Concrete is the most versatile man-made construction material of our times on an account of its flowability in most complicated forms while wet and its strength and durability characteristics when set or hardened.  Concrete constructions are economical considering the longevity of the structures.  Durability of concrete is therefore a function of the performance of concrete with respect to time.  The concrete is said to be durable when it can withstand the conditions, for which it is designed, over a period of time without getting damaged or deteriorated.  Strength alone is not alone an indication of concrete durability.  Two concrete specimen of the same strength having different permeabilities can possess different degrees of durability.  Admixtures in general can lower the permeability of concrete either by lowering the water cement ratio or enabling better compaction of concrete by providing flowable concrete, which is homogenous.  Admixtures as air entraining agents when incorporated in the mix have proved to increase the resistance to freeze-thaw cycles in addition to lowering the permeability on account of microscopic air bubbles.  Protected concrete has shown more resistance against carbonation when compared to which at later date are constant sources of moisture and oxygen ingress, which accelerates the reinforcement corrosion, lowering the durability of concrete.  However, it should be ascertained that the admixtures employed in the construction are totally free ingredients adversely affecting the durability of concrete. Concrete, therefore should be designed for durability in addition to the strength characteristics.

 

FACTORS AFFECTING THE DURABILITY

Before getting further about how the admixtures and curing will be beneficial for the durability of concrete, it is necessary to ascertain the major factors that can lead to deterioration of concrete.  It is beyond the scope of this treatise to deal with all the factors contributing to the deterioration or damage of concrete, but an attempt is made only to concentrate on some major factors, to find out the influence, and thereby to ascertain how the admixtures and curing can combat the deterioration of the concrete leading to the durability.  Detailed topics like creep etc. are beyond the scope as limited data is available, the more stress is laid on the four “C” as commonly called viz. Cement content, cover, compaction and curing.  And the concept is slightly extended to cement/water ratio (inverse of water cement ratio), carbonation, chlorides and chemical attack (internal: chemical attack like sulphate).  And to extend the same matter further it includes a passing reference to the coatings with resistance to diffusion of carbon dioxide.  The factors can once again be enlisted as under:

 

·        Permeability of Concrete

·        Water cement ratio

·        Quality of materials for construction

·        Cement content

·        Curing of concrete

·        Carbonation of concrete

·        Chloride and sulphate in concrete

·        Admixtures in concrete

·        Quality Control for concreting practices

·        Cracks in concrete-structural and non structural

·        Design parameters for concrete

·        Environmental conditions

·        Chemical and mechanical attacks on concrete

·        Corrosion of reinforcement

·        Natural and other stresses (fire etc.)

·        Creep and different shrinkage

·        Air and vapor permeability

 

PERMEABILITY

Though the strength of concrete is one of the parameters that influences the durability of concrete permeability is another most important factor that can affect the durability or longevity of the surface.  In the present context, it has to be accepted that such an important factor is highly neglected as compared to the strength of the concrete.  It is wrong to judge the performance of concrete only on the basis of development of strength and it should be made compulsory to also generate data about permeability properties of concrete either before accepting or condemning the concrete.  It is heartening to note that at the moment permeability tests are also conducted on important structures especially bridges and similar structures constructed in adverse conditions.  This knowledge should reach the common constructor in the benefit of the whole industry.

 

The permeability of concrete has the bearing on the following deteriorating factors.

 

·        Penetration of chlorides

·        Penetration of deicing salts

·        Depth of carbonation

·        Acceleration  of corrosion of reinforcement

·        Affecting the therman insulation of concrete

·        Dampness, moss and fungus growth

·        Surface absorption of water

 

The permeability of concrete depends on the following major factors :

 

·        Water cement ratio

·        Capillary porosity of cement paste

·        Curing of concrete

·        Air entrainment

·        Compaction

 

The permeability of concrete can be reduced either by addition of admixtures or by surface coatings. It is however advisable that the permeability is lowered by addition of integral admixtures rather than coatings as the coatings are prone to puncture or debonding leaving the concrete vulnerable to penetration of water.  Typical results for water proofing (integral types) and the coatings are tabulated.  Tests for permeability can give an indication about the concrete durability subjected to the water pressure.  Permeability also gives estimate for determining the corrosive action by percolating water.

