IABSE Congress Ghent 2021 ‐ Structural Engineering for Future Societal Needs

Development of ductile cast-iron bridge slabs for onsite replacement
Shinsuke Akamatsu
Chief Researcher, Hanshin Expressway Research Institute for Advanced Technology, Osaka, Japan
Hironobu Tobinaga
Section Manager, R&D Center, Hinode,Ltd., Saga, Japan
Kunihiro Oshima
Manager, Taisei Corporation, Tokyo, Japan
Norihiro Nohara
Engineering & Development Office Manager , SATO TEKKO CO., LTD., Toyama, Japan
Contact: shinsuke‐akamatsu@hit.or.jp

Abstract
Most of the road bridges in Japan were constructed in the 1960s and 1970s, and many of them havebeen in service for more than 50 years. The reinforced concrete slabs of such bridges have beendamaged and deteriorated by heavy traffic. Although these slabs have already been reinforced withsteel plates, they have deteriorated further and need to be replaced.
The renewal of reinforced concrete slabs leads to increase of dead load, the fatigue damage at welds
has not been completely solved in the case of steel slabs, and the social loss due to long‐term roadclosures at the renewal construction of the slabs on urban highways. Therefore, there is an urgentneed to develop an alternative slab that is lighter in dead weight, has good fatigue resistance, andcan be installed rapidly.
The proposed slabs in this paper are made of ductile cast‐iron instead of mild steel. Ductile cast‐ironbridge deck can be light‐weighted like mild steel, can show high fatigue resistance with improved
residual stress and detail by integrally forming into complex shapes, and can be rapidly installed tothe existing bridges by bolts.
The Hanshin Expressway has been conducting research and development for the practicalapplication of ductile cast‐iron slabs. This paper will introduce a ductile cast‐iron deck, and willshows the study results of the details of the slab panels, the slab‐to‐slab connection method, andthe slab‐to‐girder connection method.
Keywords: ductile cast‐iron bridge deck; slab renewal; connection method; construction method

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IABSE Congress Ghent 2021 ‐ Structural Engineering for Future Societal Needs

12Introduction

The reinforced concrete slabs of Japanese roadbridges have been damaged and deteriorated byheavy traffic. Although these slabs have alreadybeen reinforced with steel plates, they havedeteriorated further and need to be replaced.

2.1 Structural properties
The cast iron slab is a weldless structure that takes
advantage of the high degree of freedom informing. As shown in Figure 2, the root of thelongitudinal ribs and the intersection of the mainand sub ribs are arc‐shaped in order to reducestress concentration. The radius of curvature of theR shape was set to 10 mm based on FEM analysis
to determine the optimum shape [1]. The unit area
weight of the slab is 250 kg/m2, which is about thesame as that of a steel slab.

However, there are some problems with therenewal of reinforced concrete slabs: 1) it leads toan increase in dead load, 2) the fatigue damage ofwelds has not been completely solved with steelslabs, and 3) the social loss due to long‐term roadclosures is a problem with the renewal of slabs on
urban highways. Therefore, there is an urgent needto develop an alternative slab that is lighter in deadweight, has good fatigue resistance, and can beinstalled rapidly.
The proposed slabs in this paper are made ofductile cast‐iron instead of mild steel. As shown in
Figure 1, ductile cast‐iron has excellent ductile
performance, which is equivalent to that of mildsteel. It can also be integrally molded into complex
shapes and has high fatigue resistance due toimproved residual stress and detail. However, themethod of connecting the slab to the existing girder
and the construction method to realize rapid
construction have not been developed.

Figure 2. arc shape of the corner

2.2

stress[N/mm2] σ

600×××mild steel SM490(number refer to σu)cast‐iron FC200

200Slab-to-slab connection method

The size of the slab panels to be cast isapproximately 1200 mm in the longitudinal
direction and 1700 mm in the transverse direction
due to manufacturing constraints, and multiple
panels are joined to form a slab member. The
structure of the joint between the slab panels is nota welded joint, which may cause cracking, but ahigh‐strength bolt friction joint. The friction
coefficient of the cast‐iron member is 0.45, whichis specified in the Japanese Specifications andCommentary for Highway Bridges II, Steel Bridge
Edition [2], when the contact surface is coated withinorganic zinc‐rich paint [3].

