Honeycombs are a common issue that occurs in concrete when hollow spaces and cavities are left on the surface or inside the mass where concrete could not reach. These spaces are characterized by their resemblance to honey bee nests. The primary causes of honeycombs in concrete are improper vibration and workability during the pouring process.
Honeycombs that appear on the sides of the concrete are easily detectable by the naked eye and can be identified as soon as the shuttering is removed. However, detecting honeycombs that exist inside the concrete mass requires advanced techniques such as ultrasonic testing. These techniques are necessary because these internal honeycombs cannot be observed through visual inspection alone.
Causes of Honeycombs in Concrete
Honeycomb is a phenomenon that occurs when concrete fails to reach all areas, resulting in the formation of cavities and hollow pockets. There are several reasons why honeycomb may occur, including improper vibration during the concrete pouring process, inadequate cover for reinforcement bars, and the use of very stiff concrete. To prevent the latter, water control can be employed in accordance with the slump test.
Honeycomb is commonly found at junctions where beams meet columns or other beams. This is often due to the entanglement of reinforcement rods at these locations, which requires special attention during the concrete pouring and vibrating process. Additionally, the presence of larger aggregate sizes in the concrete can prevent the material from filling narrow spaces between the reinforcement rods, contributing to the formation of honeycomb.
Remedies for Honeycombs in Concrete
Honeycombs observed in concrete are a defect that should be addressed promptly. They can reduce load-bearing capacity and allow water to reach reinforcement rods, leading to rust and corrosion. Corrosion can weaken the grip between rods and concrete, posing a significant risk to the safety and lifespan of reinforced concrete structures. Honeycombs have caused failures in structures within 20 to 30 years of their construction, which is less than half of their expected lifespan. Therefore, it is crucial to take extra precautions in areas such as columns, machine foundations, rafts, and beams. The affected area should be broken, and the portion should be re-concreted after applying a grouting chemical to the old surface.
In the case of honeycombs on the surface, pressure grouting with cement-based chemicals can be adopted after taking the designer’s opinion and following their advice. However, contractors and their supervisors often hide honeycombs by applying superficial cement plaster, which can be deceptive. Thus, site engineers must be cautious in identifying and addressing honeycombs.
To prevent honeycombs, concrete with strictly 20mm and down aggregates should be used with slightly more water and cement at the junction of columns and beams. Tapping the sides of shuttering with a wooden hammer from outside during concreting and vibrating can significantly reduce honeycombs in columns and beams. The use of a thinner needle, such as 25mm or less, with a vibrator at intricate places of concreting, can also help in reducing honeycombs. Overall, it is crucial to address honeycombs promptly and take necessary precautions to prevent them from occurring.