MATERIALS AND TECHNIQUES FOR REPAIR

PRESENTING YOU

A PART OF:MAINTENANCE AND REHABILITATION OF

STRUCTURES

POWER POINT PRESENTATION ON

SELECTION OF REPAIR MATERIALS The repair material should have following properties:Low shrinkage and expansion propertiesGood Workability and DurabilityGood bond strengthGood hardening and setting propertiesGood Mechanical PropertiesLess permeableEconomicalNon-hazardousGood Aesthetics

REPAIR MORTARSPortland Cement Mortar

As the name suggests in this type of material a mixture of cement, sand and water is used. This type of mortar is used for repairing defects on exposed new concrete surfaces only. This method is used only when the defect is small and too shallow for concrete replacement. The mortar generally used is of 1:3 or 1:4 (C:S)

Polymer Modified Cement Mortar

These mortars are used for repair on old hardened concrete for repairing defects on exposed concrete surface only. The mortars shall consists of cement, sand, polymer and water in specified proportions.

For preparing polymer modified mortar, first of all the cement and sand are mixed in the proportion of 1:2 to 1:3 and then polymers at the rate of 5 to 20% of the weight of cement are added. W/C ratio shall be 0.3 to 0.6.

The polymers used may be in the form of :

Polymer latexes Water soluble

polymers Liquid resins Re-dispersible

polymer powders

Epoxy Mortar

Epoxy mortars consists of epoxy resins, hardener and silica sand and are applied over an epoxy bonding coat over the old hardened concrete surface. These mortars attain strength in few hours.These mortars are used for repairs where it is difficult to use epoxy bonded concrete, when depth of repair is less than 40mm or when the repair areas are small i.e. less than 0.1 sq.m.

The advantages of epoxy mortar are:

Very High Strength Abrasion

Resistance Good Water

Resistance

CLASSIFICATION OF REPAIR MATERIALSA. Patch Repair Materials

Cementatious Mortar/Concrete Polymer Modified Mortar/Concrete Polymer Mortar/Concrete

B. Injection Grouts Polymer Grouts Gas Forming Grouts Sulpho-Aluminate Grouts

C. Bonding Materials Polymer Emulsion Type Polymer Resin Type

D. Resurfacing MaterialsE. Sealing MaterialsF. Water Proofing Materials

CONCRETE REPAIR CHEMICALS: EPOXIES

Epoxies belong to the epoxy group of organic chemicals. In case of epoxies, the polymerization process takes place when two materials Epoxy Resin Curing Agent Come in contact by thoroughly mixing in specified proportions.EPOXY RESIN:

Epoxy resins are the substances of low viscosity and can be injected into small cracks too. The higher viscosity can be used for surface coating or filling of large cracks or holes.

EPOXY RESIN AND HARDENER

USES OF EPOXY RESIN Epoxy Grouts, Mortars and Coatings are extensively used for repair. They develop excellent strength and adhesive properties rapidly and provide toughness that give both durability and crack resistance. Epoxy coated steel bars are used for corrosion resistance in RC works. Epoxy Group of chemicals are used for stopping leakages.

PROPERTIES OF EPOXY Good Mechanical Strength Good Chemical Resistance Good Workability High Tensile Strength

Other materials used for concrete repair are:Polymer and LatexAcrylic Polymer Polyester resins

Chemicals Used in Concrete Construction are:Corrosion InhibitorsPlasticizersShrinkage Reducing CompoundsRetardersAcceleratorsCuring CompoundsAir-entraining AgentsBonding agents Quick settings compounds

ADMIXTURES-ACCELERATORS

Accelerator is an admixture that causes an increase in the rate of hydration of the cement and thus shortens the time of setting, increases the rate of strength development, or both.

Accelerating admixtures are added to concrete either to increase the rate of early strength development or to shorten the time of setting, or both. Chemical compositions of accelerators include some of inorganic compounds such as soluble chlorides, carbonates, silicates, fluosilicates, and some organic compounds such as triethanolamine. The accelerators also reduce the risk of damage by freezing when concreting in cold weather.

HYDRO SET

TYPES OF ACCELERATING ADMIXTURESAccelerating admixtures can be divided into groups based on their performance and application: 1. Set Accelerating Admixtures: Reduce the time for the mix to change from the plastic to the hardened state. Set accelerators have relatively limited use, mainly to produce an early set. 2.Hardening Admixtures: Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least 130% at 48 hours. Hardening accelerators find use where early stripping of shuttering or very early access to pavements is required. They are often used in combination with a high range water reducer, especially in cold conditions.

The most commonly used accelerator is calcium chloride (CaCl2). When it is used under normal conditions, and in regular amounts 2% by weight of cement. IS : 7861 (1981) recommends a maximum of 1.5 percent of CaCl2 for plain and reinforced concrete works in cold weather conditions. CaCl2 can be used with ordinary Portland cement or rapid hardening cement.

