For information makes interesting reading
http://www.ntc.gov.au/filemedia/Reports/NatStatExhaustNoiseTestSept2006.pdf
I wouldnt trust the iphone app on high levels.... doubt the mic in an iphone was designed to handle anything above 90 - 95 db (voice).
I read somewhere and i can't find it that they are not allowed to test on concrete as it is considered a noise reflective surface. i'm pretty sure that bitumen has been argued in a USA court and also found to be not acceptable but I could be wrong maybe the Boss will know.