In this article we will discuss about:- 1. Special Features of Under Water Repair 2. Steps Involved in Under Water Repair of Concrete 3. Modification in Mix Design 4. Placement Methods.
Special Features of Under Water Repair:
The special features of underwater repairs are as follows:
1. The underwater repairs are more complex and highly costly. Hence the repair operations should be as simple as possible. The choice of technique depends on the available mode of access to the damaged area.
2. The adequate preparation of the damaged area may need special technique to be adopted.
3. The repair materials must be compatible with under water use both during placing and curing. Cementitious materials have been found to be better suited for under water repair.
4. Form work and placement method adopted should be such that the mixing of repair material with the water should be minimum.
5. The supervision of underwater repair is costly and difficult.
Steps Involved in Under Water Repair of Concrete:
Following steps are involved in under water repair:
1. Preparation of the Damaged Area:
The preparation of surface consists of removal of cracked or badly damaged concrete and cutting of distorted reinforcement.
The damaged concrete may be removed by any of the following methods:
(a) By the Use of High Pressure Water Jetting:
In this method water jet with a pressure between 200 to 1000 atmospheres is directed on the concrete surface to remove the hardened cement paste mortar from the spaces between the aggregates. In this process the reinforcement is itself cleaned and can be used in repair. If the reinforcement is also to be cut, then abrasive slurry is injected into the cutting jet.
(b) By the Use of Splitting Technique:
In this case the damaged concrete can be cut by inserting the hydraulic expanding cylinders into pre drilled holes and then increasing the pressure in these cylinders till the splitting of the concrete.
(c) By the Use of Expanding Cement:
In this case the expansive cement is mixed with water to form a cement paste. This cement poste is filled into plastic bags. In turn these plastic bags are deposited in the pre drilled holes in the structure. The expansion of the cement during the next 12 to 24 hours generates stresses of the order of approximately 30 MPa, which is quite sufficient to split the concrete.
(d) By the Use of Soft Explosion:
In this case the pressurized carbon dioxide cartridges are placed in pre drilled holes. The cutting dust and earth is filled in the holes and rammed with wooden tools to make it water tight. The pressure is then released by electrically detonating a small initiating charge in each cartridge producing comparatively a gentle explosion, which results in controlled splitting of concrete by cracks formed between the prepared holes.
(e) By the Use of Diamond Tipped Saw:
This method is useful for minor works only as for core cutting. In this case a hydraulically powered diamond tipped saw and drill is used for cutting the concrete under water. The reaction force to the tools can be provided by strut or strap arrangement bolted to the structure.
(f) Cutting of Steel:
For cutting the steel under water usually following methods are adopted:
(i) Oxygen fuel gas cutting.
(ii) Oxy-arc cutting.
(iii) Mechanical cutting.
After the removal of damaged concrete, all distorted or broken steel has to be removed and replaced before restoring the cover. The reinforcing bars are replaced with new lengths, joined either by couplers or lapped with the existing bars. Immediately before replacing the damaged concrete, the surface must be flushed with clean water to remove any microbiological growth which may reduce the bond significantly between the repair material and the surface.
2. Application of Materials:
(a) Placement of Mortar:
For the prevention of future deterioration or in case of minor damages cementitious or resin based materials may be filled into the defects or cracks. This patch work is suitable only for small volumes. To reduce wash out of cement from conventional cementitious grouts and mortars adhesive admixtures should be used. The high performance mortars mixed above water can be poured by free fall through water to fill the form work.
Normally the mixes are formulated to be self levelling to ensure good compaction without vibration. These mixes can be laid in thickness varying from 20 mm to 150 mm. To safeguard against wave actin damage on vertical faces the repair may be carried out using either form work and a free flowing grout or a hard epoxy putty.
The normal epoxy or polyester resin mortars are totally un-suitable for under water use. For under water use special formulations have been developed. Normally these are free flowing. Hence they can be poured through water into the form work directly. For vertical face works, special types of underwater grade epoxy putty have been developed.
(b) Grout Injection into the Crack:
The general principles for injecting grouts into the cracks are the same as that for above water injecting. But due to the risk of washout cementitious material, generally non-conventional epoxy resin injections are preferred. As shown in Fig. 27.23 epoxy putty is used to seal the cracks between injection points. Epoxy putty is used to seal the cracks between injection points. The epoxy resins must be of low viscosity solvent free under water grade so that water in cracks may be replaced by a structural material. For small repair work, the hand held cartridge injection gun may be used satisfactorily.
(c) Large Scale Repair:
For placement of bulk repair material under water, following requisites are necessary:
(i) The form work complete with inlet pipes and external vibrators should be easy to erect.
(ii)The existing structure should be able to tolerate the vibration.
(iii) To ensure a leak tight joint, flexible seals should be used.
(iv) For vertical repairs, to secure the form work with the concrete, a positive attachment of steel straps or rock bolts fixed into the hole drilled in concrete as shown in Fig.27.24 should be used.
(v) In between the concrete face and form work a thick layer of compressible gasket as neoprene rubber should be used to form a final seal.
(vi) Finally the gaps due to unexpected variations in levels should be sealed.
Modification in Mix Design for Under Water Repairs:
For under water repair, certain modifications in the mix design may be done depending upon the nature of work. On the basis of experience, the concrete mix proportion should be selected to give the desired strength having slightly more sand in the mix i.e., the mix should have more fine aggregate.
To have a provision for washout of cement, the cement content is increased by 25% of the designed quantity of the mix. Lean mixes having cemented less than 350 kg/nr have not been found suitable for under water repairs. Round river gravels and well washed marine dredged aggregates have been found quite suitable for tremie and pump placing methods of concrete.
For heavily reinforced concrete for small repairs to obtain necessary flow characteristics super plasticizing admixtures should be used in the mix.