The basic repair procedures and materials for underwater concrete structures are similar to that of a typical concrete repair. But, conducting the repair procedures underwater brings many complex problems. The underwater concrete repair procedures are performed by highly qualified and experienced professionals, whose details and requirements are provided in ACI SP-8 and SP-65.
Before the repair of underwater concrete, proper evaluation of the present condition of the structure is essential for designing long-term repairs.
Steps in the Repair of Underwater Concrete
The basic steps involved in the repair of underwater concrete are:
- Evaluation and Inspection of Underwater Concrete
- Preparation for Repair
- Formworks
- Repairing of Underwater Concrete
- Final Inspection of Repairs
The severity of the damage in concrete often determines the type of surface preparations, the forming system required, the repair medium, and the method employed.
Repair of Underwater Concrete
After proper preparation of the repair area, and providing proper formwork support, a suitable repair method with respective material is used to rehabilitate the structure.
The method used to repair underwater concrete is dependent on the type of repair. For example, a minor spall or crack can be repaired by a simple patch or crack injection system. For large repair works, where the load-carrying capacity of that element must be regained, the repair method must bring a new load path around the damaged areas.
Following methods and materials are used for the repair of underwater concrete:
- Preplaced Aggregate Concrete
- Tremie Concrete
- Pumped Concrete and Grout
- Free Dump Through Water
- Epoxy Grouting
- Epoxy Injection
- Hand Placement
- Concrete with Anti-Washout Admixtures
All the procedures are detailed in ACI340R.
1. Preplaced Aggregate Concrete
As per ACI 116R, preplaced aggregate concrete is concrete produced by placing coarse aggregate in a form and later injecting a portland cement-sand grout, with admixtures to fill the voids between the coarse aggregate particles.
Applications
- repairing railway and highway bridge piers
- for encasing and underpinning piers weakened by such factors as weathering, riverbed scour, exposed piling or cribbing, floating ice, and overloading
- repair of piers supporting control gates on spillways and hydroelectric outlet structures that have suffered damage from ice abrasion or freezing and thawing.
Procedure
- In this method, all the damaged or weakened concrete is initially removed to a predetermined depth or until a sound material is formed (which is greater). The corroded reinforcement rust is removed, or the bar is replaced or supplemented as the situation requires.
- Forms are placed and sealed at joints and at points of contact with concrete surfaces.
- Now, coarse aggregate is poured in the form of 2 to 4 ft lifts to avoid segregation.
- Next, the grout is pumped into the preplaced aggregate starting at the lowest points. It is pumped either through the forms or through preplaced vertical pipes as given in ACI 304R.
- Once the forms are full, it is a good practice to spill some grout over the top or through vent holes or a venting section at the top of the form, to expel out trapped air, water, or diluted grout.
- Once the concrete has achieved adequate strength, the form is removed.
2. Tremie Concrete
Tremie concrete is defined as the concrete that is placed underwater using a pipe called tremie or tremie pipe.
Applications
- At Tarbela Dam more than 90,000 yd3 (68,800 m3 ) of tremie concrete were placed to repair damage caused by cavitation (Holland, 1996).
- The Corps of Engineers has used tremie concrete to repair damage to stilling basins at several of its structures (McDonald, 1980).
- best suited for larger-volume repair placements where the tremie does not need to be relocated frequently, or for deeper placements where pumping is impractical.
Installation Procedures
- For successful tremie concreting, the concrete in the tremie must be separately placed from water.
- Once the concreting starts, the tremie must stay embedded in the concrete to prevent concrete from dropping into water or getting dispersed.
- ACI 304 R provides recommendations for tremie placement for concrete.
3. Pumped Concrete and Grout
Pumped concrete is
manufactured above water and pumped into place underwater during a repair. This concrete depends upon the pressure
of the pump, and sometimes upon gravity flow, to reach its
final position in the repair. . Fluid grouts used to penetrate fissures, lenses, and small defects are made from very finely ground cement, known as “microfine cement.”
Applications
- It can be used in most applications where tremie concrete is applicable but has the added advantage of having a smaller, more flexible hose that can reach difficult locations.
- Grouts are most commonly used to fill voids between concrete and forms or jackets such as in pile repair.
- repair smaller voids and larger cracks in and under concrete structures.
Installation Procedures
- The initial concrete or grout that is placed at the start of the pumping from the water and upon maintaining is separated throughout the placement.
- The separation must be reestablished whenever the pump outlet is relocated
4. Free Dump Through Water
This is the placement of freshly mixed concrete by allowing it
to fall through water without the benefit of confinements
such as a tremie pipe or pump line. Anti-washout admixtures
may or may not be used.
Applications
- placing concrete containing anti-washout admixture underwater in new construction and repairing old concrete.
5. Epoxy Grouting
Epoxy grouts consist of epoxy resins that are curable underwater. It is used either without aggregate for narrow void
grouting, or mixed with specially graded silica sands and
sometimes with larger aggregates to form an epoxy-polymer
mortar or concrete.
Applications
- Plastic jackets and underwater-curable, epoxy-resin systems are used for the repair of eroded or structurally damaged splash zone concrete and underwater concrete structures.
- Epoxy systems are used for patching, grouting, and crack repair.
- They are also used to bond such items as anchor bolts reinforcing steel, and protective safety devices to concrete under water.
- underwater Underwater-curable epoxy coatings are used to provide protection to concrete and other building materials from erosion and aggressive waters.
6. Epoxy Injection
Injection of epoxy resins into splash zone and underwater cracks and honeycombs
in concrete structures. The injection
process may be accomplished from the interior of pipes, tunnels, shafts, dams, floating-precast-box bridges, and piers.
Piles and backfilled walls must be serviced from the waterside.
Applications
- Non-moving joints can be bonded together with epoxy resins, just like a crack.
- Anchor bolts and reinforcing steel can be grouted into concrete structures in the splash zone and underwater with the injection process
7. Hand Placement
In an isolated location
where the repair area is small, patching by hand placement
may be preferable to other methods.
Also Read: What is Underwater Concreting?