Since concrete structures are often exposed to different aggressive environments, they require high quality protection systems. Reinforced concrete structures are built to serve many generations to come. However, concrete faces many threats of natural deterioration, damage and reinforcement corrosion.
Concrete Structures Have a Lot of Stress
Depending on their location and use, concrete structures are subjected to a wide range of exposure conditions – from normal atmospheric carbonation to the aggressive influences in polluted urban and industrial environments, plus marine atmosphere and liquid or gaseous chemicals, along with influencers that can damage or attack the concrete and embedded steel reinforcement.
Water can penetrate naturally through the capillary pore structure of reinforced concrete.
Reinforcement corrosion, cracks or spalling can occur in areas of carbonated concrete or where there is high chloride content on the surface of steel reinforcing bars.
The freeze-thaw process creates stress in the concrete matrix due to free water expansion in the capillary pores during freezing conditions. This can result in surface scaling in poor quality concrete, and may be greatly accelerated by chlorides in the water.
Overloading due to increasing traffic loads, inadequate design, damage to the structure, stress/ fatigue failure, earthquake effects, or any other mechanical impact such as vehicle impact, can all exceed or reduce the structure’s load capacity.
Some structures, such as chemical plants, sewer systems or waste water treatment plants, are exposed to varying levels of chemical attacks. Special coatings may be required.
Buildings and bridges may be subjected to a wide variation of temperatures from day to night / winter to summer, or between different sides or surfaces of the structure. These repeating cycles result in thermal stresses and movement in the concrete structure that can also cause cracks.
Reinforced concrete may be damaged from fire exposure. Special intumescent coatings may be used to protect structures against fire. Coatings should not add fuel to the fire to avoid increasing its intensity. Some structures like tunnels have special considerations with this particular risk.
Carbon dioxide (CO2) reacts with calcium hydroxide (Ca(OH)2) in the cement matrix pore liquid of concrete structures, and it is deposited as calcium carbonate (CaCO3). This process, known as carbonation, reduces the strength of embedded steel reinforcement once it reaches them.
Chlorides come from de-icing salts used in winter, or from saltwater in marine environments. They can penetrate the concrete structure, and once they reach the reinforcement bars, they can locally destroy the passivation film causing fast pitting corrosion.