CoatingsPro Magazine

MAR 2015

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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COATINGSPRO MARCH 2015 69 So what does that mean? The Science of a Slab W hen a slab is in a curled condition, the outer edges of the slab, or the components within the slab, are raised slightly. Te amount most commonly seen is usually trivial: 1/16–⅛ inch (0.2–0.3 cm). More severe cases can be as much as ¾ inch (1.9 cm). And then there's the really severe cases where slab corners or adjacent panels are lifting of the ground so much that stepping or climbing from one panel to the next is necessary. Te unfortunate fact of slab curl is that it's almost impossible to diagnose because it really cannot be seen by the average foor covering or foor coating contractor. But a careful fatness survey prior to the start of a foor covering installation will identify the presence of slab curl. As a slab dries from the top down, the top becomes drier than the bottom. Tis puts the top of the slab in compres- sion while leaving the bottom of the slab in tension. Te result is a slight lifting of any edges — the outer perim- eter of the slab as well as those edges along all control joints and any random feld cracking. Slab curl has a secondary efect on those control joints and random cracks. Te size of a control joint becomes slightly larger, and the profle of each control joint changes from rectangular to slightly V-shaped. Tis happens with random cracking as well. As stated, the slab curl occurred long before the foor covering installer or coating applicator ever showed up to the job. But once on the job, the applicator performed his standard subfoor preparation duties, carefully cleaning out then flling each control joint and random crack with a cementi- tious patching material. Once all foor preparation was complete, the new foor went down, and everyone agreed that it looked beautiful. But, as I've seen in the past, before the fnal high fves of the project are ever shared, someone says, "W hat's that?" and then someone else says, "Look, there's another one over here!" So just how and why does a huge concrete slab "relax"? Wel l, once a slab is covered w ith an imper v ious f inish f loor cover ing , the moisture w ithin equa lizes w ithin the body of the slab so that an equa l amount of moisture is present f rom the top to the bottom of the slab. A nd as this moisture equa lizes w ithin the body of the slab, the top sur face and the bottom sur face are no longer at odds v is-a-v is the forces of tension and compression. Now, both sur faces are in tension (or compression) and when that happens, the slab rela xes to a point of no longer being curled; it is now f lat. Understanding the slab relax- ation, if we revisit our control joints, the slightly V-shape is now rectan- gular, the width of the control joint has narrowed, and the cementitious patching material within the control joint (or random cracks) fractures as the joint edges compress against the patching material. Te cementitious fll material becomes loose from the subfoor but is attached (via adhesion) to the underside of the coating or the resilient fooring. Te end result is a telegraphing of the control joint and random crack locations through to the surface of the fnish foor covering. Te raised ridge that we see in the resil- ient foor is actually caused by loose cementitious material that is glued to the back of the resilient foor but is no longer attached to the subfoor. Fixing the control joints was prohibitively expensive, and the time necessary for such repairs would require closing the library for at least two weeks. Te library decided to live with the condition while working with the district architect on designing curl-resistant concrete slabs. Key Takeaways If we look at this project as a common example of slab curling, then there are a few things that we can take away as coatings applicators. Checking for foor fatness is an integral part of any surface preparation prior to a coating application or foor covering instal- lation. Pay special attention to the edges of all control joints — when a straight edge is placed over a control joint, is there a bulge? If so, mention (in writing) that there is some slab curl and that the curl will have an efect on the foor coating. How much of an efect cannot be known with any degree of certainty, but there will be one; it will be bulging at the control joints and any random existing cracks. I would also recommend that you keep lines of communication open with the company that placed the concrete. Asking them a few questions about timing, materials, etc., may help you avoid rework in the long run. CP Roland A. Vierra is president and CEO of FLOOR ING FOR ENSICS Inc., which is an independent consu lt ing f ir m specia l i zing in t he science and forensic eva luat ion of f loor cover ing per for mance fa i lures. Vier ra has been a t hird- par t y c la ims consu ltant for more t han 30 years. For more infor mat ion, contact: Roland Vier ra , (408) 999-0922, Roland@Floor, w w w.Floor The slab showed signs that it had curled prior to the installation of the new floor covering (i.e., the epoxy and vinyl), and that after the floor coverings were installed, the slab relaxed. Curling Slab Consequences

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