ACI 210R-93 Erosion of Concrete in Hydraulic Structures.
Microfissures in the surface and between the mortar and coarse aggregate are believed to contribute to cavi- tation damage. Compression waves in the water that fills such interstices may produce tensile stresses which cause microcracks to propagate. Subsequent compression waves can then loosen pieces of the material. The simultaneous collapse of all of the cavities in a large cloud, or the supposedly slower collapse of a large vortex, quite pro- bably is capable of suddenly exerting more than 100 at- mospheres of pressure on an area of many square inches. Loud noise and structural vibration attest to-the violence of impact. The elastic rebounds from a sequence of such blows may cause and propagate cracks and other damage, causing chunks of material to break loose. Fig. 2.5 shows the progress of erosion of concrete downstream from two protruding bolts used to generate cavitation. The tests were made at a test facility located at Detroit Dam, Oregon. Fig. 2.6 shows cavitation damage on test panels after 47 hours of exposure to high-velocity flows in excess of 100 ft per second (ft/sec) [40 meters per second (m/sec) ]. A large amount of cavita- tion erosion caused by a small offset at the upstream edge of the test slab is evident.
3.1-General Abrasion erosion damage results from the abrasive effects of waterborne silt, sand, gravel, rocks, ice, and other debris impinging on a concrete surface during operation of a hydraulic structure. Abrasion erosion. is readily recognized by the smooth, worn-appearing con- crete surface, which is distinguished from the small holes and pits formed by cavitation erosion, as can be com- pared in Fig. 2.8 and 3.1. Spillway aprons, stilling basins, sluiceways, drainage conduits or culverts, and tunnel linings are particularly susceptible to abrasion erosion. The rate of erosion is dependent on a number of fac- tors including the size, shape, quantity, and hardness of particles being transported, the velocity of the water, and the quality of the concrete. While high-quality concrete is capable of resisting high water velocities for many years with little or no damage, the concrete cannot with- stand the abrasive action of debris grinding or repeatedly impacting on its surface. In such cases, abrasion erosion ranging in depth from a few inches (few centimeters) to several feet (a meter or more) can result depending on the flow conditions. Fig. 3.2 shows the relationship be- tween fluid-bottom velocity and the size. of particles which that velocity can transport.ACI 210R pdf download.