Waterjetting 24a - Cutting concrete - 1

There are a number of differences that take place when high-pressure waterjet operators change from a lower pressure, higher volume flow rate to one where the jets are operated at a higher pressure, with a smaller jet size. One way of illustrating the difference is in the way that the jet will interact with concrete, and that is the theme of this particular article.

Concrete is made up of two different material types, there is the cement and there is the aggregate.

Figure 1. Slot cut into concrete, showing the pebbles of the aggregate (brown) that are held in place with the finer cement (grey)

In an earlier post I wrote about the use of waterjets to remove damaged concrete from bridge decks and garage floors. In this short series the focus is going to be more on cutting through the concrete for whatever reason that it is necessary. It is the reason, however, that will likely help select the best way to cut the material.

In a typical concrete the cement paste is considerably weaker than the pebbles that make up the aggregate. Using the compressive strength of the material as a guide that of the cement may, for example, be less than a tenth of the value of that of the aggregate. And yet, when repairs are to be made to the concrete, or when pieces must be cut out, the systems are generally designed to cut through the harder aggregate.

Figure 2. Conventional approach to cutting through concrete.

The system that is used has to be capable of cutting through the hardest material in the mix, and that is usually the individual aggregate particles. (We will cover the rebar in the mix in a later post).

The slot to be created, is often not that critical in itself. For example we needed, at one time, to insert an opening in a series of concrete walls. Because this was done in the center of a university campus, the benefits of the relatively quiet waterjet cutting over jackhammers and other means of removal were significant, as was the amount of time required for set-up of the equipment. But one immediate aspect of the job was that the outlines of the hole that had to be cut were not that critical.

This is because, after the hole was to be cut, then carpenters would install a frame to hold a door, and they needed some space at the edge of the hole for adjustments, so that the tolerance on the cut was roughly plus or minus half-an-inch which covered the size of the aggregate particles.

This meant that it was not necessary to cut through these pebbles in the wall, but rather it meant that the system could be designed purely to remove the softer phase of the concrete, the cement, without needing the pressure to cut through the harder aggregate.

Figure 3. Concrete schematic showing where water jets have removed the cement (central white zone) from around the aggregate (darker blocks).

If all the cement is removed from around a piece of aggregate (Figure 3) then there is nothing holding it in place, and so the force of the waterjet (if that is used for the removal) will be enough to lift the pebble out of the slot. As a result the slot can be created at a much lower pressure than would be the case if the pressure had to be adjusted to cut through the aggregate.

Figure 4. Schematic of a slot created in concrete through removing the cement from around the aggregate particles without the need to cut through the aggregate.

The edges of the hole are not as smooth as they would be if the cut were made through the pebbles, but on the other hand the rough nature of the surface means that any later infilling of the slot with fresh concrete will have a rough surface to bond to so that the adhesion between the two layers will be much greater than that from a conventional repair.

Because the jets do not have to cut through the aggregate the cuts can be made a t much lower pressure (in the case of the University walls at less than 10,000 psi). This makes it easier to build relatively simple equipment at low cost to do the job. Back when this particular series of cuts were made it was not possible to buy reliable swivels that would allow the jets to spin and cover a larger area of the slot surface. Instead Dr Clark Barker, who designed the tool, used a four-bar linkage to allow the jet to sweep out an oval path on the wall, with the overall platform for the system mounted on a shop lifter.

Figure 5. Simple tool used to slot concrete. The high pressure hose is connected to the cutting lance on the rhs of the picture. The lance is held in a pivot at the back of the beam, and caused to oscillate through the rotation of an off-center connection to the wheel at the front of the beam. Drive to that wheel is through a chain from a motor that is not shown. The orange frame is a conventional shop lifter.

The connection to the driving wheel shown in Figure 5 could be adjusted, as could the position of the wheel along the beam, in this way adjusting the width and height of each orbit of the lance.

Figure 6. Slot cut through an 11-inch thick concrete wall using an orbiting waterjet.

The exposed rebar was cut later, using a cutting torch. A number of walls were cut in this fashion, and though the slots went through the walls in each case, the jet was large enough (around 0.05 inches diameter) that it was able to rebound within the cut and undercut the pebbles and remove them without the jet being directed directly at the cement under the pebbles.

Figure 7. Slots cut through a concrete wall using a high-pressure waterjet. Note aggregate pebbles are sticking out of the cement.

The walls were cut through to a height of about six-feet in less than an hour of cutting time, though there was some additional time needed to move the cutting platform up to cover the top of the slot. The nozzle was moved into the cut after each two passes, with the assembly being slowly raised over the cut length, using the shop lifter, and then lowered again before moving the lance into the slot. Changing the distance also changed the angle of the jet to the cut surface, and helped in getting the jet under any of the pebbles still attached to the concrete.

I’ll continue on this topic next time.

Waterjetting 1d - Not quite that simple!

When I first began the research on the applications of high-pressure water that was be one of the major parts of my professional life I must confess to a certain naïve innocence in regard to other folk’s work. One assumed that other folk had made similar mistakes to mine, and then corrected them, so that when different systems were compared that the early, obvious, mistakes had not been made.

One of the first times I found that this wasn’t the case was when we were asked to go and demonstrate that high-pressure waterjets could economically cut granite, in quarries located in the heart of the Granite industry, in Elberton, Georgia. We were working with Georgia Institute of Technology (Georgia Tech) at the time and were asked if we could, at very short notice, go down to a couple of quarries and run a demonstration.

