Economics of Post-tensioned Circular Concrete Tanks in South Africa

When is strand cheaper than rebar as circumferential reinforcement in circular concrete tanks?

In circular tanks smaller than a certain critical size, hoop tension is sufficiently low for effective crack-control by only moderate rebar. In this size range post-tensioning is uneconomical, but with increasing size, a critical point is reached where post-tensioning becomes the economic choice. So then, when is strand cheaper than rebar?

In South Africa, it is common for reservoirs above 10Ml to be post-tensioned, but rarely does one find a 5Ml post-tensioned reservoir. This observation may give the impression that prestress is an economic choice above the 10Ml mark. Regardless of local trend, it can be shown that the critical point, in terms of reservoir size, is much lower than 10Ml.

A comparative study should not be over-simplified. There are numerous variables that influence economic function and it would be a mistake to assume that the optimal tank shape is the same for both prestressed and passively reinforced cases. From shape-optimization analysis I find that the prestressed tank tends to be deeper than the passively reinforced option. Taking this and other factors into account, my comparative costing analysis of a range of tanks shows that the prestressed option is always cheaper, right down to the 5Ml mark (and below).

Economics aside, there are other factors that influences choice of reinforcement type: above the 10Ml mark it becomes nearly impossible to practically fit hoop rebar without prestressed reinforcement – and a prestressed tank is less likely to leak.