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I have concrete wall (plastered), 8 mm holes, plastic wall plugs, intact Philips head screws, and a bracket outside.

Accidentally I screwed the screws too tight. They stuck at one point, and won't loosen with the same force. I cannot apply enough force with Philips screwdrivers. Not even with a ratchet wrench (because the head simply pushes the bit backward when turning, and it slips out).

Can not use pliers to pull the screw because I can not prop the tool against the wall (can only prop it against the bracket, which is on top). I tried to use a wedge under the bracket, to strain the bracket and the screws outward. So far, the plaster on the wall would break and collapse. I can grab the higher screw with pliers but can not even grab the lower screw, both screws are stuck.

Can anyone provide some advice?

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    Those may not be Phillips head screws. The four tick marks indicate they are a newer type called Pozidriv. Trying to unscrew them with a Phillips will not work well or at all.. With the correct Pozidriv screw driver they will come right out. – Jim Stewart Dec 2 '20 at 19:41
  • wow, good eye Jim. – Glenn Willen Dec 2 '20 at 20:15
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I would try these things in this order:

  1. Use an impact driver to loosen the screw. You can either use a full impact screwgun or air impact tool to remove the screw (make sure you apply lots of pressure against it so the driver does not slip out of the screw head). There are also hammer impact tools available that are more used to loosen stuck screws, and then you use a regular screw driver to fully remove. With these, you place it on the screw, and pound on it with a hammer. If you don't have an impact tool (electric or air-powered) available, this would be a good solution as these hammer impact drivers are relatively cheap (can get them for around $10 US).
  2. Use a cutting tool to make a slot for a flat-head screwdriver. These allow you to apply more torque then a Philips head.
  3. Cut the head off the screw altogether or drill it off. This will allow you to remove the bracket at least, but you will have to abandon the screw shank in place and move your bracket slightly if you need to re-install it.

One more thought, since you said you are using plastic wall plugs. If all that's holding the screw right now is the wall plug, you could also try some gentle heat on the screw, before unscrewing or pulling it out. You might be able to heat the plastic enough to where this will work. But it's also possible the end of the screw is stuck in the concrete. Concrete will also be destroyed quickly by too much heat, so you would need to be quite careful with this method.

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    You could use a sodering gun to heat the screw if you decide to try that route. – Willk Nov 25 '20 at 22:19
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    I would not use an impact driver on a hollow wall. – isherwood Dec 2 '20 at 16:52
  • OP said it's a concrete wall that is plastered. I take that as solid concrete with plaster over it, nowhere does it say it's hollow. – PhilippNagel Dec 2 '20 at 17:57
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Vice grips and phillips head bit.

If you had a screwdriver with the bit set at a right angle to the handle you could get leverage on it. I have seen these but do not own one. But I made one when I needed it and maybe you can too.

I used a vice grips and a phillips head bit that came with a drill. I grabbed the bit good with the vice grips such that the tip of the bit was perpendicular to the long axis of the vice grips. Then I had more leverage than what I could get with a plain screwdriver. I bet that gets them out.

Maybe you could do the same thing just grabbing your screwdriver near the tip with the vice grips. I think the bit was better because it had facets, and screwdriver shafts are usually round and smooth.

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Those are almost certainly not Phillips head screws. The four tick marks indicate they are a type called Pozidriv, which is an improvement on the Phillips that has improved engagement and allows more torque. Trying to unscrew Pozidriv screws with a Phillips driver will not work well. With the correct Pozidriv screw driver they will come right out.

Look at the inscription on the driver bits you are using. A Phillips driver will be inscribed PH followed by a number (1, 2, . . .) which specifies the size. A Pozidriv driver will be inscribed PZ (or reportedly sometimes PSD) followed by a size number.

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