The rise of high-rise towers requires the use of special materials earlier to achieve maximum benefits, argues Ignacio Escobar.
A staggering 436 towers are proposed in London alone over the next decade.
Prime development space is at a premium, so it’s no surprise that the trend for multi-storey buildings is on the rise. How to deliver these superstructures, often 40 storeys and above, presents an interesting challenge for the industry as a whole.
From our point of view, this trend for taller structures requires the use of materials with significantly higher compressive strengths – up to 90 MPa, when the norm for anything up to 12 storeys is around 60 MPa.
Developers often specify high-strength concrete for the vertical elements of many new tall buildings to satisfy more challenging load paths, robustness and stability criteria.
But there’s an opportunity for the industry to adopt a holistic approach when specifying building materials by examining their effect on overall build timescales, rather than considering each constituent material in isolation.
High-strength and high early-strength concretes can be used for more than just the vertical elements of a new build. But too often special concretes are used only if and when a project falls behind schedule. That doesn’t need to be the case.
“It also makes financial sense to specify a special concrete in the first place that will provide the high strength required but at an earlier stage”
If they’re specified from the outset, these products could have a positive effect on the overall build programme and maximise usable space within a structure.
However, some engineers continue to design different elements of the building – such as floors – to allow 28 days for the concrete to reach its optimum strength. Again, this needn’t be so. They could be benefiting from using materials with higher mechanical properties at the early stages of the concrete and high strengths at 28 days.
Tarmac laying special concrete 2
Increased costs are to be expected when design commands a high-strength conventional concrete, so it also makes financial sense to specify a special concrete in the first place that will provide the high strength required but at an earlier stage, alongside many other benefits.
Cost savings to be found
The ability to reduce element sizes, speed up construction and facilitate an easy placement of the product at height by using special concretes all create cost savings across the broader project.
The latest concrete developments
Segregation in the pump line is a common problem when attempting to pump a non-flowing high-strength concrete mix to the heights that are now becoming common, therefore a self-compacting concrete is the only solution to this regardless.
“Long-term deflections can be controlled by using high strength concrete while keeping floor thickness to a minimum”
We’re now in a position where long-term deflections can be controlled, not only by using span-to-depth ratios but also by assessing the theoretical deflections. There has been a significant shift since the introduction of Eurocodes in the UK. These codes provide deflection calculations that are more advanced than span-to-depth ratios, as they consider the effects of early-age construction loading and high early-tensile strengths in crack formation.
Long-term deflections can be controlled by using high-strength concrete while keeping floor thickness to a minimum. The risk of differential stiffness between vertical and horizontal elements can be reduced by using the same product for both the floors and walls.
Tarmac delivering special concrete in London
Laying thinner material for both the floors and walls not only has a beneficial outcome on the available space and light in the subsequent rooms, but also makes the building weigh less. Cost savings can be achieved as a consequence of smaller foundations and a reduced facade surface because less material is needed for construction than when using traditional concrete.
Only ways to meet the challenge
There is a step-change opportunity for the construction industry to move away from using conventional concrete to deliver commercial value through special concrete solutions.
Concrete specification should be considered as part of a building’s critical path to cut build timescales, maximise space and ultimately deliver financial savings. Material costs shouldn’t be considered in isolation.
Early engagement is essential in meeting the challenge of tall building construction head on. We need to ensure we’re building smarter to forge a new skyline that showcases the best of tall buildings.
Ignacio Escobar is national engineering manager at Tarmac Readymix