The use of calcium chloride in form of accelerating admixtures should not be done as it results into the corrosion of the reinforcement.The accelerators suitable for reinforced concrete one can find are:Sodium thiocyanates and other thiocyanate saltsTriethanolamine and other alkanolaminesProducts based on sulfates, nitrates and formats

Advantages of accelerators1. Quick setting of concrete.2. The formwork can be removed early.3. The concrete gains strength at early stage.4. Reduces the segregation of concrete.5. The hydration of heat is increased therefore suitable for the cold regions as it stops the water in concrete from freezing.6.The repairing of structures can be done rapidly and can be subjected to load early.

Dis-Advantages of accelerators1. The excess use of accelerators may lead to drying shrinkage and swelling.2. The excess use may result into flash set of concrete.3. The excess use of accelerators containing chloride result into corrosion of concrete.4. The excess use of admixtures reduce the resistance of concrete against sulphate.

ADMIXTURE: RETARDERS Retarders are added to the concrete to slow down the process of

hydration of cement to delay or prolong the setting of cement in concrete. Retarders keep the concrete workable for a longer period.Purpose of Retarders:To overcome the accelerating effect of high temperature on setting properties of concrete in hot weather concreting.To delay setting. of cement. when concrete 15 to he placed in t conditions. When concrete is required to be transported for a long distance. In grouting oil wells, where at a depth of about ()0le meter temperature may be about 200°C and cement grout is required to be in mobile condition for about 3 to 4 hours.

RETARDER

Commonly used retarders are: Calcium sulphate (Gypsum). Starches. Sugars. Cellulose products. Acids or salts of acids like lingo, sulphonic acid, hydroxylated carboxylic acid, mucic acid calcium acetate, tannic acid.

Calcium sulphate in the form of gypsum is generally added during the manufacture of cement to retard the setting. Normally 2 to 3% gypsum is used. Calcium sulphate in the form of plaster of Paris can also be used. Addition of excess amount of gypsum may cause undesirable expansion and indefinite delay in the setting of concrete.

At normal temperature, 0.2% addition of sugar can delay the final setting time to about 72 hours or more.

ADMIXTURE: PLASTICIZERWorkability is an important property of concrete. The optimum workability of fresh

concrete varies from situation to situation. A high degree of workability is required in situations like deep beams, this walls with congested reinforcement. beam column Junctions, tremie concreting, hot weather concreting, pumping of concrete etc. The conventional methods adopted to obtain high workability are use of higher percentage of fine aggregate, using good grading of aggregates, or by increasing cement content. Sometimes, unengineered contractors use extra water to improve workability of concrete.

Use of plasticizers for improving workability without using excess of water is becoming popular practice all over the world.Calcium, sodium and ammonium lingo-sulphonates are the most commonly mad plasticizers. They are used in the amount of 0.1% to 0.4% by weight of cement.

The action of plasticizers is to fluidify the mix and to improve the workability of mix. Portland cement will have a tendency to flocculate in wet concrete. These flocculation entraps certain amount of water used in the mix. When plasticizers are added, they get absorbed on the cement particles. The absorption of charged polymer on the cement particles It will results in creating particle-to-particle repulsive forces, called zeta potential and the entrapped water gets released.

PLASTICIZER

SUPER PLASTICIZERJapan was the first country to develop super—plasticizers in 1960 and

subsequently Germany in 1970. The use of Super-plasticizers permit the reduction of water to the extent of 30 percent without reducing workability of the mix. They are also called high range water reducers. They are chemically different from the normal plasticizers. They are more powerful as dispersing agents. Advantages of using Super—plasticizers : Very high workability can be achieved. Hence, self levelling, self-compacting, flowing concrete can be produced. For the same workability, it has made possible to use w/c ratio as W 0.28 to obtain strength of the order of 100 MPa., With low w/c ratio, it also permits a reduction of cement content. The use of Super—plasticizers has made it possible to use fly ash. 513% “n silica fume to make high performance concrete. The Super—plasticizers produce a homogeneous, cohesive concrete general without any tendency for segregation and bleeding.

ADMIXTURE: WATER PROOFINGThe leakage of roofs, bathrooms, toilets, walls, kitchens, water tanks,basements etc. is still a headache for civil engineers. There are various materials and methods available in our country for waterproofing purposes. But most of them fail due to one or the other reasons. The success of waterproofing depends upon the quality of materials durability of materials, workmanship, environment etc. A waterproof concrete has to fulfil two separate and distinct functions:To be impervious to the water under pressure. To resist the absorption of water.

Waterproofing admixtures amy be obtained in powder, paste or liquid form. There are two types of materials available namely, pore filling and water repellent materials.

The Pore filling materials are chemically active pore fillers. In addition they also accelerate the setting time of concrete. Examples of pore filling materials are silicate of soda, aluminium and zinc sulphates and aluminium and calcium chloride.

WATER-PROOFING

The water repelling materials are soda, potash soaps, calcium soaps vegetable oils, Waxes, fats and coal tar residues. Butyl stearate, heavy mineral oil free from fatty or Vegetable oil may also be used as waterproofing materials. The chemical available for waterproofing concrete structures are : Integral waterproofing compounds Acrylic based polymer coatings. Mineral based polymer coatings. Silicon based water repellent materials.Waterproofing adhesives for marble, granite and tiles.

