Concrete pre-cast post-tensioned water tanks. J. Gavranic* and R. Premakumara**

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1 Concrete pre-cast post-tensioned water tanks J. Gavranic* and R. Premakumara** *Georgiou Group, 68 Hasler Road, Osborne Park, Perth, Western Australia, 6017, ( ** Parsons Brinckerhoff, 1 King William Street, Adelaide, South Australia, 5000 ( rpremakumara@pb.com.au) ABSTRACT Georgiou was appointed by the Water Corporation in to construct three tanks, with the capacity to hold vast amounts of water ranging from 15ML to 32ML that will enable adequate support for water infrastructure needs in Western Australia. Georgiou advised Water Corporation that concrete pre-cast post-tensioned tanks would provide greater resilience than steel tanks. With this, Georgiou worked with designers Parsons Brinkerhoff on all three projects. The basic philosophy behind a concrete post-tensioned tank is that the post-tensioning keeps the tank in. If the tank is in compression, it cannot crack and therefore there will be no leakage (i.e. no cracks no leaks). The pre-cast pre-stressed tanks are watertight and work with the natural durability of concrete (gets stronger when exposed to water over time). Bullabulling Tank is a 15ML ground level tank constructed to improve capacity for the Water Corporation s pipeline to Kalgoorlie. It is used to balance the demand of infrastructure in the area. Bullabulling was the first pre-cast post-tensioned tank constructed for Water Corporation and was completed in December Harvey Tank is a 32ML summit tank which has a storage capacity twice the size of Bullabulling and was the second tank constructed. Harvey stores water from the Southern Seawater Desalination Plant (SSDP) at Binningup. It was a critical component of the plant and needed to be fully commissioned before the SSDP could produce any water. Harvey was completed May Denmark Tank is a 15ML summit tank and almost identical in design to Bullabulling. The Denmark Tank was constructed to help meet water shortage demands and improve water infrastructure in the South West. Denmark was completed April 2011 This paper will discuss how Georgiou and Parsons Brinckerhoff provided Water Corporation with reliable long-term storage of large volumes of water in Western Australia.

2 INTRODUCTION Three recently constructed ground level tanks at Bullabulling, Denmark and Harvey in Western Australia have storage capacities between 15ML and 32 ML. The need for large and sustainable water tanks is becoming an increasing common demand in order to meet the population growth. Early prestressed tanks built in Australia prior to 1980 s were post-tensioned by winding galvanized high strength steel wires continuously around the tank from top to bottom and finished with sprayed concrete protection layer. This method of tank construction was adopted from United States where it is still being practiced. The Australian Standard AS 3735 for circular liquid retaining structures first published in 1988 requires bonded internal prestressing that meets durability requirements. Today, liquid retaining structures for water and waste water treatment plants have a choice of shape and can be designed as rectangular or circular. Furthermore, the owners of such key infrastructure expect it to be built cost efficiently, be an asset of low maintenance and have longer design life. Rectangular tanks are generally constructed in-situ using reinforced concrete and are therefore associated with higher construction costs. The wall height to length aspect ratio at each face is non-identical and analysis and design tools are available for determining stresses due to externally applied lateral hydrostatic load. Larger rectangular tanks consist of taller and thicker walls where temperature loading requires higher percentage of reinforcement. In contrast, a circular tank shape by nature is axi-symmetric and when subject to axisymmetric hydrostatic loading allows identical stress distribution and uniform thickness all around. Such tanks can be constructed either in reinforced concrete or post-tensioned where walls are symmetrically prestressed. Liquid retaining circular structures when prestressed allow the concrete to gain reserve compression capacity counteracting cracking. The prestressing option also provides the designer the ability to choose minimum wall thickness and accordingly reduce the impact of temperature loading. It is predominantly accepted by designers that prestressed circular tanks are the most efficient and the preferred shape for larger storage containers of present day treatment plants. Multi-cell circular donut shaped tanks under non-symmetrical loading conditions have been successfully designed and constructed using prestressed concrete in recent years. METHODS The three tanks discussed in this paper have been constructed to be cost effective through using horizontal prestressing of precast wall panels with in-situ wet-joint infill between them. The precast method provides faster construction and a higher level of quality control as the precast walls are fabricated in a controlled workshop environment. After prestressing, ducts containing the high strength steel strand wires are filled with cementicious grout acting as an additional layer of corrosion protection. Critical to the long term performance of the tank are the details of all vertical joints between precast

