CHRYSO products used to build international airport in St Helena
Products from CHRYSO Southern Africa have travelled 2 300 km over the South Atlantic Ocean to St Helena – one of the most remote islands in world – to be used in BRSHAP (the Basil Read St Helena Airport Project).
The only way to travel to St Helena is by sea. A round trip between Walvis Bay and St Helena (on the Basil Read Cargo Ship called the NP Glory 4) or the Cape Town Harbour and St Helena (on the RMS St Helena) can take roughly 20 – 22 days, weather permitting.
Brenton Brouard, CHRYSO Southern Africa’s Technical Manager explains: “It was extremely important to keep quantities of materials to a minimum as there was limited space on the NP Glory 4. All of the materials used in the production of concrete (except the crushed aggregate) were sourced off the island. The dune sand was obtained from Walvis Bay, the cement came from Ohorongo Cement in Namibia, the fly ash from Ash Resource’s Lethabo plant in Vereeniging and admixtures from CHRYSO’s plant in Cape Town. When designing different concrete mix designs, for example, we could not use vast quantities of dune sand because that dune sand still had to be transported to the island.”
BRSHAP used concrete in constructing the airport runway, the terminal building, air traffic control building and fire department building and the permanent wharf. CHRYSO new generation plasticiser – CHRYSO®Plast Omega 101 – was used in all of the general concrete, the concrete for the runway and the precast concrete used to construct the precast core-loc armour units and hollow blocks for the wharf.
The 100 m long, 10 m high and 13 m wide wharf has a rock breakwater that has to be protected from any possible damage caused by ships. Therefore, over 700 units of precast core-loc armour units (7 tonnes per unit) and hollow blocks (27 tonnes per unit before filled with stone) were placed by crawler cranes via GPS around the wharf from the surface bed to just above sea level.
“When formulating the concrete mix design for the precast units, it was important to achievea mix with optimised properties. The concrete had to fill complex mould shapes with limited bleed and settlement. Excessive bleed water would lead to unsightly voids in certain element sections, as well as increasing the risk of both plastic settlement and shrinkage cracking. Therefore, 12 mm polypropylene micro fibres called CHRYSO®Fibre Plus were used to increase the cohesiveness of the mix, while CHRYSO®Plast Omega 101 assisted in creating an optimised slump. CHRYSO®Dem Oleo SM was used on all of the moulds to ensure an easy release once the concrete had set – without causing damage to the moulds or concrete,” adds Brouard.
Sometimes concrete was transported over long distances on the island – affecting the slump retention and workability of concrete. A retarder called CHRYSO®Tard CE was used to retard the concrete setting time. When necessary, a slump revival admixture called CHRYSO®Rescue Pack was added to the concrete in a readymix truck immediately before discharge. “CHRYSO®Rescuse Pack increases workability and makes it easier to pump or discharge concrete from trucks that may have travelled long distances or have been stationary on site for extended periods,” states Brenton.
CHRYSO®Environmentally Friendly Cleaning Agents (Barracuda, Fusion and Truck Wash) are biodegradable and were used to clean and line the readymix trucks and keep them in good condition.
In order to reduce the need for future maintenance, it was decided to build the runway with concrete instead of asphalt. Manufactured from 27 000 m³ of concrete, the runway is 1 950 m in length, 45 m wide and has a maximum thickness of 350 mm in parts with reduced thickness to the ‘off-keel’ sections and is mostly unreinforced.
The prevention of plastic shrinkage and plastic settlement cracking during the runway’s construction was particularly important with regard to concrete pavements and St Helena experiences a fair amount of wind and sun – conditions that promote cracks in concrete.
“Therefore, CHRYSO® Fibre Plus was added to the concrete mix to improve its rheology and the cohesion of concrete. This prevents plastic settlement cracking and reduces the potential of micro cracks becoming wider ,” states Brouard.
In addition to the fibres, CHRYSO®Cure WP (a white pigmented emulsion curing compound) was applied to the pavements immediately after the surface finish was achieve.
Commenting on the complexity and size of the project, Jimmy Johnston, the Project Director of BRSHAP says: “As a construction, mining, development and engineering group, Basil Read was well positioned to design and construct the airport because the entire project required a diverse range of expertise in the different disciplines of project management, design, blasting, logistics, road and civil construction, open cast mining, crushing, concrete technology and commercial building. The long logistical chain made planning vital, and BRSHAP needed reliable suppliers like CHRYSO that can provide the correct product, at the required amount on an agreed date and time.”