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cathodic protection of reinforced concrete

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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.

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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.

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Conventional Steel Corrosion and Durability Design

Conventional corrosion is an electrochemical redox reaction, thus when steel is in contact with an electrolyte and oxygen, then steel mass will be lost, this is more pronounce in sea water. Corrosion, compared to time is generally a linear process and is uniformly spread over the exposed area.

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Table 1. Recommended value for the loss of thickness (mm) due to corrosion for piles and sheet piles in fresh water or in sea water

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On the basis of this table the common method utilised in accounting for corrosion is to utilise a sacrificial thickness by increasing the thickness of the pile by at least 4mm.

However, for construction in the Arabian gulf this method may not be the optimal solution due to the climatic and seawater conditions. The gulf coastline experiences some of the most extreme weather conditions with summer temperature reaching up to mid to high forties, with the salinity of the Gulf generally being highly variable with some sections near the coast reaching a concentration of 10 % (Fookes et al). In general, the salinity of the Gulf, at 4 %, is also higher than the open ocean, at 3 %.

The sacrificial thickness specification for a pile in sea water in zone of high attack is 3.75 mm, which means that a corrosion rate of 0.075 mm/year is adopted. However, according to research presented in CIRIA C634 that is the minimum rate of corrosion reported. The average corrosion rates reported range from 0.08 to 0.2 mm/side/year. For the harsh aggressive environment of the Arabian Gulf compounded with high and variable salinity of sea water, with the high temperatures a higher corrosion rate in design is recommended for optimal durability. The highest corrosion rates range from 0.17 to 0.34 mm/side/year. For a worst-case scenario, the highest corrosion rate will see a loss of 17 mm of steel, and if a sacrificial thickness of 4 mm is utilised, it will only protect the integrity of the member for 12 years.

Table 2. Corrosion Rates found in Literature

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Concrete Cancer

The Middle East is well known for the presence of a very aggressive salty water table that sits barely a few meters below the surface. As we all know, salt and water coupled with heat are the perfect blend to create corrosion nightmare of concrete structures.

Some Facts

Concrete Cancer, often identified by flaking concrete or rust stains, which originate deep within the concrete is a serious problem caused by corroding/rusted reinforcing steel from within the concrete. As steel rusts it can expand up to 7 times its original size causing the surrounding concrete to crack. As the steel pushes the concrete away, more water gets to the steel expediting the process.

The process is generally due to:

·       Presence of large quantities of water and salt

·       The ends of reinforcing being too close to the surface allowing water to seep through concrete and react with the steel

·       Poorly treated reinforcing steel being used in the original pour of the slab

·       Fractures in the concrete allowing water to penetrate the concrete and react with the steel

What do we do?

Spalled concrete can be a safety hazard. Concrete cancer and delaminated concrete should be treated immediately as deferring the treatment will inevitably lead to increased problems into the future.

Similarly, treating the visual aspects such as rendering over the steel are short-term solutions as the rusting process will continue below the surface causing the steel to again displace the concrete and in some cases, rust so badly the steel eventually needs replacement. This approach – we call it the ‘make up’ approach – is aesthetic. In essence, the ugly bits are removed and given a nice clean looking finish, however the underlying problem is very much still present. Within a short time, the area adjacent to the area repaired is cracking and breaking and requires repair. You are back to square one.

The Real Stuff…

The appropriate and effective treatment necessary is cathodic protection – an electrochemical method of arresting corrosion for an extended period of time – ranging from 5 years to 50 years.

Ducorr’s SHIELD™ technology is easy to install into dilapidated atmospherically exposed concrete areas and achieve excellent corrosion protection. The system uses permanent power to provide sustained protection by simply making the corrosion reaction impossible to occur. There’s lots of thermodynamic theory behind, which would be too long for this article – but in essence cathodic protection is the ONLY method that address corrosion at an elemental level eliminating the possibility of any further damage.

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The Dubai Water Canal is key infrastructure project that involves the construction of water canal that routes just east of Sheikh Zayed Road to the Jumeirah beach. The canal mainly consists of block wall construction. However, in a minor section of the canal, the construction incorporates a reinforced concrete diaphragm wall. The project specification requires that the reinforcing steel of this diaphragm wall be protected from corrosion using cathodic protection designed and installed by DUCORR.

Contact us to deploy your system now.

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