When Portland cement based concrete is cured, the pore solution is extremely high in Ca+2 ions. This leads to the formation of Ca(OH)2 among many species. This saturated pore solution consequently has a very high pH of about 12.5. If you look at the Pourbiax diagram (Eh vs. pH) for copper in water, you'd see that at such a high pH, copper forms a copper oxide layer and further reactivity drops. That implies copper metal in a high pH environment is very passive. Having said that, the caveat here is that the pore solution consists of many other chemical species thus can affect the solubility of copper metal in the concrete pore solution. The type and amount of other species depends on the raw material used to batch the concrete. However, I have never ever seen concrete pore solution to be acidic. As a matter of fact, if your concrete pore solution is acidic, your concrete would not set because you would not be able to form any calcium-silicate-hydrate, which is the material that binds the concrete into a solid mass. There is only one way that I can think of where copper embedded in concrete would corrode, and that is localized acidic regions due to the selection of the aggregates that were used in the concrete. Personally I would use PEX tubing instead of copper pipes in this application because PEX tubing is flexible and thus allows me to more easily manage the routing of the tubes. More importantly though, by using PEX tubing I do not need to have any joints (as with copper pipes) in the concrete and thus minimize any chances of corrosion at the joints and also poorly made joints causing leaks. I would trade off thermal conductivity with less chances of leaks any day.