 

PRECAUTIONS

While designing the concrete structures due weightage should be given to the factors affecting the durability of concrete.  In addition to specifying the 28 days strength as the sole criterion for concrete acceptance, permeability tests should also be specified to harsh environmental stresses; chloride prone areas and sulphate affected environments.  Proper flowability of the concrete should be specified by considering actual site conditions for congested reinforcements, slender sections etc.  The mixes designed at the laboratories should incorporate admixtures, if any, for desired flowabilities.  Same brand of cements and type of aggregates should be used as in practice, as admixtures have different effects on different cements.  The admixtures should be checked at least for chloride contents to ascertain that the chloride contents are within the specified limits.  Air content and retardation times should be checked while using the plasticicizers.  If curing compounds are used they should be checked for solar reflectance factor. Durability of concrete can be surely increased by incorporating good quality admixtures.  It should be borne in mind that the admixtures used are not only for modifying the fresh concrete properties, as problem to severe environmental stresses, it is advisable to coat the concrete with coating materials having high resistance to the diffusion of carbon dioxide.  In addition all measures should be taken to ensuring better quality control while mixing and placing of concrete.  Proper and timely maintenance of deteriorated structures coupled with all precautions can provide long lasting and durable concrete.  Durable concrete should be the aim of the construction industry.

 

THE AIM OF WATERPROOFING

As permeability retards the durability of concrete and reduces the life of a structure, the aim of waterproofing materials, or systems, is to lower or block the entry of water:

 

Absolute waterproofing is, however, neither possible nor advisable, as some water must be present in the capillaries for the constant hydration of the cement paste.  Most structures require damp proofing to prevent the passage of water that carries efflorescence, deleterious salts and chemicals, and promotes mould and fungi growth.  As such water will also accelerate reinforcement corrosion and the alkali-aggregate reaction, waterproofing-whether conventional or unconventional is undertaken to specifically seal unwanted water out of the system.

 

THE CONCEPT OF WATERPROOFING

Concrete is wettable and has great affinity towards water-gel pores, capillary pores and entrapped air exist in all concrete.  Permeability is, thus, a function of such capillary porosity and the water-cement ratio. Correlated, the water-cement ratio has direct bearing on capillary discontinuity, and one of the basic roles of waterproofing is to lower the concrete’s wettable characteristics.  As the pressure required to force water into concrete surfaces is directly proportionate to the contact angles, higher contact angles increase the pressure required to force the water into the body of the concrete.  Since the pressure required is positive, the capillary action is then negligible.

 

Waterproofing materials are, therefore, either permeability reducers or those that impart hydrophobic properties to concrete.  The former are effective for waterproofing, even under hydrostatic pressure, while the latter serve best in the damp-proofing of structures where water enters through capillary action.  Popular modern proprietary materials normally contain both anti-permeability and water-repellant capillary blocking chemicals and substances.

 

CONVENTIONAL V/S UNCONVENTIONAL

Conventional waterproofing admixtures are fatty acids or stearates.  These line the capillary pores with their water-repellant molecules or fill finer pores in low, fine-grade concrete when mixed with hydrated lime.  They are more suitable for stopping water ingress by reducing capillary action.

 

Conventional coating systems also have limitations, as they are bitumen-based, leading to deterioration by ultraviolet (UV) radiation.  So also, intermediate-generation coating products, like epoxies and polyurethane, have been ineffective on account of their lower flexibility as well as their blockage of the concrete’s breathing capacity.

 

Advances in polymer technology have changed waterproofing radically.  The modern integral polymer admixtures enable better water reduction and dispersion, and pore block significantly.  The pozzolanic properties of the incorporated fillers develop a denser gel formation with hydrated calcium silicate products instead of the conventional calcium hydroxide crystals.

 

Present generation coatings are also products of polymer technology.  Water-based, they form elastomeric seamless membranes with ultra-high flexibility and resistance of UV radiation.  The availability of these and other new raw materials have encouraged the development of waterproofing technology, slowly replacing conventional materials with technical superiority.

 

RANGE OF MATERIALS

When concreting or plastering, depending on the application time, integral waterproofing admixtures, impregnators like silicon-based sealers, coated to impart water repellency to the surface during the rains, or surface coating to protect the concrete under hydrostatic pressure, are applied/added, Basically, there are three types of waterproofing materials available.  These contain:

 

·        Permeability – reducing chemicals

·        Water repellant or hydrophobic characteristics imparting chemicals.

·        Combinations.

 

Depending on the structure’s physical form, either powders, liquids, pastes or suspensions are used.  The choice of material depends on the pattern or form of water ingress.  In the case of integral waterproofing of concrete with a low cement content and low fines, power-waterproofing compounds should be used.  Liquid waterproofing compounds are suitable for high strength concrete with higher cement content and adequate fines.  These also enable better dispersion and subsequently better compacted concrete with a water content as low as 15 percent.

 

Another unconventional waterproofing materials is mineral slurry modified with polymers.  A 2mm thick coating resists a 20mm water head.  This is so because its reactive chemicals react with the soluble unhydrated lime crystals in the cement capillaries and convert them into insoluble deposits.  These repel water by constricting the capillaries, thereby increasing the penetration pressure.  In the presence of existing moisture, this reaction continues to render the capillaries waterproof, while retaining the concrete’s breathing capacity.

