AWhy does a cake get crumbly at the edges?

It's been heated longer than the middle area so it's a bit drier. If it's crumbly just sitting there then it's overbaked.

If you slice off the browned edges on a freshy fresh cake you can run into crumbly issues just due to freshness --a cake actually needs to season a bit--which is why I like to freeze my filled cakes. Filled meaning already made into tiers--all the layers have filling in between and the cake is ready to be iced.  

Freezing and thawing evenly redeposits all the moisture in a cake. Then I can easily shave off that outside crust  because the frozeness holds my cake edges for me. Which is why if you seriously over bake a cake you can still rescue it in some cases by cutting off the really dark part immediately and freezing it. Because the overdark area is gone, it is not robbing the moisture left in the main cake part.

Is that anywhere near what you meant?

It's possible your oven control is off so the cake is baking up much too fast &/or unevenly.  That causes the problem as K8 describe. Lower your oven temp.  I have baked at 300 degrees for about 20 minutes (for a one mix cake any size/shape) then up it to 325 for an equal time.  That usually does just right.

I have baked 1,000s of cakes w/o using bake even strips nor a flower nail but for some it seems to help and certainly doesn't hurt :)  Exception is sq cakes.  For some reason they seem to bake faster on the corners so I have wet paper towels and places them around the corners.  If they fall off they are not wet enough.  No, I have never had any fall off the burn in the oven.  When they are wet enough the stick to the pan even when they dry out.

Bake even strips can be purchased OR made yourself.  Rather than using paper towels, some use an old bath towel cut into strips; some also wrap those wet strips in aluminum foil but that stops the effectiveness of the strips somewhat :(

AThanks all your answers. I ll try your advice. Btw, how long does it take to bake a cake? on every recipes it says 20-30 mins however last night I waited for more than 45mins!!!

It just takes as long as it takes. Bravo for not underbaking!

A

Both gas and electric each heat to the same temperture - whatever you have it set to.  

As for how long, it depends on the size of the pan.  Any pan that uses the batter from one cake mix should take 20 to 30 minutes.  The fact that it took yours 45 min means something is probably wrong w/your oven.   As I mentioned above, your dial can be off OR if you have an elec oven one of the elements can be out throwing off the whole thing :(   

Yes and some ovens let the temperature drop 50 degrees before it cycles back on and kicks up the heater to reheat and each oven is different depending on how old it is and how clean it is and lots of variables. What I meant was you did a good job baking till the cake was done rather than going blindly by what the recipe says. You just gotta get a feel for your oven which you seem to be doing.

Like someone else said, first of all, buy an oven thermometer.   I'm sure there's nothing wrong with your oven itself, but rather isn't perfectly calibrated.  An oven thermometer is an easy $5 fix!

Then buy a large pizza stone and keep it in the bottom of your oven at all times (hopefully you have a gas oven - I've owned both residential and commercial electric ovens and absolutely hate them!).  The pizza stone will help your oven maintain a steady temp.