Civil engineering and infrastructure contracting company Road-Tech Engineering recently successfully completed the construction of  Mia Boulevard road and access bridge project for property developer Balwin Properties unlocking the Munyaka Lifestyle Estate in Waterfall, Midrand, by end of February 2020.

The scope of the project included the construction of a 1.6 km-long dual carriageway with associated services such as 8.0km of Bulk Water supply, 1.0km of Bulk sewer, 2.1km of storm water ranging in different sizes from 450mm up to 1500mm in diameter a five-span 100 m-long dual carriageway concrete bridge, a 67m steel pipe bridge supporting a 400mm diameter bulk water supply pipe, as well as two 50m-long cast in-situ concrete underpasses, constructed under the dual carriageway for future links to other developments.

The  original timeline as per Balwin Properties to complete the project  included seven months for the completion of the road and associated services, with  ten months allocated for the completion of the bridge.

The tight non-negotiable deadline from Balwin Properties was due to their planned launch date of the Munyaka development.  All aspects of the project were completed within the ten months.

Road-Tech project  manager Deon Mackinlay says that the only way to achieve completion within ten months was to construct the bridge with an alternative design. The original bridge design was a five span post tensioned cast in-situ concrete deck, with a twelve-month construction duration.

The alternative design incorporated 120 precast beams which were subcontracted to precast beams contractor Civilcon, and two full time concrete teams.

Mackinlay explains that the decision to use precast beams came down to three main reasons.

The first reason is that the bridge spans the Jukskei River in Midrand, which floods on a regular basis during the summer months. Therefore, it was essential to finish the construction before flooding could occur.

Moreover, the use of precast beams would allow a easier construction method of the cantilever  edge design, allowing a hanging platform and box out design off the two outer pre cast beams to be used over the total span of the bridge but mainly assisting over the river section as well, thereby avoiding potential future flood damage and losses to staging during construction phase.

Even with the accelerated timeline, flooding was a challenge, as the temporary access bridge to access either side of the river washed away about ten times, Mackinlay reveals.

The second reason for the use of precast beams is that it accelerated the construction timeline significantly by reducing the need for conventional staging methods.

The third reason is that it mitigated the lengthy process of on-site post tensioning of the in-situ cast concrete decks.

The decision to use precast beams meant that 120 precast beams had to be manufactured within four months.

The 100 m-long bridge has two 30 m-wide abutments, averaging 5m high, with four 30 m-wide piers, averaging 6m high. Each 25m-long span consists of 24no post-tensioned precast concrete beams, averaging 23m in length, totalling 120 beams.

The bridge also includes five cast in-situ concrete decks separated by construction joints, 115m of cast in-situ concrete parapets on either side, as well as 115m of concrete hand railing. The two underpasses average at about 50m long and 6m high, with eight in-situ cast concrete down-stand beams and four wing walls.

In total, about 3 000 m3 of concrete was poured for the bridge, and about 1 100 m3 for the two underpasses. The largest single pour was for first and second deck spans, totalling about 212 m3.

Despite disruptions by different business forums during construction – including the so-called “construction mafia” – and the challenges associated with working in a confined space with many different contractors alongthe road section, and including the challenge with the road whereby Balwin Properties insisted on a clean and free from defects premix surface on the day of their launch of Munyaka.

 To mitigate the problem usually associated with chips and small defects on the premix layer post construction phase, Road-Tech proposed constructing the premix layers in two separate layers of 20mm premix.

The first layer was constructed and paved using a 20mm medium mix, this allowing all building and construction vehicles to access the site first to complete there works. Once all contractors were off site, the final 20mm premix layer was paved with a fine premix giving a new premix look finish for the clients launch  , Mackinlay confirms that the project was completed to specification and on time, all to the clients expectations.