Teichmann Structures and AfriSam excel in day-long concrete pour over N3 Highway

A continuous concrete pour has been successfully completed to create a bridge deck across one of South Africa’s busiest freight highways, demanding meticulous planning and execution from Teichmann Structures, AfriSam and other key participants.

As part of much-needed upgrades by the South African National Roads Agency Limited (SANRAL) on the N3 highway through Pietermaritzburg, a new Market Road Interchange bridge is under construction. This is being built by Teichmann Structures as subcontractor to Grinaker-LTA in the New England JV (Grinaker-LTA). The consulting engineers on the project are WSP.

Supplying the large volume of readymix concrete for the 103 metre bridge deck was AfriSam, who delivered over 1 300 cubic metres of concrete in 218 truckloads over a 24-hour period in April this year.

According to Kieresh Singh, Site Manager for Teichmann Structures, the critical aspect of the project was to avoid disrupting the traffic on the busy N3, as well as the traffic over the existing Market Road bridge close to the N3. The solution was to place protective safety netting on the sides of bridge, so that concrete could be safely poured without the risk of any spillage onto the live highway below. At the same time, the existing Market Road bridge would be closed overnight, from 8pm on Thursday April 10^th to 6am on Friday April 11^th.

“In line with our carefully calculated schedule, we began to place concrete onto the first half of the bridge at 1pm on the Thursday, so that we could achieve the maximum pump reach of a third of the pour by 8pm,” Singh explains. “With the existing bridge closed, we could then move the pumps onto the existing bridge, completing the central spans of the new bridge. The pump and supply rate needed to be maintained to ensure that traffic on the existing bridge was re-opened to the public by 6am the next morning.”

He highlights that the pour had to be continuous to ensure no cold joints, which meant there was no room for error on this task. Everyone involved had to play their role in perfect collaboration, he notes, so it required intensive planning meetings to ensure that every conceivable risk was mitigated – and there was always a Plan B for any eventuality.

“We developed a detailed ‘pour plan’ with all parties, including AfriSam, Grinaker-LTA, WSP and SANRAL, to ensure that everyone was on the same page,” he says. “The New England JV (Grinaker-LTA) conducted the traffic accommodation as the main contractor, while SANRAL kept the community and motorists informed about the planned road closure times, resulting in minimum disruption to motorists.”

He emphasises the importance of ensuring the safety of employees on site as they poured the concrete, as well as the concrete truck drivers, motorists and the general public in the vicinity.

Large pours like this one demand considerable resource capacity and flexibility from a readymix provider, according to Randal Chetty, Regional Sales Manager at AfriSam. Having the Pietermaritzburg and Umlaas Road readymix facilities in close proximity to the project was an important advantage, as was the company’s nationwide truck fleet.

“We allocated 36 trucks to the pour, primarily from our KwaZulu-Natal operations with additional support from Gauteng to ensure sufficient capacity,” says Chetty. “The project also called for two 12 hour shifts, so we mobilised personnel from our other KZN sites to maintain safe and efficient working practices.”

Among the key risks assessed for the readymix delivery was the potential for delays caused by road accidents or other disruptions between the AfriSam plants and the site, he explains. To mitigate this, a detailed route analysis was undertaken to ensure deliveries could continue smoothly, even in the face of unforeseen challenges.

Concrete quality was critical to the project’s success, highlights Xolani Mbatha, Regional Product Technical Manager at AfriSam. For the bridge deck, a 40MPa durability mix was a requirement on the project to ensure the high compressive strength required to support heavy vehicle traffic and durability to resist deterioration.

“There were sections of the bridge deck with dense steel reinforcement, which meant the concrete needed to be sufficiently workable to flow into cavities as deep as a metre,” explains Mbatha. He adds that maintaining a concrete slump consistent within the specification was essential.

“We also had to manage the heat of hydration by incorporating fly ash into the mix to minimise the risk of thermal cracking,” he says. “Consistency of aggregate materials was another priority, so we isolated specific stockpiles identified through our testing regime. This was made easier by the fact that AfriSam’s two local batching plants are located on our quarry sites, where we crush the required stone ourselves.”

Stringent concrete testing was carried out throughout the project by AfriSam in collaboration with WSP, the consulting engineers and RoadLab, a SANAS accredited laboratory on the project. Due to the high volumes of concrete being poured in a short period, AfriSam also engaged an additional independent laboratory to ensure all testing requirements were met without delay.

“Over 200 test cubes were made during the pour to ensure sufficient compressive strength data was attained and quality was ensured. “We also exceeded standard testing requirements by monitoring strength development at 1, 3, 7, 14 and 28 days.”

With aggregate sourced from its own quarries and cement supplied from its Roodepoort plant, AfriSam maintained close control over the quality of all materials - ensuring that approximately 97% of the bridge deck concrete mix comprised AfriSam’s own products. Every truck underwent slump testing before leaving the batching plant, resulting in a 99,2% acceptance rate of concrete deliveries on site. Durability panels were also cast on site and will be cored and tested for properties such as chloride conductivity and permeability, amongst other requirements, adds Mbatha.

Singh highlights that the project was successfully completed within the tight timeframe, enabling traffic to resume along Market Road into Pietermaritzburg from 6am on the Friday morning. The operation was completed without any delays or injuries on site, and the result was a high-quality concrete deck ready to form part of the new bridge.

This concrete milestone was achieved with the collaboration of the entire team which makes successful projects.