Is the word trod a concrete noun or an abstract noun?

The Word 'trod' is not a noun. The Word 'trod' is the past tense of the verb to tread (treads, treading, trod, trodden).

Example: We trod the slippery rocks with care to reach the dry ground.

The Word 'tread' is also a noun, a concrete noun, a Word for the horizontal upper surface of a step in a stair; the surface of the sole of a shoe; the outer surface of a tire; the sound of a footstep.