 

APPLICATION METHODS SURFACE TREATMENT

Firstly, all damp and loose plaster must be removed.  Then, the joints must be tested to ensure that they are sound and properly filled.  Should any cracks exist, they sealed at once.

 

The surface to be treated must be clean.  It should be free of all-loose particles, oil, grease, efflorescence, traces of form oil curing compounds, and any other prior treatment or contamination’s side effects and residue.  The mix-prepare by following the manufacturer’s instructions – can be applied with a trowel, brush or spray.  Applications at temperatures be +5⁰ C, and no frozen surfaces, should not be made.

 

COVERAGE

The consumption of waterproofing material largely depends on the application method followed, the prevalent climatic conditions, and the texture, type and general condition of the treated surface.

 

QUALITY ASSURANCE

There is some resistance in the minds of customers to switch over from conventional waterproofing materials and methods to modern ones.  This is partly owing to their previous experiences, when new materials were introduced into the market without proper technical backup, services and specialized applicators.

 

Price has been another major decision-making factor, as is the request for guarantees and their supply.  But have such practices prevented waterproofing failures?  In fact, guarantees asked for by customers have been honored by unscrupulous contractors, who have only supplied them to secure contracts.  What is necessary, is the creation of trust between the manufactures, specifier, consultant, applicator and client.  The quality of waterproofing should be a prime concern.  Economics should be worked out not only by considering the initial cost, but also the repairing cost of maintenance.  A Quality Assurance System should be thus developed.

 

Whether the materials or systems are conventional unconventional, the concept of waterproofing remains the same.  Waterproofing requires sound knowledge of Civil Engineering principles as well as material science.  Modern materials based on polymers are successful in doing away with the several limitations of conventional materials and systems, and should be applied, by specialized contractors trained in the application of such materials. The diagnosis should be thorough and the waterproofing should be viewed as a system.

 

When the materials used are unconventional, detailed specifications should be laid out as well as a work rhythm plan drafted.  Modern test methods should be incorporated in the codes of practice so that the customer is convinced about the quality of the materials, offers less resistance to new materials.  Rather than relying on guarantee drafts, quality assurance systems should be resorted to so that trust develops between the concerned parties whose common prime concern must be quality waterproofing.

 

REPAIRS & MAINTENANCE

Many a times even after taking all the due precautions and all quality control measures the structures deteriorate.  One of the practical reasons is due to the fact’ that at the time of planning or at the design table it is very difficult to estimate fully the real stresses in practice that the concrete will have to carry.  It is also not possible to suggest all the different types of coatings and membranes, also on account of economical considerations.  The concrete then becomes vulnerable to ingress water and other aggressive chemicals.  Such problems can only be identified by being vigilant and by inspections conducted from time to time.  Most of these problems can be identified at an early date by visual inspection.  The maintenance on earlier date can not only be economical but also will preserve the durability of concrete.  Rest of the defects could be rectified by usage of particular coating or minor repairs.

 

If neglected then the problems manifest themselves into serious defects like of plasters, corrosion of reinforcement and in some cases the degree of corrosion is so high that the repair is of structural nature.  Such repairs are normally costly but are necessary for preserving the structures.  A thorough diagnosis has to be done as to the cause and degree of damage and thereafter suggesting a treatment, which is practical as well as economically feasible.  The materials should have features like very good bonding characteristics, good strength development characteristics, the materials should be non-shrinking type, and above all they should be as impermeable as possible because the corrosion is accelerated only in presence of oxygen and moisture.  The repaired surfaces should be finally coated with coatings in which the basic property should be that the protective coating should have high resistance towards diffusion on carbon dioxide, which will further protect the concrete from the effects of carbonation.  The reports of maintenance and the records should be preserved to determine the suitability as well as the durability of the repair materials.  Rehabilitation is in fact the extension of waterproofing and requires very high degree of expertise. In addition to having knowledge of structural behavior, the rehabilitation expert should also possess thorough knowledge of materials science not only in terms of advantages but also the disadvantages and the limitations of the materials employed therein.

 

MATERIALS

Waterproofing system can be no better than the materials of construction employed therein.  This calls for a thorough understanding of the properties of every individual material used in a particular system.

 

The material should possess following basic requirements to provide effective treatment:

 

·        The material should provide watertightness to the system.

·        The material should have flexibility and elasticity to combat thermal and other stresses.

·        The material should be resistant to UV radiation’s and should not sag or loose other

         characteristics when used to a coat.

·        The material should have excellent bonding and adhesion properties both to the substrate as well

         as to ensuing treatments.

·        The material, as far as possible, retain the breathing properties of concrete.  The material should

         have abrasion resistance if used as a topcoat.

