Water covers more than 70% of the planet and provides transportation that keeps seven continents and communities within countries and cities connected via water passageways. Whether exposed to salty seas or freshwater rivers and lakes, concrete has in the past, and continues to play major roles in fulfilling our civilization's needs.
An amazing concrete property is its ability to perform in these marine environments. These heavily braced concrete dams, seawalls, breakwaters, canals, piers, abutments, columns, foundations, canals, docks, and other marine-exposed structures, which even include concrete barges, boats, and ships, are taken for granted. Concrete may encounter varieties of aggressive chemical solutions, aggressive chemical salts, cyclic wetting and drying, cyclic expansion and contraction, cyclic freezing, parasites that use it for food and lodgings, and it does well in resisting these intrusions on its well being. Most of the time it does quite well, sometimes doing moderately well, and, on occasion, it falls prey to its environment.
Seawater is classified by ACI as a moderate sulfate environment and concrete must be designed to accommodate salts (in solution), some of which are halite (sodium chloride, NaCl); sylvite (potassium chloride, KCl); thenardite (sodium sulfate, Na2SO4); gypsum (calcium sulfate dihydrate, CaSO4·2H2O), anhydrite (CaSO4); epsomite (magnesium sulfate, Mg(SO4)·7H2O); and the granddaddy of them all, polyhalite (K2Ca2Mg(SO4)4·2H2O). The most aggressive of seawater chemicals are the chlorides and sulfates.
Among design criteria for concrete in marine environments are use of cements low in tricalcium aluminate (3CaO·Al2O3), low water-cement ratios, and pozzolans to reduce permeability. Perhaps concrete's most effective property for maximizing durability is low permeability. The slower the infiltration by aggressive chemicals the better its performance—and its increased strength better resists physical forces trying to wear it down.
In a previous column, a dam evidenced slow surface erosion due to chemical leaching, and because of unsightly efflorescence, all the effects of lime-deficient water and subsequent calcium carbonate precipitation. The leaching occurred only during winter months because calcium carbonate (carbonated paste) is more soluble in cold water.
Concrete exposed to seawater can become laced with wormhole-like tunnels due to barnacles that eat it up and redeposit calcium carbonate on surfaces to form their shell homes. One concrete bridge found its foundation, columns, and appurtenances all performed well except for the below-water exposures. Pattern cracking accompanied by popouts developed in tidal and splash zones. The concrete was made using low-alkali cement so, theoretically, deleterious alkali silica reaction should have been mitigated. It gradually became overridden, succumbed by seawater alkalies that infiltrated progressively deeper as the exposure period lengthened until it was plagued by alkali silica aggregate reactions and had to be replaced.
Sometimes seawater becomes an integral concrete component. During World War II on the Pacific islands and even the northern Aleutians, when time was short and speed was priority, concrete breakwaters lined island embayment shores; and buildings, airport runways, and outposts flourished, hastily constructed to meet the time demands of fighting a war and safeguarding the country. There was no time to experiment, so sea-water became concrete mix water. Years later, this expeditious necessity followed Mother Nature's course and embedded reinforcement corroded, as we now know it would.
Brackish water is always cheaper than fresh water and has been used to wash aggregates, resulting in corrosion of embedded reinforcing steel.
Fresh and salty marine water exposures, whether external or internal, usually require special concrete precautions above and beyond the norm to weather potential adverse effects. So beware and consider the adversities of these exposures and use concrete designed to last. After all, the history of concrete shows its overall good, and usually unheralded, performance in always-aggressive marine environments.