Cavitation damage to concrete structure surface is most common in case of spillway in dams. This leads to cracks on the concrete surface which further increases the risk of damage to concrete by means of sulfate attack, freeze-thaw, alkali-silica reaction and other means.
Cavitation damage occurs on concrete surface when discontinuity or irregularities is encountered in the path of high velocity water flow. This discontinuity or irregularity in the flow path cause the water to lift off the flow surface, creating negative pressure zones and resulting bubbles of water vapor. These bubbles travel downstream and collapse.
If the bubbles collapse against a concrete surface, it sends a very high pressure impact over an infinitely small area of the surface. Such high pressure impacts can remove particles of concrete, forming another discontinuity which then create more extensive cavitation damage.
The initiation of cavitation damage requires irregularities along the flow surface and a low cavitation index associated with a spillway flow. Cavitation is typically initiated by singular isolated irregularities or roughnesses along a flow surface.
Typical examples of irregularities in hydraulic structure flow surfaces include the following:
- Offsets into the flow.
- Offsets away from the flow.
- Holes or grooves in the flow surface.
- Protruding joints.
- Calcite deposits on the flow surface.
The cavitation resistance of many different repair materials has been tested by the laboratories of Reclamation, the U.S. Army Corps of Engineers, and others.
To date, no material, including stainless steel and cast iron, has been found capable of withstanding fully developed instances of cavitation. Successful repairs must first include mediation of the causes of cavitation.
Fig: Extensive cavitation damage to Glen Canyon Dam
A standard rule of thumb is that cavitation damage will not occur at flow velocities less than about 40 feet per second at ambient pressures. As flow velocities approach this threshold, it becomes necessary to ensure that there are no offsets or discontinuities on the surfaces in the flow path.
Reclamation’s specifications for finishing the surfaces of concrete structures that will experience high velocity flows are very strict. Repairs to newly constructed concrete that fail to meet these requirements can sometimes be accomplished by surface grinding.
More likely, however, the concrete that does not meet surface specifications must be removed and replaced with replacement concrete or epoxy-bonded replacement concrete
Cavitation damage at, or adjacent to, control gates can usually be repaired with epoxy bonded epoxy mortar, polymer concrete , or epoxy-bonded replacement concrete.
After performing such repairs, it might be a good idea to apply a 100-percent solids epoxy coating to the concrete, beginning at the gate frame and extending downstream 5 to 10 feet. The glasslike surfaces of epoxy coatings may help prevent cavitation damage to the concrete.