Durban Heights Reservoir 3 is a 350ml reservoir situated just outside the Durban CBD, in the suburb of Reservoir Hills, Kwa-Zulu Natal. This reservoir was constructed in 1971 and is one of the biggest concrete reservoirs in Southern Africa. It supplies potable water to over five hundred thousand people in Durban and the surrounding area.
The reservoir design is unique in the sense that it has a single support in the middle and the roof is shaped like a circus tent. The roof is made up of one hundred and twenty steel post-stressed cables that radiate from the centre roof support to the outer base support, which again supports 4 440 precast panels. The total roof area is about 26 000m². Each post-tensioned cable consists of 41 x 7mm mild steel strands, protected by a metal duct filled with a bitumen mastic. An engineering masterpiece in its entirety.
In 2003, Umgeni Water engaged with civil engineering consultants, requesting a full diagnostic survey of the concrete roof panels. The objective of the survey was first to establish the current condition of the roof panels, and then to establish an effective repair and rehabilitation strategy, which would also be applied to future repair projects. The varied diagnostic survey included testing of chloride ingress, carbonation ingress, and existing cover to reinforcing. A variety of issues were identified but of interest was the existence of a micro-climate within the reservoir. It was noted that where the roof soffit at one point totally saturated, was found to be perfectly dry in a few hours. This wet and dry cycle was found to be a daily occurrence. This micro-climate with wet dry cycles creates a favorable environment for contaminants to be absorbed into the reinforced concrete panels causing corrosion of the reinforcing.
Based on the original diagnostic survey, rehabilitation proposals and the subsequent product trials, the Sika solutions and innovative products were deemed the best performing repair system for this project. Following the final trial results, Umgeni Water appointed Royal HaskoningDHV to take on this rehabilitation project, and Sika products were specified on the tender. The project engineer Royal HaskoningDHV compiled Tender Documentation for the repair and rehabilitation of the Durban Heights Reservoir 3 concrete roof structure. Smart Civils Construction (Pty) Ltd, a specialized civil contractor, was appointed as the successful tenderer.
As the work was being done in an operational reservoir the product choice was paramount to health and safety in ensuring they posed no risk of water contamination. The micro-climate phenomenon presented a challenge where the high humidity and moisture on the roof soffit threatened the successful application of the repair products. The nature of the work changed when the client, Umgeni Water, agreed to drain the 350ml reservoir during the off-peak season in mitigation of these concerns.
One of the main challenges on site was to find a repair mortar that met the required specified properties but could be easily applied to the soffit of the roof under difficult conditions. Having consulted Sika, the decision was made to use SikaTop® Armatec®-110 EpoCem® as the primary rebar protective coating for the concrete repair to protect the reinforcing of steel and inhibit corrosion. Due to the difficult conditions, Sika MonoTop®-610 was used as a primer to the concrete substrate. The decision to use Sika MonoTop®-412 NFG as a repair mortar was because of its unique properties as well as an excellent wet spray mortar.
This application method improved the compaction of repair mortar and increased the application rate of the product. Sika MonoTop®-412 NFG also differentiates itself as a 1-component, polymer modified, fiber reinforced, low shrinkage structural repair mortar with Sika’s world-renowned Sika®FerroGard®-903 Plus corrosion inhibitor. That is a 3-in-1 advanced product solution.
Sika® FerroGard®-903 Plus innovative technology delays the start of corrosion and reduces the corrosion rate when it occurs. It also increases the service life of a reinforced concrete structure.
Due to the age of the structure and minimal concrete cover, excessive spalling occurred on 15% of the surface area of the panels with additional external damage to the 20 radial expansion joints. Repairs had to be carried out on the spalled concrete areas and in some cases the badly corroded rebar had to be removed and externally strengthened. Sika’s carbon fiber fabric impregnated with a structural epoxy resin was used. The carbon fiber fabric of choice was SikaWrap®-230 C, which is a unidirectional carbon fiber fabric of 230g aerial weight.
Smart Civils consulted with Sika and the engineer to determine which product could be used to refill the ducts. This application was unique as the cables and cable ducts could not be removed and could not be drained of the existing bitumen protective filler.
By carefully opening sections of the cable duct and exposing the cables, Smart Civils were able to conclude that the existing cables were still partially protected by the remaining bitumen. Because of movement around the cable ducts, minor cracks had developed in the sleeve. On hot days, when bitumen becomes more fluid, these cracks allowed it to leak. Some investigation and product research culminated in Smart Civils and Sika identifying the perfect product for refilling the ducts - Sika® Injection-307. Sika® Injection-307 is a low viscosity elastic polyacrylic injection resin that can be adjusted to set at a pre-determined time, forming a passivating permanently elastic matrix. With community health and safety being top priority for Umgeni Water, they require a Potable Water Certification for any product that is used in the reservoir. Sika arranged for the Sika® Injection-307 to be tested at a local lab to ensure that it was safe to use with potable water and to determine whether it was compatible with the existing bitumen in the ducts which is not a normal injection application requirement. Smart Civils conducted on site trials and a test result report was compiled and then submitted to the engineer and Umgeni Water for approval. Approval was granted by all these relevant parties. Constant monitoring was done on site during the application of the Sika® Injection-307. This was to ensure that the product was penetrating through the partially filled ducts and creating a complete matrix around the all-important steel cables. Due to the current state of the rebar in the roof panels, two protective coating systems were applied to the panels.
The first part of the protective barrier is the Sika® FerroGard®-903 Plus. This corrosion inhibitor penetrates down to rebar level over time and forms a protective monomolecular layer on the surface of the reinforcing steel.
The final protective coating in the solution is the Sikagard®-705 L. A single component, low viscosity, solvent free and reactive impregnation product for concrete and cementitious substrates based on silane. It also has a 99 % active ingredient water repellent. Sikagard®-705 L forms a hydrophobic barrier to further concrete saturation. However, its unique properties allow moisture vapour to escape from the concrete panels, ensuring that they stay dry and thus preventing further ingress of contaminants.
Other Sika systems used on site was Sika® Primer-3 N and Sikaflex® PRO-3 i-cure, a one part, moisture curing elastic joint sealant with high mechanical resistance used to seal the expansion joints of the soffit.
Sikadur®-43 ZA is a solvent-free, three-component, repair and filling mortar used to do roof repairs next to the cable ducts.
The use of highly durable and high performing Sika products in the refurbishment of this reservoir will have lessened the future refurbishment cycles, saving on material consumption and time throughout the structure’s lifespan. A unique project such as this, required innovative product technology, complicated logistical planning and good collaboration amongst the project team Inventive, effective, and safe solutions were essential for this one-of-a-kind water retaining structure. Sika was honoured to be a part of such a challenging and rewarding project. Not only could we showcase our innovative product technology, but most importantly, also contributing to ensuring the safety and longevity of one of the biggest concrete reservoirs in Southern Africa.