Submerged durability

Wood-boring molluscs and crustaceans are the most destructive agents of timber submerged in seawater. Indeed the marine environment is recognised as the most hazardous situation for exposed timber. The rate at which these organisms attack is dependent primarily on water temperature and the natural durability of the wood species, notably the heartwood which has greater natural resistance than sapwood. 

The truss is partially submerged in salt water and is expected to be used by up to 20,000 pedestrians and cyclists per day. RPs was chosen over traditional materials to provide the necessary durability and corrosion resistance. The structure is said to cost about a third of an equivalent steel truss, with its low weight (5 tonnes) making construction and installation much easier than if metal had been used. 

For the durability tests, individual rovings were cast into 8 dog-bone shaped concrete specimens (Figure 1). The roving was allowed to protrude approx. 20 mm from the ends of the concrete specimen in order to provide proper anchoring in the next step. The mould was covered and kept in a controlled environment of 22 °C/65% relative humidity. The concrete specimens were unmoulded after 24 hours and placed into a second mould to cast the epoxy ends. On the third day, the final specimens were unmoulded and submerged in water at 20 °C. After 7 days the specimens were transferred to a water bath at 70 °C and kept in there for 7 days before being removed and allowed... 

There is no problem with durability of the geosystems when they are submerged or covered by armor layers. However, in case of exposed geosystems the UV radiation and vandalism are the factors, which must be considered during the design. 

Geotextile tubes are used for both exposed and submerged revetments. When the revetment is to be continually exposed, care should be taken to ensure the geotextile tube skin has the required durability. Alternatively, a rock armour layer can be placed over the outer surface of the geotextile tube to ensure long-term performance in exposed conditions. Geotextile tubes are particularly suited for use in submerged... 

Considerable attention has been given to the corrosion of SFRC in severe environments, which is of particular interest in marine structures. According to Hoff [128], in completely submerged SFRC, no durability problems were encountered. In SFRC exposed in the splash zone, the corrosion was largely dependent on the extent of surface cracking. In sound, uncracked components, no adverse effects of the marine environment were observed. However, when cracks developed at the surface, ordinary carbon steel fibres tended to corrode [132,133]. Presumably, this could be prevented if stainless steel fibres were used. 

The use of composite repairs is an alternative to metals. Compared to metal repairs, composites present several key advantages. Firstly, there are weight savings coming from the use of lower density composites and these can permit simplified installation procedures. Secondly, the smaller relative density difference between the water and composite material leads to reduced submerged installation weight for water applications. Lastly, greater corrosion resistance of the repair allows enhanced durability and hence obviates the need for corrosion protection. 

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