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Scanning before damming

Water is a precious commodity in Australia, making dam construction, maintenance and enlargement a critical part of every town and city?s infrastructure. It is crucial that maintenance and enlargement projects are completed quickly and accurately.

Australia has almost 500 large dams, many of which are old and in need of significant maintenance or upgrades. Regular maintenance to repair damage from the harsh Australian climate, including drought or flooding, is required to ensure ongoing water supply for the population.

Australian dams are predominantly made of concrete that has been reinforced with steel or iron bars to provide strength and durability. On old concrete structures, such as the Cotter Dam, where plans and blueprints may no longer exist, scanning and detection technologies have become essential to ensure errors are avoided when cutting and drilling into the concrete.

Detection systems make steel reinforcement, or ?rebars?, in concrete visible quickly, easily and without the need for destructive procedures, ensuring accuracy when drilling or cutting concrete.

This detection process was needed on the enlargement project at Cotter Dam in the ACT. The project is part of a three-phase, federally funded water sustainability upgrade to ensure an ongoing supply of fresh, drinkable water for future generations in Canberra.

The Cotter Dam enlargement project includes reinforcing work on the dam wall to strengthen areas of erosion and construction of a new dam wall 26m downstream.

The project also includes an upgrade to the Googong Dam spillway, a new water transfer pipe from the Murrumbidgee River to Googong Dam.

When finished, the enlarged Cotter Dam will hold 78 gigalitres, nearly 20 times its current size of 3.9 gigalitres, and will be the third-largest concrete gravity dam in Australia. At 80m high, the dam?s roller-compacted concrete wall will be the highest of its type in the country.

The Hilti Group develops and manufactures heavy-duty tools and anchors for the construction, mining and energy industries. Hilti has been involved in all three phases of the project, providing engineering expertise, high performance power tools and specialist anchors to the Bulk Water Alliance, comprising GHD, Abigroup and John Holland.

Maintaining the integrity of concrete is the underlying consideration for all elements of dam construction and maintenance. A thorough understanding of the make-up of the concrete is critical when builders work at considerable heights and operate with powerful equipment.

During reinforcement work on the original Cotter Dam, constructed in 1912, workers had to abseil down the dam wall to check for cracks in the brittle concrete. Hundreds of Hilti anchors were needed at various points of the wall to attach the abseiling equipment.

The drill holes for the anchors needed to be positioned correctly to avoid costly delays and damage to rebars. Hilti?s Ferroscan PS200 detection system was used to rule out the possibility of damage to the steel reinforcement.

The cordless Ferroscan system uses a scanner, monitor unit and PROFIS software for PC to determine the number of rebars present, their positions, depths and diameters.

The system employs the magnetic resonance principle to detect the reinforcing metal bars. The scanner generates a magnetic field and interprets the variations in magnetic resonance caused by the reinforcing bars within this field.

After passing the scanner over the surface of the structure, the positions of the reinforcing bars were marked directly on the concrete and the applicable data recorded and transferred right away, on site, to the portable monitor unit for viewing and evaluation.

The workers received a reliable image of the concealed reinforcements simply by scanning over the surface of the structure; they then correctly identified suitable rebar-free sites in which to drill holes and insert the anchors.

Hilti is dedicated to best practice in drilling, cutting and reinforcing on construction projects, to ensure integrity of the structure and help builders enhance accuracy and efficiency.

Non-destructive concrete scanning technology now allows builders to drill in the correct position first time, avoiding damage, because they have instant access to information on what lies beneath the surface.

Understanding the position and depth of rebar on the old Cotter Dam wall has allowed builders to keep the still-functional structure intact, while providing reinforcements that will allow it to continue providing water to future generations in Canberra.

Zac Bouchabake is a field engineer at Hilti Australia.

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