Ammonium nitrate begins its attack on concrete by reacting with the small fraction of calcium hydroxide produced by the hydration of Portland cement.
One unfortunate by-product of this reaction is a reduction in the alkalinity of the cement matrix, which increases the risk of reinforcement corrosion.
Much more damaging is the reaction between the calcium nitrate from the initial reaction and the strength-giving calcium aluminate in the hardened cement matrix. This yields calcium nitroaluminate, a weak, expansive compound. Affected concrete spalls, loses aggregate, and exposes reinforcement. Compressive and tensile strengths deteriorate. Concentrations of as low as 0.5% of ammonium nitrate by weight of cement can cause significant damage.
A further risk comes from ammonium nitrate's direct effect on the reinforcing steel. Steel under tension becomes brittle, especially if surface cracks are already present. Tensile reinforcement failure leading to catastrophic structural collapse is a distinct possibility.