National Concrete Pavement Technology Center

Project description

Incorporating bacteria into concrete has been proposed as a method of mitigating the negative impacts of concrete cracking, and may also lead to increased strength and durability through calcium carbonate precipitation and porosity reductions by the bacteria. This study aimed to increase understanding of the use of microorganisms in concrete by investigating: performance of axenic and environmentally-derived bacteria cultures to increase resiliency of bacteria for use in upscaled use in concrete systems; effects of curing method and exposure to hot and cold temperatures and salts; and performance of bacteria in a pilot scale pavement application. In this work, B. subtilis was used as an axenic strain and a non-axenic bacterial system was produced from Columbus, Ohio soil. The effects of pH, growth time, incubation temperature, and nutrient solution concentration were compared with their effectiveness in generating dense bacterial cultures capable of biomineralization. Changes in compressive strength, electrical resistivity, sorptivity, and drying shrinkage in paste, mortar, and concrete samples incorporating bacteria were compared to controls. Varying curing regimes (ponding of the samples and daily spraying) were evaluated for their ability to induce biomineralization and crack healing. The effect of environmental factors including cold and hot weather temperatures, and exposure to deicing salt solution were evaluated. Numbers of living bacterial cells were tracked across all samples to link numbers of cells with changes in performance. The results demonstrate that bacteria are capable of healing cracks <0.3mm in width, and non-axenic bacterial cultures can be used to promote biomineralization and crack healing. Existing concrete mixture designs using portland cement and fly ash can be used with bacterial solutions. Calcium sulfoaluminate (CSA) cement can also be used, and provided a better environment for preserving living bacterial cells within the paste over time. CSA showed slightly better improvements to microstructure and crack healing over time compared to OPC mixtures. A lightweight aggregate must be incorporated into mortar and concrete mixtures, replacing a portion (~10%) of the fine aggregate, in order to protect the bacteria during the initial mixing process. Nutrient solution provided to the bacteria to induce biomineralization and heal cracks should be applied through spraying in order to generate the best crack healing potential. Exposure to hot and cold temperatures and salt solutions resulted in bacterial cell death and reduced the crack healing potential of the bacteria. Finally, simple life-cycle evaluations indicated only minor improvements in service life, although the evaluation method used could not account for cracking healing effects. Production and use of bacteria approximately doubled the cost of concrete during the initial construction period.