Epoxy Asphalt for the New Second Yangtze Bridge at Nanjing
In October 2000, Chinese contractors completed installation
of approximately 6700 metric tons of Epoxy Asphalt Concrete pavement
on the world's third longest cable-stayed bridge across the Yangtze
River at Nanjing in Jiangsu Province, China. As of 2012, a total of
over 30 new spans in China have successfully employed Epoxy Asphalt
and the pavement condition on the Second Nanjing Bridge is reported
as excellent (see picture below).
The Second Yangtze River Bridge (Jiangyin Bridge) is 1238 meters long
and 30 meters wide. The twin towered, steel box girder structure uses
a 14 mm thick orthotropic deck plate with 360 mm wide trapezoidal longitudinal
stiffeners on 660 mm centers.
Nanjing Construction Command Post of the Second Yangtze
Bridge, responsible for building the bridge, conducted an exhaustive
research and test program before choosing Epoxy Asphalt. The Command
Post sought a paving material with superior resistance to fatigue cracking
and rutting than materials previously used in China on orthotropic steel
decks. The Command Post engaged T.Y. LIN of San Francisco to develop
an evaluation and testing program and design the pavement. Southeast
University in Nanjing was selected to conduct the test program.
Nanjing has extremely hot summer weather. Because
the bridge deck temperature was frequently expected to reach 70°C,
the evaluation emphasized elevated temperature tests. In addition, the
bridge was expected to have very high wheel loads as Chinese trucks
are commonly overloaded, sometimes by a factor or 2 or 3. The program
included tests of mastic asphalt, several modified asphalts and Epoxy
Asphalt Type V. Type V Epoxy Asphalt is version of Epoxy Asphalt improved
for high service temperatures.
|2nd Nanjing Bridge after 11 years
T.Y. LIN and Southeast University recommended Epoxy
Asphalt because of its outstanding performance in the test program with
respect to fatigue resistance, delamination resistance and lack of rutting
at high temperature. Southeast University also conducted extensive tests
of locally available aggregates to evaluate polished stone, Los Angeles
Rattler and crushed stone values among other characteristics. They determined
that Maodi basalt from the Jiyong Hua Mountain region was the most suitable
Manufacture of Epoxy Asphalt
ChemCo Systems shipped the Epoxy Asphalt binder components to the plant in 55-gallon drums (2700 drums in all). The quarry crushed washed, dried and packaged the aggregate into 50 kg. sacks for delivery to the batch plant. (Bagging is not usually necessary for Epoxy Asphalt aggregate, but the Command Post quality control requirements were strict that there be no chance of contamination of aggregate.) In Nanjing, the contractor mixed the Epoxy Asphalt concrete in 1.4 metric ton batches in an asphalt batch plant.
ChemCo Systems also supplied a special "Meter/Mix" machine that automatically metered the two Epoxy Asphalt components, mixed them and injected the 110º to 121°C mixture into the batch plant pug mill.
Because temperature control is critical for successful Epoxy Asphalt installation, the contractor discharged each mixed batch of Epoxy Asphalt concrete from the pug mill into a hopper where the temperature was checked with infrared thermometers. Any batches outside of the allowable temperature range were discarded. Acceptable batches were dumped into the haul truck to be transported to the bridge approximately 20 kilometers away.
Project Specifications required strict control of
delivery time to the job site because the Epoxy Asphalt components begin
to react and increase in viscosity when mixed. Excessive time in the
truck can cause the mix to become too stiff and hard to properly pass
through the paving machine and to be fully compacted.
Prior to placing the paving mix, the contractor sprayed a bond coat of Epoxy Asphalt
Type Id on the steel deck previously coated with inorganic zinc. Epoxy
Asphalt Type Id is a more viscous version of the Epoxy Asphalt binder
that cures to a stiffer polymer than does the binder. The inorganic zinc,
Epoxy Asphalt bond coat and the paving mix which is compacted to less
than 3% air voids form a very effective multi-layer corrosion protection
system for the steel deck. A "Spray Distributor" machine, also supplied
by ChemCo Systems, heated the bond coat components, metered and mixed
the components and sprayed them onto the deck at about 150ºC through a
|Nanjing demonstration test
The contractor placed the pavement in two 25 mm lifts with an additional bond coat between the two lifts. After carefully checking the temperature of each truckload as it reached the bridge deck, the contractor executed the paving as a normal asphalt paving operation with minor modifications.
Project specifications required that initial breakdown compaction be completed before the paving mix temperature dropped below 82°C and final compaction be completed before the mat temperature dropped below 65°C. These temperature limits assure that the paving mix receives all of its compaction before chemical reaction of the binder components significantly increases the viscosity of the binder.
| Checking pavement temperature
The Command Post had earlier required the contractor to produce a "demonstration section" by paving a three-lane 150-meter section of concrete bridge and roadway to test the equipment and methods to be used. Once paving started on the bridge there were no significant delays except those from rainy weather.
On average the contractor paved one lift about 5 meters wide the entire 1238 meter length of the bridge each day. Twelve paving passes, one per day (plus one day at the beginning for starting the bond coat), completed the project.
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