Repair and Rehabilitation of Reinforced Concrete

REPAIR AND REHABILITATION OF REINFORCED CONCRETE \

INDEX

SR.NO. CONTENT

PAGE NO.

1

ABSTRACT

03

2

INTRODUCTION

04

3

CORROSION OF STEEL&CONCRETE

05

4

CHEMICAL REACTION

08

5

REPAIR & REHABILITATION

11

6

CONCLUSION

14

7

REFFERENCES

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ABSTRACT The paper emphasize on Rehabilitation of Reinforced concrete. The purpose of the paper is to highlight the methods of repair and rehabilitation to be undertaken for structures with defects and deficiencies that necessitate rehabilitation. Repair and Rehabilitation methods currently used are reviewed on the basis of present knowledge and the merit of a holistic system approach, which takes into account not only the individual processes and phenomena but also most importantly their interaction. This paper focuses on visible symptoms of the problem rather than on visible and invisible problems as well as the possible causes behind them. Also the repair materials and the techniques used, since the use of appropriate repair materials and techniques is essential for the satisfactory performance of the repaired structure. This paper presents an analysis of effects of different problems leading to unsatisfactory performance of reinforced concrete structures. An attempt has been made in this paper to discuss the techniques which are used for rehabilitation & repair. The paper highlights the problem of corrosion of reinforcing steel in concrete structures and attempts to provide the measures available in design to mitigate the effects of corrosion. The various types of coatings available and the precautions to be taken in the selection of coating systems in view of these limitations are also discussed. In particular, the latest information on important technical findings pertaining to hot-dip galvanizing is discussed. This highlights the importance of epoxy resins and systems in the construction/civil engineering applications such as structural adhesives, anti-corrosive linings, etc. This also discusses how electrochemical repairs of reinforced concrete structures are proving to be highly effective in terms of durability, life cycle costing and the ability to extend concrete protection beyond the boundaries of localized patch repairs.. The paper concludes with a typical session of the expert system, which is a system for diagnosing causes and repairs of defects in reinforced concrete structures.

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INTRODUCTION : Construction of building & other type of structures with reinforced cement concrete elements has been in vogue for almost century. Initially reinforced cement concrete was considered as wonder material. However its susceptibility to certain chemicals came to fore with passage of time. The strength of Reinforced concrete structure depends upon various factors like quality of steel, cement, aggregate, water for creep & shrinkage properties, climatic changes in temperature & humidity, salinity of water, type of plasticizers etc. These factors should be studied in detail to assess qualitatively the extent of damage caused to RC structure. The damage to concrete or steel reinforcement would cause deterioration of structure. Cracking & spalling of concrete induced by steel corrosion depends upon concrete tensile strength bond or condition of the interface between the rebar & surrounding concrete. Iron metal is very weak due to oxydation of iron, low protective property of iron corrosion products etc. Improper compaction results in honeycombing. Presence of Chlorides also increase shrinkage cracks in concrete further accentuating corrosion of reinforcement in aggressive environment. Some biological species are also harmful to concrete structures. Since, the affections were varying in nature, varied approaches had to be adopted to counteract the problem. Failures are inevitable in sphere of life and they are steps to success if learns from the failures. Building structures cannot be an exception to this wellknownfact of life. If we have to minimize failures we must learn from the post failures by detailed investigations analysis for arriving at the response for failuresand take remedial action to avoid them in future. Along with the growth of concrete, steel associated structures, the problem of structurally deficient and functionally absolute structures is being encountered.This had led to the development of rehabilitation and restoration, which is relatively new ascompared to our knowledge about design and construction. It not only demands a thorough knowledge of design detailing and construction, but also of other claviers disciplines such as chemistry of materials, agnostic technique and a high level of engineering and supervision. The term repair & rehabilitation in broad sense implied restoring a structure to its original condition.

