Waterjetting 29c - If at first you don't succeed

In my last post on this topic I wrote about two of the most influential papers that were published in the Proceedings of the First International Symposium on Jet Cutting Technology (hereafter ISJCT) back in 1972. Yet there was one more paper that I remembered over the years. And it has been for an entirely different reason.

The paper was “Application of Water Jet Cutting Technology to Cement Grouts and Concrete,” by L. McCurrich and B. Browne of Taylor Woodrow in the UK. The company had looked at the use of waterjets as a means for cutting concrete, either for demolition or inserting the grooves used, for example on highways for “rumble” strips. They concluded that such a tool would have to operate at a pressure of 54,000 psi, and using 3 jets in a cutting head, would require a power demand of around 550 hp. To quote from the paper:

This scale of research to develop a practicable cutting tool would be at least one hundred thousand English pounds. (Then around $250,000). No single firm involved in demolition is likely to be able to afford this sum on a speculative development of this nature, and if a commercially viable proposition can be shown to be likely, funds will have to come from a central Government or Trade Association body.

Jake Frank and I visited the company in London after the conference was over, and the authors were explicit in their views that a waterjet tool would be too expensive for any individual company to purchase for use in concrete work. Skip forward a few years and I was at the Liquid Waste Haulers Show in Nashville TN. (This became the Pumper & Cleaner Environmental Expo International and this year is, I gather, the Water & Wastewater Equipment, Treatment & Transport Show and will be in Indianapolis next month). A friend of mine was chatting with me on his booth, when a salesman come over. He suggested I join him while we wandered over to the sales table where the customer happily signed an order for a $250,000 unit that would leave the show and be used for the hydro-demolition of concrete.

Times had certainly changed in the intervening years, and I rather suspect that the estimate that the two authors had given for the project development costs were exceeded as both the Gas Research Institute (now the Gas Technology Institute) and the Electric Power Research Institute, as well as the National Science Foundation, helped to develop technologies, both through funding research and in encouraging companies to develop the needed tools for the industry that has since grown to use it. Not that the research effort was limited to the United States.

By the time of the 2nd ISJCT in Cambridge, two years after that first paper, there were three papers dealing with studies on concrete cutting. These included studies carried out in the United States, Japan, and Canada. There were two drivers to this growth in effort, despite the pessimism of the original paper, the first being the size of the market. McCurrich and Browne had pointed out that back in 1970 the UK was emplacing about 1 ton of cement for each inhabitant of the nation, much of which would later have to be demolished or repaired. One of the other drivers was that, in contrast to rock and other materials that were being used as targets, cement properties can, to a degree, be controlled by the manufacturer so that the effect of changing concrete properties on the cutting performance could also be established.

The work was, however, constrained to the laboratory for these studies at that time and focused on jet slotting of the concrete at pressures ranging up to 60,000 psi.

There was only one paper on concrete cutting at the Third ISJCT, and that dealt with cutting underwater, which at pressures of 60,000 psi jet pressure and shallow depth appeared to be little different to cutting in air, where the nozzle was held close to the target surface.

The fourth ISJCT was held in Canterbury, UK and marked a change in emphasis for the research on concrete removal. The teams reporting differed from those of the earlier papers, and now included funding from NSF. The emphasis for the three papers was also more focused on concrete demolition, using pulsed waterjet systems in two cases and on a portable system for removing concrete and asphalt for utility repair in the third.

The idea of using a pulsed jet to shatter concrete due to the impact of the jet on the surface, the rapid generation and penetration of cracks from that impact, and the consequent rupture of a block into pieces had a number of advantages. Tools could be built with relatively simple charging mechanisms (the simplest of which – that came later from Germany – used a small cartridge similar to a shotgun shell to generate the pressure) and without the noise and dust generation of impact breakers. Unfortunately, as these tools were developed over the subsequent years, a consistent problem arose for the devices being developed. This was that the pulse that generated the damage had to be repeated relatively rapidly if it were to be able to match the performance of the impact breaker. This required that the pressure chamber holding the water had to be rapidly refilled, and this in turn required a valve between the water supply and that chamber. The valve then had to withstand the repeated high-pressure cycles each time that the device fired. This turned out to be a bigger problem than had been anticipated, and there were several efforts to develop the pulsed waterjet concrete breaker that foundered because of the complexity of the problem.

It was only at the 5th Conference, held in Hanover in Germany, that the first paper appeared noting the benefits of removing damaged concrete. Concurrently the paper that discussed this also described field trials carried out in Chicago, demonstrating that the waterjetting method was able to remove damaged surface concrete preferentially, and to a controlled depth at a rate more than twice that of existing jack hammers, while using roughly the same amount of power. It had taken ten years to reach this point, which presaged the development of the hydro-demolition industry, although it took several more years and the interest of larger companies before the technology finally took off.

Unfortunately the work on pulsed-jet concrete demolition which was still ongoing at the 5th ISJCT did not lead to a commercial product, for the reasons cited above, while concrete trenching and more detailed contour cutting, although developed by the this conference into a field portable device, also was later subsumed into the overall development of hydro-demolition.

These developments took much more money that the original authors had foreseen, but the final devices put into the field ran at lower pressures and required less power than those original experiments had anticipated. It also took a number of years for the capabilities of the technical equipment to reach to capabilities needed to field the tools that are now ubiquitous.

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.