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Concrete wharf work gets complex

Don Procter
Concrete wharf work gets complex

Rehabilitating three fishing wharves by the ocean in southeastern New Brunswick has pitted the constructor against obstacles “quite different from those when your feet are firmly on solid land.”

Unpredictable storms, high tides and fishing seasons, for example, all put a squeeze on construction (much of which is done from a barge) and scheduling, says Jean Depres, civil engineer, of Landco Construction Ltd. of Cap Pele, N.B.

Depres is referring to three federal government contracts Landco was awarded to rehabilitate wharves at Murray Corner, Cape Tormentine and Petit Cap.

He says complicating the working conditions is that concrete — a material incompatible with corrosion-causing saltwater — was specified for underwater foundations. It created a recipe for "interesting and unusual jobs."

Depres says the wharves require burlin walls with inset eight-by-12-foot precast concrete wall panels to form a retaining wall. Weighing about seven tonnes each, the panels are fabricated by Maritime Precast Concrete Products of P.E.I.

To meet the saltwater conditions, the precast concrete mix specified by the government meets a C1 exposure classification and includes the additive silica fume used for high-performance concrete.

"It’s like a bridge design concrete but probably a little bit stronger because of the silica fume," Depres says.

The impermeable precast concrete includes rebar embedded three inches deep.

While the first two wharves have been completed, rehabilitation at Petit Cap is underway. Work commenced last October and is slated for completion this summer, says Depres, who has worn several hats on the projects, including foreman and estimator.

At the project, about 150 cubic metres of poured-in-place concrete forms eight-inch beams on top of H-piles above the precast concrete panels. That concrete also makes up a 12-inch deck.

The same high-strength silica fume concrete mix was specified for the poured-in-concrete, says Depres, adding the high-strength, impermeable mix is vital to the longevity of the wharf.

He cites one case where a C1 mix was not used — a previous wharf rehabilitation contract Landco did where concrete and rebar were beyond repair from salt water damage. "It ended up being a major change order to that project."

Depres notes that most of the heavy equipment at the Petit Cap contract is on a barge next to the wharf.

"When we see a storm coming we tie everything down. Usually we’re inside the wharf which acts as a breakwater."

Scheduling is critical around tides because work such as bolting protective timbers to the concrete foundation walls is done under water at high tide. Installing protective fenders sometimes starts at the top, during high tide, and moves down as tides go out, he says, adding the process typically takes about two weeks.

Constructing the 24 pile footings every eight feet for the wharf at Petit Cap involves driving three-foot diameter steel pile caps (sleeves) into bedrock, and then drilling holes about five feet into bedrock, says Depres.

A pumper truck "sucked up the mud inside the hole" before concrete was pumped into each hole, he says.

"It was a special admixture for under water — with a molasses-like texture — that wouldn’t let the seawater penetrate the concrete."

The steel pile caps or sleeves were removed two days after the concrete pour. Steel H-pile beams were set in place and anchored to 20 to 30-foot long tie rods inside the wharf that are fastened to anchor blocks. The area is then filled with earth, he says.

About a week after the concrete was poured, the precast concrete panels are installed.

Finishing the concrete is more difficult than conventional concrete because "it sticks to the trowel. It was only a little issue for us and I know it is only a wharf but we still want to make it look good."

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