On track to becoming South Africa’s second-largest retail development, the recent Fourways Mall expansion also represents a national ‘first’ for its use of a unique precast concrete solution for most of the concrete work; construction materials leader AfriSam was the preferred cement provider.
As a key regional retail development northwest of Johannesburg, the 260 000 m2 Fourways Mall expansion boasts an impressive scale, with 24 016 precast concrete elements, 135 000 m3 of in-situ concrete and 11 000 tonnes or reinforcing steel. Main contractor Mota-Engil Construction SA (MECSA) has applied the precast system successfully in projects elsewhere in the world and was able to bring the concept home at Fourways Mall through an intensive skills transfer process.
Despite the large scale of the project, the expansion was required to be conducted alongside the existing mall facility, so that retail operations could continue unhindered. Added to the challenges was a confined workspace in a busy urban environment as well as a tight timeline. This led to a 24-hour work cycle being adopted for six days a week for the past 24 months, with over 1,300 personnel and 21 tower cranes on site at its peak. Concrete work began in March 2016, structural work was completed by November 2018 and final mall completion is expected by April 2019.
Advantages of precast
Under these demanding conditions, the precast solution proved to be a key strategy enabling the construction to deal with the various challenges related to logistics, space and time. For instance, using precast elements meant fewer ready-mix concrete deliveries to site, which in turn eased the access to the road network around the shopping centre.
It also meant that areas in the mall could be opened more quickly to public parking. Using conventional construction techniques, a back-propping system would need to be in place, but with precast there is no back-propping required. Another advantage of the precast system was that it allowed the project to run with fewer workers on site, an important benefit given the space constraints; instead, much of the concrete-related labour was effectively deployed off-site in the precast yards.
In terms of project pace, the precast construction allowed work to begin on the finishes as early as the very day that a slab was laid, also allowing brickwork to begin earlier. In addition, it minimised environmental issues such as airborne dust and dirt underfoot, which could discourage shoppers.
The project’s precast approach involved large structures such as 10,5 metre beams, 8,75 metre slabs and 24 metre columns to be cast off-site, and then transported to site as required and efficiently installed. Prefabrication was done at two precast manufacturing yards serving the project – one in Laezonia and one opposite the construction site. The Laezonia yard used its own batch plant, while readymix supplier Scribante delivered concrete to the manufacturing yard opposite the construction site; in total, about 200 000 m3 of concrete was supplied by MECSA and Scribante.
Emphasis was on high-strength 65 MPa concrete that could attain early strength, as the elements were sizeable and had to be lifted and placed as soon as possible. This concrete was required to develop a 32 MPa strength specification at 18 hours, so the team worked with cement and additive experts to conduct trials on mixes and identify the best solution. AfriSam providing 42.5R cement which performed well in terms of the high early strength requirement. The company also contributed to conventional mixes (such as the 15 MPa and 30 MPa concretes) used for toppings on the fabricated slabs – with a waterproofing additive to waterproof the slabs.
The overall quality of concrete accuracy for precast application was critical, as the columns had to fit exactly into a foundation socket. Column ends were designed with serration or tooth mechanisms, while the foundations had the ‘female’ or inverse serrations; this allowed the column to be socketed into the 1,4 metre wide base and then grouted together. The column was vertically aligned before grouting and was then stable without props. This efficient technique allowed for five to six towers had to be placed per day, with the pace of construction uninhibited by the traditional curing time of in-situ columns.
Precision was also called for in the foundation excavations, as precast columns have set lengths; this is not always straightforward, given the varying geology of the site. In some cases, the columns had to be cut shorter, or further excavation conducted to place the beams in positions that ensured a level result.
The innovation went beyond the precast system itself to the extensive quality assurance and quality control procedure applied by the contractor on site to ensure the safe installation and finishing of the concrete elements. At six tonnes each, the columns had to be navigated safely through the busy and confined site after delivery by flatbed trucks. Results in terms of concrete finish also exceeded expectations; while the project specification was for a Grade 2 finish, the use of MECSA’s steel moulds for the precast concrete elements resulted in a Grade 1 finish for 90% of the project.
Skills return to SA
While precast concrete is extensively used in South African construction, the country has lost many of the original skill-set that was present in the 1970s. With MECSA’s specialised expertise in precast, many of these skills have returned through the project’s skills transfer in precast-related fields. This upskilling has applied across the board with not just labour as the beneficiaries, but also professionals, engineers and architects. This has represented a significant learning experience, with each person on the job given the opportunity to acquire new knowledge and to put this into practice on the project.
Another important aspect of the Fourways Mall expansion was its innovation in dealing with the limitations of precast; while retail environment architectures often like to bring in free-form shapes especially around the courts, precast generally provides a rectilinear solution. The project managed to successfully marry the in-situ system with the precast system, with the in-situ elements providing the circular and free-form shapes.