In this study, the connection method andconstruction procedure between the slab and theexisting girder were investigated for the practicaluse of the ductile cast‐iron slab.

400

Cast iron slab

ductile cast‐iron FCD500

The joint in the longitudinal direction of the bridge
is double‐sided frictional joint using double
connection plates, as shown in Figure 3(a), because
the joint resists the bending moment caused bytraffic loads. The upper surface of the deck plate
was made to be less than the thickness of theconnection plate (13 mm), taking advantage of thehigh degree of freedom in forming of cast iron, so001020strain[%] ε3040Figure 1. Stress‐strain curves of mild steel(σu=490MPa) cast‐iron(σu=200MPa) ductile cast‐iron(σu=500MPa)

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IABSE Congress Ghent 2021 ‐ Structural Engineering for Future Societal Needs

that the pavement over the joint area would not bethin.

190 1335 60 60

Since the bending moment due to traffic load issmall and the main girder action stress iscompressive in the case of simple girders, the joint
at the joint line perpendicular to the bridge axis isan L‐shaped structure that can resist the shear
force between the slab panels, as shown in Figure3(b). For this joint structure, FEM analysis with axial
force introduced to the high strength bolts
confirmed that there was no separation between
the panels under truck load [1].

Figure 4. Fatigue test of slab panela－alongitudinal direction

A－A
transverse direction
30035 70 90 70 35
a951452.3.2

The fatigue resistance of the slab members,
including the joints, was verified by wheel loading
tests of the slab members on the main girder at 3m intervals, as shown in Figure 5. After 2 million
cycles of loading, there was no fatigue damage andno change in the stress and displacement at themeasured position, confirming the high fatigueresistance.

95
A

TCB M22(S10T)

10 a35 70 90 70 35
300

main rib

Wheel loading fatigue testA(a) Joint line of longitudinal direction(b) Joint line transverse direction
Figure 3. Structure of the joint between slabpanels.

2.3

Fatigue resistanceFigure 5. Wheel loading test of slab member

2.3.1 Fatigue test
The fatigue resistance of the slab panels wasconfirmed by fatigue tests, as shown in Figure 4.
The load was applied with an amplitude of 154 kN,
which is the allowable stress at the tip of the main
rib. No cracks were observed at the base of thelongitudinal ribs or at the intersection of the mainand sub ribs under 10 million cycles of loading[1].

3Construction Method for Renewalof Existing Reinforced ConcreteSlabs

3.1

Conditions for construction methodstudy

In the renewal of slabs, it is required to ensure theserviceability as much as possible where it isdifficult to close the entire road. Even when partial3

IABSE Congress Ghent 2021 ‐ Structural Engineering for Future Societal Needs

serviceability is ensured, it is necessary to minimize
the construction period while ensuring highconstruction safety in order to further reduce thesocial impact.

【Step1: Remove the existing reinforced concrete slab】

In this paper, the construction steps is studied forupgrading the reinforced concrete slabs of simple
live load composite I‐girder bridges, which were
mostly constructed in the 1960s and 1970s, to castiron slabs while ensuring serviceability. In addition,
the construction ability was verified by a large‐
scale construction test using a full‐scale specimen
under the conditions of the removal method of theexisting reinforced concrete slab as well as theconnection structure between the existing main
girder and the cast iron slab which can minimize
the construction period and ensure reliable
construction safety.

3.2【Step2: Treatment of the top surface of the existing girder】【Step3: Installation of cast iron slab members】

Construction steps

The construction steps for replacing the reinforced
concrete slab with a cast iron slab on a four‐laneroad are shown in Figure 6. In Step 1, one lane isclosed on each side and the existing reinforced
concrete slab is cut out and removed. Then, in Step
2, the top surface of the existing girder was treatedby removing stud bolts and surfacing to install thecast‐iron slab. In Step 3, the new cast‐iron slab
members were installed.

