I am building an exterior bench with 2x4 to be mounted on a low concrete wall. The concrete is over 35 years old and seems very dense. A concrete drill bit seems to hardly effective. Is there a mastic of some sort by which the wood could be attached to the concrete with out a physical penetration?
Possibly like Liquid Nails or the like?
An exterior construction adhesive might work to a point but you cannot count on it for lateral stability if the lumber flexes against it for many seasons.
Are you trying to lay 2x4 lengthwise to the concrete directly? If so, I would shift thinking and put some nailers in place perpendicular first. A hammer drill, carbide tipped masonry bit, and some patience should get you into the concrete so you can get some expansion anchors and bolts in place. This would be the way I would approach the project.
A faster way would be to ramset or otherwise shoot the cross member suggested in place. Just warn the neighbors the gunshots they are hearing are not doing in the family, just anchoring for a bench. You can apply construction adhesive too if you want but I think it will be overkill.
Then screw your long 2 by to the cross members. This raises your bench height a bit but should work out better than construction adhesive alone.
You should be able to rent a ramset gun at a tool supply company and they will have the appropriate charges and fasteners. Just tell them what you are trying to accomplish.
If you have never used a ramset gun before? You will want ear and eye protection and gloves. You hold, or have a helper, hold the wood in place and press the face of the tool firmly on the lumber, pull the trigger and a cartridge---22 caliber---fires an anchor appropriate and matched to the charge through your wood and into the concrete.
A concrete drill should easily drill into the concrete. The only thing that would slow down a quality masonry bit and a roto-hammer is steel inside the concrete. Here is an article describing how to rotohammer and install concrete screws or expansion anchors to fasten regular screws to concrete.
It also has pictures of using the screws to screw a piece of wood to concrete or masonry. When using a roto hammer to drill concrete make sure that the drill setting is on rotate and hammer, this helps clean the hole out. Installing a bench ledger for regular seating will need a screw about every 12 inches or less. Make sure that the masonry screw goes into the concrete at least 1 1/2 inches. If you use a expansion anchor make sure it is metal, plastic will not last as long for something heavy like a bench.
This job is screaming for a ramset to be used. It's much easier/faster than messing with a hammer drill. I'm assuming this wood is pressure treated, if so be sure to get the "RamGuard" fasteners that are coated so as not to be corroded by PT lumber.
The .22 can be had for under $100 and can handle what you're trying to do:
Just follow their guidelines as far as what fastener and charge to use.
Can't see the bench nailed to the wall.
I'd get a hammer drill and some good bits and drill the holes you need to have the bench bolted to the wall.
If you want to bolt the bench so that it is removable, ramset has threaded studs that can be driven into the wall.
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