Water for the people

The South African government has just declared a national state of disaster to deal with the country’s drought and water crisis. Are we doing our part?

Throughout the 19th century, technological innovation was used to generate capital wealth. In the 20th century, it was primarily used to win wars. We now live in a new era where technological innovation is measured against its social utility.

Within this paradigm, there is little point to innovating in technology unless there is a clear benefit to society. In our era inventing a navigation system that helps a parent to locate a lost child is likely to be more lucrative than inventing one that helps to deliver a warhead.

One key social need is access to clean drinking water. The problem is that this resource is scarce and unfortunately humans have to compete for it with other uses. For instance, we still use drinking water to transport our waste, whether the waste originates in our bathrooms or in industrial processes. The scope for innovation to eliminate this competition is huge.

In South Africa, only 8% of the land provides a staggering 50% of its surface water. South Africa’s mean annual rainfall is only 490 mm, around half the global average. Furthermore, high evaporation rates result in less than 9% of the rainfall ending up in its rivers.

South Africa made considerable investments in concrete dams and steel pipe inter-basin transfer schemes throughout the 20th century. This network supplies water to drier parts of the country and urban centers. The current cost to maintain and upgrade this critical infrastructure is estimated at over R60 billion per year and will use much concrete and steel.

Moody’s Investors Service recently warned that Cape Town alone would need to spend up to R12.7 billion over the next five years on water and sanitation infrastructure to deal with its water crisis.

In fact the wider construction sector accounts for one in every ten Rands spent in South Africa and the industry itself competes with people for scarce clean water. With all the impending infrastructure and housing work, the sector is expected to keep expanding for the foreseeable future.

Cement production consumes approximately 317 litres of water per tonne of cement. Aggregates consume around 150 litres/tonne of aggregate. Mixing concrete takes another 100 litres/tonne of concrete. Assuming that cement makes up 15% of the final concrete by weight then it takes a total of 275 litres of water to produce one tonne of concrete.1 Unfortunately much of this water must be as clean as drinking water.

The average water intake for an integrated steelworks – where steel is produced from virgin raw materials – is 28 600 litres/tonne of produced steel, with an average water discharge of 25 300 litres/tonne of steel. For the electric route – which produces steel by melting scrap in an electric arc furnace – the average intake is 28 100 litres/tonne of steel, with an average discharge of 26 500 litres/tonne of steel. 2

The good news for steel production is that much of this water is used as coolant and can be drawn directly from the sea. The overall net water loss from steel production varies between 1 600 and 3 300 litres per tonne depending on the method of production – and this is mainly due to evaporation.2

In typical reinforced concrete construction, concrete makes up over 95% of the weight while the rest is steel. This means such construction consumes approximately 400 litres of water per tonne of reinforced concrete – 140 litres for steel and 260 for concrete.

Thus, leaving aside water that is used for site cleaning, thankfully as little as 260 litres of fresh water can be used when casting reinforced concrete. However this still means that a regular solid floor slab of 250mm thickness will consume 156 litres of fresh water to construct just one square meter of floor.

Steel composite slab

A hundred square meters of suspended slab in a new Cape Town house can eat up over 15 000 litres of fresh water. According to city regulations that’s over a month of water consumption for a four person household. This should be unacceptable.

This presents an opportunity to innovate so that the volume of fresh water used in construction is minimized or even eliminated. For instance anything that one can do to optimize the weight of steel and concrete used in a project will provide a great service to society. Moreover the less waste that is produced on site the less water that will be required to clean and transport the waste. This speaks to modular construction where much work is done in a factory and only assembly occurs on site.

Innovation in the use of ultra-high strength steel and concrete is one way of reducing water usage by cutting the volume of steel and concrete used in construction. One source of concern when using such high strength materials is to make sure that the lower weight and stiffness does not adversely affect the serviceability or fire resistance of structures.

A related innovation involves the use of hollow precast planks. They are typically made in a factory using ultra-high strength steel tendons and concrete and installed on site using cranes. For comparison a typical 250 mm thick floor slab that is constructed using precast planks can weigh as little as half of a solid slab of the same thickness and will thus consume half the water. One disadvantage of this system is that it cannot be transported far from the factory due to the heavy weights involved.

Hollowcore plank on steel

One way to address the issue of weights is to use Light Steel Framing, an innovation which arrived in South Africa a decade ago and makes use of very thin and light steel frames that form the skeleton of buildings and other structures. They are typically dressed in fibre cement and gypsum boards and can be constructed very fast by fabricating the frames and boards in factories and assembling them on site. This method of construction probably results in the lowest usage of water of all other competing methods.

Some South Africans prefer to live and work in heavy buildings that feel like old stone, brick or concrete structures. Composite structures can satisfy such customers without having to waste scarce water resources. Such systems use steel both as formwork and an integral part of the structure. As such one can reduce the volume of concrete used while eliminating waste on site.

A comparable composite floor to the solid and precast planks above can use even less fresh water than the precast planks. The main advantage over precast however is that the steel materials are easy to transport far from the factory and much of the heavy concrete pour occurs on site.

Steel plate shear wall

Composite walls for instance involve an innovation that is rarely used in South Africa but is being deployed widely to construct power plants in South Korea and the US. A typical wall will have two steel plates that are connected by struts in a factory and concrete is poured between the plates only upon site installation. A related innovation in South Africa makes use of a light steel frame stud walls cladded in expanded metal

and then sprayed and filled with light weight concrete on site.

An exciting innovation that will arrive in South Africa soon makes use of composite columns that utilize concrete filled hollow steel tubes. Such columns can allow for huge savings in the use of water by optimizing the weight of steel and concrete used. Moreover as in most composite construction very little waste is produced on site and this should result in the use of less water.

The common thread in all these innovations is the use of high strength alternatives where slightly more steel and less concrete is used. This saves fresh water in three ways. Firstly it reduces the overall weight of structures and there are savings in water usage simply because less material is used. Secondly the use of slightly more steel and less concrete allows for savings because much of the water used in steel production need not be fresh drinking water. Lastly the use of steel both as formwork and permanent structure reduces waste and debris on site and this has serious implications on the use of fresh water for site cleaning and waste removal.

Innovation does not always require the invention of new products. In many cases it involves using existing products in new ways and markets. In this sense the use of high strength concrete and steel, precast concrete, light steel framing, composite floors, walls and columns should all help to reduce the use of fresh water in construction. In order to completely eliminate site waste all of these systems should preferably be fabricated in a factory and assembled on site.

The South African government has just declared a national state of disaster to deal with the country’s drought and water crisis. We must keep asking ourselves if we are all doing our part by innovating in our industry to help solve this urgent crisis.

Please let us know what you are doing to help!

 

  1. Lafarge Company
  2. World Steel Association