Concrete Corrosion & Commercial Building Inspections

Concrete can be damaged by many processes, such as the expansion of corrosion products of the steel reinforcement bars, freezing of trapped water, fire or radiant heat, aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by fast-flowing water, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distillate water.

 

Carbonatation is a slow process that occurs in concrete where lime (calcium hydroxide) in the cement reacts with carbon dioxide from the air and forms calcium carbonate.  The water in the pores of Portland cement concrete is normally alkaline with a pH in the range of 12.5 to 13.5. This highly alkaline environment is one in which the steel rebar is passivated and is protected from corrosion. According to the Pourbaix diagram for iron, the metal is passive when the pH is above 9.5. The carbon dioxide in the air reacts with the alkali in the cement and makes the pore water more acidic, thus lowering the pH. Carbon dioxide will start to carbonatate the cement in the concrete from the moment the object is made. This carbonatation process will start at the surface, then slowly move deeper and deeper into the concrete. The rate of carbonatation is dependent on the relative humidity of the concrete – a 50% relative humidity being optimal. If the object is cracked, the carbon dioxide in the air will be better able to penetrate into the concrete. Eventually this may lead to corrosion of the rebar and damage to the construction.

 

As a Commercial Building Inspector knowledge of Concrete and ability to the report on its condition is an important factor of the building report.  In larger buildings there can be concrete roofs, walls and floors,  which if damaged,  could cost a prospective buyer a prohibitive amount of money to repair.  Most investors would probably choose to continue shopping is major structural repair is required or will be required in the near future.

 

Here is a common list of causes and problems that may be encountered during a Commercial Inspection:

 

1.  Corrosion of steel reinforcement is probably the most common form of deterioration in cold climates, it’s difficult to deal with because once the conditions are right for corrosion, and just fixing the damaged areas does not stop the ongoing corrosion.  Corrosion-induced deterioration of reinforced concrete can be modeled in terms of three component steps: (1) time for corrosion initiation, Ti; (2) time, subsequent to corrosion initiation, for appearance of a crack on the external concrete surface (crack propagation), Tp; and (3) time for surface cracks to progress into further damage and develop into spalls, Td, to the point where the functional service life, Tf, is reached.  Corrosion on this nature in a supporting structure may necessitate a design build repair specialist to ensure integrity of structure is not compromised.

 

2.  Deterioration – The deterioration occurs because the by-product of this electrochemical process (rust) takes up many times the volume of the original uncorroded steel. The resulting pres­sure created inside the concrete will cause cracking and severe deterioration to the structure over time. The most common cause of steel rebar corrosion is exposure to de-icing salt used for roadways. If the concrete and rebar are not protected, the salts will eventually reach the depth of the rebar and cause corrosion. Exposed concrete structures such as parking garages, sidewalks, and bridges in cold climates are most at risk.

 

3. Protection of Concrete –  Luckily, there are several ways to protect steel reinforcement from corrosion. First, make sure to provide at least 11⁄2 to 2 inches of concrete cover over the reinforcement. In addition, create a concrete mix that is highly impermeable by using a mix with a low water-to-cement ratio (typically no greater than 0.40) “so that it takes longer for the chlorides or carbonation to reach the steel. Other internal protection options include adding corrosion inhibitors to the fresh concrete and using epoxy-coated reinforcing steel. External protection measures such as penetrating sealers or waterproof coatings applied to the exposed concrete can also inhibit ingress of chlorides and moisture.

 

4. Crevice Corrosion – Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas). Oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal. Oxygen content of liquid at the mouth of the crevice which is exposed to the air is greater, so a local cell develops in which the anode, or area being attacked, is the surface in contact with the oxygen-depleted liquid.

 

5. Sulfate attack typically occurs when the concrete is exposed to water that contains a high concentration of dis­solved sulfates. “We see this most often where there’s sulfate-bearing groundwater,” as in the Western states and the Northern Great Plains, and near industrial areas and seawater.

 

6. Attack Types – The two most common types of sulfate attack are physical attack, where the sulfate-containing water enters the surface of the concrete, crystallizes, and expands, disrupting the hardened concrete; and chemical attack, where the sulfate salts react with the Portland cement paste, causing it to dissolve, soften, and erode. Another type of sulfate at­tack, internal sulfate attack, occurs mainly in precast concrete, and has been attributed to high curing temperatures or cement chemistry.

 

7Pitting – Theories of passivity fall into two general categories, one based on adsorption and the other on presence of a thin oxide film. Pitting in the former case arises as detrimental or activator species, such as Cl- , compete with O2 or OH-  at specific surface sites. By the oxide film theory, detrimental species become incorporated into the passive film, leading to its local dissolution or to development of conductive paths. Once initiated, pits propagate auto-catalytically according to the generalized reaction, M+n + nH2O + nCl- → M(OH)n + nHCl, resulting in acidification of the active region and corrosion at an accelerated rate (M+n and M are the ionic and metallic forms of the corroding metal).

 

8.  Mechanical Loading Cracks –  Cracks in concrete formed as a result of tensile loading, shrinkage or other factors can also allow the ingress of the atmosphere and provide a zone from which the carbonation front can develop. If the crack penetrates to the steel, protection can be lost. This is especially true for concrete under tensile loading, for debonding of steel and concrete occurs to some extent on each side of the crack, thus removing the alkaline environment and so destroying the protection in the vicinity of the de-bonding.

 

9. Finish-related de-lamination can occur when water or air gets trapped and accumulates just below the surface of the concrete. The accumulation of water raises the local water-to-cement ratio, which decreases the concrete strength in that area. In addition, air bubbles can be elongated and interconnected by the finishing process, thereby creating a weakened horizontal plane in that area.

 

10.  Common Causes – Common Causes of Corrosion – The two most common causes of reinforcement corrosion are (i) localized breakdown of the passive film on the steel by chloride ions and (ii) general breakdown of passivity by neutralization of the concrete, predominantly by reaction with atmospheric carbon dioxide. Sound concrete is an ideal environment for steel

but the increased use of deicing salts and the increased concentration of carbon dioxide in modern environments principally due to industrial pollution, has resulted in corrosion of the rebar becoming the primary cause of failure of this material. The scale of this problem has reached alarming proportions in various parts of the world.

 

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