Dimensions and Weights: Precast Concrete Box Sections

Dimensions and approximate weights of precast reinforced concrete box sections manufactured to ASTM C 789 and ASTM C 850. Box sections are specified where the waterway area required exceeds what a practical single round or elliptical section provides, or where a rectangular opening suits the hydraulic or structural situation better than a curved one.

C 789 and C 850 are not interchangeable

Both standards cover box sections in the same 3 ft to 12 ft span and rise range, and at a glance the two tables look similar. They are not the same product.

• ASTM C 789 is the general case, for boxes under normal cover.
• ASTM C 850 covers box sections with less than 2 ft of cover subject to highway loading — the shallow crossing where wheel loads reach the top slab almost directly, with little fill to distribute them.

The difference shows up immediately in the tables. A 3 ft × 2 ft section under C 789 has a 4 in. top slab and weighs about 600 lb/ft; the same nominal opening under C 850 has a 7 in. top slab and weighs about 830 lb/ft. Nearly 40 % more material, all of it in the slab that takes the wheel load. Specifying C 789 sections on a crossing with less than 2 ft of cover is a real design error and one that the identical span-and-rise designation makes easy to commit.

Reading the tables

Sections are listed by span (the horizontal clear dimension) and rise (the vertical clear dimension), both in feet, followed by three separate thicknesses — top slab, bottom slab and wall — then waterway area and approximate weight per foot of barrel length. The three thicknesses are given separately because they are genuinely different in the C 850 family, where the top slab is thickened against the wheel load while the walls and bottom slab follow the general requirement.

Note how the range works: for each span there is a series of rises, so a 10 ft span is available from 5 ft to 10 ft of rise. That is what makes box sections useful in constrained crossings — waterway area can be increased by widening the span while the rise stays fixed by the available clearance, which is not possible with any round section.

ASTM C 789, English units

ASTM C 850, English units

ASTM C 789, metric units

Practical notes

• Weight is a site constraint. A 12 ft × 12 ft C 850 section runs into thousands of pounds per foot of length; laying lengths, crane capacity and haul route all follow from that number rather than the other way round.
• Joints differ from pipe joints. Box sections are normally sealed with preformed flexible sealants under ASTM C 990 or with external sealing bands under ASTM C 877; a compression gasket in a circular groove has no equivalent on a rectangular section. See joints.
• End treatment. Precast flared end sections are manufactured for many box sizes; see flared end sections.
• Agency requirements govern. Box culverts on state routes are subject to the requirements of the Illinois Department of Transportation, which may be more restrictive than the product standard.

Weights are approximate and given for estimating, handling and transport planning. Availability of any given span, rise and cover class varies by plant; confirm with local producers. Current dimensional data is maintained by the American Concrete Pipe Association, and the governing designations are indexed at standards: box sections, joints, connectors and testing.