Concrete is widely used as a construction material for several purposes like building seawalls, docks, groins, breakwaters, bulkheads, and other structures exposed to sea water.Freezing and thawing cycles are one cause of marine concrete deterioration. Other causes are the chemical that relates to the deterioration of the reinforcing steel, sulfate water attack on the concrete or aggregate reaction.
In areas where severe winter weather prevails, marine concrete is especially vulnerable to the deterioration due to freezing and thawing above the mean tide level. Here it is frequently subjected to two cycles of freezing and thawing per day. Due to the expansion of ice crystals concrete under such conditions tends to crumble and spall. Laboratory and Field tests have demonstrated that air entrainment will obviously improve concrete durability under these conditions.
Corrosion of reinforcing steel rate is high among the chemical causes of deterioration of concrete in a marine environment. Such corrosion results from electrochemical processes induced by the action of salts on the steel. Corrosion causes expansion and pressure which in turn causes concrete breach and spalling along the plane of the reinforcing bars. Sea salts absorbed by reinforced concrete produce corrosion cells wherein anodic (positive) and cathodic (negative) areas are established.
Corrosion products formed on the anodic areas of the steel are enough to break the concrete. Such corrosion has occurred in concrete that is permeable due to a high water/cement ratio. It has also occurred in cases where there is inadequate concrete covering over the steel. Thus, to avoid conditions that result in corrosion of steel, it is recommended that the water/cement ratio be 5.5 to 6 gallons per sack of cement and that the steel is embedded at least 3 inches in the concrete.
Proper placement of marine concrete, especially within the tidal zone which is 2 feet below and at high tide, is important to prevent deterioration. Settlement in this zone should be a continuous operation and horizontal joints and seems to be avoided whenever possible.
If construction is used then the concrete should be left to solidify undisturbed. The bonding surface should then be cleaned carefully, cement grout must be broomed into the wet surface, and new concrete should be placed immediately.
Here we will discuss the practices that are recommended to produce marine concrete of exceptional durability :
- Proper mix proportions using the optimum cement content yields a dense, water-resistant and relatively unabsorbent concrete.
- The optimum concrete is 61⁄2 to 71⁄2 sacks per cubic yard and the water/cement ratio should not exceed 6 gallons per sack of cement to produce a mix that is plastic and workable.
- Reinforcing steel must have at least 3 inches of concrete cover.
- Non- reactive aggregates should be used. The alternative is to compensate with low-alkali cement and/or pozzolans.
- An air entraining agent will reduce the danger of deterioration due to freezing and thawing.
- Especially in the tidal zone, concrete should be placed in a continuous operation; if construction joints are required, the bonding surface should be cleaned and grouted and new concrete should be placed immediately.
- Concrete should be compacted thoroughly to prevent honey combing and to provide a dense, homogenous mass.
These recommendations have been specially derived from an extensive laboratory and field tests which show that properly designed concrete when placed with care in sea water is sound and durable.
Concrete in marine environments has proved to be more economical than steel or wood which are often subject to substantial maintenance than concrete.