Latex-Concrete Mix May Fix Transit Center Flaws

Concrete with latex in it is looking like the most likely candidate, so far, to fix the flaws in the Silver Spring Transit Center.

Latex modified concrete (also known as LMC)—rather than a polymer overlay—is recommended by project engineer Parsons Brinckerhoff as the material to use to fix the flawed structure, which is plagued by cracks and thin concrete sections, David Dise, director of the county’s general services department, told the county council at a Tuesday afternoon briefing.

Both the county’s department of general services and the Washington Metropolitan Area Transit Authority, which was represented at the council briefing, agree that LMC is the best material to fix the $120-million structure.

But, project contractor Foulger-Pratt disagrees, and does not support the use of LMC to fix the structure, Dise and the council said on Tuesday afternoon. Dise and council members received letters to that extent from Foulger-Pratt shortly before Dise was scheduled to update the council about the center on Tuesday. 

The transit center was supposed to open in 2011, but the opening was delayed indefinitely after inspectors found cracks and disparities in concrete thicknesses throughout the structure, The Gazette reported.

The latex modified concrete is a mix of Portland cement and latex concrete that is durable, flexible and more structurally sound than usual concrete, and is used by (among others) the Virginia Department of Transportation in bridge and roadway repairs, Dise told the council.

To apply it, the current surfaces would be scarified (i.e., chipped down) to provide enough of a “tooth” to accept the new layer of concrete, and then the LMC would be poured on top of that.

The LMC will help strengthen the thinner stretches of concrete, and will have a lifecycle of 25 years or more, Dise said.

WMATA had planned on needing to resurface the center’s thoroughfares every 15 years, he added.

As for the cracks in the structure, they will be injected from below and filled in from above with sealant, Dise added.

“Ths is the best solution that provides the combination of durability as well as adequate coverage and infill in thin areas,” Dise said.

Altogether, the estimated time to repair the flawed structure will be about six weeks, but all work must be done in temperatures at or above 40 degrees. That may make it difficult to finish by the end of the year, if temperatures get too cold in the fall, Dise and council members said.

It’s still not determined who is at fault for the structure’s cracks and thin concrete sections—the engineer, the contractor or both. In any case, “we shouldn’t be paying for [the repairs],” Montgomery County Council Member Marc Elrich said.

“Whoever ends up being determined responsible should pay,” not WMATA or Metro riders or county taxpayers, Elrich added.

The new Paul S. Sarbanes Silver Spring Transit Center is located at the corner of Colesville Road and Wayne Avenue. When completed (and deemed structurally sound), it will replace Silver Spring's current transit center, which is more than 30 years old, and will provide bus bays; access to Metrorail, MARC and local trails; and taxi and kiss-and-ride spots.