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.

Waterjetting 24c - angled jets in cutting concrete

In this short section of the series I have been discussing some of the issues that relate to cutting through concrete. In today’s piece the discussion will continue, focusing on the angles that the jets are set at, when making repeated passes over an area to deepen the cut. The basic premise of the discussion holds true regardless of jet pressure, provided that the concrete is being removed by a moving continuous jet stream, rather than a pulsed jet system, which I will discuss next time.

As was mentioned at the beginning, the main way in waterjets remove concrete most efficiently is by washing out the cement around the individual particles of the aggregate, which, in turn, causes the particles to fall out of the slot, since they are no longer supported.

If a cutting head is built with the jets pointing vertically downwards, so that, as the head moves, so the jets spin over the surface and wash out the cement over a wider path, any cement that underlies a particle is not removed, and the particle remains held in place by the underlying cement column.

Regardless of how the nozzle is moved over the surface, with only vertical jets the path of the assembly very rapidly becomes blocked, and the nozzles can no longer move into the slot to deepen it.

The obvious solution to this is to incline the nozzles so that as they rotate over the surface, they can reach under individual particles and wash out the cement beneath them, removing their support. This also has the advantage of cutting a path into the concrete that is wider than the cutting head itself, so that on later passes the head can be lowered into the cut, shortening the standoff distance to the fresh surface and improving cutting efficiency.

Moving the head down a little on each pass also has the advantage that it exposes fresh layers of cement to the jet action and makes it more likely that all the cement within the desired slot is removed (and the aggregate with it) leaving a clear path for the assembly to move deeper into the slot.

So the question then arises as to what the most efficient angle is to tilt the nozzles to, relative to the perpendicular axis of the target surface. (I use that awkward phrase because not all targets are going to be flat horizontal bridge or garage decks).

Very shallow angles don’t work very well. The best demonstration of this was when we started cutting slots in granite, with an initial divergent jet angle of around 8 degrees. After the first few passes we noted that the slot was developing walls that sloped into the cut. As a result the slot was getting narrower with depth, and the nozzle assembly would no longer be able to move into the cut.

Figure 1. Tapering cut into granite. The nozzle had been advanced about a third of an inch after each pass of a dual-nozzle rotating head. Nevertheless the cut tapered as the cutting continued.

We had chosen that initial angle because it worked well when cutting slots in coal, but clearly in harder, less jointed material that was not the case. And so we, and others, have carried out tests to find out what the best angle would be for the cutting tests.

And, before I show the results, let me emphasize that these only hold true for a certain concrete mix. Where aggregate particle sizes are larger, the jet angle may need to change to make it easier to get around. The pattern of the jets on the surface, (affected by the ratio of the rotation speed of the head relative to the movement of the entire assembly over the surface) and the jet parameters themselves (jet pressure, nozzle diameter and standoff distance) also play a part. In this latter regard remember that the effective range for many waterjet streams is not that much more than a hundred diameters from the orifice, so that expecting some of the smaller nozzle sizes (say 0.005 inches) to cut cement more than half-an-inch from the nozzle may be an exercise in futility – and raising the jet pressure in that circumstance is unlikely to fix the fact that the target is simply out of range.

So, with those caveats, here is the result that was obtained by Puchala, Lechem and Hawrylewicz*:

Figure 2. The effect of nozzle angle on cutting performance in removing concrete (*Puchala, R.J., Lechem, A.S., and Hawrylewicz, B.M., "Mass Concrete Removal by High Pressure Waterjet," Paper 22, 8th International Symposium on Jet Cutting Technology, Durham, UK. September, 1986, pp. 219 - 229.)

Nevertheless it is clear that there is much better performance where the jets are inclined at an angle between 25 and 35 degrees to the normal to the target surface. This is reflected in the improved efficiency of cutting (as shown by the second line in figure 2, showing a more significant change with angle than is evident from the depth of cut measurements). In all cases we have found that the jet angle needs to be 15 degrees or greater to make sure that wall taper does not occur.

Correlating the rotation speed against the traverse speed of the head over the surface to find the optimal cutting performance is a little more difficult, and should generally be assessed for given concrete targets with a short test run, before the major effort is undertaken. One reason for this is the wide range in performance that can be found with different cements. We have worked with cement that was sufficiently weathered that it could almost be removed using one’s hands, on the one hand, and the new cements that contain silica fume, or small wires or fibers pose a different and more difficult challenge on the other.

This also holds true over setting the advance rate of the nozzle assembly into the slot after each pass (where the head is cutting in a series of passes to penetrate the slab). Here the advance is going to be controlled in part by the size of the aggregate, though it should be noted that even with little apparent progress the nozzle assembly should be advanced after each pass, since this exposed a fresh layer of cement to attack, and this will lead to more aggregate release and help in clearing the cut.

