The construction industry has witnessed the widespread use of Reinforced Concrete Construction (RCC). Thanks to its exceptional strength and durability. However, one often overlooked element that plays a critical role in ensuring safety and durability of RCC structures is concrete cover. This article will highlight importance of adequate concrete cover in RCC constructions for proper bonding between steel and concrete and to safeguard against corrosion, deterioration, structural failure, fire, carbonation and chemical attacks
Understanding Concrete Cover in RCC Constructions
Concrete cover refers to the layer of protective concrete that surrounds and encases reinforcing steel bars (rebars) within RCC structures. It serves as a crucial barrier, shielding the rebars from the harmful effects of environmental elements, while also providing structural stability. It is very important factor for the structural integrity and durability of RCC constructions. Various factors determine the amount of cover required. These include structural design requirements, exposure conditions, and construction codes and standards.
Role in Durability
One key aspect of concrete cover is its ability to protect rebars from corrosion caused by exposure to moisture, chloride ions, and other aggressive substances present in the environment. It prevents direct contact between the reinforcing steel and external elements. So It reduces the risk of rust-induced expansion, which can lead to the deterioration and eventual failure of reinforcing bars. It also acts as a barrier against spalling and cracking of the underlying concrete, which can compromise the integrity of the structure.
Ensuring Structural Strength
The thickness of concrete cover significantly influences the structural strength and load-bearing capacity of RCC members. Adequate concrete cover reduces the risk of premature structural failure and collapse. It will form a protective layer which distribute the applied loads evenly and prevent concentrated stress on rebars. Moreover it has got a significant impact on the structural behaviour of reinforced concrete elements, influencing their ductility, flexural strength, and overall performance under various loading conditions.
Protection from Corrosion
Concrete cover acts as a protective mechanism against various chemical attacks, such as exposure to chlorides and sulphates. It ensures that aggressive substances cannot reach the reinforcing steel, safeguarding against their corrosive effects. Different environments require different levels of protection, and therefore, the appropriate selection of concrete cover thickness plays a critical role in maintaining the chemical resistance and durability of RCC structures.
Bonding between steel and concrete
Proper concrete cover is important to achieve good bonding between steel and concrete. In a research it was found that 50% increase in cover thickness lead to 90% increase in bond strength. In the absence of proper concrete cover bond-slip between steel and concrete can occur, which can be determinantal for the strength and safety of structure.
Enhancing Fire Resistance
In the event of a fire, concrete cover plays a vital role in preventing the rebars from reaching critical temperatures. Excessive heat exposure can cause rebar exposure, loss of strength, and even structural instability. Therefore, it is important to ensure appropriate concrete cover , especially in fire-prone areas. By incorporating fire-resistant materials and maintaining sufficient concrete cover, the risk of fire-related damage can be minimised. This can help promote the safety and integrity of RCC structures.
Preventing Carbonation
Carbonation is a chemical process in which carbon dioxide from the atmosphere reacts with the alkaline components in concrete, gradually reducing the protective pH of the concrete. Concrete cover plays a vital role in preventing carbonation-induced corrosion. This is done by creating an alkaline environment that hinders the penetration of carbon dioxide into the concrete matrix. It is thus crucial to ensure the long-term durability. As it prevents the corrosion-related degradation of reinforced concrete structures.
The Role of Concrete Density in Concrete Cover Adequacy
Concrete density plays a crucial role in determining the necessary cover thickness for reinforcing steel. A higher density of concrete can provide enhanced protection, reducing the potential for corrosion and cracking. There is direct relation between concrete density, strength, and thickness of concrete cover. It’s important to achieve optimal performance of the RCC structure. Failing to achieve the appropriate concrete density can compromise the effectiveness of the concrete cover and, ultimately, the durability and safety of the structure.
Environmental Sustainability
Proper concrete cover significantly extends the lifespan of RCC structures, reducing the need for major repairs or reconstructions. It can contribute to sustainable construction practices by minimising potential deterioration and failures of structures. This reduction in resource consumption and material waste aligns with the goals of environmental sustainability. So it is an essential aspect of responsible construction.
Summary of Benefits and Importance of Concrete Cover in RCC Constructions
In summary, maintaining adequate concrete cover in RCC constructions offers numerous benefits. It plays a crucial role in ensuring strength, safety and durability of structures. It creates a protective layer that guards against corrosion, fire, carbonation, chemical attacks, and water penetration and contributes to the longevity and reliability of RCC structures. Adhering to best practices, conducting regular inspections, and implementing quality control measures are key to obtaining these benefits and ensuring the optimal performance of RCC constructions.
On Site Practices And Challenges In Maintaining Concrete Cover
Initially, materials such as bricks, stones, pebbles, marble, and wooden blocks were used to provide concrete cover in RCC constructions. However, they were generally not available in the desired sizes. Most of these materials are not suitable to be placed in concrete and could have been detrimental to the strength, safety, and durability of buildings. To overcome these problems, site-made or factory-made mortar blocks of the desired sizes are used as cover blocks to provide concrete cover under reinforcement.
Challenges With Mortar Blocks And Advent Of Plastic Spacers.
Inherent porosity of mortar blocks results in a higher rate of carbonation reducing its alkalinity. This rate can be as high as 5 times than the concrete used in structure. Generally these blocks are weaker than concrete being used. Break of these blocks at the time of casting is not uncommon. So plastic spacers / PVC cover blocks are gaining popularity as material of choice for maintaining gaps around reinforcement. They are corrosion resistant, non porous and chemically inert in concrete.
Building Codes Specifications About Cover blocks
As per Indian building code IS 456 : 2000 under section 12.3.2 “Spacers, cover blocks should be of concrete of same strength or PVC.” British standards BS:79673 – 2001 on Spacers and chairs for steel reinforcement states that if material is non cementious then at least 25% area under its periphery should be void.
Lack Of Fixity Of Cover Blocks Leading To Reduced Concrete Cover
Lack of fixity spacer is the common problem with all of the methods. In the absence of proper method to secure cover blocks with the reinforcement leads to their displacement due to human movement, vibrations due to vibrator and flow of concrete. In case of site made cover blocks binding wire is grouted to bind them with steel bars. As this binding wire is in the cover area of a porous block corrosion will start from this point and then propagate to reinforcement due to cell reaction. Thus reducing effective cover.
Addressing displacement with the use of self locking spacers
Displacement of cover blocks can be avoided by use of self locking Cross lock spacers. They have a self locking mechanism by which they fix themselves with the top steel bar at the crossing of two steel bars. This eliminates any possibility of displacement or rotation around steel bars. They are designed to withstand loads of up to 4 Kn. Their hollow structure ensures that concrete flows through it making it part of the structure, ensuring monolithic construction.
Conclusion
Concrete cover plays important role in ensuring strength, safety and durability of structures, which can be achieved by use of Cross lock cover blocks. It can protect from corrosion, structural failure, fire, carbonation, chemical attacks, and water penetration. It can help in achieving safety and longevity of structures. This will contribute to sustainable construction practices and resource conservation. This will mean increased strength, safety and durability of buildings.