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Cracked gypsum underlayment

Cracked gypsum underlayment

Cracked gypsum underlayment

I am currently investigating a project where the gypsum underlayment has cracked.  It was installed on the 3rd-floor of an existing 60 year old structure, over a cast-in-place concrete slab & joist system.  The thickness is around 3/8" and has been topped with an epoxy coating.  Some of the cracks are longitudinal, sometimes occurring on beam lines, or occur at door openings or intersections of hallways.  The later, to me, would seem to be shrinkage issues.
Others are a "spider web" effect, radiating outward from a point.  
The cracking is extensive and varied.  Some have been repaired and have not recracked.  The existing concrete slab in the repaired areas was not cracked, so it is not believed to be a telegraphing issue.
I have been scouring the internet to find a resource on gypsum underlayments, with little success.  Does anyone have experience with a similar issue or a resource you'd suggest?


RE: Cracked gypsum underlayment

The spider-webbing is either due to a hard point below, or from something heavy dropping on it.

3/8" seems awful thin.  Is it likely that this was just a leveling course?

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Cracked gypsum underlayment

Yes, it was for leveling.  The original intent was to apply the epoxy directly to the existing slab, but it was deemed too rough.  There were areas where old walls had been removed, etc.

RE: Cracked gypsum underlayment

I guess I've never heard of using gypsum-based underlayment as a base for an epoxy coating.  This doesn't sound right to me.

But in any case, gypsum underlayment is not meant to be exposed or meant to be a structural entity...only a fill layer.  

I would definitely expect it to crack at 3/8" thickness for minor deflections in the supporting structure below.


RE: Cracked gypsum underlayment

The epoxy manufacturer recommended this product and another system, which was not selected, that had been used successfully in the past.  However, I do not know at what thicknesses.  What thicknesses have you typically seen?

RE: Cracked gypsum underlayment

Agree with comments from Mike and JAE.  Also, you are probably looking for the wrong thing on the internet.  This is not a gypsum underlayment.  It is, perhaps, a gypsum based leveling course.  As JAE noted, I wouldn't use this under an epoxy topping.

Since you have cast-in-place concrete below this and if you want to use an epoxy topping, I would use a cementitious leveling course such as Laticrete or SikaTop 123.

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