Installed quickly with less labor, maintenance-free precast concrete box culvert bridges deliver a long service life. These eight box culvert designs demonstrate why precast is the best choice for a variety of site conditions.
By Gary K. Munkelt, P.E.
Maintenance and replacement of bridges over small streams are ongoing concerns for transportation agencies responsible for their upkeep. In recent years, however, infrastructure funding has not kept up with maintenance needs, and when bridge maintenance is not performed, the structures fall into disrepair.
A decision must be made as to when a bridge needs to be replaced. Should the bridge be replaced in kind, or are there better alternatives? The decision will be based on considerations such as cost, complexity of design, available materials and amount of time the highway is closed to traffic. Future maintenance should always be a part of the decision-making process.
Maintenance-free service and ASTM design assurance
Several systems are available for consideration before replacing a bridge. One of the alternative bridge systems to consider is precast concrete box culverts. They offer a range of sizes and configurations to fit specific site conditions. When properly installed, a precast concrete bridge replacement can provide maintenance-free service for many years. There are concrete structures in operation today that are 100 years old, and many of these structures don’t require an annual maintenance budget. The concrete sits in place year after year and does its job.
Design of box culverts is not difficult and is normally performed by the precast manufacturer. Standards such as ASTM C1577–11a1 provide guidelines to ensure that the product design is adequate. By producing the same product repeatedly, the precast concrete industry can offer reliable, high-quality products. Precast manufacturers are located in most areas of the country, making the product readily available. Competition between producers helps to promote economy of cost.
Advantages of precast bridges over CIP installations
Replacing a bridge over a stream using a conventional cast-in-place (CIP) installation can close a road for 10 to 12 months due to the time required for curing concrete on site. For a typical CIP job, footings must be installed first. After they cure, the pedestals formwork can be made, followed by another concrete pour. After that, more curing time is required to form and place the concrete riding surface. Waiting three to four weeks between pours significantly extends the time required to finish a project.
Less time and labor: A precast concrete box culvert can be installed much faster than CIP construction, because the three- to four-week curing time is spent at the fabricator’s plant. Precast box culverts are often manufactured before a project is started. In many cases, preparation at the site takes less than one week. Installation of the finished precast box culverts is complete in a matter of days. There are box culvert installations on county roads where the road is closed for only two to three weeks. Fast installations provide an added advantage in cost savings, because labor hours are kept at a minimum.
Design agencies should consult with precasters in the project area to determine their capabilities. Using product sizes that are standard for a producer will normally enable them to provide more efficient pricing for quotes.
Design flexibility: The box culvert concept has been modified over the years to solve many job-site problems. Standard rectangular boxes are produced in many combinations of height and width. ASTM C1577 provides a table for sizes ranging from 3 ft wide by 2 ft high to 12 ft wide by 12 ft high. Other sizes are available and are limited only by the ability of local precasters to adapt their forms.
Eight designs meet a variety of site conditions
Many precast concrete box culvert configurations may be used to span crossings that vary from narrow rivulets to fairly wide streams. Advantages of each these structures are usually unique to the job site, installation contractor or precast manufacturer. Here are eight precast box culvert designs:
1. Single box culvert: One structure, 3 ft to 12 ft wide with wing walls, is installed (end-to-end as needed for road width) above narrow streams. The height of the box culvert is dependent on amount of flow and site conditions.
2. Double box culvert: Two 12-ft-wide box culverts are used together under highways that span medium-wide streams.
3. Triple box culvert: Three 12-ft-wide box culverts are set side by side for larger streams. The length of the culverts depends on the width of the highway.
4. Bottom slab trough: Some designers require a modified bottom slab to provide a small trough for low flows during periods of drought. This modification consists of a second pour of concrete that can be formed to any desired shape.
5. Clamshell design: An innovative method of reducing the weight of each component is to utilize a “clamshell” design, where the box is made in two pieces. The advantage to the contractor is that product weight is cut in half, requiring lighter, less-expensive equipment during installation.
6. U-shaped design: Some manufacturers make a box by using a “U” shaped piece and putting a flat slab on top. Or they use a flat slab for the bottom and set a U-shaped piece upside-down on the top.
7. Three-sided arch design: This is an adaptation of the box culvert concept that has no bottom slab. It is popular where there are environmental issues or a desire to avoid disturbance to a stream bed. This structure can sit on a CIP footing situated on opposite sides of a stream. The three-sided arch spans the stream without adversely affecting the stream’s natural state.
8. Wildlife crossings: Single, clamshell, U-shaped or three-sided arch box culvert designs may be used to provide a safe crossing for wildlife under roadways.
1 ASTM C1577-11a, “Standard Specification for Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains, and Sewers Designed According to AASHTO LFRD”
Gary Munkelt, P.E., is a consulting engineer with Gary K. Munkelt & Associates in North Wales, Pa. Contact him at [email protected].