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What's a "mud slab"?

Americandown's picture

Been building here in New Zealand for the past 3 years.The foreman comes up to me last week and asks me what a "mud slab" is. He saw it on the plan and reckoned it was some American term.I said I'd never heard the term.Spent 5 years in the foundation and flatwork business exclusively(4 in Connecticut,1 in Oregon)before turning mostly to renovation and new homes in later years.Never heard the term there either. Any enlightenment would be much appreciated.

(post #53196, reply #1 of 18)

I can't swear that the way I heard it used is the way he's using it. But the one time I heard it... We were digging a footer and pulling out big pieces of granite. It was too rocky and uneven to form as is. No way to set the rebar pins we needed to secure the forms. So we talked to the engineer and inspector and poured concrete to level the unevenness and give us a base from which we could accurately form. They all called it a mudslab and it worked better for our forming requirements than gravel would have, be/c we could hammerdrill into this and get accurate placement of our stakes.

Here's the only pict I could find...

Edited 4/20/2002 7:47:22 AM ET by Cloud Hidden

(post #53196, reply #2 of 18)

I have heard the term meaning using mud and portland to do nearly the same thing you are talking about. Any application that is substrate . I have also heard doing the same for under a walk of pavers to the garden for example.


(post #53196, reply #3 of 18)

check with the author of the plans for a definition.

I'm speculating only here, I've heard a brick lip or brick ledge called a mud lip. Maybe this is a monolithic slab with a recessed edge at the perimeter of the building for a brick veneer wall on the exterior?

Excellence is its own reward!



Oh Well,

We did the best we could...

(post #53196, reply #4 of 18)

Here in the Boston area, a mud slab is what we pour in a crawl space foundation.  Usually 3-4 inches thick and floated but not finished troweled.  It's just a rough slab and is better than a dirt or crushed stone floor.  Most of the Architects here spec it .

(post #53196, reply #5 of 18)

Mud generally refers to a mixture of Portland Cement and Sand, with no aggregate (rocks).

Mud is applied on walls and floors for ceramic tile installation. It can also be used over concrete for exterior tile installations.

The mixture is generally 5 parts sharp sand to 1 part portland cement. On verticle applications, 1/2 part lime is added for stickiness.

Mud generally has a maxium thickness of about 2 inches, whatever that is milimeters.



"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927

Regards, Boris "Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934

(post #53196, reply #6 of 18)

That reminds me, what's a dirt farmer? Is it just a really bad wheat farmer?

(post #53196, reply #7 of 18)

A mud slab is a thin layer of wet concrete (no structural value) that is poured in a bottom of a footing or basement on nasty subsoil. The purpose is to have a dry area to set forms and foundation. It is usually use in a wet muck layer. We use them daily around here. for example, at the paper mill all the fondations are on piles but because of the mud, acid, falling sand, pile caps are hard to form. So you demuck around the piles, pour a mud slab and you ready to go.

(post #53196, reply #8 of 18)

I'm with Cloud and DiamondBuild.

The way I've heard it, a mud slab is just an unreinforced slab cast prior to the structural foundation to deal with a problem condition. Say very uneven ground like Cloud had. Or, very sensitive subgrade soils that shouldn't be exposed to weather for the time it takes to place your steel. Or to facilitate some heavier const. operations on site. I've seen it on jobs where final fndn. with be a structural mat. Best advice though was already given. Ask the people who prepared the plans.


(post #53196, reply #9 of 18)

Right. As you guys have described (and I'm glad to have learned a new trick) a mud slab would be a facilitation technique and not something that would seem like it should be specified in plans. It would be the judgement of the builder or concrete sub about whether to use it or not based on actuall conditions. Right?

Excellence is its own reward!



Oh Well,

We did the best we could...

(post #53196, reply #10 of 18)

Yes, right. It's a response to unpredicted, and likely unpredictable, site conditions. And in our case, it involved a discussion with the engineer and inspector--just to be safe.

(post #53196, reply #11 of 18)

Kinda rare around here but when the term is used it tends to be any non-structural slab poured cheaply, typically thinner and with less, if any, reinforcement, and primarily for the convenience of having a cleaner, drier and flatter surface to work on. A rat slab, a term not used around here, seems to be similar but located primarily in a basement or crawl space. This isn't a dictionary definition but rather a summery of the state of the term or as close as I can get.

Mostly what everyone else said. And so it goes.

(post #53196, reply #12 of 18)


Usually, I think you're right that it's the choice of the contractor to put in a mud slab. The only time I've seen it required in the plans was for a structural mat placed on sensitive silts in the Conn. River valley. The mud slab here had to be placed immediately after exposing final subgrade to avoid disturbing this material.


(post #53196, reply #13 of 18)

a "Dirt Farmer" is a farmer who's poor as dirt.

Truss Designer Extraordinaire

(post #53196, reply #14 of 18)

Found this definition in : "Architectural and Building Trades Dictionary". Referenced this book more than once since completeing apprenticeship back in 1990.

Mudslab: A 2" to 6" layer of concrete below a structural concrete floor or footing over soft, wet soil.

should clear up any arguements.


(post #53196, reply #15 of 18)

My background is tunnels, so my sense of scale is different. Mud slabs are required in deep excavations to form a seal over the wet material during dewatering operations. The mud slab is also used to place waterproofing and to set rebars or steel frames before concrete pours. In typical cut-and-cover construction, 6" minimum is specified on the plans, however it is always neglected in the design, both for structural and stability (bouyancy).

...that's not a mistake, it's rustic

...that's not a mistake, it's rustic

(post #53196, reply #16 of 18)

Thanks for the answers.I always make it a point to hang with guys who know more than me.I appreciate your time taken to respond.

(post #53196, reply #18 of 18)

My back ground is in Heavy Construction.  We use the term "Mud Slab" or "Mud Mat" to describe a concrete slab that is poured on the ground.  This slab allows you to have a solid area from which to tie your forms down too.  It also allows you to keep you rebar and formwork clean and out of the mud.  It has not structural value, but gives you a good work area.  I have also used them over rocky ground where it is hard to set you forms.  In this case you would pour a low slump concrete mix on the rocks just to tie everything together, screed it of level and then come up with you forms. 

Hope I could help

(post #53196, reply #17 of 18)

In chicago a mud slab is used in crawl spaces.  A 2" - 3"  unreinforced (maybe fiber) concrete slab just to cover the gravel .  It is mainly use to provide a more finished surface than leaving the gravel exposed.