Biomineral sealing of cracks in degraded concrete structures

Civil Nuclear sites contain significant volumes of concrete infrastructure, including both external and internal structures. As a consequence, different concretes are exposed to differing environmental conditions resulting in variable mechanisms and rates of concrete degradation. For example, external structures may be exposed to salt water and freeze-thaw cycles, while internal structures may be exposed to high temperatures and/or high levels of radiation.
Degradation can take the form of spalling, mineral dissolution and ultimately, cracking. These mechanisms increase concrete permeability, which can lead to two significant issues for decommissioning operations. First, cracking of the concrete increases water ingress, accelerating rebar corrosion and reducing the structural integrity of the buildings. This increases the complexity of decommissioning operations, which frequently involve the use of heavy plant, particularly for waste retrieval operations. Second, cracking of concrete waste containment structures can lead to contamination of the subsurface, increasing the complexity of environmental remediation operations.

Microbially-induced calcite precipitation (MICP) may provide a low-cost, durable method for the reduction of permeability in aged or damaged concrete infrastructure. The method relies upon the ureolytic capacities of the bacterial strain Sporosarcina pasteurii to metabolise urea in the presence of calcium, resulting in precipitation of calcium carbonate. Bacteria are injected into the fractured concrete, alongside urea and calcium in solution, and layers of calcium carbonate are precipitated. In the research presented here, we treat fractured concrete cores in the laboratory, reducing the hydraulic conductivity by 2-3 orders of magnitude in concrete samples collected from UK Civil Nuclear sites. We then utilise X-CT imaging to quantify and visualise the calcite deposited within the fracture network. Our research indicates this treatment protocol can significantly reduce concrete permeability as well as improving structural integrity, thus increasing the longevity of degraded concrete nuclear assets.