Civil Engineering October 2022 | Vol 30 No 9

12 October 2022 Civil Engineering added to the top surfaces of the walkways, rendering them flush with the 40 mm premix layer on the bridge deck. This additional layer includes several service ducts and slopes at 1.5% for stormwater drainage. Bridge drainage is effected with a 1% east-to-west slope. The M-type beams were cast at Cape Concrete’s yard in Cape Town as a portal framework based on a bending schedule and rebar drawings supplied by AECOM. They comprise a wide bottom flange, a narrow web portion and a flared upper flange section. “We used a W50 mix with a super plasticiser in a 100 slump flowable concrete which complied with three dura- bility criteria: oxygen permeability, water sorptivity, and chloride conductivity. The plasticiser reduced water consumption and gave us better workability. What’s more, we achieved a transfer strength of 40 MPa which was reached in 18 hours using the high-strength mix design and steam curing,” says Cape Concrete Director Johan Nel. “After de-tensioning, the beams had a vertical positive camber of between 15 mm and 20 mm, but once they were positioned on the bridge and the in-situ deck slab had been cast, they assumed a neutral camber. “Reinforcing is congested when using prestressed strands, which is why we used a 13 mm stone in the mix. It is vital for the concrete to flow through the rebar and fill the entire structure without air pockets, and smaller stones facilitate this process. We used external vibrators and pokers to remove all the air, but we only used pokers on the top flange sections. “One of the plusses in M-type beam casting is that the bottom flange dimen- sions remain constant. If larger spans are required, one simply adjusts the web or top flange depths. This means one can use an adjustable mould from an M2 to an M10 beam, and M3 beams were used at Distillery Road. “In this instance the webs were a bit higher and the top flanges a bit deeper. But because the profile did not vary, the beam casting was economical. Engineers often design using available moulds and if only small adjustments are required, costs are saved and lead times reduced,” concludes Nel. Approximately 60 m of feeder road on either side of the bridge is being resur- faced and these new road sections will be bordered with new pavements. Author Text and photographs by David Beer on behalf of the Concrete Manufacturers Association. david.bigsky@gmail.com Project team Client Stellenbosch Municipality Consulting engineers AECOM SA Main contactor Martin & East Precast beams Cape Concrete Support work Form-Scaff Heritage architects Rennie Scurr Adendorff Architects Heritage consultants CTS Heritage The underside girders and Econo-Form panelling