3.2.repair of cracked concrete.pdf

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CHAPTER 3
REPAIRS TO CRACKED CONCRETE

PURPOSE OF REPAIR
Cause of the crack should be ascertained first. Commonest reason for repair : Prevention of corrosion to reinforcement. Crack widths less than 0.3mm does not cause corrosion. Cracking at right angles to the bar is relatively unimportant. Cracks along the length of the bar are more serious. Hiding cracks is difficult and filling them nearly always fail. Surface coating is a more successful method. Coating materials vary in their crack bridging properties and in their elasticity.

CLASIFICATION & DIAGNOSIS
Cracks may be classified as ‘live’ cracks and ‘dead’ cracks. Live cracks: cracks vary with time Dead cracks: No further movement Cracks may further be sub-divided as follows: Progressive cracks: Expected to become longer. Static cracks: Unlikely to become longer. Thorough diagnosis and classification is essential before repairs are carried out.

NO FURTHER MOVEMENT EXPECTED
Dead cracks are a result of accidental overload. Cracks wider than 1mm in horizontal surface can be sealed by filling them with cement grout. Finer cracks and those in soffits and vertical surfaces may be sealed by injecting a polymer. Epoxy resins are used when Pressure injection is necessary. For structural integrity. Cheaper polymers are used when Purpose of repair is to protect reinforcement form corrosion. Polymer grout can be run into the crack through gravity.

CEMENT GROUT
Cracks 1mm-2mm Brush with dry cement Followed by light water spraying.

Cracks greater than 2mm wide
Preferable to use a cement & water grout. Alternatively, cracks can be chased out to a width of 5-10mm and pointed up with sand & cement mortar.

Cracks become conspicuous after the repair

POLYMER SEALING, WITHOUT APPIED PRESSURE
Low viscosity liquid polymers are used. Mode of application: Adequate seal can be obtained by brush application on level surface. Temporary bunds can formed with clay, plasticine or similar materials to surround the crack, and it can be flooded with polymer. Surplus materials and bunds are removed after application. Penetration upto a crack width of 0.1mm can be achieved. Repair of this nature will not be structural.

POLYMER INJECTION
GENERAL PRINCIPLES
When full depth of crack is to be penetrated, injection of polymer grout under pressure is the most common method used.

Gravity head of a few 100mm
Hand operated pumps Mechanical pumps Pressure pots are the few modes of polymer injection.

GENERAL PRINCIPLES
Sealing cracks by injection: Start at one end and work progressively along the crack. For vertical or inclined cracks, injection should start at the lowest point and proceed upwards. A series of injection points are formed at intervals along the length of the crack and grout is injected into each point until it starts to flow out of the next one. The point in use is then sealed off and injection is started at the next point, so on until the entire length of the crack is treated.

INJECTION POINTS
First the cracks must be surface sealed. Rapid curing, polymer based materials can be used for this purpose. Adhesive tapes can be used if injection pressures are low. Holes are drilled along the crack at intervals and injection nipples are grouted in. There is a risk of holes being blocked by drilling, which can be reduced if hollow drills with an applied vacuum are available. Spacing of the injection points depends on:
Depth and width of crack Variation with depth Viscosity of grout Injection pressure.

MATERIAL PROPERTIES & PRESSURE
Properties of the grout, the injection pressure and spacing of the injection points are interrelated. When treating vertical or inclined surfaces or soffits, thixotropic grouts may be used in order to prevent its flowing out again. Injection pressure is determined by experience. It is common practice to increase injection pressure during the course of the work.
Resistance against flow increases progressively as the crack is filled, Setting of the polymer, being the reasons.

‘Gravity feed’ is the normal method of injection where the cracks are greater than 0.5mm wide.

EQUIPMENTS
Funnel attached to a length of flexible tubing so as to provide a gravity head. Simplest mechanical equipment: A hand held gun that holds cartridge of resin grout. Hand held pump with the suction hose immersed in a container of resin grout. Pressure pots and pneumatic guns pressurized by compressed air. All the equipment described above work on a batch principle (A batch of grout is mixed and
injected, after which injection must stop while another batch is mixed and placed in the equipment.)

Lately Power driven equipments are used with resin & hardener not coming into contact with each other until they reach the injection head. This is basically used in the paint industry.

FURTHER MOVEMENT EXPECTED
If there are signs of continuous movement at a crack, it is necessary to make provision for it to continue after repair.

The crack can be regarded as an unplanned
movement joint, and if locked solid, another crack will form nearby.

FURTHER MOVEMENT EXPECTED

One way of achieving this is to cut a chase along the line of the crack. The sealant must then be adhered to the sides of the chase but debonded from the bottom so that movement is spread over the full width of the chase as shown in the figure. D is depth of sealant, S is the surface available for adhesion, W is the width of the joint.

FURTHER MOVEMENT EXPECTED
Figure a shows a case where W is equal to D, so any movement which places the sealant in shear or tension will exert considerable stresses on the

adhesive interface with the concrete. If
movement is excessive the seal will probably fail.

FURTHER MOVEMENT EXPECTED

Figure b shows a situation where although D is still equal to S, the width of the sealant is twice that value. This means that for any one situation, the force exerted will be considerably reduced.

FURTHER MOVEMENT EXPECTED

In case c, S has been doubled and the top surface of the sealant W1 is twice the value of W. The depth of the seal is half the width of the joint, half of the area available for adhesion and a quarter of the top surface measurement. In this situation the face seal will cope with extensive movement without exerting extensive streses on the adhesive surfaces.

VACUUM IMPREGNATION
This is a patented process called Balvac process. Essentially, the affected part of the structure is enclosed within an air tight plastic cover that is sealed to the surface of the concrete at its edges. Vacuum is applied to exhaust all the air from cracks and crevices in the concrete within the cover. Resin grout is then admitted and atmospheric pressure forces it into cracks and pores in the concrete surface. It is particularly used for treating surfaces with a large number of cracks. It is also used extensively as a means of reducing permeability of weak concrete.

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