Iowa has many small bridges in need of significant repair or replacement, but no one wants to see a “road closed” sign on the road ahead. Long-term road closures can be common on traditional bridge projects where an existing bridge is replaced with a new bridge. There is an innovative method that the Iowa Department of Transportation has been using for small bridge replacements which uses culverts constructed inside aging bridges. This method allows traffic to keep moving and eliminates the need for maintenance for decades to come.
Iowa DOT engineers are looking at aging bridges, mostly on rural two-lane roads, to see if they can be replaced with a box culvert that fits under the existing bridge. For bridges that meet the criteria, a concrete culvert is cast in place underneath the existing bridge or the culvert can be precast and slid into position underneath the existing bridge. When possible, precast concrete box culverts are cast at manufacturing facilities and shipped to the construction site, saving some on-site work time for the contractor.
Once the culvert is built or placed, soil is filled near the ends. Holes are drilled in the existing bridge deck and a soupy concrete mixture, known as flowable mortar, is allowed to flow in, filling the space between the top of the new culvert and the existing bridge deck. The soil near the ends of the culvert acts as a dam of sorts to contain the flowable mortar until it sets up and hardens.
John Colle, a design technician in the Office of Bridges and Structures who works on designing bridge replacement projects using this flowable mortar method, said, “Typically replacing an old bridge with a culvert using flowable mortar allows for traffic to continue to move over the road, although often only on one lane at a time. Even though we’ll need to close down one lane, that brief inconvenience for the traveler is still better than a long-distance detour.”
Over the last seven years, the Iowa DOT has used this construction method to replace 15 bridges with culverts. Jim Nelson, an engineer in the Office of Bridges & Structures, said, “Increased safety is another reason we would use a culvert to replace on old bridge. The bridge barrier rail and bridge approach guardrail can then be removed, eliminating obstacles that can be struck. If a car would go off the road, they would have the room to get back on the road without hitting anything.”