CHAPTER 9

UNDERWATER REPAIRS

INTRODUCTION

Many of the techniques available for above water repair can be used underwater with only minor modifications. The materials specified for use in air, however, are often completely unsuitable for underwater use.The major effects that underwater working has on repair operations are summarized as follows:

The cost & difficulty of underwater working requires that operations to be carried out at the repair site be minimized & made as simple as possible. The method of repair must be tailored to suit the available methods of access.

INTRODUCTION

Preparation of the damaged area requires specially adapted techniques. Care must be taken to ensure that the area is not contaminated prior to application of the repair material.The material selected for the repair must be compatible with underwater use both during placing & curing. Many resin-based repair materials are not suitable for use underwater; cementitious systems are however in an ideal medium.Placement methods & formwork must be adopted that minimize mixing between repair material & water.

INTRODUCTION

Checking during the repair operation and regular inspection on completion is difficult and costly to achieve underwater.

The cost of underwater repair is far greater than for similar repairs carried out in the dry. The cost of failed repairs is also therefore high. Hence it is important that laboratory trials on both repair methods and materials be carried out to identify possible problem areas and ensure smooth site operations.

PREPARATION OF DAMAGED AREA

SURFACE CLEANINGAll marine encrustations to be removed.Necessary to determine extent of damage and to ensure bond.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.

PREPARATION OF DAMAGED AREA

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.

PREPARATION OF DAMAGED AREA

SPLITTING TECHNIQUES•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.

PREPARATION OF DAMAGED AREA

MECHANICAL CUTTINGUnderwater cutting using hydraulically powered diamond tipped saws & drills have been used extensively for minor works for many years.

THE CARDOX SYSTEMHoles 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.

PREPARATION OF DAMAGED AREA

THERMIC LANCE Comprises long steel tube packed with steel rods.O2 is passed down centre of the tube & lance is ignited by external heat source.Temperature goes up to 3500 deg C, enabling the tip of the lance to quickly melt through concrete or steel.

EXPLOSIVE CUTTINGExplosives using contact demolition charges have been used for many years.The size & placing of charge is matter of experience. The resulting cut/tear in concrete is irregular. Shaped charges provide precise cut.

CUTTING OF REINFORCEMENT

OXY-FUEL GAS CUTTINGAs Acetylene becomes unstable with risk of explosion, Oxy-hydrogen flame is used for underwater steel cutting.

OXY-ARC CUTTINGSame as above, only that an electric arc instead of a flame is used.

MECHANICAL CUTTINGDiamond tipped saws can be used for small works.

FINAL PEPARATIONReplace reinforcement rods surface flushed with clean water to remove bacterial or microbiological growth.

PATCH REPAIRS

Minor damages can be repaired by special cementitious or resin based materials.

CEMENTITIOUS MORTARS

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.

PATCH REPAIRS

RESIN MORTARS

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 crackLine 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.

INJECTION INTO CRACKS

Start with lowest point till resin flows out of the next highest injection point.Lock lower injection point and injection is transferred to the next point.

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.

FORMWORK

Should be simple to erect under waterShould 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.