Precast concrete system accelerates reservoir construction

Precast concrete system ACCELERATES RESERVOIR CONSTRUCTION

This system enables various trades to work simultaneously, saving construction time in the process. Using cast-in-place methods, work starts with the construction of the concrete floor slab, followed by the wall and ending with the roof.

The roof and wall of a 25 Mℓ reservoir were erected recently, in as little as three months after the cast-in-situ floor, as were the bases for the precast concrete roof structure and ring foundation, which were completed by the principal contractor. This reservoir has an inside diameter of 65.2 m and water height of 7.8 m. The roof column grids for the roof slab and beam span are 10.87 m by 10.87 m.

The system harnesses the precast concrete design skills and experience of Infinite Consulting Engineers and Corestruc’s know-how in concrete design and application, as well as rigging and erection. It is manufactured by precast companies that have been approved by Corestruc to ensure a final structure of the highest possible quality.

These reservoirs have been designed to provide up to 100 years of service if they are appropriately maintained. This is considering that the concrete used to manufacture the various prefabricated elements that make up the system has a permeability index that far exceeds 10, a sorptivity index well below 6 mm/√h, and a chloride conductivity of less than 0.75 mS/cm.

Self-compacting concrete is used to manufacture elements that contain significant reinforcing and where conventional vibrating pokers cannot be used effectively. Trial mixes are prepared and tested for strength and durability, while also taking into consideration the workability of the concrete. Working in controlled factory environments, the perfect dosages of admixtures are also achieved to ensure timely strength development, which is imperative for uninterrupted production.

In these factories, the perfect water-tocement ratio is also maintained to ensure concrete strength. Moreover, aggregates from suitable suppliers are graded appropriately to avoid early concrete shrinkage. The prefabricated elements are inspected and approved for dispatch to ensure the accuracy of the placement of the cast-in components. Reinforcement bar size and placement are also verified as part of the quality control procedures at the approved factories.

The construction of reservoirs ranging between 10 Mℓ and 60 Mℓ in size is being significantly accelerated using a unique precast concrete system that was designed in South Africa. By Willie de Jager, Kobus

Kotze & Tian de Jager Setting out and installation

These steps are complemented by the checks and balances introduced by Corestruc ahead of mobilising to site to commence erecting the structure. The company manages the setting out and installation of the column anchors for the precast concrete roof. By confirming the dimensional accuracies before erection, the company ensures that the beams of the roof structure fit the first time round. The precast concrete wall panels are placed and positioned in a similar manner with the assistance of permanent locating plates that are fixed to the ring foundation. As an extension of the principal engineering consultant’s office, Infinite Consulting Engineers provides professional project input. The firm also collaborates

The precast concrete system consists of a prefabricated roof and wall

with the engineering team to ensure that the various aspects of the work scope integrate seamlessly to avoid delays. Moreover, Infinite Consulting Engineers approves and accepts liability for the final precast concrete structure.

Constructing the roof structure

The roof structure consists of hollow-core slabs that are placed on precast concrete beams, which are supported by prefabricated columns. They are connected to the in situ bases by components that are cast into the precast concrete columns, also referred to as column shoes.

Similar to the construction of a conventional steel structure, the precast concrete columns are connected to hold-down bolts in the base. Suspended precast concrete beams are then installed on to the columns. They are secured with dowels that protrude from the columns.

The hollow-core slabs are then installed on to the beams and secured in place. This is achieved by tying the steel reinforcing and placing in situ concrete into the cores of the prefabricated slabs. Stirrups protruding from the precast concrete beam act as a mechanical interlock to form a composite mechanism with infill concrete.

Using traditional methods, a reservoir roof is the most complex and timeconsuming aspect of the entire build. It is undertaken once the wall and in situ columns reach their final height. Many tonnes of scaffolding must be installed inside the reservoir to support the forms for the frame of the roof structure. The extensive associated reinforcement and concrete works, including placement and stripping of the shutters, also requires close and careful management. This is in addition to the timely coordination of the concrete trucks to site for the continuous concrete pours.

