Vibrations - How to adjust them?

[areks]

I produce stone imitations that are made from concrete.They are made in very soft polyurethane and silicone moulds. My products are no more than 2cm thick. I have been having problems with getting rid of air form concrete. Inside as well as outside of my products I can see bubblesthat are the results of the air. I have tried different plasctisers, diffretnt rations of mixture as well as moisture and diffrent length of vibrations. I have also been thinking if the temperature of components and concrete plays any role. After having those diffrent possibilities tried I have reached a decision if vibrations as well as my vibrating table are adequate. My vibrating table is not big (1/1m).it has one electric vibrator with revolution of 3000mpr/min and force of 1000kg.Unfortunately I don't have a possibility to regulate the speed of the engine.I have the feeling that my moulds are soft and light due to this fact they don't receive vibrations which lead to proper consolidation of concrete and getting rid of air bubbles. I am thinking where the clue is. Maybe the higher frequency of vibrations (speed engine per minute)will give postive effects?I will be very garetful for any cluses and advice. Greetings to all users.