Gulf Construction Online - Minimising the corrosion risk

Corrosion

Corrosion of metals in concrete is a very serious form of concrete deterioration.

Minimising the corrosion risk

The deterioration of concrete continues to plague the region, which suffers a harsh environment that includes saline conditions and high ambient temperatures. The American Concrete Institute (ACI)* highlights what can be done to minimise the risk.

01 June 2019

Corrosion of metals in concrete is one of the most serious – and most visible – forms of concrete deterioration and can be seen in parking structures, marine structures, industrial plants, buildings, bridges and pavements.

Chemical reactions, mostly caused by chlorides, are responsible for corrosion.

When embedded metal (steel being the most commonly used) becomes corroded, it can cause cracking and spalling of the surrounding concrete which, in turn, exposes more metal surface area to be corroded.

Although many causes of corrosion are beyond the control of designers and contractors, adequate concrete quality and detailing (especially concrete cover and crack control) can minimise corrosion of embedded metals.

Environmental conditions in the Gulf make corrosion a significant threat for concrete structures. The proximity of saline ocean water means most structures are exposed to sea spray as well as corrosive groundwater. Furthermore, high ambient temperatures accelerate the corrosion process in built structures.

Chlorides and/or sulphates can be introduced into a concrete mix by contaminated sand or aggregate. When placing concrete in some Gulf states, it is important to test aggregates for contaminants before use. If contaminants are found, mitigating steps such as washing the aggregates or selecting different cementitious materials can be taken.

Water used for concrete during mixing or curing can also introduce chlorides. High temperature conditions during placement of concrete in the region can cause a decrease in concrete workability, which may cause installers to add excessive amounts of water to the mix, increasing the number of contaminants and increasing the water-to-cementitious materials (w/cm) ratio.

Using admixtures to control workability for long periods, as well as using ice and chilled water to control temperature, is a common and successful strategy used in most construction projects in the Gulf.

Designing for Protection

Adequate concrete cover is critical for corrosion protection. The hardened cement paste in uncontaminated concrete provides an alkaline environment to protect embedded metal. However, carbon dioxide in the atmosphere reacts with concrete, producing calcium carbonate and gradually neutralising the concrete’s pH. Areas of carbonation can travel to reinforcing steel, especially if cracks are present.

ACI 318-14: ‘Building Code Requirements for Structural Concrete and Commentary’ summarises minimum concrete cover.

And according to ACI 222.3R-11: ‘Guide to Design and Construction Practices to Mitigate Corrosion of Reinforcement in Concrete Structures’, where concrete will be exposed to external sources of chlorides or to other aggressive environments, a minimum concrete cover of 2 inches (50 mm) for walls and slabs and 65 mm for other members is required for corrosion protection.

For precast concrete manufactured under plant control conditions, a minimum cover of 40 and 50 mm, respectively, is recommended for walls and slabs. Concrete cover specifications for bridges are defined by AASHTO HB-17.

Special attention must be paid to the construction and sealing of joints to prevent water penetration. For embedded items such as weld plates, additional concrete coverage and protection should be provided.

Although often overlooked, designing in adequate drainage can reduce the risk of corrosion, particularly in parking structures and bridges. Improved drainage reduces ponding, thereby reducing the amount of water and salts that can otherwise penetrate the concrete.

Concrete mixtures

ACI 318-14 highlights three exposure classes which dictate conditions requiring corrosion protection:

• C0: Concrete dry or protected from moisture.

• C1: Concrete exposed to moisture but not to an external source of chlorides.

• C2: Concrete exposed to moisture and an external source of chlorides from deicing chemicals, salt, brackish water, seawater, or spray from these sources.

ACI 318 gives requirements for concrete mixtures that include a maximum water-soluble chloride ion content in the concrete. Non-prestressed concrete has limits of one per cent for C0, 0.3 per cent for C1 and 0.15 per cent for C2.

The chloride limit for all prestressed concrete is much lower – 0.06 per cent – because prestressed concrete is more vulnerable to corrosion.

ACI 318 also gives a maximum water-cementitious material ratio (w/cm) of 0.4 for exposure condition C2, since concrete with a high w/cm and high permeability is more vulnerable to attack.

Considerations associated with admixtures and type of cement are addressed in ACI 222.3R-11. Various substitutions can be made to alter chemical reactions or change concrete’s permeability.

For example, blended cements, in which the portland-cement clinker is interground with a supplementary cementitious material, can reduce permeability. Water-reducing admixtures and corrosion inhibitors are additional strategies.

Aggregate selection is also critical for concrete durability. Avoiding use of aggregates that introduce chloride ions into the mixture is one of two main considerations; the second is proper selection of aggregate size and gradation to enhance the workability of the mixture and reduce the amount of water that needs to be added.

During mixing, the moisture content of both the coarse and fine aggregates should be monitored. Errors in assessing the moisture content can lead to substantial increases in the w/cm of the mixture, resulting in dramatic increases in permeability.

*American Concrete Institute (ACI) is a non-profit technical society and standards developing organisation. It is a leading authority and resource worldwide for the development, dissemination, and adoption of its consensus-based standards, technical resources, educational and training programmes, certification programmes, and proven expertise for individuals and organisations involved in concrete design, construction, and materials.