Deterioration of steel reinforcement and prestressing stands is a common issue that can compromise the load-carrying capacity of structural elements. Corrosion, chemical attacks, fire, and accidental cutting are some of the causes of this damage. To restore the original load-carrying capacity of these elements, repair techniques can be used, such as replacing deteriorated bars or supplementing partially deteriorated ones.
Before commencing with the repair process, it is essential to determine the cause of the steel damage and evaluate the condition of the reinforcement bars. This evaluation is necessary to determine the appropriate repair method and to prepare the steel bars for the repair process. Proper procedures must be followed to expose and prepare the damaged steel bars; otherwise, the repair method may not meet the required durability standards.
By following appropriate procedures to expose and prepare damaged steel bars, the repair method can be effective in restoring the load-carrying capacity of the structural element. This is important to ensure the safety and longevity of the structure, as well as to prevent further deterioration and costly repairs in the future.
Procedure for Repair Reinforcements
1. Remove Concrete Around Steel Bars
When removing concrete around steel bars, it is important to exercise caution to prevent any further damage to the reinforcements. One way to achieve this is by first determining the location, depth, size, and ratio of the steel bars using a bar locator or covermeter. Additionally, the proper concrete removal method should be used.
Before beginning the concrete removal process, it is important to provide proper shoring to release the member from any loads. The shoring should be thoroughly checked prior to starting the concrete removal process. Furthermore, it is important to avoid vibrating the reinforcement to prevent any damage to its bond with the concrete around the repair zone. It is also crucial to take care to avoid cutting the steel bars during the concrete removal process.
During the concrete removal process, damaged and loose concrete around the steel bars should be removed. If all the deteriorated concrete is removed and the steel bars are only partially exposed, then it is not necessary to remove the entire concrete around the bars. However, it is important to continue the concrete removal process to clear a space of maximum aggregate size dimension plus 6mm behind the steel bars if they are loose, rusted, corroded, or not properly bonded to the concrete.
2. Clean and Inspect Reinforcement
When concrete is removed, the steel bars within it must undergo a thorough cleaning and inspection to ensure that they are fit for purpose. If the bars are located in an area that is not easily accessible, wire brushing can be used to clean them. However, in most cases, sandblasting is the preferred method for cleaning steel bars of debris and other contaminants. This process involves using compressed air to propel small abrasive particles at high speed onto the surface of the steel bars, effectively stripping away any unwanted material. By carefully cleaning and inspecting the steel bars, it is possible to identify any issues and ensure that they are capable of performing their designed function.
3. Mild Reinforcement or Prestressed Strand Repair
The context provided talks about the repair of reinforcements, which can involve both mild reinforcement and prestressed strand. In order to address these types of steel repair, there are two methods that can be used: replacement of reinforcements or supplemental reinforcements.
When it comes to repairing mild reinforcement and prestressed strand, both methods of replacement and supplemental reinforcement can be utilized. The repair of these types of steel requires careful consideration and attention, as they are critical components in many structures.
Whether replacing the reinforcement entirely or adding supplemental reinforcement, it is important to ensure that the repair is done correctly in order to maintain the strength and integrity of the structure. With the right approach, repairs to mild reinforcement and prestressed strand can be effective and long-lasting.
3.1 Repair Mild Reinforcements
When the reinforcements have been exposed and cleaned, a crucial decision must be made. The decision is whether to replace the steel bars entirely or to add new ones to supplement the partially damaged reinforcements. This decision is significant as it will determine the structural integrity of the building or structure being worked on. If the damaged reinforcements are not adequately addressed, it could lead to further problems down the line, such as the structure’s collapse or other safety concerns. Therefore, it is essential to carefully assess the condition of the reinforcements and make an informed decision on whether to replace or supplement them.