EXPANSIVE CEMENTConcrete shrinks while setting due to loss of free water. This

is known as drying shrinkage. The important property of expansive cement is that it suffers no overall change in volume on drying. Such type of cement is used by adding an expansive compound known as Sulpho-aluminate in the form of clinker at the time of grounding cement clinkers. Generally 8-20 parts of clinkers of Sulpho-aluminate are added to 100 parts of Portland cement and 15 parts of the stabilizer.There are two types of Expansive Cement:Shrinkage compensating cementSelf stressing cement

EXPANSIVE CEMENTUses of Expansive Cement are:

Grouting anchor bolts Grouting machine

foundations Grouting Pre-stress

Concrete Ducts

POLYMER CONCRETE Polymer concrete can be classified into three groups:Polymer-impregnated concretePolymer Portland Cement ConcretePolymer Concrete

POLYMER IMPREGNATED CONCRETE

Polymer impregnated concrete is produced by impregnating or infiltrating a hardened Portland cement concrete with a monomer and subsequently polymerizing the monomer in situ. It is one of the widely used polymer composite.

The partial or surface impregnation improves durability and chemical resistance while total or in depth impregnation improves structural properties of concreteThe monomers used for impregnation are:Methyl methacrylateStyreneAcrylonitrileT-Butyle stryeneEpoxy

STEPS FOR IMPREGNATIONCasting Conventional Concrete ElementsCuring of ElementsDrying and EvacuationSoaking the dried concrete in monomerSealing the monomerPolymerizing the monomer

polymerization is carried out by thermal-catalyst technique. In this method after monomer-catalyst mixture penetration and then heating the concrete to 70 to 90 degree Celsius for few hours.

POLYMER IMPREGNATED

CONCRETE

Application of Polymer-impregnated concrete: Prefabricated structural

elements Surface impregnation of

bridge decks Hydraulic structures Marine works Desalination plants Nuclear power plants

POLYMER PORTLAND CEMENT CONCRETE

Polymer Portland cement concrete is a conventional Portland cement concrete which is usually made by replacing a part of the mixing water with a latex/polymer emulsion. Earlier latexes were based on polyvinyl acetate or polyvinylidene chloride, but these are now not used because of the risk of corrosion of steel in concrete.

Two types of polymers are generally used:1. Elastrometric polymers2. Glassy polymers

1. Elastrometric polymers:they are characterized by their rubber like elongation and low modulus

of elasticity.Examples are:Natural rubber latexStyrene butadiene rubber latexNeoprene2. Glassy polymers:

they are characterized by higher strength, higher modulus of elasticity and relatively brittle type of failure.Examples are:Polyester-styreneEpoxy-styreneFuransVinylidene Chloride

POLYMER CONCRETEPolymer concrete is a mixture of aggregates with polymer as the sole

binder. There is no other bonding material present i.e. Portland cement is not used. It is manufactured in manner similar to that of cement concrete. Monomers

or pre polymers are added to graded aggregates and the mixture is thoroughly mixed by hand or machine. The thoroughly mixed polymer concrete material is cast in moulds of wood, steel or aluminum etc.

To minimize the amount of expensive binder it is very important to achieve the maximum possible dry packed density of the aggregates. For example, using two different size fractions of 20mm maximum coarse aggregates and 5 different size fractions of sand, higher density can be achieved.The polymerization can be achieved by any of the below methods:Thermal-catalytic reactionCatalyst-promoter reactionRadiation

SULPHUR INFILTRATED CONCRETEIn the past, attempts have been made to use Sulphur as a binding material instead of cement.Sulphur is heated to bring it into molten condition to which coarse and fine aggregates are poured and mixed thoroughly.On cooling, this mixture gave fairly good strength, exhibited acid resistance and chemical resistance, but it proved to be costlier than ordinary cement concrete.Recent studies shows that Sulphur impregnation into lean porous concrete improve its strength and other properties. It is reported that compressive strength of about 100 MPa could be achieved in about 2 days time.The quantity of Sulphur used in this method is also comparatively less making the process economical.A coarse aggregate of size 10mm and down well graded, natural fine aggregate and commercial Sulphur of purity 99.9% are used. A w/c ratio of 0.7 or more may be adopted.

PROCEDURE-AIn procedure ‘A’, the test specimens after 24 hours of moist curing, dried in heating cabinet for 24 hours at 121˚ C. Then the dried specimens are placed in a container of molten Sulphur at 121˚C for 3 hours. Specimens are removed from the container, wiped clean of Sulphur and cooled to room temperature for 1 hour and weighed to determine the weight of Sulphur infiltrated concrete.

PROCEDURE-B In procedure ‘B’, the dried concrete specimen is placed in an airtight container and subjected to vacuum pressure of 2 mm mercury for two hours. After removing the vacuum, the specimens are soaked in the molten Sulphur for half an hour. The specimens are removed, wiped clean of Sulphur and cooled at room temperature.

APPLICATION

Precast roofing elements, fencing posts, sewer pipes. Railway sleeper. For industrial applications. Precast concrete units are cheaper than commercial concrete.

