Reinforced concrete (RCC) is one of the most widely used materials in the construction of buildings. Its strength and durability make it an ideal choice for residential and commercial structures. However, despite its advantages, reinforced concrete buildings are still susceptible to various types of failures. These failures can often be traced back to poor construction practices, inadequate supervision, or the use of substandard materials. Understanding the common causes of failure in RCC structures is crucial for builders, engineers, and homeowners alike. In this article, we will explore different types of failures in reinforced concrete buildings, supported by case studies, and discuss preventive measures to avoid such issues.
Common Causes of Failure in Reinforced Concrete Buildings
There are several reasons why reinforced concrete buildings may fail. Some of the most common causes include:
- Poor Concrete Quality: If the concrete mix is improperly prepared, it may fail to reach the required strength, leading to structural issues over time.
- Reinforcement Corrosion: Exposure of steel reinforcement to moisture and air can cause rust, which weakens the steel and reduces its bonding with the concrete.
- Soil-Related Issues: The quality of the soil beneath a building plays a critical role in the stability of the structure. If the soil is not properly tested, it can lead to foundation failure.
- Improper Construction Practices: Lack of proper supervision and improper construction techniques can result in defects such as honeycombing, poor load transfer, and weak bonding between the concrete and reinforcement.
- Use of Substandard Materials: The use of poor-quality materials like beach sand or untested water can lead to failures in the concrete mix, affecting its strength and durability.
Case Studies of Failures in Reinforced Concrete Buildings
To better understand how these failures occur, let’s examine a few real-world case studies.
Case 1: Failure of RCC Column Due to Corrosion of Reinforcement
- Observation: In October 2015, an RCC column was found with excessive rust on the dowel rebars.
- Site Condition: The building was located 1 km from the sea, and the soil quality was good.
- Cause: The dowels in the column were left exposed to the atmosphere for three months during the rainy season before the concrete was poured. This exposure allowed moisture to corrode the steel reinforcement.
- Investigation: The delay in covering the exposed rebars during construction led to the corrosion of the reinforcement, weakening the column structure.
- Precaution: It is essential to cover exposed dowels with a cement-sand slurry to protect them from atmospheric conditions. Furthermore, the coating should be reapplied every fortnight to prevent rust formation.
Case 2: Failure of Building Due to Improper Transfer of Load
- Observation: In November 2015, a building experienced failure due to improper transfer of loads.
- Site Condition: The site was on good quality soil, but no engineer was consulted during the construction.
- Cause: The centering (formwork) for the beams was done by an untrained mason who did not follow proper techniques. Additionally, due to congestion of rebars at the beam-column junction, the mason avoided proper compaction, leading to honeycombing in the concrete.
- Investigation: The improper centering led to poor load transfer from the beams to the columns, causing structural instability.
- Precaution: Centring should be done by trained masons, and engineers must supervise all aspects of construction, particularly during the foundation work, centering, and casting stages. Additionally, proper compaction should be ensured at all stages to avoid honeycombing and weak spots in the concrete.
Case 3: Failure of Reinforced Concrete Structure Due to Poor Concrete Mix and Water Quality
- Observation: In August 2015, a reinforced concrete structure was found to be weak due to a poor concrete mix and substandard water quality.
- Site Condition: The soil quality was good, but the construction was not aligned with the engineer’s design.
- Cause: The concrete was mixed with poor-quality ingredients, and the water used in the mix was not tested. Testing revealed that the concrete did not achieve the required 20 MPa strength due to the use of unfit materials.
- Investigation: The use of low-quality water and a poorly mixed concrete reduced the strength of the structure, leading to cracks and potential collapse.
- Precaution: It is critical to test the quality of all building materials, including cement, sand, stone chips, and water, before mixing. The pH of water should be more than 6 to avoid introducing acidity that could weaken the concrete. Any deviation from the engineer’s design must be approved by a qualified structural engineer.
Case 4: Failure of RCC Structure Due to Use of Beach Sand in Construction
- Observation: In August 2015, a building was found with deep cracks in its ground-floor columns and concrete chipping throughout the structure.
- Site Condition: The building was located 1 km from the sea, and the soil quality was good.
- Cause: The construction materials included beach sand, which contains high levels of salinity. Additionally, groundwater, which had not been chemically tested, was used in the concrete mix.
- Investigation: The presence of salt in the beach sand and untested water compromised the integrity of the concrete, leading to structural damage.
- Precaution: Building materials must meet the quality standards set out in IS 456:2008, which outlines the requirements for cement, aggregates, and water used in concrete. Salty beach sand and untreated water should never be used in construction, as they can lead to corrosion and weakening of the concrete.
Case 5: Structural Damage Due to Soil Failure
- Observation: In August 2015, a building’s columns were severely damaged due to soil failure after a thunderstorm and landslide.
- Site Condition: The site had good construction quality, but no soil testing was done before construction.
- Cause: The failure occurred when the untested soil beneath the building could not support the weight of the structure during a sudden downpour and subsequent landslide.
- Investigation: The soil had not been tested for its chemical properties or strength before construction. The lack of proper soil preparation led to the failure of the foundation when the soil became unstable.
- Precaution: Soil testing for both chemical properties and strength is essential before construction begins. Additionally, the sidewalls of any excavation should have a slope of more than 30 degrees to avoid landslides. In case of weak soil, micropiles or wooden sheet piles can be used to stabilize the ground.
Conclusion
The case studies discussed above highlight the various types of failures that can occur in reinforced concrete buildings. While these failures may seem like isolated incidents, they point to common issues such as poor quality control, improper construction techniques, and a lack of engineering supervision.
To prevent such failures, it is crucial to follow proper construction practices, ensure that all materials meet quality standards, and regularly inspect the work at each stage. Builders and homeowners must prioritize the involvement of qualified engineers in the design and construction process, as this can significantly reduce the risk of structural issues.
By learning from these case studies and adhering to best practices, engineers, contractors, and homeowners can ensure the safety and durability of reinforced concrete buildings, minimizing the risk of costly failures in the future.