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Investigating the effect of biochar on the frost durability and durability assessment of concrete
Dunne, Adam B.
Dunne, Adam B.
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2025
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Abstract
As the key binding constituent, cement plays a critical role in determining the engineering properties and durability of concrete. Since its production is responsible for 8% of the global CO2 emissions, there is significant interest in technologies that reduce cement’s carbon footprint. One emerging solution is the integration of biochar, a carbon negative, renewable material produced from the thermochemical conversion of biomass. Biochar’s impact on the mechanical and fresh properties of concrete has been widely studied, while biochar’s effect on concrete durability remains largely unknown. This thesis seeks to address this research gap by investigating the effect of biochar on the frost durability and durability assessment of biochar concrete.
In the first effort, biochar was used as a partial replacement of cement in mortars to investigate the validity of bulk electrical resistivity as an assessment of biochar mortar permeability. In conventional cementitious composites, bulk electrical resistivity measurements can be used as an approximation of permeability because electrical charge is only carried via electrolytic conductivity; dissolved charged ions in the pore solution migrate more easily (higher conductivity) in more permeable composites. However, cementitious composites that incorporate conductive materials can display both electrolytic conductivity and electric conductivity, both of which can affect bulk electrical resistivity measurements. Resistivity measurements cannot distinguish between these conductive pathways; thus, decreased resistivity may indicate increased electric conductivity through the solid phase or increased permeability, which is typically associated with decreased durability. This work found that while biochar can reduce matrix permeability, it also influences the electrical properties of mortars through changes to both electrical and electrolytic conductivity, making bulk electrical resistivity measurements a poor indicator of permeability changes.
In the second effort, biochar was used as a partial replacement of sand to evaluate compatibility between biochar and an air-entraining admixture and to explore biochar concrete’s resistance to rapid freeze-thaw testing. Entrained air voids are typically integrated into concrete to provide resistance against frost damage, but reduced concrete permeability can also enhance this protection. To entrain the required 6 vol% air in concrete, concrete mixes with biochar required 970 - 2800% more air-entrainer compared to a control. Additionally, despite improved permeability at some replacement levels, non-air-entrained biochar concrete failed in less than 15 freeze-thaw cycles, compared to conventional concrete which failed in 97 freeze-thaw cycles. Air-entrained biochar concrete showed improvement, but still failed between 165 and 185 freeze-thaw cycles. This degraded freeze-thaw performance was attributed to biochar concrete’s higher sorptivity, associated with pore refinement. When water freezes in small pores, higher tensile pressures are developed than when water freezes in large pores, leading to greater damage.
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