Repair of Underwater Concrete Structures – Methods and Procedure
Following are the different methods to repair underwater concrete structures:
- Surface spalling repair
- Large scale repair of underwater structural concrete
- Preplaced aggregate concrete
- Injection technique for restoring underwater concrete structure
- Guniting or shotcrete method to repair underwater concrete structure
- Steel sleeve repairing technique of underwater concrete
Surface Spalling Repair of Underwater Concrete Structures
Cover of underwater structural elements can spall off due to accidental damages. The damaged concrete cover must be replaced and repaired to prevent reinforcement corrosion in the future.
Slightly deteriorated regions will turn to more severe and dangerous damages in short time, especially in splash zones. The deteriorated area of underwater structure should be cleared from both marine growth and loose concrete before repairing procedures are began.
After that, based on the amount of damages, the boundary of spalled area should be saw-cut to a depth of 1.2-2 cm. In splash zones, cementitious mortar can be used for the damages region and water tolerant epoxy mortar may be employed in the case of small damage area.
For large repaired area, formworks might be used to hold the repairing material at its position. This could postpone enhancement work and prevent epoxy coat utilization because if it hardens it would produce smooth surface and consequently the bond will be weak.
Figure-1 and Figure-2 illustrate two different types of surface repairing formwork.
Procedure of Surface Spalling Repair of Underwater Concrete
The basic procedures of surface spalling repair technique might include:
- Flush damaged region with fresh water completely.
- Apply a bonding coat.
- Apply the repair mortar before the coat is set.
- Apply a curing membrane to the applied repair mortar.
- Protect the repaired area against wave action until it hardened adequately.
Fig.1: Formwork for Placement by Pumping
Fig.2: Bird’s-Mouth Type Formwork for Surface Spalling Repair
Large Scale Repair of Underwater Concrete Structures
This technique is suitable option when damages caused by structural overloading, fire, ship impact, or reinforcement corrosion especially in the splash zone.
In the case where large areas are required to be restored, repair method and material selection is considerably important if shrinkage or bleeding lead to leakage path at the top of parent concrete and repair material interface.
When repairing materials have great thickness, thermal cracking may develop due to rise of temperature even though surrounding water decline the temperature rise.
Furthermore, repair of reinforcement is frequently needed because of distortion and considerable corrosion of reinforcement.
Procedure of Large Scale Repair of Underwater Concrete
The large-scale repair procedure is usually as follows:
- Prepare the damaged region
- Clean reinforcement adequately
- Determine formwork typed based on the placement method of repairing material
- Decrease concrete contamination with salts by flushing formwork with fresh concrete short period before pouring of repair concrete
- Pumping is used most of the time for placing repair material and it should start at the bottom of the formwork to push water out of the formwork from the bottom.
Preplaced Aggregate Concrete
After the installing the formwork at the area intended to be repaired, a well graded aggregate is placed and compacted in the formwork. It is recommended that, fresh water is employed to clean the aggregate prior to grout placement.
Then, appropriate grout is injected into the base of the well compacted aggregate in the formwork. In this process, water and voids are expelled out of the aggregate by the grout.
It is essential that the formwork is grout proof in order to prevent leaking from the formwork in addition to provide proper venting at the top to permit escaping voids and air.
It is substantially significant to sufficiently fill the formwork to the top of damage region with aggregate because when the grout is placed and aggregate is not present, the gout will shrink and cracks will develop.
It is advised that, vibration is not applied during injection to avoid washing out of grout.
Injection Technique for Restoring Underwater Concrete Structures
Similar to the steps used for repairing dry structures, injection of cementitious grout or resin can be employed to repair cracks and or voids in the concrete structure underwater.
Not only does the material selection is substantially based on void or crack size but also on possibility of anticipated movement of the member in the future.
Epoxy resin is appropriate for crack width of 0.1 mm whereas cement grout is suitable for crack width of greater than few millimeters and when crack width is smaller than 0.1 mm, the injection is not required.
Applied pressure and the time for which the pressure is kept prior to solidify repair material. There are two methods of injection that include pressure injection and gravity feed.
It is necessary to break the concrete to reinforcement if corrosion evidence can be seen and complete repairing should be suggested instead of injection method.
The procedures for injection technique are
- Prepare concrete surface along crack length
- Along crack length, fix inspection nipples at specific intervals
- Seal crack surface along the whole length of the crack
- Remove contamination using fresh water and be sure that injection path is open
- At one end of the crack, inject epoxy resin or cement grout into the crack through nipples
Guniting or Shotcrete Method to Repair Underwater Concrete Structures
This technique is the best option when large surface area or columns or beams are encased and usually dry process is used. In the dry method, dry mix is transferred by a hose and water is added to the dry mix at the nozzle.
Despite the fact that, Guniting method is not suitable for underwater repairing but it can be employed in splash or tidal zones if seriously rapid setting additives is introduced.
The success of this technique depends on nozzle-man skill and experience in adjusting water addition, pressure, and thickness uniformity. The maximum thickness of shotcrete should be restricted to fifty millimeter even though second layer can be employed if thicker layer is required.
Steel Sleeve Repairing Technique of Underwater Concrete
In this method, a steel sleeve is used around a pile or column after that the space between the sleeve and pile or column is filled with mortar or concrete. The sleeve could be designed to make rooms for further reinforcement corrosion.
The sleeve need to exceed top and bottom of the damaged length of the pile and withstand the force of the pile in the case that the bars are ineffective due to corrosion. Typical arrangement of steel sleeve is shown in Figure 3.
Fig.3: Arrangement of Steel Sleeve Repair
A steel sleeve repair technique procedure is as follows
- Prepare the damaged pile by loose concrete and marine growth
- Clamp a temporary support or sealing ring around the pile below the damages area
- Both the two semi circular sections of the sleeve
- Pump grout or cement at the bottom of the sleeve
- Remove the temporary support and employ corrosion protection to steel sleeve