Coreslab received a special mention in the Specialist Contractors or Suppliers category of Construction World’s 2020 Best Projects Awards for its stellar work on the Kaalfontein and Diepsloot pedestrian bridges.
The pedestrian bridges are part of the Johannesburg Road Agency’s significant investment into low-income communities in the larger Midrand area to create jobs and provide access to service delivery. They also serve a more critical function by providing a quick means of evacuating areas during heavy flooding, as well as swift connections between highly densified nodes in these areas.
The precast-concrete specialist was initially approached to manufacture and supply rib-and-block suspended slab systems for the superstructure of the two bridges.
However, Coreslab suggested a simple but novel alternative design based on hollow-core slabs (HCS) to also overcome some of the site constraints on these two projects. This included working within proximity to many informal dwellings and limited access to both construction sites. For example, the construction site in Kaalfontein could only be accessed via an extremely narrow and steep gravel road. As the only means of entering and exiting the informal settlement, it is very busy and had fallen into a serious state of disrepair due to heavy flooding early in 2019. This added a further level of complexity to transport logistics planning for the professional team.
Coreslab’s design entailed using standard HCS planks, significantly lighter than a large solid concrete floor slab of equal thickness or strength due to the use of fewer raw materials. This reduces the cost of manufacture and transportation of the slabs to site where they are lifted and placed, levelled, and grouted.
The HCS were placed on top of the 14 precast-concrete beams, two per span, and each 15m in length and weighing four tons.
They were then covered by a thin 150mm-thick concrete slab. Services were first installed and then covered with polystyrene moulds with voids and a steel mesh. The concrete was then placed in an approach that reduced the load of the final slab on the beams.
The HCS were manufactured and cut-to-size at Coreslab’s factory in Limpopo according to BMK Consulting Engineers’ engineering drawings, before being transported to site and placed directly on top of the precast-concrete beams that spanned the bridges piers. Coreslab mobilised to site once Axton Matrix’s team had completed the two in-situ concrete piers and abutments, as well as the approach ramps and their walls.
The precast-concrete elements were dispatched from the company’s factory to site in seven separate loads.
They were lifted directly from the truck trailers using the company’s own mobile crane. It was established on site early in the morning ahead of the arrival of the various elements.
Coreslab installed an element every six minutes, lifting and placing them directly from the truck trailers as they arrived. The crane was equipped with special lifting tackle that was designed by Coreslab’s engineers to facilitate the rapid and efficient placement of the slabs.
Certainly, one of the challenges was constructing a secure platform for the mobile crane in the marshy terrain.
Constructed by Axton Matrix, the principal contractor, the platforms comprised a 700mm-thick G5 material that was compacted to provide the necessary stability to lift and place the heavy precast-concrete elements.
Axton Matrix then completed the in-situ sides and the 150 mm-thick concrete slab on top of the hollow-core elements.
Notably, despite the use of less labour-intensive methods of construction, many locals still gained essential skills and experience working alongside Axton Matrix’s core team. In Kaalfontein, for example, 17 locals were trained to work on this project.
A typical Expanded Public Works Programme project, it was imperative that a balance be achieved between delivering a high-quality final structure to the community without delays and creating ample work opportunities for local businesses during the construction phases. The construction of the sides of the bridge and ramp walls provided ample opportunity to train inexperienced community members without the risk of delays and compromising the overall quality of the final structure.
Notably, the installation of the decks was completed in only a day to enable Axton Matrix to commence working on the remaining portion of the bridges immediately. Using rib-and-block technology, the construction of the bridges would have taken significantly longer. This approach would have entailed installing, assembling, and removing props, steel fixing, as well as pouring the concrete and waiting for it to cure before being able to access the superstructure.
The contractor also achieved cost-savings for the client by doing away with the need for construction materials, including propping and concrete.
Another major advantage of Coreslab’s solution was the extremely high-quality outcome that was achieved the first-time round by outsourcing the manufacture and installation of the floor structure to a specialist. This eliminated the risk of having to redo work, whereas the principal contractor would have had to install the rib-and-block system.
Notably, there were no safety or injury-related incidences on these projects, despite working at heights and close to many dwellings. Part of this success can be attributed to the use of HCS, which were manufactured at ground level in a controlled factory environment that is far removed from the many variables encountered on a traditional construction site. They were also installed by experienced and skilled Coreslab personnel.
Working in a wetland, the use of HCS mitigated some of the environmental risks associated with the project for the principal contractor. This included managing the batching and then pouring of concrete over and between the blocks of standard rib-and-block systems to create a single monolithic structure in this sensitive area. Instead, the HCS were manufactured off site and the final elements merely installed on top of the piers.
Jaco de Bruin, Managing Director of CoreSlab, says that he is proud of his team’s work on these projects, which have again demonstrated the many merits of precast-concrete technologies.