We are putting in a concrete driveway around our home in Hawaii (200 inches of rain a year.) The house itself is complete. The large construction trucks will not be back. In other words, the concrete driveway only needs to support regular sized passenger vehicles.
My builder has put 5 inch concrete slab in the plans. Could we get away with 4 inches? Its a large area. The proposal is $48,000. For this large a project would we save a significant amount of money by going with 4 inches and still have a stable driveway?
If the slab is engineered for 5 you need 5. No one here will be able to intelligently tell you with the scant information you supplied. Take your plans to an independent engineer. For a couple of hundred dollars they will be able to tell you.
White SIFL is correct. The soil conditions and other issues would require you to have an local independent Engineer review the plans and that would be cheap dollars for you.
A couple additional thoughts to consider.
Concrete comes in strengths. 3000 psi is most common but you can also get it in 3500, 4000 or more PSI. You would pay more per CY for the concrete but if you can have the engineers spec a higher strength concrete you might get by with the less thick slab. Notice the word…might.
You not mention how your contracts are set up, but in construction always get a minimum of 3 quotes to keep everyone on the up and up.
Also don’t be thinking the cost will be reduced on a % basis per CY. Your contractor has some fixed costs here such as getting the guys to the job site, setting up the forms, the reinforcing methods and finishing cost. All of that will not change for a 4" verses a 5" slab.
Well, we don’t know the square footage of the driveway or the cost of cement in the middle of the Pacific Ocean. I’m guessing that is probably driving up costs.
I’m assuming they are putting some reinforcing fabric in the slab. Generally, the requirements are that you must have a minimum of two inches of concrete above and below the metal reinforcement. I know that in some places the inspectors will let you get away with a 4" slab, but perhaps in your area that’s not the case.
The 3 other bids have come in around the same cost. We’ve decided to use a good friend’s nephew for the project. Its not really a slab; its a large 12’ wide driveway that circles the house and more. The property is 8 acres; we’re thankfully only covering a small part of it.
If there will ever be a large truck on it, you should go with the thicker slab. In my area (NC) they spec 4" for sidewalks/walkways and 6" for driveways. At my workplace a roof maintenance truck drove across a 4" slab walkway to get close to the building, broke it, and had to pay to replace it.
even though the others have hit all the key points, I would chime in and agree that more details are needed for both
You to be able to assess the risk or otherwise of the change and,
Before any accurate assistance could be rendered from this board.
You need to know the intended concrete compressive strength, fabric (rebar) size and design, ground soil conditions, compaction requirements of substrate. My personal experience tells me in relatively sandy soils, for general residential and light commercial use (perhaps maximum axle weights of around 2000kg or so), 4" (100mm) of NC32 concrete with a single layer of SL72 reinforcing mesh positioned 35mm underneath the top surface is appropriate in my situation. I just pulled those figures off a preengineered slab drawing for a residential driveway i had sitting around. I am in no way saying 4" is therefore suitable for you
The real point is that there are several variables you need to control before you can just swap from 5" - 4", or indeed make any educated assessment as to this change.
Also with 8 acres, you really should consider the fact it’s likely you will have light and heavy commercial vehichles on your property in the future, doing work or deliveries or anything else. You don’t want to save a few bucks now to end up with a damaged driveway later.
