MANDOVI RIVER BRIDGE COLLAPSE
A Case Study:-
The Collapse of the Mandovi River Bridge at Pahai cab be solely attributed to the corrosion of the prestressed cable that attached the precaust concrete segments to the piers. The segmental balanced cantilever prestressed concrete method was used to construct superstructure on the bridge, i.e. one 3m. length percaust concrete segment at a time was added to either side of rapier and prestressed cables anchored them ti it (pier), keeping the pier always in a balanced condition.
REASONS FOR COLLAPSE:
The main reason for the corrosion thin layer of deck “ in-situ “ concrete wherein the perstressed High Tensile steel cables were embedded and the fact the most of the cables were not cement grouted to protect the wires and seal of the voids between the individual HT wires (a bundle of 12 wires of 7mm. diameter from a cable) and the sheathing pipe through which the cables were threaded.
Sixteen years of water seeping through the porous in-situ concrete deck, past the prestressed cable, corroded them leaving only rest powder in many places and thin corroded HT wire sections to hold the precaust concrete segment in place. There were complains in areas under the bridge during the rains just like roofs leaking in modern RCC buildings.
The bridge was already under repair in January 1986 when a gaping hole was observed in the deck slab somewhere midway along the bridge length . The corroded prestressed cables were visible
.MECHANISM OF FAILURE:
Once the corrode cables snapped or gave way and some of the concrete segment came down from one side of the pier, the other side being unbalanced, topped over blongwith the top portion of the cellular concrete pier, which most likely was not designed for unbalanced conditions. This occurred as only a few nominal diameter reinforcing bars were provided between one concrete pour (lift) of the osier and the next pour above.
The most portable collapse mechanism was that of the prestressed cable above pier 1 ( panaji side) were totally corroded. This could be seen in the inspection at the top of the inclined deck, as well as cable that were laying below. Adjacent to the pier 1. The five percaust concrete segments on the Porvorim side cantilever attached to pier 1 collapsed. Therefore on the panaji side, the precaust concrete segment along with the massive counterweight concrete block caused an unbalancing force on pier 1 and the pier bars snapped at one lift below the cantilever topping the unbalanced load onto the ground, crushing amongst people a boat below.
While the Porvorim side cantilever precaust elements of pier 1 came down, they also pulled down the adjacent precaust segment of pier 2 on the panaji side cantilever, as they were linked by means of the deck concrete wearing coat and bitumen coat and bitumen top . Obviously, the pretressed cable above pier 2 were also corrode . This resulted in unbalancing pier 2 that also snapped at one lift below the cantilever which topped and somersaulted into the water below on the Porvorim side.
Fortunately the link between pier 2 and pier 3 cantilevers must have snapped at the deck level or else a chain reaction would have caused all the other piers to topple all the way to the Porvorim side bank of the Madovi Piver similar to a pack of cards
CORROSION DUE TO POROUS CONCRETE:
The main reason for the deck “ in-situ “ concrete bearing porous is because the thickness the concrete over the precaust segments was hardly 100mm (4inch) where in perstressing cables were embedded. It is virtually impossible to compact such a thin layer of in-situ concrete without the use of a high frequency plate surface vibrate and skilled trained workers, Most contractors tend to make a watery concrete (“PATLA MAAL”) to make the concrete flow, by adding excess water ( tarmed high water – cement ratio ) during mixing instead of using “ plasticizers “ .
Plasticisers were not extensively used in India. When this collapse occupies occurred Excess water, not needed for chemical hydration of the cement, bleeds to the surface and evaporates leaving excessive minute capillary pores. Through this pores, rain water ( in coastel areas, saline water spray ) enters and starts corrosion of the steel bars or perstressed cable. Rust formation occupies retimes its’ original steel volume, causing the concrete to split as concrete is weak in tension. This splitting of concrete allows more moisture to enter, further corroding the steel bars, resulting in more rest formation, expansion and concrete re-craking, etc.
Rudting of steel spreads throughout the concret structure , leading to total collapse within 15 to 20 years as happened to the Akashdeep building near Opera House in Mumbai and as is happening to most Reinforced Cement Concrete (RCC) and prestresed concrete (PSC) structures throughout the country . This problem is not restricted to Goa only.