Waterjetting 24b - Cleaning and Cutting concrete - a cautionary tale

The control of cut depth is one of the more difficult aspects of using high pressure waterjets in places where the aim is not to cut all the way through a part. The ability of an abrasive jet to continue cutting beyond the expected target depth can first be evident to an operator when they leave the jet running, but stop the motion while they go and do something else. On their return they discover that the jet has cut, not only through the part, but also the bottom of the cutting tank, and in some circumstances also into the concrete floor beneath it. Honesty compels me to admit that the table in my old lab had at least one (repaired) hole in the bottom and a concrete mark to show where. I know of a least one very prestigious university with a waterjet that has the same sort of feature (they actually did it before we did).

Cut depth control with a plain waterjet is a little easier, since the water will run out of energy – or the jet structure can be tailored to control its effective range more easily than with the higher density abrasive particles.

Life becomes a little more complicated where the traverse speeds are slower, where the bottom of the slot will become very irregular as the cutting jet tracks backwards and forwards as the nozzle moves at a steadier pace. Henning has divided the cut section into three zones:

Figure 1. The division of the cutting edge into three zones (Henning et al 18th ISJCT)

The fluctuating patterns if the jets are cutting down to zone three make it more difficult to retain control of depth, which is most easily achieved if the cutting is restricted to zone one and the abrasive is restricted to primary impact , without the additional cutting that comes where the jet and particles bounce further down the cut, as shown in the pictures on the right of figure 1.

Restricting the cutting depth in this way (and reaching the required depth of cut with multiple passes) works quite well for abrasive jet cutting of different materials and is the technique often used in milling pockets into a variety of materials, as discussed earlier.

There are, however, some risks to this in the use of plain jets, particularly when working with target items that are made up of different materials – such as concrete. One of the problems was identified fairly early on, in the use of high pressure jets to clean surface runways at airports.

The aim for jet use on runways is to remove the surface coating of rubber that is laid down on the tarmac when planes land and in that first instant of contact as the wheels come up to speed, a small amount of rubber is moved from the tire to the pavement. However, if the jet parameters for cleaning this surface layer are not picked correctly then the jet will remove not just the rubber, put also some of the cement from around the aggregate particles in the surface.

The problem that this raises is that the cement is rough, while the pebbles of cement are usually smoother (since the often come from river deposits). Thus if the cement around the surface exposure of the pebbles is removed, a smoother surface is left on the runway. This is not good, since the point of the rougher surface is to provide friction that will slow the plane down, and the polished surface removes that traction.

The pressure of the jet can be adjusted so that, at the point where it is hitting the cement it no longer has the power to remove it, but this is a value that is going to change with the pump operating pressure, the nozzle diameter, and the standoff distance between the nozzle and the runway. It will also vary with the type of materials that are in the runway itself, so it is very smart to try some test runs at different control values before going onto the field to do the actual removal.

Concrete properties change quite a lot from place to place. In some of the earlier work that was carried out on showing how jets could cut through concrete, tests were carried out at an airfield in the southern United States. For the purpose of the tests cuts had to be made through the pavement, so that pieces of it could be easily removed.

Our approach was similar to that used when we cut the walls at the University using a rotating waterjet on a small carrier (though as memory serves this was a modified riding lawn mower) to traverse back and forward over the cut, moving the nozzle down each time.

The problem that we ran into was that we wanted to cut a slot that was about 2 inches in width, which we had presumed would be wide enough to liberate the pebbles and give access to the deeper parts of the slab. Unfortunately in this case the pebbles that had been used in making the concrete were more than two-inches in size, and so when there were parts of these sticking out of each side of the opening there was not enough of a gap between them to get the assembly into the slot and to deepen the hole, without a lot of adjustments.

It was possible to cut through by making the cut slot wider by making a second, adjacent cut, and with the jets cutting down about 2 inches into the material on each pass, it was possible to work down through to the bottom of the slab, although the large size of the aggregate meant that the nozzle path itself had to be at a greater distance from the wall than we had planned. The combination meant that it was not nearly as rapid an operation as we had anticipated. (The traverse rate was about 2 ft/minute, which was much slower than expected to allow the jets to undercut the larger pebbles). Much more material had to be cut out of each slot in order to achieve full cutting through the slab and this slowed the cutting process – plus there was the time needed to work out how best to change the cutting patterns on site so as to make the process work at all. (And the pebbles were a quartzite aggregate so that even increasing the jet pressure would not have effectively cut them, without adding abrasive to the mix, which was not – at the time – a viable alternative).

The point in mentioning this is that, while the job seemed initially to be a relatively simple one, because we did not know enough about the target material we were caught off-guard when it turned out to differ from our assumptions. We have been caught that way a number of times. We were asked at one time to demonstrate precision cutting of a piece of metal – assumed it would be no more than two-inches thick, and set up a cutting time based on that assumption, and then were faced with a block of eight-inch thick Hastelloy. Which we did cut, as requested, but it took some changes in the cutting plan, which had not been built into the day’s schedule. Asking those few extra questions, in both cases, would have saved us some embarrassment and time.

