While there are several useful material options available for bridge overlays, one of the longest-running is latex-modified concrete (LMC).
The Virginia Department of Transportation (VDOT) is one of the first DOTs to implement the use of LMC overlays in bridge preservation projects. VDOT has been using LMC overlays for more than half a century in a wide variety of bridge repair and preservation projects.
The first noted use of LMC in Virginia was in 1969 on a bridge deck with insufficient cover concrete, according to the Virginia Transportation Research Council (VTRC) (Balakumaran, S., Weyers, R. E., Brown, M. C. (2017), Performance of Bridge Deck Overlays in Virginia: Phase I: State of Overlays, VTCR Report 17-R17, Charlottesville, VA). LMC continues to be a regularly used tool in the VDOT toolbox to this day. It will be one of the materials used to assist the state in its five-year, $535-million bridge deck preservation program funded by the Infrastructure Investment and Jobs Act (IIJA).
The State’s plan for federal bridge funding focuses on restoration of bridges on interstates and high-volume primaries with 75 percent of the funding directed towards preserving bridges rated in "fair" or better condition, while the remaining 25 percent is earmarked to go towards "poor" bridges.
"Our focus is on preserving as many of the decks as we can, before they deteriorate to a stage where they cannot be preserved," says Adam Matteo, Assistant State Bridge Engineer, VDOT. "We want to keep the decks in service as long as we can at an acceptable level of service. We're at a stage now where our statewide effort is a preservation-first strategy. We're going to preserve as many of our structures as possible and rigid overlays are a big part of that. We're focusing first on interstate highways, and we'll be moving into primaries and secondaries. But right now, if you drive the interstates of Virginia, you will find many concrete overlays that have been successfully placed, and that's a point of emphasis, and a big part of our Infrastructure Investment and Jobs Act funding is going to those overlays."
LMC overlays have been one of the most common types of overlays used across the state due to the material’s many advantages in the field, according to the VDOT report: In an extensive field study of 16 different overlay systems, it was concluded that LMC overlays were the best overall performing overlay type, since they showed the second-lowest permeability and chloride diffusion coefficients and the lowest chloride concentration after 10 years of service (Sprinkel, 2009).
In a study in which curling, delaminations, and tensile bond strengths were compared for different high-performance overlays, LMC overlays were recommended as good performing overlays (Ray et al., 2008). Celik Ozyildirim, Principal Research Scientist at the VTRC, observed in a four-year study (1996) that LMC overlays had very low permeability, low chloride ingress, satisfactory compressive strength, and higher flexural strength.
According to the report, the expected service life of LMC rigid overlays is between 22 to 26 years. However, in interviews with personnel from VDOT District Bridge Office, observed LMC overlay service life has ranged from 15 to 50 years (p. 34).
The biggest challenge of using LMC isn’t considered to be the material itself, but ensuring the contractor hired to complete the project uses the material correctly. "Our biggest issue is workmanship. We have to do it right to be successful," says Ozyildirim.
Matteo says his favorite feature of LMC is the material’s durability and ability to preserve an existing deck.
"When we do an overlay in conjunction with a joint elimination, or link slabs or deck extensions, or both, then we've created something that can last a very long time," he says. "It has low permeability, which keeps the water and salts from permeating into the deck. It preserves the existing deck. It's something that we can come back to in three decades and do another overlay. So, as long as we take care of the joints at the same time, we've put ourselves on a path to sustainability, and at an acceptable level of service."
Rapid-setting LMC has also become a regular tool for VDOT in its bridge rehabilitation program, since its first use in Virginia in 1997/98.
"We evaluated those, and the results were good, and we've continued to move forward using more and more rapid setting overlays, particularly when we want to minimize cure times because of traffic," explains Michael Sprinkel, Associate Director at VTRC. "It’s been comparable to regular LMC (with Type I/II Cement), as long as the workmanship is good. On I-64, we’ve got two 5,000-square-yard overlays that are eastbound and westbound. One of those was done on weekends, and one was done with a full-blown lane closure. They were done in 2006. And while there are patches in these overlays, they're generally performing very well."
Sustainability is key
While VDOT has found rehabilitation results will vary from bridge to bridge, in general, Matteo says they can preserve a bridge – eliminate the joints, paint the superstructure, repair the substructure, and do a rigid overlay – for about 15 to 20 percent of the cost of replacement.
"We feel that we can get at least 30 years out of that work. That's an important thing because we have an aging inventory. The average age of bridges in Virginia is approaching 53 years right now, and most of those bridges had a 50-year anticipated service life when they were built. We can't replace all those bridges right now; it’s way too much money. Nobody has the billions of dollars to do that. So, we need to sustain what we have," Matteo says.
As one of the original providers of latex for LMC products, VDOT has been working with BASF for decades.
"We’ve been involved with BASF since the beginning," Sprinkel says. "They tend to be on committees that deal with bridge preservation and LMC overlays. And so, we're involved in the overlay preservation committees, and you naturally work with them. We all have the same goal and that's to produce overlays that last longer and longer."
Sprinkel believes that LMC overlays will continue to have a strong role in bridge preservation going forward. "As long as the contractors can successfully produce latex overlays for us, and we're getting a good product, and nothing else comes along that we decide is better, I think the future's bright," he says.
To learn more about BASF products for LMC overlays, click here.