A. Replacement of Reinforcements
When replacing reinforcement in concrete structures, the deteriorated parts of steel bars are typically removed and replaced with mild reinforcing steel splices. The length of the splice must comply with relevant codes, such as the American Concrete Institute’s ACI 318. However, instead of a lap splice, a welded splice can be used, but it must also adhere to the codes and standards set forth by the American Welding Society and ACI 318. It is important to note that a welded splice is not recommended for bars greater than 25mm due to the possibility of the welding process causing the embedded bars to expand and crack the surrounding concrete. Butt welding, which requires a high degree of skill to perform a full penetration weld, is typically avoided because the back side of the bar is often inaccessible. Mechanical connections are another option for splicing steel bars, but they must also meet the requirements of ACI 318. Proprietary mechanical connection devices are available commercially and can be found in ACI 339.3R.
B. Supplemental Reinforcements
When it comes to replacing reinforcement in concrete structures, the standard method is to remove the deteriorated portions of steel bars and substitute them with mild reinforcing steel splices. To ensure safety, it’s essential to comply with applicable codes such as the American Concrete Institute’s ACI 318, which specifies the appropriate splice length. While a lap splice is the typical choice, it’s possible to use a welded splice instead. However, it must comply with the regulations set forth by the American Welding Society and ACI 318. Note that a welded splice is not recommended for bars larger than 25mm since the welding process could cause embedded bars to expand and crack the surrounding concrete. Additionally, full penetration butt welding is often not feasible since the back of the bar is usually inaccessible and requires significant expertise.
There are other alternatives to welded splices, such as mechanical connections. However, it’s essential to follow ACI 318 requirements for such connections. Proprietary mechanical connection devices are available commercially and can be found in ACI 339.3R. It’s crucial to remember that any splicing method must be done with safety in mind and in accordance with applicable codes and standards.
3.2 Repair Prestressing Steel
There are various reasons why the strands or bars of a structure may become damaged or deteriorated. These include impact, design error, overload, corrosion, and fire. In the case of fire, the high temperatures can anneal cold-worked, high-strength prestressing steel, causing it to lose its strength and structural integrity.
When repairing damaged or deteriorated strands or bars, it may be necessary to de-tension unbonded high-strength strands before carrying out the repair work. This is important to ensure that the repair is carried out effectively and the initial structural integrity of the member is restored. Once the repair work is complete, the unbonded high-strength strands may need to be re-tensioned to ensure the structure remains safe and stable.
A. Bonded Strands
When it comes to repairing a prestressed strand that has become damaged, only the section that has been exposed and affected by the damage will need to be restressed. The repair process involves removing the damaged section of the strand and replacing it with a new section that is then connected to the existing ends of the undamaged strands.
To ensure that the repaired strand is properly restored to its original level of stress, both the new section of strand and the exposed lengths of the existing strands will need to be post-tensioned. This will ensure that the repaired strand matches the stress level of the bonded strand, allowing it to function effectively once again.
B. Unbonded Tendons
Unbonded tendons in structures are protected from corrosion by either sheathing or a corrosion-inhibiting material, or sometimes a combination of both. However, the end connections of these tendons are the main cause of failure. If a portion of a strand deteriorates, the concrete needs to be excavated and the sheathing needs to be cut to expose the damaged area. To confirm the tendon’s ability to carry the design load, a lift-off test is performed by attaching a chuck and coupler to the end of the strand, which usually requires at least 20 mm of free strand beyond the bulkhead. If excessive corrosion is present in the strand, it needs to be replaced or spliced. The span being repaired and adjacent spans up to several bays away may require shoring before the unbonded prestressed strands can be removed or re-tensioned. The damaged portion of the strand is cut on both sides, and a new section of the strand is put in its place. The new and existing strands are spliced together at the location of the cuts, and the repaired strand is then prestressed.
To supplement reinforcement in structures made of prestressed, post-tensioned, and mild steel reinforced materials, carbon fiber or equivalent systems are available. These systems are typically glued onto the exterior surface. However, unless the component being reinforced is unloaded, the strengthening system only provides reinforcement for future loadings. When reinforcing columns, especially in earthquake zones, fiber wrapping is commonly used. There are also systems available that can recover the dried and damaged protective barrier within the sheathing.