EQUIP-WORLD-RS-MAY-2015-FINAL

Eliminate approach ‘end bumps’
with new slab system
Differential settlement in Louisi-
ana leads to what are called “end
bumps” at the approaches to bridg-
es there, posing a safety hazard as
well as being disruptive to motor-
ists. But researchers at Louisiana
State University say they’ve come
up with a new construction design
incorporating precast deck slabs
paired with geotextile-reinforced
approach embankments that will
solve the problem.
These bridge approaches, which
connect the bridge deck to the
adjacent pavement, are normally
built with reinforced concrete
slabs. The Louisiana Department of
Transportation and Development,
however, received complaints
about the roughness of the transi-
tion between the bridge abutments
and embankments:
“Field observations indicated
that either faulting at the roadway
pavement/approach slab joint (R/S
joint) or a sudden change in the
slope grade at the approach slab/
bridge joint (S/D joint, shown in
Fig. 1) causes this ‘bump’,” say the
research authors1
.
To solve the problem the LaDOTD
considered changing the design of
approach slabs where differential
settlement is expected. The state
wanted to find a solution that makes
approach slabs strong enough to
allow them to lose a portion or all of
their contact supports without caus-
ing detrimental deflection.
“In this solution, the flexural
rigidity of the approach slabs is
increased through increasing both
the slab thickness and the rein-
forcement ratio,” the authors write.
“Consequently, some embankment
settlement will be allowed without
decreasing the ride quality. As a
result, the slab dead load and traffic
live loads will be carried by the two
ends of the slab rather than distrib-
uted over the length of the slab.”
Accordingly, the local soil pres-
sures beneath the strip footing (or
“sleeper slab”) increase, resulting in
an increase in the faulting deflec-
tion. This can be solved by use of
road science | by Tom Kuennen
EquipmentWorld.com | May 2015 41
TRB: NEW RESEARCH CHANGING
HOW YOU WILL BUILD BRIDGES
For more information about the 2016
meeting, or to obtain the full papers,
visit www.trb.org.
E
liminating the “end-bump” in construction
of bridge approaches ... precast deck pan-
els speeding bridge construction ... quick
seismic retrofits for bridge piers ... and how
rockets can speed construction of suspen-
sion bridges in isolated areas.
Each of these topics will change how bridges will be
built in the future, and they were among the themes in
bridge design and construction that were presented at
the 2015 Transportation Research Board meeting.
1
Mitigating the Bridge End Bump Problem:A Case Study of a New Approach Slab System with
Geosynthetic Reinforced Soil Foundation, by Qiming Chen, Ph.D., P.E., and Murad Y.Abu-Farsakh,
Ph.D., P.E., Louisiana Transportation Research Center, Louisiana State University, Baton Rouge.
Fig.1: Illustration of approach slab and its interaction with soil in Louisiana.
Image:ChenandAbu-Farsakh

geosynthetics to reinforce the soil
beneath the sleeper slab, thereby
increasing the soil’s bearing capac-
ity, helping redistribute the load
to wider area, and thus reducing
the sleeper slab’s settlement, they
researchers say.
To validate this approach, several
research projects were initiated by
LaDOTD. Field performance – in-
cluding deformation and internal
stresses of the concrete slabs, contact
stresses between slab and embank-
ment, stress distributions within the
reinforced soil foundation, and strain
distributions along the geogrid – was
monitored for two years.
The authors concluded:
• The west approach slab of the
Bayou Courtableau Bridge, with
the new design method, retained
its contacts and supports from the
embankment soil during the first
static load test (at the time when
the bridge was ready for traffic).
However, during the second static
load test, after about a year and
half, the west approach slab lost
most of its support from the em-
bankment soil.
• On the other hand, the east ap-
proach slab, with the traditional
design method, showed slightly
gradual loss of its contacts from
the embankment soil starting from
bridge abutment side towards the
pavement side.
• The roughness profiles demon-
strated better performance of the
new approach slab system (west
approach slab) compared to the
traditional design method (east
approach slab) with much lower
International Roughness Index
values.
Connections, grout influence
prefab deck panel success
Accelerated bridge construction
depends on the use of precast, pre-
stressed and otherwise prefabricated
bridge elements and systems, but
their success depends on how well
they are implemented. New research
from TRB2
indicates that connections
and grouts used are the weakest
links in the process, but application
guidance is available that will ensure
long-life performance.
Noting that prefab deck panels
have gained popularity, the authors
of the paper point out the primary
challenge for deck systems is the
connectors, typically made using
interlaced connector elements and
field-cast grout. “Poor detailing and
design considerations have been
shown to pose problems with deck
durability, serviceability and ultimate
capacity,” the authors say.
For this research, a series of precast
deck panel connection tests were car-
ried out at Federal Highway Admin-
istration’s Turner-Fairbank Highway
Research Center to further understand
deck-level connections under realistic
performance demands. Several de-
sign variations – including shear key
shape, reinforcement type, joint mate-
rial type, and precast surface prepara-
tion – were investigated.
“Seventy two large-scale, deck-level
connection assemblies were tested in
four-point bending and subjected to
cyclic crack loading, fatigue loading,
and monotonic loading until failure,”
the authors say. The three loading
protocols allowed researchers to as-
sess the durability, serviceability and
ultimate strength of the connections.
Their conclusion: the performance of
these connections “is heavily influ-
enced by the grout and the precast
component interface surface prepara-
tion.” They also determined:
• The selection of grout materials is
a critical design consideration for
deck-level prefabricated bridge
elements and systems connec-
tions. This assumes that lap splice
lengths have been selected such
that bond failure does not occur.
• Depending on the grout material
selected, surface preparation of
precast concrete deck panels can
have a significant impact on tensile
bond resistance of the panel-to-
grout interface.
• Although epoxy grout and ultra-
high performance concrete grout
systems have higher initial cost,
they could provide better value
when long-term performance
and required maintenance are
considered.
• Non-shrink cementitious grout and
magnesium phosphate grout may
lead to durability issues as a result
of poor bond to precast concrete
regardless of surface preparation,
and poor performance under
repeated loading.
• An exposed aggregate surface
preparation preformed the best for
promoting bond between precast
concrete components and cementi-
tious grout connection materials.
• The epoxy grout exhibited good
bond with precast concrete regard-
less of the surface preparation.
Hybrid jackets for quake-
damaged bridge columns
Hybrid jacketing – made up of a thin
cold-formed steel sheet wrapped
around an earthquake damaged
reinforced concrete column – of-
fers a fast solution to getting bridges
reopened following a seismic event,
says one research team.3
Such a jacket, which is also sup-
ported on the outside by prestress-
ing strands, was tested using a
large-scale RC column with a defi-
cient lap splice (where two pieces
of rebar are overlapped to create
continuous reinforcement), typical
road science | continued
May 2015 | EquipmentWorld.com42
2
Experimental Evaluation of Prefabricated Deck Panel Connections, by
Zachary B. Haber, Ph.D., Professional Service Industries, Inc., and Benjamin A. Graybeal, Ph.D., P.E.,
Team Leader, Bridge and Foundation Engineering, at FHWA’s Turner-Fairbank Highway Research
Center, McLean,Va.
3
Hybrid Jacketing for Rapid Repair of Seismically Damaged Reinforced Concrete Columns, by
Mostafa Fakharifar, Genda Chen, Ph.D., P.E., Mahdi Arezoumandi, Ph.D., and Mohamed ElGawady,
Ph.D., Missouri University of Science and Technology, Rolla.

