Hi Ignacio,
Your application won't use "jca threads" to handle the db requests. These are handled by your application threads.
Example:
* Connection pool size is 10.
* You have a maximum of 20 http connections.
* Handling of an http requests will involve getting a connections from the pool.
If you fire up JMeter, a maxiumum of 20 connections will be allowed, each connection will try to get a DB connection from the pool and is blocked while none is available.
Eventually a connection will become available and the blocked thread can continue.
So typically you will have 10 of your 20 http threads blocked waiting for a db connections from the pool.
The blocked threads are those named "http--0.0.0.0-8080-"x ...
Hope that helps.