A STUDY ON USE OF WATERJET IN CONSTRUCTION
Abstract Use of abrasive waterjet has been in existence for over twenty years, but it is yet to reach its full potential in the construction industry. This study examines the role of waterjet in heavy construction in general and particularly how it
affected the regional contractors in the northeast. A survey was conducted among 215 civil contractors of the Northeast region of the United States and the results are documented in various categories. Introduction Waterjets were introduced in the United States during the 1970’s, and were utilized merely for cleaning purposes [1]. As the technology developed to include abrasive waterjets, new applications were disco
vered. However, until recently this tool had not been used to a great extent in the construction industry. The abrasive waterjet offers several advantages over conventional cutting
techniques. With stricter environmental laws in most states, waterjet technology may be the only method that can replace conventional tools. The following is a summary of some important benefits [2]. Low levels of Dust Generated: The water car
ries away the abrasive particles and the kerf material making the process virtually dust free. No Sharpening Needed: There is no direct contact between the waterjet and the material being cut. Therefore there are no parts to be sharpened. N
o Heat Created During Cutting: The waterjet is safe to use in explosive environments, such as fuel tank demolition. High Quality Cut: Finishing steps are avoided or reduced in metals. This saves both time and material. Compatible with R
obots: Low reaction forces allow robotics to be used. This also allows for easier handling of the material. According to Jet Edge, a manufacturer of the waterjet, some other advantages include [3], Portable Cutting and Easy to Us
e features. Along with the advantages of waterjet cutting there are also some disadvantages. The waterjet is a sophisticated machine: With the depth of cut dependent on many parameters, it takes trained operators to get the best performance at the lowest cos
t. Other disadvantages include: High Initial Cost of Machinery: Depending on the system, the price can range from a low of $20,000 to a high of $500,000 [3]. High Noise Level: With a proper catcher the abrasive waterjet is as loud
as 90 dB. In open air the noise level can reach 110 dB or higher [4]. Mixing Nozzle Wear: The mixing nozzle needs to be replaced every 2-6 hours depending on the abrasive type being used and abrasive flow rate [5]. Components The abrasive waterjet consists of a stream of ultra-high pressure water carrying abrasive at velocities of up to 2500 feet per second. The cutting is done by an erosive and shearing action of the material. The abrasive acts as tiny knives cutting
away the material, in the same manner as saw cuts, only on a much smaller scale and in rapid succession [6]. Pump High pressure intensifier pumps are used to pressurize the water as high as 55,000 psi. For the abrasive waterjet, the operating pressure ranges from 31,000 to 37,000 psi. At this high pressure the flow rate of the water is reduced greatly [6].
The flow rate for Flow International Corporation’s "Jetpac Intensifier Pump" - model 40Dt is only 4.0 gpm at an operating pressure of 35,000 psi [7]. This is significantly less than the flow rate of traditional hydroblasting methods. Nozzle Once the water is pressurized, it is forced through a sapphire nozzle which is composed of the natural sapphire stone due to the strength of the stone. The nozzle can have diameters ranging from 0.003 to 0.026 inches. The diameter of the nozzle
can be varied depending on the application for which the waterjet is being used. A damaged nozzle leads to poor cohesion of the stream, thereby reducing the cutting ability greatly. The nozzle typically will last 100 to 200 hours before it needs to be r
eplaced [8]. Mixing Tube The stream of water which emerges from the nozzle is then mixed with the abrasive. This takes place in the mixing tube, which is usually constructed out of tungsten-carbide. Wear of the mixing tube, due to the abrasive, is a problem and it ne
eds frequent replacement. When the tube becomes worn, the jet no longer is cohesive and loses power and cutting ability rapidly. At this point the stream with the abrasive leaves the mixing chamber and is ready to cut almost any material. The diameter of the stream varies from 0.020 to 0.090 inches, which accounts for the small kerf width associated with abrasive waterjet c
utting [6]. The abrasive type and size varies depending on the application. Garnet sand is the most common abrasive type due to its abundance and relatively low cost. Aluminum oxide is also fairly common. It is harder than garnet sand, but it is also more e
xpensive. Catchers
Main Construction Applications
Concrete Deck Removal
The removal of deteriorated concrete from bridges and parking decks is an area where waterjets have already been used successfully. The process of selective removal is defined as "...the capacity to remove completely only the deteriorated concrete, independently of depth to which the damage has penetrated [12]". The waterjet is able to remove the damaged concrete without causing microcracks in the healthy concrete or damaging the reinforcement bars. Although it is more costly to operate a wat erjet tool than a jackhammer for deck removal, there are savings by the shortened time span needed to complete the job and from the need for fewer laborers.Paint Removal
In the United States the removal of lead-based paint from bridges is causing big problems. Traditionally, sand blasting was carried out in the open with no concern for the surrounding environment. Today many states require the complete containment of the removed produce as well as the waste generated from blasting [14]. Waterjet technology has been used successfully for coating removal. The abrasive waterjet is able to remove the coating without creating dust or damaging the base metal, unlike tool s such as mechanical brushes [16]. In the removal process, the metal is also cleaned and prepared for a new application of paint. The surface is cleaned of soluble corrosives and blasted to the specified surface roughness.
