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Some brief information on the causes of cavitation damage to concrete was given in Section 2, Sub-section 2.3.4. Generally cavitation damage occurs on the surface of spillways, aprons, energy dissipating basins, penstocks, syphons and tunnels which carry high velocity water. There is no agreed figure for the critical velocity below which cavitation damage will not occur, but a figure of about 15 m/s has been quoted. Damage to concrete caused by cavitation can be fairly easily distinguished from normal erosion, as the damaged areas have a jagged appearance while erosion by grit-laden water results in a comparatively smooth and /or rounded surface.
From published information on the subject of repairs to cavitation damage, it appears that unless the basic cause of the trouble can be removed, and this is very seldom possible, no repair method yet devised will guarantee lasting success. It is hoped that the following suggestions will prove useful.
1. All defective and damaged concrete must be removed. It is essential that the new concrete or mortar should bond very strongly to a sound high strength base. This may involve cutting away a considerable quantity of concrete. The new concrete or mortar must be high strength; as stated previously in this guide, attempts to bond a high strength topping to a weaker base will almost certainly result in failure.
2. The concrete used for repair should have a minimum cement content of 400 kg/ m^3 unless the thickness of the new concrete exceeds about 750 mm when the cement content can be reduced to 360 kg/m^3 a maximum w/c ratio of 0.40 and the aggregates should be a high quality crushed rock or flint gravel with a well graded clean concreting sand. Full compaction of the concrete, followed by adequate curing, is essential. Special care must be taken to provide as smooth a surface as possible, and there must be no lipping with adjacent concrete. Any high spots which remain should be ground down, and depressions filled with a fine epoxide mortar.
3. The junction of the old and new work is particularly vulnerable as a fine shrinkage crack usually appears around the perimeter of the new concrete. It is advisable to wire brush a band of a total width of about 300mm (150mm on the new concrete and 150mm on the old) to remove all weak laitance, and then to apply two coats of epoxide resin.
4. Usually the repairs have to be carried out as quickly as possible, but it's , important that speed should not be allowed to adversely affect the quality of the work. Consideration can be given to the use of ultrarapid hardening Portland cement or HAC. In both cases the minimum cement content should be the same as for ordinary Portland (as recommended above), but with HAC, the maximum w/c ratio should be 0 Advice on the use of both these cements should be obtained from the manufacturers; the reader should also refer to Section 1.
5. If the damaged area is extensive, but of shallow depth, consideration can be given to the use of high strength gunite having a minimum thickness of 75 mm. Depending on the area, it's usual for the gunite to be reinforced with a light galvanized fabric. The gunite should be specified to have a minimum equivalent cube strength at 28 days of 50 N/mm Gunite work is highly specialized and should only be entrusted to an experienced firm.
6. Experience in the USA on repairs of spillways and stilling basins has shown that the provision of a sprinkle metallic finish to high quality concrete gives improved resistance to abrasion and cavitation. This type of finish is referred to in more detail in the next section.
7. Recent work in the USA suggests that polymerized fiber reinforced concrete may be particularly resistant to both cavitation and abrasion. Severe damage was caused to the spillway of the Dworshak dam in Idaho and after extensive laboratory research, the Corps of Engineers decided to use fiber reinforced concrete which is polymerized insitu for the repair of the most badly damaged areas. Brief information on polymerized concrete is given in Section 1.
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