Concrete topping on Precast Double tees 1

[Trillers]

Greetings:

I am designing the structure for a 360’ x 124' warehouse. The structural system consists of pre-cast concrete wall panels and double tees on the roof. The PC DT's are supported by the PC wall panels along the building exterior and by a continuous inverted T beam at the apex ridge line. We will top the roof system with a 4" cement topping with rebar mesh. The cement topping will then be painted over with a double coat of elastomeric reflective paint.

Because we are in a high seismic zone and subject to monsoon rains the owner is concerned about cracks between the DT panels. The architect has suggested a fiber mesh product be laid over all PC roof joints (beneath the rebar mesh)prior to pouring the topping.

I am not sure if this will prevent crackS in the topping. The owner is concerned because this warehouse will be storing wholesale dry goods. Any good recommendations on how to waterproof the DT joints?

 

[BridgeSmith]

For bridges similarly configured, we would provide weld ties between the DT sections, especially in a high seismic region. It would improve the seismic performance by making entire roof a single diaphragm and should prevent any relative movement between sections that would crack the topping.

Fiber reinforced concrete for the topping layer may also reduce the crack width to the point where the elastomeric coating can bridge the cracks.

Another option would be one of the membrane-type roofing systems. Often they are combined with a rigid foam that is supplied with a sloping profile to provide drainage. If the DT sections were connected, you could likely dispense with the concrete topping and have less weight to manage in the seismic resisting system.

 

[dik]

You have no roofing membrane other than the elastomeric coating? Is this a high build, rubberised product? You likley need control joints between the panels, else, something that size is going to shrink. Best to deal with the shrinkage that to reinforce to resist it... Your building will be subject to thermal cycling, too.

Dik

 

[JedClampett]

If I was the owner I'd be more comfortable with a membrane roofing system. It doesn't have to bridge cracks. Plus you didn't mention any insulation. It seems like depending on the location, condensation might be a nuisance just as much as stormwater.

 

[Ingenuity]

...with a 4" cement topping...

That cement topping is going to get trashed in the first rain I know you meant to state concrete.

This is Guam, right? Every one uses/specs paint-on elastomeric waterproofing roof systems, and never (seldom) membrane-type systems.

Go visit the local Tamuning K-Mart store where precast DT's were first used (more than 20 years now) for the parking deck over the store and see how that is holding up.

And also talk to your local precaster - there input is often helpful.

It has been my experience that the topping cracks up over every DT joint, and given the building dimensions those cracks will be significant and really 'stretch' the capabilities of the elastomeric to bridge the cracks.

In some parts of the US, the DT joints to parking structures are purposely jointed (partial depth saw cut) and sealant filled. The sealant is another maintenance item,

What exactly is the preplaced fiber mesh product under the rebar mesh that the architect refers?

There are woven fabric fiber-mesh products that are wet-applied in strips with elastomeric coatings designed to 'reinforce/bridge' joints - but these are applied after the topping is cured.

 

[dik]

That cement topping is going to get trashed in the first rain smile I know you meant to state concrete...
And also talk to your local precaster - there input is often helpful.

Their?

Dik

 

[Ingenuity]

Touché, dik.

 

[Trillers]

Hafa Adai Ingenuity and All, yes - this is Guam.

The KMart deck has done quite well but they have experienced some cracks and leakage over the years.

I am really concerned about this because of the solar panel racking installation. We will be installing sealant over a backer rod in the DT joints, then the fibermesh and finally the topping. The DT joints will be connected.

Membrane roofing systems actually work quite well here until the 200+ MPH winds hit and the membrane peels, rips or splits. Our experience is that the elastomeric will hold up well on a flat surfaced slab - but we have never had to design a slab of this size just about totally populated by the solar panels.

I've discussed this again with the architect and he is proposing the product made by FiberMesh (FibreMesh?). We are also working with the concrete supplier on the possibility of using a fiber-reinforced concrete topping but I am awaiting data on how the FRC has responded to similar installations with the solar system racking.

Appreciate all the input -

 

[Teguci]

I'd think a closely spaced, small diameter welded wire reinforcement placed close to the top surface would provide a watertight system. My concern would then be dealing with thermal and shrinkage changes in the topping slab. Crossing out thermal (13 deg variation for Guam) that leaves shrinkage and creep which puts you at less than an inch towards the middle. With bracing walls around the outside, that will be tricky to detail but, if detailed properly, should provide a watertight system.

If you want to disburse this volume change over the length I'd be concerned with cracks collecting at the DT to DT joints. Perhaps consider a smooth finish within 6" from the edge of the tees with the raked finish in the middle? Is the fibermesh smooth so that bonding between the DT and topping is prevented near the joints (I would want that or a width of adhered EPDM)?

As for fiber reinforced concrete, I'd want to do that as a replacement to the WWR. But, I like WWR and at a 4x4 or 6x6 spacing I don't think fiber in the concrete would improve much.

 

[BridgeSmith]

I agree with the small-opening WWR as an alternative to FRC. For a topping slab, not subjected to significant bending, I believe either would work well. Depending on the maximum aggregate size for the topping concrete, you may be able to go smaller than 4x4, which would minimize the width of the cracks even more. (With 3/4" max aggregate, you could go as small as 2x2)

To handle the movement due to shrinkage, consider a neoprene waterstop around the edges of the slab if there is a good way to attach to or embed it in the walls. Other options to consider are elastomeric compression seals, such as the DS Brown CV series (probably overkill for your application) or compressed joint material, such as Illbruck/Willseal 600.

 

[Trillers]

Teguci, could you clarify the depth at which you suggest the WWM should be located? This is actually my first preference for the topping temperature control. Did you mean 1" from top or from in-depth? We do have a "stop" if you will in the form of a CIP FASCIA which will be cast with the topping. Since there will be continuous horizontal bars in the fascia it should provide the bracing restraint to prevent shrinkage movement.

 

[haynewp]

The DT parking deck where I work leaks like a sieve. It cracked along all the interfaces of the adjacent DT's. They tried to caulk the cracks but it still leaks. I don't know what reinforcing they put, if any, in the topping.

We didn't design that deck but a building design that hasn't had any leaks used a 2-ply modified bit membrane system over the DT topping slab. I don't know if fiber mesh over the joints would prevent cracks, I would be suspicious.

 

[BridgeSmith]

haynewp's bituminous membrane suggestion seems like a good one to me. Using a membrane that adheres tightly to the deck should hold up, even in high winds, as long as the substrate is clean, so it bonds properly.

 

[Teguci]

I'd put the WWR at the minimum clearance from the top.

 

[haynewp]

I found the arch detail (attached) that was used and there was also an insulation layer that would have helped to isolate the bit membrane waterproofing system from any cracking in the topping over the DT's. I don't know if this is an option on your project or not.