Waterjetting Technology – Repairing Concrete

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

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 got to work and removed those patches.

Automated removal of damaged concrete with water pressure

Figure 1. Automated removal of damaged concrete with water pressure

The change in the sound that he heard and used to find the bad patches in the concrete 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.

Damaged area of bridge after jet passes

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.

Scarifying jets with the head raised above the deck so that their location can be seen

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 if the damaged material had been removed mechanically with a grinding tool.

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

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 contrast to 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, the waterjet did not exert a 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, 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, namely 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.

Scarified garage floor showing the rough underlying surface

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.

Waterjet Technology – Cleaning with Heat

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

Water is used almost everywhere as a way of cleaning surfaces. Several times a day, we typically rub our hands together with water and usually with some soap to clean them. Pediatricians and others suggest that children recite a short rhythm such as a chorus of “Happy Birthday” while doing so to allow the water, soap and mechanical actions to combine and effectively remove dirt. That teaches the child that it takes some 20 seconds for the cleaning action to be effective. The cleaning action is not to sterilize germs, viruses and other obnoxious things on the hands. Rather it is to ensure that they and other dirt particles are physically removed, leaving the hands clean. (This is a different action to the chemical washes that are becoming popular.)

This is not an instantaneous process since the soap and water must reach into all the dirt-collecting parts of the hand – hence the need for the nursery rhythm. The same basic sequence occurs in the cleaning action of a high-pressure waterjet on a surface, although the pressure of the spray means that the water can penetrate faster. But it is why, in using a car wash lance in cleaning a car, it is smart to spray the body of the car with a detergent first, and then allow this to work in creating micelle clusters around the dirt particles, so that the mechanical action of the subsequent jet spray will dislodge and remove them. Merely adding detergent to the cleaning water as it goes through the cleaning lance and strikes the car surface does not give the chemicals in the water time to act before they are gone. Bear in mind that the jet is moving at several hundred feet per second and that it hits and rebounds from the surface over a path length of perhaps an inch or two. As a result, the residence time of the jet on the surface is measured in fractions of a millisecond. This is not enough time for the chemicals to work. (On the other hand it does help keep the sewers under the car wash cleaner than might be otherwise expected.)

With an increase in jet pressure, the speed of the mechanical removal of dirt and other particles from a surface can be fast and effective. The ability of the jet to penetrate into and flush out surface cracks and joints means that it becomes a good tool for removing debris from the joints in concrete decks, and, at a little higher pressure, it can also be used to remove deteriorated concrete from surfaces. But I am going to leave that topic until next week.

The other “treatment” that we use when we wash our hands is to heat the water. When used with soap, it helps to remove the surface oils on the skin that act as a host to bacteria. Heat is becoming a less common tool than it used to be in high-pressure jet cleaning. At one time, steam cleaning which was followed by hot pressure-washing had a larger sector of the market. It is a bit more difficult to work with (the handles of the gun get hot, and the operator needs more protection) but for some work it is still the more effective way to go.

Steam, however, loses both heat and mechanical energy very quickly after it leaves the nozzle. It will, for example, lose some 30% of its temperature within a foot of the nozzle. Hot sprays of water can thus be more effective, but when cleaning grease and oils, a lower temperature spray will merely move the globs of grease around the surface. Heating the water to around 185 degrees Fahrenheit (or 85 degrees C) will stop that happening and works much more effectively in getting the surface clean.

The effect of water temperature on cleaning different surfaces of different types of dirt

Figure 1. The effect of water temperature on cleaning different surfaces (A, B and C) of different types of dirt.

But, as with many tools, heated water needs to be applied with a little bit of background knowledge. I mentioned that just pointing a large jet of water at, for the sake of discussion, a boulder covered with an oil spill would, at lower water temperatures, just move the oil around the surface. At higher temperatures, the oil would break into smaller fragments that are removed from the surface, but they need to be captured, otherwise the treatment is just spreading the problem over a larger area. This is why it becomes more effective to use smaller, higher pressure systems that have lower contained jet energy and which can be used with a vacuum collection system to pick up the displaced water, oil and debris.

Using hot, pressurized water streams in cleaning up after the Exxon Valdez oil spill

Figure 2. Using hot, pressurized water streams in cleaning up after the Exxon Valdez oil spill (NOAA )

With the streams used in the picture shown in Figure 2, the energy in the jet will move the oil, but without containment it was being washed down to the water, where it was collected using booms. This is not particularly effective since in the process, the jets also washed the silt out of the beach and drove some of the oil down into the underlying beach structure, so that it continued to emerge in later years contributing to an ongoing problem.

What is needed is to provide enough energy to drive the oil away from the surface and yet not enough to move it great distances or to disrupt the surrounding material. This can be achieved by using a higher-pressure but lower flow rate jet. Because some of the water will turn to steam as it leaves the nozzle, Short (PhD U Michigan, 1963) showed that the droplet size will fall from 250 microns to 50 microns when the water is heated above 100 degC.

Obviously, that also will reduce the distance that the jet is effective, and so a balance needs to be achieved between the heat put into the water and the size of the orifice(s) if the jets are to remove the contamination but in such a way that it can be captured. And here again there is a benefit from having a suction tool associated with the cleaning spray. Because of the problems that oil and grease can cause, it will require special care in designing the capture systems downstream. Incidentally, it is generally better if the water is heated downstream of the pump, since there are higher risks of cavitation in the inlet ports if the water is too hot.

And sometimes the two can be combined in ingenious ways. For example Bury (2nd BHRA ISJCT, Cambridge, 1974) added a steam shroud around a conventional waterjet at 5,000 psi as a way of cleaning hardened plastic from the insides of a chemical plant pipe.

Wrapping a conventional waterjet in a steam shroud

Figure 3. Wrapping a conventional waterjet in a steam shroud (Bury et al 2nd BHRA ISJCT, Cambridge, 1974)

Without the steam assist, the plastic was not removable even at higher jet pressures, but with the steam to soften the plastic the pipe was successfully cleaned.

High-pressure water fails to remove hardened plastic

Figure 4. High-pressure water fails to remove hardened plastic, (lhs) but with a steam shroud a lower-pressure jet effectively cleans the pipe (rhs). (Bury et al 2nd BHRA ISJCT, Cambridge, 1974).