·        The material should be easy to apply, preferably free from solvents, pot life limitations etc.

·        The material should be cost effective.

·        The material should be resistant to algae, fungus and other microbe attacks.

 

However in the actual practice it is quite difficult to locate a material possessing all above requirements.  The fact therefore becomes quite evident that there can be no universal material to solve all the problems and a combination of materials can only serve a fruitful purpose.  Judicial compromises are necessary but the problems become simpler when the waterproofing treatment is viewed as system and not as a material alone.

 

CARBONATION OF CONCRETE

Carbonation is the effect of C0₂ from the atmosphere reacting with the alkaline component in the concrete, Ca (OH)₂ in the presence of humidity thereby converting the calcium hydroxide to calcium carbonate.  The pH value of the pore water is generally between 12.5 to 13.5 but due to carbonation the pH value is reduced to less than 9.  The reinforcement is therefore no longer in the passivating range and corrosion occurs.  Though carbon dioxide and humidity are external factors leading to carbonation of concrete affecting the durability it has been found that the grade of concrete as well as permeability has the effect on the rate of deterioration.  The existences of cracks also increase the rate of corrosion by providing the moisture and oxygen.  The durability is actually affected only when the depth of the carbonation is more than the depth of the cover.  Protected concrete is more resistant to deterioration than unprotected concrete.  The use of admixtures can lower the permeability of concrete and mitigate the effect of environment.  Concrete in regions of severe environmental stresses should be coated with suitable coatings to lower the depth of carbonation.

 

CHLORIDES IN CONCRETE

Another very important factor that affects the durability of concrete is the presence of chlorides.  The chlorides can enter the concrete at two different stages.

·        At the time of casting: in aggregates, in water and in admixture.

·        During service conditions: Concretes exposed to marine condition and due to deicing and other

         salts.

 

The maximum water-soluble chloride ion content for corrosion protection is laid down in various standards.  While concreting, the aggregates should be checked for chloride contents as well as the water and the admixtures.  Similarly, permeability plays an important role during the service conditions of the concrete structures.  The deterioration caused by chlorides in concrete is more difficult to be rectified at the later date and therefore, precautions are better to counteract this phenomenon.  The presence of sulphates.

 

CRACKS IN CONCRETE

Depending upon whether the damage is structural or non-structural, injection techniques are to be used for structural repairs.  Cracks occur in the concrete despite the fact that quality is controlled.  Cracks are one of the signs that give the indication of damaged or distressed structure. However, it is fortunate that all cracks are not a sign of structural failure.  Basically the cracks have to be repaired for two reasons viz. For structural purposes and for durability purposes.  One of the most prevalent techniques of repairing the cracks is by injection of different type of materials depending upon the nature of the defect.  The selection of material for injection requires thorough understanding of the properties of the material and functions that such a repair has to perform.  In all the cases, it is imperative that the cause of crack is properly determined otherwise the selection of material can be totally faulty.  Basically, the injections can be of three categories.  First, injections that are undertaken to restore the structural stability of structures.  Second, injections that are undertaken to protect the reinforcement to avoid the moisture and air entering the concrete and to lower the rate of corrosion.  Third, the injections that are undertaken to stop the water entering the structure.  Moreover, it should be borne in mind that the injection techniques are not only the function of materials but also the pattern and rhythm of application.  Under all circumstances it is advisable to trust theses type of jobs to experienced contractors having the knowledge of materials as well as experience in the use of several equipment.  The repair of cracks along cannot guarantee the structural stability or durability of concrete and therefore, if necessary should be complimented with other treatments as per the established practices of civil engineering.  An adequate inspection of cracks can provide valuable information about the reasons for cracking.  The information about the location of the crack, the pattern of cracking, inclination of crack, the depth of crack etc, is absolutely necessary to determine the material as well as the technique to be adopted to be adopted for the remedial measure.

 

WHY CRACKS OCCUR?

The cracks can occur in structures at two different stages.  Firstly before and during construction and secondly during the service conditions of the structure.  To a great extent, through proper measures, it is possible to arrest or minimise the cracks in the first case.  But in the second case, many of the factors are beyond control and therefore the cracks occur due to excessive mechanical and thermal stresses, chemical attacks (C02 and S02-leading to carbonation and corrosion) and biological attacks like plant growth and microorganisms.  It should be first established whether the cracks are structural or nonstructural.

 

In general, the major factors affecting the formation of structural cracks are:

·        Errors in stress calculations

·        Faulty construction, form work alignment, removal etc.

·        Excess loading under service conditions

·        Settlements

·        Unforeseen physical damage like fires, explosions etc.

·        Lowering of section of reinforcement in the second stage of corrosion.

