Project Description

STRUCTURAL TECHNOLOGIES were appointed under STRUCTURAL to carry out a condition assessment to ascertain the current corrosion activity and future corrosion risk of the reinforced concrete elements within Basement 3 (Raft, Walls and Columns).

The condition assessment determined that Chloride ions had migrated into the concrete elements in sufficient concentration and to a depth which enabled corrosion of the embedded steel reinforcement. The source of the Chloride ions was from the surrounding ground water which has penetrated into the building due to failures of the waterproofing and dewatering infrastructure.

The surface area of the Raft was 8,500m². The concrete cover for around 75% of the total area was delaminated or spalled due to the applied stresses from the expansive corrosion products from the Chloride-induced corrosion of the embedded steel reinforcement.

Although significant Chloride was also measured within the first 1m of the vertical elements (walls and columns), the resultant corrosion activity was not aggressive enough to instigate widespread delamination at the time of the assessment. Notwithstanding, corrosion activity was prevalent and with time, delamination and spalling would have occurred.

The following scope of works was developed by STRUCTURAL, the client and an independent consultant. STRUCTURAL successfully managed to work directly with the concerned authorities to achieve approval on the repair solution to secure the building permit.

1. 1. Concrete repair of vertical elements – 1,500m² (walls and columns)
   2. Crack injection – 2,000 lm
   3. Removal of Chloride contaminated raft and re-skinning – 8,500m² / 1,000m²
   4. Cathodic Protection for 1 m height of vertical elements – 2,000m²

Concrete repair was carried out on the walls and columns in accordance with ACI using STRUCTURAL pressurized form and pump concrete placement. Cracks were treated with epoxy injection or route and seal methods in accordance with ACI RAP1. The reinforced concrete raft was removed to a depth of 50mm behind the steel and recast with 100mm min. concrete to steel cover as the client opted against Impressed Current Cathodic Protection (viable alternative solution).

A Second Generation Sacrificial Anode system was designed and implemented by STRUCTURAL TECHNOLOGIES under STRUCTURAL to provide an optimized Cathodic Protection system which would mitigate corrosion for at least 15 years. The discrete Zinc anodes were installed into holes drilled into the concrete cover before being encapsulated by the manufacturer’s proprietary grout. The protected area was divided into zones and the anodes from each zone were connected to the steel via a Junction Box; each zone can be monitored by its permanently installed reference electrodes and the galvanic current can be measured during monitoring.

The hybrid anodes arrest corrosion via pit re-alkalization treatment impressed of the sacrificial anodes followed by subsequent galvanic protection from the same anodes. This type of system is referred to as a HYBRID ANODE system which combines the power of an Impressed Current Cathodic Protection (ICCP) system with the low maintenance requirements of galvanic protection.

The ICCP phase is powered by a state of the art STRUCTURAL TECHNOLOGIES power supply system which delivers the required charge (measured in Coloumbs) before automatically switching to galvanic mode. Unregulated power supplies such as batteries should not be used for such applications as the charge passed cannot be monitored accurately.

Having all of the concrete repair items and Cathodic Protection implemented by a single specialist contractor, STRUCTURAL, increased the efficiency of the project and lead to a faster handover of the structure back to the main contractor so they could complete the construction of the building.