For Colonial Lake, a renovation in mid-stream

Scott Parker, with Design Works, points out the original tabby wall that surrounds Colonial Lake. The wall had been covered over in the pass with concrete. Workers are now making new tabby to make repairs to the existing wall.

Finding a recipe to recreate 150-year-old tabby — a sort of primitive concrete made from lime, ash and chunks of oyster shells — isn’t easy.

That’s one of the main lessons that Wildwood Contractors of Walterboro has learned halfway into its $5 million project to improve Colonial Lake in downtown Charleston.

Some of the earliest work has included stripping off 1970s-era Portland cement from the original tabby walls of the 19th century man-made lake, then repairing and adjusting those walls.

Jerry Crosby of Wildwood said the biggest challenge has been making about a dozen batches of tabby and subjecting them to more than 50 tests to find the one that would best match the appearance and strength of the original. The Scottish Lime Centre Trust was consulted to help measure the sample’s compressive strength.

Complicating the equation is that tabby can take up to 90 days to cure completely — and that the oyster shells had to be imported from Georgia and the Gulf of Mexico states because South Carolina environmental officials encourage recycling oyster shells by placing them back in the environment.

Wildwood, the city, landscape architects DesignWorks and the Charleston Parks Conservancy eventually agreed on a recipe early last month — about six months after work began.

“What’s being blended back in is very hard to tell from the original wall,” Crosby said, as crews dumped oyster shells, ice and other ingredients into a big mixer nearby. The ice kept the mixture from setting too fast in the July heat.

Those working on the lake’s transformation offered up a tour of their construction site Thursday and announced they were only about a month behind schedule, partly because of unexpected drainage work on Rutledge Avenue. The chain link fence is now expected to come down in late February.

Before then, Wildwood will add a new pipe under the Ashley Avenue side of the park to create a second point where water from the Ashley River can enter or exit the lake. It also will replace the broken control structure that governs water flow between the river and those two points.

And it will add an improved promenade and other pathways, about 90 new trees and about 32 more benches, plus a brick seating wall that already is taking shape near Rutledge. The workers already have begun laying new curb on Rutledge, which will be about 7 feet narrower.

The work not only includes repairing the tabby wall but also making its height more uniform. The two steps near Beaufain Street and Ashley Avenue have been removed; the wall is being raised as much as 18 inches in some sections and lowered as much as 18 inches in other parts. The wall will be topped by granite blocks along the water’s edge.

Once Wildwood’s work wraps up, the Charleston Parks Conservancy will plant the new beds with flowers and shrubs. The conservancy raised $1.2 million toward the work.

“The crown jewel of all of this will be the new plantings,” said Scott Parker with DesignWorks. “That will be the last thing that happens.”

Reach Robert Behre at 937-5771.