In 2013, our nation’s bridges were rated and we got a C+ on our scorecard! With the majority of our bridges approaching the ripe age of 42 years, United States is officially dealing with issues caused by an aging road infrastructure. We have over 607,380 bridges spread across the nation’s 102 largest metropolitan areas (where most of the traffic takes place with commuters and freight vehicles) and one out of nine bridges has been declared structurally deficient. The Federal Highway Administration has its work cut out for itself! The current annual budget of $12.8 billion won’t address the existing backlog by 2028! An additional $8 billion a year is needed to repair them appropriately, an additional budget that states and counties need to find to speed this process.
So what are the main issues with our aging bridges? They are either deficient, structurally deficient and/or functionally obsolete. So what’s does this mean? Well, a bridge that is considered structurally deficient will require significant maintenance, repair or even replacement and annual inspections. A deficient bridge is both structurally deficient and functionally obsolete.
A functionally obsolete bridge does not meet the current standards like load carrying capacity and width of lanes.
States that top the list of structural deficient bridges (with over 20%) are Pennsylvania followed by Iowa and Oklahoma. However, when you combined the number of bridges that have structural deficiency and are functionally obsolete, the District of Columbia leads all 50 states with 77%!
The first signs of aging bridges are potholes, cracks, missing concrete chunks, posted signs with load restrictions and can extend in extreme cases to the closure of the bridge. The right bridge lane usually shows signs of damage faster that the left lane used for accelerating and decelerating, the shoulder lanes. To the naked eye, spalls or chips of concrete appear, they are usually caused by thermal strain due to rapid freeze thaw weathering. Dangerous explosive spalling can occur of refractory concrete and render the structure not usable as a result. Delamination or blisters is another common problem. When excess water and air in the mix are trapped under the surface mortar, voids are created along with weakened areas just below the surface that will come apart in the future. This problem stems from starting the finishing phase before the bleeding process is complete or when concrete is placed on cold substrates when ground temperatures are below 40F.
Corrosion of steel reinforcements in concrete
This occurs when chloride ion that is found in sea water, ice melt water and deicer salts (made of sodium chloride and calcium chloride) starts corroding the concrete-steel contact surface. This can happen through drying shrinkage, cracking or the concrete’s pore water. The higher the temperature, the faster the corrosion! So regions like the Florida coast encounter constant and rapid degradation due to its warm temperatures and exposure to sea water. The corrosion produces rust which causes internal pressure and creates cracks, potholes…
Rust must be removed from the exposed reinforcing steel before repairing or the corrosion will continue under repaired patches. A layer of corrosion inhibitor must also be applied for the repair to last.
Unprotected concrete elements
Some bridges were built using bare concrete elements with black steel. Others used reinforced steel without epoxy or galvanized coating or a polymer concrete overlay. Others were made with a low slump dense concrete that results in low permeability concrete and allows chloride ions to do corrode the structure.
Deck patching: for temporary partial depth repairs, bitumous concrete, quick-set hydraulic concrete, polymer mortar can be used. For full-depth patch repairs, Portland cement is the choice material.
In this method, repairs are done without removal of the chloride contaminated concrete.
Material used for these temporary repair methods are: latex-modified concrete (LMC), low slump dense concrete (LSDC) and hot mix asphaltic concrete with a preformed membrane.
Patching with Cast-in-Place PCC for Superstructure and Substructure:
This method requires the removal of loose concrete identified by sounding with a hammer. Formwork may be needed, which excludes the application of bonding agents but requires keeping forms cool by providing a cover during the curing phase.
Patching with shotcrete:
Removal of lose concrete is required. Repairs on superstructure and substructures tend to use dry-mix mortar. No bonding agent should be used. A single layer of shotcrete should be applied to avoid cold layers. A bottom up application technique should be followed to fill vertical cavities. Overhead surface may require multiple layers, 1 to 2 in. thick so that sagging doesn’t occur. Moist curing for 7 days should be provided using a cover or sprinkling system.
Encasement and Jacketing:
When column and piers have greatly deteriorated over time, concrete can be place to fill cavities, providing a new encasement for the element. In worse cases, a concrete jacket can be added after the damaged concrete has been removed.
For deck, some of the techniques involve Microsilica concrete overlays, corrosion inhibitor overlays, polymer impregnation and more.
For Superstructure and substructure elements, patching with Corrosion inhibitors is mostly used.
Anything that will prevent chloride ions from entering and diffusing into concrete will work. It needs to be breathable so that water vapor can pass but not liquid water.
Deck sealers are a good option, they are either solvent or water based. Penetrating sealers are the only via options for the job (a combination of silanes and siloxanes). However, they should not be used on structure with active corrosion or high chloride contaminated concrete.
In general, large scale bridges in urban areas should be repaired first since the demand on them is higher than in rural areas. So if you are looking for a good shotcrete machine at a competitive price, give us a call at: 503-283-2105.
Infrastructure Report Card (2013)
Concrete Bridge Protection Repair & Rehabilitation Plan from the Strategic Highway Research Program