Dr Christof Kemmann explains that concrete mixing is not as simple a process as it might appear
ASKED what ‘mixing’ means, people will normally have a general idea. But describing what this process really does involve is not so easy. The aim of any mixing process is to combine substances of different properties, usually in different ratios, as evenly as possible. What you get is a homogeneous substance with changed properties. This is produced by introducing relative motion of the highest possible intensity into the ingredients being mixed. A merely uniform motion with inadequate mix acceleration would therefore not merge the substances in the desired way that would be the equivalent of stirring, rather than mixing. Other conditions for a successful mixing process of course include being able to contain the entire mix volume completely and being able to achieve as high a probability as possible that particles of one substance introduced at one point at the start of the mixing process may be found at any other point in the mixing vessel at the end of the process. Only in this way is it possible to ensure that the process will repeatedly produce the same consistent result.
Mixing cycle
A mixing cycle is made up of three activities: filling, mixing and discharging. The order of sequence in which the constituents are added may facilitate or aggravate the mixing process. The mix should be discharged in such a way that there is no risk of segregation. A mixing process should incur low operating costs, particularly in terms of energy and wear. The efficiency of concrete production is determined by the time required per mixing cycle, energy costs, as well as the costs incurred by wear and maintenance. This is all part of successful mixing.
In concrete production, choosing a suitable mixing technique has a crucial effect on both the quality of the concrete that is made and the economic efficiency of the production process. Increasingly stringent demands are being made on both of these parameters. In each batch mixed, the binding agents must be able to completely react with the water and the aggregate must be enveloped with the binder slurry as completely as possible.
Furthermore, most concrete formulations involve the use
of minute quantities of chemical additives that must also be
evenly distributed.
Twin-shaft mixing technique
The twin-shaft mixing technique has become a widely accepted approach for making concrete in many countries of the world. But, what are the reasons for this development?
The twin-shaft mixer for making concrete was invented at the end of the 19th century which was when BHS-Sonthofen applied for a patent. The starting point for the invention was the use of
a paddle or blade to mechanise manual mixing in which an
aggregate/cement/water mix is repeatedly shovelled from right
to left. This mechanisation concept was later abandoned and,
via a series of development stages, superseded by today’s highly dynamic motion process.
Today, the two mixing shafts carry mixing blades that are arranged to follow the pattern of an interrupted spiral. This transports the mix in helical fashion along the mixing shafts, which turn in counter directions. Towards the end of each mixing shaft, the mixing blades are positioned in the opposing direction so that they are able to transfer the mix to the opposite shaft. This way, the mix is conveyed around the mixing trough in circular fashion, whereby the movement sequence at the same time turns inwardly in a screw-like manner. This generates an intensive three-dimensional motion. To increase the share of relative motion, the two mixing circles overlap in the middle
to produce a high-turbulence zone that significantly intensifies
the mixing effect.
It has only been possible to achieve today’s optimised
mixing-tool design after extensive experience. In the BHS
twin-shaft mixer, the mixing elements can achieve homogeneity levels of around 95% within 30sec. Moreover, this can be attained at a relatively low mixing-shaft speed of 20-30 revolutions
a minute, saving energy, and reducing wear and stress on the
mix particles, according to the company.
The most costly raw material in producing concrete is the cement. Various tests have shown that a twin-shaft mixer can produce
concrete of a given strength with less cement than, for instance,
a drum-tilting mixer.
The twin-shaft batch mixer is discharged by a rotary slide gate along the middle of the trough. The majority of the mix is discharged under the force of gravity on opening the rotary gate, with the remainder being moved to the discharge opening by the mixing blades. This keeps the risk of mix segregation extremely low. The rotary-gate system also allows the discharge quantity to be metered and prevents the risk of jamming when closing off.
The twin-shaft mixer is also distinguished by its ability to permit a relatively high filling level compared with other mixing systems. This means the mixing vessel can be kept very compact. For the same batch size, the area of mixing trough exposed to wear is much smaller than in mixing systems that permit only lower filling levels. It also has advantages in terms of installed drive power. The BHS mixer is furthermore distinguished by an extremely robust drive system. It can even be restarted under load.
Finally, the twin-shaft can handle an out-of-the-ordinary demand or operating status. From a technical point of view, it is possible to add ice for cooling concrete or superheated steam to heat it. Coarse-aggregate concrete formulations (up to 180mm) can also be mixed. Studies show that even self-compacting concrete (SCC) can be reliably mixed within 30sec.