Tribology of steel/concrete interface [51 ]

When concrete is pushed through a steel pipe of constant diameter, coarse aggregate particles tend to concentrate in the axis of the pipe, so that a slip motion happens in a thin layer of cement paste, located at the steel/concrete interface (see Figure 10). In general, all shear deformations are located in this zone with the rest of the concrete being transported as a plug. Thus, the study of the interface mechanical behavior, which is the purpose of tribology, must be carried out. It is possible to reproduce the slip phenomenon in the laboratory, using a large-gap coaxial viscometer with a smooth inner cylinder. At low rotation speed, a linear relation is found between shear stress and slip rate, corresponding to the following equation:

 =  _{0, i}  + k.v

where is the shear stress, _{0, i} is the interface yield stress, v the slip rate, and k a viscous interface constant. At higher rotation speeds, the shear deformation tends to propagate into the bulk of concrete. Therefore, a rheometer test has to be performed first, in order to know the Bingham constants of the concrete considered. Then, a numerical analysis of the experimental relationship between torque and rotation speed in the coaxial viscometer leads to the determination of the two interface parameters. It is especially important to have a gap large enough in order to be able to keep the external concrete layer at rest in the viscometer.

Figure 10: Flow of fresh concrete in a pump and in a large-gap coaxial viscometer. The arrows represent for the speed of particles.