Fill for Concrete Driveway Extension

[david71]

I live in Charlotte, NC. I am adding a 34'L x 34'W x 4" deep concrete pad to the end of my driveway upon which I will place an All Weather Shelter to house my truck, truck-camper and boat . The maximum weight is about 12,600 lbs. when truck-camper is loaded in bed of truck.
I have had a recommendation to build the concrete slab.
The following recommendations included binging in 5 to 7 truck loads of fill dirt.

Question 1 : Should fill be a grandular substance to provide for better drainage ?

The soil and fill is to be graded with a 'Bobcat'.

Question 2: Should a 'plate' compactor be used on the soil and fill and to what specification of compaction ?

Question 3 : Should a geo-thermal fabric and some gravel be placed on top of sub-grade ?

I intend to specify a 2:1 ratio of Cement:Water to provide for 4,000 psi of support.

Question 4 : Is there a General Contractors Contract form where I can record specifications ?

I have estimated that I will need 14.27 cubic yards of concrete without pouring footers.

Question 5 : Given this information, do I need to make any other specifications and requirements for the Ready-Mix company that delivers the concrete ?

Thank you for your attention and expertise

 

[Focht3]

Are you doing the work yourself, or hiring a contractor?

Please describe your site and soil conditions. I'm not familiar with the Charlotte area -

Do you care if you get a cracked slab? If you do, then you need to find a local civil engineer to help you with this. Or find one of the better local concrete contractors who will guarantee his work.

It sounds like the fill will be used to raise the site less than 6 inches. A pad compactor should be fine to compact the fill; but what about the soil beneath the pad?

Avoid prescriptive specs (2:1 cement:water) and use performance specs (4,000 psi at 28 days.) Do limit the ratio to 2:1 to reduce the risk of excessive shrinkage.

Don't forget to use crack control joints; you'll need to divide the slab in half each way. That will give you 17x17 ft panels - good crack control.

 

[dicksewerrat]

first you have to level the site of the pad. dig a 1 foot wide by 1 foot deep footing around the whole pad.
Find out where the anchors for your building should be located. Then make a drawing of the anchor locations. This is important because in the hurry to place the anchors, you may not remember the correct placement.
set your expansion feltin place inside of the 17 x17 forms. This concrete placement should take two seperate pours a few days apart if you are doing this yourself. As Focht3 said you will want four seperate 17 x17 slabs. Pour the two that are on opposite corners from each other.
You may want to place steel reinforcing mesh in the slab to reduce cracks.
While placing the concrete you will want to place a 'screed pipe' down the center of the slab to help keep the slab surface level.
Protect the slab from drying too quickly by using a 'curing' compound.

 

[dicksewerrat]

I almost forgot. A water/cement ratio of 1:2 is not good. It should be more in line with 1:3 or so. More water equals weaker concrete.

 

[david71]

Thank you for your advice . I am hiring a contrator to do the work for me. The soil conditions are red clay. I have a lot of rock and dark brown soil on the surface of my lot.
Some contractors have recommended the use of fiber in the concrete in lieu of wire mesh. The fill will require about 18" at the back of the pad.

 

[cvg]

fill should be similar to the insitu material. Granular material does allow water to flow through, thus providing better drainage. however, you need to limit the water flowing onto or across the driveway in the first place. Is the driveway sloped? Is the surrounding ground sloped away from the driveway?

subgrade should be compacted prior to placing fill and a plate compactor or small walk behind roller is fine. place fill in thin lifts - 6 inches thick or less and then compact

fabric and gravel should not be necessary. this is mostly used on roads with very poor subgrade. repeated cyclic traffic loadings cause structural failure. you won't have this type of loading.


Steel reinforcement mesh is normally not needed for driveways and would be a waste of money. fiber reinforcement is also probably a waste. neither is normally done on driveways.

Civil engineer is probably also overkill for this small homeowners project.

how will you know if concrete meets the strength requirement? this is normally done by compressive strength tests of specimens - this will cost $$. is it worth it? probably better off just requesting class A concrete (3,000 psi) and be done with it. 4,000 psi concrete is probably not needed, however I would still require a maximum water cement ratio between 0.45 - 0.50. Water cement of 0.33 is extremely low and not normally used. You could also specifiy the minimum amount of cement - say 520 lbs per cubic yard.

 

[dicksewerrat]

I would disagree with 'cvg' on acouple points. You are building a contractors and the equipment and skill they bringfloor for a building. If I were the engineer on the project I would still go with the mesh and a low water/cement ratio. .33 is low for the weekend do-it-yourselfer but for the contractor and the skills they bring, this will give a better finished product. I have seen fiber used for small crack control, but it isn't as good as steel.
When I was specing concrete in my former job, I wanted 2" slump, high early concrete, and chloride for fast cure.

 

[CarlB]

I agree with dicksewerrat on all items except the water:cement ratio. A ratio of not more than 0.45 should be sufficient to minimize shrinkage (cracking) and durability concerns; 0.50 is pushing the limit.

 

[Rjeffery]

dicksewerrat has the right idea about using the welded wire fabric. (WWF) 2.5 6 X 6 WWF is cheap insurance if the subgrade begins to fail.

Was it Ben Franklin who said 'Mind your pennies and the dollars will take care of them selves.'?

In other words pay me now (let me do it right, today) or pay me a lot, later (pay me to do it right, again and be mad at me for doing it wrong last time because you wanted to scrimp and 'save' and it fell to crap like I told you it would so I gotta haul out the old work or spend time half @ssed [oops] fixing it and you still won't be happy!)

 

[cvg]

I agree with the cheap insurance idea, but better to start from the ground up. Good subgrade and good fill compaction and good drainage give a good foundation for the concrete. If this is done right, no need for the steel. But if the insitu material is poor quality with low bearing capacity, then it might be impossible to get a strong foundation - then steel might be an option.

I still disagree with .33 w/c and 2 inch slump. I have inspected a project where .45 w/c with 3 1/2 inch max slump was specified and both contractor and ready mix supplier had difficulty supplying and working with the mix. ACI recommends 0.53 max for concrete slabs on ground.

I suggest a good idea would be to talk with the local city or county engineer and ask him for his concrete spec...