yogender shashi harender

8/7/2019 yogender shashi harender

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Introduction:Concrete expands and shrinks with changes in

moisture and temperature. The overall tendency

is to shrink and this can cause cracking at an earlyage. Irregular cracks are unsightly and difficult to

maintain but generally do not affect the integrity

of concrete. Joints are simply pre-planned cracks.

Joints in concrete slabs can be created by

forming, tooling, sawing, and placement of joint

formers.

Some forms of joints are:Contraction joints - are intended to create

weakened planes in the concrete and regulate

the location where cracks, resulting from

dimensional changes, will occur.

Isolation or expansion joints - separate or

isolate slabs from other parts of the structure,

such as walls, footings or columns; and driveways

and patios from sidewalks, garage slabs, stairs,

light poles and other points of restraint. They

permit independent vertical and horizontal

movement between adjoining parts of the

structure and help minimize cracking when such

movements are restrained.

Construction joints - are surfaces where two

successive placements of concrete meet. They

are typically placed at the end of a day's work

but may be required when concrete placement

is stopped for longer than the initial setting timeof concrete. In slabs they may be designed to

permit movement and/or to transfer load. The

location of construction joints should be

planned. It may be desirable to achieve bond

and continue reinforcement through a

construction joint.


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Why are Joints

Constructed?Cracks in concrete cannot be prevented entirely, butthey can be controlled and minimized by properly

designed joints. Concrete cracks because:

Concrete is weak in tension and, therefore, if its

natural tendency to shrink is restrained, tensile

stresses that exceed its tensile strength can develop,resulting in cracking.

At early ages, before the concrete dries out, most

cracking is caused by temperature changes or by the

slight contraction that takes place as the concrete

sets and hardens. Later, as the concrete dries, it will

shrink further and either additional cracks may form

or preexisting cracks may become wider.Joints provide relief from the tensile stresses, are

easy to maintain and are less objectionable than

uncontrolled or irregular cracks.


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How to Construct JointsJoints must be carefully designed and properly constructed if uncontrolled cracking of concrete

flatwork is to be avoided. The following recommended practices should be observed:The maximum joint spacing should be 24 to 36 times the thickness of the slab. For example, in a 4-

inch [100 mm] thick slab the joint spacing should be about 10 feet [3 m]. It is further recommended

that joint spacing be limited to a maximum of 15 feet [4.5 m].

All panels should be squarely or nearly so. The length should not exceed 1.5 times the width. Avoid :-

shaped panels.

For contraction joints, the joint groove should have a minimum depth of 1/4 the thickness of the

slab, but not less than 1 inch [25 mm]. Timing of jointing operations depends on the method used:

Preformed plastic or hard board joint strips are inserted into the concrete surface to the

required depth before finishing.

Tooled joints must be run early in the finishing process and rerun later to ensure groove bond

has not occurred.

Early-entry dry-cut joints are generally run 1 to 4 hours after completion of finishing, depending

on the concrete's setting characteristics. These joints are typically not as deep as those obtainedby the conventional saw-cut process, but should be a minimum of 1 inch [25 mm] in depth.

Conventional saw-cut joints should be run within 4 to 12 hours after the concrete has been

finished.

Raveling during saw cutting is affected by the strength of the concrete and aggregate

characteristics. If the joint edges ravel during sawing, it must be delayed. However, if delayed too

long, sawing can become difficult and uncontrolled cracking may occur.


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Use premolded joint filler such as asphalt-impregnated fiber sheeting, compressible foam strips,

or similar materials for isolation joints to separate slabs from building walls or footings. At least 2

inches [50 mm] of sand over the top of a footing will also prevent bond to the footing.To isolate columns from slabs, form circular or square openings, which will not be filled until after

the floor has hardened. Slab contraction joints should intersect at the openings for columns. If

square openings are used around columns, the square should be turned at 45 degrees so the

contraction joints intersect at the diagonals of the square.