CORROSION OF STEEL REINFORCEMENT: 4

Cement concrete reinforcement with steel bars is an extremely popular construction material. One major flaw, namely its susceptibility to environmental attack, can severely reduce the strength and life of these structures. In humid conditions, atmospheric moisture percolates through the concrete cover and reaches the steel reinforcement. The process of rusting of steel bars is then initiated. The steel bars expand due to the rusting and force the concrete cover out resulting in spalling of concrete cover. Due to direct environmental attack on reinforcement the rusting process is accelerated. Along with unpleasant appearance it weakens the concrete structure to a high degree. The spalling reduces the effective thickness of concrete. In addition, rusting reduces the cross sectional area of steel bars, thereby reducing the strength of the reinforcements. Moreover, the bond between the steel and the concrete is reduced which increases the chances of slippage. The rusting related failure of reinforced concrete is more frequent in a saline atmosphere because salinity leads to a faster corrosion of the steel reinforcement. In a tropical country like India, where approximately 80% of the annual rainfall takes place in the two residential and industrial structures. India also has a very long coastline where marine weather prevails. Typically, a building requires major restoration work within fifteen years of its construction.

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RUSTING OF STEEL

Corrosion of steel reinforcement is amongst the most important causes of deterioration of reinforced concrete which causes spalling of concrete due to increase in volume of oxidized compounds when compared with the volume of base metal dissolved. The increase in volume causes tensile forces leading to cracks in concrete around steel reinforcement .Cracking & spalling of concrete depends upon concrete tensile strength, diameter of reinforcing bar etc.

CORROSION OF CONCRETE: Concrete gets corroded due to a number of factors. A few of the important modes of deterioration of concrete are briefly dealt here under.

COMPOSITION OF CONCRETE: This perhaps is the most important factor determining durability of concrete. A well formulated fresh concrete is a high alkalinity material with pH ranging from 12 to 13. In this range of alkalinity the passivating film on the rebar remains protected from further attack of oxygen. A well designed concrete should have a minimum quantity of 350 kg of cement per cubic meter of concrete. Cement should be chosen suiting particular application of concrete

& depending on the nature of aggressive environment prevailing in or 6

concrete. For concreting in aggressive marine environment or sewage environment ,it is preferable to use sulphate resisting cement or granulated blast furnace slag cement. Further it is essential that w/c ratio be kept to minimum in aggressive environment or where alternate cycles of wetting & drying are likely to be experienced. To obtain proper workability it would be desirable to use superplasticizers.

PERMEABILITY AND PLASTIC CRACKING: Permeability results essentially from low compaction, low cement content, high water cement ratio, high temperature of concrete at the time of placing etc. Plastic cracks, however, result from excessive mechanical vibration at the time of placement of concrete. The effect of permeability or plastic cracking is identical i.e. to increase ingress of water or moisture.

Ingress of water may result in leaching of calcium hydroxide thus

reducing the alkaline protective environment to steel against corrosion. Water as well as moisture provide electrolytic medium highly conducive to galvanic cell action resulting in corrosion of steel reinforcement

Cracks can be repaired However, when a concrete structure has cracked for any of those basic reasons, it does not necessarily mean that the structure has failed. On the contrary, any cracked concrete structure can be repaired effectively and most often permanently, provided the structure has not been wholly deformed and the original cause of cracking has stopped.

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IMPROPER COMPACTION: Improper or inadequate compaction causes honeycombing and marked capillary channels in the concrete thus resulting in easy ingress of water and other aggressive fluids leading to rusting of steel and corrosion of concrete. The extent of honeycombing can be seen in following photograph.

REACTION WITH SULPHATE: In hardened cement, tricalcium aluminate hydrate can react with a sulphate salt from outside the concrete. The product of addition is calcium sulphoaluminate forming within the framework of the hydrated cement paste. Since the increase in the volume of the solid phase is 227 percent, gradual disintegration of concrete results. A second type of reaction is that of base exchange between calcium hydroxide and the sulphates, resulting in the formation of gypsum with an increase in the volume of the solid phase of 124 percent. The sulphate attack is greatly accelerated if accompanied by alternate cycles of wetting and drying. The vulnerability of concrete to sulphate attack can be considerably reduced by limiting the percentage of tricalcium aluminate.

