a minimum, the properties of percentage (of 28- day strength) gain in compressive strength and impermeability of OPC concrete.
Inadequate curing means inadequate hydration of cement. Inadequate hydration means fewer and smaller sizes of Ca(OH)2 crystals or calcium silicate hydrate particles inside concrete. All of these mean more porous concrete, which in turn means less durable concrete structures.
Besides being deficient in chemical and strength properties, inadequately cured concrete structures are much more porous than properly cured structures are. Porous concrete will facilitate the ingress of water or moisture, along with carbon dioxide and oxygen, into the concrete structure. Porousconcrete will also make it easier for the ingress of chlorides and other harmful elements into the structure. The surface of inadequately cured concrete, particularly PPC concrete, is easily eroded, thereby creating conditions for early decay and distress, specially when used in roadways. Today’s inadequately cured concrete structures, built more particularly with blended cement, cannot thus be expected to match the durability of concrete structures of earlier decades, which were built with OPC and cured for 28 days or so.
The ProblemThe problem of structural decay and distress, which can be related most directly1-5 to the use of high strength reinforcing bars with surface deformations, is today a worldwide phenomenon as Papadakis, et al12 have stated “The last two decades have seen a disconcerting increase in examples of the unsatisfactory durability of concrete structures, specially reinforced concrete ones.” The problem of early corrosion in high strength rebars with surface deformations and the consequent early structural distress is particularly severe in coastal areas of India with warm climates. A picture of the severity of the problem, caused by the use of high strength rebars with surface deformations, can be found in the words of Dr. C. S. Viswanatha13 : “It generally takes 4-5 years for the formation of hair cracks in locations away from the coastal belt. But another 5 years is sufficient enough to render the structure unserviceable. In coastalbelts, the corresponding figures are 1-2 years and 4-5 years, respectively.” The problem of early decay and distress in concrete structures is further compounded when inappropriate cement is used and it is further aggravated when the curing is inadequate.
Cracks in Bridge GirdersDuring the last couple of years this writer has come across many new concrete bridge girders, slabs on grade, buildings, etc. with prominent cracks where there should have been none.
Since the cracks in absolutely new structures could not have developed due to problems with rebars, it is felt that the crackshad developed due to the problems inherent in modern cements and the inadequacy in curing.
The SolutionThe current version of the Indian concrete code recognizes the problem of early decay in concrete structures and it has mandated solutions to the problem14. Under Clause 8 Durability of Concrete,the code has recognized in Clause 8.1.1 that “One of the main characteristics influencing the durability of concrete is its permeability to the ingress of water, oxygen, carbon dioxide, chloride, sulphate and other potentially deleterious substances”. Further on in Clause 8.2.1, it has stipulated that “The life of the structure can be lengthened by providing extra cover to steel, by chamfering the corners or by using circular crosssections or by using surface coatings which prevent or reduce the ingress of water, carbon dioxide or aggressive chemicals.” The writer has shown that of the four alternative solutions, suggested in the code, the provision of surface protection systems is the only viable option15.
The provision of waterproofing systems on the surface of concrete structures for the protection of such structures has thus become essential16. It has become essential also due to theshortcomings in rebars and cement and due to inadequate curing of concrete.
Waterproofing systems will not only provide protection against early corrosion in rebars and prestressing elements, such waterproofing systems will also prevent alkali-silica reaction in today’s concrete with excessive amounts of water soluble alkalis, thereby preventing another form of distress in concrete structures. Some waterproofing systems will also provide protection against chlorides and other aggressive elements.
Surface Protection of BridgesIn the year 2002, in recognition of the importance and essentiality of surface protection systems on concrete bridges, the Public Works (Roads) Directorate of the Government of West Bengal19 published, as a part of its Schedule of Rates, a comprehensive set of specifications for such surface protection with the provision of waterproofing systems. In 2005, Eastern Railway20 included, as a part of its Schedule of Rates, a specification for the surface protection of concrete bridges. The emphasis, as in the case of PW (Roads) Directorate SOR’s, was on coatings with phenolic resin based preparations.
Over the years, some of the specifications in Refs. 19 and 20 were followed to provide surface protection to concrete bridges. A few case histories are given here.
Case HistoriesFigure 6 shows Bankim Setu, an R.O.B. of Eastern Railway at Howrah Station, Howrah, West Bengal. The bridge is also over the very busy railway tracks at Howrah Station. The underside of the reinforced concrete deck, theprestressed concrete girders and piers were given surface protection with one coat of phenolic resin based coating compound PERMAPROOF, followed by a finish coat of phenolic resin based PERMACIL-GA of dove grey color matching the color of concrete. The highly distressed bridge had to be repaired before the application of the surface protection system. The specification for the surface protection treatment was included in both the SOR’s in Refs. 19 and 20.
