Sunday, October 02, 2016

Mastic Waterstops

Most 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 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.

Obviously, this waterstop function is very limited as the only barrier to migrating fluids is the adhesion to the concrete and compression of the mastic waterstop product within the joint. For this reason, mastic waterstops tend to be more effective in construction situations where large concrete loads are applied on top of the waterstop such as wall on footing or burial vault lid, and far less effective when installed in vertical applications such as a wall joint.

Mastic waterstops are the lowest performing of any commercially available waterstop and are price accordingly, being the lowest cost waterstop as well. Because of the low cost and ease of installation, mastic waterstops are the favored products of commercial constructors and home builders.

Excerpt From: David Poole. “The Little Book of Waterstop.” Waterstop King, 2013. iBooks.

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