1. Water curing
2. Membrane curing
3. Steam Curing
4. Self Curing Concrete

1. Water Curing:

This is by far the best method of curing as it satisfies all the requirements of curing, namely, promotion of hydration, elimination of shrinkage and absorption of the heat of hydration.

It is pointed out that even if the membrane method is adopted, it is desirable that a certain extent of water curing is done before the concrete is covered with membranes.

Water curing can be done in the following ways:

1. 1 Immersion:

The precast concrete items are normally immersed in curing tanks for a certain duration.

1.2 Ponding :

Pavement slabs, roof slabs etc. are covered underwater by making small ponds.

1.3 Spraying :

Vertical retaining walls or plastered surfaces or concrete columns etc. are cured by spraying water.

1.4 Wet covering :

In some cases, wet coverings such as wet gunny bags, hessian cloth, jute matting, straw etc., are wrapped to vertical surfaces for keeping the concrete wet.

2. Membrane Curing:

Sometimes, concrete works are carried out in places where there is an acute shortage of water. The lavish application of water for water curing is not possible for reasons of economy. Curing does not mean the only application of water; it means also the creation of conditions for the promotion of uninterrupted and progressive hydration. In this type of curing, concrete could be covered with a membrane which will effectively seal off the evaporation of water from concrete.

The idea is to obtain a continuous seal over the concrete surface by means of a firm impervious film to prevent moisture in concrete from escaping by evaporation.

Some of the materials, which can be used for this purpose, are bituminous compounds, polyethylene or polyester film, waterproof paper, rubber compounds etc.

When waterproofing paper or polyethylene film are used as a membrane, care must be taken to see that these are not punctured anywhere and also see whether adequate lapping is given at the junction and this lap is effectively sealed.

3. Steam Curing:

3.1 Steam curing at ordinary pressure

This method of curing is often adopted for prefabricated concrete elements. Application of steam curing to in situ construction will be a little difficult task. However, in some places, it has been tried for in situ construction by forming a steam jacket with the help of tarpaulin or thick polyethylene sheets.

But this method of application of steam for in situ work is found to be wasteful and the intended rate of development of strength and benefit is not really achieved.

3.2. High-Pressure Steam Curing

In the steam curing at atmospheric pressure, the temperature of the steam is naturally below 100°C. The steam will get converted into water, thus it can be called in a way, as hot water curing. This is done in an open atmosphere.

High-pressure steam curing is something different from ordinary steam curing, in that the curing is carried out in a closed chamber. The superheated steam at high pressure and high temperature is applied to the concrete. This process is also called “Autoclaving”. The autoclaving process is practised in curing precast concrete products in the factory, particularly, for lightweight concrete products.

4. Self Curing Concrete

Self-curing concrete is one type of modern concrete, which cure itself by retaining water (moisture content) in it.

The use of POLYETHYLENE GLYCOL in conventional concrete as an admixture helps better hydration and hence the strength of concrete.

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Sedimentation is a water treatment procedure in which there is little or no movement in the water and suspended materials descend to the bottom under the force of gravity and form sediment.

Settling’s Purpose:

1. To get rid of the coarsely distributed phase.
2. To eliminate contaminants that have coagulated and flocculated.
3. To eliminate contaminants that have been collected after chemical treatment.
4. After the activated sludge process/fooling filters, settle the sludge (biomass).

Settling Principles:

1. When turbulence is reduced by providing storage, suspended particles in water with a specific gravity greater than water tend to settle down by gravity.
2. A settling tank is a basin in which the flow is slowed.
3. The detention period is the theoretical average time for which the water is held in the settling tank.

Types of Settling:

Type I: Discrete particle settling

Particles settle one at a time, without interacting with one another.

Type II: Flocculent Particles

Flocculation increases the bulk of the particles, causing them to settle more quickly.

Type III: Zone or hindered settling

Individual particles tend to retain in fixed positions with regard to one another while the mass of the particles settles as a unit.

Type IV: compression

Because the particle concentration is so high, sedimentation can only happen if the structure is compacted.

Sedimentation Types:

Plain sedimentation refers to the process of causing suspended materials to settle by gravity under still water conditions.

