Transforming ‘ordinary’ edifice into an architectural landmark

By Eamonn Ryan

A 26m-long, highly unusual concrete portico over the entrance of the Council for Geosciences head office, located in Silverton, Pretoria, transforms what was a fairly ordinary looking building into an architectural milestone.

The R3-million GeoSciences project entails R1.6-million of formwork, just 70m3 of concrete and a substantial 15t to 18t of steel. The project was a tremendous learning curve for all involved – sadly, as a once-off these lessons may take a long time before they are needed again.

Architect Pieter Breytenbach, of Deter Architects & Designers, was given free imaginative rein on the concrete portico cover over the entrance. “It was a fairly old building, so it was a challenge to get a structure which would relate to the existing building while contributing something to its aesthetics. To achieve that I employed one of the oldest structures ever designed – the arch.”

Making it especially challenging was the fact that the ground ramped up towards the entrance, so the design had to accommodate the gradient slope up to the building plinth. In fact, it was his unique design which ultimately won him what was an open tender. The only guidance he was given, he says, is “they said they wanted something ‘impressive’.”

“I like working with monolithic structures and with concrete, in terms of not adding too much to the structure while improving its appearance. The structure rises up towards the building entrance and narrows down to a diminishing point – indicating the way to the door almost like a target. That was the biggest challenge from a design point of view. Once I had decided on that design, I had to look at what I could take away – one usually has heavy buttresses on an arch, but we were able to do away with this due to the capabilities of the concrete and the expertise of the engineers we were working with,” says Breytenbach.

“We wanted to open up the western side of the structure and have sufficient natural light, and we were lucky in that protection from the elements was afforded by the existing building. We used polycarbonate as a roofing material, because it doesn’t transfer a lot of heat, clad over ribs tucking into the structure on the sides which turned three-dimensionally.”

As you approach the building the portico opens up in a well-lit manner, closing in as the entrance is neared, tilting up slightly. Two large pillars and the gutters act as the keystone to the structure. There are long spans of unsupported cantilevered concrete between the pillars, “and that’s a homage to the material we’re working with and what it is capable of,” explains Breytenbach.

Scaffolding

The precision achieved by AJ King. Photo by AJ King.

Scope of work

“I believe this is one of the only structures where every element of rebar has its own bar-mark – every single piece is individual, because everything is either diminishing or increasing in size and the whole structure is moving in three directions. It’s not like a normal column.” The steel design was done by engineers SCIP (Francois Deysel and Jacques le Roux).

Peri was responsible for the formwork, lay out and assembly. Each was cut by laser into 200mm sections so that Peri could get the stop ends correct, because each one turns. “We didn’t want to place the covering material on top but below in order to create a draught through it. It was impossible to cast it as a sleeve in situ, and that is where Shotcrete came in when we looked at how to practically build it,” says Breytenbach.

With sufficient admixtures such as Penetron added to the concrete, a slump of just 55 to 65 was achieved. The columns are 800mm deep and 2.4m wide, and a whole column could be erected in a single day without shuttering at the back. “It is quite amazing,” enthuses Breytenbach. With lateral support, the result was that the structure did not have a weight of concrete pushing down.

From Peri, Callie Middel (estimator), Shane Treherne (sales engineer) and Jacques van der Walt were responsible for the project. Middel explains that Peri was responsible for the design work for the formwork and shoring, which was done by Jacques van der Walt, senior formwork designer at Peri.

“All formwork elements for the soffits were pre-fabricated by Peri off-site to be delivered for assembly on site. This was however not possible in the case of the formwork and many items were attended to by AJ King (site manager for Shotcrete Africa SCP) on site. This project was unique to say the least as well as challenging in terms of formwork, reinforcing and the shotcrete operation. The solution to constructing the canopy had to be found in a close collaboration of the trades involved, architect Pieter Breytenbach and his team, the team at Peri, the rebar team and the team at Shotcrete,” says Middel.

“The structure is asymmetric so section and formwork design work had to be done in 3D taking existing structures into account. From a formwork point of view, this project was highly unusual and many contractors took the decision not to get involved. The placement of concrete is only possible by the wet-shotcrete method,” says Middel. Shotcrete Africa SCP, with assistance from Peri, installed all the formwork.

The construction method started with the installation of a designed network of props and scaffolding to support the ribs of the canopy. Each individual rib was laser cut and no two are the same. The shutter ply was then placed and fastened to the support structure. At all times strict health and safety was maintained. A blinding layer of shotcrete was then put down so as not to use ‘spacers’ that would have negatively affected the smooth finish required by the architect.

AJ King and Alphious

AJ King, site manager for Shotcrete Africa SCP and supervisor Alphious Mahlabela.Photo by Eamonn Ryan.

Shotcreting

“It would not have been possible to cast the concrete in the conventional method of closing up the vertical faces. An alternative approach was needed and the casting of concrete by shotcrete with a controlled water cement ratio was the key to success. The canopy is asymmetric in terms of every formed surface and the development of the formwork had to be done off a 3D model. Some of the formed elements, like the curved stop ends had to be built in place by AJ King and his team,” says Middel.

The design and fixing of the rebar was also a challenge and a hands-on approach was the only way to get the job done.

“From my personal point of view, I would say that this project was a question of complete commitment from all involved and required clear vision from the architect in terms of what had to be achieved. The progress meetings we had required detailed technical discussions and the various trades worked much closer together than normal,” says Middel. “The steel bending schedule was very complicated and required many clarification meetings to ensure it was designed and installed correctly.”

Pieter BreytenbachShotcrete’s King says, “Every aspect of the project has been a challenge. Everything changes everywhere – there is no duplication anywhere.” In fact, the Shotcrete application turned out to be the most simple aspect of this build.

Shotcrete Africa SCP is a specialist shotcrete contractor and has been involved in the construction of many concrete structures where the speed of construction and minimal formwork requirements offered by the wet-shotcrete method of placing concrete, have accelerated their clients’ programme leading to significant time and cost savings.

“Wet Shotcrete is just a method of placing concrete,” says MD and founder, Dustin Strever. “We have invested heavily in best of breed REED shotcrete equipment that enables us to tackle landmark projects such as this one. With our pumps capable of 145 bar of pumping pressure at the piston, there is not much these pumps can’t push.

“The mix design was critical to achieve an ideal slump that enabled us to shoot the 800mm thick columns through two layers of heavy rebar with a minimum of rebound and excellent encapsulation of the rebar. The addition of Penetron to the mix will ensure a very durable concrete structure that lasts,” he says.

Unlike many construction projects, the job was internal and had no impact on surrounding roads and communities other than the need to implement a traffic go/stop system in the access roads to the GeoSciences building. The construction also did not affect the landscape – all the trees were left in space with abundant gardens. 

List of professionals
Client GeoSciences
Main contractor Bhekanani
Engineers SCIP
Formwork design and shuttering Peri
Concrete work Shotcrete Africa SCP
Concrete supplier Metier
Architect Pieter Breytenbach
Steel Piovesan

 

 

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