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I am constructing a building from reinforced concrete and AAC blocks in Indonesia.

The building will have a flat (2% sloping) roof of reinforced concrete. This will be exposed to tropical rain, as will a overhanging canopy. It will also have foot traffic, leaves, detritus, etc.

I understand that it is important that the roof not leak, in order to avoid water entering into the building, and the canopy also not leak, in order to prevent the rebar rusting and failing over time.

As the roof and canopy are yet to be constructed, I may have the choice of coatings after pouring, concrete additives/modification as part of the pour, or both.

I can find in terms of coatings:

  • 'waterpoofing slurry' based on resin modified cementious material (this is supplied as a bag of cement-like powder with a solvent) - this needs to be protected by cement screed as it is not UV stable
  • waterproofing mortar - (cement to which water is added)
  • ' a cold applied, one component waterborne liquid applied waterproofing membrane', a 'Polyurethane modified Acrylic Dispersion'. Apparently this may also enhance solar reflectivity. There is an optional 120g weight polyester fleece, which is a polyester fleece product, you can apply at the same time.
  • A combination of non-woven polyester membrane, and 'UV-resistant acrylic emulsion'
  • " one component, liquid elastic waterproofing coating, based on styrene-acrylics copolymers in aqueous dispersion reinforced with fiber to increase the tensile strength of the product. It forms a flexible, waterproofed and durable film layer"
  • ATACTIC POLY PROPYLENE (APP) MODIFIED BITUMEN-BASED WATERPROOFING MEMBRANE WITH NON-WOVEN POLYESTER FELT REINFORCEMENT WITH SAND BROADCAST (i.e. you melt bitumen onto the roof, and there's sand on top)
  • self-adhesive, rubber modified, bituminous sealing tape - this is a roll of tape, so I guess you would not use it for a whole roof, but only for small areas
  • steel corrosion inhibitor - surface-applied (for the canopy only, which technically doesn't need to be waterproof)

Or concrete admixture:

  • Watertight/superplasticising (HRWR) concrete admixture (Combination Fatty Acid and Polycarboxylate )
  • Crystalline admixture

What's the best way to go? It seems like an admixture might save time and effort? But is it sufficient?

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    +1 for a well asked question! Do all these methods meet building code in Indonesia? Are they all (code) legal and commonly used there, and you're trying to determine which is the best for you? Are these simply coatings you've found online and want to pick one with no local code knowledge? I'm not trying to imply anything, just trying to get some additional info that someone will need to help you out.
    – FreeMan
    Jun 3 '16 at 20:06
  • Hi, there might be a building code in Indonesia, but none is followed in my area. People just do what they like (often, things leak), and there is no specific insight from my builders.
    – thelawnet
    Jun 4 '16 at 5:18
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I would use the HRWR admixture to help seal the concrete. Alone, it might not be the best roofing solution, but it's well worth the minor cost for the extra protection.

The biggest problem that I heard was foot traffic. Normally I would recommend EPDM, but I'm not so sure about foot traffic. This sounds more like a terrace, in which case, silicone cement is most commonly used.

However, something like a layer of asphalt would be more durable. This could be layered over steel. I would consider using coal tar (sealer) on top of the asphalt (which works better than bitumen/petrol tar) for shedding water. However, using coal tar is against roofing codes (causes cancer) in most of the world.

But one alternative that was not mentioned would be a green or living roof. I don't really know much about the materials involved, but in a tropical climate, it might be best to just go with the flow (in a sense). Fighting the elements can sometimes be harder than working with them.

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I would not trust admixtures to make concrete waterproof. Concrete in inherently porous and hygroscopic. Adding things to concrete still leaves it concrete with these properties. Point is it is the wiser to cover the concrete with something water proof.

Waterproof materials typically will be a solid sheet of something like metal, plastic or "poly...", elastomerics, or EPDM (rubber), and finally bituminous (tar/asphalt).

Which waterproofing material is best is a matter of many variables and then many opinions based on those variable and the pros and cons of each option.

I personally like a well "lapped" steel solution, next is a EPDM or rubber solution in my book.

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This question is 5 years old so I will give my comments.

'Tropics' covers a lot of countries, but many have poor construction standards. Concrete is supposed to have slump; i.e. it it doesn't collapse, but most I see is just a watery slurry and has very little strength.

It is extremely difficult to get workers to change their practices, when they are used to making very weak concrete that is easy to pour. If the concrete does not flow easily, then you can use a vibrator, but again if this is not usual then providing a vibrator does not necessarily mean it can be used. The problem is formwork which is often made out of cheap thin plywood. If you buy phenolic plywood, (which is dipped in phenolic resin (plastic) and is therefore essentially waterproof apart from the edge), then it helps to make the plywood stronger, and is also more reusable, although if you use plywood that is both thicker and phenolic then it will increase the waterproofness by a lot.

Reducing water increases strength but workers are resistant to making their lives more difficult. In addition, wetter concrete is more likely to leak as the water evaporates leaving voids.

Besides simply reducing water, adding more cement increases the mixture strength. Here there are a lot of issues with people mixing concrete with inaccurate and random measurements. If mixing concrete with buckets of sand and gravel then weighing the sand is a good approach, but you might want to take into account if your sand is damp that you are getting more water.

There are waterproof concrete recipes:

https://www.building.co.uk/cpd/cpd-26-2014-introduction-to-waterproof-concrete/5071050.article

but essentially waterproof concrete is that which contains a very low amount of water relative to the concrete. I believe water : cement ratios here reach up to about 1:1 (and hence not really possessing any strength), but 0.4:1 is said to be waterproof, and is very strong.

It is essentially impossible to get workers used to adding ridiculous amounts of water without use of a superplasticizer. Plasticizer will make your 0.4:1 concrete flow like it's 0.6:1, it's magical stuff, the only problem is that it's not free, so in an ideal world you would have workers trained to make concrete to a proper slump, have good strong formwork (perhaps rented) and heavy use of a vibrator to get it to go into place (without the vibrator or superplasticizer it will not magically do that), but in practice things may be compromised.

The problem here is that you need a lot of cement (if you are targeting a ratio better than 1:2:3 of cement:sand:aggregate), and a lot of quite expensive superplasticizer. I also added cooking oil https://iopscience.iop.org/article/10.1088/1755-1315/16/1/012077 in a similar quantity to the superplasticizer. Note that the different fatty acids make a difference here - I think saturated fatty acids are the desired type, so rape or sunflower oil probably are not good, and also used frying oil is better in that the frying process results in the fatty acids becoming more saturated.

I used Siska Viscocrete 1003 as my superplasticizer but doubtless there are many alternative products.

My concrete has been totally waterproof without any coating; I daresay if you do not need concrete that is strong, then using lower quality concrete (less cement) without superplasticizer and then a coating system will work, but in a way the coating seems like a bandage, whereas it's 'nice' to have a good quality base product that doesn't need the coating, but obviously economic factors quite possibly make the cheaper concrete and coating the cheaper option for single-storey buildings etc.

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