Monday, November 28, 2016

What are Mastic Waterstops?

Commercially available mastic waterstops are sold in rolls or strips. The profile is usually a small rectangle or trapezoidal shape, with 3/4”  x 1” being somewhat of a standard. These strips are adhered to existing concrete using an adhesive or primer, or alternately concrete nails at 12” on-center spacing. This adhesion is important, as only three sides of the waterstop will then be exposed to fresh concrete. If the waterstop is displaced during the concrete pour it can easily lose most if not all of its effectiveness. Also, like their similar hydrophilic cousin, mastic waterstops are designed for non-moving construction joints only. No expansion, isolation, or contraction joints.
Unlike the hydrophilic waterstops, mastic waterstops are simply a strip of tacky, rubbery compound (usually based on bitumen and butyl rubber) that is designed to “stick” to a primed surface of a cured concrete cold joint on one side, and have fresh concrete cast against the remaining three sides, with the heat from the hydrating concrete causing the product to become even tackier, and therefore sealing the joint by acting as an internal, adhered sealant.

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