FERRO CEMENT In 1943 pier luigi nervi tested and presented and in his

paper, a new structural elements, an extremely thin plate of concrete made of layers of small diameter wire mesh and cement mortar with sand used as the binder.

Ferro cement is a type of thin wall reinforced concrete, commonly constructed of hydraulic cement mortar, reinforced with closely spaced layers of continuous and relatively small size wire mesh. The mesh may be made of metallic or other suitable materials.

FERRO CEMENT MATERIALS USED IN FERRO CEMENT Cement mortar mix Skeleton steel Steel mesh reinforcement or Fiber

-reinforcement polymeric meshes

TECHNIQUES OF MANUFACTURES Hand plastering Semi- mechanized

process Centrifuging and Guniting

CONTENTS OF FERRO CEMENT Cement Mortar:

Normally Portland cement and fine aggregate matrix is used in Ferro cement. The matrix constitutes about 95% of the Ferro cement. The cement mortar is in the ratio of 1:2 or 1:3 with w/c ratio of 0.4 to 0.45. The fine aggregates conforming to Zone 2 and 3 are used. The fine aggregates greater than 2.36mm are used, i.e. coarse sand is used. The fine sand is not recommended to be used in Ferro cement.

Plasticizers and other admixtures can be used to improve workability, increase durability, reduce permeability and increase strength. Fly ash can be added up to 30% as cement replacement to increase the durability.

Reinforcement There are two types of reinforcement Skeleton Steel

The skeleton steel consists of large diameter steel bars upto 8mm dia spaced at 75 to 100mm. It may be tied reinforcement or welded wire fabric.Wire Mesh

The wire mesh consists of galvanized wire of 0.5 to 1.5mm diameter and 6 to 20mm c/c spacing. The wire mesh are formed by welding, weaving or twisting the wires. The welded wire mesh can be square or hexagonal in shape. The meshes with hexagonal opening are known as chicken wire mesh. The square woven meshes consists of 1-1.5mm dia wire spaced at 12mm c/c spacing.

ADVANTAGES OF FERRO CEMENT It is highly versatile and can be formed in to almost any shape for a wide wire range of used 20% saving on materials and cost Suitability for pre – casting Flexibility in cutting, drilling and jointing Good fire resistance Good impermeability Low maintains cost

Disadvantages of Ferro cementlow shear strength Low durability stress rupture failure corrosion of reinforcement materials large number of labors required high time consuming

FIBER REINFORCED CONCRETE

Fiber-reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented.

Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers – each of which lend varying properties to the concrete.

Fiber reinforced concrete (FRC) is a new structural material which is gaining increasing importance.

Addition of fiber reinforcement in discrete form improves many engineering properties of concrete.

Uses of Ferro Cement Water tight structures Silos and bins Biogas plants Pipes Wind tunnel Swimming pool

STEEL FIBER REINFORCED CONCRETESteel fiber-reinforced concrete is basically a cheaper and easier to use form of

rebar reinforced concrete. Rebar reinforced concrete uses steel bars that are laid within the liquid cement, which requires a great deal of prep work but make for a much stronger concrete. The steel fibers have diameter ranging from 0.25mm to 0.75mm. The use of steel fibers make significant improvement in the flexural strength of concrete.

Steel fiber reinforced concrete is used in construction of roads, air fields, bridges etc surfaces subjected to wear and tear

GLASS FIBER REINFORCED CONCRETE(G.F.R.C.)

Glass fiber-reinforced concrete uses fiberglass, much like you would find in fiberglass insulation, to reinforce the concrete. Major problem with the glass fibers is breaking of fibers and surface degradation by high alkalinity.

The commonly used glass fibers are E-Glass fibers and AR-Glass fibers. Glass fibers are made from 2000 to 4000 filaments which are lightly bonded to make up a strand. These strands can be chopped or combined to make cloth or mate.

PLASTIC FIBER REINFORCED CONCRETE

Plastic fiber reinforced concrete uses plastic fibers such as nylon, polypropylene, polyethylene to increase the tensile strength of concrete. The nylon and polypropylene and nylon fibers are found to increase the impact strength of concrete and thus reduce the tendency of cracking and their propagation. Generally 0.25-1% of fibers by volume are added to concrete.

CARBON FIBER REINFORCED CONCRETE

Carbon fibers possess high tensile strength and high youngs modulus. The modulus of rupture of an aligned carbon reinforced concrete cement composite with 8% fiber volume can be as high as 1623 N/mm2.

ASBESTOS FIBER REINFORCED CONCRETE

Asbestos is a mineral fiber and has proved to be the most successful fiber, which can be mixed with OPC. The minimum length of asbestos is 10cm. The asbestos fiber reinforced concrete has high flexural strength. Tensile strength of asbestos fiber is 500-980 N/mm2

CORROSION PROTECTION OF REINFORCEMENTMethods used are:Use of corrosion resistant steelCoating to reinforcementCathodic protectionHot dip galvanizingElectrochemical chloride removalImproving the cover to reinforcement

USE OF CORROSION RESISTANT STEEL

Stainless steel is the name given to a family of corrosion resistant steels containing a minimum of 12% chromium. On contact with air, the chromium forms a thin oxide layer on the surface of steel. This layer is inert and resists corrosion.

