What is my 3/8" concrete drill bit hitting in the basement wall and not penetrating when I try to mount a TV?

Question

I have a 48" flat screen TV that I'm trying to mount into a pre-assembled then-installed permanent basement shelves. It appears that the back wall of the shelves was glued onto the cinder blocks. Across the top of the shelves is a support beam and air-ducts. I took the template that comes with the TV mounting kit, after centering the TV and removing the shelving in that area, and drilled the first two upper holes and they penetrated the cinder block for the 3" needed well. It actually drilled into airspace after about 2 1/2" in. The 3rd hole will not let me drill farther than 1/2 ". Even relocating the template higher or lower by a hole size, involves losing the ability to use one of the holes. What could be about 3" wide around neck level behind the cinder block wall and impenetrable going horizontally across the wall? House was built in 1984 and I'm certain is cinder block throughout the basement as it is visible anywhere support walls belong and the other 3 walls and areas that meander around the shape of the basement shape are cinder block. There is a furnace room about 10 feet to the right and a separate cinder block room about 5 feet to the left. The other side of the wall is simply the foundation outside. So with all that, what gives? Josh-U

Answer 1

You could be dealing with a vertical cell filled with concrete or a vertical piece of rebar. If you are not using a hammer drill, try that first and see if it starts going deeper. If not, take a loot at the hammer drill bit and see if it looks like it's dulling. If it is, you will need to try a Carbide Rebar Drill Bit.

Answer 2

First thing to check is that you are using a hammer-Action Drill or Impact Action Drill.

the Rotary/Immpact drill is much more powerful for drilling holes in walls (cinder/concrete/brick) but not so cool as a general all purpose drill, since they use special SDS bits which dont scale to small drill sizes very well.

once you have one of those drills, make sure its running the correct way (clockwise rotation) and at the hammer action is engaged (you will know it is when you depress the bit onto a hard surface and the drill makes a MUCH louder sound) Lastly.. check that you have inserted a masonry bit (that's the one with the wings at the tips)

now.. i have used hammer drills which are just too weak for the wall (i.e the wall is nice and old and the bricks are super hard) - in this case, you just drill and drill, and end up with a nice red tip and completely melt the bit into a blunt bit... I would recommend getting a good hammer action drill if you can. Personally, i would stay away from battery drills if you are looking for a large diameter hole... im happy to use a chord for the extra reliability and power.