How to prepare bridges to survive winter
Little lasts forever, and concrete bridges under heavy use are no exception. But properly protecting the surface and structure of bridges can add ears, possibly decades, to their useful lives.
Proper maintenance is also an investment in reduced future repair costs. According to one estimate, every dollar spent on bridge maintenance today, produces a savings of $3 in future, more drastic, repairs.
As hard as it may be to find maintenance money each year, over time it costs less to keep a bridge in good repair than to rehabilitate or replace it.
Finding the money and balancing new construction, repair work and preventive maintenance is a constant challenge for every government agency responsible for roads and bridges.
Sodium chloride is sometimes called the friendly enemy. It protects winter travelers but shortens the life of steel-reinforced concrete bridges. The path of deterioration is simple: Water enters the pores in concrete and freezes, breaking off small pieces.
This process, called scaling, is repeated, leading to cracking and spalling.
Eventually, water reaches the steel reinforcement bars. When the water carries chloride, the steel corrodes and expands at a greatly accelerated rate.
Chloride also gets into the non-traffic bearing surfaces when contaminated water splashes or runs off the deck onto the superstructure. Jersey barriers, columns, bents, girders, abutments, guardrails and other structural elements can suffer from chloride-induced deterioration.
At the beginning of “Huck Finn,” the title character states: “It’s better to stay out than to get out.” That is certainly true of chloride intrusion into concrete.
Once in, desalinization treatment or removal of the contaminated concrete, both of which are expensive, are the only ways to get rid of the chlorides. Cathodic protection can halt corrosion, but it is even more costly.
However, according to one study that measured percent of chloride intrusion two inches below the surface, proper maintenance can reduce water and chloride intrusion into concrete by 86 percent or better.
The benefits of a consistent maintenance program are most dramatic when the structure is new. But existing structures benefit as well, even if the steel bar is already corroding. If salt cannot be removed from around the steel, maintaining the lowest possible moisture levels can prevent water-borne chlorides from reaching the steel.
Even properly designed and installed, new concrete should be protected before being exposed to traffic and de-icing salts.
Silane sealers, by far the most widely used products for protecting new and existing bridge decks, penetrate and chemically attach to the concrete, to reduce water and chloride intrusion.
Only as the concrete wears away do the sealers lose effectiveness. For example, wheel ruts need more frequent re-treating than does the rest of the surface.
Silane sealers are available in three forms: organic solvent, water based and 100-percent reactible (without any carrier) versions. Studies using NCHRP 244 and the Alberta tests show all versions provide the same basic protection.
Penetrating sealers are good for traffic-bearing surfaces because they become part of the concrete. But for non-traffic bearing surfaces, more impermeable products, that generally cost less, last longer and are more pleasing to the eye, are available.
DOTs that prefer to work with a minimum number of products often choose to use Silane sealers on traffic and non-traffic surfaces. But there are additional choices for surfaces free from tire abrasion.
Silane is a film-forming sealer that is slightly less expensive than Silane sealers. It stays more on the surface of concrete and will wear off quickly if used on traffic surfaces.
Both Silanes and Siloxanes reduce water and chloride intrusion, but neither change the appearance of concrete, which most people consider unsightly. Consequently, some departments opt for pigmented epoxy sealers that have low permeability and are extremely durable – a big advantage on bridges.
They are available in any color, although most road departments prefer whites and grays. One drawback is that epoxies can chalk and discolor with UV exposure.
Water-dispersed epoxies can be applied on green concrete just like a cure-and-seal wax product. But unlike wax seals, epoxy will remain in place as an effective screener of water and water-borne chlorides.
Solvent-based epoxies can be applied in colder weather but not on green concrete. Like all solvent-based products, they raise VOC compliance issues.
High-build latex modified cements are rubbery liquids applied in a relatively thick coating. Because of the latex additive, they are much more effective at reducing water and chloride intrusion than untreated concrete.
These cementitious materials, the most aesthetic products for coating non-traffic surfaces, dry to a uniform textured surface and are available in any color imaginable. While these are the least expensive and easiest to install, they can be worn away relatively easily by the whipping action of water and road debris.
Cracked when new
No one likes to hear this about a brand-new, multi-million-dollar bridge, but concrete may begin to crack during curing or soon after construction. An example is map cracking caused by rapid-drying shrinkage during cure.
Structural cracks must be repaired to return the element to the original design intent. The most common materials used for this purpose are structural epoxy adhesives, including pastes and low-viscosity injection resins.
Non-structural cracks simply need to be closed. One of the best products currently around for this purpose is High Molecular Weight Methylacrylate (HMWM), which not only seals cracks but bonds the concrete back together. HMWM can be accelerated and used whenever concrete is not frozen. Overlays return the bridge deck to a uniform driving surface and improve traction.
Cementitious latex modified overlays are relatively impervious, have a similar coefficient of thermal expansion to concrete and are less expensive than other overlays.
But because they are water-based, they have a long curing time and are less resistant to chemical attack.
Stress-relieving epoxy overlays are impervious, impact-resistant and withstand gas, oil, brake fluids and other chemicals that might spill on them. Epoxy overlays are often used where high volumes of vehicle traffic stop. Toll booths are a good example.
They are also favored when the curing time for cementitious overlays is unacceptable.
Polysulfide epoxy overlay can bond to damp concrete, cure at low temperatures, have superior flexibility and show better chemical resistance than other epoxies, yet they do cost slightly more.
Cementitious compounds are starting to emerge as the most durable patch materials. Formulated differently than overlays, holes patched with this material can be reopened to traffic in a few hours.
Some epoxy patches have had disbonding problems, but the right formulations have proven durable. Asphalt patches should not be used on concrete because the technology is so different.
Potential challengers are magnesium phosphate-based products. But unlike epoxy patches, they must be confined on all sides to avoid disbondment. They cure rapidly even in cold weather.
One of the best ways to protect the underside of a bridge is to keep it free from runoff water. This can be done with sealed expansion joints and a good drainage system.
It also makes sense to check the joint assemblages on a regular basis to make sure the seals are intact, and nothing is jammed in the works.
Still, when faced with the choice of allocating funds for a badly needed new road or for maintenance and repair, the new project often wins. But, the price for deferring maintenance, at least with bridges, is major renovation or replacement much sooner and at higher overall costs.