Bitumen naturally becomes stiffer over time due to oxidation, evaporation, exudation, and physical hardening.
bɪtˈuːmɛn nˈætʃɚɹəli bɪkˌʌmz stˈɪfɚɹ ˌoʊvɚ tˈaɪm dˈuː tʊ ˌɑːksɪdˈeɪʃən, ɪvˌæpɚɹˈeɪʃən, ɛɡzjuːdˈeɪʃən, ænd fˈɪzɪkəl hˈɑːɹdənɪŋ.