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Logic behind Mud Pack vs. concrete mix?

Streamline's picture

What is the logic behind mud pack for the shower pan rather than using pouring a concrete mix as we would do in any flat concrete work?  How does mud pack cure as it seems so "dry."

(post #104822, reply #1 of 25)

Don't know, but I have NEVER used anything but fiber-reinforced Sakrete mix.  Probably 10-15 pans over the last 18 years, and never a bit of a problem.  I find the concrete easier to float and finish.


Curious to know the answer myself.


Forrest - not a tile guy

(post #104822, reply #3 of 25)

Your fibre helps prevent cracking. Drier mud paks also help resist freeze/thaw problems, which are probably not a concern in your necl of the woods.

I'm sure the John Bridge forum has something to say re this.
http://www.johnbridge.com/vbulletin/archive/index.php/

There is also a tile guy and author named Micheal Byrne who is a dry mud pak afficianado...

 

 


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(post #104822, reply #2 of 25)

The more water that is added to any Portland type mix after it has what is required to cure only serves to make it easier to work while it weakens the end product and increases the odds of shrinkage cracks.

Packing it is to compress it hard into place where it will cure as placed without shrinking.

 

 


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(post #104822, reply #4 of 25)

You need 2-1/2 times the diamter of the biggest aggregate outside the aggregate, or the mix is weakened.  For a mud bed you might only have an inch of thickness at the drain, not enough with 3/8" aggregate.  Sakrete even says their concrete mix is for 2" or thicker applications.

(post #104822, reply #5 of 25)

<Sakrete even says their concrete mix is for 2" or thicker applications>


Ah - forgot that.  Good point - mine usually go from 3" at the edge to around 2" at the drain; that will take care of the diagonal of a 6-7' square shower, and is about all the height I can get out of the subdrain I use.


Forrest

(post #104822, reply #6 of 25)

That would do it.  Do yo do a pre-slope then a pan or pan on the bottom?

(post #104822, reply #7 of 25)

Yes - preslope the plywood (maybe 1/2"drop in the center), then liner, then slab.


Forrest

(post #104822, reply #8 of 25)

A "dry" pack mud base is not flat.


It's done with sand and cement because that's the right way to do it.


If your mix is so dry that it scares you, then maybe it is too dry.


This is NOT a DIY project.


Some things are best left to professionals to complete Mon -Fri during normal working hours.


 

 

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(post #104822, reply #9 of 25)

The mud pack for a shower pan is sloped.  A sloping pan (and the pre-slope) is easy to do with mud pack and more difficult with with concrete.  Plus, you can "shave" the mud pack easily after it is dry but before it has fully hardened.  This is more difficult with concrete.


In a traditional shower pan some water drains through the grout (if it's not epoxy) and into the mud bed above the membrane, and it eventually drains through weep holes into into the drain, assuming the installer used the correct drain and didn't block the weep holes.  Regular concrete wouldn't accomodate this water drainage, and it would clog the weep holes when you pour it.


The mud pack doesn't have as many shrinkage problems as concrete.


Mud pack isn't all that different from concrete in terms of ingredients, except there is no aggregate and less water.


That's all I can think of for now, but it's an interesting question.


Billy

(post #104822, reply #10 of 25)

Good intellectual discussion.  In application, I have 3 torched down outdoor decks over living space.  The door threshold is 1 1/2" above the deck and I want to tile the deck.  The tile guy suggests "mud packing" but without much conviction, while the concrete guy says "pour."  The decks (5'x12') at 1 1/4" thick at thinnest level, sloped toward scuppers. 


One deck is the front entry, which ties in with the concrete stairs system (see pic).  Making the deck and the concrete pour one piece would make sense, but it appears to have disadvantages you mentioned (mud pack less likely to shrink).


If it were your project, how would you have this done - one pour for both deck and stairs (continuously flow together) or 2 different tasks (mud pack deck to the edge, then metal mesh tie in with the concrete stairs?)


I plan to Ditra the deck top and tile to bring the top-of-tile to flush with the door threshold.  I am aware of the risk of cracking when the deck turns to steps so at that juncture, I am making the stairs riser 4" thick, with ample rebars from sides and toward house, and tying with the mesh at deck, along with control joints so the cracks are encouraged at front joint.  I think Ditra would help with the tile above. 

(post #104822, reply #13 of 25)

In a shower, you want the tile base to be water permeable so the water that passes through the grout can escape.

On your deck, you don't want water to get into the base because there is no place for it to escape to.

I would pour the deck. Specify BB- aggregate and a waterproofing additive like Xypex.

The junction of the steps and deck *is* going to be a movement and drainage problem. I would pour the Riser to the deck as part of the deck and put the expansion joint between that riser and the top stair tread. You will have to figure out how to drain that junction.

Perhaps run the Ditra down the riser and lapping over the tread. If you pour the stairs first, you could ramp the tread up in the area under that lap. Put some water permeable expansion material on top of the lap.