 

Non structural cracks are mainly due to: 

a.  Plastic shrinkage cracking –rapid evaporation of water

b.  Drying shrinkage cracking

c.  Plastic settlement cracking-settlement of concrete in formwork

d.  Thermal contraction cracking

e.  Cracking caused due to poor workmanship

f.   Alkali aggregate reaction.

 

In both the cases, repairs are necessary to be undertaken.  The first type structural cracks, can lead to structural failure.  The second type non-structural cracks lower the durability of concrete.  Non structural cracks, if neglected, can lead to corrosion of structural cracks.  Actual repairs will largely depend upon the type of cracks and the reason for cracking.

 

SELECTION OF MATERIAL

The selection of material for injecting in the crack. Largely depends upon the investigation and primarily the following

 

Factors:

1.      Pattern of cracks

2.      Width of the crack

3.      Movements in the crack faces

I)       Due to temperature variations

II)      Due to dynamic loading.

4.      Moisture in the crack

5.      Dirt in the crack

 

The pattern of the cracks decides the reason for cracking which in turn reflects on the selection of base material.  Width of the crack, which reflects on the type of material, required whether it should act as structural injection or just an elastic seal.  When the injection is a structural one, it should be able to transfer stresses from one crack, which reflects on the type of material required whether it should act as structural injection or just an elastic seal.  When the injection is a structural one, it should be able to transfer stresses from one crack face to the other and should have adequate compressive and flexural strengths of at least 10-15% of the surrounding concrete.  The moisture in the crack calls for a water compatible system of injection.  Existence of dirt in the crack will guide the crack preparation system.

 

In general, the material for injection should have the following basic properties:

a.      Good compressive and flexural strengths

b.      Excellent bonding properties

c.      Lower viscosities

d.     Workable at wide range of temperatures

e.      Compatibility with moisture

f.       Non shrinking

g.      Low modulus of elasticity at higher temperatures

h.      Longer pot life and workability time. 

 

THE PROCESS OF INJECTION

After completion of diagnosis and selection of material for injection the work of injection passes through following stages: 

a.      Preparation of the crack

b.      Location of points (nipples) for injection

c.      Fixing of injection points

d.      Surface sealing of cracks

e.      Injection of resin proper

f.       Removal of nipples and plugging

g.      Removal of sealing material

h.      Final surface treatment after injection resin/grout hardens.

 

The preparation of crack is required to ensure perfect bonding of the injection material to the crack surfaces.  The preparation of crack should aim at removal of dirt, loose material and moisture in the crack, if the system chosen is not compatible with moisture.  This can be done with compressed air and solvents depending upon the width of the crack and contamination.  Before getting into details about the spacing of the injection points i.e. nipples, it is necessary to know the types of nipples available.

 

It is immaterial, whether the nipples are in form of metal or plastic tubes or just the holes in the structure.  They should be able to be connected to the injection nozzle, so that the pressure, if any should not be lost.   Thereafter, it should be possible to tie or seal the nipples, so that the resin is not lost and they should be removable to enable the surface smoothening.   Normally there are two types of nipples, which can be stuck to the surface of the structure along the line of crack, if the surface is even, and nipples, which are to be introduced in the structure after boring and inclined at 45 C to the crack plane.  The figures illustrate both the types.  The spacing of injection points depend upon the width of crack as well as the porosity of concrete.  However, as a thumb rule, in case of adhesion nipple, the spacing should be about 50% of concrete cross section.  Adhesion nipples should not be used for high-pressure injections exceeding 60 bars.

 

The work rhythm is of utmost importance, in case of vertical cracks, the injection should start from the bottom most point and it should be continued until the resin flows out of immediate top point.  Then the lower nipple should be sealed.  In case of horizontal joints different patterns are possible.

 

The simplest of the injection method is the brush injection.  The resin is brushed on the non-moving surface cracks and is absorbed in capillary action.  In case of pressureless injection the material is poured into the nipples especially in case of pipes acting as nipples, the use of such injection depends largely on the dimensions of the crack. In case of structural cracks of the width 0.2 – 1.0 mm, it is advisable to resort to low pressure injection.  This low pressure can either be created with handguns (sealant guns, grease guns etc.) or a normal compressor used at site.  The pressure developed is around 6 –10 bars.  Depending upon the crack widths and depths, high pressure injections can be resorted to for structural crack repairs.  It is possible to develop pressures to the tune of 500 bars using mechanical or pneumatic transmissions.  The injection method should be clearly specified prior to the commencement of the work and should be supervised to conform to specifications.

 

After the injection resin or grout has hardened and after the removal of the nipples, the surface sealing material, which is normally quick setting hydraulically system or thermoplastic resins should be scrapped off completely and the surface should be prepared for further cosmetic or strengthening treatment.

