Preventing Corrosion in Bridges
Corrosion is essentially defined as the destruction of metal or alloy by chemical or electro-chemical reaction with its environment. The major factors affecting rate of corrosion in Bridges are:
FFPs – Frequently Faced Problems
Bridges are constantly exposed to the moisture and salts from the water bodies and air. In concrete bridges the process of electrolytic corrosion decreases the shelf life.
Bridges are constantly subjected to immense wear and tear due to dynamic vehicular load s Over loading due to increase in wheel loads and regular exposure to aggressive external environment aggravate the situation further.
Poor quality of construction and lack of regular maintenance leads to major retrofit in bridge structure. Defects in constituent material may be manifested in form of cracking spalling of concrete, excessive deflection of structure, rusting of steel components etc….
Ferrous metals such as iron and steel that form the main building blocks for bridges corrode in the presence of moisture and oxygen to form rust.
Presence of high levels of chloride in the coastal/ marine areas aggravate the deterioration of steel.
Furthering the damage is the presence of all crevices, cavities, bolts and small air gaps( as in wire bridges) that becomes the collecting point of agents of corrosion as soil, water and other atmospheric pollutants.
The presence of dissimilar metals, stray currents can lead to intense localized corrosion.
General Recommendations: RH to be maintained
It is generally recommended that the RH to be maintained for preventing corrosion in Bridge s is below 40%.
Dew Point of the Air:
The Dew Point to be maintained is -10ºC at ambient temperature.
In order to prevent corrosion of ferrous parts, the controlled space is maintained at specific relative humidity and temperature so that the products or material in the controlled space do not adsorb moisture from the surrounding.
Metalizing: One efficient option for increasing the life-cycle cost and durability is through metalizing with active metals such as zinc, aluminium, and their alloys. This is helpful in producing long-lasting protective coating on all concrete and steel structures.
Wire Bridge: Also known as suspension bridges these are made up from small steel wires which are bundled up to increase the strength and thereafter anchored on the ground and the top of the bridges. These bundled wires have small air gaps in between and these gaps are continuously fed by dehumidified air to avoid any type of corrosion.
In general suspension bridge/ wire have benefited from dehumidification in two areas:
At the base of the bridge where these wires are anchored with girders and concrete, the small rooms are dehumidified. At the anchorage of main cables the dehumidifier maintains a RH of 40% all the time. At this humidity level corrosion rate drops to negligible value.
Booster fans are required to maintain the flow of the air across the wires.
Girder Bridge: These bridges are without the wire.
Damp air inside the chambers is drawn through a Metal Silicate Fluted Synthesized Rotor which absorbs moisture from the air through a process of continuous 'physical adsorption'.
Moisture is adsorbed in the dehumidification sector by the fluted metal silicate desiccant synthesized rotor and is exhausted in the reactivation sector by a stream of hot air in the counter flow. Following the reactivation process, the adsorption sector is again ready to adsorb the moisture. Positive sealing between chambers prevents mixing of process and reactivation air stream.
The relative humidity inside the chambers is thus reduced to about 35%, which is low enough to stop the cables corroding.
Dehumidification is an effective way to reduce life cycle cost because the cost of painting the girder is reduced. This allows for uniform removal of humidity without concentration of moisture.
All the concrete block rooms have to be dehumidified at the anchoring positions.
Atmospheric moisture causes condensation on the metal surface, which results in oxidation and corrosion of the precision parts . This affects their reliability and impairs their performance, which can lead damage of machinery and to loss of inventory.
Bry-Air's recommendation is to install a Bry-Air desiccant dehumidifier as they are capable of maintaining dew point as low as -60ºC regardless of ambient conditions and thus are ideal for preventing corrosion in bridges.