1. Plain sedimentation: This is the process of sediments and contaminants in raw water settling to the bottom of a sedimentation basin using only gravity and no chemicals. This technology is very inexpensive, and it is commonly employed in all water filtration and purification systems.
2. Sedimentation with coagulation: In this procedure, chemicals are combined in water, which is then cycled by pumps for two hours each day, settling suspended solids in the bottom of the reservoir or tank.

Zones of Sedimentation Basins:

The following zones are found in most sedimentation tanks:

Zone of the Inlet

The inlet or influent zone should evenly distribute flow throughout the tank’s intake. Baffles are used in the standard design to gently disperse the flow throughout the tank’s entire entrance and prevent short-circuiting. (A condition in which a portion of the influent water departs the tank too soon by flowing across the top or along the bottom is referred to as short-circuiting.) Sometimes the baffle is constructed as a wall across the intake, with holes perforated across the tank’s breadth.

Zone of Settlement

The sedimentation basin’s settling zone is the greatest part. This zone offers the stillness that the suspended particles require to settle.

Zone of Sludge

The sludge zone, positioned at the tank’s bottom, serves as a holding area for the sludge before it is taken for further treatment or disposal. Inlets in basins should be built to reduce high flow velocities towards the tank’s bottom. Sludge could be swept up and out of the tank if high flow velocities are allowed to enter the sludge zone. Scraper or vacuum devices that move along the bottom remove sludge from the sludge zone for further treatment.

Zone of Outlets

The basin exit zone (also known as the launder) should enable a seamless transition from the sedimentation zone to the tank’s outlet. The depth of water in the basin is likewise controlled by this region of the tank. The overflow rate is controlled by weirs at the tank’s end, which prevent sediments from rising to the weirs and leaving the tank before they settle out. The weir length in the tank must be sufficient to manage the overflow rate, which should not exceed 20,000 gallons per day per foot of weir.

Types of Settling Tanks:

1. Fill and draw

The intermittent tanks also called quiescent type tanks are those which store water for a certain period and keep it in complete rest.

2. Continuous flow type

In a continuous flow type tank, the flow velocity is only reduced and the water is not brought to complete rest as is done in an intermittent type.

3. Rectangular

The most basic design is a rectangular basin, which allows water to flow horizontally through a long tank. Large-scale water treatment plants frequently use this sort of basin.

4. Circular

Clarifiers are circular sedimentation basins that have horizontal flow. The functional zones of circular settling basins are the same as those of long rectangular basins, but the flow regime is different. The horizontal velocity of the water decreases as the distance from the centre grows when the flow enters at the centre and is confounded to flow radially towards the periphery. In contrast to the straight line journey in the long rectangular tank, the particle travel in a circular basin is a parabola.

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It’s worth noting that the elements in both good and bad concrete are the same.

The final concrete will be of poor quality if diligent attention is not exercised and good regulations are not followed.

With the same material, if great care is used to maintain control at every stage, good concrete will result.

As a result, we must understand what are the best practices to follow at each stage of the concrete manufacturing process in order to produce high-quality concrete.

The various stages of manufacture of concrete are:

1. Batching
2. Mixing
3. Transporting
4. Placing
5. Compacting
6. Curing

1. BATCHING

Batching of concrete is the process of measuring ingredients or supplies in order to make a concrete mix. To obtain a high-quality concrete mix, proper batching is required.

There are two methods for batching:

1.1 Volume batching:

Materials are measured on the basis of volume in volume batching. It is a less exact batching method. Materials are measured using measurement boxes or gauge boxes of known volume. Cement is packaged in bags, with a volume of 35 litres for each bag of cement (50 kilogrammes). The volume of the gauge box is set to the volume of one bag of cement (35 litres) or a multiple thereof. To make a 1:1:2 ratio concrete mix according to volume batching, one should take one bag of cement (35 litres), 1 gauge box of fine aggregate (35 litres) and 2 gauge boxes of fine aggregate (70 litres). If the water-cement ratio is 0.5, then half of the volume of cement which is 25 litres of water should be taken.

1.2 Weigh batching:

Weigh batching is the correct way of measuring the ingredients strictly speaking. Weigh batching systems should always be used for essential concrete. The use of a weight system in batching improves accuracy, flexibility, and ease of use.

2. MIXING

For the manufacture of homogenous concrete, thorough mixing of the materials is required.

