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Re: Water Proof Membrane

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I have not seen an example of this kind of sidewalk that did not allow some
amount of moisture into the basement.  It may be possible to prevent
moisture intrusion, but the entire system, including the basement walls
would need to be designed and built to exclude moisture.

I've worked on this kind of side-walk-over-basement system -- built around
1900.  Routing and filling cracks may not repair all sources of water
entering the basement through the sidewalk.  The need to provide a membrane
without changing the elevation of the top of the sidewalk slab doesn't allow
any net thickness for a membrane or topping.  A topping over a membrane
needs to have a thickness in the range of inches to minimize cracking due to
concentrated loads.  Any new surfacing will not match the appearance of the
existing -- would  patches of contrasting color and texture be a problem?
In the case of my projects, it would not have been acceptable because they
were in a historic district, and matching the historic appearance was
important.  Alternatively, providing a uniform appearance approximating the
historic appearance using a uniform new surface over the entire sidewalk
would have been acceptable

Corrosion of reinforcing steel in the beams supporting the sidewalk was the
principal problem in our case.   The concrete was carbonated and corroding
rebar was causing significant spalling/delamination damage.  A proposed
solution was to temporarily shore the system, clean of corrosion or replace
the corroding rebar by exposing it, installing new rebar, lapped with
existing undamaged rebar, and trowel-applying fresh concrete/mortar to embed
the new rebar.

The upper surface would have been completely removed by sawing a grid of
closely-spaced kerfs into the surface, chipping away all of the surface to
the bottom of the kerfs, providing a very irregular surface for bonding the
new topping.  I judged it unwise to prepare the surface by chipping or
bushhammering because of the potential damage to the remaining structure by
micro-cracking.  The topping would have been made with a non-shrink portland
cement mortar such as Eisenwall by CTS [714-379-8260].  However, a finish
matching the historic appearance would have been very difficult, if not
impossible, using the Eisenwall topping material.

The final solution was to remove and replace the sidewalk structure as part
of a Public Works project.

Nels Roselund
Structural Engineer
South San Gabriel, CA

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