DeWayne Wilson, Washington State Department of Transportation
Background
WSDOT has a comprehensive Bridge Deck Program with the primary goal of economically repairing and overlaying concrete bridge decks to prolong their lifespan and avoid expensive deck replacements (sustainability). WSDOT manages 3,109 vehicular bridges over 20 feet in length as part of the state highway system. The majority of these bridges have reinforced concrete decks.
The use of salt in winter deicing practices causes premature deterioration in many concrete bridge decks through corrosion of the reinforcing steel. Once the rebars start to corrode they cause the concrete to spall and deteriorate. Each summer WSDOT Regional Maintenance crews repair any of these spalled areas. These repairs are considered to be temporary and typically last 1-3 years. Once the total areas of repairs and / or patching exceed 2% of the total deck area then the bridge is added to the list of future needs for adding an overlay. When funding becomes available then a contract is developed and advertised for a contractor to perform deck repairs and add a protective overlay (normally a 1.5” thick modified concrete).
WSDOT Modified Concrete overlay types
WSDOT has developed five separate modified concrete overlay mix designs for deck rehabilitation, two of which has been discontinued. The mix designs consist of either Latex or Microsilica (silica fume) or Fly-ash (42 hour cure time). WSDOT also installed a few rapid-set Latex Modified Concrete (LMC) overlays (4 hour cure) but their use has been discontinued. The following modified concrete mix designs provide over 5,000 psi compressive strength and a permeability value of less than 1,000 coulombs:
- Low Slump Dense Modified Concrete (LSDMC) was first applied in 1979 and has been used on 35 bridges to date (0.4 million sq.ft.). This overlay type has been discontinued due to poor performance.
- LMC was first applied in 1979 and has been used on 324 bridges to date (8.0 million sq.ft.).
- Microsilica Modified Concrete (MMC) was first applied in 1987 and has been used on 126 bridges to date (3.4 million sq.ft.).
- Fly-Ash Modified Concrete (FAMC) was first applied in 1995 and has been used on 43 bridges to date (1.2 million sq.ft.).
- Rapid-Set Latex Modified Concrete (RSLMC) was first applied in 2002 and has been used on 5 bridges to date (0.2 million sq.ft.). The use of this overlay has been discontinued due to excessive cracking. Difficulties with the supplier prevented a mix design that could be verified during construction.
WSDOT Modified Concrete overlay types
The overlay process begins by setting up traffic control and closing all or part of a bridge. The amount of time a contractor can have to do the project is a very important issue with more emphasis being made toward rapid construction. WSDOT requires a contractor to use a hydromilling machine with at least 7,000psi of water pressure to remove ½” of good concrete and any previous patches. The removal of the top ½” of concrete also removes a high percentage of the salt in the bridge deck. The contractor must do a trial on a portion of the deck with good concrete and then use the hydromill setting for the good concrete on the rest of the bridge. These settings will remove concrete in poor condition up to several inches. The contractor has to properly contain and dispose of the waste water used during the hydromill process. The next step is to fill repair areas below the top mat of reinforcing steel with a standard 4,000 psi concrete (WSDOT does not allow fast curing patching materials). These areas have to be cured for about 24 hours to achieve the strength desired of 2500 psi prior to applying the modified concrete overlay.
The construction process is nearly the same for any of the modified concrete overlay types. The main difference is that LMC is mixed and delivered to the bridge deck with a mobile mixing truck verses MMC and FMC that are mixed at a r plant and then delivered to the site in a ready mix truck. After a hydromill is used to remove ½” of the existing concrete and prepare the surface the contractor uses a finishing machine to place the concrete overlay and to ensure a uniform placement for the desired 1.5 inch thickness. The temperature of the existing bridge deck must be more than 45 degrees and less than 75 degrees prior to placement. WSDOT also sets a criteria for the evaporation rate at the time of placement. The modified concrete overlay is wet cured under burlap for a minimum of 42 hours. The overlay is then checked for strength per ASTM C805, and if the concrete is above 3,000psi then the contractor can remove the curing blankets and open the bridge deck to traffic. More details on the WSDOT modified concrete overlay specifications are available in section 6-09 of the WSDOT 2014 Standard Specifications for Road, Bridge, and Municipal Construction.
Concrete overlay service life
Modified Concrete Overlays are a very effective part of WSDOT’s bridge deck preservation strategies as evident by how few number of total deck replacements have been necessary (only 14 bridges to date). There are 165 bridges with modified concrete overlays that have provided more than 25 years of service. WSDOT has replaced 13 modified concrete overlays to date (0.8 million sq. ft.) and has identified another 30 (1.1 million sq. ft.) that will need to be replaced over the next 8-10 years.
Further Information
For further information about this article, contact the author at [email protected].