The mixing process should result in a homogenous, colour- and consistency-uniform mass.

Concrete is mixed using one of two methods:

• Machine Mixing 
• Hand mixing

3. TRANSPORTING

Concrete can be moved using a variety of techniques and equipment. The precaution to be taken when transporting concrete is to retain the homogeneity established during mixing while being carried to the final deposition location.

The methods adopted for the transportation of concrete are:

1. Mortar Pan
2. Wheel Barrow, Hand Cart
3. Crane, Bucket and Ropeway
4. Truck Mixer and Dumpers
5. Belt Conveyors
6. Chute
7. Skip and Hoist
8. Transit Mixer
9. Pump and Pipe Line

4. PLACING CONCRETE

It is not enough to have a well designed, batched, mixed, and delivered concrete mix; it is also critical that the concrete be put in a methodical manner to achieve the best outcomes.

The precautions to be taken and the strategies to be used when putting concrete in the conditions below will be described.

1. Pouring concrete into a mould made of dirt. (For instance, foundation concrete for a wall or a column.)
2. Pouring concrete into a big earth mould or a piece of wood formwork. (For instance, a road slab or an airfield slab.)
3. Adding layers of concrete to wooden or steel shutters. (For example, mass concrete in dam construction or concrete abutment or pier construction).
4. Placing concrete within the confines of normal work. (For instance, columns, beams, and floors)

5. COMPACTING

Definition : Compaction of concrete is the process adopted for expelling the entrapped air from the concrete so as to make the concrete dense. In the process of mixing, transporting and placing of concrete air is likely to get entrapped in the concrete. The lower the workability, higher is the amount of air entrapped. In other words, stiff concrete mix has high percentage of entrapped air and, therefore , would need higher compacting efforts than high workable mixes. If this air is not removed fully, the concrete loses strength considerably. That is why its necessary to eliminate the voids.

6. CONCRETE CURING

Curing is the process of keeping the concrete moist and warm enough to allow the cement to continue to hydrate.

More precisely, it is the process of maintaining a satisfactory moisture content and a favourable temperature in concrete during the period immediately following placement, so that cement hydration can continue until the desired properties have developed to a sufficient degree to meet the service requirement.

The quality of concrete will be poor if curing is neglected during the early stages of hydration.

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Fresh concrete, often known as plastic concrete, is a substance that has been recently mixed and may be moulded into any shape. Fresh concrete contains a variety of qualities that are necessary for constructing concrete structures on the job site. Fresh concrete’s most important qualities are addressed.

1. Workability

The phrase “workability” or “workable concrete” has a considerably broader and deeper meaning than the term “consistency,” which is sometimes used interchangeably with workability. Consistency is a broad concept that refers to the degree of fluidity or mobility. Concrete with high consistency and mobility does not have to be suitable workability for a certain job.

Concrete that is regarded workable for mass concrete foundations is not workable for roof building, and even when used in roof construction, concrete that is considered workable when vibrated is not workable when compacted by hand.

Similarly, concrete that is considered workable in thick parts is not workable when used in thin sections.

As a result, the term workability takes on the full meaning of the type of work, section thickness, reinforcing extent, and compaction mode.

To develop a mix, a concrete technologist needs a thorough understanding of workability.

Workability is a parameter that a mix designer must specify in the mix design process, along with a thorough grasp of the type of work, transport distance, slump loss, manner of placement, and many other variables.

Factors affecting workability

Workable concrete is the one which exhibits very little internal friction between particle and particle or which overcomes the frictional resistance offered by the formwork surface or reinforcement contained in the concrete with just the amount of compacting efforts forthcoming.

The factors helping concrete to have a more lubricating effect to reduce internal friction to help easy compaction are given below:

• Water Content Mix Proportions
• Size of Aggregates Shape of Aggregates
• Surface Texture of Aggregate Grading of Aggregate
• Use of Admixtures.

2. Segregation

The separation of the constituent ingredients of concrete is referred to as segregation.

Good concrete is one in which all of the elements are evenly distributed to produce a homogenous mixture; if a sample of concrete shows a tendency for coarse aggregate to separate from the rest of the materials, that sample is said to demonstrate segregation. Such concrete would not only be weak, but it will also have all of the bad features of hardened concrete due to its lack of homogeneity.