REACTION WITH CHLORIDES: Presence of calcium chloride even in small percentage can lead to rapid corrosion of reinforcement as it reduces the electrical resistivity of concrete and helps to promote galvanic cell action. The absence of corrosion of reinforcement in a normally dense. 8

Portland cement concrete is due to complete anodic incompetence as steel in an alkaline environment of such concrete, passivates. The passivating film on iron consist of ferric oxides, which retard the anodic process. The chloride iron is noted for being highly activating and disturbing the passive state of steel even in an alkaline environment of concrete. The activating ions absorbing on the steel surface replace the oxygen participating in the formation of protective films or layers. This ability of chloride ion to de-passivate the protective film makes it imperative to limit the presence of chlorides. The concentration of chlorides necessary to promote corrosion, among other factors, is greatly affected by the concrete’s pH. It was demonstrated that a threshold level of 8000 ppm of chloride ions was required to initiate corrosion when the pH was 13.2. As the pH was lowered to 11.6, corrosion was initiated with only 71 ppm of Chloride ions Presence of chlorides also increases shrinkage cracks in concrete further accentuating corrosion of reinforcement in aggressive environment

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ALKALI AGGREGATE REACTION: In ordinary Portland cement alkalis namely, sodium oxide and potassium oxide are present to some extent. These alkalis chemically react with certain siliceous mineral constitute of some aggregates and cause expansion, cracking and disintegration of concrete. Opaline silica, microcryastalline silica, chalcedony, tridymite, crystobalite, certain cryptocryastalline volcanic rocks such as rhyolites and andysites, some zeolites, and certain metamorphic schists are responsible for reactivity of aggregates.

Preventive

measure consists of avoiding aggregates which and which on testing have been found to be alkali reactive or using cement with a maximum of 0.6% of alkali (equivalent Sodium Oxide). Use could also be made of some suitable puzzolanic material, which prevents alkali-aggregate reaction, by itself combining with the alkalis present in the cement.

EFFECT OF ENVIRONMENT: The capillary porous structure of concrete of concrete renders inevitable the interaction of concrete renders inevitable the interaction of environment. Ambiency of inland waters provides one of such environments The inland waters may be corrosive due to leaching of 9

calcium hydroxide, presence of sulphates, chlorides, carbonic acid (dissolved carbon dioxide) or dissolved sulpher dioxide etc. Since calcium hydroxide is soluble in water, it is readily leached concretes. The reduction of calcium hydroxide below a certain level diminishes stable existence of hydro silicates, hydro ferrites of calcium responsible for cemetitious properties. The attack by seawater on the hardened concrete is very severe. The sulphates of magnesium and calcium present in the sea water combine with the hydrate of tricalcium aluminate or calcium hydroxide to form calcium sulphoaluminate hydrate or calcium sulphate hydrate (gypsum) thus causing disruption of concrete.

BIOLOGICAL ATTACK: Biological organisms, particularly anaerobic bacteria often exert a marked accelerating influence on the electrochemical process of corrosion. Certain bacteria excrete acids, which are capable of dissolving rocks & concrete. Concrete foundations in shallows marine regions are sometimes attacked by a species of marine sponges of the family “clionidae”.

CARBONATION: Normal air is not harmful to a hardened, dense concrete. However, even Ordinary air, at a certain temperature and humidity, may present danger to reinforced concrete, for it contains carbonic acid, though in incredibly low concentration (.03%). The air borne carbon combines with water in humid air to form carbonic acid. This carbonic acid gradually neutralizes calcium hydroxide in the surface layer of concrete, thereby reducing the protection it can afford to steel reinforcement. As a result of this reaction, the alkalinity of concrete is reduced to a pH value of about 10, and consequently, concrete protection of the reinforcing steel is lost the passivity of the protective layer on steel is destroyed.

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When steel is depassivated and the environment is acidic or mildly alkaline, corrosion begins if moisture and oxygen gain access into the concrete, The rate of carbonation directly depend upon the density of concrete, water cement ratio, concentration of carbon dioxide, humidity and temperature. Higher the density of concrete, lower is the penetration of carbon dioxide because gas permeability of a denser concrete is relatively low. In good quality concrete the carbonation process is very slow. It has been estimated that the process proceeds at a rate up to 1mm per year. The process requires constant change in moisture levels from dry to damp to dry. Carbonation will not proceed when concrete is under water.

