A major project comprising the structural retrofitment, rehabilitation and maintenance to the bridges over the spillways of the Van der Kloof and Gariep Dams has benefited from the expertise of Sika’s innovative systems in the field of construction materials.

The Van der Kloof dam (constructed in 1977) is the second largest (composite gravity concrete arch) dam in South Africa with a capacity of 3175 million cubic meters and is currently the highest dam in South Africa with a wall height of 107m and a crest length of 765m. The bridge over the spillway is curved horizontally, has a total length of 204m along the centerline and comprises 15 simply-supported spans of 13 meters. Due to the bridge developing vibrational serviceability concerns, it was closed to public traffic and the structure was then inspected, comprehensively tested and modeled by consulting engineers BKS (Pty) Ltd, with specialist support from the University of Cape Town.

The technical solution to these vibrational concerns required the installation of two new reinforced concrete transverse beams at third points to effectively tie the longitudinal bridge beams together. The bridge deck slab was then thickened using a fully-bonded, reinforced concrete pavement that required specialized concrete finishing techniques. The project was undertaken by Ibhayi Contracting CC.

Sika technical services assisted the contractor with the economic and performance optimization of the Self Compacting Concrete utilized to construct new reinforced concrete transverse beams. The self compacting concrete mixture, which contained Sika’s third generation concrete admixture Sika ViscoCrete-3088 (for 70 m3 of SCC to construct 30 new transverse beams) had to be introduced into the formwork from the top of the deck through 125mm diameter holes cored through the Existing deck slab. Technical mix parameters required a flow from 580 to 650mm, with an associated flow time range of 6 to 8 seconds, which was consistently achieved on site by the Contractor.

Sika ViscoCrete-3088 is a high range water reducing admixture with retarding properties for concrete and mortar and is used for the production of site batched and ready mix concrete. The advantages of using this concrete admixture for the reinforcing of the bridge are that it has an extremely powerful water reduction thereby creating high concrete density and high strengths; it has excellent plasticising effects, resulting in improved flow, placing and compaction characteristics as well as greatly improved watertightness. It is believed that this is the first time that Self Compacting Concrete has been routinely batched by a contractor in South Africa, who utilized locally available materials.

In addition numerous spalls at the beam-slab connection were repaired using Sika MonoTop-612 (2900 litre) high strength repair system; a cementitious, polymer modified, low permeability mortar containing silica fume and synthetic fibre reinforcement. This repair mortar is particularly suitable for application on overhead and vertical surfaces, it was placed by hand but can also be placed using the wet spray method. Sika MonoTop-612 has adjustable consistency and excellent slump resistance; it has good mechanical strength and resistance to water and chloride penetration.

Other conventional bridge maintenance items, such as the replacement of bridge expansion joints and replacement of bridge bearings, with associated bridge jacking operations, were performed on both the bridges over the spillways of the Van der Kloof and Gariep dams, using various Sika repair and reinforcing materials. At Van der Kloof 326 elastomeric bearing pads, and at Gariep 270 elastomeric bearing pads, were affixed between the longitudinal beams and supporting piers of the bridges using Sikadur-31 and Sikadur-32N epoxy bonding agents.

The Gariep dam is the largest (composite gravity concrete arch) dam in South Africa with a capacity of 5341 million cubic meters and is situated 130km upstream of the Van der Kloof dam on the Orange River. This dam is the central structure of the original Orange River Project, which involves the supply of water to parts of the Vaal, Fish and Sundays River Catchments as well as irrigation along the Orange River itself. The original construction was completed in 1971 and is 88m high with a crest length of 914m. The bridge over the spillway is curved horizontally, has a total length of 245m along the centerline and comprises 15 simply-supported spans of 14.5m.

Owing to the development of a transverse hogging mechanism induced by prestressing in the transverse beams, the loads applied to the bridge deck were concentrated on the outer longitudinal beams, resulting in cracking of the affected beams. The outer longitudinal beams were strengthened using steel plates that were epoxy bonded with 1000 litres of Sikadur PBA/SBA. This epoxy paste adhesive has excellent adhesion to concrete and its thixotrophic nature facilitates application in both vertical and overhead situations, making it the ideal bonding agent for the steel plates on the underside of the bridge structure. All remaining superstructure concrete was coated with 2350 litres of Sikagard-550W Elastic ZA, (primed with 985 litres of Sikadur 550 W Primer) a plasto-elastic coating that has crack bridging properties and can resist the further ingress of carbonation. Sikagard-550W Elastic ZA is used for the protection and enhancement of concrete structures, especially exposed concrete surfaces with a risk of cracking and gives excellent resistance against weathering and ageing.

A specialised suspended scaffolding system was erected under each bridge span. All structural retrofitting and rehabilitation tasks below the bridges were performed from these platforms, including installation of steel strengthening plates, fixing of reinforcement and formwork to cast the new transverse beams, bridge jacking and bearing replacement activities and the application of protective coatings.