Repair of corroded or deteriorated steel reinforcement and prestressing stands is one of the techniques by which deteriorated structural elements are rehabilitated to regain its original load carrying capacity. Corrosion of reinforcements is frequent due to chemical attacks, fire, and accidental cutting.
After the cause of steel damage is determined, it can be repaired by either replacing deteriorated bars or supplementing partially deteriorated bars. Before commencement of repair process, the reinforcement bars are exposed to evaluate their condition and prepare steel bars for the repair methods.
It is necessary to consider proper procedure to expose and prepare damaged steel bars otherwise the repair method would not meet the durability requirements.
Procedure for Repair Reinforcements
1. Remove Concrete Around Steel Bars
The removal of concrete around steel bars should be conducted cautiously to prevent further damage to reinforcements. This can be achieved by firstly determining location, depth, size, and ratio of steel bars using bar locator or covermeter, and secondly using proper concrete removal method.
Frequently, proper shoring should be provided release the member from loads. The shoring has to be checked before concrete removal begins. Vibration of reinforcement should be avoided to prevent damage to its bond with concrete around repair zone. Moreover, care should be practiced to avoid cutting steel bars during concrete removal process.
Damaged and lose concrete is removed around steel bars. If all deteriorated concrete is removed and steel bars are partially exposed, then it is not required to remove the entire concrete around the bars.
However, concrete removal process should continue to clear a space of maximum aggregate size dimension plus 6mm behind steel bars when they should loose rust, corrosion, or not bonded properly to concrete.
2. Clean and Inspect Reinforcement
After removal of concrete, steel bars are cleaned and carefully inspected to find out whether they are capable of performing their designed function. Wire brushing can be used if the area cannot be accessed easily otherwise sandblasting is mostly desired to clean steel bars from debris and other contaminants.
3. Mild Reinforcement or Prestressed Strand Repair
The repair of reinforcements includes the repair of mild reinforcement and prestressed strand. Replacement of reinforcements or supplemental reinforcements are the two methods which can be used for both types of steel repair:
3.1 Repair Mild Reinforcements
After exposing and cleaning reinforcements, a decision should be made whether to replace steel bars or supplementing partially damaged reinforcements.
A. Replacement of Reinforcements
When replacing reinforcement is selected, deteriorated parts of bars are cut out and then mild reinforcing steels are spliced in. The length of the lap needs to be according to applicable codes such as ACI 318.
It is possible to use welded splice instead to lap splice. Similarly, weld splice must be conducted according to codes such as ACI 318 and American Welding Society.
One should be aware that weld splice is not suitable for bars greater than 25mm. This is because welding process may lead to expand embedded bars and causes cracking of surrounding concrete.
Butt welding is usually avoided due to the high degree of skill required to perform a full penetration weld because the back side of a bar is not usually accessible.
Mechanical connection is another method which can be used to splice steel bars. It should also conform the requirements of ACI 318. Commercially available propriety mechanical connection devices can be found in ACI 339.3R.
B. Supplemental Reinforcements
Supplemental reinforcement is used when embedded steel bars lost cross section or inadequate, or the member need to be strengthened. It is the responsibility of engineer in charge to decided whether supplemental reinforcement is used or not, and each member need its own decision.
Deteriorated bars should be cleaned and concrete need to be removed to permit the placement of supplemental bars adjacent to damaged reinforcement. The length of supplemental bars is equal to the length of deteriorated portion of deteriorated bars plus lap splice length of each side.
Moreover, if coating such as epoxy, polymer cement slurry, or zinc-rich coats is applied to reinforcement to prevent corrosion in the future, then coating thickness should be smaller than 0.3mm to decline lose of bond development at the deformations.
Finally, spillage of coating on parent concrete should be prevented since it would decrease bond development.
3.2 Repair Prestressing Steel
Deterioration or damage to the strands or bars can result from impact, design error, overload, corrosion, or fire. Fire may anneal cold-worked, high-strength prestressing steel.
The unbonded high-strength strands may need to be de-tensioned before repair and re-tensioned after repair to restore the initial structural integrity of the member.
A. Bonded Strands
Because the prestressed strand is bonded, only the exposed and damaged section is restressed following repairs. The repair procedure requires replacing the damaged section with the new section of strand connected to the existing ends of the undamaged strands.
The new strand section and the exposed lengths of the existing strand must be post-tensioned to match the stress level of the bonded strand.
B. Unbonded Tendons
The strands are protected against corrosion by the sheathing, corrosion-inhibiting material, or combination thereof. The main cause of unbonded tendon failure are corrosion of the end connections.
A deteriorated portion of a strand can be exposed by excavating the concrete and cutting the sheathing. Unbonded tendons can be tested to verify their ability to carry the design load.
This can be done by attaching a chuck and coupler to the exposed end of the strand and performing a lift-off test. This usually requires at least 20 mm of free strand beyond the bulkhead.
If there is excessive corrosion in the strand, failure occurs and the strand must be replaced or spliced. Shoring of the span being repaired and adjacent spans up to several bays away may be required before removing or re-tensioning unbonded prestressed strands.
The strand is cut on both sides of the deterioration and the removed portion of the strand is replaced with a new section. The new strand is spliced to the existing strand at the location of the cuts. The repaired strand is then prestressed.
Carbon fiber or equivalent systems are available to supplement the reinforcement in prestressed, post-tensioned, and mild steel reinforced structures.
This system is normally glued onto the exterior surface. Unless the component being reinforced is unloaded, the strengthening system only provides reinforcement for future loadings.
Fiber wrapping is commonly used for reinforcing columns, especially in earthquake zones. There are systems available that recover the dried and damaged protective barrier within the sheathing.