Innovative Strategies for Enhanced Concrete Durability: A Comparative Analysis of Autogenous and Autonomous Self-healing Mechanisms

Abstract

This study explores incorporating self-healing mechanisms into concrete to address cracks compromising structural integrity. Two self-healing techniques, autogenous and autonomous, are compared. Specimens with excessive cement were created to investigate the impact of cement content on autogenous healing. The autonomous technique used Bacillus subtilis bacteria with a constant concentration, using two insertion methods: direct and immobilization. Crumb rubber was used to immobilize bacteria, assessing its effect in preserving them over long periods. Calcium lactate was used as a source of calcium in some of the autonomous mixtures. Healing techniques were evaluated using compression tests, chloride permeability, visible crack inspection, and microscopic investigation. The test results showed a reduction of 32% in compressive strength at 7 days when using calcium lactate with bacteria. However, after 28 days, the strength was enhanced by about 15% compared to the control mix. Despite the strength reduction, the crack healing ability exceeded the control mix in 7- and 28-day cracked specimens. The best results were achieved using bacteria immobilized in rubber without calcium lactate by healing 2mm cracks with a 71% compressive strength increase and 89% strength recovery. The immobilization method outperformed the direct method in preserving bacteria, as confirmed by scanning electron microscope images revealing bacterial presence after 100 days, accompanied by a substantially reduced chloride permeability to very low levels. For the autogenous mixture, adding excess cement enhanced the crack healing efficiency by 67%, healing a 0.5mm crack width. SEM results showed calcite precipitation in both autogenous and autonomous mixtures. However, autonomous mixtures exhibited denser and sharper crystal morphology, confirmed by energy dispersive spectrometer results since the bacterial concrete achieved a calcium weight of 36.52% compared to its control mixture of 8.69%.