shower tile grout cracking on floor


I have a shower and the grout on the floor has hair line cracks in it. The shower wall is fine, no problems. Upon looking closely at the grout I found that there are holes through out it. The holes don't seem too deep, the don't seem to be all the way through it more like bubbles in the grout at the time of the application.
Can I simply just cut out the grout from the hair line cracks or should I try to cut all of the grout from the shower base? Can I spread grout across all of the tiles even ones that I didn't cut the grout from? Will it bond?
Any advice would be greatly appreciated.
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Since no one else has responded...
Since you are posting this Q you already know it needs to be taken care of ASAP. No matter how much it involves it's nothing compared to water getting through and doing structural damage.

Yes and yes. How much faith do you have where it's not cracked...yet?
Info that might help to an approach:     How big is the floor area?     What size are the tiles?     About how wide are the grout lines.
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k wrote:

If removing bad grout, you need to remove it to sufficient depth that the new grout will have "tooth" to hold. In doing a partial replacement of grout, the main issue IMO would be a possible color difference. I redid a shower stall, removing old grout with Dremel grout attachment. I think the finest bit for that is 1/8". Or was it 1/16"? Worked very nicely and fairly fast, with a good deal of dust, of course. The point at which I decided to regrout was when I noticed quite a few pin-holes. A couple of the condos in our complex, with same construction, had badly damaged and leaking walls from letting it go too long. They generally were very lax in maintenance for a long time. Our condo was used only for family vacations, so most of it was pretty pristine and just kind of faded and dingy - still had original paint in most of it, age 30+ years.
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If this is a quality shower, it may have a waterproof membrane under the tiles. This could be damaged if you dig the grout out too deep or too vigorously and cause leaks. If it is a builders special, there could be anything under the tiles, probably water saturated concrete over a vinyl waterproof sheet.
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k wrote:

I can't help but picture a shower with poor support and a "spongy" floor. It will just crack again and again. Hopefully I'm wrong.
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Thanks for the input. From your responces I am going to try to remove all the grout with a dremel multimax tool and re grout it. I think it will take some work but hopefully the grout won't crack again?
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