The use of corrosion resistant steel is done where humidity is more and where the concrete is subjected to chemical attacks.

COATING TO REINFORCEMENT: CEMENT SLURRYThe purpose of coating of steel bar is to provide a sufficiently durable barrier to aggressive materials such as chloride.

This method was developed by CECRI, Karaikudi, Tamilnadu. It consists of application of two coats of cement slurry to rebars. The entire process of coating is : De—rusting : Removal of rust, dirt and oil from steel surface. Phosphating : Phosphatization of steel surface by phosphating jelly offers temporary protection during time lag between de—rusting and application of first coat of inhibited cement slurry. Cement slurry : Two coats of cement slurry made by OPC cement and inhibitor solution to protect steel surface from the attack of sulphate and chloride ions and to keep the steel surface in high pH environment. ( Sealing : Two coats of sealing solution to increase the adhesion between steel and cement coating and to seal the surface. Sealing coat is applied after each application of slurry coat.

COATING TO REINFORCEMENT: EPOXY COATINGThe Fusion Bonded Epoxy Coating (FBEC) is a process where epoxy powder is applied by electrostatic spray on hot steel at pre-set temperature level The powder, when in contact with the hot bar, melts, flows, gels, cures, cools and produces a well adhered continuous corrosion resistant protective coating. The coating thickness typically varies from 130 micron to 300 micronAdvantages : As the technique is factory based, it gives uniform thickness and better quality control. It has excellent adhesive property. The coating is flexible not broken due to bending of bars during fabrication. Disadvantages:There is reduction in bond strength between coated rebars and concrete. Patching may not always be effective.Even the smallest damage in coating can initiate corrosion in severe environment.It requires careful handling as coating may get damaged.

EPOXY COATED BARS

CORROSION INHIBITORSA corrosion inhibitor is defined as a chemical substance that

reduces the corrosion of metals without a reduction in the concentration of corrosion agents, i.e. chlorides.

Corrosion inhibitors work by reducing the rate of the anodic and/or Cathodic reactions thereby Suppressing the overall corrosion rate. The examples of corrosion inhibiters are The addition of calcium nitrate extends the time of corrosion initiation, and also reduces the rate of corrosion once started. Organic corrosion inhibitors such as amino-alcohols are believed to suppress corrosion by primarily being absorbed on to the steel surface thereby displacing corrosive ions such as chlorides.

CATHODIC PROTECTIONCathodic protection is a technique by which the electrical

potential of the steel is decreased to level at which corrosion cannot take place. It is widely used for concrete offshore structure and steel structures, while on land it has been used for the protection of pipelines. Two methods are used:Impressed current methodSacrificial anode system

CATHODIC PROTECTION: IMPRESSED CURRENT

The Cathodic protection comprises of application of impressed current from an external power source to an electrode laid on the concrete above steel reinforcement. This electrode serves as anode and the steel reinforcement which is connected to the negative terminal of a DC source acts as a cathode. In this process the external anode is subjected to corrosion and the Cathodic reinforcement is protected against corrosion and hence the name ‘Cathodic Protection’ .

In this process the negative chloride ions which are responsible for the damage to “it passivating layer are drawn away from the vicinity of steel towards the anode where they are oxidized to form chlorine gas. The environment around the steel reinforcement reverts back to alkaline condition which protects the steel.  

CATHODIC PROTECTION: SACRIFICIAL ANODE

In this system the reinforcement is connected to anodes with a more negative corrosion potential than steel, such as zinc and aluminium. The external anode corrodes preferentially to the steel and supplies electrons to the Cathodic steel surface. Sacrificial anode system are more effective in submerged structures where the concrete is wet and resistivity is low. Warm temperature i.e. above 20˚C is generally required for this system.

DRY PACK TECHNIQUE

In this technique , dry cement sand mix in proportion of 1:2.5 is used and sufficient amount of water is added to it so as to form a ball by hand. The ball should neither slump nor crumble.

DRY PACK PROCESSBefore actual operation the hole should be thoroughly cleaned and ensured free from broken pieces of aggregates, washed and dried.A thin layer of cement grout is applied on the surface. The mix for bonding grout is the mixture of cement and fine sand in ratio of 1:1 to a consistency like thick cream.After this, the dry pack material is put into place before the bonding grout has dried up. Packing material consists of the volume or by weight a mix of 1 part of cement to 2.5 parts of sand.Water requirement for mix is such as to produce a mortar which, when used, will stick together on being molded into a ball, and will not exude water but will leave the hands damp. If less water is added, it will not make sound solid pack. If more water is added excessive shrinkage will occur and the material becomes loose.

