RESTORATION OF EXISTING MAJOR BRIDGE ACROSS RIVER BHADAR ON NATIONAL HIGHWAY N0. 8E BETWEEN PORBANDAR AND SOMNATH- CASE STUDY

Date : 21/09/2015
S. R. SHAH J. N. PRAJAPATI
Assistant Engineer Assistant Engineer
Design Circle (R & B) Design Circle (R & B)
P. C. GANDHI S. K. PATEL
Executive Engineer Superintending Engineer
Design Circle (R & B) Design Circle (R & B)

General Information of Bridge :
Bridge location

 The bridge situated across river Bhadar on National Highway No. 8E,
between Porbandar and Somnath National Highway No. 8E, previously
known as coastal highway ,runs along the Arabian sea coast.
 The transportation of heavy vehicle becomes frequent due to proximity of
the port near by the bridge location and the bridge is only option of
connectivity .
 The overweight vehicles from mines also passes frequently over the
bridge structure in this coastal region.
 Location of bridge adjoining to Arabian Sea creates the adverse saline
environmental condition.

EXISTING BRIDGE
BRIDGE AFTER CONSTRUCTION IN THE YEAR 1985.

Details of Bridge Structure:
 The river bridge is of 480m length having 12 spans of 40m.
 The bridge has square crossing i.e. bridge is having zero degree skew
 The bridge has Post-tensioned Pre-stressed type superstructure & it was
constructed in the year 1985.
 Post tensioned prestressed type superstructure has three girder systems
with centre to centre spacing of girder at 2.5m.
 Main girder, intermediate cross girders,end cross girders & Deck slab are
provided with concrete grade M 35.

It has total 18 nos. 12T13 cables for prestressing.
 Four stage Prestressing was done.
The total depth of super structure was 2.92 M.
The super structure rests on Elastomeric bearings.
RCC pier cap with cut & ease type shape.
The wall type mass concrete pier.
 Well foundation having 8.0 m diameter is provided.

 The bridge was designed for load combination under IRC loadings
of Class AA Tracked vehicle single lane or Class A vehicle 2 lane
or 70 R vehicle single lane.
 It has been under use for over 25 years without any problem of
super structure.
 Span no. 9 collapsed from centre of span on Dated 7/04/2015.
.
Distressing of Bridge & Failure of Span 9

 Overweight vehicle produce large bending moment in the middle
portion of the span, which eventually results in flexural cracking.
 This cracks leads to corrosion of strands
 Under saline environment, these flexure crack allows more
penetration of chloride ions. Therefore, severe corrosion and spalling
of concrete is observed in the bottom portion of the girder at mid
span.

 The cracks may not harm in short term period but in long term it
leads to failure of structure.
 In severe exposed condition, Load causing Tension crack of width
0.2mm(IRC SP37-2010)is permitted.
 As per provisions of IRC:SP37-2010, over weight movement should
be infrequent, but in the present case, due to large scale mining
activities, movement of over weight vehicles become frequent, which
has resulted in severe distressing of prestressed girders.
 Result of Chloride test of concrete supports severe corrosion of
steel.

 Investigations & Tests
 After collapsed of span no 9 ,visual inspection was carried by the
committee members.
 During visual inspection, sever distressing is observed at bottom
middle portion of the prestressed girders of the span no. 7 and 8.
Minor distressing is observed on other spans, also.

 Visual observation
 The NDT test
 Rebound Hammer test
 Core test
 Carbonations, PH and Chloride content test were carried out
 Concrete Quality of existing girders were evaluated from N.D.T test.
 Probable Cube strength of concrete worked out by consultant on
basis of N.D.T & rebound hammer test.
TYPE OF TESTS PERFORMED:

Result of NDT Test:
Location Evaluation of Concrete
Quality of Girders as per NDT
Test
Probable Cube compressive
strength based on UPV/RH in
N/mm2
Span No. 1 Good 52 + 25 %
Span No. 7 Good, Medium to Good at
most place, Doubtful at few
place
Girder 1 33 + 25 %
Girder 2 38 + 25 %
Girder 3 47 + 25 %
most place
42 + 25 %
most place
35 + 25 %
Span No. 11 Good 49 + 25 %
Span No. 12 Good 42 + 25 %

 NDT results indicate that concrete strength is good at most of the
locations, except near bottom portion in middle of the span nos. 7 and 8.
Thus, probable concrete cube strength of girders are nearer to the
design requirement of M35.
 Core Test : Core test results of Girders shows compressive strength of
15 N/mm2 to 63 N/mm2 having large difference between minimum and
maximum strength.
 Carbonation, PH and Chloride Test: These tests were carried out on
web portion of girder. Carbonation was observed in 0 to 15 mm depth,
PH value was above 11 which is well within normal limit. Chloride content
of existing concrete was between 0.36 Gm/Kg to 1.2 Gm/Kg (1200
Mg/litter) which is more than 0.4 Gm/Kg as per IS:456-2000.