3.3

Figure 6. Construction steps for reinforced concreteslab renewal

Removal of existing reinforced
concrete slab

between the cutting height and the top surface of
the upper flange to avoid interference with theupper flange and main girder joints. In thehorizontal cutting test using the specimen,
concrete removal tests were conducted on the top
surface of the upper flange with a margin height of
25 mm and 60 mm. When using a small chipper, thechipping speed was 6.5 m2/h with a clearance of 25mm, while it was 0.75 m2/h with a clearance of 60mm. The influence of the margin height on thechipping time is significant, and it is desirable toreduce the margin height as much as possible,taking into account the condition of the remained
reinforcement of the concrete blocks and theconnection plate on the upper flange of the target
bridge.

After the slabs between the main girders areremoved, the concrete blocks left on the main
girders are cut horizontally with a wall saw, asshown in Figure 7(a). Water is used as a splash
prevention measure when cutting the concrete
blocks, so consideration must be given to thesplashing of muddy water. The noise level is 85 to90 dB at a distance of 6 m. Therefore, it is necessary
to take soundproofing measures according to thesurrounding environment. The horizontal cutting
speed was about 3 m2/h, which is about 60% of thevertical cutting speed, as a result of cutting aspecimen with welded studs of 22 mm diameterusing a large wall saw with a blade diameter of1200 mm.

After concrete block removal, the studs on themain girder were cut at the base of the stud using

The horizontal cutting height of the concrete blocksof the main girder should have a margin of height4

IABSE Congress Ghent 2021 ‐ Structural Engineering for Future Societal Needsa special saw as shown in Figure 7(b), and theremaining welds were removed using a disc sander
and then finished with a spark disc as shown inFigure 7(c).

per piece, and the finishing of the stud weld was
124 seconds per piece. However, the cutting speedby the special saw varied greatly depending on thewear and tension of the saw blade.

In the trial construction using the specimen, thecutting speed by the special saw was 87 seconds

3.4

Connection between Slab and GirderAs shown in Figure. 8, the main girder and slab
members are connected by high strength bolts
through an inverted T‐shaped support, after theupper surface of the girder is drilled and a filler
plate is installed. The height adjustment during
construction is done by the web height of theinverted T support, and the filler plate is tapered toaccommodate the cross‐slope. The inverted Tsupport is made of cast iron with corner radius toensure durability against deflection of the slab
under live load and change in deflection angle on
the main girder due to temperature effect.
The connection structure at the end of the girder
for the composite girder slab was designed tostrengthen the horizontal shear resistance so thatthe required number of bolts could be secured
against shear forces due to temperature effects.
Scratches and unevenness on the top surface of theupper flange are corrected by using epoxy resin
adhesive. In this case, the effect of the adhesive is
not considered because the design method has notbeen established yet.

(a) Horizontal cutting of concrete blocks on main girder

cast‐iron slab(b) Cutting of studs on main girder
Girder End

Span center

Connection plate（Steel）
Inverted T support（cast‐iron）
Filler plate（Teper machining）

Existing main girder

Figure 8. Connection structure of slab and girder(c) Surfacing with spark disk

Figure 7. Removal of existing reinforced concreteslab
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IABSE Congress Ghent 2021 ‐ Structural Engineering for Future Societal Needs

4studied from the viewpoint of improving safety
during construction and shortening theconstruction period by reducing the crane
operation time.
3) Tolerance in the bridge transverse direction:
The error during assembly in the direction
perpendicular to the bridge axis was less than
1.5mm for 8.0m. This error could be absorbedby the bolt holes, so it was confirmed that it
would not be a problem during construction.
4) Tolerance in the bridge longitudinal direction:
It was confirmed that the filler plates installed
between the slab members in the bridge axial
direction could be installed and that the error
could be absorbed by varying the plate
thickness. However, it is necessary to consider
the procedure to prevent the plates fromfalling because the plates are installed from thetop and supported at the bottom of the slab
after the slab members are placed in position.

