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 17a - Runways and discriminatory cleaning

Welcome back as we begin a New Year, and I hope that it brings Prosperity and Happiness to you and yours and that it brings much success.

Somewhere within the celebrations of the last couple of weeks water has played a considerable role, probably largest in the many different ways in which it was used to clean items –either before or after use. Water, often with a bit of soap, has been one of the earliest cleansers helping to loosen and dislodge dirt as the flow passes over the surface. There have been studies in the past which suggest that there is little advantage to the use of antibacterial soaps over less expensive conventional soap. Although the initial study was in 2005 it has only been within the last week that the FDA has told manufacturers of antibacterial soaps and washes that they have to demonstrate that they are both safe and effective. They note:

In fact, there currently is no evidence that over-the-counter (OTC) antibacterial soap products are any more effective at preventing illness than washing with plain soap and water, says Colleen Rogers, Ph.D., a lead microbiologist at FDA. Moreover, antibacterial soap products contain chemical ingredients, such as triclosan and triclocarban, which may carry unnecessary risks given that their benefits are unproven.

"New data suggest that the risks associated with long-term, daily use of antibacterial soaps may outweigh the benefits," Rogers says. There are indications that certain ingredients in these soaps may contribute to bacterial resistance to antibiotics, and may have unanticipated hormonal effects that are of concern to FDA.

In light of these data, the agency issued a proposed rule on Dec. 16, 2013 that would require manufacturers to provide more substantial data to demonstrate the safety and effectiveness of antibacterial soaps. The proposed rule covers only those consumer antibacterial soaps and body washes that are used with water. It does not apply to hand sanitizers, hand wipes or antibacterial soaps that are used in health care settings such as hospitals.

The debate over the use of chemicals with water, rather than just relying on the mechanical force of the water to dislodge dirt (used generically to include bacteria and other undesired coatings) or the combination of water with a mechanical action (using a brush or rubbing hands together) has been evaluated in many circumstances, with different results. One path will form the thread for the next few posts, and it will take us to a perhaps unexpected product line. Given that this is the season when a lot of us travel, let's begin at an airport.

Figure 1. Plane landing at airport – note the puff of smoke from the tires (Seattle Times)

When a plane first makes contact with the end of a runway, as it comes in to land, the wheels are not rotating very fast on contact, and so there is a small smear of rubber left on the end of the runway as the tires are dragged up to the right speed by the movement of the aircraft over the tarmac. (A plane can lose a pound of rubber per tire on landing).

Over a period of time this thin layer of rubber starts to build up over the surface, covering and smoothing the rough asperities that allow subsequent tire impacts to grip the runway, and making the surface slick and less tractive as a way of slowing the aircraft. As a result planes can start to hydroplane in rainy conditions and several major accidents have been blamed on this layer being allowed to grow too thick and become a dangerous surface.

For many years the practice was therefore to bring out different chemical trucks to spray the surface and a recent operation in Port-au-Prince describes the operation.

The chemicals that the team used were environmentally safe and effective in cleaning rubber deposits from the surface. The chemicals were sprayed onto the airfield and then scrubbed, brushed and worked into the rubber deposits for approximately four hours. The chemicals loosened the sticky rubber buildup into a soft gel that was then removed from the runway with a brush and water onto the shoulders of the runway. . . . . .

The AOS Research Group has developed a C-130 deployable chemical rubber removal system that is light, compact and requires no in-theater support other than fuel and water. AFCESA provided $125,000 in Research and Development funds to evaluate the rubber removal system in actual expeditionary environment. Four team members (Figure 2) traveled to Haiti and used the rubber removal technology to remove rubber build-up from the runway. In three days (approximately three eight hour shifts) they cleared 125,000 square feet on the west end of the runway and 75,000 square feet on the east end.

Figure 2. Results from chemical removal of rubber at Port au Prince (ARA)

The process is relatively slow and somewhat manually intensive, which may be an advantage in a labor-rich environment such as Haiti, but which makes the process very expensive in places such as Europe and the United States. And this provided an opportunity for a new business. As I wrote first in “Waterjetting Technology”:

One of the fascinating stories of the waterjet cleaning industry is that of Bob White, and his wife Donna. In 1972 Mr. White was a small painting contractor who unsuccessfully tried to clean the rubber deposits left from the wheels of landing aircraft from the runway at McClellan Air Force Base in California (now closed). Although able to remove material at only 60 sq ft/hr, he was convinced, particularly after seeing that the state of the art system was a chemical treatment, that waterjetting was the answer. Through a combination of loans from a variety of sources to the tune of $180,000 he built a four-pump, 24 nozzle spray bar system, initially operating at 10,000 psi and went into business. By driving himself through the night and thus being able to underbid the competition he took the first 25 of 28 jobs which were bid, and by the end of that year he had paid off his loans. By 1977 he had 5 rigs on the road around the country and was anticipating his first million dollar year.

Figure 3. Bob White (From the Duck Book).

After the first year on the road it was found, for most applications, that the jets removed the rubber better at 5,000 to 6,000 psi rather than at 10.000 psi and there was the added risk that at the higher pressure the water would either damage or polish the underlying concrete, depending on its quality and that of the aggregate contained. By supplying the flow from each pump to a six nozzle section of the spray bar it was possible to isolate a section should it have a problem, while still operating the rest of the units.

Figure 4. Bob's early rig (ibid)

The rig used spray bars fitted with 36 nozzles of hardened steel, 0.062 inch orifice diameters with a 30 degree spread, required to give 1 inch of coverage at a 0.75 inch standoff used. Other available equipment used 2,750 hp pump units to supply 0.078 inch diameter jets held one-inch apart, and with up to 96 nozzles on the spray bar. Such a unit could clean paint build up over 0.18 inch thick at a rate of 13,000 sq ft/hr, at a cost of $0.064/sq ft. Lowering the nozzle diameter to 0.04 inches resulted in a bar 94 inches long which when held an inch and a half above the surface, at 7,000 psi would allow cleaning at the rate of 60 ft/min, for a combined rate of 40,000 – 50,000 sq. ft./hr. The cost was estimated at $0.037 to $0.05 per sq ft.

In 1974 White was winning contracts at $0.045 per sq ft, and going below $0.032 per sq ft to get others. He had at that time 34 competitors, and yet grossed $280,000 that year. In 1975 he was bidding runway cleaning at less than $0.022 per sq ft for larger areas. Nozzle life was on the order of 50 hours for the stainless steel tips, and 250 hours for the steel holders. In January of 1978 Bob White discovered he had cancer and although successfully treated by 1980 he had sold off his rigs to highway painting contractors while he himself had turned to publishing.

He was sadly reported as being murdered in Belize in 1988. He was a true Pioneer of the industry.

Runway cleaning has progressed considerably since those early days and modern systems use higher pressures, in part to reduce the amount of contaminated water that is produced, since the cost of collection and disposal becomes significant. There is also a marriage of chemical pre-treatment and waterblasting which is shown in a video here.

But the ability of the waterjet system to discriminately remove the overlying coating (rubber) without damage to the underlying surface led on to other applications, which I will cover in the next few posts.