Water jetting as a means of achieving concrete rehabilitation is gaining acceptance with construction companies becoming increasingly aware of the benefits the technology has to offer.

This is primarily due to the fact that the technique causes no damage or vibration to existing rebars and prevents damage or loosening of adjacent concrete and parts. In addition, the technique is also found suitable for a wide range of applications as it ensures high performance rates with a good finished surface.
For quite sometime, hydrodemolition techniques have been 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.
The technique can involve cleaning, roughening, removal of deteriorated concrete and even cutting.
Other applications of its use and benefits include the removal of 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.

Basic principles
A key element of hydrodemolition involves pressurising and widening existing pores and micro cracks in the weakened concrete structure using high-pressure water penetration.
As the built-up pressure exceeds the tensile strength of the damaged or weakened concrete, it gives way and the debris can be easily cleared.
In addition to pressure, the volume of water is also a contributing factor to the efficiency of the technique and the rate of removal of the matter is dependent on the amount of water directed towards the concrete surface being treated.
This combination of water pressure and flow, together with the controlled kinetic and geometric movements of robotic equipment  used in hydrodemolition techniques, creates the necessary ‘effect’ criteria for the hydrodemolition process – leaving sound concrete undamaged.
Research into water jet erosion has shown that the concrete resistance against water jet removal is dependent on concrete strength, method of finishing the concrete, aggregate size and the content of steel reinforcement bars.
The lower the concrete strength, the higher the penetration and pressurisation of the material and the consequent material removal rate.
Additionally, an increase in the strength of the cement matrix is accompanied by a corresponding increase in concrete resistance. This is due to the increased difference in the compressive strength of the construction, and the penetration effect of the water jet, which is a cumulative effect of its pressure, water volume and jet movement.

Techniques & tools
The pump pressure range required for concrete hydrodemolition applications is between 70 MPa (10,000 psi) and 135 MPa (20,000 psi), while the water flow rates range from 150 l/min (40 gpm) up to approximately 400 l/min (105 gpm) for heavy concrete removal machines.
Removal capacities in deteriorated concrete averages between 0.4 to 1 cu m/h, but rates above 1.5 cu m/h are also possible in some low strength concrete.
It is possible therefore to evaluate the hydrodemolition capacity in different concrete structures by measuring the concrete strength compared with the fluid-dynamic impact energy directed towards the material to be removed.
Mechanically-guided robotic tools, which are used for precise guiding of the water jet, can also be used on rotating nozzle carriers. But, rigid nozzle heads are generally used on commercial machines for selective removal. They will however, all work with much higher water flow rates.
The units can therefore be applied for heavy concrete removal on large structures such as bridges, jetties, locks or multi-storey garages. They are chiefly used for removing several centimetres-thick concrete layers of deteriorated low strength concrete in order to expose the corroded reinforcement bars.

Surface quality
Hydrodemolition has proven to be an excellent technique to produce excellent clean surfaces. The pull-off strength, which is an important indicator of the surface structure conditions, exceeds the required 1.5 MPa (N/sq mm) value with a 95 per cent reliability.
If the surface is prepared with water jets, the interfaced zone between the remaining concrete and the new cast overlay very seldom constitutes a plane of weakness.
This compares favourably with a surface prepared with hand-held tools, which results in a higher probability on interface failures at pull-off testing.
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 left unaffected by the water jet.