REHABILITATION OF REINFORCED CONCRETE STRUCTURES: Rehabilitation of reinforced concrete structure, could be achieved effectively if the factors influencing the durability of the structure & the extent of damage are investigated systematically & various problems & causes afflicting the structure are isolated. Various techniques such as guniting, epoxy mortar coating, polymer grouting etc. could be used to undertake rehabilitation of concrete structures damaged by corrosive environment. The methods of repairs to be adapted should be specifically suited to arrest further corrosive action of the environment, which almost invariably continue to ravage the structure even after its rehabilitation. 11

GALVANIC PRITECTION SYSTEM: Galvanic protection systems utilize sacrificial anodes that naturally generate an electrical current to mitigate corrosion of the reinforcing steel. In concrete structures, zinc anodes are typically used. Galvanic protection for concrete can be classified into two categories: targeted protection for concrete repair and distributed systems for blanket protection. Discrete anodes are used to provide localized protection around concrete patches or places into drilled holes on a grid pattern to provide targeted concrete corrosion control. Galvashield® XP and Galvashield XP+ embedded zinc anodes are examples of discrete galvanic anodes that are used to provide corrosion mitigation for concrete repair. Galvashield CC zinc anodes are installed into chloride contaminated concrete, carbonated concrete or areas with high corrosion potentials to achieve corrosion control in targeted areas of concrete structures. Distributed systems generally consist of galvanic anodes that are placed over a wide area to provide corrosion control or cathodic protection. Examples of this are Galvanode® ASZ+ activated arc spray zinc metalizing, Galvanode Jacket systems for galvanic protection of concrete piles and galvanic protection of columns, and Galvanode DAS distributed anode system for use with concrete overlays and reinforced concrete jackets.

Electrochemical treatments: Electrochemical treatments directly address the cause of the corrosion activity by applying a high current density to the structure for a limited period of time. The selected electrolyte and the duration of the treatment depend on the cause of the corrosion problem. After the treatment is complete, the structure is left in a long-term passive condition with no further system maintenance or monitoring required.

Chloride Extraction : Electrochemical chloride extraction (ECE) is a treatment which a) extracts chloride ions from contaminated concrete and b) reinstates the passivity of steel reinforcement. Chloride 12

extraction is carried out by temporarily applying an electric field between the reinforcement in the concrete and an externally mounted anode mesh. During the process chloride ions are transported out of the concrete. At the same time, electrolysis at the reinforcement surface produces a high pH environment. This process returns the steel reinforcement to a passive condition.

Applications:  Decks  Beams  Columns  Pier caps

Features and Benefits:  Mitigates active corrosion addresses the cause of corrosion by reducing chloride content and repassivating the reinforcing steel.  Long term protection corrosion will not re-initiate unless recontaminated with sufficient chlorides.  Environmentally friendly vastly reduced concrete break-out reduces noise, dust and environmental pollution.

Re-alkalization : Norcure Re-alkalization is a treatment that restores the alkalinity of carbonated concrete and reinstates the passivity of steel reinforcement. Re-alkalization is carried out by temporarily applying an electric field between the reinforcement in the concrete and an externally mounted anode mesh. When the process is complete, the pH of the concrete is greater than 10 which is sufficient to provide lasting protection to the reinforcing steel.

Applications:  Columns 13

 Beams  Other carbonated areas

Features and Benefits:  Mitigates active corrosion - addresses the cause of corrosion by reestablishing a high pH environment and repassivating the reinforcing steel.  Long term protection structure will not re-carbonate in the treated areas.  Environmentally friendly vastly reduced concrete break-out reduces noise, dust and environmental pollution.  Low maintenance no permanent system to maintain or monitor.  Global protection treats the entire structure, not just isolated patches

CONCLUSION •

Proper workmanship, low water cement ratio, adequate compaction, proper formwork & strict quality control on the designed ingredients is a must to achieve durable reinforced concrete.

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In case of development of cracks, treatment with epoxy injection or non shrink cement grout injection should be resorted to at the first sign of distress.

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If the damage is extensive then treatment with epoxy mortar overlain with cement mortar could be adopted. 14

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Cracks are not likely to develop at the junction of old & new concrete if the workmanship &surface preparation is proper

REFERENCES •

www.icivilengineer.com

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New building materials & construction world [monthly journal]

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www.chemcosystem.com

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