If the slab contains wire mesh, cut out alternate wires, or preferably discontinue the mesh, across

contraction joints. Note that the wire mesh will not prevent cracking. Mesh tends to keep the

cracks and joints tightly closed.Construction joints key the two edges of the slab together either to provide transfer of loads or

to help prevent curling or warping of the two adjacent edges. Galvanized metal keys are

sometimes used for interior slabs, however, a beveled 1 by 2 inch [25 by 50 mm] strip, nailed to

bulkheads or form boards, can be used in slabs that are at least 5 inches [125 mm] thick to form

a key which will resist vertical loads and movements. Keyed joints are not recommended for

industrial floors. Metal dowels should be used in slabs that will carry heavy loads. Dowels must

be carefully lined up and parallel or they may induce restraint and cause random cracking at the

end of the dowel.

Joints in industrial floors subject to heavy traffic require special attention to avoid spalling of joint

edges. Such joints should be filled with a material capable of supporting joint edges.

Manufacturer's recommendations and performance records should be checked before use.


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Sealing of jointsjoints are the areas most susceptible to leaks in buildings and structures. To repair and establish

the joint seal that lasts its expected life select the right quality of sealant, prepare the joint properly

and install the sealant correctly. Repair treatment for sealant of joints comprises of following

necessary steps.

Choosing a suitable sealant

preparing the joint properly

installing sealant at proper temperature

providing bond beaker tape for shallow jointsproviding backer rods for deeper joints

backer rod placement

application of sealant

field molded hot applied sealant

field molded cold applied sealants

choosing a suitable sealantthe flexible sealants are able to deform to accommodate joint movements. Rigid materials or thosethat harden with age or at low temperature may fail when the joint opens and closes. It the sealant

material shrinks after it is installed, tensile stresses develop causing it to split or lose bond with the

joint face. The type of sealant available for construction industry have already been discussed in

detail in chapter-6 on materials for repair. The physical properties listed for various materials can

help users to choose a sealant that will perform well for a given situation of environment and loads


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Preparing the joint properlyif the existing sealant has performed well for the given environment then fill the same sealant in small

gaps and cover with hard and soft sealants. However, if the failure is extensive remove and replace

the sealant completely.Remove the sealant with hand tools from the groove or plough the joint with suitable mechanical

joints for larger projects. If the joint needs to be widened to improve its shape factor, saw cut the

sealant reservoir of required size.

Remove loose aggregate and embedded foreign material from the joint. Fill minor edge spalls with a

repair mortar.

Sand blast the faces of the grooved joint to remove contaminants and to clean the pores of the

concrete. Then remove any remaining dust by air blasting or vacuuming. These steps are important to

develop a good bond with the field molded sealant and to provide uniform contact of compression

seals. Solvents intended to remove oil contaminants may sometimes carry the contaminants further

into the pores of the concrete. Careful cleaning with solvents, brushes, or oil free compressed air is

important to obtain good adhesion to the sides of a joint. Solvent cleaning Is required to remove

many contaminants from non porous substrates. Porous masonry substrates can be cleaned by

applying solvent oil, free compressed air, saw cutting, mechanical scrapping, grinding, or sandblasting, while sand blasting can aggressively erode the surface of joints in porous masonry materials.

In addition to cleaning of a joint, application of surface primer on certain substrates is necessary to

assure good adhesion. Sealant manufactures usually recommended specific primers for each sealant

and type of substrate. Some sealant material and not be substituted because of chemical

incompatibility. As a general rule, joint faces must be clean and dry since the sealant has to bond to

the concrete surface. Better results are achieved if the sealant is installed in a dry joint .


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Installing sealant at proper temperaturejoints open and close with changing temperatures. As the temperature increases, the concrete

expands and the joint closes. As the temperature decreases, the joint opens.

If possible, install the sent when temperature is close to the average annual temperature Thisminimizes the stress due to temperature changes on the sealant during its service life.

