Originally Posted by
mattekure
The concentration isnt the concentration of the spell, its concentrating on the act of holding the spell. Once the spell is cast, it may or may not require continuing concentration.
From the PHB:
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.