Waterjetting 24d - Impulse breaking of concrete

The most popular applications of high-pressure water on concrete deal with the removal of dirt and undesired coatings from the surface, or the removal of layers of the immediate surface for repair, hydro-demolition. There is, however, also an application where the concrete has to be removed in its entirety. Most often, this is done with jackhammers, wrecking balls and impact breakers of varying description. And yet, superficially, it might seem that waterjets might play a similar role in breaking the concrete pad into small pieces that can be removed.

There were a number of projects, back in the day when waterjet technology was first being developed, where a number of tools were developed aimed at generating the high-energy pulses that can be used to break the concrete slab. Yet none of them found a niche in the marketplace. It is perhaps instructive to explain what the different tools were, and the reasons why they never fully succeeded.

When waterjet technology was first being developed in the United States and in the UK, many of the devices used to generate the ultra-high pressures relied on the sudden release of large quantities of gas behind relatively small slugs of water in order to create the jet stream. In the extreme this was exemplified by the (then)* UMR water cannon. A 90-mm howitzer had been converted into a waterjet generator, and after filling the barrel with 12 gallons of water, a cartridge with about 4.4 lb of black powder was placed in the breech, and the charge ignited. Pressures of up to 50,000 psi were generated, and the jet drilled holes more than 6 inches deep into limestone test samples.

Figure 1. The UMR* Water cannon

The use of the large volume of water in the cannon was an attempt to overcome one of the disadvantages of similar devices which had been developed at the Safety in Mines Research Establishment in the UK, at IIT Research Institute in Chicago, and by William Cooley at Terraspace. The disadvantage was that, while the initial impact of the jet was at the very high pressure, as the driving gas expanded, so the pressure dropped dramatically, and the pressure in the water fell accordingly.

Figure 2. Pressure pulse from the UMR water cannon

The result was that while the initial impulse would generate a large number of cracks around the impact point in the surface, there was not enough energy in the water slug that followed to grow the cracks to the point that large volumes of rock were broken out.

This was illustrated when Bill Cooley took his water cannon underground to try and drive a tunnel in a limestone mine.

Figure 3. The Cooley cannon in a mine

The cannon generated pressures of up to 500,000 psi, determined by measuring the speed of the leading edge of the water slug as it broke successive pencil leads. Yet the fragments produced were not of great volume, relative to the energy expended.

And yet there was an application that did, for a short while, seem promising, and that was developed in Germany. There are a number of situations in the mining industry where large boulders can get into the transport system, and where the conventional application of a small stick of dynamite can have collateral damage effects that can be expensive. German investigators therefore developed a small tool, where (as with the UMR cannon) a deflagrating cartridge was used to generate the gas behind a slug of water, and this could be driven into the boulder, and split it, without damaging any of the equipment surrounding the rock.

Figure 4. The German impulse boulder breaker

Figure 4b. Schematic of the boulder breaker

An alternative approach was developed by Briggs Technology in Pittsburgh, following a different concept – a line of development that others had also developed (as will be discussed in a late post). Rather than generate a single pulse of very high energy, the concept was to develop a simpler tool that could be rapidly recycled. In this way, while the individual cracks from single impacts would not liberate that much material, by having a series of these it would be possible to get the individual crack patterns to intersect and in this way to break out the concrete pieces. (This is a similar concept to that of an impact breaker, although using water as the impacting device).

Figure 5. Schematic of the Pittsburgh device

Single shot tests of the tool were promising, as were the early tests on slabs of concrete. Unfortunately the high-pressure pulses travelled both ways, and thus the valves and fittings that were necessary to allow the tool to rapidly recycle were also exposed to the high-pressure loading. The materials that were in use at the time, for these parts, was insufficient to give the long-life under the loading cycles that it saw, and as a result the project, unfortunately, never reached the commercial market.

This problem of high-cycle loading is made worse where the pressure is allowed to decay back to ambient pressure between cycles, and the more modern tools that use a cyclic change in pressure to improve on jet performance (such as those from Mohan Vijay in Canada) do not drop the pressure within the delivery line and thus get around the problem of the earlier systems where it was the high range of pressures seen in a cycle that led to the valve problems. Although I should be careful there to differentiate the impulsive cannon type devices from the early ones where the flow to the nozzle was intermittently stopped. In the latter case it was the hydraulic shocks to the delivery line from the flow blockage that pulsed back down the line and (as rumor had it at the time) drove the pump pistons through the cylinder wall within the first few minutes of operating time.

As a result of these past developments there has been less emphasis on developing ultra-high pressure impulse devices over the last few years, particularly as the pressures of continuously operating equipment have risen, and the use of abrasive in the water has meant that most objectives can be effectively met with the new equipment.

*The University of Missouri-Rolla (UMR) has changed its name to Missouri University of Science and Technology (MST) – since the tests were carried out some decades ago, the older designation for the cannon has been used.