There are three types of segregation:

1. Coarse aggregate separating out or settling down from the rest of the matrix
2. Paste or matrix separating away from coarse aggregate
3. Coarse aggregate separating away from paste or matrix.

Reason for segregation

A poorly balanced mix in which there isn’t enough matrix to bind and contain the aggregates.

Concrete that has been improperly mixed and contains too much water has a higher tendency to segregate.

3. Bleeding

Water gain is a term used to describe bleeding.

It’s a type of segregation in which some of the water in the concrete escapes to the surface, owing to the fact that water has the lowest specific gravity of all the concrete elements.

Bleeding is most common in the extremely wet mix, poorly proportioned, and inadequately mixed concrete.

Excessive bleeding occurs in thin parts such as roof slabs or road slabs, as well as when concrete is put in bright weather.

Water rises to the surface as a result of the bleeding. Along with the water, a small amount of cement occasionally rises to the surface.

Bleeding water will most likely collect below the aggregate.

Water voids are created as a result of the buildup of water, which weakens the link between the aggregates and the paste.

In the case of flaky aggregate, the feature mentioned above is more evident.

Water that collects below the reinforcing bars, especially below the cranked bars, weakens the bond between the reinforcement and the concrete. Delay in finishing operations can reduce the production of laitance and the resulting negative effect.

4. Setting Time of Concrete

Setting time of cement is found out by a standard Vicat apparatus in laboratory conditions. Setting time, both initial and final indicates the quality of cement.

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It is the deliberate application of a predefined compressive force in a structural member’s tension zone to overcome the concrete’s natural weakness in resisting tensile stress created owing to superimposed loads.

Prestressing is usually done using high-strength steel, which creates compressive stress, which causes the beam to bend upwards, balancing the downward deflection generated by superimposed loads. As a result, the member experiences minimal deflection and becomes uncracked over time.

Benefits of PSC:

• Under service loads, the member’s cross-section stays uncracked.
• Corrosion of steel is decreased, resulting in increased durability.
• The member’s entire cross-section is used, which enhances the moment of inertia and stiffness, resulting in less deformation and increased serviceability.
• The member’s sheer resistance is raised.
• The PSC structures can be used in pressure vessels, liquid retention structures, and other applications.
• PSC structures are used for long span bridges because they have better performance under dynamic and fatigue loads.
• For the same time period, PSC members had a lower depth than RCC members, resulting in a lower death rate for PSC members. PSC members have lost weight, are more attractive due to their slim sections, and are the most cost-effective.

PSC’s Limitations:

• Because PSC is more technologically savvy than RCC, it is less common.
• Equipment that is relatively more expensive and costly.
• The initial investment is substantial.
• Strict material quality control is a must.
• It must be handled with care.

Pretensioning is analysed:

In the analysis, the following assumptions are made:

• Plan section persists even after bending since concrete is a homogeneous and elastic material.
• Hooks law is applied.
• Both under tension and compression, the material has the same properties.
• Initially, all members are straight.

Concentric prestressing:

In concentric prestressing tendon passes through the centroid of the cross-section of the members only direct compressive stress is induced in the member due to the prestressing member along with bending stress due to dead and live load resultant stresses are obtained At the extreme fibers the stress distribution diagram are drawn due to prestressing force, dead load and live load.

Eccentric prestressing:

Eccentric prestressing tendon is placed at an eccentricity “e” from the centroid of the section in the tension zone. Prestressing force “P” induces direct compressive stress and bending stress along with the bending stress due to dead load and live load resultant stresses are obtained at extreme fibre and stress distribution diagram are drawn due to the prestressing force, dead load and live load.

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A crack is a complete or incomplete separation of concrete into two or more parts produced by breaking or fracturing.

Reason for cracks in structures:

1. Initial Shrinkage:

Building materials shrink in the beginning. This is a partially irretrievable shrinkage. Shrinkage cracks in structures can be reduced by using less rich cement mortar in the masonry and by application of plastering on brickwork. Shrinkage cracks in plastering can be minimized by using less rich motor and provides resistance to durability.