Large-scale assembling test

In order to verify the assembly of the slab panels
and the erection of the assembled slab memberson the girder, construction tests were conductedusing full‐scale members. The dimensions of theslab panels were determined based on theperformance of the current manufacturing
equipment. The main girders to which the slab
members are attached are steel I‐girders (threemain girders) with 3 m spacing between the maingirders, which have field joints and verticalstiffeners.
Figure 9(a) shows the assembly of 10 panels, two in
the longitudinal direction and five in the transverse
direction. The dimensions of the assembled slabmembers were 2.3 m in the longitudinal direction
and 8.0 m in the transverse direction fortransportation purposes. The bolt holes on thesides of the slab panels were drilled duringassembly instead of during the casting stage inorder to ensure assembly accuracy.
Figure 9 (b) shows the erection of two blocks ofassembled slab members on the girder. A 10‐mm
filler plate was installed at the field joint of the slab
members in the longitudinal direction of the bridgeto accommodate construction tolerance. The
following is a summary of the results of the large‐
scale assembly test.
1) Construction ability: It was confirmed that thefield joints of the main girders and the areas
where the vertical stiffeners and inverted T‐
shaped supports were close to each other
could be installed by adjusting the placementof the supports. In the narrow area where thejoints between the slab members and the main
girder are close to each other, the bolts can beinstalled without interfering with each other,but it is necessary to pay attention to thetightening order.
2) Time required for slab erection: After the slab
members of the first block were installed, thetime required for erection of the adjacent slab
members of the second block wasapproximately 80 minutes. In this test, theheight of the slab was adjusted while the slab
was suspended by a crane. The structure of thesupport for height adjustment needs to be(a) Assembly of slab members(b) Erection of slab members

Figure 9. Large‐scale assembly test

6IABSE Congress Ghent 2021 ‐ Structural Engineering for Future Societal Needs

3) Field errors can be dealt with by installing filler
plates between the slab members.
4) In case of damage after in‐service, partial
replacement is possible by preparing filler
plates in advance.

In addition, it was verified whether this slab can bepartially replaced when the slab member isdamaged by some unexpected influence afterservice. In this verification, the slab panels werepartially removed from the specimen after thelarge‐scale assembly test and reinstalled. Figure 10
shows the removal of the slab panel using a jack.
The partial removal of the slab panel was possibleusing a jack, but the filler plate for absorbing theconstruction error could not be installed during thereinstallation due to the release of internal stress.
This indicates that partial replacement of cast ironpanels is possible by preparing new filler plates.

There are still some technical issues to be resolved
before applied in the field, such as the detailing of
the attachment points for accessories such as wall
railings, telescopic devices, and drainage basins,
but the ductile cast iron slabs is being developed
quickly and carefully with the goal of applying it tofuture slab renewal projects.

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Acknowledgements

The financial support for the present study fromCommittee on Advanced Road Technology,
Ministry of Land, Infrastructure, Transport, andtourism gratefully acknowledged.

7[1] Ministry of Land, Infrastructure, Transport andTourism: New Road Technology Council,
Research and Development of Technology for
Improving the Quality of Road Policy,
Development of High Performance Cast IronSlab
https://www.mlit.go.jp/road/tech/jigo/h26/pdf/report26‐5.pdf[2] Japan Road Association: Specifications for
Highway Bridges: Part 2 Steel Bridges. 2012.
[3] Y. Ikeda, T. Yamaguchi, E. Saeki, H. Tobinaga, T.
Hidaka: Slip resistance of high strength bolted
frictional joints with spheroidal graphite castiron, Journal of Structural Engineering, Vol.
62A, 2016.3.
[4] Committee on Steel Structure, Japan Societyof Civil Engineers, Subcommittee 1 ofResearch and Study Subcommittee on Road
Bridge Deck Slabs, Wheel load running test offloor plate, Rationalization of Design andImprovement of Durability of Road Bridge
Deck, 2004.

Figure 10. Removing a slab using a jack

5ReferencesConclusion

Ductile cast iron slabs, which are light and highly
durable due to their weld‐less structure that takes
advantage of their high degree of freedom in
forming characteristics, have been verified in termsof load‐bearing performance and durability as slab
members.
In this study, a method of removing the existing RCslab, a method of connecting the ductile cast‐ironslab to the main girder, and a full‐scale large‐scaleassembly test were conducted in order to apply thecast‐iron slab in the field. The results of thesestudies are described below.
1) Horizontal cutting at the time of removal of theexisting RC slab can be done as close as
possible to the top of the top flange to reducethe construction time.
2) The connection between the slab and thegirder can be made by using cast iron inverted
T‐shape supports and avoiding the vertical
stiffeners and contact plates.

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