Conjet - General - Croatia, Hydrodemolition Robots Used in Concrete Bridge Repairs
Date: 27 Oct 2006
The multi-span reinforced concrete bridge over the river Sava at Ivanja Reka, in the south east suburbs of the Croatian capital Zagreb, is a vital structure on the major E70 highway link through the Balkans. The bridge was opened in 1980 but the use of de-icing salts during winter months has since caused considerable calcium chloride damage, threatening the integrity of the structure and forcing the Croatia Highways Authority, Hrvatske autoceste, to carryout extensive repairs. The restoration, which is expected to cost over €10 M, is focusing on the concrete deck, parts of which have to be completely replaced and Hrvatske autoceste has specified the high-pressure water jetting technique of hydrodemolition to remove the damaged areas.
Hydrodemolition is being used to remove approximately 1500 m3 of calcium chloride (salt) infected concrete and as this is too extensive for one contractor, the work has been split in to two separate contracts. Specialist hydrodemolition contractor Carin d.o.o is working on one-half of the bridge with Spegra Konstrucktor Inzcnjering, based in Split, on the other half. Four Conjet Robot high-pressure water jetting hydrodemolition machines are playing a major part assisting in repairs to the bridge deck. Carin is using two of its existing model 362 Robots, while Spegra bought one of the Conjet 322 and 363 Robots specifically for its €2 M Sava bridge repair project.
"This is a major bridge repair project and hydrodemolition was the specified method of removing the damaged concrete," says Spegra project engineer Boris Pavic. "Using very high pressure jets of water to remove the poor concrete does not cause any damaged to the good concrete left behind and it also takes away concrete from above or below any reinforcement, which is also cleaned of any rust. Hydrodemolition also produces a very rough and uneven surface giving a good bond for the new concrete to key onto. Following an extensive evaluation, which included talking to experienced hydrodemolition contractors in Switzerland, we initially bought the Conjet 322 Robot to start off the job and followed about three months later with the bigger Conjet 363."
The approximate 1 km long, 29.4 m wide bridge, with its combined approach viaducts and main span, carries the busy dual two lane E70 highway over the River Sava. The bridge is supported on pairs of reinforced concrete columns 15 m apart and spaced at about 40 m centres. Each pair of columns is linked with a transverse concrete crosshead, which in turn supports six, 2m deep longitudinal spanning precast concrete beams, three for each carriageway. Each set of three beams is topped and spanned by rows of two precast concrete deck slabs, which are 5.3 m long, 1.8 m wide and 200 mm thick. The slabs, or plates as they are known, are spaced end to end with a central 900 mm wide gap and a 200 mm space between each row. The gaps are in filled with mass reinforced concrete, together with the cast in-situ parapet edge beams, to form a monolithic carriageway deck.
Bridge owner Hrvatske autoceste has scheduled the rehabilitation project to repair one carriageway at a time and Spegra has to remove about 750 m3 of concrete on its half of the bridge deck. Spegra first planes off the combined asphalt wearing and base courses to expose the surface of a 50 mm to 70 mm thick layer of light mesh reinforced deck levelling concrete. Here Spegra follows on with its Conjet 322 Robot connected to a 330 kW Volvo powered Hammelmann model HDP 333 high pressure pump delivering clean fresh water at a maximum pressure of 1180 bar and flow of 138 litres/min. Spegra runs the pump at a slightly reduced 1100 bar and at this setting the Robot 322 is removing an average 7-8 m3/10 high pressure hours of the 30-40 MPa strength concrete. In addition it is also removing the original rows of 200 mm deep in fill concrete at about 2 m centres across the deck between the lines of the precast deck panels. Spegra is using the Robot to make 450 mm wide cuts to break into each side of adjacent deck plates to make way for additional reinforcement and new stronger 45 MPa concrete. In addition about 60 of the original precast deck panels have suffered so badly from chloride attack that they also have to be completely removed and replaced with new cast in-situ deck panels.
Spegra started its contract initially working two shifts/day with the Conjet 322 Robot. This dropped to single shift working when the larger Conjet 363 Robot arrived with its 400 kW Deutz powered Hammelmann 403 pump, which is capable of operating at a maximum pressure of 1500 bar. Spegra operate this pump at a lower pressure of 1200 bar and flow of 157 litres/min using a 2.6 mm diameter jetting nozzle on the Robot. "The 363 Robot was bought primarily for the areas where the concrete is deeper than 200 mm and especially along the central reserve where we have to remove about 500 m of concrete about 400 mm wide and to a depth of 450 mm," adds Boris Pavic. "We are very satisfied with the Robots, which are very easy to operate and control. They are performing very well."
After Spegra has removed the salt infected concrete, added extra reinforcement and replaced with fresh 45 MPa concrete, the entire section of repaired deck is recovered with a new layer of 45 MPa deck levelling concrete up to 100 mm deep. This is followed by an epoxy resin insulating layer and waterproofing prior to the final asphalt base and wearing course. To complete the restoration the repaired structure will be impregnated with a special polymer sealant to prevent future salt attack and carbon dioxide penetration.
Conjet Robot 363 in action during bridge repairs in Croatia.
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