Question about Underpinning a slab (per Rod Gervais' book)

I was talking to a friend of mine regarding the underpinning of an existing slab. He brought up a good question that I didn't really have an answer for. (My venture into construction is brand new, obviously). The question: How does one clean the dirt off the bottom of the existing slab making for a nice bond to the new pour? So as not to create more of a mess? Please be kind in your responses (if you choose to). I'm still learning here. Thanks. EDIT: spelling

There are several products that are used for bonding old to new concrete; each will have its own prescribed methods as far as surface preparation. Generally though, once you've removed as much of the larger pieces of dirt adhering to the underside of your old concrete you can use a wire brush on an electric drill (corded drills work better for this, since it will be pretty time-consuming) - you'll be laying down for this, so find a pad that will let you get comfy :? If you have a mirror and flashlight you can see what you're working on - then just run the wire brush in an overlapping pattern under the exposed slab area until you've covered everywhere - re-inspect with the mirror and light. Re-brush areas you missed. I've not worked on this type project for many years; the OLD way was, you bought muriatic acid (rubber gloves, respirator and caution required) which is really Hydrochloric acid; you brushed this onto the surface to etch it, then washed it off. This was really a messy, DANGEROUS job. Not recommended for home-owners, period. I doubt whether current environmental laws even allow this any more. Newer products get mixed with the new concrete, and apparently form their own bonding, usually with epoxies. Any more and I'm "talking out my hat" here - if Rod doesn't see or respond to this, you might google "concrete bonding agent underpinning" as I did - one link that mentions a few company names - (Rod mentions one of these at the top of page 44) http://www.nps.gov/goga/history/seafort ... ncrete.htm Basically, for any add-on you need to clean and roughen (the wire brush helps here) and WASH the dust off after; obviously this will make some mud - you can minimize this by laying 6 mil black plastic in the trench first, and by vacuuming the debris out of the trench with a shop vac before washing the wire-brushed surface. Then you can empty the shop vac, convert it to wet pickup and use it to keep the water level on top of your plastic under control while spraying off the cleaned areas. Personally, if you find NO rebar in the location you're working I would (in addition to the 8" underlap Rod mentions) drill into the existing slab horizontally, centered top to bottom, and slide rebar EXTENSIONS into these holes using epoxy mortar or Concrete FixAll, giving you an extra method of making sure your new and old concrete stay tied together. HOpefully Rod will chime in here with any other insights, since I've probably sold at least a thousand copies of his book for him... :wink: Steve

See why a brand spankin' new slab is probably more worthwhile?

Yes, Keith........ :roll: It's becoming quite clear........... :wink:

Heh -- you forgot the long and pronounced SIGH at the beginning of that comment! ;-) :) --Keith :mrgreen: