Granite Staircase Grout Failure

[Civ-StructEIT]

Ok all, I have just come across a problem on a project that I did not personally work on, but wanted to see if others have had similar issues and how they resolved them.

Problem: There is a concrete staircase with granite treads for a University in the Northeast US. The construction was completed approximately 2 years ago, and within the past 6 months they have started noticing significant grout failure in the steps.

Design: Steps were designed with concrete slab and piers, and 5.5" x 6" x 16" 5.5" x 16" x 48" granite treads that are grouted onto concrete steps and doweled in with stainless steel dowels into the concrete slab. I am working on getting some design drawings and photos right now, but this is all I have to go off of at this point.

Thoughts: Initial thought is that the grout was not spec'd out properly, and the constant rain, snow, and salting the steps have encountered the past couple years have sped up the failure process.

Any thoughts?
Thanks in advance.

 

[dik]

You need to do some petrographic studies to see what you are working with, and, take it from there...

Dik

 

[Tmoose]

WAY more info is needed up front.

They sound like exterior steps. Water management, and like you said, salt/de-icer can be real tough. The profile of the finished grout can make a big difference in how it sheds water too. Shed roof vs bird bath.


Cementitious grout? Was some kind of sealer included in the specification? For the grout and even the granite?


I don't really know anything about this, but as a practical matter I don't have much faith in cementitious grout in easy indoor industrial machine installation applications.

I'd think superb surface preparation and a highly flexible grout would be needed to have a chance of long term survival.

 

[hokie66]

What is the geometry of the grout between the granite pavers? How wide and how deep?

 

[dik]

I don't know about this project, but, often joints are 1/4" thick for stonework.

Dik

 

[hokie66]

The reason I asked is that the granite pavers in this case are very thick in comparison to the norm. A cross-section of the monumental stair would be helpful, including the concrete details, as the granite dead load is considerable.

 

[Civ-StructEIT]

I have received a photo and basic information!

Please see attached for the photo and a detail for the stairs. It appears that they are calling out 1/2" mortar on the detail, with no real specific "water repellent" described.

 

[Civ-StructEIT]

Photo attached here.

 

[hokie66]

From your photo, it looks like the main location where the grout is coming loose is at the tread to riser horizontal intersection. The section shows no overlap of the pavers at that point, so there is not really a receptacle for the grout if it is built that way.

You indicated that the pavers are 16" x 5.5" x 6", but the treads are long blocks. Maybe those are the ones in the landing.

 

[Civ-StructEIT]

Hokie: I will correct my initial statement, I have checked to confirm and the granite treads are actually 5.5" x 16" x 48". I'm not sure where that original dimensioning came from, but it is definitely not what is out there. The 5.5" x 16" x 48" dimensioning is correct.

I am planning to get out to the site in the next couple days to ensure that the steps were built properly and have a pitch to them. The more I look at the photo, the more I'm thinking that the water is not running off the step, and just sitting in the crevice at the horizontal intersection.

 

[cvg]

all the grout lines show degradation, water is clearly not draining off well and design slope is less than the ordinary minimum of 1/4 inch per foot. grout seems to be turning brown. two questions: 1) is the slope constructed per design and 2) is water draining off the roof or other area towards this landing

 

[hokie66]

From this one photo, I think the grout looks as expected where there was a gap to grout. The staining is probably just dirt, and can be cleaned.

Where there was no gap to grout, along the horizontal lines at the bottom of the risers, grout will not work. What remains should be raked out and that corner joint sealed. Sikaflex 11FC comes to mind.

 

[dhengr]

Civ-StructEIT:
Any water getting into the grout and down behind/under the granite treads will be subject to freezing and thawing action, and will concentrate at that horiz. joint from inside and outside. I wonder if those horiz. grout joints at the tread/riser juncture shouldn’t be raked back so a proper caulked joint can be applied. The butt joints btwn. treads are probably o.k. as shown. Also, they appear to be draining the whole upper plaza onto those steps. You can see drainage stains on the treads and the risers on the left of the photo. Should there be a drain at the top of the steps?

 

[dik]

Couple of comments... did they include rebar as part of the DL? and running a gas line through a pier is a bit silly...

Dik

 

[Civ-StructEIT]

The drawing was a sketch I found, but I do not believe it was the final IFC drawing, which I am still trying to acquire. The point of draining the whole landing area to the steps is a good one, I will have to check a Utility Plan to see what the actual intention was for stormwater. I spoke to one of the contractors I regularly use, and he suggested raking back the grout joints, and use a backer rod and caulk joint instead.

First and foremost, I am going to confirm that the slope was constructed as designed, and does not have a negative pitch back to the horizontal joint.

Thanks for all the valuable insight.

 

[dik]

It really needs draining for longevity as well as possible ice build up.

Dik

 

[bridgebuster]

@Civ-StructEIT - I'd be interested to know if the slopes are correct. It could just be lousy mortar. I have a project under construction that uses similar stair details. Our setting bed is 1.5 inches and we're sealing the edge of the joint. However, the GC isn't the best in the world and the stone contractor leaves much to be desired. They haven't started stone work yet but I know him from other projects.