Figure 7 shows the central span of the multi-span Katakhali Bridge on the Hasnabad- Hingalganj Road, not too far away from the coasts of West Bengal. The bridge is over a wide river with fast flowing water. The newly constructed bridge had already shown early signs of minor distress. The surfaceprotection treatments with PERMAPROOF and PERMACILGA were provided after local spotsof the bridge were repaired. Though the superstructures of Katakhali bridge and Bankim Setuhad similar surface protection treatments, the piers of the Katakhali bridge, in the river bed were provided with cement-based PERMAKAR waterproofing treatment. The PERMAKAR treatment is ideal in the environment of fast flowing water.
Figure 8 shows one of the spans of Charki Bridge over the Ajoy river in West Bengal. The bridge was provided with two coats of PERMAPROOF. Though phenolic resin based PERMAPROOF has excellent resistance against water and many chemicals, the polymer is susceptible to decay when exposed to ultra-violet ray. As a solution, dry coarse sand was sprayed on the second coat of PERMAPROOF on the exterior faces of girders.
All the types of treatment, when tested under a water head of 20 metres, were found to have zero permeability.
Many concrete bridges of East Coast Railways and those of other organizations were provided with acrylic coatings. PERMACRYLIC is one such coating material in the line of PERMAKAR Technology of Engineering Services Internationalas PERMAKAR, PERMAPROOF and PERMACIL-GA are.
With the experience gained during the execution of projects at bridge sites, which were open to traffic at the time of execution of the work, over railways and roadways and some on tidal rivers, a shortened list of five practical specifications are given below. This small set of specifications will cover the requirements of surface protection of most cases of concrete bridges.
- Piers and Substructures in River Beds
Thorough cleaning; etching the surface with muriatic acid; providing the surface with a two - layer plaster; the first layer shall be 7 mm thick with cement (OPC/slag), admixed with a cementitious waterproofing compound, e.g., PERMAKAR 3 or equivalent, in the weight ratio of 50:3 for cement and PERMAKAR 3; finally providing the second layer in the form of a protective plaster (12 mm thick) of cement-sand (1:2.5), containing polyester fibre as secondary reinforcement @ 0.20% by weight of cement, followed by curing and providing a wearing course.
- Underside of bridge deck, railing, etc. and Piers and Substructures which are not in River Bed
Thorough cleaning of the surface; drying the surface; removing dust particles; applying one coat of a single component high performance polymeric waterproofing compound of modified phenolic resins, e.g. PERMAPROOF or equivalent; followed by the application of one coat of a modified phenolic resin based top-coat of grey or other shades, e.g., PERMACILGA or equivalent.
- Bridge deck Thorough cleaning; etching the surface with muriatic acid; providing the surface with a two layer plaster; the first layer shall be 5 mm thick with cement (OPC/slag), admixed with a cementitious waterproofing compound, e.g., PERMAKAR 3 or equivalent, in the weight ratio of 50:3 for cement and PERMAKAR 3, with added deformed galvanized steel fibre (285 gm per square metre of surface area), e.g., KARFIB or equivalent; finally providing the second layer in the form of a protective plaster (12 mm thick) of cement-sand (1:2.5), containing polyester fibre as secondary reinforcement @ 0.20% by weigh of cement, followed by curing and providing a wearing course.
- Sidewalk Thorough cleaning; etching the surface with muriatic acid; providing the surface with two - layer plaster; the first layer shall be 5 mm thick with cement (OPC/slag), admixed with a cementitious waterproofing compound, e.g., PERMAKAR 3 or equivalent, in the weight ratio of 50:3 for cement and PERMAKAR 3; finally providing the second layer in the form of a protective plaster (12 mm thick) of cement-sand (1:2.5), containing polyester fibre as secondary reinforcement @ 0.20% by weight of cement, followed by curing and providing a wearing course.
- Underside of bridge deck, Inside of Box Girders and Other Shaded Areas Thorough cleaning of the surface; drying the surface; removing dust particles; applying two coats of a single component high performance polymeric waterproofing compound of modified phenolic resins, e.g., PERMAPROOF or equivalent; the second coat to be applied before the first coat becomes tackfree.
