Waterjetting 29d - Fixing an Oops.

There was a story that is told in Mining Engineering classes about a tunnel that collapsed, even after there had been a whole series of tests carried out to make sure that the rock was strong enough before the tunnel excavation was started. In working out why the tunnel had collapsed some questions were raised about the tests made on the rock samples. It turned out that the testing technicians had received the samples and struggled to find good enough quality pieces of rock in the sample from which they could extract the required sample sizes to run the standard strength tests.

When they reported the results of their tests these predicted that the rock would be strong enough to stand, without collapse, for a long enough time for artificial supports to be placed under the rock and to hold it in place. But it was not that strong segment of the rock that failed, rather it was the rather more rotten rock that surrounded it which provided the weakest link in the tunnel wall. That material had been too weak to make into a sample, and the technician had therefore not reported the lack of strength.

Knowing the properties of a target material before starting a job is an important part of correctly forecasting how how long it will take to perform the cutting tasks required and, as a result, how much to charge for the work. And further to ensure that there is no unanticipated cost that will come from the use of the waterjet tool at the parameters planned.

These unintended consequences have, for example arisen in the past when a high-pressure waterjet system was being used to remove damaged concrete from the surface of a bridge. (As with the tunnel we’ll keep the bridge as an unidentified example).

In repairing a bridge deck it is usually required that the top layer of concrete be removed just past the top layer of reinforcing steel (rebar). This allows a good bond between the previous concrete and the repair pour, which also bonds to the rebar giving a repair that will last for some time. (More conventional repairs leave a weakened joint between the repair and the old concrete which fails more rapidly in many cases).

However the waterjet system is only discriminatory to some extent. The jet pressure can be set so that it will only remove damaged concrete, for example, but does not have sufficient pressure to remove healthy (and less cracked) material. But if the material is weaker than expected, or the damage extends further into the deck than was expected, then the waterjet system will continue to remove damaged concrete, even if this means it ends up removing material all the way through the deck. This can be a real problem, given the extra money and time that must now be spent in replacing that additional concrete, and ensuring that full integrity is restored to the deck. This additional cost can be more than the price of the original repair work, and do serious damage to the economic health of the waterjet company. Unfortunately with many of the systems today becoming more and more automated, it requires close attention the machine at all times to ensure that only the required amount of material is removed and no more.

One solution to the problem is, at least initially, the very opposite of what you might think would be the best answer. It is to increase the pressure of the jets removing the material. For a system with the same horsepower as the machine that was being used first, this means that the amount of water used will be less, and the nozzle diameters will, as a result, also shrink in size.

The smaller jet diameters and higher pressures mean that the cutting distance of the jets themselves will become shorter, as the jet decays more rapidly with distance. (To give an extreme example a 1200 psi jet at a diameter of about an inch-and-a-half can throw a jet about 125 ft. At 50,000 psi and at a diameter of about 0.005 inches the range of the jet is usually less than 2 inches*.) Within their effective range, the higher pressure jets will cut much faster, and so it is possible, by mounting the cutting nozzles in an array that spins around a common axis, to rapidly clean a swath of material (say up to 2 ft in width) as the head moves across a traffic lane at an advance rate of roughly 1 pass a minute as it moves up over the bridge. The higher rotational speeds will also restrict the depth to which the jets can cut on a single pass, so that the depth of material removed can be relatively accurately programmed into the machine by adjusting some of the operating controls. (After first finding out what the best parameters will be for THAT bridge concrete in a small test area off the main work site).

There is an additional advantage to using the higher pressures, and that comes with the smaller volumes of water that will be required to take the damaged layer of concrete from the surface. This water will be contaminated by the different fluids that may have soaked into the bridge over time, and by the corrosion products of the deterioration. For these reasons all the debris and water from the demolition operation will have to be collected, removed and properly (and expensively) disposed of. The higher the volume of water then the greater the collection and disposal cost, and the lower water volumes needed with higher pressures will thus carry a lower disposal price.

The example given here is for the removal of damaged concrete from bridge decks and garage floors, but the underlying principle also applies in the milling of pockets into materials of differing composition, where a controlled depth of cut needs to be held, even if the material strength changes.

*The word “usually” is used since there are ways of increasing the jet throw to several thousand orifice diameters.