3 panels and wet-joints as well as horizontal joints at wall base and requires highperformance waterproof material. The design approach of all three tank walls is driven by the construction method. Precast wall panels are fabricated off site, delivered and then vertically positioned in a floor slab rebate. In the case of the Harvey Tank, which at 32ML is the largest of the three tanks mentioned in this paper, the tank consisted of: 60 standard precast panels which were 14 metres in height and weighed 20 tonnes; and 6 buttress panels which were 14 metres in height and weighed 47 tonnes. Figure 1. Buttress panels Once the precast panels were erected, the wet joints which stitch the precast panels together were poured in-situ to close up the tank structure. Following the completion of the tank wall erection, the entire wall is stressed in two stages: 1. During first stage of stressing where the walls are stressed to 50% capacity, the tank wall is free to slide within the footing rebate. At the completion of the first stage of stressing, the internal portion of the floor slab rebate is grouted to seal the gap between the wall and the floor slab. 2. The second stage of stressing where the walls are stressed to 100% capacity is applied only after the waterproofing and grouting of the internal portion of the floor slab rebate is complete. This places the internal grouting in compression to prevent cracking, and therefore leaking, through the wall / floor joint. The external portion of the floor slab rebate is then grouted to close out the wall / floor connection and this forms a pinned base condition. The level of prestressing and

4 wall base restrain conditions at each stage are modelled in the design and combined with other transient load effects that include temperature, swelling and shrinkage. Water Corporation s specification for three projects required the tank walls to be precast post-tensioned and designed to owners design manual DS61 which requires no residual tension in the tank wall under all loading conditions. Figure 2. Formwork and wet joint preparation works The design methodology for the three tanks is essentially a partial prestressed concrete design for horizontal stresses and a more conventional reinforced concrete design for vertical; but with a conservative approach to limits referred in AS 3735 to minimise risks. Under thermal load combination in hoop direction, the stresses remain well within allowable limits in the prestressed walls, with crack control over joints using normal reinforcements. The work on all three tanks incorporated a belts and braces approach to the wallfloor joint detail of the tank. In addition to the compression mechanical joint design mentioned above, the following was also incorporated: The use of hydrotite which was applied along the wall / floor joint; and The grout used at the wall / floor joint was enhanced with Xypex as an additional mechanism to prevent leakage through the joint. The two points above were also incorporated in the vertical wet joints. During the design phase of each of the three tanks, minor improvements were incorporated into each new design to improve both the cost efficiency and the constructability of the tank. This included:

5 Reduction of the post-tensioning duct sizes within the walls to prevent induced hairline cracking; Increasing the amount of reinforcement bar in the wet joints; and Installing localised wall thickenings within the wet joints to improve concrete placement and compaction. Figure 3. Harvey Water Tank - roof installation and panel completion works RESULTS AND DISCUSSION This paper describes design and construction aspects of three operational, large circular post-tensioned tanks which were successfully completed recently. The analysis and design of the tanks were driven by the construction method described in this paper and structurally modelled to represent wall boundary conditions at two stages of construction. Various loading conditions considered were hydrostatic pressure, prestressing, temperature, creep, shrinkage, wind and earthquake. The two dimensional models for axi-symmetric loadings and more accurate three dimensional finite element models for asymmetric loadings are most commonly used by engineers for similar tank design. All the three tank walls were horizontally prestressed and vertically reinforced. When concrete tanks are post-tensioned, they become more durable than reinforced concrete due to the presence of residual hoop compression. The design code requirements for crack width control are fulfilled by satisfying stresses in reinforcement steel to the specified limits.

6 The durability of prestressing strands is guaranteed by implementing best practice and high performance grouting procedures. The use of hydrotite sealants in combination with the Xypex admixture in grout for all in-situ joints have proven to be successful in waterproofing. For circular tanks with symmetrical load configuration, simplified analysis methods using 2-D software tools and design tables in Australian Standard AS 3735 have proven to demonstrate sufficient accuracy in comparison with more rigorous FEM analysis. The hoop stress and vertical bending moment plots are illustrated below in Figure 1 and 2. Figure 1: Harvey Tank - Variation in inside face hoop stress in full tank due to liquid, prestress, and positive temperature Figure 2: Harvey Tank - Variation in inside face hoop stress in full tank due to shortterm and long-term load combinations to AS 3735

7 Figure 3: Harvey Tank - Variation in vertical bending moment in full tank due to liquid, prestress, and positive temperature CONCLUSIONS The construction of pre-cast post-tensioned concrete tanks in Western Australia is proving to be a successful solution to the long-term water infrastructure requirements for the Water Corporation. The design has enabled the Contractor to provide tanks with a 100 year design life that need minimal maintenance due to the durable nature of concrete and the posttensioning to minimise the effects of concrete cracking. The examples at Bullabulling, Harvey and Denmark show that the design philosophy of no cracks no leaks is possible to construct through the use of a precast concrete system. REFERENCES 1. Australian Standard, Concrete structures for retaining liquids (AS 3735), Australian Standard, Concrete structures for retaining liquids commentary (supplement to AS 3735), 2001.

8 Figure 3. Aerial view of the completed storage tank