Methods of demolition

When the decision of partial or full demolition of a concrete structure is made, the process of selecting a demolition method starts. To be considered are local circumstances influencing the process as well as the kind and volume of the building material which should be demolished. If the purpose is to repair damaged parts and the construction is a sensitive structure like a road, bridge, dam wall, multi-story car park walls and ceilings, airfield landing ground, concrete surfaced tunnel or similar there are just a few alternatives: hand held jackhammers, road milling machines or hydrodemolition.

Hand held Jackhammers create micro cracks
The disadvantage with hand held jackhammers are several i.e. low efficiency, noisiness and difficulty in separating bad concrete from sound. Vibrations in hit rebars will cause loosening in bondage to sound concrete and thereby speeding up the corrosion process and cause lamination, i.e. the concrete separate like slices along the rebars. Micro cracks will normally be created in the undamaged concrete. The result of how well the chipping turns out depends on a very great extent on the skill of the operator and his feeling for both the equipment and the concrete.

To be considered are also the restrictions in working time due to risk of operator's injuries in form of circulation disturbances (white fingers) and hearing reduction.
This method can for economical reasons be chosen when the area is very small, since the investment and start up costs are quite low.

Road Milling Machines create micro cracks
The same disadvantages of creating micro cracks applies for road milling machines as well, but the risk for the operator is eliminated. Additionally, it has no capability to remove the concrete under the rebars, and the disadvantage of this is obvious.

Hydrodemolition does not create micro cracks
Though some decade in rapidly progress the technique is still unknown for many contractors and commissioners. It was invented as a way to render a faster and more effective system for removing damaged concrete without the disadvantages listed above. A description of the technique will show its uniqueness;
Preferable is to start with a determination of the quality status of the concrete. The best way to get a good quality status is to examine cores drilled out from the structure in a concrete laboratory. The core examination is of great interest to both the owner of the construction and the contractor as it can guide to a better specification of the contract.

After the results from the laboratory is obtained, a small test area of approx. 1 m2 is then worked out to determine the right quality depth of the removal, i.e. all concrete weaker than a decided specific level will be removed, no matter of depth.
The removal to a specified quality depth is a major difference compared to other methods which remove the concrete to a certain depth measured in centimetres or inches, no matter if the concrete is sound or not, and no matter if bad concrete still remains under that depth. What makes this possible is that the demolition is made by a robot delivering a high pressure computer controlled water jet stream, with the settings determined by the result of the test area. The water jet stream literally blasts the bad concrete away. No matter if it is situated under layers of rebars or not. At the same time the rebars will be blasted free of corrosion without any damaging vibrations.

Thanks to the appeared irregularity of the surface, the bond between the old sound concrete and the new replaced one is extremely strong. Independent laboratory tests have time after time shown that test samples are not breaking at the bonds when pulled to their breaking point.

According to this the hydrodemolition has shown that the method is more than 25 times faster compared to hand held jackhammers. One robot will create a faster and better demolition than 25 workers with jackhammers can do at the same time, and without any working time restrictions. This means that any project of some size will be done much faster, more profitable and of higher quality than with any other method.

The standardisation organisation of the European Union, CEN/ CENELEC is developing the EN 1504 system of standards for products and systems for the protection and repair of concrete structures. Section 3, prEN1504-3 specifies the minimum pull off strength for concrete overlays for structural and non-structural concrete structures. In order to meet these requirements in EN 1504-3, hydrodemolition has to be used for removing the old concrete. The EN1504-3 draft specifies that the pull off strength shall be 1.5 MPa for non structural and 2.0 MPa for structural structures.

High pressure water-jet hand lance
Hand lances are the simplest and less effective form of water jetting concrete sometimes incorrectly referred to as hydrodemolition. The maximum allowed reaction force created by the water jet is 250 N for hand held equipment compared to robots that are built for reaction forces from 1000 N up to 4000N. Hand lances are normally not recommended, but in certain conditions where the concrete otherwise is hard to reach due to lack of space it may be the only option, especially as they do not create micro cracks. Other disadvantages with hand lances except their lower efficiency are several and include: They can not be pre-set to a certain quality depth; they are very hard to work with, which requires frequent pauses or two operators taking turns; risk for accidents.