 The bridge have no problem of super structure since 1985 when it
open for the traffic.
 As the bridge is designed in year 1980, thickness of concrete
members of existing bridge super structure are deficient
compared to present design criteria .

Details of Existing Super structure

Details of cables at midspan of Existing
bridge(2 nos of cables grouped horizontally
& verically )

 Existing concrete member dimensions are as below:
 Uniform 175mm Thickness of Deck slab
 Thickness of Web 175mm.
 Thickness of End cross girder 200mm & Intermediate cross
girder 150mm.
 Width of bottom bulb at mid span 480mm & at support .
600mm with horizontal & vertical grouping of Cables.
 Steel of Intermediate & End cross Girder have not been
Anchored in deck slab.
.

Details of Super structure as per MOST
1990 for 40.0m span

Restoration Measures
Restoration measures are planned in
two stages are :
 Immediate Restoration
 Long-term Restoration

Immediate Restoration:
 Immediate restoration measures includes reconstruction of
collapsed span no.9.
 Based on NDT results and visual observations it has been
decided to go for reconstruction of span no. 7 and 8.
 For Reconstruction of prestress super structure two options
 Post tension prestress girders.(Cast in situ)
 Precast Pre tension /Post tension girders.(Precast)

 Reconstruction of post tensioned prestress 40.0m superstructure
using cast in situ method is not feasible due to inadequate width
pier cap.
 The distance for prestressing available is hardly @ 0.50m (C/C of
bearing 38.90m) against 1.10 to 1.20m minimum requirement.
 The other option was to opt for precast post tensioned girders.
The weight of girder section is @ 140.0t . 3-4 m depth of water
and tidal condition makes it difficult to cast girder under span &
launch it on piercap. The practical difficulty is that for 9 nos. of
girder (for three spans) financially it is not viable .

 In the above circumstances, it has been decided to construct two 20.0m R.C.C. span
in lieu of 40.0m Pre stress span.
 The additional piers P6A between pier P6 & pier P7,pier P7A between pier P7 & pier
P8, and pier P8A between pier P8 & pier P9 are supported on 6 No.1.2 M dia pile.
 Total 120.0m length of super structure, 6 span of 20.0m c/c having R.C.C. type
superstructure with three girder system having centre to centre spacing of girder at
2.5m. with deck slab resting on girders are proposed.
 Neoprene bearings are proposed.
 R.C.C. piercap & R.C.C. wall type pier is proposed.
 Replacement of all expansion joint with Strip seal type joint.
 Replacement of existing wearing coat of the bridge with uniform thickness on entire
bridge.

 Regarding the saline adverse condition following special treatments are suggested
for reconstruction.
(1) Use of slag cement for Reconstruction.
(2) Use of Fusion Bonded Epoxy coated steel in Reconstruction.
(3) Application of Deck Guard Aliphatic Anti carbonation paint on Entire
super structure of bridge.

Long-term Restoration :
 Long term restoration measures include external prestressing of the
girders.
 Remaining spans need to be restored with application of corrosion
inhibitor & polymer concrete.

Conclusion:
 Flexural cracking due to frequent plying of over-weight vehicle coupled
with aggressive environment have aggravated corrosion of prestressed
structure which eventually lead to failure of prestressed girder.
 Provision of higher cover in such saline environment shall be strictly
followed.
 For the bridge structures on the coastal road connecting to the port, they
shall be design as per IRC class special vehicle loading having Weight of
385 Tone (20 axle of 18.0t + wt of vehicle)mentioned in amendment No.
1/IRC 6 2014/ January 2014
 Strictly restricting passing of over loading vehicle on the bridge structure.

RESTORATION OF EXISTING MAJOR BRIDGE ACROSS RIVER BHADAR ON NATIONAL HIGHWAY N0. 8E BETWEEN PORBANDAR AND SOMNATH- CASE STUDY

RESTORATION OF EXISTING MAJOR BRIDGE ACROSS RIVER BHADAR ON NATIONAL HIGHWAY N0. 8E BETWEEN PORBANDAR AND SOMNATH- CASE STUDY

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RESTORATION OF EXISTING MAJOR BRIDGE ACROSS RIVER BHADAR ON NATIONAL HIGHWAY N0. 8E BETWEEN PORBANDAR AND SOMNATH- CASE STUDY