Bonded Concrete

Breaking tests: how strong is ultra-strong?

Opus caementitium (opus = work, caementitium = aggregate, quarry stone) was the name of the building material developed by the Romans. This “Roman concrete” or “lime concrete” was made of quicklime, water and pozzolanic ash, the mortar, which was mixed with an aggregate of pumice. Roman concrete was used, for example, to build the dome of the Pantheon in Rome, which is still standing to this day, which is 43 metres in diameter. A lot of progress has been made to get from that material to today’s ultra-high performance concrete (UHPC) though.

Structures made of normal concrete are heavy due to their physical dimensions and, just like other building materials, consume large amounts of energy and raw materials. On the one hand this is due to the dimensions required for the building components to have the necessary strength and the high energy consumption, amounting to about 70 GJ each year to make cement. On the other hand, the durability of normal concrete against environmental effects such as frost, acid rain, or salt is limited. Steel reinforced concrete uses steel reinforcements to absorb the tensile forces, while the compressive forces are borne by the concrete. The amount of force the concrete can withstand is primarily governed by the thickness of the concrete part. The thickness of the part thus needs to increase as the span covered by the part increases. For structures with a very large span – such as bridges or halls – the maximum possible span of the structure is governed by the weight of the concrete itself rather than by the load-bearing capacity.

Ultra-high performance concrete (UHPC), on the other hand, is an entirely new type of concrete with a very dense structure that has a compressive strength of about 200 N/mm², which is comparable to steel, with strengths of as high as 400 N/mm² being possible with heat treatment, and a bending tensile strength of up to 50 N/mm2 (1 N/mm2 = 10 kg/cm2). It is between 5 and 10 times harder than normal concrete and is thus ideal for parts that are subject to compressive stress such as prestressed beams, hybrid cross-sections or pillars in high-rise tower blocks. The dead weight of beams can be reduced by a half or even two-thirds using UHPC, and the cross-section of pillars can be reduced to a similar extent. Structures made using UHPC can therefore be much more filigree, lighter and more aesthetically pleasing while having the same load-bearing capacity. Although the raw materials used to make UHPC are more expensive than for normal strength concrete, this is offset by a reduction in the total amount needed. The potential energy savings for making parts of equivalent strength are estimated to be up to 40%, with the CO2 emissions being reduced by the same amount and, thanks to its high density, UHPC also protects the reinforcement embedded in it better against corrosion.

There are a number of research projects involving UHPC at the Institute of Structural Concrete at the RWTH Aachen University. The problems being addressed by these projects include:

• How does UHPC behave in combination with other materials?
• What new shapes can be made using UHPC and how could they be used in practice?
• What are the load-bearing and deformation characteristics of prestressed concrete beams and composite beams made using UHPC?
• And finally: How does it react under dynamic load, for instance for use of this new method of construction in bridge building?

The researchers begin by making educated assumptions, to estimate the load-bearing capacity from an engineering point of view. Using computer simulations they are able to vary a large number of parameters to predict the behaviour of the finished components as accurately as possible. Using the results of these simulations they can then choose the right dimensions of both the components and the test rigs for the component tests. Together, these tests and simulations provide all of the information that the researchers need to derive the dimensioning rules needed for its use in real life applications.




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