Complex concrete silo structural repairs

Citi-Con is completing complex structural repairs on bins 21 and 22 for a leading steward of raw materials in the food production industry. These interventions represent the most critical component of the extensive specialist concrete repair and corrosion protection solutions that the company has already undertaken for this client across various silo complexes.

The scope involved reinforcing the primary support beams – 10 on the rigid side of bin 22 and two on the flexible side of bin 21 – which tie into the steel beams which interconnect the structure linking the machine tower to the concrete silo. Over time, differential movement between the machine tower and the concrete silo structures caused shear-related distress on the metal beams and concrete spalling at the concrete anchoring points (beam-wall) connections. If left unaddressed, this deterioration posed a significant risk of structural failure.

Work began on bin 22, replacing the five front and five rear H-plate assemblies in sequence. Citi-Con started at the lowest level, completing one H-plate assembly per day.

The engineered solution required cutting back each existing beam by 50 mm and installing a new H-plate reinforcement system.

Work started by cutting back each existing beam, removing it, and then using it as a template for precise measurement and drilling.

Six 200 mm-deep holes were then drilled through the concrete using one of Citi-Con’s powerful industrial Hilti drill machines. Thereafter, threaded bars were inserted and secured with chemical anchoring mortar that achieved full strength in just 30 minutes.

A 600 × 600 × 12 mm internal plate was then hoisted and aligned with a 100 × 100 × 6 mm washer plate and nut to secure the internal anchor point.

This was followed by reattaching the new A4 stainless-steel plate using high-tensile bolts, spring washers, and lock nuts.

All plates were CO₂ welded at 45° and 90° angles. Offering deep penetration and high deposition rates, CO₂ welding is particularly suited to thick steel. A sealant was also applied between the concrete silo walls and the new H-plates to prevent water ingress and possible rusting in the future, followed by anti-corrosion and final protective coatings.

By the time the project is completed, Citi-Con had safely handled and installed more than 924 kg of structural steel as part of this innovative turnkey structural engineering solution.

iti-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.