Water that contains oxygen and contaminants can cause an aggressive attack on concrete. Underwater repair of concrete is a specialized and highly technical part of concrete repair technology. It presents problems of selecting appropriate repair materials and methods, and of maintaining quality control not normally associated with repair above water. Sound engineering, quality workmanship, and high-performance products and systems are extremely important. Successful repairs can be achieved when these factors are considered carefully and properly implemented.
Proper evaluation of the present condition of the structure is the essential first step for designing long-term repairs. To be most effective, long-term evaluation requires historical information on the structure and its environment, including any changes, and the record of periodic on-site inspections or repairs. Comprehensive documentation of the cause and extent of deterioration, accurate design criteria, proper repair techniques, and quality assurance of the installation procedures and the repair will result in a better repair system. Longevity of the repair is the ultimate indicator of success.
The design objective of the repair largely dictates the type of repair used on a project. For a minor spall or crack, a simple surface patch or crack injection system may be adequate to provide protection to the reinforcing steel. For major damage, where the load-carrying capacity of the element is compromised, the repair may either re-establish the strength of the original element or perhaps even establish a new load path around the damaged area. The severity of the damage often determines the type of surface preparation, forming system, reinforcement arrangement, and repair medium used for the repairs.
Preplaced aggregate concrete
Preplaced aggregate concrete is defined as concrete produced by placing coarse aggregate in a form and later injecting a Portland cement-sand grout, usually with admixtures, to fill the voids between the coarse aggregate particles.
The physical properties of preplaced aggregate concrete are essentially the same as conventionally mixed and placed concrete with respect to strength, modulus of elasticity, and thermal characteristics. The permeability of preplaced aggregate concrete can be significantly reduced when fly ash or silica fume are added to the grout.
Preplaced aggregate concrete has been used extensively for repairing railway and highway bridge piers for many years, particularly for encasing and underpinning piers weakened by such factors as weathering, riverbed scour, exposed piling or cribbing, floating ice, and overloading. In many cases, piers have been enlarged to increase capacity to accommodate heavier deck loads, or to resist masses of floating ice or the impact of runaway river traffic. The method also has been widely applied to the repair of piers supporting control gates on spillways and hydroelectric outlet structures that have suffered damage from ice abrasion or freezing and thawing.
Tremie concrete
Tremie concrete is placed underwater using a pipe, commonly referred to as a tremie or tremie pipe. Tremie concrete differs from pumped concrete in that the concrete flow away from the tremie pipe is caused by gravity acting on the concrete mass in the tremie and not by the pump pressure.
Tremie concretes typically have a high cementitious-materials content, which results in adequate compressive strengths for most underwater repair work.
Tremie concrete is probably 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. However, tremie methods have successfully been used for small grout placements such as filling cavities.
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.
Grouts are more fluid than concrete and generally do not contain coarse aggregate. Most grouts consist of portland cement and sand and may contain fly ash and silica fume. Proprietary grouts may contain selected admixtures for pumping, adhesion, acceleration, dimensional stability, or other properties. Fluid grouts used to penetrate fissures, lenses, and small defects are made from very finely ground cement, known as “microfine cement.”
Pumped concrete is the most common method of placing concrete in underwater repairs, including those that are formed. 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. Other uses include filling voids in or under structures. Grouts are most commonly used to fill voids between concrete and forms or jackets such as in pile repair. They have also been used to repair smaller voids and larger cracks in and under concrete structures.
Free dump through water
The free dump method is used for placing concrete containing an anti-washout admixture underwater in new construction and to repair old concrete. Free-fall concrete has been most successful in shallow water applications, where self-leveling concrete is not required.
Concrete that is to be allowed to free-fall through the water should contain anti-washout admixtures. The admixtures contain some or all of the following ingredients: high molecular weight polymers, superplasticizers, cellulose derivatives, and gums. When added to fresh concrete, the anti-washout admixtures improve the flow characteristics of the concrete.
Free dump through water 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.
Epoxy grouting
Epoxy grouts are used for splash zone and underwater repairs. The materials typically consist of an epoxy resin that is curable underwater; the resin 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.
The epoxy grouting process usually includes the epoxy grout and a jacket, creating a composite system. The jackets, concrete, and epoxy grouts have different physical properties. Adhesion of the epoxy grout to the concrete is important to the overall composite design. The jacket system protects the outer surface of the concrete structure against abrasion, reduces oxygen flow into the damaged area, and defends the structure from physical and chemical attack.
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 underwater. Underwater-curable epoxy coatings are used to provide protection to concrete and other building materials from erosion and aggressive waters.
Epoxy injection
Injection of epoxy resins into splash zone and underwater cracks and honeycombs in concrete structures has been successfully practiced since the 1960s in fresh and saltwater environments. The injection process may be accomplished from the interior of pipes, tunnels,
shafts, dams, floating-precast-box bridges, and piers. The purpose of the crack injection is to restore the integrity of the concrete or to seal cracks. Honeycombed areas within the concrete also can be repaired by the injection process. The injected epoxies fill the cracks and bond the crack surfaces together, restoring, at least in part, the concrete's original integrity and preventing any further water intrusion into the structure.
Limitations of the underwater environment such as light, currents, temperature and contaminants on or in the crack limit the size of crack that can be injected from a practical standpoint. Considering these limitations, a low viscosity the epoxy may penetrate cracks 0.015 in (0.38 mm) and larger; when the crack size is 0.10 or larger a gel consistency epoxy resin may be used. Both consistencies of materials are generally considered capable of bonding and repairing a cracked section when there is adequate adhesion to the surfaces of the crack and when at least 90% of the crack is filled.
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
