We, in our workshop use several different mix designs for different type of models. What about others? Do you guys put agregate in your mixes or sand and cement mainly? Any chemicals or glass fiber maybe?

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In Lubbock, TX, we use different aggregate ratios depending on the application. For exposed aggregate work, we blend local stones that handle our Texas climate well. Resurfacing and overlay projects need modified sand-to-cement ratios, often with fiber reinforcement.
Garage floors require different considerations than outdoor repair work. The key is matching mix design to environmental conditions and intended use.
We've shared some of our approaches here: https://concretecontractorstxlubbock.com/
What specific applications are you working on?