Gravel / rock under concrete.

[AlanB]

OK, I am about to pour my concrete driveway / workpad.

18' X 30' X 6"

current base is red clay, but will be digging it out and putting compacted rock / stone down about 8" thick.

I also know that few people in this area do anything for a base on a drive.

Now I have heard several explanations as to why the rock was so important, drainage, stability, etc. etc.

Can someone offer some insight or point me to a website of the why's and wherefores of a crushed rock base under the concrete?

Thanks.

 

[pat32rf]

It prevents the base from becoming waterlogged and heaving when it freezes

 

[praha]

Well,
my neighbor only used a sand and he thinks he has the best base ever./he compacted it heavy.

Joe

 

[Steve_Miller]

I'm not sure of my state abbreviations, if it is Tennessee then you don't have to worry about the ground freezing, so, if the slab is on higher ground and runoff is not a problem then I would say that the red clay base should be fine. If it is in a hollow then yes, you probably want some rock for stability when the ground may get saturated with water. If it is at all possible try and get the water to drain away from the slab. The main thing when pouring concrete for a slab or footing or any base is to have the water run away from the area. Water is the worse enemy mother nature can throw at you, well maybe a hurricane or a tornado could be worse.


Steve

 

[AlanB]

Yes, I am in Tennessee.

My ground is very wet (normally) and we are having an exceptionally dry summer this year, so right now it is dry.

This driveway is too low. But shy of pumping the water uphill, there is not much way to get much drainage.

Because of my concerns over drainage, fall, flooding etc. I am electing to go with pavers. At least that way, I can do it over if needed.

I am laying in about 6" of 2"/3" rock (already done) then about 2" of Crusher run (small rock with fines) then about 1" of gravel sand for a bed to lay the pavers in.

I am compacting it all in small lifts, maybe about 1" or so with my too small plate compactor (3500 lb force) I am hoping / expecting that by doing it often in small lifts it will be sufficient.

I think the pavers will also give me some wiggle room if needed with a particular bad place.

I am laying in a french drain beside the area, to help get the water flowing away, but realisticly, there will be water in this area.

 

[Toiyabe]

The main purpose of an aggregate base under a concrete pavement is to keep the subgrade from pumping or erroding, leading to a loss of support under the joints and corners.

Pumping happens when you have a fine grained subgrade (including clay) that is very wet. Moving wheel loads can cause that material to liquify and move around under the slab, often shooting up through the joints. This leaves voids under the joints and corners, which then causes them to break. Movement of subgrade material also leads to faulting (vertical mis-alignment of joints).

Frost heave is a complicated and misunderstood phenomenon. It is caused by moisture moving up from underneath a structure (not surface moisture), and can't really be cured by drainage. Frost heave only happens when air temperatures are well below freezing - near-surface water will be frozen, so can't migrate under the structure at that time and cause problems. It is the unfrozen water from deep in the soil (which can't easily be drained away) that gets sucked up and causes frost heaves.

 

[bamatoolmaker]

Around here all that is used under concrete is compacted chert of course we dont really have any building codes either.As a matter of fact perk tests have been omitted to cut county costs.