<i>Jamuna Bridge </i>
The report entitled "Cracks on Jamuna Bridge" was published in your newspaper on Feb. 13, 2008. I have examined the photograph that you have printed with the report as well as other photographs printed in other newspapers. I do not have any information about the design analysis of this bridge. By surfing the Internet I came across some information about soil condition, piling system, girder type etc., but not the design criterion or the thickness of slab. The photograph you have printed is of a contraction joint and not of an expansion joint as is generally thought to be by the non-technical people. When water curing stops after about 28 days, the concrete hardens and shrinks. Then when the temperature is high it expands. At lower temperatures it again shrinks. This cycle repeats every day throughout the year. Cracks occur when contraction joints, necessary to absorb the stress of shrinking, are at improper distance or not sufficiently grooved. Test observations have shown that a 100 meter long concrete contracts by about 55 millimetre when the temperature is lowered by 38 degree C. If there are no contraction joints at all in this 100m stretch, cracks may appear in many places. The combined width of these cracks will be about 55mm. In order to avoid appearance of cracks and as a result the destruction of concrete in a short period of time, contraction joints are provided at about 15m to 25m distance and reinforcing steel bars are placed about 20mm below the top surface, even if by structural analysis these are not required. This provision can be confirmed by the ACI (American Concrete Institute) codes. A contraction joint, as specified by AASHTO (American Association of State Highway and Transportation Officials) and practised throughout the world, has the following features: 1) It is supposed to be placed at around 15 meter to 25 meter centre-to-centre. 2) The groove for this joint shall better be V-shaped, its top and bottom widths being 12mm and 8mm respectively. 3) The minimum depth of groove shall be 25mm or at least one-fourth the slab thickness - whichever is larger. For example, if the slab thickness is 300mm, the depth of groove shall be 75mm. A shallower depth is superficial and is not effective to control cracks. 4) Top steel reinforcing bars - never the bottom bars - shall be cut at the grooves. If the top re-bars are not cut, the slab surface cannot move freely during shrinkage, causing cracks at the slab surface. 5) The grooves shall be filled with a remoulded flexible material to prevent rainwater from entering the groove. 6) In each side of the long groove, a steel made double- L shape ( half of Nazi swastika mark) shall be placed at the top of slab and fixed with the use of anchor bolts. 7) Cement grouting in between slab top and swastika bottom may be needed for proper alignment. 8) The top of steel shapes shall have saw-teeth. These shapes shall be placed in such a manner that the teeth of each shape can enter to or come out from the teeth of shape at the opposite side whenever there are movements of concrete slabs in the horizontal direction. 9) The height of these swastika-shapes shall be 40mm from the surface of slab. 10) There shall be two layers of asphalt paving on the top of concrete slab, the total thickness being 40mm. This height is same as the height of steel swastika shape (see above in 9). I do not remember the source, but somewhere I have read that a total of only seven contraction joints have been provided to the whole slab length. If it is true, then, in my opinion, it was a gross mistake. However, it can be somehow rectified. I would like to suggest the following: 1) Cut the concrete slab along transverse direction with the help of electric or gasoline powered circular saws and create grooves. 2) The centre-to-centre distance shall not exceed 25 meter if there are top steel bars in the already built concrete slab. If steel bars have not been provided, my suggestion is that the centre-to- centre distance shall be kept within 15 meter. 3) Since the slab is already built, therefore, the groove can be made U-shape if it is difficult to make V-shape. The groove width can be maintained at 12mm. 4) Other features shall remain the same as suggested. In all the river crossings that I have seen in Bangladesh, be they box culverts, girder or truss bridge, the top concrete slab is not covered with asphalt paving. It is very much an unusual practice. I suggest that not only the Jamuna Bridge, but also all other bridges, be paved with asphalt. Without this measure the concrete slabs wear out easily.
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