SCIENCE/ENVIRONMENT
Adding bacteria to recycled coarse aggregate concrete can significantly improve its freeze-thaw resistance, scientists from the Department of Civil Engineering at Xi’an Jiaotong-Liverpool University in China have found.
Concrete is one of the most frequently used materials in the building industry because it’s cheap, strong and easy to make. However, natural aggregates for mixing with concrete, such as sand and gravel, are in limited supply. Although recycled materials can be used instead to create recycled coarse aggregate concrete, in colder climates daily freeze-thaw cycles can damage concrete and compromise its structural integrity.
The constant cycle of freezing and thawing is a big problem for concrete. During these cycles, water penetrates the concrete, creating cracks in the structure, and reducing its durability. When the water freezes, it expands. The more water, the more swelling, and the more swelling, the more damage.
In a study published in the Journal of Cleaner Production, scientists added denitrifying bacteria to recycled coarse aggregate and found they significantly improved the concrete’s strength and durability. It means this treated concrete is now ideal for widespread use in colder climates because it can endure as many as a third more freeze-thaw cycles.
Professor Chee Seong Chin, one of the study’s authors, says traditional ways to improve concrete’s freeze-thaw resistance are unsustainable in the long-run, chiefly because they require more chemicals. “Our method uses denitrifying bacteria and doesn’t contain or create poisonous or polluting substances,” he says.
Chin notes bacteria can improve the capacity of concrete to resist water-freezing expansion by creating a steadier structure. “Based on our experiment, denitrifying bacteria can improve the compressive strength and tensile splitting strength by 30.3% and 20.3%, respectively,” he notes.