Common Causes of Cracks
Question:
Why did my concrete countertop crack, and how can I fix it?
Answer:
Cracks in concrete are a fact of life. However, you can minimize or prevent them if you understand how and why they occur.
Most cracks are caused by tensile failure of the concrete. Thermal cracks, shrinkage cracks and flexural cracks are all tensile failures. Hairline cracks, which are very small and relatively short, are often caused by long-term drying shrinkage and are the most common type of tensile failure seen in countertops. They are generally considered inherent characteristics and not flaws, unlike full-depth structural cracks which can be quite large and noticeable.
Proper reinforcing can significantly reduce the likelihood of cracks. Unreinforced concrete countertops are more vulnerable to cracking because they are very weak in tension. Out-of-level cabinets, improper supports, improper lifting of the slab during transport and installation, and incorrect sizing and placement of reinforcement will also increase the likelihood of cracking. If a concrete countertop is improperly reinforced, or if a properly reinforced countertop isn’t supported right, then the tensile stresses that develop in the slab will be resisted only by the concrete, not by the reinforcing steel.
Once a crack occurs, it’s often challenging to repair the crack invisibly. However, cracks can be filled to minimize their appearance using a color-matched epoxy, cement grout or caulk. Be sure to use pigments and fillers that closely match the color and texture of the countertop to get the crack to blend in.
Keep in mind that a crack happens because something caused a tensile failure, so the repair may be more aesthetic than structural. For example, if the cracks were caused by excessive movement, and if that movement continues to occur, then the cracks could re-form or even widen. For full-depth cracks, it may be very difficult to find an epoxy to fully penetrate the crack and preserve and restore the original appearance of the unbroken concrete.
Let your clients know about the possibility of hairline cracks, so they will have realistic expectations. Also reassure them that if minor cracks do occur, repair methods are available to disguise them.
Why did my concrete countertop crack, and how can I fix it?
Answer:
Cracks in concrete are a fact of life. However, you can minimize or prevent them if you understand how and why they occur.
Most cracks are caused by tensile failure of the concrete. Thermal cracks, shrinkage cracks and flexural cracks are all tensile failures. Hairline cracks, which are very small and relatively short, are often caused by long-term drying shrinkage and are the most common type of tensile failure seen in countertops. They are generally considered inherent characteristics and not flaws, unlike full-depth structural cracks which can be quite large and noticeable.
Proper reinforcing can significantly reduce the likelihood of cracks. Unreinforced concrete countertops are more vulnerable to cracking because they are very weak in tension. Out-of-level cabinets, improper supports, improper lifting of the slab during transport and installation, and incorrect sizing and placement of reinforcement will also increase the likelihood of cracking. If a concrete countertop is improperly reinforced, or if a properly reinforced countertop isn’t supported right, then the tensile stresses that develop in the slab will be resisted only by the concrete, not by the reinforcing steel.
Once a crack occurs, it’s often challenging to repair the crack invisibly. However, cracks can be filled to minimize their appearance using a color-matched epoxy, cement grout or caulk. Be sure to use pigments and fillers that closely match the color and texture of the countertop to get the crack to blend in.
Keep in mind that a crack happens because something caused a tensile failure, so the repair may be more aesthetic than structural. For example, if the cracks were caused by excessive movement, and if that movement continues to occur, then the cracks could re-form or even widen. For full-depth cracks, it may be very difficult to find an epoxy to fully penetrate the crack and preserve and restore the original appearance of the unbroken concrete.
Let your clients know about the possibility of hairline cracks, so they will have realistic expectations. Also reassure them that if minor cracks do occur, repair methods are available to disguise them.
posted by Jeff Girard
at
4:04 PM
2 Comments:
This is John Truax ACS Impressions In Concrete Inc.
This letter serves as a follow up on my conversation with Bob Harris , pertaining to hairline cracking that
could occur on the applications of concrete counter tops.
The Construction manager of the project : Deco management had request for us to send them information on voids in concrete and hairline cracking. We were referred to by the architect, the construction manager to
hired us to install in place a two level concrete counter top with no color added or glass chips applied, just a light polish finish and inserts of stainless steel rods inserted at interval parts of the block and brick face structure.
After approximately 20 days after the polishing had been performed , (Resin Pads) Some signs of hairline cracking was noticeable. After the Construction manager had requested for us to do an on site visit, it was clear to me that the areas where the hairline cracking had occurred, was at the intervals where the stainless steel rods had been installed, as where the Architects plan had shown on the drawings as where to place the inserts. I had spoken to the Construction manager about the steel rods had performed not only structure support but also at the same time had created a void in the 2 inches of low slump 5000 PSI, Fritz Pak and counter Flow concrete mixture we had used in the Mix.
We are asking you to send us information on Voids and hairline cracking that could possibly occur on Concrete Counter Top. Also it would be more noticeable to the naked eye on smooth polished finishes rather then on a conventional rough finish.
I am looking forward in meeting you again at the North Carolina show. Thank for you input.
John Truax CEO .
please email us at impres1983@aol.com 1 (800) 546-7737
www.impressionsinconcrete.com
By
Anonymous, at 10:40 AM
It sounds like the cracks you saw are due to drying shrinkage that occurred over the 20 days or so.
Concrete countertops are especially sensitive to insufficient curing and mix designs more prone to shrinkage.
You don't mention how long the piece cure for. Longer wet curing allows the concrete to gain strength (compression and tensile) before it dries out. Drying causes large internal tensile forces that can cause cracking. If the shrinkage forces exceed the tensile strength, the concrete will crack.
Generally good mix design and construction practices are the key to preventing problems. Using a low w/c ratio (usually below 0.4) and continuous wet curing for at least 7 days will help reduce the possibility of cracking.
By
Jeff Girard, at 11:38 AM
Post a Comment
Home Page