I think one big flaw in the grout/cement atop polystyrene method, that nobody ever thinks of or takes into consideration when planning, is the fact that these aggregates will most likely not stand up to the test of time, and will begin to crack.



By simply glopping a grout or cement on top of a solid object, you are offering nothing to strengthen the structure, and nothing to hold the structure together. This is the same reason why doctors do not use plaster alone when making a cast, or the reason why a fiberglass shop doesn't use just resin.



These two situations call for backing; a material which adds support, and connects/affixes the mixture/aggregate together. In the case of a doctor and a plaster cast, a net-like muslin/cloth tape is used to bond together with the plaster, rendering the cast strong, uncrackable/unbreakable. In the case of the fiberglass shop, they use glass fiber cloth or matting to mix with their resins for added strength, and stability. Without a fiber cloth, the resin would simply crack and break apart.



The same thing does, and will happen in these grout or concrete-atop styrofoam situations. In cement construction, masons do not lay down just cement for a sidewalk, they use a network of reinforcing wiring and rebar. Even for small scale cement jobs, a backing is used to prevent cracking.



This is why I stand behind my approach(which has been tried and true for decades now within the exhibitry industry), as it produces favorable long term results. By using a lathe underlayment, the cement becomes one continuous stucture, which is bonded and rigid.



Most of the people who swear by the grout/cement atop styrofoam technique, have only a limited amount of time with it in their situation(months, maybe a year or two). Over time, these facades will not stand up to the everyday wear and tear that it is put through, and the simple fact that it is a mixture set atop a solid structure, it has no stability or reinforcement to prevent it from cracking.



Cheaper and faster is usually not the best choice, especially when you want to do things the right way, in order for something to last. Not to mention I have yet to see any project done with the cement/styro or grout/styro method that resulted in an authentic looking finished product. It usually turns out looking like some unnatural object covered in cement or grout(which it is).



I am not saying there is one and only one way of doing things, as there surely isn't. There are in fact many ways of doing things correctly. In my opinion, and based on the further evolved and researched exhibit industry, structural integrity and realism are the two utmost important aspects governing faux rock construction. I just do not see the styrofoam approach satisfying either one of these 'requirements', no matter what size or scale the project is.



These are my thoughts on the subject at hand. I have 'been around the block' in the fabrication industry for several years now, and have seen my fair share of good work, shotty work, and utterly poor work. "Quick, Cheap, and Easy" usually end up in the latter.
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