2. Thermal Movement:

Building materials undergo expansion and compression. Thermal movement creates cracks leading shear stresses. Thermal movement are strongest, demand attention. Daily changes have a more damaging effect than the seasonal changes, which are regular. On the other hand, in seasonal changes, the pressure gets relieved to a large extent on account of creep.

3. Elastic Deformation:

Unevenly loaded walls undergo large extent stress development, leading to cracks in structure. Building material with different elastic property develop different shear stress resulting to cracks at the junctions. Elastic deformations are caused depending on loading. Extent of deformation is based on the elastic property of the building material.

4. Creep Movement:

Gradual and slow time-dependent distortion of the concrete structure under constant loads is known as creep. It leads to development of great amount of stress ultimately developing cracks in structures. Creep also is the reason for the increase in cement and water content. the increase of temperature in the steel bars surges creep as well.

5. Chemical Reaction:

Internal stress and chemical reaction are directly proportional, and leads to increase in overall volume. Chemical reactions lead to weakening of building material. Carbonation in cement-based materials, sulphate attack on cement products, alkali-aggregate reaction, and corrosion of reinforcement in concrete are few of the most common types of chemical actions on building materials.

6. Foundation Movement and Settlement of Soil:

Differential settlement in the foundation causes shear crack. Expansive soils which are vulnerable to swelling on absorbing moisture and shrink on drying due to change in moisture content of the soil are highly prone to cracking in structure.

7. Cracking Due To Vegetation:

The presence of any vegetation leads to formation cracks in the walls of a building. Major reason for this is expansive action of roots growing in gaps of brick masonry or in foundation. The cracks occur in clay soil due to moisture contained by roots. Growth of roots develops tension and also cause cracks.

8. Permeability Of Concrete:

Depletion of concrete begins with movement of aggressive substance. Depletion is many caused by weathering action, chemical attack, or any other process of deterioration. Several factors which control concrete permeability, water-cement ratio, curing, air voids due to deficient compaction, use of admixtures, micro-cracks due to loading, cyclic exposure to thermal differences, and oldness of the concrete.

9. Structural Design:

For any structure to have good performance it is very important to consider all the environmental features including soil investigation, which helps the deisigner to comeup with best and funtional structural design.

10. Poor Workmanship:

Skilled labour and good structure goes hand in hand. Inferior mixing of construction materials, like sand, cement, and aggregate cause cracks on the walls, slabs, beams, etc. All of these lead to development of cracks in structure.

11. Poor (Or) Lack Of Maintenance:

Performance of any structure depends on the care taken. Regular maintenance would reduce the chance of cracks in any structure.

12. Natural Forces:

flooding’s, Earthquakes, tremors, winds, rains and many others can cause cracks in the buildings.

Types of cracks in structures:

We can majorly see two types of cracks in any structure,

• Non-Structural Cracks
• Structural Cracks

Non-Structural Cracks:

The stresses induced internally in any building material lead to non Structural cracks. These cracks do not cause major default in the structure but aesthetically is not pleasing.

Structural Cracks:

Incorrect design, faulty construction or overloading affect the structure and cause weakening. These cracks cause major problems in the structure.

There are many types of structural cracks, Here are 7 types of cracks and their prevention:

1. Plastic shrinkage cracks:

Loss of moisture quickly in slabs or structural member forms these cracks. concrete in plastic state it is full of water with time water evaporates leaving voids, these voids or spaces decrease the strength of concrete. These cracks are very fine and barely visible. That plastic shrinkage cracks don’t just exist on the surface, they extend throughout the entire thickness of the slab.

Measures to prevent plastic shrinkage in structure:

• By providing Control joints.
• By use of plastic sheeting to cover the surface between finishing operations.
• Choose Water ratio according the Is code or according the temperature.

2. Crazing & Crusting Cracks:

Crazing :

The top of a concrete slab loses moisture too quickly, this leads cracking. These cracks are rarely more than 3 mm deep. These are very fine, surface cracks that resemble spider webs or shattered glass. These are random cracks on surface.

Crusting :

During stamped concrete finishes, surface crusting is one of the most difficult problems to manage in the field. This leads to formation of larege number of cracks on the surface and is not aesthetically pleasing. On windy days where the top of the slab dries out quicker than the bottom, the top of the concrete surface can become crusty. This can be reduced by using admixtures increase the setting time.

