Concrete box culverts are extremely popular due to its availability in a wide range of sizes and its versatility with many uses. In addition to the conventional conveyance of stormwater or wastewater under roadways or railways, box structures are also used as pedestrian or wildlife crossings, utility corridors, or stormwater retention. Like circular concrete pipe, the availability of factory-made precast concrete box sections has simplified the process of constructing box culverts over traditional cast-in-place construction methods. Precast concrete box sections also offer a consistently high-quality product to contractors and owners.
Due to the infinite number of cross-sectional dimensions and structural reinforcing details for box, the precast concrete industry has worked closely with State and Provincial highway departments in both the United States and Canada for the past 40 years to standardize the structural designs and develop manufacturing standards for precast concrete box sections. Current precast box standards with additional features and special fittings such as bends, and a variety of end treatments allow design engineers to customize a precast box structure to any site condition with very little to no structural design calculations required. Other reference material available to design engineers include the CHBDC Section 7 Buried Structures, and the Ontario Concrete Pipe Association (OCPA) Precast Box & Culvert Guideline.
In Ontario, the Ministry of Transportation (MTO) and Ontario Provincial Standards (OPS) collaborated with the producer members of the OCPA to develop OPSS 1821 for the design and manufacturing of precast concrete box, and OPSS 422 for the installation. OPSS 1821 lists 9 standard box sizes that should suit most hydraulic requirements and provides reinforcing steel area tables for a range of burial depths based on the Canadian Highway Bridge Design Code loads, including the CL-625-ONT design truck live load. This standard also addresses product quality concerns by requiring every manufacturing facility be audited by a recognized third-party plant certification program.
Similarly, parts of Western Canada and the US specify ASTM C1433 or C1577 which provide standard designs that meet various design codes and loading conditions. With ASTM C1675 covering the installation requirements of precast box sections.
The development of these design and manufacturing standards in OPS and ASTM, has prompted many precast concrete producers to invest in equipment for common dimensions to mass produce box sections using dry-cast, or zero slump, concrete combined with structural welded wire reinforcing. This has resulted in a more efficient production process and shorter delivery lead times which ultimately leads to faster construction
projects, cost savings, and less inconvenience to the general public.
For these reasons, it is strongly recommended that design engineers specify the standard precast box sizes available in their local market. Highway departments and municipalities would also benefit from using standard precast box sizes from the perspective of lower initial construction costs, ease of maintenance, and less complicated extensions of box culverts in the future.
By: Paul Imm, P.Eng. Director of Engineering, Forterra Pipe & Precast -Canada
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