Cold joints can be a very costly thing to repair. Make sure that you arrange enough number of trucks for your concrete pour so that you will not have to wait for the next truck in the middle of your pour. At the extreme, you will need to demolish the whole thing and form it and repour again from scratch but in most cases you can avoid doing this by taking certain precautions. Lets say the next concrete truck is delayed, in that case before you pour the next load, you will have to poke the existing poured surface with a piece of rebar so that it will have ins and outs at its surface, and it is also a good idea to wet the surface a little. Of course you can do this to a certain point. I am wondering if there are any admixtures to use during this wait, if this happens to you.