Back during my graduate studies I had found that Russian claims were true that said that it was possible, with a 10,000 psi jet pressure to cut through a rock with a compressive strength of 30,000 psi. (I'll tell you how later)

Figure 1. 9-inch thick block of granite drilled through by a 10,000 psi waterjet at Leeds University. It took over 30 minutes. (Summers, D.A., Disintegration of Rock by High Pressure Jets, Ph.D. Thesis, Mining Engineering, University of Leeds, U.K., 1968.)

Knowing this, and having a suitable pump at Rolla, our group ran some tests at the RMERC to get the angles right between the two jets that we were to use, and then, about a week later, we went down to Elberton and set up a system in the quarry.

Figure 2. Starting to cut a 1-inch wide slot in granite, pressure 14,000 psi, 90 rpm, linear cutting speed around 9 ft/min, areal cutting rate around 20 sq. ft./hour.( Raether, R.J., Robison, R.G., Summers, D.A., "Use of High Pressure Water Jets for Cutting Granite," 2nd US Water Jet Conference, Rolla, MO., April, 1983, pp. 203 - 209.)

The trials demonstrated that high-pressure water could cut granite at commercial rates, we cut a slot some 11 ft long and about 2-ft deep, and, after a couple of days of work, we went home. Georgia Tech then went to one of our competitors who set up to run a similar test. We had been done in 2 days, it took them two weeks to cut a slot about 2 ft long and 6-ft deep. They were running a jet system at 45,000 psi, roughly 3 times the pressure of our system. Why did they do so badly?

Well it turned out that they connected their pumps to the nozzle through a very narrow length of high-pressure tubing, and we calculated (as later did they) that of the 45,000 psi being supplied at the pump, some 35,000 psi had been lost in overcoming friction between the pump and the nozzle, As a result they were trying to cut the granite with jets at a pressure of 10,000 psi effective pressure, and it was much slower than our system which retained most of the 14,000 psi from the pump to the nozzle. (Hilaris, J.A., Bortz, S.A., "Quarrying Granite and Marble using High Pressure Water Jet," paper D3, 5th International Symposium on Jet Cutting Technology, Hanover, FRG, June, 1980, pp. 229 - 236.)

Now you may note that I said something about mistakes – it turns out that we had made an identical mistake a few years earlier and had added a second 10-ft length of narrow diameter tubing to the nozzle, and suddenly a system that had cut adequately with 10-ft of tubing did not work with 20-ft. The reason was the pressure loss in the tubing was too great at the longer length, and the pressure fell below that required to cut into the rock. (But at the shorter length we were drilling the hard sandstone at 12-ft/minute).

It is a very simple mistake, and many folk have made it over the years. The system has to be designed from one end to the other to ensure that all the parts are properly sized for the systems that are to be used. (And I will refer to other cases such as that above as we go through this series.)

It is not just the diameter of the feed lines that is important. In 1972 it took, on average, 150 man-hours and about $2,000 for the U.S. Navy to clean a single ship boiler using chemicals and mechanical scrubbing and cleaning. An enterprising company showed the Navy that it was possible to use waterjet lances to clean the tubes. In the demonstration they cleaned a boiler in 10 hours, and it cost around $700. This being Government work, the Navy then arranged a competition to find the most effective contractor. Based on the performance of the system that had been used in the first demonstration they asked 5 companies to compete in cleaning boilers. The operating equipment was designated as having to operate at 20 gpm, at a pressure of 10,000 psi. The results were not even close, even with systems nominally the same.

Figure 3. Relative cleaning efficiency in areal percentage cleaned, of five competing systems in cleaning heat exchanger tubes in Navy boilers. (Tursi, T.P. Jr., & Deleece, R.J. Jr, (1975) Development of Very High Pressure Waterjet for Cleaning Naval Boiler Tubes, Naval Ship Engineering Center, Philadelphia Division, Philadelphia, PA., 1975, pp. 18.)

One of the differences between the competing systems, you won’t be surprised to hear, was that some had smaller feed hoses than others.

There are many different reasons that the various systems performed as they did. One of the aims of this series is to ensure that, should you be asked to engage in such a competition, you will know enough to follow the path of company A, rather than company E.

As systems have become more sophisticated the different factors that control the performance of the jets have increased in number. As a simple example, when abrasive particles are mixed with high-pressure water in streams of abrasive-laden waterjets at pressures that can run up to 90,000 psi in pressure, for high precision cutting of material, the factors controlling performance now include not only the delivery system for the water, but also that for the abrasive, the type of abrasive and the configuration of the nozzle through which that final cutting jet is created.

Again, when we were asked to compare the performance of these different systems we set up nominally identical test conditions under which to determine which nozzle system would perform better. If I were honest I would tell you that before the tests began I expected that the variation in performance of the systems would vary by perhaps 10% between the best and the worst. We were quite surprised by the result.

Figure 4. Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

I use these last two figures to show that all the details of a high-pressure waterjet system are important, when it comes to optimizing performance. One of the reasons to write this series is to ensure that folk that use these systems in the future do not make the mistakes that we made, as we learned how to tune the systems from getting poor performance to the commercially viable rates that are achieved today.

Unfortunately much of the early research and tests that are the basis for this knowledge were performed before the Internet existed. As a result I will have to use references to books and papers (as above) rather than using the electronic references that are the more common habit now.

This concludes the basic introduction to the series, which will now focus on more specific subjects.