The request for long-term concrete structures without premature need for maintenance and repairs is growing rapidly all over the world. Clients have asked for structures, tunnels, or bridges to be designed to satisfy a specified service life, usually around 100 years and sometimes even 200 years. This surpasses the design life of traditionally used codes and standards. Usually, the durability is ensured by adopting rules and standards such as Eurocode, AASHTO LRDF, BS, or DIN. Unfortunately, these rules that are based on experience and research have many faults and can very often result in durability design that is not adequate. Codes and standards that we use today are too often insufficient, and we cannot rely just on those standards. The term ‘durability’ started being considered just as important as ‘structural safety’ about 40 years ago. Engineers, contractors, and owners had to learn more about the fact that environmental impacts are irreversible and they worsen with time. This happens, for example, because of the accumulation of chloride ions in concrete exposed to salts or sea water.
How to approach durability design?
Most standards as well as design codes for concrete structures have a “wish-to-satisfy” approach to the design and specification of reinforced concrete in various exposure environments, including exposure to chlorides. Design life has not been included as a consideration except the obvious time-dependent nature of the risk of concrete deterioration such as reinforcement corrosion. Today, design for the durability of new reinforced concrete structures is based on a prescriptive approach.
Environmental impacts are characterized in exposure classes in EN 206 and the resistance of the structure to these impacts is defined by a set of requirements, e.g., concrete strength class, w/c ratio, cement content cover depth, crack width needed to achieve the required service life without major repair work (for bridges, usually 100 years). The design, construction, and planned maintenance of a concrete structure have to lead to the intended level of safety and serviceability throughout its entire service life. It is very important for designers to understand the basic deterioration mechanisms and the potential types and rates of damage development. For example, different types of corrosion cause very different damage developments, some of which reduce structural safety.
According to certain engineering regulations, the durability and design of structural lifetime is often around 50 years. It can be greatly extended with regular maintenance; otherwise the structure should be demolished and rebuilt. By using MCI® Technology in severely corrosive environments, structures will have a stronger resistance to corrosion and therefore longer durability. Increased durability means fewer repairs, enhanced structural integrity, and a longer service life, all leading to greater sustainability. To put it more simply, maintenance is essential to avoid fatal tragedies like the one we witnessed when a residential building in Florida suddenly collapsed. Structural engineers were shocked that this would happen to a building that had stood for decades. Engineers are conducting a preliminary review in order to help understand the collapse. Such a failure suggested a foundation-related matter—potentially corrosion or other damage at a lower level. Corrosion is the deterioration of materials over time. It is a serious problem for engineers who use metal products in their structures, because it can be a huge safety hazard. Ignoring this powerful force can have tragic consequences.
The three major potential consequences of corrosion are
- Life threatening accidents resulting in loss of life
- Economic costs involved in rectifying the corrosion damage
- Environmental damage threatening the ecosystem
How to Build and Maintain Durable Structures
During the last two decades there have been huge advances in technology to extend the lifespan of structures and avoid possible tragedies. Patented MCI® Technology was designed to protect reinforcing metal in concrete from corrosion and is widely used around the globe. The application of MCI® products has experienced rapid growth in recent years due to a number of factors such as proven efficiency and environmental advantages. By using this technology, corrosion initiation is delayed, and the lifecycle of structures is significantly extended. One of the most efficient uses of Migrating Corrosion Inhibitors (MCI®) is when applied directly during the construction phase as well as being used as a part of the maintenance repair system in existing structures.
Sustainable construction has become a goal for owners across the globe. Often overlooked is the aspect of durability and service life for the final structure. However, this is undoubtedly one of the main factors influencing structural sustainability. Many MCI® inhibitors are made from a renewable raw material, enabling users to earn certain LEED credits.
