From the moment concrete is poured it is under attack. It is under attack not just from industrial pollutants in the atmosphere but also from naturally occurring gases such as carbon dioxide.
It is an attack which is both gradual and relentless. Worst of all, by the time it becomes apparent to the naked eye, the damage has spread to the reinforcing steel, demanding expensive repair and renovation.
This attack is known as carbonation. It is this carbonation of concrete, leading to the corrosion of steel reinforcement that has led to the eventual collapse of concrete structures, causing increasing concern among Architects and Construction Engineers throughout the world.
ALKALINITY PROTECTS REINFORCEMENTS
Concrete is alkaline - when new its alkalinity approaches pH-13 - and it is this alkalinity that protects any steel reinforcement from corrosion. However, over the years acid gases in the air attack the concrete reducing its alkalinity, thus its ability to protect steel.
OUR ENVIRONMENT-THE INVISIBLE ENEMY
The acid gases in the air are carbon dioxide (CO2) naturally present in all air and increased by industrially produced gases such as sulphur dioxide (SO2). These two gases combine with the moisture in the atmosphere and react with the calcium hydroxide in the concrete producing calcium carbonate and calcium sulphate (gypsum). When this happens, the natural alkalinity of the concrete is lost, leaving the way free for corrosive attack to steel reinforcements. The worlds increased levels of environmental pollutants accelerate these degradative affects to masonry.
THE AFFECTS OF ACID ATTACK ON CONCRETE
Acid rain places high demands on concrete structures. Corrosion starts when the pH value of the concrete falls below 10 and it is at this point that the damage to the structure becomes obvious.
Carbonation of the concrete leaves the surface friable and unsound. Where the concrete is reinforced rust occupies a greater volume than its parent steel.
The initial hair line cracks rapidly develop into major damage as corrosion products of the steel reinforcements expand with explosive effect on the surrounding concrete.
During construction of masonry structures, poor on site practice can leave reinforcements closer to the surface than originally intended or specified and concrete substandard in nature.
Naturally it is impossible to reposition the reinforcing bars for greater cover. Therefore, the only solution is to prevent alkalinity loss of the concrete is by applying a surface coating which eliminates the diffusion of acid gases.
Astec Elastomeric wall coatings are designed as a sophisticated "molecular sieve", allowing moisture to defuse out but preventing the entry of acid gases such as Co2 and So2.
Astec EC-2000 Boasts the ability to have a protective equivalent of 65 metres of air or 160 mm of pH 13 concrete.
In addition, EC 2000s superior crack bridging properties offer absolute confidence in long term masonry protection, by it state of the art ability to withstand forthcoming cracks 4.9 times the dry film thickness.
( 350 microns dry will accommodate a forthcoming
crack of 1.75mm) Read More