چكيده به لاتين
The abutment is a component of the bridge substructure that is placed to stabilize and support the ground behind the structure, on both the upstream and downstream sides. One of the most significant factors leading to the failure of this element is scour, which occurs as a result of bed erosion by flowing water and the transport of material detached from it. This phenomenon ultimately leads to the collapse of bridges, resulting in loss of life and economic damage. Previous studies on scour have presented various methods, each with its strengths and weaknesses, often based on flow dynamics such as suction and jets, while less attention has been paid to the materials that make up the structure, such as concrete. Due to the negligible effect of ordinary concrete on scour, the impact of concrete composition has not been investigated in these studies, often disregarding the roughness of concrete and consistently using non-concrete materials. By adding hydrophobic carbon powder to the concrete mix, which can increase surface roughness at micro and nano scales, the contact angle between water droplets and the surface is enhanced, leading to improved hydrophobicity of the concrete. Therefore, this research investigates the effect of this material’s hydrophobicity in concrete and its impact on increasing surface roughness on local scour. In this experiment, carbon powder was added in amounts of zero, four, six, and eight percent by weight of sand, and tests were conducted to observe the hydrophobic effect of each mix design. It was found that with increasing amounts of carbon, the contact angle increased and the hydrophobicity of the samples improved. After constructing the abutment, the scour depth around each abutment was examined in a channel measuring 9.80 meters in length, 0.42 meters in width, and 0.6 meters in depth. Results indicate that with increasing carbon content, scour pit depth decreases by 13.8%, 32.2%, and 45.97%, respectively. Additionally, the equilibrium time increased from 24 hours in the control sample to 50 hours in all hydrophobic samples. Upstream of the abutment, due to the interaction of flow with the rough hydrophobic surface, flow is dissipated, and flow intensity decreases downward, subsequently reducing initial vortices. This change is also evident in the profiles of turbulence intensity, where greater order is established in turbulence levels upstream, and reductions are observed in areas related to horseshoe vortices and wake vortices. Furthermore, Reynolds stresses significantly decrease with increasing percentages of carbon powder. In samples containing carbon powder, Reynolds stress changes from a convex pattern to a downward trend, and in some angles, the stress sign does not change from negative to positive, indicating the influence of carbon powder on flow patterns and stress behavior near the bed.
