Contractors in the region should be looking at proven hydrodemolition techniques for an efficient, safer and longer lasting solution to removing corroded concrete, says a senior official at the UAE-based Asia Waterjet.
The company’s managing director Mike Biddle says contractors conventionally have, for many years, relied on jackhammers for concrete removal and repairs on structures, bridges, silos and highways, and other projects but today there is a growing recognition of the benefits of using hydrodemolition techniques that offer increased efficiency with greater productivity, significant labour cost saving and operator safety.
Asia Waterjet was founded by Biddle who, after the Gulf War in the early 1990s, was involved in supplying high-pressure waterjet cold cutting equipment during the fire-fighting and capping of damaged oil wells.
The company has evolved from that experience and from its base at Jebel Ali Free Zone in Dubai acts as the exclusive Middle East distributor for leading market suppliers such as Swedish leading hydrodemolition equipment manufacturer Aquajet Systems.
These systems have been used worldwide and proven to provide significant time and labour savings, says Biddle. Citing examples, he says CanMar Contracting, a Canadian concrete restoration specialist in underground car-parking garages, took on its competitors head on.
In a recent garage contract, CanMar met its target of treating 1,000 sq ft per day of concrete slab using one of its three Aqua Cutter robots from Aquajet. This was directly compared with a contractor using jack hammers on an identical project in an adjacent apartment block.
According to CanMar’s hydrodemolition manager David Porciello, a single jackhammer can achieve, at best, up to 50 sq ft per day.
“This means at least 20 operators would have been required to match the production of the hydrodemolition robot on the car-park project,” he says. “Jackhammers are labour intensive. This apart, hydrodemolition provides a cleaner finish and does not damage the rebar.”
In Singapore, it has been estimated that a single Aqua Cutter robot helped achieve significant labour cost savings by eliminating the need for 50 jackhammer operators on an MRT (mass rapid transit) contract. The Austrian tunnelling contractor on the project removed about 14,000 sq m of concrete in three underground stations.
“In the event, not only was the contractor able to save on labour costs by using hydrodemolition techniques, he was also improving productivity with the system, removing between 110 cu m and 150 cu m per day,” says Biddle.
In a Middle East port application, an Aqua Cutter was used to strip deteriorated concrete from its East Pier to allow it to be rebuilt. Compared with traditional methods using jackhammers, the Aqua Cutter 710V was able not only to replace between four and five jackhammer crews – meaning up to 20 operators – but also leave a cleaner finish on the piers rebar, which was left in situ for the reconstruction.
The contractor’s CEO confirms that the Aqua Cutter increased accuracy and production with reduced manpower. “It also eliminated the risk of ‘white finger’ from vibration for the operators,” he says.
Concrete rehabilitation using hydrodemolition or water-jetting techniques is becoming an increasingly important aspect of civil engineering and building renovation projects, says Biddle. Applications can involve cleaning, roughening, removal of deteriorated concrete and even cutting.
“Water jetting is actively used for the removal of chloride-damaged concrete layers down to the reinforcement bars or to a depth where the chloride content is at or below the permissible level,” he adds.
Other examples of its use and benefits include the removal of any loosened or easily peeled layers; pull-off strength of an overlay cast to the prepared concrete surface above 1.5 MPa; and minimised removal of good concrete. Similarly, it causes no damage or vibration to existing rebar and prevents damage or loosening of adjacent concrete and parts, Biddle says.
The systems are suitable for a wide range of applications and ensure high performance rates with a good finished surface.
Biddle explains that the key element of hydrodemolition is to pressurise and widen existing pores and micro-cracks in the weakened concrete structure using high-pressure water penetration. The material is easily removed as the build-up pressure exceeds the tensile strength of the damaged or weakened concrete.
In addition to the water pressure, the volume of water is also a contributing factor to the system’s efficiency. The rate of removal, for example, is dependent on the amount of water directed towards the concrete surface in order to rapidly and continuously pressurise the areas being treated.
“This combination of water pressure and flow together with the controlled kinetic and geometric movements of the robotic equipment creates the necessary ‘effect’ criteria for the hydrodemolition process, leaving sound concrete undamaged,” he says.
Biddle continues: “Research into water jet erosion has shown that the resistance of concrete against water jet removal is dependent on its strength, the method of finishing the concrete, aggregate size and the content of steel reinforcement bars. The lower the concrete strength, the larger the material removal rate that can be achieved due to larger penetration and pressurisation of the material. Additionally, the higher the cement matrix strength the higher the concrete resistance.
“This is due to the increased difference in the compressive strength of the construction and the penetration effect of the water jets pressure, water volume and jet movement.
“It is also proven that hydrodemolition produces an excellent clean surface quality. The pull-off strength, which is an important indicator of the surface structure conditions, exceeds the required 1.5 MPa value with a 95 per cent reliability.”
With water-jet hydrodemolition, once programmed, the jet moves rapidly and continuously over the selected area for removal. There is no percussive effect on the surface with the water jet penetrating the deteriorated concrete.
“Extensive investigations have proved that there is no modification of the concrete microstructure during the water jet treatment. Similarly the concrete pore structure is not affected by the water jet.
“The surface geometry achieved after hydrodemolition depends on the type and size of the aggregate. With limestone, for example, the surface is comparatively smooth and characterised by a high degree of fractured aggregate gains. In contrast, quartzite containing concrete exhibits an uneven surface and a high amount of undamaged aggregates.
“Compared with other removal methods, hydrodemolition generates a very large contact surface between the concrete and applied coating system,” he adds.