Concrete expertise for landmark development

Citi-Con successfully solved abnormal discolouring of exposed concrete of the new 38 000m² Amazon head office in Precinct 2 of the R4-billion River Club mixed-used development. Notably, we finalised a project that should have taken three months to do so in only two months.

The company designed a system that would remove the efflorescence. Importantly, this had to be done in an extremely short timeframe to avoid delaying the opening of the building to its tenant.

The repair method was tested on various sample areas and approved by the architect. It was not only effective, but also practical and feasible. Bear in mind that our operations coincided with the various specialist trades which were completing the final aspects of the build.

About 8 000m² of exposed concrete was significantly stained. This included the full length of the entire concrete beam around the perimeter of the building. There, the efflorescence was so severe that it had to be removed by grinding off a thin layer of concrete. Other areas were washed by hand with a weak-acid solution cleaner.

The concrete was then sealed with StonCor’s Pro-Struct 670. Pro-Struct 670 is a modified siloxane solubilised in aliphatic hydrocarbon for deep penetration in concrete substrates. This system is very effective in resisting water and salts that are normally encountered in natural-weathering conditions.

The substrate was first thoroughly cleaned to remove all dirt and loose particles. Pro-Struct 670 was then applied to a dry surface with a low-pressure spray. As per StonCor’s instructions, the surface areas were flooded with the solution, with Citi-Con’s team ensuring a run-down on the masonry of between 150mm and 200mm. To avoid a high spread rate that would greatly reduce Pro-Struct 670’s effectiveness as a water repellent, a litre was used for every 2m² to 4m². This was dependent on the irregularities of the substrate surface.

Two coats of Pro-Struct 670 were applied with the second placed after about six hours and then allowed to cure over seven days.

Before the concrete could be sealed in this way, all of the windows had to be masked to avoid permanently staining them. This was an extensive undertaking that entailed covering 8 500m² of glass within the vicinity of the various working areas.

Moreover, various areas of the building had to be prepared simultaneously. This is so that the programme could continue when the Citi-Con team was unable to work between the two towers due to very high wind speeds.

Accessing the various work faces was a significant challenge. Initially, Citi-Con intended undertaking the entire project with rope access.

However, the building’s many overhangs obstructed the rope technicians. They were, therefore, equipped with window-suction cups to pull themselves closer to the working areas. While effective, it was a slow and tedious process. Thus, Citi-Con also deployed suspended cable systems and mobile scaffolding towers that could be easily moved around the perimeter of the building to the worksite. Adjusting them to the site elevations, the mobile scaffold towers provided safe and quick access to the fourth-floor level.