LOADING AND VIBRATIONS

Structures fail and crack due to excessive loading beyond the load-bearing capacity of the reinforced concrete elements. Excessive loading can be static due to faulty design and construction or dynamic caused by vibrations in industrial plants or earthquakes, in general.

Following a damage assessment, the structure should be demolished and reconstructed if the damage is severe and extensive, as the repair will not be cost effective, and the safety of the structure will not be restored

The five types of structural strengthening methods are as follows: • Internal restoration,

• Exterior reinforcement, • Exterior post tensioning, • Jackets and collars, and • Supplemental members.

Each method is well suited to a particular field condition.

Internal restoration: Internal dowel-type of repairs cannot be made in deteriorated concrete. The concrete must be strong enough to develop a full bond with the reinforcing steel. This requires that the concrete strength is determined before each repair.

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The most common way to provide strengthening across cracked sections of structural members is to install a new interior reinforcement. The dowels are usually deformed steel bars, stainless steel bars or bolts. Galvanized or epoxy coated steel rods or graphite fiber reinforced or glass fiber reinforced plastic bars are also acceptable so long as they are chemically compatible with the bonding agent.

Figure 5.5.1 illustrates the method of internal restoration of a cracked structure.

Exterior reinforcement: Steel plates or channels are effective in stopping the spread of cracks. They can also be used for providing exterior reinforcement for structural elements. Steel plates (or glass fiber graphite fiber reinforced plastic wrapping) will provide excellent shear and moment resistance when bolted or epoxy bonded to the cracked area of the structural members, such as overhead beams or columns. In the case of walls, failure can be caused due to the following reasons:

a) Pivotal settlement,

b) Differential settlement, or c) Lateral pressure.

The failed wall can be rehabilitated by steel rod staples. In both the repair methods, the arrangement of drilled holes should be staggered to avoid creating a line of weakness. Figure 5.5.2 is an illustration of a reinforced concrete beam strengthened with a bonded steel plate.

External post tensioning: Pre-stressing strands or tie rods with threaded ends may be used very effectively as external post tensioning. The post tensioning tie rods or strands can be steel or graphite fiber reinforced plastic rods.

Figure 5.5.3 is an example of external post tensioning.

Jackets or Collars: Concrete members that are cracked or deteriorated throughout their entire cross-section, may be restored either by constructing a new reinforced concrete collar or by installing a series of tensioned steel straps around the existing member. Concrete jackets are commonly utilized to restore cracked or deteriorated concrete compression members such as columns and piles. Beams that have failed in shear can be restored by installing a series of tensioned steel straps around the cracked section. Glass fiber or graphite fiber reinforced plastic rapping also can be used for retrofitting, such as structural members. These exterior reinforcing methods are commonly used together with epoxy injection of the cracks.

Figure 5.5.4 is an example of rehabilitating a deteriorated concrete component by the use of external strap.

Figure 5.5.1. Internal restoration of a cracked structure.

5-12 Figure 5.5.3. External post tensioning of a beam.

Figure 5.5.4. Rehabilitation of a deteriorated concrete component by the use of external strap.

Supplemental members: These are simply new columns or beams installed to support damaged structural members or systems. They are obvious and distracting, and therefore, are the least desirable methods of repair.

The five structural repair methods discussed above can be used singly or in various combinations. If none of the repair methods is feasible, then it may be necessary to either reduce the allowable working loads on the area or remove the damaged area and completely rebuild that part of the structure.

5.6 REPAIR OF DETERIORATION DUE TO EXPOSURE TO CHEMICALS

Following procedure is recommended for repairing concrete structures affected by chemical exposure:

1. The unsound concrete should be marked as a rectilinear shaped area. The marked area should be saw cut and the concrete removed from this area to reach the sound concrete by sandblasting or other suitable means. If mechanical tools are used, damage to the surrounding sound concrete should be avoided. In case of reinforcement corrosion, the concrete should be removed to a depth of 25 mm behind the steel bars.

2. Prior to the application of the repair material, the surfaces of the existing substrate should be roughened to provide an adequate key for the repair material. The surfaces should be made free of loose, broken and unsound material.

3. The substrate should be washed with potable water to remove dust and loose material. Alternatively, the dust and loose material may be removed by air blasting.

4. For repairs using a cementitious material, a bonding agent should be applied on the old concrete. Alternatively, the substrate may be saturated with potable water and kept wet for 24 hours prior to placement of the repair material.

5. The repair material should be placed when the bonding agent is still tacky.

6. Resin-based repair mortars can be used for the repair of concrete affected to a depth less than 50 mm. These may be used for the hand patching of horizontal, vertical, and overhead surfaces. The repair material should be placed in layers, and the maximum thickness of each layer should be as recommended by the manufacturer of the material.

7. If cement-based repair materials is utilized it should be cured by covering the repaired surface with a wet burlap for a minimum of seven days.

8. An acid-resistant coating should be applied on the repaired surface after it has been cured and dried.

5.7 REPAIR OF DETERIORATION AND CRACKING DUE TO EXPOSURE