Thin polymer overlays to protect bridge decks - E470 explains
The E470 tollroad, a half belt route around the east side of the Denver metro area, has bridges between six and 21 years old. They were built using best practices (1) all the steel rebar that provides the tensile strength is epoxy-coated to inhibit corrosion (2) the bulk of the concrete of the deck is protected by a waterproofing membrane (3) the wearing surface is a 3in, 75mm concrete or asphalt overlay.
It's by the book.
But still they're getting cracks in their bridge deck. The cracks are the result of temperature changes day and night and season to season - sometimes called thermal cycling - as the deck expands in heat and contracts in the cold. Also vehicles on the bridges, especially heavy trucks cause movement that stresses the decks. The more cracking that's left untreated the more the likelihood of water and de-icing salts getting to the rebar (steel reinforcing bar.)
The rebar's epoxy coating gives it some protection, but it too can eventually be ground away by movement.
Neil Thomson, E470 director of engineering and maintenance says that despite the corrosion resistance provided by the coated rebar and the membrane under the wearing surface it pays them to act now to seal up the surfaces. Their bridge decks are designed to last 50 years, but only if they get preventative maintenance.
They first used a polymer topping or thin overlay on bridge decks in 2010 - two of the big ramps of the 4-level E470/I-25 north interchange.
Now they doing more bridge decks each construction season.
Cost is $6 to $7 per square foot.
PSam Consult calculations
So a 1,000ft by 30ft ramp, say, would cost about $180k to $210k. But that ramp would cost $4 million to replace, an annual cost at 50 years life of $80k/year.
If the $200k thin overlay adds 5 years life it is costing $40,000/year half the price of a new bridge. And if it provides 10 years extra life it is $20k/year or four times as cost effective as a new bridge. (NOTE; these are our own back of the envelop arithmetic, not to be hung on Geist or Thomson. We don't have to bother with the complications of a discount rate since the Fed's Magician Bearded Ben has ushered in an era of zero interest rates - editor)
Polymer overlays have been around for about 25 years and there are a number of different suppliers. Key qualities are strong adhesion to the concrete base underneath and elasticity so they can take thermal and load movement without breaking up like bare concrete or - to a lesser degree - asphalt.
As they've become more popular there have been innovations in the polymers with different suppliers touting improvements in their formulation versus others.
Thomson says on E470 they're using a Dow POLY-CARB brand. The polymer is a poly-urethane formulation, which Dow claims to have an exclusive on.
As well as protecting against ingress of water and salts this 3/8 inch thick (10mm) overlay contains a topping of fines gravel which improves tire-on-roadway traction to avert skidding.
The rougher texture also has the benefit of retaining de-icing materials longer than a more polished surface.
The overlay is therefore a safety measure as well as a deck life extender.
Another approach to bridge deck preservation and traction is to apply an asphalt overlay atop a waterproofing membrane. Installed in two layers this is usually 3 to 4 inches thick (75mm to 100mm).
Christopher Waszczuk, administrator of the Bureau of Turnpikes in New Hampshire told us the asphalt overlay is their preferred method of protecting concrete bridge decks. Many other road operators use this rather than thin polymer overlays.
A third alternative, the most expensive and disruptive but some will argue the best is a 4 inch, 100mm polyester concrete overlay - a portland cement concrete given some of the elasticity and strength of polymers.
Five years or 10?
The thin polymer overlays being used on E470 bridges are only warranted for five years, less than an asphalt overlay and much less than a polyester concrete. But the comparisons get complicated. The asphalt and poly concrete overlays involve more materials and more work - two layers and laying plus rolling on each in case of asphalt, and more time for setting up .
The thin polymer application requires shotblasting to remove any loose materials then spraying and application of the fines - much quicker so less disruptive to traffic than two alternatives. Cost is lower at about $6 to $7 per square foot.
Thin is cheaper to install than thick
Dow's polymer representative in the west Mike Geist says the thin polymer overlay is about a third to a fifth the cost to install of the competing thick overlays. And he says despite the limited five years warranty in fact most of their thin polymer overlays are usually good for 8 to 10 years and sometimes as much as 15. In their later years he says they do need close inspection and may need some patching.
Neil Thomson says a problem with thick (3 to 4 inch) overlays is they add to the deadweight load on the structural girders and columns and they soon reduce overhead clearance to signs and make transitions to ground-supported pavement more difficult. Then they call for milling down and removal of the old overlay, an additional expense and more time the deck has to be closed to traffic.
New ramps are usually 2-lanes wide plus some shoulder which allows the option of maintaining traffic in one lane for work in the other. Older roads have ramps only one lane wide.
Thomson says in addition to bridge decks they are applying thin polymer overlays to concrete pavement at toll points - to protect their investment in their in-pavement electromagnetic loops that do vehicle detection, tracking and classification.
"It is expensive and difficult to replace those loops," says Thomson.
The case for thin polymer overlays on on-ground concrete pavement is weaker. They usually have no steel rebar to protect, and they don't get subjected to the same amount of thermal and load movement as bridge deck.
Dow's Geist says his biggest customers for the thin polymer overlays are New Mexico and Utah highway departments.
But he likes toll roads.
He told us: "In my experience they tend to be more interested in preventive maintenance, more willing to look at new technology and more businesslike (than DOTs.) They are usually willing to invest now to save money later if you can make a strong case."
Dow thin polymer overlays: