Acrylic tub base cupped outward inside has give, what to do?

[powerlifter405]

Howdy everyone
First time caller, 8 month lurker.

I'm installing a Mansfield alcove tub, acrylic whirlpool tub encased in fiberglass.
I have laid 3 mortar beds and I get a "spot" in the middle of the tub that has give. Each time I lay enough to compact, I set the tub in place and allow it to dry, I always wait 2 days for full cure. It get what feels like a soft spot in the exact same place every time. I took a pic of the inside of the tub, I initially outlined the area that gives when I walk/push on it so I could add extra mortar the other two times. Each time the same issue. I get mortar down, verify level and wait.

Today I pulled the tub and placed a level on the underside and lo-and-behold, the osb that is fiberglassed to the acrylic is cupped outward. The gap on the outsides are visible when I hold a level in the center of the osb. W/ the osb having a cup, the plastic will always have a little give on the tub side.

I got the tub last summer but the tear out and several other projects, family emergencies and now the gov shut down has put me behind.  Thus i'm beyond the warranty period.

Any ideas on how I can fix this or if it will be ok?

Thanks

 

[Reach4]

Rather than lay a flat "bed", you would usually be better off putting in mortar piles. That way they will squish to conform to the tub. Put the term I bolded into a search using the search box above.

 

[Sponsor]

 

[powerlifter405]

Rather than lay a flat "bed", you would usually be better off putting in mortar piles. That way they will squish to conform to the tub. Put the term I bolded into a search using the search box above.

I did that this last time. I made a grid from 1x1's.
The mortar is shaped to the underside of the tub BUT there is an air gap between the acrylic and the osb so everytime you walk in the tub it has give, it is the acrylic that is flexing, not the osb. The osb is fully supported.

 

[Reach4]

I did that this last time. I made a grid from 1x1's.
The mortar is shaped to the underside of the tub BUT there is an air gap between the acrylic and the osb so everytime you walk in the tub it has give, it is the acrylic that is flexing, not the osb. The osb is fully supported.

The plan would be to put baseball or softball globs of mortar on the OSB, and to drop the acrylic on top of that. The globs squish to the right size. You could put the globs into plastic bags if you prefer. They don't need air to harden.

You don't want to try to have a solid slab of mortar. Instead you want each glob to be able to spread out as needed. The globs have to be big enough that they each get squished down. Each will serve as a little just right height pillar in compression.

You would fill the tub to add some weight while the mortar hardens.

I have never done it. I have never seen it done. This is my interpretation of the prior posts. And it seems brilliant to me. I suggest you try that search and see what you think.

 

[powerlifter405]

The plan would be to put baseball or softball globs of mortar on the OSB, and to drop the acrylic on top of that. The globs squish to the right size. You could put the globs into plastic bags if you prefer. They don't need air to harden.

You don't want to try to have a solid slab of mortar. Instead you want each glob to be able to spread out as needed. The globs have to be big enough that they each get squished down. Each will serve as a little just right height pillar in compression.

You would fill the tub to add some weight while the mortar hardens.

I have never done it. I have never seen it done. This is my interpretation of the prior posts. And it seems brilliant to me. I suggest you try that search and see what you think.

My sub floor is fine. It is the OSB that is made/adhered to the acrylic and bonded w/ fiberglass, that is warped.
After 3 mortar bed tries, I realize my bed making is fine, it is the tub it's self. You can see in the pic that the the OSB part of the tub is warped. Mansfield used cr@p materials and/or their CQ person should have thrown it away before they assembled it. The fiberglass is varying thickness in several places, the flange for the drain is misshaped and there was a few spots in the whirlpool plumbing that wasn't sealed. At the end of the day, POOR quality control.

I did call menards a few minutes ago and the guy I talked to said this has happened before and it generally requires a swap. He said to email them and talk to a rep. Hopefully mansfield will work w/ me but who knows.

 

[Reach4]

I did misunderstand. I thought the OSB was the subfloor. So if you did use mortar blobs, I would put them between the subfloor and acrylic.

I guess the OSB would sit on the subfloor too.

 

[powerlifter405]

No prob, it's really a challenge to describe this cluster I'm dealing w/.
Thanks!

 

[Jadnashua]

I gave you my thoughts over at www.johnbridge.com.

 

[powerlifter405]

I gave you my thoughts over at www.johnbridge.com.

Thanks

The epoxy was my exact thought as the only thing that may work. It would act as filler and while simlutaniously "gluing" or bonding the acrylic to the osb. I know I have "X" mm's of osb cupping before I hit the air pocket. The flex is only about 3-5mm's so if I go this route I'll have to be super gentle and I'll work from the back.

I emailed Mansfield so hopefully they can cut me some slack and get me an exchange. Next week I may have an answer or I may be measuring and epoxying.