Sadaqat Ullah Khan

Reinforcing steel bars in coastal regions are frequently exposed to chloride-rich environments before the concrete placement, yet the mechanical consequences of this pre-embedding exposure are rarely quantified. This study experimentally investigates the corrosion progression and mechanical degradation of ASTM A615 grade 60 reinforcing steel bars subjected to natural marine exposure and accelerated simulated chloride conditions using real Arabian seawater. Bare bars of 10 mm diameter were exposed to outdoor coastal conditions in Karachi and to an electrically accelerated seawater environment. A periodic evaluation was carried out up to 270 days, including visual inspection, mass loss, diameter reduction, tensile testing, and microstructural characterisation using scanning electron microscopy (SEM). Natural exposure produced gradual general corrosion, corresponding to ~0.5% annual cross-sectional loss and minor reductions in tensile strength within experimental variability. In contrast, simulated chloride exposure markedly accelerated deterioration, causing diameter losses approaching 1 mm and reductions in yield and ultimate strength of up to 20–25% within 60 days. Strength degradation trends closely followed section loss, indicating cross-sectional reduction as the dominant observed factor. SEM observations showed porous and cracked corrosion products with limited protective capacity. A performance-based time equivalence between natural and simulated exposure was derived from degradation trends while acknowledging possible mechanistic differences. Regression models relating exposure parameters to residual strength showed strong agreement with experimental data. The findings demonstrate that pre-placement marine exposure can introduce measurable steel degradation, underscoring the need to account for construction-stage corrosion in durability management of reinforced concrete in coastal regions. The findings highlight the critical impact of pre-embedding chloride exposure on reinforcing steel performance and emphasise the need to incorporate construction-stage corrosion effects into durability-based design and marine construction practices.

Reinforced concrete (RC) members are designed to crack and the crack width usually remains within the service limit; however, these micro-cracks make structures susceptible to the infiltration of aggressive substances, especially near the coastline. Thus, the healing of these cracks is necessary before they further widen and spread. This study focused on the development and application of a crack-healing solution using microorganisms of the class bacillus; healing was observed through a crack-sensing camera. The aim was to regain the load-carrying capacity of the concrete member to meet the serviceability limit state requirements after healing the crack. The performance of the crack-healing solution was compared with the epoxy-injection method. Five full-scale RC beams of 100 × 200 × 1800 mm in dimension were cast using concrete designed with a cylindrical compressive strength of 21 MPa. After curing for up to 28 days, the beam specimens were tested and subjected to four-point bending to produce a flexural crack of width 1–3 mm. One of the beams was treated to fill the crack by injecting epoxy, while the three other similar beams were treated using a crack-healing solution consisting of bacteria (Bacillus subtilis), nutrient (calcium nitrate), and transporting agents. The healing solution was applied directly to the opened crack with silica gel and with cement slurry in three similar beams cracked under flexural load. The cracks in the beam treated with the crack-healing solution were sealed and kept moist for a further 14 days. After curing, all of the beams including the control (without treatment) were tested again and were subjected to four-point bending until failure to observe the effect of the crack repairs on the flexural response. It was observed that both systems were equally good at enhancing the serviceability limit state and improving the load-carrying capacity.

In this investigation, polyethylene terephthalate (PET) fibers obtained from the plastic bottles were used in engineered cementitious and geopolymer composites (ECC and EGC). These composites were compared with polyvinyl alcohol (PVA) and polypropylene (PP)–reinforced ECC and EGC. A total of eight mixtures (four for ECC and four for EGC) were prepared with 0% to 2% volume fraction of fibers. In each ECC and EGC series, one mixture was a control (without fibers) and a 2% volume fraction of PET, PVA, and PP fibers were added in the remaining three mixtures. The specimens prepared using these mixes were tested to failure under bending, compression, and direct tension to observe the multiple cracking and pseudo-strain-hardening (PSH) response associated with ECC. The flexural load–deflection response of fiber-reinforced ECC and EGC was observed and compared with the control mix without fibers. Besides mechanical properties, the field emission scanning electron microscopic (FESEM) images were also envisaged to observe the microstructure of the composites. It was found that the behavior of PET fiber–reinforced ECC and EGC in terms of mechanical properties was better among all combinations. PET fiber–reinforced ECC and EGC were also found competitive to PVA fiber–reinforced ECC and EGC in terms of strength and multiple cracking PSH response.