Construction World June 2018

COVER STORY

CONCRETE POSSIBILITIES REALISED Leading construction materials supplier, AfriSam, designed and supplied specialised readymix for

ing had to be constructed within the old. This could only be done once the façade had been retained. This was achieved by re-sleeving the old tubes with a new semi-circular concrete structure, 200 mm thick and separated from the old with a spray-on insulation. This re-sleeving process required concrete formwork to be specifically constructed to the internal shape of the bins; this was then lowered into the bins by crane and concrete poured from the base up in a number of stages. Innovative concrete mix AfriSam sponsored the concrete for the construction of the Zeitz MOCAA (it also sponsors the architectural audio tour for visitors). AfriSam and the University of Cape Town spent three months de- signing the optimal concrete mix design, some of which was poured with the help of a crane, 40 m above the ground, while the reinforcing concrete sleeving (to reinforce the existing silos) was between 200 mm and 75 mm thick. Aggregates were an important factor in the final mix design as the surface finish had to meet the stringent specifications of the architect but also had to comply with the workability requirements. For this project AfriSam supplied 9 800 m 3 of readymix concrete – mainly 30 MPa and 9 mm stone. Some standard concrete mixes were also supplied. With the pours the use of a poker vibrator was not possible due to space constraints. Therefore the concrete had to have good workability, flow and low shrinkage so as to avoid voids and honeycombing. According to Shaughn Smit, AfriSam’s aggregates national sales manager, the inability to vibrate the concrete to remove air pockets, made this highly technical and painstaking work. Each tube forming part of the future atrium cut had to be set out in visual isolation from the next. The concrete was then poured to the exact curve of the atrium cut. This required some major innovation from AfriSam as the tapering edge of the cut in numerous areas was less than 50 mm thick. It also tapered upwards at the base of the atrium. To set out and pour perfect concrete with these sorts of constraints is no mean feat. Each daily pour was slow discharge because of the workability and the way the complex formwork was positioned inside the silos. On average, there were two daily pours of 4 m 3 each and discharge time was two hours. Traditionally, offloading should take anything from 45 to 60 minutes, however in this instance the offloading time was extended to accommodate the complexity of pouring into the complex formwork in the silos. Accommodating specialised project requirements is one of the strengths that AfriSam brings to its customer partnerships. For this project the special design mixes also extended to the concrete for the shallow shutters to restore the external aesthetics of the bins – in line with Heatherwick Studio’s vision to celebrate the industrial heritage of the structure. The mix designs were not only suitable for use in shallow depth

A design that required innovation The design of this museum was done by the world renowned archi- tect Thomas Heatherwick and involved the cutting away of the inside of the existing grain silo. This historic complex is heritage listed and as a result as much as possible of the external façade of the building was preserved, whilst the complex of 42 concrete silo tubes and elevator building was repurposed to house the Zeitz MOCAA. This project challenges the construction norm as it was the first time extensive curved concrete work had been tackled concurrently with top-down and bottom-up demolition activities by the main contractor, WBHO. This top-down and bottom-up demolition activities continued even while the main concrete cutting and shaping of the atrium, as well as other works, were underway In executing Heatherwick’s design, the concrete grain silos were gradually carved away to create the new museum space and the iconic structure of the atrium which resembles a large grain of corn. To achieve this shape, the architects digitally scanned a piece of corn grain three-dimensionally and then supersized it to form the space – connecting this new space to its historical past. One of the biggest challenges faced during construction was the removal of portions of the silos without affecting the strength of the 100 year old structure. Many of the silo tubes were actually split or cracked right around horizontally and standing on their own weight. This meant that these particular silos needed to be structurally reinforced. In order to construct the museum inside the silo bins, a new build- 25 years, and its sponsorship of the concrete for the Zeitz Museum further anchors it in the Western Cape where it is the biggest supplier of readymix and aggregate and the second biggest supplier of cement. the redevelopment of the 100 year old Grain Silo which now houses the Zeitz Museum of Contemporary Art Africa (Zeitz MOCAA) at the V&A Waterfront in Cape Town. AfriSam has been supplying materials for the construction of various prominent buildings at the V&A Waterfront for

AfriSam's Peninsula Quarry.

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