7 Things to Consider in Box Culvert Design
By Alan Siebenthaler
HK Marketing/Territory Manager
Deciding on precast box culverts for your next project is just the first step in the decision-making process. Here’s a breakdown of the different choices in joints and sealants, fastening devices, and testing procedures, among other considerations for your application.
1. Different joints mean different gaskets
Tongue and groove joints
This is the most common type of joint. Very few of these are compatible with gaskets, and will need a butyl sealant instead. Sealants work if they are applied to the joint properly, but they are often not as resilient or as watertight as gaskets. Typical applications are where the culvert is open at both ends and hydrostatic water tightness is not crucial. For projects where erosion needs to be regulated, an exterior joint wrap in addition to the butyl sealant often provides the best solution.
Single offset joint
Single offset joints are easy to manufacture and install on the jobsite. In most cases, a pre-lubricated gasket like Hamilton Kent’s Tylox® SuperSeal™ (TSS) gasket is utilized with this joint. However, a wedge type gasket can also be installed. In either case, the resulting homed sections are consistent and watertight.
2. The recommended psi capability
ASTM International – an international standards organization – recommends that your box culverts should be able to handle water pressure up to 5 psi under ASTM 1677: Standard specification for joints for concrete box, using rubber gaskets.
This is the minimum requirement, but, depending on your application, you may need to reinforce the joints with another type of seal. If you’re using butyl rope with your tongue and groove joint, you might need to add external wrap. With single offset joints, gaskets such as our Tylox® SuperSeal™ Box Culvert Gasket have been tested to handle up to 13 psi. Examples of the types of projects that might need to handle higher psi are those with external pressure from high water tables, or internal force because of the conduit being pressurized.
3. Do I need restraining devices for my project?
Generally speaking, no restraints are needed to connect box sections. We are aware of some projects that require these devices in written specifications, such as those in New York City or underneath railroad tracks. You might consider including restraining devices to keep the box sections from moving in projects where there is significant internal pressure or external vibration.
If your project does require restraints, there are a few options: One option is to run cables through holes in each box section to connect the entire culvert. The cables are post tensioned to pull all of the sections together and lock them tight. This is not an easy task as all of the holes have to line up perfectly with each other during installation.
The second option is using bolts placed in pre-formed pockets in the culvert walls spanning between two sections of the culvert and tightening them to lock the sections together.
4. If your project needs multiple box sizes
Though some projects specify a variety of box sizes, we recommend using as few box dimensions as possible in order to simplify the production and testing at the precast concrete plant. Unless plants have invested in adjustable formwork, the purchase of additional forming equipment to produce a broader range of sizes is a big investment. Also, scheduling the production of a variety of sizes can present challenges. Furthermore, job site delivery, staging and installation are simplified with fewer sizes to handle.
5. How to put a bend in your culvert
If a culvert requires a bend, contractors will often leave a small opening in one side of the joint in a few sections. However, caution needs to be taken in this process, as opening the joint too much (greater than ½”) increases the likelihood of a joint leak.
To avoid this, we recommend using boxes that have a skew built into them. Producers are capable of creating a skew when they pour the concrete, making joint integrity in the field much higher.
One way to simplify installation of bent or skewed boxes is by working with the producer to map the sequence of each box at the job site, then mark or number the boxes for easy identification and proper installation.
6. How should box sections be installed at the job site
We recommend requiring ASTM C1675 for the proper box section installation procedures. View our brief installation video for additional guidance. If a contractor is unfamiliar with box section installation, a representative from the design engineering firm should be present to ensure procedures are followed correctly, at least at the start of the project. If gasketed box sections are being used, the box and/or gasket manufacturer can also have a representative talk with the installation crew about proper procedures as they relate to the joints and gaskets.
7. How joints should be tested
Our recommendation for testing the joints on gasketed boxes is to perform an external joint test, an ASTM recognized testing method.
Gasketed box projects should be specified with ASTM C1677: Specification for joints for concrete box, using rubber gaskets and ASTM C1619: Standard specification for elastomeric seals for joining concrete structures. These include the proof of design testing that producers should complete prior to sending box sections to a job site.
We also stress using extreme caution when performing off-center alignment testing (from section 9.1.3) as this can be dangerous to personnel on or near the box sections if the joint were to fail suddenly. Joints can also be field tested using products made by companies like Plug-it Products and Cherne Industries Inc. These companies have various products that can isolate joints and test them via vacuum or hydrostatic pressure. These devices are expensive, and are not adjustable to different box sizes, so this could be a big investment for whoever is designated to provide the testing.
With all of these choices in mind, you can take your projects in numerous directions, while making sure that your culverts remain watertight.