Concluding RemarksConcrete bridges, roadways and other structures of recent times, compared to such structures of earlier periods, have suffered early damages due to the use of inappropriate materials (both cement and reinforcing bars) and due to a lack of adequate curing.
Many of the problems, associated with the use of high strength rebars with surface deformations, OPC with high specific surface, high C3S/C2S ratios, blended cement, cement with excessive contents of water soluble alkalis and inadequate curing, can be alleviated if all exposed surfaces of concrete will be protected with waterproofing treatments.
The Indian code for concrete structures14 has mandated the provision of surface protection systems for the prevention of the ingress of water as one of the ways to make concrete structures durable. Several bridge authorities have adopted specifications for the surface protection of concrete bridges as a part of their schedules of rates. Many concrete bridges all over the country have been protected with the provision of surface coatings and other treatments in recognition of the fact that if left unprotected, as in the past, today’s concrete structures, unlike concrete structures of earlier decades, will fail to be durable.
- Kar, Anil K., “Deformed Reinforcing Bars and Early Distress in Concrete Structures,” Highway Research Bulletin, Indian Roads Congress, No. 65, December 2001.
- Kar, Anil K., “Reinforcing Bars in the Context of Durability of Concrete Infrastructure,” All India Seminar on Utilisation of Skyways and Subways in the Cities in India, American Society of Civil Engineers ¾ India Section and Institution of Engineers, UK, Eastern Region, India, 9th and 10th December 2005.
- Kar, Anil K., “Deformed rebars in concrete construction,” New Building Materials & Construction World; New Delhi; Vol. 12, Issue-6, December, 2006.
- Kar, Anil K., “Reinforcing Bars ¾ The Good and The Bad,” Steel Scenario Journal, Vol. 14, No. Q1, July-September 2004.
- Kar, Anil K., “Concrete Structures ¾ the pH Potential of Cement and Deformed Reinforcing Bars,” Journal of the Institution of Engineers (India), Civil Engineering Division, Vol. 82, June 2001.
- Kar, Anil K., “Concrete Structures We Make Today,” New Building Materials & Construction World; New Delhi; Vol. 12, Issue-8, February, 2007.
- Kar, Anil K.., “Concrete Jungle ¾ Calamity may be waiting to happen,” The Statesman, Kolkata, 4 August, 2000.
- Alekseev, S. N., Ivanov, F. M., Modry, S., and Shiessel, P., Durability of reinforced concrete in aggressive media, Oxford & IBH Publishing Co. Pvt. Ltd., NewDelhi, 1993. (Translation of Dolgovechnosti zhelezobetona v agressivnikh sredakh, Stroiizdat, Moscow 1990.)
- Durability of Concrete Construction, Technical Circular 1/99, Central Designs Organisation, Central Public Works Department, Government of India, No. CDO/SE(D)/G-29 dated 18.02.1999.
- Kar, Anil K., “Concrete in the Context of Durability,” RITES Journal, RITES Ltd., Vol. 8,Issue 1, April 2006.
- Mehta, P. K., “Durability of concrete ¾ The zigzag course of progress,” The Indian Concrete Journal, Vol. 80, No. 8, August 2006.
- 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.
- Viswanatha, C S, “Corrosion in R C Members,” Abstract of Lecture at The Institution of Engineers (India), West Bengal State Centre, Calcutta, 9 June, 1993.
- Indian Standard Plain and Reinforced Concrete, Fourth Revision, Bureau of Indian Standards, New Delhi, IS456:2000, July 2000.
- Kar, Anil K., “IS 456:2000 on Durable Concrete Structures,” New Building Materials & Construction World, New Delhi, Vol. 9, Issue 6, December 2003.
- Kar, Anil K., “Waterproofing of Structures : Challenges and Solutions,” New Building Materials & Construction World, New Delhi, Vol. 11, Issue 10, April 2006.
- Kar, Anil K., “Arresting Water Leakages in Tunnels and Other Underground Structures,” All India Seminar on Underground Construction with Particular Reference to Metro Railways, The Institution of Engineers (India), Calcutta, December, 1987.
- Kar, Anil K., “Protection of Structures as a Means to a Long Life for Bridges,” Indian Highways, Indian RoadsCongress, Vol. 28, No. 7, July 2000.
- Typical Specifications on Maintenance & Repair of Bridge for the Year 2001-2002, P.W. (Roads) Directorate, Government of West Bengal, 01.01.2002.
- Schedule of Labour & Material Rates 2005, (chapter- XXV), Engineering Department, Eastern Railway, Government of India, Kolkata.