Treating Reinforced Concrete Corrosion In Parking Structures- Part 2

Levels of Corrosion Protection for Reinforced Concrete

There are three basic levels of active (electrochemical) corrosion protection available. These are generally referred to as:

• Corrosion prevention;

• Corrosion control; and

• Cathodic protection

All levels are essentially similar in that a protective current is provided to prevent or reduce the corrosion activity of the reinforcing steel. They differ in terms of the intensity of the protective current and suitability for a given range of applications.

Corrosion Prevention

Corrosion prevention is defined by the National Association of Corrosion Engineers (NACE) as “Preventing corrosion from initiating even though the concrete may be sufficiently contaminated with chlorides to favor corrosion.”

For owners or managers who suspect corrosion is already underway and damage is occurring, the first step is to identify the extent of the problem. Unless corrosion is severe enough to force off the outer face of the concrete, reinforcing steel is generally hidden within the concrete slab, making any visual identification of early stages of corrosion difficult or impossible. Instead, the concrete is evaluated through field and laboratory testing to determine whether conditions conducive to corrosion exist within the concrete structure.

Chloride ion content testing identifies the concentration of chlorides in concrete at various depths to evaluate the probability a corrosive environment exists. Dust samples from incremental depths through the concrete slab are extracted and sent to a testing laboratory for analysis.

Half-cell potential testing determines the electrochemical behavior of embedded steel by measuring its electrical potential (i.e. the difference in charge from one area to the next). The greater the electrical potential, the greater the risk corrosion is occurring. Conducted onsite, the test involves removal of concrete cover over reinforcing bar, followed by the connection of exposed steel to an electrode through a voltmeter. Half-cell potential readings can be used to generate an electrical potential map, indicating areas with the greatest and least risk of corrosion.

Loss of steel reinforcement is a concern for areas where corrosion has progressed at an advanced rate. Where reinforcing bar is exposed or where concrete is cracked, delaminated, or spalled, a structural engineer should evaluate the remaining slab’s structural capacity to determine whether corrosion has compromised its loadbearing ability.

Where corrosion-induced spalls have been previously repaired, a characteristic ‘halo effect’ might be observed, with a ring of corrosion staining appearing around the patch site. Patching delaminated and spalled concrete with conventional concrete can lead to an electrochemical reaction at the interface between the existing chloride-contaminated concrete and the new concrete. The large difference in electrical potential between the two, combined with the short distance between anode and cathode, leads to accelerated corrosion. Usually, such patches need to be repaired again in just a year or two.

CORROSION CONTROL

Corrosion control is defined by NACE as “Providing a significant reduction in the corrosion rate of actively corroding steel in concrete.” Corrosion control can result in an increased service life of the rehabilitated targeted sections of precast members at a relatively low incremental cost. This is how sacrificial corrosion protection or mitigation is most often used. Corrosion control may or may not completely stop ongoing corrosion, but the reduction in corrosion activity will significantly extend the service life of existing corroding structures. In corrosion control applications, the conditions for corrosion (such as chloride contamination) already exist and corrosion may have already initiated in some areas, although have not progressed to the point of concrete damage. The applied current necessary to address corrosion activity (after corrosion initiation) is higher than the current required for corrosion prevention. Therefore, either larger and/or closer spaced galvanic anodes will be required to provide corrosion control.

CATHODIC PROTECTION

One way of protecting the steel is through cathodic protection. ln this method, the corrosion is stopped by reversing the processes of electrochemical action that cause the corrosion. By applying a direct current to the rebar in opposition to the current causing the corrosion, the corrosion causing current is overcome.

An effective way to achieve long-term corrosion protection of existing chloride contaminated structures, or to provide extended service life to target locations on new precast members, is to use sacrificial protection systems. Galvanic protection is achieved when two dissimilar metals are connected. The metal with the higher potential for corrosion will corrode in preference to the more noble metal. As the sacrificial metal corrodes, it generates electrical current to protect the reinforcing steel. With this type of cathodic protection system, the galvanic protection system voltage is fixed and the amount of current generated is a function of the surrounding environment. Galvanic anodes will generate higher current output when the environment is more corrosive or conductive—for example, where there is higher chloride concentrations, and where current output exhibits a daily and seasonal variation based on moisture and temperature changes. Sacrificial protection systems are low-maintenance, do not require an external power supply, and are compatible with prestressed and post-tensioned steel.

As discussed with our previous Blog article, when sacrificial anodes cannot deliver sufficient current to prevent corrosion, impressed current cathodic protection (ICCP) may be used. As with passive cathodic protection, ICCP reverses the electrochemical process of corrosion through the action of an applied electric potential—in this case, the current arises not from the inherent properties of the materials themselves, as it does with galvanic coupling, but from an external power source.