3. Settling cracks:

The cracks formed due to the settling of structure. The major reason for such cracks are, improper compaction of soil. These are the very dangerous cracks.

4. Expansion cracks:

Another reason that concrete cracks is expansion. In summer a concrete slab is more likely to expand as it gets hotter. This produces great stress in slab. As the concrete expands, it pushes the adjacent objects, such as a brick wall or an adjacent slab of concrete, the resulting force will cause something to crack. An expansion joint is a point of separation, or isolation joint, between two static surfaces.

5. Heaving cracks:

Heaving is caused by clay soils expansion by absorbing moisture. The amount of water in the ground is often uneven and so the movement in the house is uneven. Cracking is ground movement is also brought on by freeze/thaw cycles. The presence of large tree roots can also cause concrete to heave; the growing roots of these trees can lift and crack the concrete.

6. Overloading cracks:

Concrete is weak in tension. Excessive amounts of load on top of a concrete can cause cracking. If distribution of loads is not done properly develops these cracks.

7. Corrosion of Reinforcement:

As the steel corrodes, the resulting rust occupies a greater volume than the steel. This expansion creates tensile stresses in the concrete, which can eventually cause cracking, delamination, and spalling.

Methods of crack repair in structures

1. Epoxy injection:

Epoxy injection is an method of repairing non-moving cracks in concrete walls, slabs, columns. Here it requires the establishment of entry and venting ports along the cracks, sealing the crack on exposed surfaces, and injecting the epoxy under pressure.

2. Routing and sealing:

The cracks are made wider at the surface and then the groove is filled with flexible sealant. This is a common and simple technique for crack treatment. It can be done on vertical surfaces and curved surface.

3. Stitching:

It is a permanent structural repairs solution for masonry and cracked wall reinforcement. It is done by drilling holes on both sides of the crack, cleaning the holes and anchoring the legs of the staples in the holes with a non-shrink grout.

4. Drilling and plugging:

It is only applicable when cracks run in straight lines and are accessible. This method is used to treat vertical cracks in retaining walls.

5. Gravity Filling:

Low viscosity monomers and resins are used to seal cracks with surface widths of 0.001 to 0.08 inch. High molecular weight methacrylates, urethanes, and some low viscosity epoxies are used successfully.

6. Dry packing:

It is the hand placement of a low water content mortar followed by tamping or ramming of the mortar into place and also helps in producing contact between the mortar and the existing concrete.

7. Polymer impregnation:

Monomer systems is used for repair of some cracks. A monomer system is a liquid consisting of monomers which will polymerize into a solid. The most common monomer used for this purpose is methyl methacrylate.

Principles of investigation of cracks

STEP 1: Discussion with the Client of the building

One of the most important things is to discuss with client or owner about cracks, ask them:

1. When was the building constructed ? Date and year of construction?
2. Ask for building drawings ? (And the details of constructions if avaliable)
3. Ask them when the cracks first appeared? Or how long was the cracks seen?
4. Check whether the client makes complaints about pieces of concrete falling, excessive deflections, large cracks, staining, or water leakages?
5. Ask them whether any repair work was carried out if yes, what was the result?

STEP 2: Visit the Site

1. When you visit the site, always carry building drawings. Check whether the building is constructed as per the plan.
2. Check its present use of the structure or any change in the usage of building.
3. Photograph the cracks and number them
4. Mark the width of crack
5. Check for any tilting of walls or tilting of any structural members, deflections, staining, water leakage, spalling, and corrosion.
6. Collect the samples from the site.

STEP 3: Understand the Cracks And Its Causes

Find the type of crack – Is it live or dead crack.

Is it permeability of concrete, corrosion of reinforcement, moisture variation, temperature variation, poor construction practices, poor structural design and specifications, elastic deformation, creep, chemical reaction, foundation movement & settlement of soil, growth of vegetation, additional alternation of structures?

STEP 4: Monitoring And Measuring The Movements Of Cracks

1. Using tell-tale
2. Crack width gauge
3. Precision callipers

STEP 5: Finding The Suitable techniques To Repair Cracks

1. Epoxy injection
2. Routing and sealing
3. Stitching
4. Drilling and plugging
5. Gravity filling
6. Drying packing
7. Polymer impregnation and underpinning

STEP 6: Formation Of Report

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