27 bridges earmarked under road rehab. programme
Guyana Chronicle
September 5, 2003
TWENTY-SEVEN (27) bridges are being reconstructed under the main Road Rehabilitation Programme Phase 11- Civil Works lot 1 which is being finance by a loan from the IDB. Under this Programme these excising bridges, which are on the Georgetown - Rosignol Road and the Georgetown Timehri Road well be demolished and replaced by new bridges. In order to do this a single lane bridge will be constructed at each site, onto which traffic will be diverted to enable the demolition of the excising bridge to take place.
The new bridge will make extensive use of precast prestressed concrete components. These are being fabricated at a plant in the MMA Compound at Onverwagt by DYWIDAG International GmbH, which is contracted to the Government of Guyana for the execution of the Project. The precast components are comprised of piles, sheet piles, deck planks and beams. The advantage of using precast components is that precasting is usually done in a factory situation, which facilitates good quality control and is lest affected by weather conditions. Prefabrication reduces the time needed for construction on site, which in the case of bridges means a shorter period of restriction of traffic passing through the site.
Prestressing is a system of reinforcing concrete, which uses steel cables instead of steel bars to provide the tensile resistance to bending in the structural component. Both concrete and steels are stressed to higher levels than in reinforced concrete with the consequence that prestressed concrete are usually slender than reinforced concrete components of the same strength. There are two main techniques in prestressed concrete, viz pretension concrete and post- tension concrete. With pretension concrete the steel cables are tension inside the form structural component and the concrete is then cast inside the form around the steel cables. After the concrete has hardened the steel cables are cut flush with the ends of the structural component. With the post- tension concrete the concrete is cast inside the form in which cables ducts have been place previously. After the concrete has hardened steel cables are placed inside the ducts and tensioned by hydraulic jack and anchored against the ends of the structural component.
For the new bridges the piles that support the abutments and the piers, the sheet piles and the deck planks are of pretension concrete, while the beams are post- tensioned concrete. Except for the Mahaica River Bridge and the Machaicony River Bridge the superstructure of the bridges is comprised of deck planks, which span the gap between abutments in the case of single span bridges.
In the case of the bridges across the Mahaicony River the main span is comprised of 6 post- tension concrete beams each weighting 65 tons, which support a reinforced concrete deck, which is cast in situ. On each side of the main span are two approach spans each 80ft. long. These are also comprised of post- tension concrete beams with a cast in situ deck.
Steel forms are used for fabricating the precast concrete components. To minimize the overhead cost that these impose on the cost of components the curing time for the pretensioned concrete components has been reduced to 30 hours, while that for the post- tension concrete beam is 3 days. The concrete used for the pretensioned concrete components has a cube strength of 6,100 pounds per square inch, while that for post- tensioned components is 7,200 pounds per square inch. These are approximately twice the strength of concrete used for reinforced concrete structures.
This is the first project, which is using pretensioned concrete components for the structure of bridges on a significant scale in Guyana. Post- tensioned components have been used in bridge construction before in Guyana, viz on the Canje River Bridge.
On a visit to the bridge project on September 3, 2003, Prime Minister Sam Hinds inspected
** The diversion timber bridge under construction at Beehive / Greenfield.
** Abutments for the relocation of the railway bridges across the Mahacia and Mahaicony Rivers, and a rail system across the abutments at Mahaicony for moving the railway bridges from their present locations to the new locations.
** Abutments for the new bridge at Belladrum
** Piles being driven for the centre pier of the new bridge at Onverwagt
** Pretensioned piles being cured and stored at the Onverwagt Plant
** 127 ft post- tensioned beams being cured and one being fabricated at the Onverwagt Plant.