POOR IMPLIMENTATION OF GOOD DESIGN :
The Mondovi River Bridge was Russian in design and the first to be used in this country. The mathematics, calculations and drawings ( i.e. on paper) of this design are indeed commendable, leading to an economical and elegant structure. However knowing the we Indians lack practical training facilities for our workers, supervisors and civil engineers, it would have been advisable to have imported a few Ressian technicians and engineers to physically carryout and supervise the work for making, placing and compacting the thin layer of deck concrete in which the prestressed cables were embedded, to make the concrete deck impervious and durable.
The prestressing anchorage cones were located at the ends of each precaust T- Beam slab segment. The prestressing cable ducts tapped from the anchorage cones to the duck slab. Hence most of the prestressing cable were subsequently embedded in the in –situ concrete deck slab, which consisted of porous concrete.
The same Russian technicians and engineers would also have ensured that the protstressing cables were properly cement – grouted for full protection. Since the anchorage cones of the prestressing cables were located at both ends of the T-Beam slab segments, they were virtually increasable and it was most impossible for the workers as well as the supervising PWD engineers to reach these spots to carry out cables in simply supported beams, because cantilever prestressing cable ducts need airvent pipes at the topmost points of the cables to ensure that the cable are fully grouted.
POOR CONSTRUCTION JOINTS:
It was also observed that the successibe lefts of the hollow cellular Piers successive lifts were cast by the jump –form method without proper construction joints. Before the next lift of the concrete is placed, the top surface of the concrete of the previous lift has to be thoroughly hacked (roughened) to remove the laitance (cement slurry ) that comes to the surfaced with vibration, so joint, the next lot of concrete has to be placed with care to prevent segregation and then vibrated fully. It was also observed that the concrete of the upper lift of pier 1 was honeycombed at the construction join due to segregation when pouring concrete from a height . This is a common occurrence in building columns. These poor construction joints also permit ingress of moisture which corrodes the reinforcement bars and PSC cables.
LAKE OF TRAINING:
The main cause of poor construction in our country is that it is not mandatory for academically trained civil engineers to undergo a prescribed minimum three years of practical apprenticeship training programmer under the guidance of a senior practicing site construction engineer, as is done in other countries where engineers have to appear for a professional examination before acquiring the status of a Chartered Civil Engineer. There are very few senior experienced construction engineers at site to mould and train the young graduates or diploma engineers who are sent to construction work sites. Very few senior experienced construction engineers demonstrate each new item of construction work at least onece.
RCC and PSC are dangerous treacherous building materials if used with out knowledgeable and strict supervision. Concrete disintegrates slowly within 10 to 15 years, i.e. long after the normal one year guarantee period. Hence the term treacherous is used. The only guarantee that is valid is the 1.5 to 2 per cent project cost that must be spent by the owner of the project, by engaging a strong monitoreing team to strictly supervise the construction work at every stage of construction, it is useless to appoint a young graduates engineer to supervise the contractors engineers. It is more like the blind leading the blind.
LACK OF COMPETENT SUPERVISION:
A clerk of Work or a resident engineer posted at site on a full – time basis will ensure that our structures are built to specifications. Se at the site along with their experienced construction Foreman , so that our young engineers receive correct practical training. Just ensuring that our designs and drawing match up to international standards is sufficient. Our construction also must be at par with international standards.
Concerts must be made scientifically with minimum water cement ratio of 0.45 to prevent porous concrete, placed correctly without segregation, compacted (vibrated) fully without honeycombing, cured adequately to get desired hydration of ailment and tested regularly to ensure that the design compressive strength has been achieved. Th monitoring must be done throughout the construction period if our concrete structure are to last a hundred years or more free free from maintenance and repairs. Otherwise we should stop building with unscientific manmade concrete and revert to back using natural stone or bricks, which last thousands of years !.
Precaust T-Beam concrete segments used in the Mondovi River Bridge made under factory conditions in a casting yard were excellent as could be seen from the few extra precaust concrete segments discarded in saline water for 16 years when the bridge was under construction. From (shutter) vibrators must have been used under strict supervision for the concrete to be dense and impervious. It is a pity that an excellent bridge design and reasonably good workmanship during then initial stage of construction (well foundations, piers and precaust concrete segments) were brought to naught at the fag-end of the construction period when the difficult deck “in-situ” concrete had to be done, along with subsequent grouting of the prestressed cables from inaccessible locations of the anchorage cones of each segment.
RESTORATION NOT RECOMMENDED:
As the prestressed cable in most of the balanced cantilevers were corroded, the precast concrete segment of the superstructure were in a precarious condition and likely to fall by on account their own weight. Hence river traffic below the bridge was not safe. The superstructure could not be repaired. Hence the bridge had to be demolished and rebuilt.