Curb making machine have to use proper concrete mixing ratio to produce smooth and strong concrete curbs.
The concrete mixed ratio is different according to curb size,curb height,curb position.
For example,flat and wide curbs can use fluid concrete. Higher concrete slump.
High and narrow concrete curb have to use thicker concrete with lower concrete slump.
It is also depending the curb positions. If under ground, the concrete mixing radio can have more water.