Optimization of Planting Concrete Thickness and Grass Species for Roadbed Side Slopes

Planting concrete is a composite material used for erosion control on roadbed side slopes. However, excessive concrete thickness creates an unfavorable environment that prevents the survival of some grass species. This study aims to optimize the thickness and grass species of planting concrete. The stress scenarios of planting concrete, including pedestrian loads and frost heave stress, were analyzed. The maximum internal stress under pedestrian loads and the frost heave stress during freezing were determined using finite element analysis and frost heave tests, respectively. Nine groups of planting concrete specimens with different porosities and water–cement ratios were prepared and tested. The measured compressive and splitting tensile strengths were compared with the maximum stress of planting concrete to determine the optimal mix proportion. Using the optimal mix, planting concrete specimens with three thicknesses were prepared, and six common grass species were selected for planting experiments. Vegetation coverage, plant height, root length, root number, and root biomass were measured for each grass species at three thicknesses to determine the optimal thickness and grass species. The results show that the maximum tensile stress of planting concrete under pedestrian loads and frost heave stress is 0.86 MPa. The optimal porosity and water–cement ratio are determined to be 30% and 0.33, respectively. Ryegrass exhibits the highest vegetation coverage and plant height, thereby determining that ryegrass is the optimal grass species. Planting concrete of 4 cm thickness demonstrates the best root development, thereby determining that 4 cm is the optimal thickness. These findings provide a scientific basis for optimizing ecological slope protection with planting concrete.