The minimum thickness for a slab should be 4". A vehicle bearing by slab such as a garage floor or driveway should be further reinforced with steel rebar.
The upfront cost for a proper slab may be slightly greater than what you may have budgeted in terms of proper thickness, concrete quality etc. However, long term costs are far less given that a poorly constructed slab which is too thin and made with inferior hand mixed concrete is prone to cracking, damage and failure at points.
The readymix options are also possibly the same cost and also possibly cheaper when you factor labour into the mix. However, in terms of quality, a premixed design far exceeds any hand mixed or on site mixer design as premixed concrete is done in the correct ratios to achieve proper strength and slum. While one may argue that the ration is 4 barrow or gravel to one bag of cement, there is no accuracy in the size or fill of a barrow.
My advice is do it properly the first time, 4", with steel if necessary and readymix. Also consider setting conduit and pipe for future use as you never know if there will be a future need for these which saves from having to tear up your floor at points.
Cost wise, a 28 X 18 X 4" slab should be about 5 cubic metres which may run between $4000 - 6000 for a 3000psi compressive strength, depending on supplier and wastage of capacity in truck.
Assuming the area to be cast is currently earth, no trees and no existing concrete:
Grubbing area - 2 labourer, 1 mason, 3 days. $3000.
Prep area for casting - 2 labourer, 1 mason, 1 day. $1000.
Cast is half day but pay them for a day. Same crew $1000.
Remove formwork and clean up is a next day for same crew $1000.
Conservatively and roughly you are looking at about 10-12k.
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jsr wrote:I was looking at it being 3 inches thick with brc .
Also what about the option of renting a concrete mixer and do it myself?