Creative use of precast

The completed Val de Vie Berg River bridge

Weighing 64 t each, 18 prestressed beams were employed during the construction of the Val de Vie Berg River bridge in the Western Cape, each measuring over 35 m long and 2 m high. Their manufacture required a creative mix design.

The large volume and tight spacing of the reinforcing steel, coupled with the presence of stressing cables, meant conventional concrete would not suffice and the local Chryso Southern Africa team worked in conjunction with the precast contractor to design, test and submit a selfcompacting concrete mix.

Chryso Premia 310 superplasticiser and Chryso Dem Bio 10 release agent formed part of the solution for the successful casting of these beams and were also used in the project’s large facing panels, coping 16 of the 18 prestressed beams in place and culverts. between the abutments and central pier

Once all the beam strands were evenly stressed, readymix concrete was placed and a combination of external and poker vibration was used for compaction. This provided a Close-up view of the precast facing good surface finish with minimal blow holes. panels and the prestressed beams After pouring, a tarpaulin was placed over the mould and steam was injected under the cover to accelerate hydration. Using this method, a strength of 40 MPa was achieved in 20 hours, enabling the production of one beam a day. The final strength required was 60 MPa, which was achieved within 28 days.

Off-shutter finish

Before the casting process, Chryso Retarder paste was applied at the end of each beam shutter and the concrete washed off with water the following day after the shutter was removed. This created a rough, exposed aggregate finish that provided a good bonding surface for the assembly of the bridge.

The facing panels were also steamcured and were cast with a glass-smooth finish. They were attached using ‘fingers’, eliminating the use of visual fixing on the front face. Some panels were curved, such as those used for cladding the central viewing cove.

Once all precast beams were in position, five diaphragm transverse beams were cast in situ between the precast beams. Permanent deck shutters, measuring 1.2 m by 300 mm by 50 mm, closed the remaining gaps between the precast beams, resting on recesses cast into the beams for that purpose.

Readymix concrete using Chryso’s waterreducing plasticiser Chryso Plast Omega 126 was then poured on to the shutters to create the deck.