On January 31, the Uganda National Roads Authority (UNRA) boss, Allen Kagina, returned from leave.
On arrival at her workplace, Kagina asked aides and engineers to prepare to move to Jinja for a physical inspection of the New Nile Bridge.
Earlier, photographs of a cracked asphalt layer sent shivers down the spine of many a Ugandan.
It was feared that the historic bridge, which was commissioned recently amid fanfare, had structural defects.
An investigation by UNRA engineers has now revealed that the main reason for the wearing off of the surface was “poor bonding between the water proofing material and AC14 Asphalt.”
According to a classified official briefing prepared for Kagina, which ChimpReports has seen, construction of New Nile Bridge (Source of the Nile Bridge) started on 14th April 2014 and by May 2018, the works on the main bridge structure were completed.
The contractor started working on the bridge structural attachments which were purposely designed to improve the functionality of the bridge and protect the structural surfaces of the bridge.
These include asphalt, bridge barriers and street lighting poles.
In short, the commissioning of the bridge did not necessarily mean that all work on the structure was complete. The defects liability period will run for two years.
The Bridge Deck Asphalt
In order to protect the bridge concrete deck, an investigation by engineers revealed, “water proofing material was introduced in between the surface of the concrete deck and the asphalt during the design stage.”
Sikalastic – 822 water proofing materials was applied on the bridge deck and to facilitate bonding between the Sikalastic 822 and Asphalt, Sikalastic-823 Tack coat was applied.
Sikalastic 822 is a 2-part elastic, crack bridging, hand applied polyurethane membrane.
In simple terms, it is used as a waterproofing membrane underneath hot poured asphalt on a bridge or car park decks. It serves as a waterproofing membrane for other concrete structures.
Kagina was further informed that before the application of AC14 Asphalt, the waterproofing company informed the contractor that their materials only bonds with mastic asphalt not AC14 asphalt.
In general terms, mastic asphalt is one of the oldest construction products, and the oldest waterproofing membrane system.
It has been in use for centuries.
It consists of a graded limestone aggregate, bound together with asphaltic cement (i.e. bitumen), heated to 210 degrees C and applied by qualified installers with a wooden float.
Kagina was further told that the trial test was done and the Subcontractor, Chinese firm CICO, wrote to the main Contractor, Zenitaka Corporation Company, on the same.
Zenitaka then instructed CICO to continue at the main contractor’s risk as they monitor the behavior (performance).
ChimpReports understands at the trial test (braking test) was done on September 21, 2018 using 32 tonne trucks and contractor proceeded with the application of asphalt.
Interestingly, a few weeks after opening of the road to traffic, there were defects seen and the contractor kept on monitoring.
Engineers further told Kagina that during monitoring, the contractor conducted several tests of which part were for the bridge structural health monitoring system – both in-situ and laboratory test.
“The test findings show that there was poor bonding between the water proofing material and AC14 Asphalt,” the brief reads in part.
Going forward, engineers told Kagina that the contractor has now submitted a “method statement to replace the AC14 Asphalt with modified polymer asphalt.”
This type of asphalt caters for today’s increasing loads, greater traffic volume and longer-lasting road.
An engineer who spoke to us on condition of anonymity so as to speak freely said modified polymer asphalt can fight the four main ways pavement fails: rutting, stripping, cold cracking and fatigue cracking.
As of Friday morning, the contractor was undertaking works on the trial section using modified tack coat as they finish the procurement and shipping process for the modified polymer asphalt.