Author Topic: Pouring concrete pad on dirt?  (Read 1369 times)

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Offline ryanjg117

  • Posts: 250
Pouring concrete pad on dirt?
« on: October 10, 2019, 10:05 PM »
I need to pour a smallish (3' x 10') concrete pad for the side entry of a workshop. Not a concrete guy, but a few of the tutorials I've seen online suggest a base layer of 3/8 or 5/8 minus (rocks/pebbles). Others I've spoken with suggest it's not necessary as Pacific Northwest soil tends to be quite hard. I would describe the soil under the pad as fairly "loamy" but not very springy. Attaching some pictures below.


Not the best pics as it was getting pretty dark, sorry.

Just wondering if I need to add that base layer of rocks, or if compacting the current soil will be fine.

This is a side entry to a garage and won't ever see a tremendous amount of weight. I'm also planning to reinforce with some wire mesh.

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Offline denovo

  • Posts: 67
Re: Pouring concrete pad on dirt?
« Reply #1 on: October 10, 2019, 11:09 PM »
If there is any risk of the concrete being exposed to freezing temperatures then I would definitely excavate down 4-6" and install a granular base.  The gravel will allow proper water drainage; helping to avoid frost damage/heaving of the concrete.

Offline ryanjg117

  • Posts: 250
Re: Pouring concrete pad on dirt?
« Reply #2 on: October 11, 2019, 12:29 AM »
Thanks, that's what I'll be doing.

Offline Cheese

  • Posts: 7504
Re: Pouring concrete pad on dirt?
« Reply #3 on: October 11, 2019, 12:32 AM »
Ryan, think about how the city does business. When they pour sidewalks they usually don't use an underlying base of class 5 (crushed rock or cement fines) however, they do firmly compact the area underneath the sidewalk. This is usually done with a weighted, driven roller that weighs several tons. The 4" cement pad is then poured on top.

The typical homeowner doesn't have access to this type of equipment and so we are left to come up with an alternative. This usually becomes a 200# plate compactor (Wacker) that's used to compact a 4"-6" base of class 5. Class 5 when compacted properly becomes like concrete, it's incredibly solid because all of the fines interlock with one another.

Also remember that the wire mesh will only reinforce the concrete pad, it will make no difference when it comes to restricting any movement of the pad. If the heaving isn't too excessive, it will allow the entire pad to move as one continuous element.

« Last Edit: October 11, 2019, 09:18 AM by Cheese »

Offline Rob Z

  • Posts: 1005
Re: Pouring concrete pad on dirt?
« Reply #4 on: October 11, 2019, 01:58 PM »
Hello Ryan

To follow up on what Cheese said, the plate compactor might be the best option for you because you can rent one fairly inexpensively.

You might already know this, but in case it helps, get a tool that will cut a control joint and place those so you divide your slab in thirds.

Offline ryanjg117

  • Posts: 250
Re: Pouring concrete pad on dirt?
« Reply #5 on: October 12, 2019, 11:10 PM »
I built the forms today and grabbed a half yard of 5/8 minus crushed rock. Compacted nicely with a hand tamper I rented from the box box store for just $18 a day. My plan is to place expansion joints against the foundation but I'm concerned about the slab pulling away from the house over time. There is a downward slope not far off the pad and I'm a bit worried about long term erosion. So, my plan is to run a few of the bigger tapcon screws through the expansion joint material and into the foundation, just sticking out a few inches so they'll be captured in the concrete slab. I know you can drill bigger holes and put rebar and anchoring epoxy to accomplish the same thing.

Offline Cheese

  • Posts: 7504
Re: Pouring concrete pad on dirt?
« Reply #6 on: October 13, 2019, 01:14 AM »
After working with concrete and all things that float on compacted class 5 over the last 25 years, I can only be in awe of how Mother Nature slowly and deliberately proceeds despite my most serious interventions.  [smile]

I installed a 500 square foot bluestone patio using a Wacker plate tamper and it took 15 years before the bluestone started to heave.

I also installed a 60 square foot paver sidewalk using a hand tamper and it took 4 years before it started to heave.

To this day, I'd rate the patio at 90% while I'd rate the sidewalk at 25% or less. Mother Nature does what she does and a few Tapcons will not change her disposition. She's like the Mississippi...she just keeps on rolling along.  [smile]

Offline Holzhacker

  • Posts: 967
Re: Pouring concrete pad on dirt?
« Reply #7 on: October 13, 2019, 10:23 AM »
Another easy option to prevent movement is to pound a few rebar into the ground so they stick up into the pad. Works pretty well. 2 or 3 pieces of rebar don't cost that much at HD. You could also dig a hole at each outside corner as a sort of footer but that seems overkill for this size pad. The footer works really well for patio pads.
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