What are the effects of the marine environment on concrete and reinforcement?
The marine environment – due to its particularities – has several effects on concrete and its reinforcement which are caused by various factors:
Composition of seawater
Seawater contains many dissolved salts, some of which affect the durability of concrete.
Concentrations vary from one sea to another.
For example, the Nordic Sea has a much lower salinity than the Mediterranean Sea.
It should be mentioned that seawater also contains dissolved oxygen and carbon dioxide.
The amount of these gases can vary greatly depending on local conditions.
Depending on the amount of dissolved carbon dioxide (CO2) and other dissolved gases they can cause decreases in the seawater pH from the normal values of 8.2-8.4 to 7 or less.
Acidic waters reduce the alkalinity and strength of concrete and increase the electrochemical corrosion of steel contained in reinforced and/or prestressed concrete structures.
Marine organisms
Marine organisms that may interact with concrete structures include algae, molluscs, sea urchins, barnacles, and bacteria.
Both algae and molluscs can be a problem when the concrete surface is porous and can cause a significant loss of alkalinity.
Barnacles, sea urchins and molluscs can generate organic acids that can cause holes in the concrete surface that can lead to the onset of pitting corrosion.
Exceptionally, an anaerobic bacterium (“Theobacillus Concretivorous”) is known to be present in oil-containing sediments attacking permeable concrete.
Also exceptionally, attacks by aerobic bacteria transform hydrogen sulphide (H2S) into sulphuric acid (H2S) into sulphuric acid which is highly corrosive to both concrete and steel.
It should also be noted that the growth of seaweed on the structure can increase the surface roughness by increasing the hydrodynamic load, which in turn influences the structural stability.
Temperature
Sea surface temperature in Europe varies from -2°C in very cold areas to 25°C in the hottest areas in summer.
Additionally, seawater temperature varies with depth and at depths between 100 and 1,000 metres the temperature is usually between 2-5ºC.
Water temperature is a determining factor for chemical and electrochemical reactions in concrete structures.
In general, hot or very hot climates accelerate the processes of initiation and progression of concrete deterioration mechanisms.
The air temperature to which the marine structure will be subjected must also be taken into account since the thermal gradient generating thermal stresses in the air temperature is much greater than that of the seawater temperature.
In the tidal zone, part of the marine structure, there can be combined cycles of warming and cooling, freezing (ice) and thawing (ice melt) and wetting and drying.
The synergistic effects of these cyclic phenomena must be taken into account when designing the structure from a durability point of view.
Hydrostatic pressure
Hydrostatic pressure acts as a force that drives the seawater to try to enter any seawater to attempt to enter any marine structure that is submerged.
The greater the depth, the greater the hydrostatic pressure and the more force the water will have to try to enter the structure.
This can lead to a big problem, as if water ingress into the structure, the reinforcement will probably be corroded.
In this sense, the action of hydrostatic pressure must be taken into account when planning and when designing and constructing a marine structure.
Structures with medium or high porosity concrete, fissured structures, or structures with holes should not be accepted.
Tidal action
Marine structures are exposed to the action of astronomical tides generated by the interaction between the Earth and the Moon.
This phenomenon causes twice-daily exposure to wet-dry, hot-cold and, in cold climates, freeze-thaw (freeze-thaw) cycles.
The tidal range can vary from 0.5 to 15 metres.
On average, in the Atlantic Ocean, in deep areas, the range of variation is less than 1 metre and 4-5 metres at the coast.
The range of the astronomical tide must be studied on a case-by-case basis.
Meteorological tides and storm action
In addition to the astronomical tide, squalls/low-pressure storms can generate significant sea level rises of up to 4-5 metres.
These situations of deep squalls/low pressure are also often associated with major storms with large series of waves generated by the friction of wind and water.
The amount of energy that a large storm can transmit to a marine structure is of enormous magnitude.
The part of the structure that is subjected to intense wave action is called the splash zone and can be attacked by erosion by any suspended solids carried by the waves such as sand, gravel, ice, timber, etc.
Fog and spray
In summer or hot weather, sea fog forms when warm air from land passes over the cold surface of the ocean.
In winter or cold weather, cold air from land crosses over the warmer and more humid seawater environment, forming sea fog and low clouds.
These coastal fogs generate a spraying action of small seawater droplets over the areas where they occur.
The wind, acting on the waves, generates the spray effect. Strong storms can generate the transport of small seawater droplets several kilometres inland.
Ice effect
In areas of extreme cold, close to the Arctic, there can be a double phenomenon of ice impact on the structure and the abrasion it produces on the concrete.
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