Sealant in a rigid control should be installed over a bond beaker type, while sealant in an open

expansion joint should be installed over a sponge backer rod

Providing bond breaker tape for shallow jointsthe mortar in a rigid control joint must be raked out to the proper depth while it is still plastic.

Bond breaker tape preventsadhesion to the back of three sided shallow joints, which can cause splitting failure in the sealant.

Polyethylene tape is the most commonly used material, but it can be difficult to handle in cold

weather. Butyl tape is a good substitute for easier handling at lower temperature.

Providing backer rods for deep jointsdeep joints require placement of a backer rod to control sealant depth to provide a firm surface

against which the sealant can be applied and tooled. Backed rods can be made up of either open

cell polyurethane foam, or closed cell expanded polyethylene foams.

Backer rods should be sufficiently stiff and immovable to absorb the pressure applied during

tooling of the sealant and large enough to maintain compression during the full cycle of joint

expansion and compression. Closed cell backer rods should be sized one third to one quarter of

the joint size, so these develop atleast 25% compression. Open cell backer rods should be sized at

least one half the widht of the joint, so these develop at least 50% compression.


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Backer rod placementit is critical to install backer rod to the correct deoth in the joint. If the rod is positioned too deep,high sealent stressses can cause adhesive faliuyre . Many technicians judge placement depth

visually, but tools designed to control placemnet can assure proper width to depth ratios for better

sealant performance. Tools should be smooth surfaced to avoid puncturing or tearing closed cell

rods

Application of sealantmulti component sealants require job site mixing to ensure proper curing and development ofphysical properties. Both under mixing and over mixing can lead to sealant and joint failure, so

manufacturer's instructions should be followed carefully.

The nozzle of the caulking gun should be kept parallel to the plane of the joint surface to avoid

over or under filling the joint. Holding the nozzle tip tightly to the surface also prevents the sealant

from squeezing out. The sealant must fill the joint completely.

All sealant joints must be tooled. Tooling should be used to seal the joint by pressing the sealant

firmly against sides eliminating voids and air pockets to create a clean surface. Wetting agents such

as solvents, spirit solutions, and water are not recommended.Wet tooling can inhibit the curing

process if solvents or soaps are used. Because wet tooling products are frequently misused and

misapplied, most manufacturers suggest the sealant use with dry tooling.


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Field mould hot applied sealent:Each manufacture has recommended a proper pour temperature

As well as a safe heating temperature , which should not be exceeded. The safe heating

temperature usually is 20 degree above the recommended pour temperature . Subjecting the

sealent to temperatures above the safe heating limit result in impairing the compound which

may

Ultimately result in poor field performance.These materials are usually heated in double boild type mealting kettle equipped with asuitable agitation system in the sealant mealting chamber . The kettle also contain a postive

pessure delivery and re-circulation system and a recording thermometer. The inner tank

should be oil jacked and the temperature of the heat transfer oil should be thermostatically

controlled. The application pipe and the nozzeles be insulated.

Hot poured materials are normally suitable for installation in horizontal joints only. They can

be placed in vertical joint , but adequate dams are necessary to prevent the sealant fromflowing out from the bottom before it cool and sets. Fill horizontal joint slightly belows the slab

surface.


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Field moulded cold applied sealants:The equipment used to install filed moulded cold applied sealants is the hand operated caulkinggun. The sealant is supplied prepackaged in cartridges to suit the gun. Sometimes the chamber or

cartridge is loaded on the job from bulk containers, or in the case of two component materials,

they are filled with the compound after mixing.

Mix two component sealant thoroughly to ensure proper curing and uniform properties. Patches

of sealant that do not harden due to improper mixing are required to be replaced with properly

mixed material.

Small quantities of two component sealants can be mixed manually with a broad bladed puttyknife while significant quantity of material requires mechanical mixing. For small batches, hand

held electric drills fitted with paddle blades can be used for mixing. For larger projects, more

sophisticated equipment is available in which the components are brought by individual pipe lines

to the nozzle where they are mixed in a small chamber before they are extruded.