Treating Reinforced Concrete Corrosion in Parking Structures - Part 1

Facility Management Contractors are often tasked with maintenance of parking structures. Made of concrete and steel, these multi-level hubs provide visitors and their vehicles with shelter from the elements and often provide access to housing or office space. However, protecting the structure itself from the constant attack of environmental stressors and wear-and-tear comes with its own set of challenges.

Vehicles regularly entering parking garages leave water, oil and dirt behind that can corrode the structure’s concrete and steel support system.

One of the greatest issues related to the deterioration of parking structures is the corrosion of embedded reinforcement. Structural concrete used in parking structures is strengthened by means of steel reinforcement bars, or “rebar,” which is embedded into the concrete to improve resistance to tensile and compressive stresses. Ordinarily, the surrounding concrete protects this embedded steel from the corrosive effects of water and dissolved salts in the environment. However, breaches in the concrete, whether due to cracks, flaws, thin coverage, or poor concrete composition, can allow steel reinforcement to come into prolonged contact with corrosive elements. As the steel corrodes, it expands, leading to further damage to the concrete, greater water infiltration, and additional corrosion in a self-perpetuating cycle of deterioration. If not arrested early on, the progressive nature of the cracking and corrosion can eventually lead to an unsafe structure and can cause costly repair.

There are several ways contractors can retrofit concrete parking structures to ward off the effects of chloride-induced corrosion. 

One of the effective ways to stop corrosion is the use of a cathodic protection system.

Corrosion is the electrochemical process of reinforcing steel losing electrons and decomposing to iron oxide. Reinforcing steel that loses electrons acts as an anode. One way to stop further loss of electrons, and t h e re f o re stop corrosion, is to reverse the current flow and turn the steel into a cathode.

Passive cathodic protection controls steel corrosion by connecting the reinforcing bar to a sacrificial anode, a metal that is more active than steel and so will corrode preferentially. In the presence of the sacrificial metal, the steel surface becomes polarized to a more negative potential, until the driving force for the oxidation of the steel is removed. The galvanic anode will continue to corrode until it is consumed by the electrochemical reaction and must be replaced. Galvanized rebar is one example of passive cathodic protection, where the zinc coating acts as the sacrificial anode. Other commonly used galvanic anodes include magnesium and aluminum-based alloys.

Where galvanic anodes cannot deliver sufficient current to prevent corrosion, impressed current cathodic protection (ICCP) may be used. As with passive cathodic protection, ICCP reverses the electrochemical process of corrosion through the action of an applied electric potential; in this case, the current arises not from the inherent properties of the materials themselves, as it does with galvanic coupling, but from an external power source. Care must be taken in designing and installing ICCP systems in parking structures, however; excessive current density may cause the alkaline concrete to react with acid generated by the anode, leading to concrete damage. In an ICCP system, it is difficult to provide protection at any significant distance from the anode, since current distribution within concrete is poor. Therefore, anodes must be placed no more than about a foot apart, and the anode material must remain continuous throughout the structure. The ICCP system must take into consideration differing proportion and placement of reinforcement throughout the parking structure, so as to avoid voltage drops from one area to another.

Choosing the Right Strategy

Different approaches nowadays may or may not guarantee protection against reinforcement corrosion for all parking structures. Determining the best way to prevent and treat the underlying causes of corrosion involves consideration of garage conditions and exposure, concrete quality and construction, environmental contaminants, and other factors specific to the structure and situation. Initial cost and maintenance demands are also important decision criteria. Often, the most successful strategy involves a multi-component approach, one which combines preventive treatment with an ongoing program of assessment and repair to keep corrosion at bay. Ultimately, the time and expense required to prevent corrosion and treat early warning signs is far less than that of rehabilitating a garage that succumbs to corrosion induced structural failure.

The Impact of Corrosion on Concrete Infrastructures

In the past 50 years, U.S. Department of Transportation’s Federal Highway Administration (Washington DC & Florida) have done research on the bridges and offshore platforms that have aggressive chloride environments and show evidence of corrosion after short service periods. They found that, since mid of 1970’s, the cost of repairing or replacing of deteriorated structures has become a major liability for highway agencies. $20 Billion was spent on repairing corrosion problems in the past 10 years and it is increasing at $500 Million per year. The primary cause of this deterioration (cracking, delamination, and spalling) are due to the chloride attacking the reinforced steel.

Various Cathodic Protection techniques were developed to prevent corrosion in their bridges & offshore platforms. The U.S. Department of Transportation’s Federal Highway Administration (Publication no. 00-081, August 2000) is applying cathodic protection on their major bridges/tunnels, etc. The advantage of deploying Cathodic Protection System are:

1.    100% guaranteed service life (10 to 100 years life span)

2.    Easy installation

3.    Low maintenance

4.    Decreases (stop) the risk of corrosion in the reinforced concrete structures

In recent years, Road and Transportation Authority (RTA) of Dubai, have taken the approach of deploying Cathodic Protection System on their assets such as Dubai Water Canal and Shindagha Tunnel since it is a simpler option that allows to decrease the risk of corrosion on their reinforced concrete assets.