The 1km long multi span steel box girder Moerdijk Bridge, over the Hollands Diep river near Dordrecht in the southern Netherlands, is a major structure on the vital E19/A16 European highway through Holland and Belgium. The current 45m wide, dual three lane bridge, opened in the late 1970s and built on the original concrete piers of the previous structure, is believed to be one of the most 
heavily trafficked bridges in Europe, carrying around 120,000 vehicles a day. During routine inspections of the nation’s steel bridges fatigue cracking was found in Moerdijk’s thin steel deck plate threatening the structure’s integrity, leading the Dutch Ministry of Transport to carry out extensive repairs and strengthening involving Doornbos and Conjet hydrodemolition equipment.
The restoration, costing around €10.5 M and designed by the Dutch Ministry of Transport, involves repairing the cracks followed by what is believed to be one of the first major applications of a special thin very high strength concrete overlay wearing course of 105MPa. The 50mm to 70mm thick high performance, steel mesh reinforced concrete overlay is based on a secret blended recipe and additionally reinforced with steel and polyethylene fibres. It aims to spread the traffic loading and stiffen the 12mm thick steel deck plate, eliminating future fatigue problems and extend the life of the bridge.
Main bridge repair contractor Haverkort Voormolen B.V. used subcontractors to place the heavily reinforced, high performance concrete in a series of 100m long and 8m wide bays. The finished surface proved to be bumpy and slippery and was scarified by contractor and equipment hirer Doornbos Equipment using high pressure water rotors operating at 2,800bar to blast and roughen the surface. Just a few weeks after completing the first northbound three lane carriageway the concrete overlay started breaking up in two of the 20 bays, forcing the Ministry to stop the entire project and investigate the cause of the failure and urgently devise a repair.
Core samples, initially taken in the affected bays, showed extensive porosity, honeycombing and voids around the steel reinforcement and gaps up to 30mm deep between the steel deck and concrete overlay. Some gaps and voids were filled with water. Total uniform bonding between the concrete overlay and steel deck plate is vital to ensure the deck is rigid and conforms to the original restoration and strengthening design. “The ministry and Haverkort Voormolen formed a partnership to investigate the cause of the problems, including the possible influence of the traffic vibrating the deck during concreting,” says Haverkort Voormolen project engineer Elwin Roelfsema. “It has been decided in the two affected bays to reduce the amount of overlap of the reinforcing bars, in the area where the truck wheels run, by changing from 2m square prefabricated reinforcing mats to hand laying larger areas. In addition we are increasing the mesh size from 50mm to 75mm square and changing to two layers of 12mm reinforcing steel instead of three layers of 8mm in the original, so providing more openings for the stiff concrete. We will also change the concrete mixing and placing method, as before we initially used an excavator with a bucket to spread the onsite batched concrete from truck mixers, prior to levelling with a pair of counter-rotating augers passing back and forth across the mat. As the concrete is quite stiff we believe there is the possibility of air being trapped as the concrete is effectively folded over on itself by the excavator during spreading. We shall just use the auger levelling system followed by a vibrating finishing beam.”
Prior to replacing the revised concrete overlay the damaged sections had to be removed. “We looked at two methods and initially tried pulling it off with a bucket on an excavator, but this was damaging the steel deck,” says Elwin Roelfsema. “So we had no other alternative but to try hydrodemolition and contracted Doornbos to remove the concrete and all the reinforcing in the two bays covering about 1,700m2.” In addition to high pressure water scarifying of the concrete wearing course, Doornbos also used one of its three Conjet 322 hydrodemolition robots to break away and roughen the longitudinal edges of the overlay to form a connecting bond with the adjacent lane.
“Although we had earlier used the Conjet 322 to successfully roughen the edges of the thin overlay for bonding to the next lane, TBI was initially sceptical and didn’t think hydrodemolition would be able to remove all the very high strength concrete from around and below the densely placed steel reinforcement,” says Doornbos project manager Jurjen Volmer. “We tested our Conjet 322 on the overlay, which worked well, but was not powerful enough so we bought Conjet’s largest 432 Robot specifically for this project and coupled it to two of our existing 500 hp Hammelmann high pressure pumps. These provide a combined flow of 360 litres/min and pressure of 1100 bar at the 432’s single jetting nozzle. We have to remove about 130m3 of this heavily reinforced, extremely high 105MPa strength concrete, which, due to the uneven deck plate, varies in thickness from 50mm to 70mm. This is the toughest hydrodemolition job I have been involved with and we are managing to remove between 3.5m3 to 4m3/10 high pressure hours on the thickest areas, rising to 4.5m3 to 5m3/10 high pressure hours on the thinnest sections.”
Water for the hydrodemolition process is pumped from the estuary into a purifier and holding tanks prior to pumping up onto the bridge and along to the main high pressure Hammelmann pumps. After passing through the Conjet 432 and removing the high strength concrete the dirty water is collected in a sump and pumped through a series of settling tanks and sand filters prior to discharging the clean water back into the estuary. After the successful hydrodemolition Doornbos follows on cutting out the steel mesh reinforcing and removing the broken concrete to fully expose the steel deck plate. The company expects to finish its approximate €350,000 contract during February 2007.
“Hydrodemolition with the Conjet 432 Robot has proved to be a very safe and effective way of removing the damaged sections of high strength concrete overlay,” says Elwin Roelfsema. “There is no other method we could have used without damaging the deck plate. We expect to finish the repair to these two sections and all the investigations on all the other sections of the east side northbound carriageway of the bridge by June. What we find during these investigations and the extent of any damage will depend on whether it can all be repaired. The results from our investigations and repairs to the east side of the bridge will also depend on whether we can overlay the west side southbound carriageway with high strength concrete and that will be no sooner than 2008.”
For further information please contact:
| Lars Göran Nilsson | Haverkort Voormolen B.V. |
| Conjet AB | TBI Infra |
| P.O. Box 507 | Postbus 20175 |
| S-136 25 Haninge | 7302 HD APELDOORN |
| Sweden. | Netherlands |
| Tel: +46-(0)8-5565-2240 | +32-(0)55-538 22 22 |
| Fax: +46-(0)8-5565-2260 | +32-(0)55-538 22 44 |
| E-mail: conjet@conjet.com | info@haverkortvoormolen.nl |
| Web: www.conjet.com | www.haverkortvoormolen.nl |
For more information on Conjet AB and their Hydrodemolition Robot visit their company profile page