best method for removing concrete ballast

i only want to take it down about a foot. the concrete is maybe 30 yrs old, small aggregate, intact and dry. my concern is using a jack hammer might lead to delam in the hull later on ( heavy frp/polyester ). anyone have any experience in this area ?

 

First question is: Why? A jack hammer is a bad idea. Use a diamond blade with a grinder to cut crossed lined and then break the small sections off.

 

yeah i knew you were going to ask....

id like a bit of a sump for the bilge pump to ride in and to keep the sloshing down. since ill be glassing over the concrete eventually thought id incorporate the well now. a grinder/circular was my first idea but the dust.... then thought of a series of holes with the hammer drill and a cold chisel to break out pieces. works but is quite tedious when working below foot level. then thought of a roto hammer sized demo tool. is delam a real concern or is it not being able to control the cracks through the concrete? ultimately all the concrete will be glassed/epoxied over to keep dry and im assuming low vis epoxy could be soaked in first to fill-up any cracks.

 

Drill a series of 1/2 inch holes in 2 rows. Put the holes full of water and place dry ice on the holes in 1 row. Water will freeze and expand, cracking the concrete along the line of holes. Repeat the procedure on the 2nd row. It won't get it all but should save a lot of labor and noise. You will have more success the closer the holes in the row are and the closer the 2nd row is to the first. Keep making rows of holes until you have removed the targeted concrete. Or you can use a series of metal wedges all aligned parallel to the row, tapped down a little at a time together.

 

My thought would be a concrete hole saw drill, then chip out the center with a impact hammer. Much more controllable than a jack hammer, and you can get exacally the size you need for the pump. Alternatively you could use a standard concrete drill bit, insert a pipe and use a self priming diaphragm pump with a time delay on the float switch.

 

you, sir, are a genius.

 

You can buy a tile/concrete saw blade for an angle grinder. They have no teeth, just diamond abrasive and work wet or dry. Make a grid of shallow cuts, say 1" squares, about 1/2" deep. Then switch to an air (or power) chisel or use the scraper attachment that's now available for a reciprocating saw. These will knock off the 1/2" thick, 1" squares and you can do it again to the full depth you need. The dry ice is unique, but a bit imprecise. If you use a diamond blade on a an angle grinder (about $15) you can be really accurate about the cuts and what's removed.

 

as the best above;Do your cutting 'Wet' & there will be no dust,untill it dries!Sponge the slurry off,where accessable!

 

Buzzly is right. All concrete work should be done wet. No dust at all, plus your blades will last more than 10 minutes too.

 

yeah...wet is not an option (long story). but i ask again...is delam of the hull in the immediate area a real concern if i use a small roto hammer? thanks again

 

It is a distinct possibility. The worst is that the damage will be hidden and you won't find out until a storm catches you.

 

If your not worried about the boats interior finish or any buildings nearby Dynamite would be quickest.

 

I understand that also works for ex-wives, if you aren't worried about collateral damage.....

 

Im not sure if you cant get little dynamites that dentists use for people that dont like drilling.

 

Why not wet, its in a boat bilge.... building industry here has contractors usually set up in a small van that come to you with diamond core drilling equipment.