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August 23 - August 29, 2015

How do you keep sealants in control joints from cracking or separating from the substrate?

Selected Answers

From Tom Murphy of Adhesives Technology Corp. on September 1, 2015:
This is an excellent question and one that has been raised frequently since the big box retailers and stained concrete have made this issue much more visible. First, it is important to clarify that "sealants" are generally used to prevent moisture migration, in this case, into the concrete. They typically have high elongation, used in expansion joints, and are classified under ASTM C920. Joint fillers, on the other hand, are used to protect the concrete edge of a control joint that is being subjected to traffic (fork lift or shopping carts). These materials typically do not have as much elongation and are relatively hard (durometer Shore A 75-85) to support traffic. The dimensions of the joint sealant/filler for treating these two applications is completely different. Sealants are only half as deep as they are wide; while control joints are at least three times as deep as they are wide. Recently there has been some developments in the treatment of control joints that minimizes and can eliminate the separation issue. This system is a two- step process using a desiccant to clean and dry the concrete bond surface, followed by the application of a polyurea filler. Laboratory and field tests have shown this system improves the bond strength by as much as 50% over traditional fillers. Simplistically, this can be visualized by improving the grip on a rubber band allowing it to stretch, versus trying to stretch that rubber band with soapy fingers.

From William Slama of International Paint/Ceilcote Products on August 31, 2015:
I need to explain my blunt response. The reason I worded it as above is that most control joints do not crack. But if they do crack, the movement is restricted by the rebar that passes through that section. However, there will be a small amount of separation even with the rebar. Because of the geometry (a narrow, deep joint), and the fact that most control joint materials have limited flexibility (typically less than 100% elongation), they will not be able to accommodate the movement, and they will either crack or pull lose from the concrete. That is because of the resulting transverse strain that happens in response to a longitudinal strain. That ratio is called the poisson ratio and, for very flexible materials, it is about 0.5. And so there is no way for the joint material to relieve that strain and resultant stress, so it acts as a much more rigid material. Whereas for an expansion joint, as previously explained, the joint allows the material to expand or compress.

From William Slama of International Paint/Ceilcote Products on August 27, 2015:
Chemical-resistant joint sealants are of course designed to be used for floor or wall slab expansion joints. They need to allow for shrinkage of the concrete as well as thermal movement. When properly installed, they are designed for stretching and/ or compression due to perpendicular in-plane movement. The standard design limit is for the joint width to be changed by plus or minus 25% due to the separation (or closing) of the space between the slabs. That performance depends on proper installation AND configuration. 1) The concrete surfaces need to be clean and abraded and primed with a compatible primer. 2) The bottom of the joint must contain a compressible foam backer rod to allow for the resultant cross section change during compression or extension. 3) The material depth should be one half as thick as the joint is wide. These sealants will work fine in thin control joints AS LONG AS THEY DON'T CRACK AND SEPARATE. If they do, they must be treated as an expansion joint.

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Tagged categories: Coating selection; Cracking; Good Technical Practice; Sealant

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