roof cement topping on waterproof

[releky]

when you do cement topping of waterproof membrane.. is it better to use one pour for the entire 150 square meter roof or partial? the membrane topping will be 2 inches cement mixed with fine aggregates.

 

[Ron]

This process is fraught with issues....
First, make sure the waterproofing is waterproof! Do a flood test at the least to make sure that no leaks occur before you start the topping process.
Secondly, you need to make sure that the process of placing the topping does not breach the waterproofing membrane. Be extremely careful with any steel reinforcement of the topping....preferably, don't use reinforcement.

Avoid this process if you can! Use pavers.

 

[releky]

In roads, they don't use continuous concrete so if a part breaks, it will be isolated. I'm asking if you use the same concept in roofdeck waterproof concrete topping noting that dynamic movement of the building may cause crack on the membrane topping and if say a 3x2 square meter topping used, the crack may be isolated. What do you think (to others too)?

The membrane used is thick so no problem with it and it is flooded tested.

 

[JAE]

Usually for a membrane waterproofing system there is a protection board on top of the membrane to keep punctures to a minimum and also allow in-plane movements (i.e. thermal) to occur over time.

 

[dcarr82775]

I think Releky is asking whether or not to provide control joints in the topping, or possibly to cast it is a checkerboard pattern to combat shrinkage cracks.

I would put in control joints, but like Ron said, pavers (along with protection board, drainage board, etc.)

 

[releky]

Below the member are 5 inches of concrete slabs. The membrane is asphalt sheet. 2 inches of topping will be used, pavers more expensive and heavy. The flow of water will be diverted above the topping to 2 inches drain. Are you saying you need to put another drainage below the topping so if the topping gets soaked, the water would flow to the drain below? But the concrete topping above membrane are sloped enough so water from rain would immediately go to the drain, so no stock up water.

Any comment of the above?

About pouring concretre in 5 parts instead of 1. The reasoning is that the building is situated in busy highway, so vibrations from the street can make the building shake a bit and this can cause micro cracks in the membrane concrete topping. If they are not isolated in parts, then the cracks may propagate the entire span. Do you not agree?

 

[aaronPTeng]

releky If I understood correct, your top of drain will be with respect to topping slab and not membrane structural slab?

The issue with this is water will leak below your topping slab, unless it is designed as a water tight slab (including connections at its edges), and it will pond. a 2" slab will crack for sure

No matter how good a membrane is you never want to encourage ponding as over the years it will find a weak spot.

I would have to agree with comments above to use something like pavers. The benefit to this is it allows future access for repairs if leaks do occur. Typically this is the process I have used, with the pavers placed on small pedestals to allow easy removal if required. The man slab below which has the water proofing applied has the falls to drainage outputs in the slab.

The main use for the pavers is to protect the membrane from birds (As they like to eat it). Another alternate is pebbles!


"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[releky]

aaron, the 2" concrete is put above the membrane. How can water leak in between them when the weight of the 2" concrete is heavy enough to bear on the membrane?? In other words, there is just no space between the concrete and membrane for water to flow between them. Unless you are saying the entire concrete be permeable to water? but the topping is sloped so water flows to drains.

 

[Ron]

Releky...drainage for this configuration occurs at the membrane level, not the top of the topping slab. You need to do a little more research and understand concrete, waterproofing and the interaction between the two. It is not a simple task, although Architects often treat it as such.

How long do you expect the structure to perform? Waterproofing membranes have significantly lower life expectancy than concrete.
How will leaks be found if they occur?
How will you repair leaks in the membrane if they occur?
How will drainage be accomplished?
How will flashings be done and how will they be accessible for repair?

Pavers are not heavier than cast-in-place concrete.

You mention an "asphalt sheet" membrane. What is that? Is it fully adhered to the structural slab?

 

[releky]

Please see this picture for illustration:

Ron. In fresh slabs, there are always areas where water can pond like in between drains, because the formworks is not always perfect. Unless you are saying fresh slabs are always levered accurately down to mm scale? So for those roof decks with commonly uneven slabs. Most commonly put asphalt membrane that is attached to the slabs by a blow torch and then concrete topping were put on top to make the final top sloped to the drain. The sides of the slabs were also waterproofed to 10 inches high and concreted too. So the whole concrete topping is attached to the membrane (see above picture). How can water flow in between them?? This is commonly done. About how to repair them. Then you have to remove the toppings and use thermal imagers on the ceiling below to trace it.

 

[Ron]

releky...your picture did not come through

 

[releky]

pls check this direct link

http://imageshack.us/photo/my-images/10/hhy4.jpg/

 

[dcarr82775]

Releky,

You have a 2" topping slab over a membrane. This topping slab WILL crack, and over time it will deteriorate no matter how well you detail it, and how well it is constructed.

Water WILL travel through these cracks and come to rest on the membrane. The weight of 2" of concrete does not prevent water from finding its way into every little available crack and crevice in the topping. Since the only drainage you are providing is at the top of the topping slab level, where does the water that is on the membrane go, how does it get removed from the system? It doesn't, it sits there and works its powers of damage corroding steel, and if you happen to live in a freeze thaw area, breaking apart your topping with efficient ease.

The topping slab protects the membrane, nothing more. The membrane protects against water so you need to drain the membrane level as well.

 

[releky]

When you guys do roofdeck slab pour, do you make it a point to not only level it but already create slopes to the drain? In practice, I found this difficult because in ready mix concrete pour of large area like roofdeck, you just pour it. Then put waterproof membrane later and put topping that is sloped to drain. This is commonly done. I'd like to know how many do the former and latter.

 

[dcarr82775]

I could cast is with the a slope similar to how any parking deck would be poured, but I have not done so in practice. Depending on its purpose and location I might cast it flat, and use tapered insulation to build up the slope with a membrane above the insulation. I have also done similar to what you describe, where there is a flat structural slab, waterproofing membrane and a topping slab. But in this case I have always provided a drainage board between the membrane and topping slab and provided drainage capability at the membrane level.

I have never done the method as you describe it.

What part of the world are you in?

 

[aaronPTeng]

releky,

Water will get through, If you have some time go look at old structures where you are from.

With the falls in the roof slab, this is easy to achieve. The min fall for water is 1:100. We know concreters cannot do this accurately, so you go 1:50 or 1:75 to make sure as a minimum when they are having a bad day you will get 1:100

I have done my above process over thousands of m^2 with ease. Especially utilizing water tight concrete design as a further water tight solution.

As per dcarr where abouts in the world are you from?

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976