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I'm moving a 8x16 shed-turned-shop onto nine 10" piers on 16" footings over a clay substrate. I'm doing a split pour. I have already dug the holes, poured the footings and set the vertical rebar a few days ago and planned to pour the piers today. Timing was based on no rain being forecast for the week. It rained 36 hours following the pour. The holes are now filled with water and the footings have a 1-2" layer of clay mud settled on them. More rain is forecast now.

When the rain clears, I'll pump the water out of the holes and pour the piers. Before I pour, how "clean" does the surface of the footings need to be of the clay that has settled on them? There 3 lengths of at least 12" of vertical rebar exposed from each footing for the piers to pour over, so there will be a good bit of mechanical bonding.

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  • This is mostly a matter of opinion. I'd at least hose them off with a strong jet. Should I assume that's not possible since you didn't just default to that approach? – isherwood Mar 24 at 21:23
  • Each footing being at least 18" below the surface to the closest firm substrate, I can't spray them off without introducing more water into the holes (and more clay mud). – pierandjimbeam Mar 24 at 21:27
  • The dirt needs to be completely removed no clay residue. I would rather have a little water in the pits than clay on the footings the concrete will displace the water but it can not bond to mud on top of the footings. – Ed Beal Mar 24 at 21:35
  • "No clay residue". That's a good way of putting how clean it should be. With the rebar as described, how important is the surface-to-surface or chemical bond vs the mechanical bond from the rebar? In other words, how much additional strength is there to gain in wiping down the face of nine footings in holes 20" deep? – pierandjimbeam Mar 24 at 21:41
  • "being at least 18" below the surface": by further compacting the earth around the poured piers you reduce the lateral/sheer force on the pier/footing interface, making a poor chemical bond less critical. – P2000 Mar 25 at 16:54
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You have to have your Footings completely clean. The only thing that is allowed to touch the bottom of a footing is a dobie. In-fact, if you have to be inspected, you will likely be handed a failed inspection tag for even slightly dirty footings, as clean footings are part of the checklist before you can pour in most areas, and furthermore; your piers will need to be totally clean as well.

Leaving loose dirt at the bottom of a footing can lead to gaps under concrete that fills the footing, this is especially true in areas that have seismically active like the Middle East, Mexico, Japan & California, or in areas that are swampy like Florida, inland New York, Brazil, and Massachusetts. When the ground shakes, or gets wet, loose dirt compacts, leaving space between the dirt, and concrete, underneath the footing where you cannot see. The dirt that hasn't been moved has already compacted for millions, if not billions of years, therefore, it is not only important to clean the footings, removing all the loose dirt, but also to make sure the dirt at the bottom of the footing has never been excavated before (in other words, the bottom of the footings should be dirt that has never been touched by man before). Also don't just re-compact the loose dirt, because no matter how much you pack it, nature will pack it more as time passes.

When loose dirt is left under fresh poured concrete, as I mentioned previously, it eventually turns into space between the concrete and the bottom of the footing. Space under the concrete leads to cracking, potentially big cracks, and in a worst case scenario, if enough loose dirt was left in the footing before the pour, it can cause the foundation to sink on one side throwing the entire home out of level. Bellow I have compiled a list that has the proper steps to take when excavating and pouring footings.

Steps for excavating and pouring footings:

  1. When all of your excavation is complete, you need to clean your footings extremely well before you tie any rebar.
  2. With clean footings you can now start cutting and tying your rebar. (usually one man/woman cutting and 2-3 men/women tying.
  3. After your Rebar is completely tied you should have nothing but dobies touching the bottom of your footings. At this point you need to clean your footings one final time.
  4. Get inspection if required.

Side Note: "If you have a small crew working for/with you it works best to have everyone stop doing what they are doing, and have them all clean the footings."

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  • Welcome to the DIY group! With "footing", do you mean the concrete footing, or its cavity? I think you mean cavity. Just trying to be helpful here, because it took me a few reads to understand. Perhaps: "The only thing that is allowed to touch the undisturbed bottom of a footing cavity is a dobie. There should be no loose mud or soil." Yes? – P2000 Mar 25 at 16:25
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    Sorry it took me so long to reply as I am usually on stackOverflow. I personally have never heard the term cavity used, we always refer to the cavity in terms of an excavated footing, anyhow, yes you cannot have anything touching the bottom other than dobbies. This is the rule of thumb (in California at-least) for any excavated ground that's going to filled with concrete. The exceptions to the rule is gravel, and waterproofing but obviously only if the architect called for one of those two in the blueprints. Rebar especially cannot touch the ground. – AbstractMechanic Apr 6 at 22:51
  • What is a "dobie" that is allowed to touch the bottom of the footing? Also, unless I'm misunderstanding you, it sounds like most of your post is addressing the ground under the footing, not the top of the footing where the post/pier is going to be poured, which is what the question is about. – FreeMan Apr 14 at 12:47
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The footing should be completely clean where the pier attaches to the footing. If clay or soil is left on the footing, the piers could settle when water enters the area and washes the dirt away.

Btw, the rebar needs 30 bar diameter to completely transfer the tension load. (1/2” bar x 30 = 15”) The bar will be in tension when the shed shifts sideways.

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  • By "completely clean", do you mean "wiped down" or just scraped? A film of clay wouldn't leave any room for settling, but it would compromise any chemical bond (right?). Is there any chance of getting a good chemical bond anymore anyway with the rough surface have had clay settling into it? As to the bar diameter, is that total combined length? I have three 1/2" bars of 12" each in a footing making for 36" total length per footing. – pierandjimbeam Mar 24 at 21:35
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    Chemical bonds and rebar ratings are only important if you're building a parking garage. You won't have much for lateral loads in a yard shed. Deck blocks, on which many sheds have been built, require neither. – isherwood Mar 24 at 21:37
  • I could definitely just throw cinder blocks into these holes and be doing far better than where the structure was before. The building will be sealed, insulated, powered, climate controlled, and housing some weight in material and tools, so I'm okay with overdoing it. And learning the rightest way reasonably possible on this micro-reno will be a good exercise. But your point is well taken. – pierandjimbeam Mar 24 at 21:46
  • @pierandjimbeam You won’t have and you don’t need a chemical bond between the footing and piers. The resistance for 30 bar diameter is for each side ...in the footing and in the pier...for a total of 30” for each bar. You can reduce that by making a “hook” in the end of the bar. Rebar needs to be 2” from the sides of the footing and piers and 3” from the bottom to avoid developing rust and so the rebar developes full bond between rebar and concrete. – Lee Sam Mar 24 at 21:52
  • Some lessons learned there. I'd heard about 30d, but for lap lengths of rebar, not for bonding concrete pours like this. As mentioned above, finding out how to get 30" per piece of rebar into piers and footings of this size seems like a puzzle. But good to know for future bigger projects. – pierandjimbeam Mar 24 at 22:12

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