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Thread: New gunite cracks - any concern?

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    Join Date
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    New gunite cracks - any concern?

    My gunite was shot yesterday. When I went out to water it this morning, I saw some cracks. I'm not sure if they're significant or not. This is the largest one, it's about 2' long in the deep end. Any advice welcome.

    Attachment 32688
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    Re: New gunite cracks - any concern?

    I had some similar to that more in the middle of the pool after our gunite, maybe a half dozen hairline cracks ranging from 2-3" to maybe a foot. I took pictures and showed our builder. He said it was normal to have a few and nothing to worry about. Definitely keep any eye on it during the time before plaster to make sure they don't get wider. Mine actually looked the same or better a couple weeks after and before they sprayed plaster.

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    JasonLion's Avatar
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    Re: New gunite cracks - any concern?

    Small cracks are to be expected. That is just part of the way gunite is.

    If the cracks open up over time worry, otherwise don't.
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    Re: New gunite cracks - any concern?

    Just keep watering it... Play it safe and water twice a day for ten days, if possible. I would take photos of each crack with a tape measure showing dimensions in photos. This way you know for CERTAIN, and can PROVE if the cracks lengthen/widen, or remain the same size.

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    Re: New gunite cracks - any concern?

    Quote Originally Posted by anderson110 View Post
    My gunite was shot yesterday. When I went out to water it this morning, I saw some cracks. I'm not sure if they're significant or not. This is the largest one, it's about 2' long in the deep end. Any advice welcome.

    Attachment 32688
    Concrete is constantly curing in the presence of water (water vapor). The first 14 days of curing are the most significant (See concrete curing curves). During curing, shotcrete experiences volume changes as a result temperature (internal & external), humidity & other weathering elements. These volumetric changes can cause shrinkage and/or swelling, especially at the surface. So you have differential changes at many levels throughout the entire matrix of the concrete structure. It's during the intial stages of curing, from the time water is added to the mixture that many rapid changes occur. This is when the paste start to form around the aggregates in the mix. Concrete curing is a very complex inorganic chemical process and there are many things that contribute to the overall quality of the final cured product. What I can tell you is that twice a day watering during the first 28 days of curing is inadequate to yield the highest possible compressive strength of the concrete. In fact, in water retaining vessels, improper curing actually can lead to latent failure. This is due to unreacted variants within the matrix that when exposed to water vapor (through normal capillary action over the hydrostatic pressure gradient that exists in water retaining vessels) will form ettringite crystals leading to deleterious cracking with within the matrix of the concrete. Other possible deleterious reactions can also occur within the matrx like ASR or AAR in this same scenario. You see, improperly cured concrete results in structures with more porosity which means they are less durable and are at higher risk of exposure to the kinds of things I am mentioning here. So, 6 hours or so after these structures are shot they should be moist cured with water misting nozzles for at least the first 14 days but best 28 days. Now, you can get around this with the use of chemicals that are state-of-the-art nanotechnology curing solutions that also render much added benefits to the concrete as a whole. Ask your builder if he knows of a solution available in the industry today that is specified for water retaining vessels and more specifically swimming pools that he can recommend for you. Don't be surprised if he scratches his head and gives you the deer in the headlights look, most pool builders do not understand concrete well enough at this level and so instead of embracing these new nanotechnologies they simply ignore them with the excuse, "I just don't know enough about that stuff".

    The crack appears to be a hairline shrinkage crack (<.010in.) that does not require any action but with out a crack guage it cannot be classified an so no guidance. Cracks >.035in. will loose their aggregate interlock and start to create a ledge effect. These types of cracks are much more problematic for water retaining vessels. Cracks > .010in. do require measures to seal that can be prescribed by a professional with credentials in evaluating cracks in water retaining vessels (i.e. structural engineer).

    All concrete cracks, yes. Are the cracks nanoscopic, microscopic or macroscopic. The question is at what point should action be taken on a WATER RETAINING VESSEL that presents with macroscopic cracking? That depends on the level of expertise of the individual you are asking and how they are associated with the structure.

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    Re: New gunite cracks - any concern?

    Just add to what poolcritic has said, if you arent prepared to cure the pool correctly via water misting, then dont bother trying to cure it at all via soaking once or twice per day for a few days as this is actually worse and will lead to greater cracking.

    Moral of the story, do it correctly, or dont do it at all.

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