Cutting
In the construction industry, the abrasive waterjet will most likely be used for cutting steel beams and concrete sections. For steel, the cutting rates are slower with the waterjet than for other tools, such as plasma arcs. However, often the cost is o ffset by the time saved by reducing or eliminating finishing steps. This is because there is no heat affected zone with the waterjet. The waterjet has also been able to cut through thick slabs of concrete. This will save in the cost of buying diamond t ipped saws and sharpening them [4].Rivet Removal
A study was conducted for the Canadian National Railway on the removal of rivets from railway bridges [17]. Pneumatic tools had been used and were found to be inefficient because the weight of the tool caused excessive fatigue of the operator. Also, a t ool was needed that would be able to be maneuvered in small spaces to remove the rivets close to the flange of the girder. Waterjet cutting was found to be a feasible method for removing rivets with thicknesses less than 2 inches. At greater thickness the quality of the cut became unacceptable, and a tool that could be inserted into the cut would be needed to perform the job . It was concluded that it is conceivable to design such a tool that would allow deeper cuts without sacrificing edge quality.Survey
In order to gain a better understanding of how much the industry knows about waterjet technology, a survey was mailed out to various companies. A total of 215 surveys were mailed along with the cover letter and a self-addressed envelope for return. The construction companies were chosen out of the list provided by Associated General Contractors (AGC) in a random manner. A total of 95 surveys were returned. Approximately 42 percent of the companies are familiar with the waterjet. Of the 42 percent wh o were familiar with its use, only about 17 percent of them actually employ this technology in their company.Of the 17% who use the abrasive waterjet, 100% of them do employ another cutting technique. Eighty-eight percent use the pneumatic hammer, 63% use the diamond blade power saw, 19% employ other techniques such as torches, hydroblasting and grinders.
Of the 17% who employ the abrasive waterjet in their business, 56% use the waterjet for concrete cutting, 38% use the waterjet for steel cutting and 63% use the waterjet for cleaning and paint removal.
Of the 83% who do not use the abrasive waterjet, 76% use the pneumatic hammer, 83% use the diamond blade power saw, 9% use the mechanical brushes and 9% use other techniques such as verneer trenches and torches.
Conclusion and Limitations
It appeared after studying the advantages and disadvantages of the waterjet, that this is a tool that the construction industry should find very useful. Unfortunately, this does not seem to be the case. Many of the regional companies do not seem to have any significant knowledge of the waterjet, thus remaining unwilling to employ this technology.The responses that we have received have left us with the inability to comment on the cost effectiveness of the waterjet in the construction industry. The majority of companies that we contacted do not employ the waterjet in their companies, nor d o most of them have any knowledge of the abrasive waterjet. These companies seem to be conservative to new technology and unwilling to take risks. This may also be due to the fact that many companies are unwilling to invest in a new technology that is n ot widely used.
The contractors that do employ the abrasive waterjet technology did not provide us with the percentage of cost benefit to their company.
REFERENCES
[1] Behringer -Plosonka, Catherine A., "Waterjet Cutting - A Technology Afloat on a Sea of Potential", Manufacturing Engineering, Vol. 99, Nov. 1987, pp. 37-41.
[2] Haylock, Rob, "Waterjet Cutting", Conference: Automach Australia "85, July 2-5, 1985, Melbourne, Australia.
[3] Slattery, Thomas J., "Abrasive Waterjet Carves Out Metalworking Niche", Machine and Tool Blue Book, July 1991, pp. 31-34.
[4] Hitchox, Alan L., "Vote of Confidence for Abrasive Waterjet Cutting", Metal Progress, Vol. 130, July 1986, pp. 33-34.
[5] Flow International, 1995.
[6] Steinhauser, John, "Abrasive Waterjets: on the ‘Cutting Edge’ of Technology", Flow Systems, Inc.
[7] "Jetpac Intensifier Pump - Model 40DT", Flow Specs, FS-130, Sept. 1989.
[8] Ayers, Gary W., "Principles of Waterjet Cutting", Tappi Journal, Vol. 70, Sept 1993, pp. 91-94.
[9] Zaring, K., "Advanced Abrasive Waterjet Hardware and Cutting Performance", 5th American Waterjet Conference, Aug. 1989.
[10] Hashish, M., "Steel Cutting With Abrasive Waterjets", 6th International Symposium on Jet Cutting Technology, April 1982.
[11] Vijay, M.M., "A Critical Examination of the Use of Waterjets for Medical Applications", 5th American Waterjet Conference, Aug. 1989.
[12] Medeot, R., "History, Theory and Practice of Hydrodemolition", 5th American Waterjet Conference, Aug. 1989.
[13] Schmid, R.F., "High Pressure Hydro-Milling of Concrete Surfaces", 5th American Waterjet Conference, Aug. 1989.
[14] Peart, John W., "Lead-Pigmented Paints - Their Impact on Bridge Maintenance Strategies and Costs", Public Roads, Vol. 52 Sept. 1992, pp. 47-51.
[15] Katausakas, Ted, "DOT Coasts Rusting Bridges With Layers of Problems", R&D Magazine, May 1990, pp. 43-48.
[16] Baldry, Ian, "Surface Preparation", Civil Engineering (London, England), April 1987, pp. 47-48.
[17] Falcone, Linda M. Tonkay, Gregory L., "Economic Feasibility of a Tool to Remove Rivets From Railway Bridges", Proposal to The Canadian National Railway, April 1990.