Structural Rehabilitation of Reinforced Concrete Structures

Cement concrete is one of the most important construction materials and is practically basic to present-day developments. It is strong enough mechanically, yet vulnerable to deterioration. It thus gets damaged and even fails. This deterioration may be due to the weathering action, fire, soil failure, defects in construction, and also due to natural calamities like flood, tsunami, earthquake etc.

Sometimes, the factors important for durability of concrete structures are not given due importance for construction and maintenance. Some measures need to reduce the effect of deterioration which greatly enhances the durability of structures. According to the consideration of time and money, repair of the damaged part of structure is the most favorable. Repair is the technical aspect of rehabilitation which refers to the modification of a structure partly or wholly which is damaged in appearance or serviceability. Repair and rehabilitation extend the life time of concrete structure. Repair of deteriorated structures involves situations or conditions which are totally different from those encountered in new construction, although every repair has unique conditions and special requirements. Hence, it is important to select more appropriate repair material for particular type of deterioration. Repair and rehabilitation is a technique by which the probability of the existing structure can be enhanced so that it will survive for a longer period of time. This can be accomplished through the addition of new structural elements, the strengthening of existing structural elements and/or the addition of base isolators. Repair and rehabilitation for concrete structures can be classified broadly into two categories: a) repair in which damages due to deterioration and cracking is corrected to restore the original structural shape and size while the core is mostly intact and b) repair which is necessary to strengthen the load carrying capacity of members which have become structurally deficient over a period of time. The main purpose of repair is to bring back the architectural shape of the building so that all services start working and the functioning of building is resumed quickly. Repair does not pretend to improve the structural strength of the building. The ancient /historical buildings require repair to regain some strength, durability and stability as it was before. Many construction materials are being used for the repair of existing structures. In this paper, different repair materials which are used for the purpose of regaining the strength of structures have been discussed. Various methods used for repair and rehabilitation of structures.

Repair: Repair is the technical aspect of rehabilitation which refers to the modification of a structure partly or wholly which is damaged in appearance or serviceability have also been highlight

Stages of repair: The various stages for the repair of concrete structures are as follows:

1. Removal of damaged concrete
2. Pretreatment of surfaces and reinforcement
3. Application of repair materials
4. Repair Procedure

1. Removal of damaged concrete
1. Before the execution of repair in any structure, one most important factor is to remove the damaged concrete.
2. The equipments and tools used for the removal of damaged concrete mostly depend on the damage.
3. Damaged concrete are normally removed by using hand tools. If it is impossible to use hand tools then it can be removed with a light or medium weight air hammer fitted with a spade shaped bit.
4. Care should be taken while removing the damaged portion that it must not damage the unaffected concrete portions.
2. Pretreatment of surfaces and reinforcement
1. Unsound material must be completely removed.
2. Undercutting along with the formation of smooth edges.
3. Surface cracks must be removed.
4. Formation of a well defined cavity geometry with rounded inside corners.
5. Uniform surface but rough for repair can be provided.
6. Before the repair, dirt, oil and all other loose particles should be removed out from the cavities. It can be accomplished by blowing with compressed air, hosing with water, acid etching, wire brushing, scarifying or a combination. Brooms or brushes will also help to remove loose material.
3. Application of repair materials
   When the concrete surface is prepared, a bonding coat such as cement slurry, epoxy, resin materials etc. must be applied to the whole exposed surface which was cleaned before without any delay
1. Repair procedure
   The repair of any damaged structure falls under two categories:
1. ordinary or conventional procedures;
2. special procedures including the latest techniques and newer materials.
It must be done with one or more undermentioned objectives:
• To increase the strength
• To improve the performance of structure.
• To provide water tightness.
• To improve appearance of concrete surface.
• To improve durability.
• To prevent access of corrosive materials to reinforcement.
4. Repair materials
Cement and steel are generally used for the repair of various types of damages. Besides these, some special materials and techniques are available for best results in the repair works. They are:
• Shotcrete
• Epoxy resins
• Epoxy mortar
• Gypsum cement mortar
• Quick setting cement mortar
5. Shotcrete
1. Shotcrete is a strategy for applying a mix of sand and portland concrete which blended pneumatically and passed on in dry state to the nozzle of a pressure gun, where water is blended and hydration takes place only before expulsion. The material bonds splendidly to the arranged surface of masonry and steel. In adaptability of use to curved or irregular surfaces, its high quality after application and great physical attributes, make for a perfect way to accomplish included auxiliary capacity in dividers and different components. There are some minor limitations of clearance, thickness, course of use, and so on.
2. Epoxy resins
   These have high tensile strength as these are excellent binding agents. Epoxy resins are prepared chemically whose compositions can be changed according to the requirements. These are blended just before the application. The final product obtained is of low viscosity and can also be easily injected in small cracks. While for the filling of larger cracks, higher viscosity epoxy resin may be used.