 

MATERIALS AND MACHINERIES

Several proprietary materials and machineries and available for treating the cracks by injection system.  They are mostly on the synthetic resin basis and cement bound.  The synthetic resins are usually two component based, on epoxies and polyurethane.  The cement based materials are invariably modified with polymers, to impart flowability, non shrinking characteristics, better bonding etc. The Table shows a chart, which compares some important properties of the injection materials.  A brief mention is as under:

a.      Solvent free unfilled epoxy for filling or injecting cracks of more than I mm width.  Though the
         viscosity is high the strengths are very high.

b.      Solvent free epoxy modified with suitable fillers is suitable for cracks of about 1mm.  It has lower          viscosity enabling better flowability than the above epoxy.

c.       Epoxy injection resin with a very low viscosity is suitable for injection of cracks wider than 0.2          mm for structural repair of cracks.

d.      Epoxy injection resin for structural repairs of cracks having still lower viscosity is suitable for          injection of cracks in prestressed concrete bridges.

e.      Polyurethane based two component injections which form gel when it comes in contact with          water within seconds is suitable for cracks where water is also pressing.  This should be used as          a primary injection to stop water and then normal PU injection can be used for sealing.

f.       Water compatible, two component polyurethane injection resin for non-rigid and elastic sealing of          cracks, it is suitable for sealing cracks subjected to differential movements and standing water.

g.      Non shrink grouts, develop high strengths and flowable at very low water cement rations.  They          are suitable for injections dry or wet over 15-mm wide joints.

h.       Polymer modified ready to use grouts remain in suspension at a very low, water cement ratio          and are suitable for wet or dry joints of above 2 mm.  The strength developed is also quite high.

 

The equipment required for crack injection can range from a simple bucket with an outlet to most sophicated pneumatically compressed machines capable of producing about 500 bar pressure and with hand controlled nozzles with a mixing assembly to mix the two components at the point of injection.  The sophisticated machineries are designed to provide better working pressures, better nozzle nipple combinations and to take care of pot life considerations and to enable proper crack sealing.  In principle, the machines for injections can be assembled to provide desirable pressures and nozzle-nipple assemblies.  The pressure is controllable through the pressure gauges attached.

 

 

FOLLOWING EQUIPMENTS ARE NORMALLY USED :

 

a.      Hand guns, sealant guns or grease guns in which two components are mixed and filled into the guns and pressure of about 6-10 bars can be exerted.

b.      Foot-pumps can be employed to create pressures upto 400 bars attached to single vessel containers into which the premixed two components are added.  These are normally suitable for small quantities of material.

c.      Machines are available in which the two components are separately introduced in two containers and automatically controlled quantities can be mixed at mixing assembly near the nozzles.  This arrangement solves the pot life problems.  These machines are connected to pneumatic or mechanical transmissions for creating pressures to the tune of 500 bars.  Since the exact quantity of different components can be preset, these machines are very suitable for continuous injection operations.

 

The more sophisticated the machinery, the better the control and therefore the performance.  Specifications written in office can be perfectly adhered to at site and control via supervision is good. Occurrence of cracks is practically unavoidable in concrete structures.   The modern building chemical technology coupled with proper equipment can solve almost all types of rehabilitation problems thereby providing economical solution in comparison to demolition and reconstruction of structures.  The specifications should be very clear and unambiguous.  The specifications should at least cover points like material, viscosity, techniques to be adopted, the equipment to be employed, type of nozzles and spacing, pressure to be applied etc.  The supervision at site is very essential to ensure that the specifications are strictly adhered to.  The temperature plays very important role in the performance of some resin based systems and therefore manufactures instructions as to die environmental temperature as well as the temperature of the component in which the material is injected are to be followed.

 

From the foregoing it is evident that only generalizations can be made for selection of materials and injection techniques.  In practice, suitable system is possible only after analysing the problem proper.  There can be more then one solution to a problem and therefore the technical appraisal should be done taking into consideration the site conditions die functions, that the system has to perform as well as the availability of materials and economic considerations.  An early action in remedying the crack can save further damage to the structures.  Neglecting a crack does not only reduce the chances of successful repair but also it makes the work uneconomical.  The repair of crack is a part of repairs of damaged and distressed structure and is not a substitute to other remedial measures required to be adopted for successful rehabilitation.  Many a types of crack can be avoided if resort is made to the advanced building chemical technology as well as proper quality control at site.  As a part of maintenance of structures, which involves inspection, it is better that the first occurrence of cracks is immediately reported to the consultant and his advice taken rather then to hide the defect by plastering, filling with crack fillers etc.  An early detection and immediate repair of cracks can ensure longer life to structures.