Dry pack material is not to be filled at a time, but should be filled in layers and properly compacted having compacted thickness of about 1 cm.Each layer is scratched & finished to secure bond for succeeding layer and is compacted over its entire surface by use of hardwood stick and hammer. The size of such stick is about 20 to 30 cm in length and not over 2.5 cm in diameter. Wooden sticks will not polish the surface of each layer and thus better bonding surface is formed. Last layer is finished to match the surroundings.For better results and when water tightness is a requisite, the holes should be sharp and square at the surface edges, but corners within holes should be rounded. Holes for dry pack should have a minimum depth of 2.5 cm

VACUUM CONCRETEHigher water-cement ratio is damaging for concrete. We always try to restrict

the water-cement ratio in order to achieve higher strength. the adopted water-cement ratio is much more than that mainly because of the requirement of workability. Workability is also important for concrete, so it can be placed in the formwork easily without honeycombing.

After the requirement of workability is over, this excess water will eventually evaporate leaving capillary pores in the concrete. These pores result into high permeability and less strength in the concrete. Therefore, workability and high strength don’t go together as their requirements are contradictory to each other.

Vacuum concrete is the effective technique used to overcome this contradiction of opposite requirements of workability and high strength.

In this technique, the excess water after placement and compaction of concrete is sucked out with the help of vacuum pumps. This technique is effectively used in industrial floors, parking lots and deck slabs of bridges etc. The magnitude of applied vacuum is usually about 0.08 MPa and the water content is reduced by up to 20-25%. The reduction is effective up to a depth of about 100 to 150 mm only.

COMPONENTS REQUIRED FOR VACUUM CONCRETING

Mainly, four components are required in vacuum dewatering of concrete, which are given below: Vacuum pump Water separator Filtering pad Screed board vibrator

Vacuum Pump: Vacuum pump is a small but strong pump of 5 to 10 HP. Water Extractor: Water is extracted by vacuum and stored in the water separator. Filtering Pad: Filtering pad consists of rigid backing sheet, expanded metal, wire gauge or muslin cloth sheet. A rubber seal is also fitted around the filtering pad. Filtering pad should have minimum dimension of 90cm x 60cm. The mats are placed over fine filter pads, which prevent the removal of cement with water. Screed Board Vibrator: To force the excess water to get collected at the top.

ADVANTAGES OF VACUUM CONCRETING

Due to dewatering through vacuum, both workability and high strength are achieved simultaneously.Reduction in water-cement ratio may increase the compressive strength by 10 to 50% and lowers the permeability.It enhances the wear resistance of concrete surface.The surface obtained after vacuum dewatering is plain and smooth due to reduced shrinkage.The formwork can be removed early and surface can be put to use early

Effect of Vacuum Concreting

ASPHALT SHEETING

Bitumen sheets are watertight. They are used for water proofing of concrete. They can be laid easily on any surface. These sheets cannot resist excessive pressure and temperature. They are available in thickness of 3mm, and applied hot on the surfaces. At the junction of two sheets minimum lap of 100mm is required.

GROUTINGGrouting is a process of injecting mixture of cement, sand

and water at a high pressure in the cracks , joints, voids. Materials generally used for grouting are:Cement + WaterCement + Stone Dust + WaterCement + Clay + WaterCement + Clay + Sand + WaterChemicals, For example: Epoxy

PROCEDURE OF GROUTINGDrilling of grouting holes: For drilling the holes equipment's like jack hammer or shot drill can be used depending on the diameter of hole and the depth of hole required.Arrangement of grout pipes Grout pipes of 4-5cm dia and 45 to 90cm length are inserted in the grout holes. The space surrounding the pipes is filled with cement mortarCleaning of cracks Before injecting grout mixture in the cracks, it is necessary to clean the cracks. The loose materials in the cracks can be removed by injecting air-water mixture by pressure in the cracks. Inserting the grout in the holes Grout is inserted in the holes at a pressure of 0.65 kg/cm2.

SOIL GROUTING

GROUTING IN TUNNELS

SCHEMATIC PLAN OF GROUT PLANT

GUNITINGGuniting is the process of pneumatically transporting the mortar

or fine concrete through a hose on to a surface at a high velocity.Guniting is an effective technique, which has been extensively

used in the rehabilitation of structurally distressed R.C.C members. Gunite is also known as shotcrete or pneumatically applied mortar.The various applications of shotcrete are:Thin overhead, vertical or horizontal surfacesSwimming pools and prestressed tanksCanal and tunnel liningRepair of damaged concreteRefractory lining worksOverlays on concrete roads

METHODS OF GUNITING Dry-mix process shotcrete In this process, cement and moist aggregate are mixed and then placed into a device that meters the mixed material into a stream of compressed air. Material is carried by the compressed air through a delivery hose to the nozzle where water is added under pressure through a perforated ring. The water thoroughly wets the other ingredients as the mixture is jetted from the nozzle at high velocity onto the surface to be shotcrete. The amount of water added is under the control of the nozzle man or placing operator and can be varied by means of a valve to produce concrete or mortar ranging from extremely dry to extremely wet .

