The materials used for concrete repair works such as replacement and jacketing of concrete structural members should have good bonding properties with the existing materials and should be non-shrinking.
Replacement of concrete is done for damaged structural members. For strengthening of existing concrete members, additional concrete including reinforcement is added in the form of jackets.
Materials for Concrete Repair, Replacement and Jacketing
There are various materials available for jacketing and replacement of concrete. We will discuss about them below.
Ordinary Portland cement Concrete and Mortar:
This is a commonly used material for replacement and jacketing of concrete as well as masonry structures. These materials are cheap and do not require any special skilled workmen for repair works. This material has similar properties as the concrete in existing structures.
This material consists of high early strength cement and an expansive component is added to concrete to prevent shrinkage cracks. The common expansive agents used are:
- Aluminium powder
- Coke powder
- Anhydrous calcium sulfo-aluminate, and
- Calcium oxide.
The above materials have good bonding with exiting concrete in structures.
Shotcrete is also an ordinary concrete which is sprayed on a prepared concrete surface or masonry surfaces under pressure with low water content through a nozzle. This pressure force compacts the shotcrete and produces a dense homogenous mass. This shotcrete used for replacement of damaged concrete and also for jacketing.
Reinforcement in shotcrete provided is generally welded wire fabric and deformed bars which are tacked onto the existing concrete surface. This improves the bond between new and existing concrete. Surface coating such as epoxy bonding agents, latex modified cement slurries or neat cement slurries are also used for improving the bond between new and old concrete surfaces.
The application of shotcrete does not require any formwork and is applied on any surfaces of concrete including inclined and vertical surface as well as roof slabs. This results in very good adhesion between old and new concrete and good compaction due to application under pressure.
The use of low water-cement ratio in shotcrete results in high strength and low shrinkage of repaired concrete. The permeability of shotcrete is also lower than that of ordinary concrete and results in better protection of steel against corrosion.
The application of Shotcrete for concrete repair can be done using either wet mix or dry mix process. In the wet mix process, the concrete materials are mixed with water before they are used for repair works.
In the dry mix process, compressed air is used to propel the mixture of concrete without water through delivery hose to the nozzle where water is added through a separate hose. The water and dry mix concrete through the second hose are spread on the existing concrete surface.
Fig: Shotcrete application in concrete repair
The application of shotcrete requires skilled workmen.
Polymer modified concrete and mortar:
Polymer modified concrete and mortar is prepared by adding polymer latexes. The polymer latexes form continuous film at drying. This film binds the cement hydrates together to form a monolithic network in which the polymer phase interpenetrates through the cement hydrate phase. The resulting matrix binds the aggregate more strongly and enhances the properties of mortar and concrete.
The polymer modified concrete and mortar has better workability and water retention properties than ordinary concrete and mortar. This helps in reduction of curing of replaced or repair concrete members. This material does not increase the compressive strength of concrete, but it increases the tensile strength of concrete member.
The advantage of using this material is that it improves the adhesion and bond with existing concrete and significantly reduces permeability. The reduced permeability of concrete increases the durability of reinforcement due to low risk of corrosion.
Fiber-reinforced plastics (FRP) Concrete:
Fiber-reinforced plastics or polymers is used for strengthening of concrete and masonry structures. This material is used as a replacement of steel plate bonding and is very effective in strengthening of columns by exterior wrapping or jacketing.
The main advantage of FRP is its high strength to weight ratio and high corrosion resistance. The commonly used fibers in concrete are carbon fibers, glass fibers and aramid fibers. The commonly used resins are polyester, vinyl ester and epoxy.
There are different types of fiber reinforced concrete and is named on the basis of type of material used such as, carbon fiber reinforced polymer concrete, glass fiber reinforced polymer concrete, and aramid fiber reinforced polymer concrete.
Epoxy is a type of polymer as discussed above. This is an excellent binding agents and have good mechanical strength, chemical resistance and ease of working.
Epoxies are used in civil engineering for high performance coatings, adhesives, injection grouting, high performance systems, industrial flooring or grouting etc. The low viscosity resins can be injected into fine cracks.
The higher viscosity material is used as binding agent for surface coating or filling larger holes and cracks. They may also be used for gluing steel plates to the distress members.
Epoxy concretes are used for the following repair works:
- To bond plastic concrete to hardened concrete
- To bond rigid materials to one another
- For patch work
- For painting over concrete to give color, resistance to chemicals, water and to give abrasion resistance.
The Epoxy mortar is made using epoxy and suitable sized aggregate (sand). They have high compressive strength, high tensile strength and low modulus of elasticity than cement concrete and can be used for repair of larger void spaces.
In cement mortar or concrete, the inclusion of epoxy can be considered as an incorporation of a second binder into the mix