Researchers use bacteria to enhance freeze-thaw resistance of concrete

Concrete is one of the most widely used materials in construction due to its low cost, high compressive strength, and ease of manufacture. However, with the acceleration of urbanization, natural aggregates such as sand and crushed stone for mixing with concrete have become in short supply. Although recycled materials can replace natural materials to produce recycled coarse aggregate concrete (RCAC), they can cause various problems. Especially in cold regions, daily freeze-thaw cycles can damage concrete and cause safety issues.

In a recent paper published in the Journal of Cleaner Production, researchers from Xi'an Jiaotong-Liverpool University's Department of Civil Engineering added denitrifying bacteria to recycled coarse aggregate (RCA) to strengthen concrete strength and durability. Treated recycled coarse aggregate concrete can withstand up to 225 freeze-thaw cycles, 75 more than untreated concrete, making it especially suitable for extensive use in cold regions.

Traditional methods are not sustainable

Corresponding author of the paper, Prof. Chee Seong Chin, said traditional methods of improving the freeze-thaw resistance of concrete are not sustainable in the long run. "Traditional approaches include lowering the water-cement ratio and increasing chemical admixtures, essentially increasing the use of chemicals that can adversely affect sustainable development."

"In contrast, we propose an environmentally friendly solution using denitrifying bacteria, which neither contain nor produce toxic or polluting substances," he said.

Less water absorption
Professor Chin said that to improve the freeze-thaw resistance of recycled coarse aggregate concrete, reducing water absorption is crucial.
During freeze-thaw cycles, water can penetrate the concrete and create cracks in the structure, reducing its durability. This is because water expands in volume when it freezes, and the more water, the greater the expansion, and the greater the expansion, the greater the damage. "The regenerated coarse aggregate has a loose structure and high porosity. If it is directly used in concrete without bacterial treatment, it will inevitably increase the water absorption rate. The denitrifying bacteria can block the pores that water enters, effectively removing the free water inside the concrete. Absorption is reduced by 33%." He said that this method can block water absorption from the outside, thereby reducing internal swelling.

More stable structure
Bacteria can also improve concrete's ability to resist freeze expansion by creating a more stable structure, Professor Chin said. "The pores in the recycled coarse aggregate concrete are filled with calcium carbonate crystals produced by denitrifying bacteria, and the denser structure reduces the swelling effect of the chilled water."
"Our experiments show that denitrifying bacteria can increase compressive strength and tensile splitting strength by 30.3% and 20.3%, respectively. In addition, bacteria consume excess calcium hydroxide during biomineralization process, making concrete more resistant to freezing. Generally, calcium hydroxide between the aggregate and the cement matrix has a detrimental effect on strength and durability.”
Professor Chin said that although this new method significantly improved the freeze-thaw resistance of recycled coarse aggregate concrete, further research is needed to improve freeze-thaw resistance by using nanomaterials or other cement-based materials with biomineralization methods . "Future studies will also need to consider economic costs and use life cycle assessment to quantify environmental impacts."

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