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The factors which are important in determining whether fast flowing water will or will not abrade the surface of concrete were summarized in Section 2. Of the four listed factors, probably the most important is the quality of the concrete. The appearance of concrete damaged by grit-laden water is quite different to that caused by cavitation. In channels and pipes the wear may be confined to bends, or it may extend along the invert for comparatively long lengths. Really good quality concrete is surprisingly resistant to abrasion. Surface water sewers laid at very steep gradients down the slopes of Mount Carmel in Haifa showed no sign of damage after more than 15 years. There was only flow in these pipelines during the winter (November to March) but during that period of five months the average annual rainfall was about 600 mm.
On the other hand, in the UK, large insitu reinforced concrete channels used to convey sugar beet mixed with earth, stone and grit, were worn away to a depth of 50—75 mm in about two years The abrasive conditions were very severe and , in addition, the water in the channels contained sugar in solution which is chemically aggressive to Portland cement concrete.
If it's established that the quality of the base concrete is satisfactory and erosion has only occurred in a number of isolated places, then these can be repaired by careful patching with high strength concrete. It is advisable for these patches to be cut out with a saw or high velocity water jet, so as to provide a thickness of new concrete not less than 75 mm. If this minimum thickness can't be provided, then a high quality cement/sand mortar containing an SBR latex should be used. Even so the thickness should not be less than 30 mm. For thinner patches, an epoxide resin mortar should be used.
Regarding the execution of the work, the general recommendations for the repair of cavitation damage will apply, with the exception that the provision of a very smooth surface is not usually required. However, this will depend on the hydraulic requirements of the structure.
Mention was made under the repair of cavitation damage to the use of a metallic sprinkle finish. This type of finish to industrial floors where high abrasion resistance is required has been described in Section 6. It is best applied to an adequate thickness of high strength insitu concrete while the concrete is still in the plastic state. The sprinkle finish consists of Portland cement, a metallic aggregate and an admixture, and is spread over the base concrete a few hours after compaction and finishing. The time factor is very important and can only be determined by experience. This thin topping is finished by careful hand troweling, and then the newly laid area is cured in the usual way. When ferrous metal is used, brown stains will appear; this is caused by the rusting of the fine particles of iron in the surface layer. The rusting has no adverse effect on the durability of the material. The thickness of the metallic finish depends largely on the weight of metal used. With about 5 kg/m^2 of metal, the thickness would be about 4 mm. For very severe conditions on industrial floors, 45 kg/ m^2 of metal with a thickness of 12 mm is sometimes used. These metallic sprinkle finishes are applied by specialist firms, and can be laid on slopes up to about 45° to the horizontal.
The type of repair so far described can only be carried out in locations where there is access for the workmen. This means, in the case of pipes, a minimum diameter of 1000 mm. For pipes smaller than this, special equipment and techniques have to be used. Equipment has been dev eloped and used for providing a special abrasion resistant invert to asbestos-cement and concrete pipes. The material used consists of calcined bauxite mixed with a specially formulated epoxide resin. The bauxite is expensive, and it's likely that carefully selected, graded and washed flint would be satisfactory in many cases. Figure 9.18 shows such an invert being laid in concrete pipes.FIG. 9.18. Equipment for laying abrasion resistant invert in concrete pipes.
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