Saturday, August 9, 2014

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.

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