A skilled operator is needed to apply sealant in a joint reservoir. Hold the gun nozzle at a 45-degree

angle and move steadily along the joint to apply a uniform bead without dragging or tearing the

sealant or leaving unfilled spaces. In large points, several may be needed, building up the sealant

in roughly triangular edges at the each run.

For nonsag sealants, tool the material after the joint has been filled. Tooling of sealant ensures

intimate contact with the joint faces, removal of any tapped air, consolidation of the sealant

material and provision of seal with uniform appearance.


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Two component sealants have a limited working life, especially on hot days. Once the accelerator

is mixed in, the curing reaction starts. Therefore, limit the batch size to what to what can be used

within the pot life of the sealant.To provide uniform contact with the joint face ,compressionseals require a uniform joint width and straight ,clean, smooth joint face .To ease insallation of

the seal,apply a neoprence or other lubricant in a bead to the upper edge of each of joint face

.AppIy. the

lubricant with hand pressure applicator immediately ahead of inserting the seal SO that it does

not dry out.

Position the seal vertically over the joint to press down and move the seal into the joint by using

a hand roller. Do not twist the seal or fold it over on itself.A small amount of stretching (upto 5%)may occur as the seal is forced in. Do not willfully Stretch and lengthen the seal to ease

installation. Stretching the seal may develop cracks and crevices which may impair the

effectiveness of the seal. Install the seal in as long a continuous length as possible. If field spilces

cannot be avoided, make them in the least critical location.


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SAFETY PRECAUTIONS IN JOINT SEALANTSThere are certain hazards in using jont sealants and must be considered before installationfion

.

Hot applied materials can cause serious burns or fire hazard if the flammable materials are

spilled.

.Excessive breathing of fumes or skin contact with coal tar compounds may cause irritation.

.Cold applied materials (other than emulsion) and primers may contain flammable solvents. Such

containers should be kept closed and also away from flames.Working areas must be well

ventilated.

.

Many elastomeric sealants contain toxic chemicals. Avoid skin, eye or internal Contact with suchmaterials. Protective gloves and sometimes aprons are necessary for working with such materials.

.Solvents used in clean up or during sealant curing may be restricted since these are atmospheric

pollutant even though these may be non-hazardous.

.Hazards in joint sealing can be minimized by adopting correct practices.


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SUMMARYJoint in a building represents a discontinuity, and serves specific purpose of free movement

without causing excessive stresses .Joints are located at suitable places. Joints are normallyspaced at less than 30m to avoid excessive stresses due to contraction or expansion.

Apart from structural behavior, these joints are also required to provide barrier to flow of

moisture, water and gases. Generally seal of the joints get deteriorated with time and needs to be

repaired or replaced based on its condition.

Sealing Of jont is done after removing the damaged seal and preparing the joint surface properly.

A suitable sealant material is selected for the purpose and applied step by step joint surface is

prepared by sand blasting the groove, cleaning and using bond breaker for unbounded shallowjoints and Backer rods for deep joints. Sealent is applied at suitable temperature Specified by

the manufacturer . Sealant are field moulded hot or cold applied depending on the situations and

the material used. Hot poured materials are normally suitable in horizontal joints. Field moulded

cold applied sealant use hand operated caulking gun. The sealant is often supplied prepackaged

in cartridges. Two component materials are mixedjust before application in a special mixing

equipment. Cartridges should not be kept filed ,beyond the pot life of mixed material.

Seal should applied uniformly and without twistor stretch.

Precaution should be observed while using hot applied materials. Proper gloves. masks, aprons

and goggles should be used. Flammable materials should not be spilled and containers should be

kept with closed lids. Avoid contact with toxic chemicals. Cleaning solvents are atmospheric

pollutants and should be used with restriction.Hazards can be avoided by simple precautions and

use of correct practices in joint scaling.

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