Precast concrete beams and hollow-core slabs supplied by Concrete Manufacturers Association member, Cape Concrete Works, have been used to roof a new
Precast concrete beams and hollow-core slabs supplied by Concrete Manufacturers Association member, Cape Concrete Works, have been used to roof a new reservoir in Piketberg, 80km east of Saldanha Bay.
Commissioned by the Bergrivier Municipality, the structure adds 4ML to the town’s existing 3ML storage capacity. Erected on the site of a disused open reservoir built in the early part of the 20th Century, a site which entailed the lowest environmental impact, the new unit is situated next to a 2ML reservoir and both are fed by a third reservoir which in turn draws its supply from the Piketberg water-purification plant.
With a roof area of just under 1 400m² and shaped as a quadrilateral, the new reservoir was constructed by main contractor, Hiload Inyanga and was designed by iX engineers, the project’s consulting engineers.
The original intention had been to build a 2Ml circular reservoir on the site of the old reservoir but a subsequent geotechnical investigation revealed an existing 200mm thick solid concrete floor which could be deployed as sub-base material for the new structure.
“That left us with three options,” said the reservoir’s design engineer, Alwyn Le Roux. “We could stick with the original plan and build a 2Ml reservoir inside the old structure, or we could opt for a 4Ml reservoir by relining a section of the old structure and adding a roof to it. The third option entailed revamping the old structure as a 5Ml storage unit but it was never given serious consideration. This was because the Bergrivier Municipality wanted to allocate some of the old unit’s floor space for vehicle access and the construction of an above-ground valve-control chamber adjacent to the new reservoir.
“Although we went to tender on the first two options it was soon apparent that the second offered the best solution because a circular reservoir would have entailed additional excavation, work which was unnecessary in the construction of the 4Ml option. What’s more, on a cost per-cubic-metre storage basis the 4ML unit proved the better proposition and that was the clincher.
“We proceeded on the basis of retaining the old structure as a liner for the new floor and the three embankment walls. Although its concrete strength was more than adequate, over the years lateral movement between panels had taken place which meant it could only be used as a sub-base layer,” said Le Roux.
Construction began with the erection of a 5.6m cantilevered wall which formed one of the four reservoir walls and divided the old reservoir into two sections, two thirds for the reservoir itself, and one third for the control chamber and vehicle access. With a tapered outer face, the wall is 600mm wide at its base and 300mm at the top.
The wall footing was anchored to the old floor with Y20 shear studs, and 25 tonnes of rebar were required to build the footing and the wall. Both are 37m across and to accommodate the slope of the old floor, footing height varies from 1.2m to 700mm. The bulk of its 3.5m width falls within the reservoir to create the cantilever effect and it was cast in two lifts with protruding rebar and vertical sides to buttress the wall.
An 80mm no-fines drainage layer, also used as a concrete correction layer, was the first of three new concrete strata cast on the original concrete surface. However, some of the old embankment walling consisted of a stone-pitched concrete conglomerate which required remedial work in preparation for the no-fines layer in those sections.
A 10mm plastered layer was trowelled to seal off the no-fines concrete layer and a reinforced concrete lining, which, depending on the existing dam’s profiles, varied between 150mm-200mm, formed the third layer.
Perforated pipes are imbedded in the no-fines floor sections for sub-surface drainage, leak detection and pressure relief, whereas embankment drainage is channelled through the no-fines layers into the flooring’s perforated pipes.
The reservoir is divided into four zones using in-situ cast concrete cut-off beams. Movement joints were filled and sealed with a SIKA Flex polyurethane sealant. Thereafter a 200mm SIKA Combiflex (hyperlon) bandage was placed over the joints. SIKA Swell sealable strips were used in the construction joints.
“We used the same rebar for the embankments and the floor and we opted for a dry mix to minimise cracking and to make the embankment casting process easier. The steep embankment slopes in some areas required shuttering at the floor-end and the casting was done in stages to prevent the concrete from sliding down. Both the floor and the embankments were cast with protruding rebar for the construction of the column bases.
“It is normal practice to construct column footings on top of reservoir flooring which is what was done in this instance. Some of the columns were erected on the sloped sides and level footings had to be constructed to accommodate them,” said Le Roux.
The columns were cast at varying heights to ensure that the hollow-core-slab roof fell 1° to either side of the central beam structure for drainage. Five rows of parallel beams support the hollow-core slab roof covering and each of the two rows on either side of the central row are slightly lower than the preceding row to accommodate the roof slope.
Cape Concrete sub-contractor, CPI, took only five days to install the 38 beams and 240 slabs using a Teemane Cranes 100 tonne mobile crane.
Each beam end was cast with a vertical groove and once the beams were installed, opposing grooves formed a cocoon around Y20 dowel bars which had been cast into the middle of columns. This arrangement provided the columns with lateral support and once two facing beams were installed on top of a column the 50mm gap between them was grouted. Besides forming a beam joint, the grouting protects the rebar from corrosion.
All the hollow-core slabs are 150mm thick and most are 5.7m long. However, some are only 1.0m while others are as long as 6.8m. Moreover, some of the slabs ends are splayed to accommodate the structure’s quadrangular shape and prior to delivery, Cape Concrete cut them to fit. The control chamber, which provides easy access to the inlet and outlet valves, was also roofed with hollow-core slabs.
Cast with protruding rebar for splicing with the roof topping rebar, the 40MPa beams were designed to bear the weight of the precast slabs and the working load during construction. Structural continuity over the column heads was achieved once the topping had been cast. The structural topping was reinforced and treated with a crystalline additive for waterproofing and crack control and Xypex was used as a sealing/crack additive. Once the roof-topping screed had been cast, beam thickness increased from 350 to 570mm, yielding additional strength for live loads and deflections.
Cape Concrete inserted polystyrene void formers into the hollow cores of the roof slabs to limit the amount of concrete which could enter the cores during the screeding process. A layer of clean-washed aggregate forms a third and final roof layer and precast coping, cast on site by Hiload Inyanga, was installed on the roof edges to create a barrier for the aggregate and to improve the overall appearance of the reservoir.
Project Team
Client Bergrivier Municipality
Consulting Engineers iX engineers
Main Contractor Hiload Inyanga
Precast Concrete Supplier Cape Concrete Works
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