If you can come up with a solution that will dissolve plastic Take out a patient and retire a multi millionaire. Plastic by it's very nature is a petrol by product derivative and it requires heat to work it into it's many forms. If the plastic you are working with has been deposited with heat, and that looks fairly obvious, then it will take heat to remove it or extreme cold. If the plastic is sitting on the top of the concrete, which I doubt you could try an ICE SPRAY from CRC to make it that hard that it will chip off with the right tool under it. On the other hand you could try to set it alight and let is burn it's self out and wire brush off the charcoal that's left. Either way good luck with it. I'd be telling the customer you will do what you can but "No guarantees expressed or implied".