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Question:

How do I deal with small cracks and fractures that occur along the edges and corners of the stone imprints in some of my stamped concrete work? They are random and do not occur on all my projects. They seem to happen with deeper, more aggressive stone patterns. How do I fix these cracks, and how do I keep them from happening in the future?

Answer:

These are shrinkage cracks caused by pushing imprinting tools into the concrete after the surface has become too hard. This surface hardening, known as "crusting," can be caused by exposure to sun and wind, overfinishing, the concrete mix design, and even the concrete color (darker colors more readily absorb heat from the sun).

A good analogy is what happens when you cook pudding. When the pudding comes off the stove, it's a viscous liquid, similar to concrete when it comes out of the truck. The pudding then goes into the fridge to cool. If it cools too quickly, the rapid temperature drop causes a skin to form on the surface. The pudding then thickens and hardens from the top down. The same thing occurs when concrete dries too fast from the top down. Remedies for reducing surface crusting include using admixtures to slow the concrete set time, using surface evaporation retarders, avoiding darker colors, and scheduling the concrete pour for the coolest time of the day. Using stamping tools with deep grout lines, such as random stone and large slate patterns, can aggravate the cracking problem.

One way to repair these small cracks is to use a colored cement paste, or color patch. Some color hardener manufacturers offer these patches in their standard color palette for use in filling and repairing these types of cracks and small popouts. Some contractors will sift the sand out of the color hardener they are using on the project and use this to make a color patching material. I recommend wetting color patch materials with a 50:50 blend of concrete bonding polymer and water. Use a wooden tongue depressor or gloved finger to smear the color patch into the cracks. Soften the edges with a damp rag or sponge to blend the patch and color into the slab. Let these repairs cure for 24 hours.

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Author Chris Sullivan, ConcreteNetwork.com technical expert and vice president of sales and marketing for ChemSystems Inc.


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