Construction World September 2016

PRESTRESSED SLABS specified for security wall PROJECTS AND CONTRACTS

Designed by Delta Built Envi- ronment Consultants in close collaboration with leading prestressed hollow-core concrete slab manufacturer, Topfloor, the wall is being erected by KP Construction, the project’s main contractor. According to Topfloor director, Wessel Prinsloo, the slabs are being precast at Topfloor’s Cape Town factory where they are prestressed with steel reinforcement before being delivered on flatbed trucks to Saldanha. “Prestressing provides hollow-core slabs with additional strength and the slabs being used for the Saldanha wall boast a strength rating of 50 MPa plus. As a result, nothing short of mechanised demolition equipment or high explosives would make any sort of destructive impression on the wall. This is why prestressed slabs are being used on an increasing basis to safeguard property of strategic importance or high value in other parts of the country,” advises Prinsloo. The wall is being built to a simple yet effective design which uses galvanised > Some 3 400 prestressed hollow-core concrete slabs are being used to construct a five kilometre security wall for the Department of Public Works at South Africa’s naval base in Saldanha.

steel H sections to support the six metre- long slabs. The H sections are bolted onto six threaded bars cast into the reinforced concrete foundations. These extra heavy duty foundations were specified by Delta to ensure that the wall can withstand the frequent and sometimes gale-force winds prevalent on this stretch of coastline. The wall follows the natural contours of the undulating fynbos-strewn land and rises 3,2 and 4,2 m above ground level; a section of approximately 400 mm is buried below ground. Construction, which began in December 2015 and is due for completion by the end of 2016, is effected by hoisting the slabs using a truck-mounted crane. A special grab mechanism had to be designed by Topfloor to lift and install the slabs. The crane has sufficient reach to service three wall bays from one location. Once installed, the slabs are caulked into the H sections with a plaster sand and cement mix. Part of the contract involves the planting of a line of various types of trees on the outer perimeter of the wall to add a green and aesthetic element to what would otherwise have been a stark monolith. Additional aesthetic appeal is achieved by mounting the panels so that the slabs’ smooth soffit sides are positioned on the outer (public) side of the wall. Moreover, the slabs have been cast with bevelled edges which form V-shaped channels where the slabs meet to further enhance the appear- ance of the walls. Prinsloo says there are several advan- tages to this type of walling, speed of construction and the superior strength of the wall being major considerations.

Topfloor contracts manager, Norlando Nomdoe, stands on one of the concrete foundations.

A Topfloor wall slab is offloaded prior to installation between the steel H sections.

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An inner section in which the attractive V-shaped indentations between the slabs are clearly visible.

A section of completed security wall built with Topfloor prestressed hollow-core slabs at the Saldanha Naval Base.

“Eight to 10 bays or 48 to 60 linear metres are completed daily (eight hours). A conventional masonry wall, which is not nearly as strong or durable, would have taken two to three times as long to build. “The cost of constructing a security wall using prestressed hollow-core slabs is consid- erably less than an in-situ wall offering the same properties. Moreover, precast walling requires no shuttering or propping, onsite curing, formwork or grouting. “As with other walls built with prestressed hollow-core panelling, the Saldanha wall will have a very long life span. It is maintenance free, and other than occasional cleaning, no other servicing is required. The slabs can also be dismantled and re-used elsewhere,” adds Prinsloo.

CONSTRUCTION WORLD SEPTEMBER 2016