REHABILITATION OF STRUCTURES

It is not always true that concrete once cast is maintenance-free during its service life.  However good the quality of concrete may be, it is liable to deteriorate and it is only a matter of time before the deterioration reaches a stage where the durability of the structure is seriously affected.  Deterioration of concrete is basically a slow process and collapses do, not occur overnight.  The concrete structure fights this process of deterioration by drawing on its reserve strength. The distress may be either structural or aesthetic and in many cases of combination of both.

 

In this article the discussion will be restricted to concrete damaged by reinforcement corrosion.  The whole concept therefore revolves around protection of rebars.  This can be achieved by several methods like catholic protection, galvanizing process, epoxy and other polymer coatings, fusion bonding process, etc. All these processes are specialized jobs and are used sparingly in our country.  Moreover the costs involved in these systems make them viable when the structures in question are specialized ones like bridges, chimneys, cooling towers, etc.  The simplest method of rebars protection is by taking advantage of concrete medium of high pH value.  In this alkaline medium, a passivating layer of gamma iron oxide is created around the rebar and this acts as a further corrosion inhibitor. Fully solvent free PCC repair system is based on this concept or recreation of high pH around the reinforcement, and creation of impervious cover.  The success of this system is on account of its simplicity and can be easily carried out by existing contractors under the supervision of civil engineers.

 

CARBONATION AND CHLORIDE DIFFUSION

Concrete is continuously exposed to adverse environmental-influences, natural, man-made and invariably, combinations of both.  Industrialization has led to stack emission acidification and the environmental pH has reached levels of 4.5 to 6.  These pollutants combined with humidity constantly attack the concrete and lead to carbonation.  Moreover exposure to marine atmospheres accelerates the chloride ion diffusion.

 

Carbonation is the effect of C02 from the atmosphere reacting with the alkaline component in the concrete, Ca (OH)2 in the presence of humidity thereby converting the calcium hydroxide to calcium carbonate.  The pH value of the pore water generally between 12.5 to 13.5 but due to carbonation the pH value is reduced to less than 9.  The reinforcement is therefore no longer in the passivating range and corrosion occurs.  Though carbon dioxide and humidity are external factors leading to carbonation of concrete affecting the durability it has been found that the grade of concrete as well as permeability has the effect on the rate of deterioration.  The existence of cracks also increases the rate of corrosion by providing the moisture and oxygen.  The durability is actually affected only when the depth of the carbonation is more than the depth of the cover.  Protected concrete is more resistant to deterioration than unprotected concrete.  The use of admixtures can lower the permeability of concrete and mitigate the effect of environment.  Concrete in regions of severe environment stresses should be coated with suitable coatings to lower the depth of carbonation.

 

Another very important factor that affects the durability of concrete is the presence of chlorides.  The chlorides can enter the concrete at two different stages.

 

·        At  the time of casting : in aggregates, in  water and in admixtures

·        During service conditions: Concrete exposed to marine condition and due to deicing and other

         salts.

 

The maximum water soluble chloride ion content for corrosion protection is laid down in various standards.  While concreting, the aggregates should be checked for chloride contents as well as the water and the admixtures. Similarly, permeability plays an important role during the service conditions of the concrete structures.  The deterioration caused by chlorides in concrete is more difficult to be rectified at the later date and therefore, precautions are better to counteract this phenomenon.  The presence of sulphates.

 

The rate of corrosion depends upon the chloride concentration and chloride levels about 1% by wt. Of cement are high and accelerate the corrosion.  Chloride infected concrete can only be repaired by cathodic protection, but the basic principles of corrosion form the base for PCC repairs.

 

CORROSION AND CRACKING

Corrosion is basically an Electro chemical process and involves consumption of the anode.  In this case iron oxide is formed and deposited at cathodic portions.  All corrosion processes require electrolyte and this is provided by aqueous solutions of carbonic acid or chloride solution depending upon the attacks on the concrete structures in question.  The rate of corrosion determines the speed of deterioration and the degree of corrosion at the particular moment determines whether the damage is structural or non-structural.

 

Physically speaking the rusted reinforcement occupies a volume of about 2.5 times that of normal rebar and this creates internal stresses leading to cracking of the protective cover.  These cracks are relatively easy to identify, as they tend to follow the line of reinforcement.  The degree of rusting determines whether the present existing diameter of bar is capable of carrying the structural stresses at that moment of time.  This will classify the repairs into structural and cosmetic.  Structural repairs are undertaken to restore structural stability while cosmetic repairs are undertaken to restore durability.  It is a normal practice to add reinforcement if the deterioration is more than 15% of the Reba area.  However, this is just a guideline.

 

If there exist cracks due to other stresses the cracks should be first repaired by either epoxy, polyurethane or polymer cement grout injections and thereafter the new design steel should be connected to the old one and repairs can be conducted by PCC mortars.

