Many of the methods for dry repair can be used to repair underwater structures with only small modifications. The materials used in aii however, are usually completely unsuitable for underwater use.
The cost & difficulty to repair underwater structures requires that operations to be carried out at the repair site be minimized & made as simple as possible. The method of repair should be tailored to match the available methods of access.
PREPARATION OF DAMAGED AREA
Preparation of the damaged area requires specially adapted techniques. Care should be taken to ensure that the area is not contaminated before application of the repair material. Following techniques may be used to for the preparation.
- All marine coatings should be removed.
- Necessary to estimate the extent of damage.
- Small areas: Hand held tools, mechanical wire brushes, needle guns or scabbing tools.
- Large areas: High pressure jets;
- Encrustations – Abrasive slurry
- Oil contamination – Detergent.
- After cleaning determine extent & type of damage.
REMOVAL OF DAMAGED CONCRETE
HIGH PRESSURE WATER JETTING
- Most common for underwater work.
- A thin high pressure jet of water directed on the concrete surface removes mortar from between the aggregates
- The reinforcement is itself cleaned but not cut by the water jet.
- Hydraulic or Pneumatic expansive devices can be used to split concrete.
- Hydraulic expanding cylinders are inserted into predrilled holes & pressurized until splitting occurs.
- Expansive cements are also effective. Cement mixed into a paste & poured into plastic bags, which are put into predrilled holes. In 24 hours it expands and splits concrete.
Underwater cutting using hydraulically powered diamond tipped saws & drills have been used extensively for minor works for many years.
THE CARDOX SYSTEM
- Holes are drilled into concrete.
- Into these cartridges of pressurized CO2 are placed.
- The pressure is then released by electrically detonating a small initiating charge in each cartridge, producing a comparatively gentle explosion which bursts the concrete apart.
CUTTING OF REINFORCEMENT
OXY-FUEL GAS CUTTING
As Acetylene becomes unstable with risk of explosion, Oxy-hydrogen flame is used for underwater steel cutting.
Same as above, only that an electric arc instead of a flame is used.
Diamond tipped saws can be used for small works.
Replace reinforcement rods surface flushed with clean water to remove bacterial or microbiological growth.
- Conventional mortar when immersed in water, washout of the top surface will occur.
- Special admixtures can be added to resist washout of cement.
- Self-leveling, Self-compacting mortars to be used.
- Normal epoxy or polyester resin mortars unsuited for underwater works.
- By careful formulation of the base resin & the curing agent, special epoxy & polyester resin mortars have been developed which can be used underwater.
INJECTION INTO CRACKS
General procedure for underwater crack repair same as for dry workings.
- Drill 50 mm deep injection points along the crack
- Line of crack thoroughly cleaned by high pressure water jetting.
- Line of crack is sealed using epoxy putty.
- Injection pipes sealed into injection points.
- Width > 3mm Cementitious grout
- < 3mm Epoxy grout (low viscosity,
- solvent free, underwater grade)
- Injection achieved by pressure pot.
LARGE SCALE PLACEMENT
- Where large volumes of material are required, consideration must be given to bulk placement under water.
- Precast high density concrete mats for renewing ballast can be obtained in various sizes & offer many advantages over wet cast systems.
- Should be simple to erect under water
- Should be tolerant to variations.
- Figure shows steel formwork fitted to damaged surface.
- Flexible seals ensure leak-tight fit.
- Formwork is complete with inlet pipes, external vibrators, etc