Continued developments in high-performance high-strength concretes are enabling taller, lighter and more cost-effective high-rise construction, reports Jenny Burridge, head of structural engineering at The Concrete Centre.
The economic benefits of using high-performance, high-strength concrete for the structures of high-rise buildings are based on the straight forward premise of ‘more for less’. Using high-strength concrete means that the column size is reduced and therefore, the amount of concrete, reinforcement and formwork required is consequently reduced. In simple terms, the use of high-strength concrete provides one of the most economical ways to carry vertical loads to a building’s foundation. It also provides performance benefits of early high-strength, volume stability and extended life cycle.
High-strength concrete can also be used to reduce slab depths and, therefore, a building’s overall height. This can result in significant cost savings whilst slimmer high-strength concrete columns can increase the overall net area; again, more for less. The sustainability benefits of high-strength concrete of using less material are further underlined by the possibility of using by-products from other industries such as ground granulated blast furnace slag (GGBS), fly ash and silica fume.
Before determining to use high-strength concrete, a definition must be understood: a high-strength concrete is always a high-performance concrete, but a high-performance concrete is not always a high-strength concrete. To be high-performance, a concrete will have been designed to have specific characteristics such as resistance to chloride ingress.
According to BS EN 1992-1-1, to be high-strength the concrete should have a compressive strength for design of greater than C50/60. Ongoing development means that high-strength concretes with cube strength levels of 80 to 100 MPa are now being supplied in the UK.
Producing high-strength concrete is not so different from producing normal strength concrete. The target water/cement ratio should be in the range of 0.30 – 0.35 or even lower. Super-plasticisers or water reducers should be used to achieve maximum water reduction. A wide range of aggregates can be used with crushed rock offering particular benefits for high-strength concrete up to levels of C60/75. The addition and use of silica fume offers a dense structure with enhanced strength to levels of C90/105 with high levels of chemical and abrasion resistance.
New developments in Ultra High Performance Concrete (UHPC) offer significant potential for even thinner/larger floor slabs and small/longer columns. To be considered as UHPC, a concrete should have a compressive strength of over 150MPa and a flexural strength of over 20 MPa.
Given the recognised benefits of using high-strength concrete, it is surprising that its use is not more widespread. A number of factors may be contributing to this including the lack of specific design codes and unfamiliarity with high strength concretes particularly UHPC. This is especially apparent where current design codes are not readily available for concretes that have many times greater strength than conventional concrete. This will change as experience grows as engineers seek to exploit the optimal designs made possible by using high-strength concrete.
Increasingly, for many commercial and residential projects the only way is up. High-strength concretes will enable taller buildings that are built more efficiently and, therefore, more sustainably.
SJ by worldcement
