Well it just keeps happening, you simply go up and ask these builders and they will give you anything ! its great and also good for the world as instead o fit all going into landfill.
I got the "waffle pod " from a building site today, they were pourin the concrete and i asked the guy if i could have his offcuts etc , he said " take what you want mate".
i got the stryo i need plus some extra strong reo and some reo supports ( you know the ones that hold it up into the middle of the concrete.
The secret is "just ask" , sometimes i don't have time to get the stuff so i just say " my names tim i'll come get it over the weekend, whats your name incase i am asked why i'm taking stuff" . I get a name and thats all i need to protect myself as stealing from building sites is probably a big problem and no doubt there is security organised so make sure you always ask.