3. Epoxy mortar
   Epoxy mortar is a combination of epoxy resins with fine aggregates which has higher compressive strength, higher tensile strength and a lower modulus of elasticity than the Ordinary Portland Cement concrete. Epoxy cannot be used alone as it is a combustible material.


4. Gypsum cement mortar
   The structural application of gypsum cement mortar is limited as it has lowest strength at failure.
5. Quick-setting cement mortar
   This material is patented and was originally developed for the use as a repair material for reinforced concrete floors adjacent to steel blast furnaces. It is non-hydrous magnesium phosphate cement with two components, a liquid and a dry, which can be mixed in a manner similar to Portland cement concrete.Methodology for repair and rehabilitation
   1. Grouting
   2. Guniting
   3. Routing and sealing
   4. Stitching
   5. Drilling and Plugging
   a) Grouting: Grouting is the technique by which the material can be placed into cavities or holes present in the structure. It is used for increasing the load bearing capacity of a structure, filling voids around precast connections, stopping leakages, placing adhesives and soil stabilization. It is a mixture of cement, water and some other material such as sand, pozzolons and water reducing admixtures.
   
   
   Procedure of grouting
   1. A mixture of cement sand grout is prepared by employing 1:2 along with the water cement ratio between 0.6 and 0.8 using a mechanical mixer.
   2. The mixture is sent down under pressure (of about 120 kg/cm²) through a 30-50 mm diameter pipe terminating into steel cages.
   3. As the grouting continues, the channel is raised slowly up to a stature of not more than 60 cm above its beginning level after which it is pulled back and set into the following cage for additional grouting by a similar methodology.
   4. After the process of grouting for a height of about 60 cm, this operation is repeated, if necessary, for the next layer of 60 cm and repeated continuously.
6. b) Guniting
   It is a process which utilizes dry material from the machine to the surface whichever needs to be repaired through the nozzle by applying high velocity and compressed pressure. It is also called as the dry-mix shotcrete process.Procedure of Guniting
   1. The cement is mixed with moist sand and afterward required amount of water is added as the admixture comes out from the gun. Quantity of water can be regulated with the help of a regulatory valve.
   2. Firstly, the surface on which repair work required must be washed and cleaned. Then the nozzle of gun is kept at a distance of about 750 mm to 850 mm from the surface to be repaired and the velocity of nozzle varies from 120 to 160 m/s.
   3. Sand and cement are initially dry mixed in a mixing chamber and the dry mixture is taken through a pipe to a nozzle, where it is forcibly projected onto the surface to be coated.
   4. The purpose of regulatory value is to provide a mix of desired stiffness which will adhere to the surface against which it is projected.
   c) Routing and sealing
   It is used for treating both larger and fine pattern. This treatment reduces the chances of moisture to reach the reinforcing steel or pass through the concrete which can cause surface stains or other problems. For floors, the sealant should be sufficiently rigid to support the anticipated traffic.
   
   
   Procedure for Routing and Sealing
   1. First a groove of depth ranging from 6 to 25 mm at the surface is prepared.
   2. Then this groove must be cleaned by air blasting, sandblasting, or water blasting and further it was dried.
   3. On the dry groove, a sealant is placed which is further allowed to cure.
      When the tensile strength of any structure needs to re-established across the major cracks, stitching must be used. It involves the process of drilling the holes on both sides of the crack and grouting in U-shaped metal units with short legs (staples or stitching dogs).
      Drilling and plugging a crack consists of drilling down the length of the crack and grouting it to form a key. This technique is applicable only when cracks run in reasonable straight lines and are accessible at one end. This method is often used to repair vertical cracks in retaining walls.
7. e) Drilling and Plugging
8. d) Stitching