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Q.While I was removing the old tile from a concrete shower pan in a slab foundation, a portion of the 1x5-inch raised concrete lip separated from the slab below. It appears that the lip was formed and placed on top of the slab, and there is nothing tying the two together (see illustration, below). How should I repair this? Should I remove the complete lip and repour the whole thing, or repair just that section? How can I bond the new concrete to the old? Also, while making the repairs, should I hot-mop the pan?
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A.Michael Byrne responds: It may be possible to repair the existing curb, but you will get better results by pouring a new reinforced curb that is bonded to the old concrete with a concrete bonding agent. You will have to ensure that there is no wax, oil, grease, or soap scum on the face of the concrete slab that could ruin or reduce the bond. After that, I recommend that you install a sloped shower pan to contain the shower water and prevent harmful organisms from becoming resident in the surrounding walls and floor.

As for hot-mopping the pan: In many areas, local building codes still allow unprotected concrete to be used as a base for a ceramic tile shower stall, even though the practice is not sanitary, is not approved by the UBC, and is not sanctioned by the tile industry. I haven't recommended hot-mopped shower pans since 1974, when health and safety concerns convinced me to switch to Chloraloy 240, a CPE pan liner (made by the Noble Company, www.noblecompany.com). You could also check to see if your local building department approves of using a surface-applied membrane system. The Schlter Company (www.schluter.com) makes a low-profile drain and membrane system that might work for you.

Contributing editor Michael Byrne is an expert tilesetter and consultant in Los Olivos, Calif., as well as author of many JLC articles and the book Setting Tile.