The Louisiana Department of Transportation & Development (LaDOTD) oversees the planning, construction and maintenance of public transportation, including roadways, canals, port facilities, and more than 12,000 bridges.
Preventative maintenance is a top priority when it comes to keeping these bridges safe. Project engineers are always looking for a preservation method that’s efficient and effective while minimizing downtime.
The Vicksburg Bridge is a nearly 2-and-a-half-mile-long bridge built in 1973 that connects Madison Parish in Louisiana to Warren County in Mississippi. When it required repairs on its aging bridge deck, LaDOTD decided to use latex-modified concrete (LMC), or concrete mixed with latex solids, for restoring the surface.
As a proven method of bridge deck preservation for over 40 years, LMC effectively extends the lifespan of bridge decks far better than unmodified concrete while also reducing traffic disruptions and saving money on repairs in the long run.
Several factors can contribute to bridge deck deterioration, including freeze-thaw cycles, moisture exposure, and de-icing chemicals such as salts, which can penetrate and corrode the steel reinforcing bars within the concrete deck. Water carries oxygen and chloride ions from the salt through the concrete to the reinforcing steel, causing it to oxidize and creating pressure within the concrete.
Over time this creates cracks, potholes and surface degradation that can lead to an unsightly, uncomfortable, and occasionally an unsafe driving surface. Deck preservation helps prevent these issues before they compound and become a safety hazard, as it’s far more expensive to replace an entire bridge deck than to perform the necessary surface maintenance.
Nearing 50 years of age, the time was right for some preventative maintenance for the famous Vicksburg bridge.
“I would say the condition of the bridge deck was a four or five on a scale of one to ten,” says Tim McCallie, Engineering Technician and lead Inspector on the project. “There were several deck panels with concrete falling off and some exposed rebar in places on the deck and the shoulder, but the worst of it was in the truss section of the bridge. In isolated areas it was quite bad, but it needed repairs overall.”
“We always emphasize preventive maintenance,” says Jenny Fu, LaDOTD Bridge Administrator. “We monitor deck conditions, especially on major bridges like those in the Interstate National highway system. We don't want the deck to become an operational issue for the district or a safety issue for the public, so we always use a proactive approach to address deck issues.”
Modifying concrete with a latex, such as BASF’s STYROFAN® 1186, provides several beneficial properties to the concrete. It adds strength and durability by creating a latex elastic membrane throughout the concrete matrix that reduces voids and slows the corrosion of reinforcing steel, extending the lifespan of the bridge deck.
It also increases flexural strength, which reduces cracking from temperature fluctuations, and lowers the required water-to-cement ratio, which increases strength and durability.
“We’ve seen LMC performing well in other projects, and we received a lot of input from other States as well,” says Fu. “There are many different options for deck overlays, but our state has been using LMC for projects and it seems to be performing well, and this happened to be a good application for it.”
Preparing for pouring
The Vicksburg Bridge project was one of the largest fast-track hydrodemolition jobs administered by the LaDOTD, with approximately 65,000 square yards of total surface restoration with an inlay of a minimum of one-and-a-half inches.
Before application of LMC, the surface must be adequately prepared. Hydrodemolition is a controlled process that removes layers of damaged concrete using high-pressure water jets. It prepares the surface of the bridge deck by selectively removing deteriorated concrete, resulting in a roughened surface that allows for a stronger bond between the latex and the deck surface.
“This was my first LMC job and first experience with hydrodemolition, and I can say it went very smoothly,” says Douglas Fleming, Project Engineer at District 5. “We tended to be a little bit deeper than the specifications, but it's better to be a little deeper than too shallow, and overall I was very pleased with the milling and hydrodemolition for surface prep.”
A few windy days caused delays in pouring and high moisture content in the supplier’s sand required some adjustments. However, once those minor issues were addressed, only the bugs put a damper in the project.
“The only real challenge we had was the mayflies,” said Fleming. “But at the end of the day, we had a good team and we didn't have problems with the materials, so overall I was very happy.”
Overall, the district and the department were both happy with the use of LMC for the project and the end results.
“Speed of application played a huge role in this project, because anytime you close a lane on a major bridge you want to reduce the impact to traffic as much as possible,” McCallie noted. “With the way the hydrodemolition and LMC application progressed, we were able to finish in a relatively short amount of time and with as little disruption to traffic as possible. It also seemed like we were getting a consistent mix, which was really workable too, so I'm very pleased with it.”