Inspection and rehabilitation of old White Nile steel bridge substructure

Abstract

The substructure of The Old White Nile Bridge consists of steel tubular piers (steel casings filled with concrete) supported by caissons drilled into the sandstone. This steel bridge which had been completed in 1928, was investigated and assessed for integrity and safety. The inspection and investigation of the piers covered visual inspection, field tests and laboratory tests. Steel plates tests included microstructure analysis for steel, tension test, bending test and hardness test to determine the chemical and mechanical properties, whereas concrete core tests were conducted for determining cubes crushing strength of concrete. The inspection tests and assessment study revealed that most of piers steel casing suffered from high corrosion problems. Thus, due to corrosion the thickness of the casing plate, which was originally 5 mm for fixed piers and 7mm for swing span pier, was reduced severely to about 1.5 mm or even in some cases to about only 0.4 mm (in the swing span piers). In some cases holes appeared in the casing plates indicating that the steel was corroded totally and all the thickness was lost. Then, the Foundation caissons which were permanently under water or fully buried in the ground were found sound and suffer no problem of corrosion. No sign of differential settlement was observed. The materil tests performed showed that the steel exceeded its expected life time and have lost their weld-ability. Moreover, cathodic protection system which had originally provided previously to protect against corrosion found damaged and not functioning. Therefore, this paper presents the results of inspection and rehabilitation of the bridge substructure.