Concrete Weak in Tension

I am a graduate student studying Architecture and am in a Seismic Design Course.  I want to know definitively why concrete is weak in tension. Given the fact that it is, I am having a hard time finding out WHY?  I am looking for as complex an answer as can be provided.  Related Formulas, thresholds, etc are all acceptable and greatly appreciated. 

thank you
You are asking for a lot...

Basically, concrete is strong in tension - it is much stronger in compression (ten times).

The reason is in the structure. Concrete is made of a collection of materials (several aggregate types, cement, pozzolans, water, air...), which are glued together with a cement paste.  The "interface" zone is the weakest link in the structure. When compressing, that interface only serves to transfer compressive stresses from one aggregate to the next. That does not require exceptional strength.

Under tension, the aggregates are trying to pull away from each other, and the glue is what holds the whole system together.  Since it is significantly weaker than the aggregates, it is where the failure starts at much lower stresses.

Steel, on the other hand, is uniform and therefore has similar strength in tension and compression.

Hope that helps.