Experimental rehabilitation of a road with intractable salt damage with a bitumen-rubber single seal

Abstract

Bituminous surfacings which have been blistered, cracked and potholed due to excess salt in the base course are notoriously difficult to rehabilitate by resurfacing with a seal or asphalt as the damage usually penetrates the new surfacing within one year. This paper describes an experiment laid in 1984 on the O’kiep-Steinkopf road (now the N7/8) in Namaqualand to rehabilitate with 7 mm bitumen-rubber single seals at application rates of 2,6 - 3,6 l/m2 . A length of 728 m of an extensively salt-damaged 19 mm Cape seal with two reseals which was still developing new salt damage more than 20 years after construction in 1961, and a very lightly trafficked dead-end remnant cut off from the 1961 road and resealed once was used. On both sections the damage on the existing surfacing consisted of patches of mostly 50 mm diameter craters and blisters averaging about 75 mm in diameter at a spacing of 0,5 - 2 m on the road (some 4 000 / km), and 300 mm on the remnant, together with 1 - 3 mm-wide cracking on a starburst to random pattern, closed blisters, and staining. Although within one year all the experimental reseals on both sections were also penetrated by salt damage, after five and eight years the bitumenrubber seals on the lightly trafficked section (intended to simulate the worst-case scenario of surfaced shoulders and airports) were in a fair condition and remained so for 13 years whereas the unresealed control sections were severely and extensively potholed. After two years the unmaintained 12 half-width bitumen-rubber sections exhibited an average of 11 blisters (closed and cratered) / 100 m and 55 blisters plus stains (potential blisters) / 100 m in contrast to 47 blisters and 83 stains on the emulsion control section and 9 blisters, 5 stains and 178 patches (some prior to 1984) on the two untreated 1980 control sections. After five years before a routine reseal of the whole road in 1989 the bitumenrubber exhibited negligible damage and no patching, whereas both were present on the emulsion and 1980 seal controls. Although after eight years in 1992 even the 1989 (the fourth) reseal had become slightly damaged, in 1997 after a further reseal no damage was visible except for a few stains on the emulsion-sealed section, and none at all after 20 years. It is concluded that an ordinary single reseal should not be used on a salt-damaged seal, that a bitumen-rubber single reseal at a minimum of 2,8 l/m2 should be an acceptable although not a perfect solution, and that the sulphide as well as the salt content and pH should also probably be controlled in base courses