I am about to rip down an old shed in a the garden and replace it with a new one. There are already concrete foundations measuring 13' by 7'. I would like to put a shed up that is 14' or 15' by 8'. I therefore need to extend the concrete base. Given that the shed will not weight that much can I just lay some more concrete alongside the existing stuff or will i need to add rods or something into the existing concrete to give it something to 'bond' on to?