DRY MIX PROCESS

DRY MIX PROCESS

Wet-mix shotcreteIn the wet-mix process, all ingredients are first mixed to produce mortar or concrete.The mortar or concrete is then placed into delivery equipment which can be of a squeeze tube, pneumatic-feed or positive-displacement type. In any c a s e, the material is forced thro u g h a delivery hose to the nozzle where compressed air is injected to increase velocity. The nozzle man can vary the amount of air introduced but has no direct control over the other properties of the concrete or mortar being placed.

WET MIX PROCESS

JACKETING Jacketing is the most popularly used method for strengthening of building columns. The most common types of jackets are steel jacket, reinforced concrete jacket, fibre reinforced polymer composite jacket, jacket with high tension materials like carbon fibre, glass fibre etc.

The main purposes of jacketing are: To increase concrete confinement by transverse fibre reinforcement, especially for circular cross-sectional columns, To increase shear strength by transverse reinforcement, To increase flexural strength by longitudinal reinforcement provided. To increase the seismic capacity of the structure

COLUMN JACKETINGJacketing of columns consists of added concrete with

longitudinal and transverse reinforcement around the existing columns.

This type of strengthening improves the axial and shear strength of columns while the flexural strength of column and strength of the beam-column joints remain the same. It is also observed that the jacketing of columns is not successful for improving the ductility. A major advantage of column jacketing is that it improves the lateral load capacity of the building in a reasonably uniform and distributed way and hence avoiding the concentration of stiffness as in the case of shear walls.

PROCEDURE OF COLUMN JACKETINGOpen the footing of columns by excavating around it and provide the supports to the slab with the help of the hydraulic jack and vertical posts.Remove the plaster from the surface of the columns.Make the surface of the concrete rough by sand blasting.Add new bars to the column externally from footing to the column as per engineer guides.Apply the bonding agent on the old concrete for proper bonding between the old and mew concrete.Erect necessary shuttering around the columns.Pour Minimum of M-25 grade of Concrete, compact and cure it.

COLUMN JACKETING

COLUMN JACKETING

BEAM JACKETINGJacketing of beams is recommended for several purposes as it

gives continuity to the columns and increases the strength and stiffness of the structure. While jacketing a beam, its flexural resistance must be carefully computed to avoid the creation of a strong beam-weak column system. The location of the beam critical section and the participation of the existing reinforcement should be taken into consideration.

Jacketing of beam may be carried out under different ways, the most common are one-sided jackets or 3- and 4-sided jackets. At several occasions, the slab has been perforated to allow the ties to go through and to enable the casting of concrete. The beam should be jacketed through its whole length. The reinforcement has also been added to increase beam flexural capacity moderately and to produce high joint shear stresses.

PROCEDURE OF BEAM JACKETINGReduce the load on the beam by removing the tiles and bed mortar from the slab.Erect the props to support the slabRemove the plaster on the beam and then plsce the longitudinal bars along with the stirrups to the beam.Stirrups are inserted by making the holes through the slab.The longitudinal bars are either passed through the column or in case if column is also to be jacketed they are attached to the external reinforcement of the columns.The old concrete surface is cleaned by air jetting. To allow good bonding between old concrete and new concrete epoxy bond coat is applied. The formwork is erected around the beam and the concrete is poured, compacted and cured.

BEAM JACKETING

SHORINGShoring is the construction of a temporary structure "to support

temporarily an unsafe structure. The main objects of shoring are : When the walls of a building shows signs of bulging or leaning outwards, shoring is necessary. When an adjacent structure is to be dismantled. When openings are to be made or enlarged in the wall. When a wall cracks due to wall needs repairs.    

RAKING SHORESThis is system of giving the temporary supports to an unsafe

wall. The construction of raking shore, varies with the condition at the site. In this method the incline members known as rakers are used to give support to the wall.

Components of raking shores Rakers Wall plate Needles Cleats Bracing Sole plate

PROCESS OF ERECTIONThe wall plate, about 20 to 25 mm wide and 5 to 7.5 cm thick is placed vertically along the face of the wall and is secured by means of needles of 10 cm x 7.5 cm section. The needles penetrate the wall by about 10 cm.In order that the needles do not get sheared ,, oil due to the thrust of the raker; the needles are further strengthened by means of cleats which are nailed directly to the wall plate. Rakers abut against the needles in such a way that the Centre line of the raker and the wall meet at the floor level. Thus there will be one raker corresponding to each floor. These takers are inter-connected by struts to prevent their buckling. The feet of takers are connected to an inclined sole plate, embedded into the ground by means of iron dogs.The feet of rakers are further stiffened near the sole plate by means of hoop iron. The wall plate distribute the pressure to the wall uniformly.

POINTS TO BE KEPT IN MINDRakers should be inclined to the ground by 45°, to make them move effective However. in practice, the angle may vary from 45° to 75°. Rakers should be properly braced at intervals. For tall buildings, the length of raker can be reduced by introducing rider raker. The Centre line of raker and the wall should meet at floor level. The size of the raker should be decided on the basis of anticipated thrust from the wall. The longer length of the wall needs support, shoring may be spaced at 3 to 4.5 m spacing. depending upon the need. The sole plate should be properly embedded into the ground, at inclination. and should be of proper section. Wedges should not be used on sole plates since they are likely to give way under vibrations which are likely to occur.

