Precast concrete drainage channels preferred for Abalone farm upgrade

South Africa’s largest abalone producer, Abagold, has installed drainage channels constructed with precast concrete sections manufactured by CMA member, Concrete Units. The installation forms part of an upgrade of Bersig, Abagold’s second Abalone farm established in Hermanus in 2002.

Abalone is one of the planet’s most desirable sea foods and Abagold cultivates the world-renowned Haliotis Midae species which it exports live, canned and dried globally. Internationally Abagold has invested in and built Oman Aquaculture in Mirbat Oman, the Middle East’s first abalone aquaculture facility. Abalone farming involves maintaining a continuous flow of fresh sea water through cultivation tanks.

The new system was designed by consulting engineer, Jean Dippenaar, who conducted extensive analyses of flow rates on the old system before he determined the optimal flow rates for its replacement. Once the new design was finalised the original plant was gutted; the old tanks, pipe work and the PVC drainage pipes were removed and were replaced with a new tank system and the precast concrete drainage channels.

“Using precast concrete drainage channels has not only improved flow rates, but has simplified operation and maintenance. What’s more, we anticipate that far
less maintenance will be required in the years ahead. For instance, after three months in operation, the channels required no maintenance at all,” said Nico Hough, site manager of main contractors, Exeo Khokela. “Originally designed as an in-situ installation, we switched to precast concrete channelling because it gives the smoothest possible concrete finish and it saved a month’s installation time. Smooth finishes inhibit biological growth, unlike porous surfaces which encourage it.”

Implemented over two phases

Implemented over two phases, Phase 1 involved the construction of 13 channels and took six months to implement, whereas Phase 2’s nine channels took just under four months to complete.

In order to minimise the number of channels, Dippenaar’s design has two rows of tanks draining into one channel. Concrete Units supplied 327 channel sections for both phases. Weighing 2.3 tonnes apiece, the channel sections were also cast with interlocking tongue-and-groove ends to ensure watertight joints and were laid end-to-end using Jointex and an expansion filler.

“One hundred and sixtyfour channel sections were used for the project’s first phase and 163 units were deployed for the second,” said Alwyn Carstens, production manager of Concrete Units.

“With external dimension of 700mm (width) x 600mm (height) X 4.04m (length), we cast the channels in inverted sections to yield smooth off-shutter finishes. We used just over 23kg of rebar on each channel section. Inner concrete cover was 75mm (typical for marine work) and outer wall cover was 60mm.

“We used a 40MPa durability mix with a GGCS slagment extender. And a special water resistant and crystalline waterproofing admixture, Sika WT 200P, was used to reduce the permeability of the concrete. The admixture will also improve the durability and design life by healing all post cracks that might form over the design period,” said Carstens.

“We created the channel platforms so that the drainage channels could be installed at a constant slope of 1%. We used imported fill for the subgrade and we raised the platforms by 350-500mm to achieve the slope. After excavating the channel trenches we cast a 15MPa blinding onto which the channel sections were installed,” concluded Hough.

Text: David Beer