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 EngineerDesign 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

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.

Span 8 GIRDER BOTTOM

Span 8 GIRDER BOTTOM

SPAN NO 8

SPAN 7 BOTTOM GIRDER

SPAN 7 GIRDER

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. 2 Good 54 + 25 %Span No. 3 Good 54 + 25 %Span No. 4 Good 46 + 25 %Span No. 5 Good 54 + 25 %Span No. 6 Good 42 + 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 %

Span No. 8 Good, Medium to Good at most place

42 + 25 %

Span No. 10 Good, Medium to Good at 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.

Thank You