Civil Engineering December 2021 | Vol 29 No 11

a solution that is maintenance free with a long-term cost saving for the client. ENGINEERING DESIGN The bridge superstructure comprises a continuous composite precast pre-stressed concrete beam and reinforced concrete slab deck, consisting of adapted U12 beams and a 150 mm reinforced concrete top slab. Concrete was particularly selected as a construction material for its versatility in fully encasing and supporting the internal sewer line and its durability in being able to resist the environmental conditions to which the structure would be exposed. Using precast pre-stressed concrete beams for the bridge deck further demonstrated the versatility in concrete and particularly pre-stressed concrete in efficiently supporting the loading of the internal sewer line. Furthermore, the precast beams would be self-supporting during construction, thus eliminating the need for temporary supports within the river floodplain and therefore reducing the risk of flooding as well as easing and speeding up construction. The encased gravity sewer line com- prised a structured wall HDPE pipe. The selection of the HDPE pipe was based on its reduced weight which was vital in reducing the loading on the structure. The HDPE pipe is also inherently flexible and can withstand larger deformations and deflec- tions than other more rigid and brittle pipe materials. This was of particular impor- tance at the abutments which incorporated free movement joints for the pipe. The bottom half of the HDPE sewer pipe is encased in concrete to restrain excessive movements of the pipe due to thermal expansion and contraction. To prevent the pipe from floating during the casting of the encasement concrete, the pipe had to be tied down. The holding down points were spaced 3 m from each other to prevent excessive local stresses developing at these positions. It was contemplated that the inside of the bridge could be flooded in case of a spill. The holding down mechanism will therefore provide for flotation of the pipe for its design life. Normally, metallic holding down straps would be used for this purpose. In this case it was considered prudent to use cast in situ reinforced concrete collars. This also eliminates the risk of damage to the precast deck units by attaching bolted metal straps. Filling the pipe before casting the encasement concrete negated the upward flotation forces completely. Using concrete also allowed the deck slab to be made continuous over the piers to eliminate the necessity for expansion joints, limiting them to the abutments only. The abutments consist of short, reinforced concrete stems on perched pile caps. The piers consist of 1.2 m × 1.3 m rectangular, rounded reinforced concrete stems on pile caps, each supported by 4 mm × 600 mm diameter piles. The piles are socketed into bedrock about 10 m below the riverbed. A peak flood of 1 200 m³/s for the 1:100 year return flood period was required. Due to the vertical alignment of the bridge, the required freeboard for this flood was significantly higher than the stipulated minimum according to the SANRAL drainage manual. This meant that no lat- eral flood forces were anticipated to act on the deck. This allowed the deck to be fixed against lateral movement at the piers and prevents any excessive horizontal deflec- tions at the abutment movement joints. GLOW IN THE DARK CONCRETE The revolutionary adoption of photo- luminescent polymer concrete added a new dimension to the eZimbokodweni Pipe and Pedestrian Bridge, as it will for infrastructure projects going forward. During the construction phase of the project the community indicated that lighting was needed on the bridge for safety reasons. Given the high cost of electrical lighting, load shedding and theft of electrical cables, an innovative solu- tion to illuminate the bridge was found in using polymer concrete infused with photo-luminescent particles which create a glow in the dark effect. Phosphorescent particles can absorb light, natural or artificial, during the day and release this stored energy at night in the form of visible light, creating a lumi- nous glow. This absorption and emission of light is only possible through materials that allow light to pass through them, hence the use of polymer concrete. The light intensity is at its brightest initially and then starts to decrease at a faster rate in the first two hours. Thereafter it stabilises and gradually decreases over the next 10 hours. As a result, the concrete will glow from 18:00 to 04:00. This presents as an ideal source of light and provides improved sight dis- tances for both drivers and pedestrians. Civil Engineering December 2021 15 Concrete was selected as a construction material for its versatility