A greying fellow reflects on... Sustainability
Part 3: Concrete George Bothamley FCInstCES
THE commercial working group for the sustainability white paper has identified concrete as a recurring topic. For environmental sustainability they suggest using less concrete in construction, applied by using either no cement or less cement as a binder.
For economic sustainability they advocate use of reduced carbon concrete and precast components, applied by using precast slabs, bridges, walls, etcetera. These are all relevant to several of the CICES sustainability pledges, including advising and supporting decision making to ensure that projects are economically sustainable; adopting life-cycle costing, whole life carbon modelling and post-construction evaluation; endeavouring to reduce activities and footfall on site; making management of environmental risks a key part of decision making. All of these will contribute to implementing a circular economy approach to products.
Pre-cast concrete
On the face of it, pre-casting components is not obviously more sustainable. As well as the additional management and overhead costs of the casting yard, pre-casting requires additional loading and transport – to the casting yard and then to the installation site compared with just to the installation site for in-situ concrete.
But there are many instances where pre-casting reduces cost and carbon effect, such as confined city sites, live railway and road works and work over water, where the additional cost of precasting off site is more than offset by the savings in installation costs and time on site.
There are also instances of pre-casting being commonly used when it is not more sustainable. In the past decade, pre-cast modular construction has become common for valve chambers, pumping stations and storage reservoirs.
But my analysis on a framework I was working on, showed that in more than 100 comparisons, only one had a site location that made pre-casting less costly than in-situ construction. Furthermore, this technique has increased risk for water-tight construction because of the increased incidence of joints. These examples illustrate why comparative costing of all potential solutions should always be done.
Pre-cast versus in-situ
In the 1980s, I worked with a project manager who was a big advocate of pre-casting; he had worked in the Middle East, where importing cement and dealing with climate issues made in-situ concrete problematic and expensive. We did some extensive analysis of pre-cast versus in-situ concrete and found that in most instances pre-casting was more expensive.
For every £100 spent on stronger concrete over £1,000 was saved on construction duration. The extra cost would be largely increased carbon, but the reduced duration would also have reduced carbon.However, we found that where space was available for the pre-casting yard to be on site, it frequently offered large savings The reduced installation costs and time saved did not need to off-set the extra cost of offsite manufacture.
We did not do carbon modelling in those days, but I would expect that the result would follow the life-cycle costing; the changed site installation costs will have a carbon effect too.
Examples of pre-casting on site I have been involved in were complex curved wall tops and simple cross beams on a steel making plant project and encasing steel beams for fireproofing on a multi-storey chemical plant.
Reducing cement content would appear to always reduce monetary cost and reduce the carbon effect. But it is worthwhile doing comparative checks. In the early 1980s, I worked on a very fast-track design and construct steel-making facility project. The designer, who was very pro-active, asked the foreman how he could modify his design to foreshorten the programme, and received the response ‘thicker concrete components of simpler shape and less reinforcement’.
The designer was ethically concerned that this would be a more expensive option for the client than the usual design concept of minimising the volume of concrete. I did some quick comparative cost calculations that showed that the simpler but thicker concrete solution was the least cost without considering construction time savings.
We did not model carbon effect, but it would not have been all negative; the reduced complexity of installation, time saved, reduced concrete strength, and reduced reinforcement would all have reduced carbon effects.
A more recent experience was to reduce the programme duration and cost of refurbishing a bank of water works clarifier tanks by increasing the strength of in-situ concrete, so that curing time was reduced. For every £100 spent on stronger concrete over £1,000 was saved on construction duration. The extra cost would be largely increased carbon, but the reduced duration would also have reduced carbon.
The above examples show that the solution with the best economy and carbon effect is not always obvious. Using less concrete by having thinner components is likely to always produce savings, providing it is not achieved by having more complex shapes. And it may have other advantages too.
An example I experienced is where a concrete storage reservoir was designed with tied walls and roof. This not only allowed thinner concrete, but also fewer joints giving less risk of leaks.
Economically sustainable
Using alternatives to concrete can have environmental as well as carbon and economic benefits. On sewage works, we used to use concrete hard standings where sewage contamination needed to be washed down, but granular or reinforced grass in other areas.
Using alternatives to concrete can have environmental as well as carbon and economic benefits. On sewage works, we used to use concrete hard standings where sewage contamination needed to be washed down, but granular or reinforced grass in other areas.Some years earlier, I was estimator and commercial manager for the construction of car parks at the only first-class cricket ground built in England in the 20th century, at Chester-Le-Street.
The car park was a series of elegant, curved bays with hedged surrounds, two-thirds in block paving and one-third just grassed for overflow at busy times.
They have the environmental benefit of being free draining, unlike concrete or tarmacadam solutions which create run-off into the surface water drainage system.
And they still look elegant, nearly thirty years later. There will be a good economic and carbon effect to this type of solution. The environmental benefit will also have economic benefits. These are hard to evaluate, but should at least be mentioned in selection decisions.
Using more pre-cast concrete, less cement and concrete, and alternatives to concrete will often be economically sustainable, have a reduced carbon effect and environmental benefits.
But it will not always be the case, so it is important to do comparative checks to ensure the best solutions are always selected. Making this consistently done is why we need a quick and simple to use calculating tool.
George Bothamley FCInstCES