Concrete structures, particularly spillways in dams, are prone to cavitation damage on their surfaces. The result is the formation of cracks on the concrete surface, which increases the risk of further damage from sulfate attack, alkali-silica reaction, freeze-thaw, and other means. Cavitation damage occurs when high-velocity water flow encounters a discontinuity or irregularity in the path. This leads to the lifting of water from the flow surface, creating negative pressure zones that produce bubbles of water vapor. These bubbles move downstream and collapse, causing damage to the concrete.
When bubbles collapse onto a concrete surface, they create a high-pressure impact that is focused on an extremely small area of the surface. This impact can cause particles of concrete to be removed, resulting in the formation of a new area of damage known as a discontinuity. The presence of these discontinuities can lead to further damage, as the process of cavitation continues and the effects of the high-pressure impacts are compounded. In this way, the collapse of bubbles can cause significant damage to concrete surfaces, which may have a range of negative consequences depending on the context in which the damage occurs.
Cavitation damage begins when there are irregularities present along the flow surface and a low cavitation index associated with spillway flow. The presence of singular isolated irregularities or roughnesses along a flow surface is the typical initiator of cavitation. These irregularities can take various forms, including offsets into or away from the flow, holes or grooves in the flow surface, protruding joints, or calcite deposits on the flow surface.
Offsets into the flow are one of the irregularities that can initiate cavitation damage. These are locations where the flow surface dips or recesses into the flow, causing a change in the local pressure and velocity of the fluid. Offsets away from the flow are the opposite, where the flow surface protrudes out from the flow, causing similar pressure and velocity changes.
Holes or grooves in the flow surface are another common irregularity that can initiate cavitation. These can be caused by erosion or corrosion of the flow surface or by other mechanical damage. Protruding joints, where two pieces of the flow surface meet but do not align smoothly, can also initiate cavitation by creating pressure differentials and turbulence.
Finally, calcite deposits on the flow surface can also initiate cavitation damage. Calcite deposits are a type of mineral deposit that can build up on the flow surface over time, causing irregularities and roughness. These rough areas can create small pockets of low-pressure zones, leading to the formation of cavitation bubbles.
Several organizations, including the laboratories of Reclamation and the U.S. Army Corps of Engineers, have conducted tests to determine the cavitation resistance of various repair materials. These tests have included materials such as stainless steel and cast iron. However, none of these materials have been found to be able to withstand instances of fully developed cavitation.
It is important to note that successful repairs must go beyond simply finding a material with high cavitation resistance. It is necessary to address the underlying causes of cavitation in order to achieve lasting repairs. This means that any repairs must include measures to mitigate the effects of cavitation.
Fig: Extensive cavitation damage to Glen Canyon Dam
When dealing with high-velocity flows, it is crucial to ensure that the surfaces in the flow path are free of any offsets or discontinuities. To prevent cavitation damage from occurring, a general rule of thumb is to maintain flow velocities above 40 feet per second at ambient pressures. To meet this standard, Reclamation has established strict specifications for finishing the surfaces of concrete structures that will be exposed to high-velocity flows.
If newly constructed concrete fails to meet these surface specifications, surface grinding may be used as a repair method. However, if the concrete cannot be salvaged through grinding, it will need to be removed and replaced with either replacement concrete or epoxy-bonded replacement concrete.
When cavitation damage occurs at or near control gates, it is typically reparable using epoxy-bonded epoxy mortar, polymer concrete, or epoxy-bonded replacement concrete. To further prevent cavitation damage from recurring, it may be beneficial to apply a 100-percent solids epoxy coating to the concrete. This coating should begin at the gate frame and extend downstream between 5 to 10 feet. The glasslike surface of the epoxy coating can provide an additional layer of protection against future cavitation damage.