Repair is the technical aspect of rehabilitation. Refers to modification of a structure partly or wholly which is damaged in appearance or serviceability.
Stages of repair
Repair of concrete structure is carried out in the following stages:
- Removal of damaged concrete
- Pre-treatment of surfaces and reinforcement
- Application of repair materials
- Restoring the integrity of individual sections and strengthening of structure as a whole.
A. Removal of damaged concrete
- Prior to the execution of any repair, one essential and common requirement is that the deteriorated or damaged concrete should be removed.
- Removal of defective concrete can be carried out using tools and equipment the types of which depend on the damage. Normally, removal of concrete can be accomplished by hand tools, or when that is impractical because of the extent of repair, it can be done with a light or medium weight air hammer fitted with a spade shaped bit.
- Care should be taken not to damage the unaffected concrete portions.
- For cracks and other narrow defects, a saw-toothed bit will help achieve sharp edges and a suitable under cut.
B. Pre-treatment of surfaces and reinforcement
The preparation of a surface/pre-treatment for repair involved the following steps:
- Complete removal of unsound material.
- Undercutting along with the formation of smooth edges.
- Removal of the cracks from the surface.
- Formation of a well-defined cavity geometry with rounded inside corners.
- Providing, rough but uniform surface for repair.
- The cleaning of all loose particles and oil and dirt out of the cavity should be carried out shortly before the repair.
- This cleaning can be achieved 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.
C. Application of repair materials
- After the concrete surface has been prepared, a bonding coat should be applied to the entire cleaned exposed surface.
- It should be done with minimum delay.
- The bonding coat may consist of bonding agents such ass cement slurry, cement sand mortar, epoxy, epoxy mortar, resin materials etc.
- Adequate preparation of surface and good workmanship are the ingredients of efficient and economical repairs.
D. Repair procedure:
The repair of cracked or damaged structure is discussed under two distinct categories, namely, ordinary or conventional procedures; and special procedures using the latest techniques and newer materials such as polymers, epoxy resins etc.
A repair procedure may be selected to accomplish one or more of the following objectives:
- To increase strength or restore load carrying capacity.
- To restore or increase stiffness.
- To improve functional performance.
- To provide water tightness.
- To improve appearance of concrete surface.
- To improve durability.
- To prevent access of corrosive materials to reinforcement.
Durability of concrete Repair
The objective of any repair should be to produce rehabilitation – which means a repair carried out relatively low cost, with a limited and predictable degree of change with time and without premature deterioration and/or distress throughout its intended life and purpose.
To achieve this goal, it is necessary to consider the factors affecting the durability of a repaired structural system as part of a whole, or a component of composite system.
Summarized some of the findings and recommendations may be grouped into three categories:
- Durable Repair Design
- Durable repair application and
- Evaluation of the repairs
A. Durable Repair Design
Modulus of elasticity and strength of repair material
- The modulus of elasticity of the repair material not only affects the resultant flexural stiffness of the repair members, but also the tensile stress present within the repair material and the debonding stress at the interface when differential movement occurs between the repair material and substance.
- A higher difference in modulus of elasticity between the repair material and substrate may adversely affect the stress distribution within the repaired composite materials cross section and may lead to considerable stress concentration. Therefore, the repair material selected should have as similar modulus of elasticity as the substrate as possible.
- Thus, considering the strength of material alone seems less fact, an overemphasis on strength may cause repairs to experience cracking arising from drying shrinkage, creep and heat of hydration.
- As the substrate as possible. Thus, considering the strength of material alone seems less infect, an overemphasis on strength may cause repairs to experience cracking arising from drying shrinkage, creep and heat of hydration.
Coefficient of thermal expansion of repair material
Tensile stresses in the repair material caused by changes in the temperature of the surround environment are proportional to the differences in the coefficient of thermal expansion and the change temperature. Therefore, the repair material selected should have as similar as coefficient of then expansion as the substrate as possible.
Thickness of Repair
The internal stresses within the repair material and substrate are affected not only by differential movements, but also by the relative thickness. A thinner repair layer is more easily cracks or deboned by the higher tensile stress which occurs in the repair material.
For most available repair materials, there seemed to be an optimum thickness of repair material which results in the lowest tense stress occurring within the repair material for a given amount of differential movement between the repair material and substrate.
This optimum value is affected by the ratio of the modulus of elasticity the repair material to substrate and thickness of repair material to substrate.
Shrinkage and creep of repair material
Differential shrinkage of the repair material and substrate is another important consideration a durable repair. The most common damages in concrete between substrate and repair materials and it proportional to the differential shrinkage. Therefore, the repair material selected should have fi shrinkage properties that are as low as possible. For the repair material under tension, creep mitigate against the tensile stresses caused by differential shrinkage.
However, for the repair material under compression, creep may decrease the compressive stress within the repair material and aggravate the compressive stress in the substrate caused by the different shrinkage. The creep of the repair material should be controlled based on the state of stress that the repair material will be subjected to service.
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