Question - Why is concrete weak in tension?
Answer -
Concrete is made up of collection of materials (several aggregate types, cement, pozzolans, water, air…), which are stick 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.