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Concrete Crack Sealant required

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hemis

hemis

Structural
Nov 10, 2008
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I am designing a reinforced concrete pavement-light (see link). The light consists of a grillage of thin beams set at 150mm c/c. I want to maximize the structural opening.

Due to deflection, the concrete cracks in the ribs at a short span (1.9m). If I ignore deflection, I could achieve a span closer to 3m but at these spans, the concrete has certainly cracked. I intend to control the size and spacings of the cracks using a fibre in the concrete (polypropylene seems favourite at the moment) but I need to find a way to seal the cracks. The cracks should be microns wide - I am working on the math.

My idea is to coat the moulds used to form the concrete ribs with a material that would bond with the concrete and form a waterproof barrier at the cure stage and remain flexible as the concrete cracks. The sealant would soak into the concrete and remain permanent. I have heard of some waterproof barriers that work by bonding at the cure stage.

Or maybe a material that could be added to the concrete that would enhance the cracking?

I would prefer not to have to coat the ribs after construction but I will if no other solution exists.

Has anyone any materials that I may research, please?

Daniel
 
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I'm not sure you will find too many concrete experts on this forum. It appears that you are still using steel rebar for reinforcement and you want to find a way to make the concrete more flexible, so that it stretches rather than cracks in areas where the it is under tensile load due to shrinkage or deflection.

Polypropylene fibers should help there. I have also seen polymer dispersions , like acrylic latex, sold as additives to toughen concrete. I'm not sure why you are looking for waterproofing coatings or how that would help prevent cracks.

The most effective approach would be to place strips of open mesh fiberglass fabric (like drywall seam tape)into your mold before you cast your concrete. This will work like wire mesh reinforcement to stop cracks and stiffen your grid except it doesn't corrode. A spray of latex can be used to tack the fabric to the mold and impregnate the fibers. This would help adhere the mesh to the concrete and provide protection to the fiberglass from abrasion and caustic cement.

Many architectural details are made from shaped styrofoam blocks that are covered with fiberglass mesh and then coated with stucco.
 
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Compositepro and Berkshire

Thank you for the response.

I am not trying to stop the cracking; the cracking is inevitable. I am looking for a "filler" which would prevent the cracks from allowing moisture in. Most pavement lights fail through corrosion of the rebar and spalling through frost (pavement lights cannot be insulated and regularly freeze).

I do like the idea of applying the fibre-mat but maybe not practical to implement on site. I will be researching the method.

Regards
 
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You could try one of the water seal products that are on the market. They are solvent with a hydrophobic additive that penetrates and binds chemically to the concrete so water can't get in. The chemical is a trialkoxy silane, most commonly trialkoxy n-octyl silane.

Chris DeArmitt

Consulting to the plastics industry
 
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I knew a guy from Ecole Plytechnique du Montreal who showed me a “self healing material”. There are micro spherical resin bubbles imbedded and also solid catalyst crystals uniformly distributed inside. When crack develop the bubbles hit by the crack will bleed and liquid resin in contact with catalyst will cure quite fast filling the crack. The problem is when a second crack grows in the same place is no resin available to refill the crack . . . Anyway I recommend to use something similar with a resin formulated to cure to a rubbery stage.
 
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Thank you all for the response.

It looks like trialkoxy n-octyl silane is going to be a favourite and I have written to Newport Industries for a specification of a product that may fit the brief.

The problem I have with post-filling the cracks is the life of the material and its maintenance. I approached Dow corning and they suggested a very good caulk but for the sealant to work, I think, the slab would have to be cracked sufficiently to allow capillary action to take place. If the sealant is applied to the un-cracked surface, I am concerned that the sealant may peel. The problem then would be, how to crack the concrete in a controlled manner.
 
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avior

Would you have a more specific name for the resin materials or which sort of company I should contact to discuss the method, please?

Regards
 
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I have written to the professor - in english.

I have written to several organisations concerning the trialkoxy n-octyl silane but so far, no response.

Thanks for all your help.
 
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Hemis
Why do you want to seal the cracks? The basic problem is that concrete in tension always cracks, and there's nothing you can do about it. Fibres make the cracks smaller, possibly to the point that they are invisible to the naked eye.

Silanes/Siloxanes will not prevent water intrusion in growing cracks, nor the resulting freeze-thaw damage. They are for bulk waterproofing of (otherwise porous) masonry and concrete. Some film-forming coatings (ie paint, basically) will span small cracks, but durability is a concern.

You may be looking for something called a "flexible cementitious waterproofing". These are very durable, maintenance-free, are usually applied as a layer maybe 1.5mm-4mm thick, and can span cracks upwards of 2mm. The flexibility comes from a polymer included in the mix, so sometimes it's called a "polymer-modified cement" coating.

You may be able to place a layer in a mold before pouring the bulk of the concrete. I don't see why that wouldn't work, though it's unusual.

Epoxy coatings are another option, but they are expensive.
 
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Demon3

Thank you for the post. Interesting material. My problem is the grillage is a complicated 3D shape and moulding the material would be difficult. I have been in contact with three chemical companies now and none have replied. I'm starting to have doubts if there is any concrete additives that I could use to seal cracks. There is always paint.

Thanks for the interest.
 
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