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Assuring Structural Integrity and Reliability of Precast Box Culverts

Precast box culverts have gained in popularity in recent years for use as underpasses, tunnels, subways, bridges, stream culverts, material handling, storage, watertight tanks and more. Available in a variety of standard sizes as well as custom designs, precast box culverts can be built with features meeting the exact needs of any project: toe walls, manhole openings, headwalls, wingwalls, pipe openings, keyed ends, watertight joints and more. While precast box culverts promise an extended service life, how can an engineer/specifier assure that precast box culverts are in fact manufactured to the exacting standards that assure service longevity? How can the engineer/specifier mitigate any risk of premature failure and its associated liability? To answer these questions, it is important to consider several factors that affect the quality and performance of a precast box culvert.

Engineering Design
Design of a box culvert by a qualified structural engineer is critical to its performance and avoidance of premature failure. While a box culvert appears to be a rather simple four-sided structure (three sides in special applications), a box culvert is subjected to complex loads in its typical application. These loads include:

  • Lateral earth loads from soil and hydrostatic loads,
  • Vertical loads from the cover soil and live loads above,
  • Surcharge loads from nearby impact loads, and
  • Seismic loads where applicable.

Because the rectangular shape of a box culvert lacks inherent rigidity, specification of sufficient circumferential reinforcement areas is critical to maintaining structural integrity under the application of these normal loads. Other critical design factors include the
compressive strength of the concrete, use of sufficient structural steel reinforcement, and proper application of joint seals. Furthermore, the box culvert design must also specify a maximum depth of cover to assure that loading remains within design parameters. To assure the integrity of a box culvert design, it must conform to the applicable ASTM and AASHTO standards.

Manufacturing and Quality Assurance
It is important that box culverts be manufactured with high degrees of quality and uniformity. This requires the use of uniform, documented, controlled processes throughout manufacturing. These processes cover every aspect of the manufacturing process:

▪ selection and testing of specified raw materials,
▪ preparation and inspection of concrete forms,
▪ fabrication of rebar structures for reinforcement,
▪ concrete casting operations, and
▪ final product testing and inspection.

Certification of box culvert manufacturers by the National Precast Concrete Association (NPCA), based on periodic unannounced inspections by an independent professional engineering firm, assures that controlled processes are used routinely throughout manufacturing and that the manufacturer’s quality program meets NPCA’s rigid requirements. NPCA certification also assures that qualified manufacturing workers perform critical manufacturing and testing operations.

NPCA’s quality requirements focus on building quality into precast components rather than placing primary emphasis on testing to identify defects. In the absence of controlled manufacturing processes, there are potential defects that are invisible and that simply cannot be discovered by post inspection methods. Some NPCA certified plants also
uniquely identify every precast component, thereby providing complete traceability to manufacturing and quality control data captured throughout the manufacturing process. Manufacturers providing this documented pedigree of every precast box culvert component afford the customer enhanced assurance that product quality meets or exceeds expectations.

The precast product quality assured by NPCA registration of the manufacturer has led many state and federal organizations to require that all precast products must come from an NPCA certified plant. Three-fifths of state departments of transportation, among the heaviest users of precast products, including box culverts, now require NPCA certification. In addition, the
U.S. Army Corps of Engineers now specifies that all precast concrete components in drainage structures must come from NPCA certified plants.

Table 1 summarizes the factors that have a significant impact on achieving the desired structural integrity and reliability of precast box culverts, as well as the methods for successfully accomplishing each factor.
Assured pedigree of each box culvert component is achieved by unique identification of each manufactured component and traceability to manufacturing and testing data. Pictured to the right is one example of assured pedigree by having a barcode placed on every precast box culvert component that links back to concrete testing data all the way to invoice number and delivery information. This is sometimes called a "Birth Certificate" in precast concrete manufacturing circles.
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