Enhancement on stability and strength of underwater concrete through liquid nitrogen freezing process: A sustainable approach for scour protection

The scour protection application of conventional underwater concrete in aquatic environments still faces the unpredictable quality control and harmful ions leaching issues due to the contact between fresh concrete and water. To tackle those issues, a novel underwater anti-scour repair technique using frozen-treated concrete as an alternative to thrown stone was proposed, which not only enhanced the bonding and cohesion of the concrete but also prevented segregation and reduced ionic leaching. In the study the feasibility of using liquid nitrogen to produce flocculants-incorporated concrete capable with underwater shaping capability was investigated with key performance parameters, including underwater morphology, water quality impact, and compressive strength, were assessed. Moreover, the micro-level characterisations were conducted scanning electron microscopy (SEM) and X-ray diffraction (XRD) to examine the influence of liquid nitrogen freezing and flocculant addition on the inherent properties of the concrete. Compressive strength tests indicated that both flocculants and freezing treatment led to an acceptable slightly reduction on the strength due to matrix imperfections and the degradation of C-S-H gel. However, the freezing treatment primarily affected early strength development, with hydration products adequately filling the matrix pore space, leading to only a 3.55% reduction in compressive strength at 28 days. Moreover, the tests of environmental impact revealed that, flocculants improved bonding to reduce material loss and ionic leaching, freezing formed a denser matrix and further minimized leaching. This work aims to draw interest from the scientific and engineering communities to further develop liquid nitrogen concrete technology for specialized applications. • The addition of flocculants enhances cohesion and internal bonding. • Freezing treatment maintains the three-dimensional shape stability of flocculants-incorporated concrete. • Flocculants-incorporated concrete with frozen-treated exhibits excellent mould-less forming capability underwater. • Flocculants-incorporated concrete with frozen-treated has less pollution to water quality.