Contribution to the study of mechanical strength and the thermo-diffusive effects on the durability of a cork bio-based concrete

Abstract

Enhancing the mechanical and thermal performance of concrete structures has become a fundamental requirement in light of current trends emphasizing energy efficiency and carbon emission reduction in the construction sector. In this context, the present study aims to build upon previous research that investigated the mechanical and thermal properties of cork-modified concrete as a non-structural material, by developing improved bio-concrete formulations incorporating virgin cork granules, which are traditionally characterized by limited mechanical strength. The research focuses on enhancing the resistance and durability of this concrete under aggressive environments, particularly in chloride-rich marine conditions, which pose a serious threat to the integrity of steel reinforcement due to accelerated corrosion processes. To achieve this objective, the cementitious matrix containing 450 kg/m³ of cement was reinforced with 10% finely ground limestone as a supplementary cementitious material, while the water-to-binder ratio was reduced to 0.38. Based on previous findings that indicated promising behavior when replacing 25% of the coarse aggregate with wet virgin cork, two experimental programs were designed to examine the influence of the wet-preconditioning method applied to cork aggregates. In the first program, dry cork was mixed with a quantity of water equivalent to its experimentally determined absorption capacity after two hours of immersion (OC+HC2h AW). In the second program, cork granules were directly soaked in water for two hours prior to mixing (OC+HC2h IW), in order to evaluate the differences between theoretical and actual water absorption and their effects on the final concrete properties. The study investigated six concrete mixtures: reference concrete (OC); concrete with wet cork containing absorbed water (OC+HC 2h AW); concrete with water-immersed cork (OC+HC 2h IW); concrete with limestone filler (OC+F); concrete with limestone and wet cork containing absorbed water (OC+F+HC 2h AW); and concrete with limestone and water-immersed cork (OC+F+HC 2h IW). Experimental results indicated that replacing 25% by volume of mineral aggregate with virgin cork granules, along with the addition of limestone filler, improved workability and reduced plastic viscosity. Furthermore, the dry density of the concrete decreased, ranging between 2008 and 2060 kg/m³. Although a reduction in mechanical V strength was expected due to cork incorporation, the addition of fine limestone resulted in compressive and tensile strengths comparable to those of the reference concrete, with a notable improvement observed in mixtures containing wet cork with absorbed water compared to those with immersed cork. The results also revealed an inverse relationship between capillary absorption and porosity when limestone filler was used. Regarding chloride diffusion, higher diffusion coefficients were recorded for mixtures with wet cork containing absorbed water, whereas those with immersed cork exhibited reduced chloride diffusion rates, demonstrating superior resistance to chloride ingress and improved durability under marine exposure. The study concluded that the addition of limestone did not exert a significant influence on chloride diffusion behavior, as diffusion coefficients remained relatively close across all mixtures. This finding confirms that enhancing the durability of bio-concrete containing virgin cork primarily depends on the preconditioning method applied to the cork aggregates. Finally, these results highlight its potential for use in reinforced concrete horizontal structures exposed to aggressive conditions.