Can caps be placed on decayed tooth? my dentist seem to be planning to on an almost fully decayed tooth and i'm concerned?

Crown/ cap: A tooth is usually shaved down to make room for the cap/crown. Discuss with your dentist. See: http://www.medicinenet.com/dental_crowns/page3.htm

Decayed Tooth: All the decay needs to first be removed. After that, your dentist will determine if the amount of healthy tooth structure that is remaining can support a crown. Sometimes root canal therapy is also necessary.

Crown and decay: Need more information but here are some basic guidelines: A temporary crown can be placed on a tooth even with some decay if this is an emergency or transitional procedure; pending further evaluation or treatment. However, no decay should be left when a permanent crown is fabricated! All decay should be removed to determine if RCT or extraction is necessary.

Crowns: If a tooth is badly decayed, and there is not enough sound tooth material left to do a filling, often the best treatment is to remove all diseased tooth material and then replace it with a construct, a crown. This is a tried and true treatment modality.