Tile grout cracking "against Marble" -- Cures?

About two years ago, I had a "cultured marble" <we are very high-class HAHA> shower enclosure installed and then had the bathroom floor donein ceramic tile.
When the tile butts up against the shower enclosure, the grout has cracked, separated, pulled away -- not sure what you call it. I posted pictures of it on my flickr page, you can see them here:
http://www.flickr.com/photos/62275472@N06 /
What I THINK is happening is that the shower enclosure "flexes" slightly when people use it (it is used rarely, btw) and it has caused the "inflexible grout" to crack.
Any thoughts about a solution? Obviously, I guess I could try to chip out the loose grout and put in more grout, but I think I'll have the same problem. Alternatively, would some sort of caulking work?
I'm pretty much unskilled in this "field" -- in other words have no idea what I am talking about. Any thoughts?
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Oh, if you look at the pictures, if you "double click" on them, they come up larger, making them easier to see....Tim
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Dig it out and put in white caulk.
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Yeah, flexible caulking is sort of where my "clueless" brain is leading me....
Does everyone else agree?
If so:
1) What type of caulking -- acrylic -- ?? (tons of different caulking at the store all made from different stuff).
2) Any tips on digging out the existing grout....I'm thinking there must be something that might make it a bit easier.
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tim birr wrote the following:

Caulk made for bathroom shower and tub enclosures.

You may not have to dig it out at all. Just caulk over it.
--

Bill
In Hamptonburgh, NY
  Click to see the full signature.
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OP, again....
Thanks, guys. I'm starting to "picture" all this.
My only issue with silicone caulk would be that I used it years ago in another house, along a bathtub glass/chrome door, if I remember correctly. Needed to remove it a few years later and the silicone was just about impossible to remove -- OK it was impossible.
While I don't plan on removing this, you never know. Would the polyurethane have good flex, as well as be somewhat "removable" in the future?
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Any change in wall or floor plane will experience movement and should be caulked. Any change in adjoining types of material will experience movement and should be caulked (with the exception of 'like' materials - stone to tile, brick to tile, etc).
Remove the old grout, clean the joints as well as you can, final wipe down with alcohol to remove any soap or grease residue. In general you should not fill deep joints entirely with caulk if it is a critical joint. As yours are not, it's probably not an issue. Use a good quality silicone or polyurethane caulk meant for bathrooms (has mildewcide in the mix), and in the color of your choice.
For critical joints deep joints should be partially filled with some type of backer so that the caulk is thinner in depth than in width so that the caulk will stretch more in the direction you want it to stretch, and not pull away from the critical attachment points. Again, not really an issue for your situation.
R
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*The joints where you have cracks all seem to be narrower than the joints between tiles. Usually in small joints an unsanded grout is used. I suspect that the regular grout was used on the whole shower. I couldn't say for sure, but I think that you should clean out the old cracked grout in those small joints and regrout with an unsanded grout.
Of course you could caulk, but you may not like the way it looks.
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On 4/28/2011 12:33 PM, tim birr wrote:

another. Flexible caulk is the right thing to use. There is enough normal expansion/contraction to produce enough movement to crack grout. I would dig out the loose grout, clean the surfaces at the joints of grout, dust, soap, etc., and caulk them.
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