 

BASIC STEPS IN A REPAIR PROGRAMME

Repair programme is an expert’s job right from the beginning and should be carried out in a planned way.  The execution of repairs is totally different from new construction as different materials as well as specialised diagnosis forms a part of the system.  Whether the distress is structural or non-structural, the following basic steps should be taken:

 

a.      Preliminary investigation and diagnosis

b.      Laying out of specifications and design

c.      Selection of materials

d.      Actual repairs

e.      Periodical maintenance and record keeping.

 

Preliminary investigation and diagnosis are carried out to ascertain the nature and the extent of distress and to establish the feasibility of repairs.  Destructive as well as on non-destructive methods is at our disposal.  Shows the examination methods and the assessments depending upon the nature of defect shows a compact kit for evaluation of damage.  One should not condemn the structure arbitrarily, and extreme care and judgement should be exercised in the interpretation of the results.

 

Since the repair is a specialised job, after taking into consideration the existing degree of damage, design should be made for restoring the structural stability.  The additional reinforcements and their mode of connection to the old steel should be carefully laid out in specifications.  The work rhythm should be specified and every ambiguity should be carefully laid out in specifications.  The work rhythm should be specified and every ambiguity should be avoided.  Working drawings should be self explanatory to enable erection and execution at the construction sites.

 

Selection of materials poses a major problem as it has a slight chemical angle.  The disadvantages of the materials should lead to material selection rather than the advantages.  The material should be selected after due discussion with material manufacturers after full technical clarifications.

 

Actual repairs can then be carried out by experienced contractors without deviating from the design as well as material types and consumption.  The work should be planned, scheduled and perfect coordination should exist in all aspects of materials, personnel, tools, work assignment including weather conditions as well as service conditions of the structure.

 

SOLVENT FREE PCC MORTARS

The selection of materials is one of the most important steps in repair and rehabilitation field.  The civil engineer is confused with the infinite number of proprietary materials available in the market and is liable to err on this count. 

 

Commercial literature is normally full of the advantages but offer minimum necessary technical data.  The selector should discuss with the manufacturer the limitations of each material and then decide.  Table 3 presented by the working party of concrete society can form an excellent base for selection of materials.  Principally speaking the repair programme involves multiple layer applications and therefore the first and foremost consideration is to ascertain whether the materials behaving as system are compatible with each other as well as the substrate.  Polymer modified cement concretes and mortars are increasingly used in rehabilitation on account of their general superiority over normal concrete.  The fact that they are cement based gives homogeneity to the system and the repaired portions move with the substrate without creating undue stress at the junction of two layers.

 

In particular the polymers used should be water compatible and resistant to saponification.  The polymers should provide the mortars with adequate workability without affecting the setting times and they should be able to harden in alkaline environments.  Polymers should be free from deleterious materials and should not attack the reinforcement.  The polymers used are of alkaline nature and therefore restore alkalinity of the applied surfaces.  The polymer mortars exhibit better abrasion resistance, elasticity and extraordinary bending and bonding values.

 

Protected concrete and repairs are more durable than unprotected ones.  Therefore all repairs should end with coating over the full surface to resist the ingress of oxygen and moisture in the concrete.  Elastomeric membrane forming materials based on acrylics have been found most suitable as they have high resistance to diffusion of carbon dioxide as well low resistance to water vapor permeability enabling the concrete to breathe.

 

CONCLUSION

Fully solvent free, polymer modified cement mortar and concrete systems are gaining popularity all over the world on account of the established properties of the polymers, ease of application and ease to adjust the work rhythms as against the pot life and film forming limitations of epoxy and other polymer mortars.  Moreover, economically PCC mortar repairs are favorable while maintaining the technical values.  PCC mortars exhibit high elasticity accompanied by higher strengths and this is a very valuable property.

 

There is a genuine need to develop testing methods in our country so that easy evaluation of the new materials can be undertaken and technical doubts can be dispelled.  The system as a whole should be tested for corrosion protection capacity, shrinkage, compressive strength, flexural strength, dynamic madulii of elasticity, coefficient of linear expansion, bonding strength etc.  The coatings should be tested for resistance to water vapour diffusion and resistance to carbonation, as these are absolutely basic requirements.  Modern technology should be adopted for the diagnosis and evaluation of distressed structures.

 

Selection of proper materials, thorough surface preparation and skilful application under a quality control engineer can lead to satisfactory repair guaranteeing strength coupled with durability, aesthetics and overall true economy.  The repair proposals should be judged on technical merits and not on guarantee periods.  Trust should be established between owner, consultant, manufacturer and applicator leading to Quality Assurance system involving testing as well as third party supervision.  Periodical maintenance and proper concrete protection is the key to durability of the concrete structures.