Corrosion Protection of Coto del Rey – El Vendrell Water Tank
The Consorci D’Aigües De Tarragona (C.A.T.) needed to construct a 25,000 m³ capacity regulating water tank in the Coto del Rey area in El Vendrell, Spain in order to increase the supply and security of their water source. Given previous problems in other C.A.T. facilities, they decided to add corrosion protection to each of the reinforced concrete elements included in the project. The tank at Coto del Rey is rectangular: 88 m long, 63 m wide, and 5 m high. Once the problem and potability considerations had been established, Cortec’s MCI®-2005 coating was selected for admixing into 3,800 m³ of concrete containing 289,000 kg of corrugated steel reinforcement. MCI®-2005 dosage rate was 0.6 L/m³. C.A.T. had previous experience using MCI®-2005 for water structures in 2003 and 2016 and chose it once again in 2021. A key factor in their decision was the admixture’s certification to meet NSF Standard 61 for use in large potable water structures. MCI®-2005 will be an important protective measure against attack by chlorides and carbonation in order to extend the durability and service life of the new water tank now and in years to come.
Repairing and Extending Service Life of Höganäs County Water Tower
The water tower in Höganäs County, Sweden was built in 1978 and was designed for at least 50 years of service. The height of the tower is approximately 45 meters (49 yd), and the total façade area is approximately 3,000 square meters (3588 yd²). The water tower’s frame is made up of six externally reinforced concrete pillars that have a U-shaped cross section. These pillars stand around a larger central pillar of reinforced concrete that houses stairs, an elevator, water pipes, and electricity installations. On top of these seven pillars is the water reservoir. The reservoir is built of reinforced concrete with 14 tension cables running through rings in the outer walls. On the outside of the reservoir, there are six pilasters, two of which serve as anchoring points for the tension cables. The concrete had been breaking down, with pieces falling off in several areas where the concrete covering the reinforcement had been consumed. The presence of chlorides in conjunction with concrete carbonation (which reduced the natural protective qualities of the concrete) had caused reinforcement corrosion. Fortunately, splint injuries had not become extensive enough to affect the bearing capacity to a greater extent. The damages were limited in size but relatively numerous. The situation was still serious because such large areas had high levels of chlorides and because concrete damage can develop at a rapid pace—especially in areas around the fastenings of the tension cables. The investigation also showed damage to fastenings of the tension cables that could affect bearing capacity. It was therefore important to slow down the development of claims in good time. The municipality wanted a repair method that would extend the structure’s durability by at least 20 years.
Croatian Overpass Preservation
The customer required protection to prevent against future corrosion and to create a better appearance of the overpass.The concrete surface was water blasted to remove dirt, oil, and grease. Once dry, MCI®-2020 Powder was applied by brush tothe surface and allowed to dry for 24 hours. Then MCI® Architectural
Coating RAL 7035 (Gray) was applied as additional corrosion protection and a more appealing appearance within thelandscape. Cortec® provided an innovative solution to protect the overpass against future corrosion and provided an additional protective coating that offered a more natural look.
The Spanish government requested renovation of the Leon bridge in Ruitelan, Spain. The goal of the project was to protect the highway bridge form corrosion. Corrosion protection of the concrete against chloride ions was conducted on of surface:8.800m2. After that passivating grout was applied followed by repair mortar. Finally MCI-2020 was applied onto the surface.
Rebar Preservation at Prince Mohammed Bin Abdulaziz Medical City
Another project was facing construction delays after the majority of concrete work had been completed. The main issue was corrosion on exposed rebars of a significant number of in-fill beams and expansion joints that needed protection from the corrosive environment during the delay. Two stages of protection were conducted: Rusted rebars were treated with CorrVerter® MCI® Rust Primer and MCI® CorShield® was then applied on all exposed reinforcement to provide the necessary outdoor exposure protection during the construction delays. This provided an economical and efficient solution to preserve exposed reinforcement and satisfied the requirements of all parties involved in the project.
Durability in the exploitation of reinforced concrete structures is the basis for the preservation of structures long-term. The durability of structures depends on a good and professional choice of materials that help to avoid damage. If damage occurs, rehabilitation should be approached with great care and responsibility and selection of a product that is compatible with material used during construction, in order to avoid major damage or demolition of the building itself. Cortec® has a wide range of products that can help to achieve greater durability and, above all, safer structures.