May 2015 | EquipmentWorld.com44
of pre-1970 bridge constructions
in the central United States. The
testing was done in mid-Missouri, a
seismically active state.
Results from original and repaired
columns were compared by several
factors, including strength, stiffness,
ductility and energy dissipation. The
authors concluded the hybrid jacket
was effective in “restoring structural
behavior of the damaged column
to prevent bridge collapse.” Other
points in its favor: it’s a lightweight,
quick to field product, cost-effective
solution that can be implemented
in hours.
The researchers also found:
• The initial stiffness is [only]
partially restored, due to existing
damage in concrete and rein-
forcement.
Seismically damaged column repair procedure with the hybrid confining jacket. From left to right, damaged
column; patched column with repair grout; sheet metal wrapping; prestressing strands application
road science | continued
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EquipmentWorld.com | May 2015 45
4
Use of Rocket Traction Method to Span Pilot Rope for Long Span Suspension Bridge Construction in Mountainous Area[s], by Dajin Guo and Xiao Li,
National Engineering Laboratory For Surface Transportation Weather Impacts Prevention,Tuodong Rd., China;Wenhuan Zhou and Mingfang Hu, Re-
search Institute of Highway, Ministry of Transport, Beijing; Zhengfu Yu and Guobang Xia , Puxuan Highway Project Headquarters of Yunnan Province;
Wei Liu, Beijing Zhongjiao Ruida Technology Co., Ltd., Beijing; and Biyu Yang, Department of Transport of Yunnan Province, Kunming.
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• The confining pressure exerted
by prestressing strands is ad-
equate for shear transfer between
the steel jacketing and column;
no adhesive epoxy or dowel
reinforcement is required
• Sheet metal provides the re-
quired bearing strength and
prevents cover concrete spalling
and penetration of the prestress-
ing cables
• Prestressing cables could sustain
the active confining pressure up
to 6 percent lateral drift with no
significant prestressing force loss.
Build remote suspension bridges
using rocket-powered cables
China’s centuries-old prowess in
gunpowder and fireworks now has a
new application: the use of rockets to
fire the “pilot” or initial guide ropes
for suspension bridges across voids in
remote, mountainous areas, accord-
ing to a 2015 TRB paper from China.4
There are many pilot rope erection
methods as part of the suspension
bridge construction; these include
water towing, manual hauling and air
traction, the authors say. But these
all may not be available for construc-
tion of suspension bridges in rugged,
remote areas, now being reached by
China’s growing highway system.
“For constructing long-span
suspension bridge[s], erection of
[a] pilot rope is the first step of the
superstructure construction, which
provides the foundation for the
load-bearing cable catwalk traction
system to be constructed [follow-
ing] the pilot rope,” the authors
note. “However, erection of pilot
rope under certain mountainous
terrain conditions can be extremely
challenging.”
The Yunnan Puli long span bridge
is a 3,412-foot suspension bridge in
the Karst region of China, cross-
ing a canyon more than 1,968-feet
and extremely steep on both sides.
Ever-changing weather and thick
vegetation covering the ground
made it difficult to erect the pilot
rope using the traditional methods.
After comparing three types of
erection methods, the researchers
experimented with the rocket trac-
tion method for erecting the pilot
rope. The project demonstrated
that the rocket traction method has
benefits, including easy prepara-
tion, fast erection, is cost-effective
and environmentally friendly, and
is particularly suited for use in the
mountainous region.

EQUIP-WORLD-RS-MAY-2015-FINAL

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