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 8b - Repairing concrete

Some years ago we were on a bridge in Michigan, working on a demonstration of the ability of high-pressure jets to remove damaged concrete from the surface of the bridge. Before the demonstration began the state bridge inspector walked over the bridge armed with a length of chain. He would drop the lower links of the chain against the concrete at regular intervals, and depending on the sound made by the contact, would decide if the concrete was good, or not. He then marked out the damaged zones on the concrete, and suggested that we get to work and remove those patches.


Figure 1. Automated removal of damaged concrete from a bridge in Michigan

The change in the sound that he heard, and used to find the bad patches in the 1concrete, was caused by the growth of cracks in that concrete. It was these longer cracks, and delaminations in the concrete that made it sound “drummy” and which identified it as bad concrete.

Now here is the initial advantage that a high-pressure waterjet has in such a case. The water will penetrate into these cracks. As I mentioned in an earlier post, water removes material by growing existing cracks until they intersect, and pieces of the surface are removed. The bigger the cracks in the surface, the lower the pressure that is needed to cause them to grow. This is because the water fills the crack, and pressurizes the water, the longer the crack, the greater the resulting force, and thus the greater the ease in removing material.

At an operating waterjet pressure of between 11,000 and 12,500 psi, for a normal bridge-deck concrete, the cracks that are long enough for an inspector to call the bridge “damaged” will grow and cause the damaged material to break off. The pressure is low enough, however, that it will not grow the smaller cracks in “good” concrete, which is therefore left in place.


Figure 2. Damaged area of bridge after jet passes.

In order to cover the bridge effectively and at a reasonable speed, six jets were directed down from the ends of a set of rotating crossheads, within a protective cover. The diameter of the path was around 2 feet, and the head was traversed over the bridge so that it took about a minute for the head to sweep the width of a traffic lane.


Figure 3. Scarifying jets, with the head raised above the deck so that their location can be seen. Normally the nozzles are positioned just above the deck, so that the rebounding material is caught in the shroud.

Unfortunately, while this means that the rotating waterjet head could distinguish between good and bad, and remove the latter while leaving the former, it could not read marks on concrete. So where the bridge inspector was not totally accurate, the jet removal did not follow his recommendations. It was, however, quite good at removing damaged concrete from reinforcing bar in the concrete, where the water migration along the rebar had also caused the metal to rust. And, since the pressure was low enough to remove the cement bonding, without digging out or breaking the small pebbles in the concrete, they remained partially anchored in the residual concrete. As a result when the new pour was made over the cleaned surface, the new cement could bond to the original pebbles, and this gave a rough non-laminar surface, which provided a much better bond than that left had the damaged material been removed mechanically with a grinding tool.


Figure 4. Rebar cleaned by the action of the jet as it removes the surrounding damaged concrete.

Waterjets had an additional advantage at this point in that, in contrast with the jackhammer that had previously been used to dig out the damaged region, but which vibrated the rebar when it was hit, so that damage spread along the bar outside the zone being repaired, with the jet action there was no similar force, so that the delamination was largely eliminated.

Now this ability to sense and remove all the damaged concrete is not an unmixed blessing. Consider that a bridge deck is typically several inches thick, and it is usually sufficient to remove damaged concrete to a point just below the top layer of the reinforcing rods. Once the damaged material is removed, then the new pour bonds to the underlying cement and the cleaned rebar. But the waterjets cannot read rulers either. So in early cases where the deck was more thoroughly damaged than the contractor knew at the time that the job began, the jet might remove all the damaged concrete, and this might mean the entire thickness of the bridge deck. And OOPS this could be very expensive in time and material to replace.

What was therefore needed was a tool that still retained some of the advantages of the existing waterjet system, that it cut through weakened concrete, and cleaned the rebar without vibration, but that it did so with a more limited range, so that the depth of material removal could be controlled.

There was an additional problem that also developed with the original concept. For though the jets removed damaged concrete well in this pressure range, the jets were characteristically quite large (about 0.04 inches or so). The damaged concrete is contaminated with grease and other deposits from the vehicles that passed over it. Thus any large volumes of cleaning water would also become contaminated, and, as a result will have to be collected and treated. That can be expensive, and so any way of reducing the water volume would be helpful.

The answer to both problems was to use smaller jets at higher pressures. Because of the smaller size, their range is limited, and at the same time the amount of water involved can be dramatically reduced. It does mean that the jet is no longer as discriminatory between “good” concrete and “bad.” This is not, however, a totally bad thing, since when working to clean around the reinforcing rods, there has to be a large enough passage for the new fill to be able to easily spread into all the gaps and establish a good bond.

Thus the vast majority of concrete removal tools that are currently in use are operated at higher pressures, and lower flow rates. This allows the floor to be relatively evenly removed down to a designated depth, and this makes the quantification of the amount of material to be used in repair to be better estimated, and the costs of disposal of the spent fluid and material to be minimized.


Figure 5. Scarified garage floor showing the rough underlying surface. This will give a good bond to the repair material, as will the cleaned rebar.

The higher pressure system has the incidental advantage of reducing the back thrust on the cutting heads, so that the overall size of the equipment can be reduced, allowing repair in more confined conditions.