Innovation is key to solving the water challenges with which the country grapples. This includes revolutionising construction practices to deliver critical municipal water infrastructure quicker and more cost-effectively. At the same time, these new builds need to be robust so that they continue to add value for many years with minimal maintenance and repairs.
Precast concrete meets all these requirements and more, including the ability to execute projects in a safer manner by restricting work at height and by placing concrete in a setting that can be controlled more easily than on a worksite. Then there is the smaller carbon footprint of industrious precast-concrete operations. To remain competitive, enterprising fabricators use energy, water and materials judiciously. This is in addition to the innovative deployment of admixtures and materials to produce concrete mixes with lower cement content. These ultra-strength mixes, together with the sophisticated curing processes deployed in a controlled factory environment, also facilitate the manufacture of high quality prefabricated concrete elements that are less susceptible to corrosion. This, in turn, reduces carbon emissions from maintenance and repair operations over the lifecycle of the structure.
This method of construction has proven itself time and again when building reservoirs and, more recently, two water towers, among the latest innovation in precast concrete from a recognised leader in the field, Corestruc. These are the first water towers to be constructed using this modular method that is based on a uniquely South African design.
They have also been built in record time in areas plagued by water shortages. It would have taken between two and three years to complete similar sized structures of the same quality using traditional cast-in-place concrete construction methods. As more municipalities and their engineers specify this method of water tower construction in jurisdictions, Corestruc will have an opportunity optimise its processes even further to construct these structures much faster.
The 18 m-diameter prefabricated tanks, each with a 2,5ML of water-storage capacity, are supported 34 m above ground level by an innovative precast concrete structure, consisting of columns, beams and hollow-core slabs.
A standout feature of both towers is the spiral beam that provides critical support to the columns. However, it fulfils an even more important role than its striking aesthetic value by providing lateral support to the columns. It was cast in U-shaped segments to complete a 30o rotation for each beam. They were made continuous by installing site-placed rebar inside and then filling them to the brim with cast-in-place concrete.
The columns were cast to individual lengths to fit between the spiral beam levels. They were made continuous via cast-in threaded sockets in which dowels were installed and then filled with non-shrink grout.
Supporting the centre of the tank and housing the stairs and pipes, the central shaft consists of individual precast-concrete rings, each about 1,6m in height. These rings are connected via bespoke cast-in mechanical connectors, which also served as a line-up and levelling mechanism during construction.
Its diameter was dictated by the allowable maximum transport width. A challenge was installing the 600 mm and 500 mm diameter inlet and outlet pipes, respectively, in addition to the access stairs, inside.
Tapered precast-concrete beams on top of the columns connect to the shaft at tank floor level where hollow-core slabs were used as a permanent shutter to cast the 350 mm-thick in-situ tank floor.
The tank floor beams were designed to support the weight of the precast-concrete slabs with that of the wet concrete while still in the virgin prefabricated state and as a composite with the cast-in-place floor to withstand service loads.
The tank is a based on a tried-and-tested reservoir system design, although a significantly smaller water structure than the company’s 50 ML and larger reservoirs.
It consists of 170 mm-thick precast-concrete wall panels with a 150mm-thick hollow-core slab roof. Suspended precast-concrete beams are connected to the dowels that protrude from the precast-concrete columns.
For temporary stability, the wall was provisionally braced back to the roof structure, until all panels were positioned to form a complete circle.
Once all the panels were placed, unbonded cables were pushed through their horizontal polyvinyl sleeves, cast into the vertically prestressed prefabricated elements at designed positions.
Hereafter, a grout was poured continuously in between the wall panels and horizontal cable sleeves. It is a high strength and flow type with an extended pot life so that it does not segregate and set to early. These characteristics are achieved by manipulating the 0:37 water-to-cement ratio with the use of admixtures.
The cables were stressed to 75% when the grout reached 80 MPa. Prestressing was undertaken via two precast concrete buttress panels that were spaced across from each other. The wall was then pinned by casting a 200 mm to 250 mm-high reinforced kicker on the 350 mm thick floor on both sides of each panel.
Corestruc uses a “slide-and-pinned” system. Post-tensioning is undertaken when the wall is not yet fixed to the base and it is, therefore, allowed to slide 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 a minimum of 1MPa in all directions.
Again, the project benefited from extensive upfront planning between all stakeholders. This is considering that there is very little scope for variations in precast concrete projects once the system has been manufactured strictly to specification. City of Ekurhuleni Water and Sanitation Department, the client; Tango’s Consultants, the consulting engineer; Infinite Consulting Engineers, the precast-concrete consultant; and RSMM Construction, the principal contractor, all assisted Corestruc with pre-construction planning.
The manufacture of the 12 columns and spiral beam elements for each of the three sections; the 15 prefabricated elements that make up the shaft; and the 12 tapered beams started during the earthworks and site terracing. By undertaking casting at ground level and in a controlled environment, concrete elements of an exceptionally high quality are manufactured. For example, the necessary modifications can be made to maintain the optimum water-to-cement ratio by accurately calculating the moisture content of aggregates and considering water from admixtures. Furthermore, admixtures are used to modify fresh or hardened concrete to increase their durability and abrasion resistance.
Once they arrived on site, the elements were thoroughly inspected again to ensure that they were not damaged during transport. Even the smallest cracks will allow contaminants to enter the concrete matrix and initiate corrosion once the passive layer around the rebar is breached. If maintained correctly, these precast-concrete structures have a lifespan of well over 100 years, reducing operating costs for municipalities.
However, precision manufacture, which includes the accurate placement of the many cast-in-components, also facilitates efficient construction once the elements arrive on site.
Corestruc uses a robotic total station, which provides unmatched precision and safety, as well as enhanced efficiency and time savings. It rotates and angles itself with pinpoint accuracy, reducing human error.
A 150 t hydraulic crawler provided the capacity to lift the heavy precast-concrete elements and reach to efficiently place them.
An articulated boom lift was placed on top of the superstructure to assist with the installation, as well as the grouting of the 34 tank wall panels. This is in addition to the two buttress panels for post-tensioning.
“The challenges that beset municipalities demand smarter ways of constructing infrastructure. With massive backlogs in service delivery systems, municipalities are under pressure to not only complete projects quicker, but to build quality infrastructure in an affordable, safe and sustainable manner. Our precast concrete solutions respond to these demands,” Willie de Jager, Managing Director of Corestruc, concludes.
