Radiator Repair

I took my bakkie to Leach Toyota on the 2nd November 2015 to replace the leaking radiator. I was asked to pay for the replacement whereas the bakkie is still under warranty. I was told that the damage to the radiator was caused by the external factors and it is not covered under the warranty. The same problem occurred on the 7th May 15 and I also paid (R6682.99). I just wonder if this radiator had been made to withstand our road conditions (gravel/sand). It means that every 21000km I will have to replace this radiator.<br> I only received my car on the 5th Nov 15 and I was not given a courtesy vehicle.