Nanodur concrete convinces with highest deformation stability

It is not the machine foundations but the machine beds, which are set up on the foundations and integrated into the machines, that are the load-bearing and supporting elements of a machine tool; this is because the individual functional elements, such as the guides and drives, are fastened to these base frames. Their size and shape essentially depend on the required process task. Machine beds can therefore be lighter than one tonne or very large. The correct selection of the materials used is of decisive importance for the functionality of a machine bed. In addition to the technical and economic aspects of production, the mechanical properties of the various materials play an important role, as they have a decisive influence on the component properties. This is above all the modulus of elasticity, which influences the elastic deflection, as well as the material damping and the thermal behaviour. Long-term geometric constancy, i.e. only minimal plastic deformation after application of a load, is expected of all materials as an indispensable prerequisite. If this is not the case, the guideways will lose their flatness and parallelism over time, which is the result of complex and expensive precision machining.

Modern high-performance concrete in mechanical engineering
In the past, machine beds were mainly made of steel, cast steel or grey cast iron. As a result of technical and economic requirements for machine tools, in the course of the last 30 years frame components made of materials such as mineral casting or epoxy resin-bonded polymer concrete have become established in addition to machine beds made of grey cast iron. Steel and cast iron have the highest load-bearing capacity, but are very energy-intensive and expensive to manufacture. Solid materials such as polymer concrete and natural stone, on the other hand, are widely used due to their lower price and technical advantages. Modern ultra-high performance concrete (UHPC for short) is now firmly established in mechanical engineering. In addition to its low cost, concrete offers advantages in terms of both the vibration and thermal behaviour of machine tools and replaces conventional materials such as grey cast iron or welded steel constructions. Ten years ago, a patent application was filed for the high-performance binder Dyckerhoff Nanodur. During this time, Nanodur has had a decisive influence on the world of ultra-high performance concretes. Above all, the innovative high-tech material has given new impetus to modern machine construction. Several specialized precast plants worldwide have been successfully producing machine parts from Nanodur concrete for several years.

Comparative studies on creep behaviour
In order to prove the special deformation stability and in particular the sufficient long-term geometric constancy of the material Nanodur concrete as a frame building material, extensive comparative tests on creep behaviour were recently carried out at the Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University. Creep" is understood to be a plastic, time-dependent deformation under load. If a force is applied to mineral bodies, they deform elastically. If the body is not directly relieved, an additional time-dependent, plastic deformation of the material occurs. The material withdraws from the applied force by creeping. The aim of the study was to compare the deformation stability of components made of Nanodur concrete with that of alternative materials such as natural hard stone and polymer concrete. In order to classify the results, test specimens made of commercially available natural hard stone and epoxy resin-bonded mineral casting were examined in parallel. For the tests, a Nanodur concrete E45 was used, a standard mix without fa- sern. Specimens without heat treatment (without WB) were stored for four weeks in a climatic room before they were delivered for testing. Specimens with heat treatment (with WB) were heated to approximately 80 °C for two days shortly after demolding. This accelerates and completes the chemical hardening process, the hydration of the cement, and increases the dimensional stability. The Na- ture hard stone came from a recognised German precision company in the measuring equipment industry and the epoxy resin-bonded mineral casting with round quartz grains from the quality control of a recognised manufacturer. All tested plates were sawn out of prisms of the size 160 mm × 40 mm × 40 mm with a test equipment saw and stored in the climatic room until the respective test. In order to determine the long-term geometric constancy over a period of 90 days, a special test rig concept was developed at the WZL of RWTH Aachen University, with which the expected minimum deformations could be measured.

Clear results
The comparative investigations showed that the natural hard rock investigated exhibits the lowest time-dependent deformations in absolute terms. The long-term stability (expressed by the creep index) of heat-treated, cement-bound Nanodur concrete is of a comparable order of magnitude. Mineral casting bonded with synthetic resin shows the highest deformations, both in short-term elastic deformations and in additional plastic deformations due to creep. The complete final report with the detailed results is available from durcrete GmbH.