RepairQ We are the Architects of Record for a multi-family rehabilitation here in Portland, OR. The existing building is three stories plus a half basement. It is a historic building and is also an unreinforced masonry building. As a result, it requires significant seismic upgrades. Our structural engineer is proceeding with a design that uses several 4 in. (102 mm) thick shotcrete walls as shear elements throughout the building. In most instances, these walls are being constructed adjacent to an existing wall to minimize their impact to the existing floor plans. This seems fine against the exterior masonry walls but we have concerns where the shotcrete is to be installed directly adjacent to a standard architectural partition (gypsum wall board and wood studs). We are planning on plywood sheathing to act as a one-sided form to prevent the shotcrete application from harming the existing stud wall. However, we were curious if we should also include a water barrier so that the application did not allow moisture to migrate into the existing walls during the installation. Is this assumption correct? And if so, is there a performance or product recommendation that you can offer for this purpose?
Shotcrete has been used in similar structural upgrades for decades quite successfully. There are many ways to ensure that the moisture from the fresh shotcrete would not impact the existing partitions. Methods for protection include use of green board (as used in a shower or bath), plywood, painted coatings, moisture-resistant sheeting, and so on.
Shotcrete is placed with a low water-cement ratio (w/c) and the water is needed by the shotcrete to hydrate the cement and harden. As long as the existing surface or treated surface does not actively absorb the moisture, the shotcrete will use the available internal moisture to hydrate the cementitious materials.