FLYING SHORES/HORIZONTAL SHORES

In this type of shoring, horizontal supports are provided for supporting temporarily the parallel walls of the two adjacent buildings, which may tend to collapse or damage when one of the intermediate buildings has to be pulled down and rebuilt.

Types of flying shore:

Single flying shore Double flying shore

If the walls are quite near to each other (distance up to 9 m), single flying shore can be constructed. It consists of wall plates, needles, cleats, struts, horizontal shore, straining pieces and folding wedges.  

When the distance between two parallel walls is more than 9 m, a double flying shore having a trussed formwork

POINTS TO BE KEPT IN MINDThe Centre lines of flying shore and struts and those of the walls should meet the floor levels of the two buildings. If the floor levels are different, the horizontal shore should be placed either mid-way between the levels of the two floor of equal strength, or it should be placed at the level of weaker floor. The struts should preferably be inclined at 45°. This inclination should not exceed 60°. The flying shores should be spaced at 3 to 4.5 m centres along the two walls, and horizontal braces should be introduced between adjacent shores. Single flying shores should be used only up 0 9 m distance between two walls. For greater distance, double flying shores should be provided. Flying shores are introduced when old building is being removed and should be kept in position till the new unit is constructed. Large factor of safety should be used for determining sections of various members of the shoring, since it is very difficult to assess the actual loads.

DEAD SHORE/VERTICAL SHORES

This type of shoring consists of vertical members known as dead shores supporting horizontal members known as needles. The needles transfer the load of the wall etc. to the dead shores.  

Process of ErectionHoles are made in the wall at suitable height. Needles, which are made of thick wooden sections or of steel, are inserted in the holes. Each needle is supported at its two ends; by vertical posts or dead shores.The dead shores stand away from the walls so that repair work is not obstructed. The dead shores are supported on sole plates and folding Wedges

Purpose of Dead shoresTo rebuild the defective lower part of the wall.To rebuild or deepen the existing foundation.To make large opening in the existing wall at lower level.

POINTS TO BE KEPT IN MINDThe needles are spaced at l to 2 m. A minimum of three needles should beused for an opening. The needles should be suitably braced. The section of needles and dead shores should be adequate to transfer the which can be estimated with fair degree of accuracy.If the opening is made in an external wall, the length of the outer dead shore will be greater than the inner ones. The floors should be supported from inside. The dead shores are supported on sole plates. Folding wedges should be inserted between the two. If the external wall is weak, raking shores may be provided, in addition to the dead shores. Shores should be removed only when the new work has gained sufficient strength. The sequence of removal should be needles-strutting from opening-floor strutting inside-raking shore. A interval of 2 days should be allowed between each one of these removal operations.

UNDER PINNING

The term underpinning is applied to the construction of a new foundation underneath the existing one for strengthening purposes, without endangering the stability of the existing structure.

Underpinning may be necessary for a variety of reasons:The original foundation is simply not strong or stable enough.The usage of the structure has changed.The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during design.The construction of nearby structures necessitates the excavation of soil supporting existing foundations.To increase the depth or load capacity of existing foundations to support the addition of another storey to the building (above or below grade).It is more economical, due to land price or otherwise, to work on the present structure's foundation than to build a new one.Earthquake, flood, drought or other natural causes have caused the structure to move, thereby requiring stabilization of foundation soils and/or footings.

UNDERPINNING: PIT METHOD

In this method existing wall over the foundation is divided into various sections generally 1.2 to 1.5 m in length. Holes are then made at adequate height in the existing wall. In these holes steel needle beams with bearing plates are inserted and supported on either side of the wall by means of crib supports (wooden blocks) and screw jacks.

The pit is now excavated up to the desired level of the proposed new foundation in sections. The old foundation may be extended up to level of foundation directly or by cutting the lower part of old foundation as desired.

PIT METHOD

The following points regarding pit method of under pinning may be noted:The entire wall foundation can be replaced or deepened by this method in sections.In case of underpinning the large sections of walls, the work is started from the centre and progress is made side ways. Proper timbering to the excavated trenches should be provided. Alternate sections are taken up in the first round. The remaining intermediate sections are then taken up. Only one section should be taken at a time.The needle beams etc. should be removed only when the new foundation hag gained sufficient strength. It is desirable to do the new foundation work in concrete. The needle holes, etc. should be closed in masonry using cement mortar.  

UNDERPINNING: PILE METHOD

Sometimes, pit method may be impracticable or uneconomical under certain circumstances, for example, pit method cannot be used for underpinning in water-logged areas, or when the heavy loads of existing structure are to be transferred to the ground at a great depth. the cost of underpinning by pit method may be excessive and hence uneconomical. Under such situations, the use of precast piles or steel piles is made for underpinning of foundations. In this method the piles are driven at regular interval along both the sides of the wall.

Generally bore hole piles or under reamed piles are used. The piles are connected by concrete or steel needles, penetrating through the wall. These needles act as pile caps also.

This method is very much useful in clays soil and in water logged areas.

PILE/PIER METHOD

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