Concrete box culverts are available with spans varying from 6 to 16 feet and rise to vary from 4 to 14 feet. Standard precast concrete box culverts are typically fabricated in 6-foot sections; however larger boxes are fabricated in 4-foot sections to reduce section weight. The designs utilize concrete strengths between 5 and 6 ksi and are suitable for fill heights ranging from less than 2 feet to a maximum of 25 feet. Box culverts outside of the standard size ranges must be custom designed. Each culvert size has three or four classes. Each class has specified wall and slab thicknesses, reinforcement areas, concrete strength, and fill.