What is Distress of Concrete?
Concrete structures can suffer from distress over time, which may be caused by various factors such as corrosion in reinforcement, loading, or settlement of foundations. One of the most visible signs of distress in concrete members is the development of cracks in elements such as slabs, beams, and columns. These cracks can indicate that the concrete has been subjected to significant stress and is no longer performing optimally. Therefore, identifying and addressing these issues early on is crucial to maintaining the safety and durability of concrete structures.
The cracking of concrete in building is developed in three stages:
Concrete structures that use reinforcement bars are prone to corrosion, which can cause damage to the structure over time. The corrosion process can be divided into three stages.
During the first stage, the volume of rust formed due to corrosion is about 2.5 times the volume of the steel reinforcement. As a result, the corroded reinforcement bar presses the concrete outwards. Since concrete is poor in tension, longitudinal cracks are developed along the reinforcement bar.
In the second stage, the longitudinal cracks in the reinforced concrete provide wide access to oxygen, carbon dioxide, and moisture. This allows excessive carbonation to occur, which further damages the structure. Users begin to fear for the safety of the structure at this point.
In the third and final stage, the concrete cover comes off, exposing the corroded reinforcement bars. This poses a serious danger to the structural integrity of the building and renders the structure unserviceable. It is important to address corrosion in reinforced concrete structures early on to prevent it from progressing to this dangerous and costly stage.
Fig: Distressed Concrete
Remedial Measures for Distressed Concrete
1. At design stage
To ensure the durability and stability of a concrete structure, several factors must be considered during the planning and design stages. One such factor is the provision of adequate cover to the concrete. This cover helps protect the concrete from external factors such as moisture, chemicals, and physical damage. Therefore, it is crucial to plan for adequate cover to ensure the longevity of the structure.
Another crucial factor in the design of a concrete structure is the thickness of its structural members, particularly non-structural members like chajjas, parapets, pergolas, and fins. These elements are often subjected to various stresses such as wind, rain, and seismic forces, and a sufficient thickness is required to ensure their stability and functionality. Proper planning for the thickness of these elements can prevent potential failures and ensure the overall structural integrity of the building.
In addition to adequate cover and thickness, proper detailing of reinforcement is also essential for the smooth placement of concrete and to avoid congestion. Reinforcement detailing is particularly crucial at junctions, where different elements of the structure intersect. Careful planning and design of reinforcement detailing can prevent congestion, which can lead to poor concrete placement and affect the structural performance of the building. Therefore, proper reinforcement detailing must be included in the design of a concrete structure.
2. At the construction stage
Concrete is a versatile material that can be used for a variety of purposes, but it must be prepared and handled with care to ensure its longevity and effectiveness. One important factor to consider when working with concrete is the water cement ratio, which should be kept at a minimum of 0.45 to ensure that the concrete is workable and can be compacted properly with a vibrator. Achieving a high density with minimal voids is crucial for the strength and durability of the concrete.
Another crucial consideration is the quality of the aggregate used in the concrete mix. The aggregate should be properly graded and free from any deleterious materials that could weaken the concrete. Additionally, only potable water should be used in the mixing process to ensure that the concrete does not become contaminated.
Proper design and construction techniques are also essential for working with concrete. The formwork used for the concrete should be leak-proof and well-designed, and the concrete itself should be mixed, placed, compacted, and cured properly to prevent any segregation or honeycombing. In congested conditions, plasticizers and super-plasticizers can be used to achieve the desired level of workability without compromising the water-cement ratio.
When using binding wires, it’s important to turn them inside and ensure they don’t touch the formwork. In aggressive environments, G.I. wires should be used. Proper cover with dense concrete or mortar is also essential, and a drip course should be provided for any projections.
Finally, the type and age of the cement used is critical to prevent sulfate and chloride attack. Cement that is no more than three months old should be used to ensure its effectiveness. By following these guidelines, the use of concrete can be optimized to achieve strong, durable structures that will stand the test of time.
3. Protection of reinforcement bars
Reinforcement bars, commonly known as rebars, are used extensively in construction projects to provide strength and stability to reinforced concrete structures. One of the major concerns in using rebars is the risk of corrosion, which can weaken the entire structure and cause serious safety issues.
To prevent corrosion, protective measures can be taken such as applying a coating to the surface of the rebar, which acts as a barrier against corrosive agents such as water and chemicals. Additionally, reinforcing bars can be made of corrosion-resistant materials such as stainless steel, which eliminates the need for external protection.
Another method for protecting rebars against corrosion is cathodic protection, which involves applying an electrical current to the rebar to prevent the formation of rust. This technique is particularly useful in areas where the environment is highly corrosive, such as in coastal regions where saltwater can cause rapid corrosion.
In summary, protecting reinforcement bars against corrosion is crucial for ensuring the long-term durability and safety of reinforced concrete structures. Different methods can be employed, including coating the surface, using corrosion-resistant materials, and cathodic protection.
(i) Using corrosion resistant steel
Steel alloys can be modified by adjusting their constituent elements to improve their resistance to corrosion. This is an important property because corrosion can weaken the material, leading to structural failures and potentially hazardous situations. By carefully selecting and controlling the amounts of alloying elements, such as chromium, nickel, and molybdenum, the steel can be made more resistant to oxidation and other forms of corrosion. This is particularly important in environments where the steel is exposed to moisture, salt, or other corrosive agents. The process of adjusting the constituents of the steel alloy to resist corrosion is an important aspect of materials engineering and plays a critical role in many industries, including construction, transportation, and manufacturing.
(ii) Fusion bonded epoxy coating
Fusion bonding of powder epoxy to reinforcement bars at a high temperature of 2500C is a successful method for preventing corrosion. This technique is particularly useful in coastal regions like Mumbai and is being used in significant projects such as Bridges, Flyovers, Shipyards, and Jetties. The loss of bond in this process is estimated to be around 20%. The specifications for this process are outlined in the IS:13620-1993, which details the requirements for “Fusion Bonded Epoxy Coated Reinforcement Bars.”
(iii) Passive coating with polymer based cement slurry:
The process described involves the preparation of reinforcement bars, which are cleaned for rust prior to being treated with a polymer cement slurry. To achieve this, brushes are utilized to apply the slurry in a fresh and prepared state. The purpose of this process is likely to improve the durability and longevity of the reinforcement bars by protecting them from corrosion and other forms of damage that can occur over time. Overall, this technique involves a thorough and deliberate approach to ensuring that the reinforcement bars are adequately prepared and treated to withstand the stresses and strains that they will be subjected to over the course of their service life.
(iv) Protective epoxy coating:
Epoxy coatings are a popular option available in the market for protecting surfaces from rust. To begin the process, any rust on the surface needs to be removed by using a wire brush or sand blasting. Once the surface is cleaned, the epoxy coating is applied using either a spray or a brush, following the specific instructions provided by the manufacturer.
It is important to note that during the application of epoxy coatings, there may be a loss of bond of up to 30%. This means that some of the coating may not adhere properly to the surface, which could potentially compromise the effectiveness of the protection provided by the coating. As such, it is crucial to ensure that the coating is applied carefully and precisely, according to the manufacturer’s instructions.