...not an expert and only comment on what I've seen and done over the years (prep and boxing up ready for the pour) before bringing in the pro's for the pour.
Also note: Whats done is done.
Carport slab is about right in terms of thickness.
- Assume they used reo bars or sheets - F82 would be a minimum sheet gauge although some may use F72 which would likely be fine if all prep'd really well but in my view is really only suited to paths.
- Would be interesting if they used the plastic reo chairs something I always insist on or use myself if I do the boxing. Quite a few operators simply say they "lift" the reo up into the middle and in my DIY view is all BS particularly when your spending your hard earned time and money to then skimp on $40 of plastic chairs to ensure the best possible result with the reo in the ~middle and therefor achieving maximum strength of the slab. Been burnt 20 years ago with this BS and never again. All my jobs, they either use the chairs or the job goes to someone that will.
- Are you planning to bolt a car port to the slab? Was this specified where you requested quotes? If so, then there should have been footings dug where the posts are going to go. Depending on your soil type and construction a general starting point would be holes of 350mm square by 350mm deep.
Hard to tell in the photo's, but i'd never just lay a slab directly on top soil, I'd always scrape off the top soil and fill with road base or compaction sand to the required depth followed by the concrete boxing/form work on top of that.
The tank pad, whilst its an even weight across the entire area under the tank, given its concrete I would have built it to the same specs as a car slab - ie: ~100mm thick, excavated top soil replaced with crushed rock or compaction sand.
What you've got will hopefully function fine.
Keep in mind there are plenty of tank "slabs" that are simply leveled compacted "compaction sand" and/or crushed rock which work fine, is cheaper and would be my preferred method unless there was some sort of site considerations that required concrete such as sitting one of those slim line tanks up against the house wall or maybe fence to tie in with paving, easier maintenance with weeding etc
The old slab - are you sure its only 60mm thick as looking at the form work in the first photo looks to be 4" (90 to 100mm) to me.