SamT


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #104822, reply #18 of 25)

Sam,


Let me see if I understand your suggestion correctly.  Pour the riser at edge of deck as part of the deck in a single pour.  I think it is a great idea as the vertical portion will add strength to the deck edge.  Then pour the stairs second (likely the same day).  At the junction where the bottom of riser meets up with the first step (tread) down, I could caulk that line as the control joint will provide room for movement? 


I don't worry about the drainage issue here because everything is sloped to drain out, there is a torch down material on the plywood, and a concrete drainage mat against the exterior wall, along with 1" aggregate structural fill in the stair well that drains to a system below.  Since I am planning to tile the steps also, does this pose any concerns? 

(post #104822, reply #19 of 25)

Leave the bottom of the riser 1/4" -1/2" above the tread tile. Caulk that joint. Leave at least 1/2" of that joint uncaulked at the end sides of the tread as weep holes.


SamT


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #104822, reply #20 of 25)

Sam,


I assume you want this gap at tread and riser as control joint.  I was thinking of placing a few sheets of tar paper in this space, and pull them out later to create 1/8" to 1/4" control joint.  This is so because all this is one pour and I don't have an option to leave the top riser hanging during pour.


Also, what's the logic of leaving uncaulked section at the control joint?  Since this is all outdoors, and the treads are sloped 1/4" anyway away from stairs, wouldn't that be adequate to control the water drainage?  thx. 

(post #104822, reply #21 of 25)

I assume you want this gap at tread and riser as control joint.

Correct. If you are confident that the deck won't settle relative to the steps, you can leave a 1/4" gap just to leave a way for the underside of the deck to drain.

"I was thinking of placing a few sheets of tar paper in this space, and pull them out later to create 1/8" to 1/4" control joint."

Is it best practice when vertical tile meets horizontal tile for the vertical to butt the horizontal. Since the bottom row of riser tile can cantilever a bit, it's ok if the control joint is a bit wide.

I'm thinking to leave the bottom edge of the riser 1/4" above the tread tile. Tile + thinset = 3/8" or better. Add the 1/4" control joint and you have 5/8", too much for a few folds of tarpaper.

Close off the bottom of the riser form with a piece of 3/4" ply, but be sure to leave it run long on the ends so you can get a grip on it when you strip the forms.

If you wrap it in plastic, just fold the edges up on the face of the riser form, it will strip 5 times easier. Pour the top step and it will support the riser bottom while you pour the deck.

"what's the logic of leaving uncaulked section at the control joint?"

Water *will* get under the deck concrete. That gives it a place to get to open air.


SamT


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #104822, reply #22 of 25)

I understand what your intentions are now.  If I told you that the entire stair well below the steps is filled with clean free draining 1" quary spalls, and drain holes are at the bottom of a 8" thick stair footing that leads to a catch basin (meaning that there is a way for the water to drain) do you think I still need the 1/4" gap anymore? 


To establish the control joint, I could just place cement board strip at the bottom of the first riser to hold up the concrete during setting, and leaving the cement board in place permanently.  To create the joint, I can place 5 sheets of tar paper under the cement board, and pull 3 out during stripping (easier to slide out), and that would leave me a thin control joint. I can just caulk over the joint to not have that gap visible but still effective as a control joint.  Thoughts?


 

(post #104822, reply #23 of 25)

What I hear you saying is that there is a channel for water that gets under the concrete of the deck to drain and not sit in the expansion gap. That's fine, caulk away.

BTW, that gap between the riser and tread is not a control joint. It is to allow for the deck settling in relation to the stairs.

Since the deck riser, from the steps to the deck, is only 8" -10" tall, stacking tarpaper under that riser, to a height enough to clear the top of the tile you place on the stair tread by 1/4" will work and you should still be able to pull the tarpaper out.



SamT


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #104822, reply #24 of 25)

very helpful graphics.  Thanks a bunch.  I understand now.

(post #104822, reply #25 of 25)


SamT


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #104822, reply #16 of 25)

i would want a minimum of 2-1/2" to leave drainage, mudpak, and tile

 

 


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We did the best we could...

(post #104822, reply #12 of 25)

<and it would clog the weep holes >


I use tiny gravel or tile spacers around the base and weep holes, as per Michael Byrne.


Forrest

(post #104822, reply #14 of 25)

I figured you would either do things right, or find a better way.  It's the other Forrest Gumps that I worry about, but they probably would clog the weep holes with dry pack as well.


Billy


Edited 9/24/2007 12:39 pm ET by Billy

(post #104822, reply #15 of 25)

"except there is no aggregate "

Actually it is that the aggregate is smaller. The sand is an aggregate.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #104822, reply #17 of 25)

Good point.  Use sharp sand in the mud mix, not the fine play sand.


Billy

(post #104822, reply #11 of 25)

A couple of good reasons for dry pack are to ease shaping and to minimize shrinkage.


Concrete that is mixed with more water is weaker. Less water, as long as it's sufficient for the hydration reaction to occur, will generally be stronger.


Dry pack mix is signficantly weaker than the 4000 PSI (maybe) that you'd get out of concrete mix. This simplifies removal in 25 years when the HO wants to redo the shower.


John Bridge has a whole FAQ on this topic.