Fill up the cracks if you think that the damage is not too deep. Use thinnest mortar and apply it on the cracks. Do not look for tiny cracks as mortar won’t fill up, just focus on the ones that are visible. The mixture you make should not be too thick. Filling up the crown cracks will save you a lot of inconvenience and money, but it is not guaranteed that the repair will stay intact for a long time.
Another problem with filling is that the concrete keeps dropping off every now and then, especially after heavy rain, extreme temperature and windy conditions.
Caulk the cracks. This can save you a lot of inconvenience and avoid permanent repair (if arranging money seems difficult for big repairs at the moment). Make sure the areas where cracks are visible should be cleaned up. If there are rock pieces in the cracked areas, caulk might not work perfectly. So, remove all the dust and rock pieces with the help of brush. Now fill up the cracked areas with caulk.
Remove the damaged crown if temporary repairs do not seem to work. With the help of chisel and hammer, gently break the concrete. Cover the roof of the chimney with some sheet so that it will remain clean.
Prepare mortar by using cement and gravel. Mix cement and gravel and then add some water to prepare mixture. Make sure that you do not pour too much water as you will probably have to use more cement if the mixture becomes loose.
Now it is time to apply the mortar mixture on the chimney. The areas that you have just detached should be filled with the mixture. Make sure you spread the mixture evenly across the broken area. Use trowel to even the surface of the chimney crown.