The issue of ocean corrosion in coastal areas, particularly in concrete structures partially buried in the soil has attracted wide attention has garnered attention due to the unique challenges it presents. This study investigates the enhanced chloride ions intrusion in concrete due to bidirectional unsaturated gradients between the parts buried in soil and those exposed to the air. A 90-day chloride salt erosion experiment was conducted on both fully buried and semi-buried concrete across five different soil environments to analyze the transport properties of chloride ions. Drilling powder sampling and the detection of free chloride ion content in the concrete specimens were performed at intervals of 30, 60, and 90 days. The results indicate that the chloride ions in semi-buried concrete exhibit bidirectional unsaturated migration toward the airexposed end, leading to significantly peak chloride ion concentrations at the air-exposed end compared to those subjected to wet-dry cycles in a pure salt solution. Specifically, after a 90-day cycle, the total chloride contents at the air-exposed end of semi-buried concrete in pure sandy soil, 50 % clay soil, and pure clay soil increased by 68.8 %, 43.1 %, and 16.4 %, respectively. Soils with lower permeability intensified the vertical unsaturation gradient within the concrete, accelerating chloride ion migration towards the exposed end and resulting in higher accumulation at elevated positions. The research work underscores the critical impact of bi-directional unsaturated transport on concrete durability in coastal environments and calls for a deeper understanding of its applications for corrosion prevention.

Coastal erosion is a global environmental concern, threatening infrastructure, human livelihoods and ecosystems. Recently, microbial-induced calcite precipitation (MICP) has emerged as a promising ground improvement technique. The present study examined the effects of adding three different fibre reinforcements, namely carbon, basalt and polypropylene, on the physical and mechanical properties of coastal soil through MICP. The fibre content used was 0.20%, 0.40% and 0.60% of soil weight. A comprehensive biotreatment investigation was conducted using Sporosarcina pasteurii (S. pasteurii) in a 0.5 molar cementation solution. The samples prepared for this study had aspect ratios of 2:1 and 1:1. These samples were subjected to biotreatment, consisting of a 24-h cycle for 9 and 18 days. Unconfined compressive strength (UCS), split tensile strength (STS) and ultrasonic pulse velocity (UPV) tests were conducted on the biotreated samples to evaluate the effect of fibre reinforcement on the mechanical properties of the biotreated samples. The amount of calcite precipitation, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to interpret biocementation. Results suggest that adding fibres to the MICP process enhances the mechanical properties of coastal soil. The optimum fibre content for carbon and basalt fibre was 0.40%, whereas, for polypropylene, it stood at 0.20%. The maximum UCS, STS, UPV and average CaCO3 were observed in a basalt fibre-reinforced biotreated sample with a fibre content of 0.40%, subjected to 18-day biotreatment. Conversely, the sample without fibre-reinforcement, biotreated for 9 days, exhibited the lowest values for these parameters. Samples subjected to 18 days of treatment have higher values of UCS, STS, UPV and CaCO3 content than 9-day-treated soil samples. SEM revealed the presence of CaCO3 precipitates on the surfaces of soil grains and their contact points, and the EDS spectrum corroborated this observation.