Circular walls

The circular precast concrete wall comprises panels that have been prestressed vertically. They feature 32 mm diameter polyvinyl chloride sleeves that have been positioned according to the post-tensioning design.

The panels are placed into position and then supported by push-pull props and steel brackets at the top of each. Unbonded cables are pushed through the sleeves, which are then grouted monolithically with the joints of the panels.

The grout is poured continuously in-between the wall panels and horizontal cable sleeves. It is important to achieve a high-strength, high-flow grout with an extended pot-life and free of segregation. To achieve this, the water-to-cement ratio of 0:37 is manipulated with the use of admixtures and the water temperature reduced and controlled to act as a chiller in the mix. Furthermore, only cement is used in the grout mix, which also contains an unhydrated cement that reacts with water and seals possible leaks.

After the grout has cured to a strength of 80 MPa, the cables are stressed to 75% via precast concrete buttress panels that have been spaced along the perimeter of the reservoir.

The wall is then pinned by casting a 200 mm to 250 mm high reinforced kicker on the wall footing on both sides of each panel. Joints between the panels are grouted with a high-flow, high-strength

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grout and post-tensioning renders them in compression to achieve watertightness.

Slide and pinned system

Notably, unlike most conventional posttensioned reservoir walls, which are designed to slide, Infinite Consulting Engineers and Corestruc deploy a slide and pinned system. Post-tensioning is undertaken when the wall is not yet fixed to the ring footing and it, therefore, slides on a steel bearing or locating plates. The coated post-tensioned cables are not bonded to the grout, with the reservoir designed to maintain a residual compression of at least 1 MPa in all directions. Horizontal reactions to the wall base are transferred to the ring foundation through the second-phase, castin-situ kicker. This is where the ring tension in the base is also activated to resist the reaction. Additional post-tensioning of the lower part of the wall reduces the amount of rebar required in the cast-in-situ ring footing. By comparison, conventional castin-place methods involve significant steel-fixing and the onerous installation of side shutters. Care also needs to be applied when managing processes such as the placement of the concrete to avoid segregation and the displacement of the reinforcement to ensure a watertight structure. Any mistakes in this aspect of the works programme will require rework and result in costly delays.

Smaller carbon footprint

There are also other important advantages offered by the system that have been noted by municipalities. This includes the precast concrete reservoir’s smaller carbon footprint. The vertically stressed precast concrete wall panels are thinner, reducing the amount of

The circular precast concrete wall comprises panels that have been prestressed vertically aggregate and concrete required to build the reservoir. The hollow-core slabs, alone, reduce the carbon footprint of the structure by up to 40%, augmented by the use of prestressed 665 mm by 460 mm I-shaped beams. The self-compacting concrete used to manufacture the various precast concrete elements also reduces the volume of the construction material required and conserves energy by eliminating the need for vibration. Notably, the system also provides a safer means of constructing reservoirs. This is considering that the various elements are manufactured at ground level and erected on-site by a skilled and experienced team. Corestruc and Infinite Consulting Engineers have designed and erected reservoirs of various sizes in the country

Community participation

The principal contractors working alongside Infinite Consulting Engineers and Corestruc have also met all their socio-economic commitments. Locals are usually employed to work on the earthworks and site terracing. They also have the opportunity to assist with the construction of the floor slab, outlet and inlet works, and pipework. At least 30% of the contract value is usually set aside for socioeconomic development on these projects.

Conclusion

Precast concrete has helped to accelerate and mitigate risks on complex civil engineering construction projects the world over. This includes demanding reservoir projects that are traditionally built using castin-place concrete methods.

South Africa has gradually followed suit, with many municipalities and their engineering teams now also specifying precast concrete reservoirs in the early phases of water augmentation projects to overcome many of the challenges associated with constructing these structures using conventional methods.