Doubell Machines makes every effort to keep customers aware of all the ups & downs of brickyards and brick-making machinery. It is with this ethos that you should understand why making bricks with vibration is different to manual compaction.
The principle is simply to create a structural building unit that can be strong enough to support a load and allow for the creation of a civil structure. Bricks could be made from straw if it would provide long-term stability for a house. Other considerations would be water penetration, heat insulation and even sound dampening; all of which would be specified for the project.
Currently the forming of bricks employs one of two methods: cement combined with aggregate (crushed stone, sand, combinations thereof, etc) to produce concrete bricks; and clay bricks that are formed from clay and heated in a furnace/kiln to "bake" the bricks. These can also be referred to as fire bricks.
There are other methods, such as soil bricks that are bonded with tight compaction; or composite materials bonded with chemical agents; but the most commonplace bricks are concrete & clay.
The materials are formed into shape with a mould and then compressed to some degree. The rigidity is kept in the brick for the duration of the hardening process (fire or water + time).
In concrete bricks, blocks and pavers, the compression works like this:
A loosely filled volume of aggregate (with typically 8:1 ratio of sand/grit to cement with 0.5 part water) will be compressed by roughly 30% (optimal compression is specific for the shape/design of each brick).
During this compression, particles are moved into a closely packed (more dense) formation. Because of this, more particles are touching one another with less void space than that of the loosely filled starting volume.
More connection allow for better cohesion & adhesive forces when demoulding. Typically, even without a chemical agent added, brick formations will hold together from the moisture content alone. This shows how well the brick will be bonded when cement is added.
Cement in the water will create micro-joins between the particles at the connection points (following the water lattice structure present). Thus when more connections can be made, the stronger the bond will be; additionally this means less void space and therefore less wasted cement to fill the spaces.
Adding a chemical like Doubell Quickmix, which is a plasticisor will greatly improve the performance of this process. It will act like a soap - reducing the internal friction between particles so that they can slide into a denser pack and do that quicker as well during compaction. Because this agent is diluted in the water solvent, the cement is propagated throughout the mix more uniformly as a result.
Essentially, as the compression reaches maximum tolerance, the manual press compaction would be no different to using vibration - all the particles are so closely packed that the force exerted is almost uniform throughout & little is lost to deformation.
However, at the level of brick-making this level of particle compaction is never achieved. Instead, the aggregate is compressed from the top & the upper layer will compress first before resisting further compression, then push on the layers below. Once they have achieved a level of compression sufficient to resist the upper layer's exertion, they upper layer will once again compress further. This is a simplified example of a realtime occurrence.
Ultimately, you may find that the less overall compaction on the block, the more uneven the distribution of compression (ie. the top is more compacted than the bottom).
As the particles are pressed to closer, the vibration will excite all the particles so that they shuffle into place together. As the particles vibrate, the volume is reduced; this compaction is more uniform from outside inward from all the surface, not just the top.
Vibration greatly assists in compaction but also helps to speed up the compaction process. The increase in brick-making results in a faster daily target achievements.
In another article I will explain the difference between high & low frequency vibration, but what you need to know right now is that you should have a vibrating unit that offers little start-up or shut-down low-frequency judder, which will shake the machine and break the bricks; or worse - create some peculiar breakages on the outer bricks.