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I am pouring mosaic concrete stepping stones to be placed in a city sidewalk. The mosaic will go face down into a form and the concrete poured from the back. I have been having problems with my test tiles in getting the sand to move all the way forward to the level of the face of the tile and have been ending up with air bubbles in the surface where I want it to be flat.

How can I alleviate this problem? Would pouring a mixture of just sand and cement first to fill cracks and then pouring the rest of the mix with the gravel behind it solve the problem or would it weaken the top surface of tile?

Normally I lay the mosaic on top of something with mortar, thin set or other adhesive and grout once the adhesive is dry. Not sure how to duplicate that with a concrete pour.

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    Hello, and welcome to Home Improvement. Maybe you could use a concrete vibrating tool, as they do in concrete forms? And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Jul 4 at 23:30
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I think you're doing it backwards and would only see them fail. You'll have very easy and perfect results with store bought bagged concrete. And, if you can lip the mosaic edges would really be ideal.

Follow the bag's instructions and fill your form or mold. Then, if desired (definitely not needed) vibrate the form/mold with a palm sander pressed on the side to remove air bubbles and settle the concrete.

Add more concrete to refill your form/mold. Float or trowel the concrete to bed aggregate down and away from the surface. Then, deeply hand-place mosaics into the concrete to sweep cement slightly over each mosaic's edges...so you aren't relying on just the cement's very poor and brittle adhesion for durability.

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It sounds like a portland + sand mix (or bagged mortar mix) to mortar the tiles first might work. Fill the remaining form with concrete before the mortar sets so they can bond. A bit of vibration should help to fill in gaps. Not sure how well this would hold up to foot traffic over time.

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