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.