Hydrodemolition: Why 'Yes' and Why 'No' 

Jerzy Remisz, Ph.D, P.Eng. Applied Fluid Mechanics, Inc., Toronto, Canada
Wojciech Remisz, M.Sc., P.Eng. Remisz & Associates Engineering Ltd., Ottawa, Canada
( 7th American Water Jet Conference, August 28-31, 1993: Seattle, Washington, USA )

ABSTRACT

Water jet technology has been known for well over 20 years. One of its best known and developed applications is hydrodemolition of concrete structures provided for rehabilitation. Using hydrodemolition in place of conventional jackhammers provides a number of advantages, such as excellent surface preparation, far superior to the traditional method; the concrete is well saturated, free of chlorides, rough, with perfectly cleaned rebars; and when new toping is placed, slowly migrating moisture enhances its curing. Hydrodemolition eliminates harmful structural vibrations, dust and high noise level associated with jackhammering operations. By avoiding structural vibrations and secondary damages caused by jackhammers to the sound concrete longer between-repairs time can be achieved.

Despite of all benefits provided by hydrodemolition, and its long history, commercial use of this technology is still rather marginal.

In this study effort was made to identify all parties involved in a typical concrete restoration project, their approach to the problem, and resulting effect on the selection of the technology finally employed.

It is not only contractor's decision to use or not to use hydrodemolition. The problem is more complex and to efficiently promote the technology it is not enough to improve its technical parameters. Efforts are needed to convince both the public (the repaired structure owners) and consultants that quality of the job done by water jets and its long-term repercussions should dominate over the short-term cost savings.

1. INTRODUCTION

Advantages of water jet technology in various applications have been well known for over 20 years. One of the domains where the technology and the equipment for its application are well commercially developed is construction industry, and particularly, demolition of concrete structures before their rehabilitation.

By using high-energy water jets one can affect concrete selectively, removing already weakened fragments while avoiding damages to the sound material. Such damages, due to transmission of vibrations are typical to commonly used jackhammers, and moreover, they can spread far away from the actually repaired area reducing considerably life-time of the whole structure. The perfect bonding between the old and the newly poured concrete in case of hydrodemolition-prepared surfaces is well documented. Despite of these predominant and other benefits such as high productivity, more quiet and comfortable operation, reduced risk of vibration-induced injuries to the personnel, the hydrodemolition is still rather of marginal use.

There are several reasons why contractors, even those aware of the benefits of hydro-demolition, in their jobs employ jackhammers. In the paper presented an effort was made to point out those reasons, and to validate them. It is hoped that the knowledge where the real barriers are can help to break them out.

It should be understood that in such a paper weaknesses of the technology are emphasized while its benefits are not discussed here.

2. STATE OF THE ART

Currently several manufacturers over the world offer hydrodemolition equipment. These are for example: Atlas Copco, Aqua-Dyne, Jet Edge, Hammelmann, Water Blast, Cristanini, FIP Industriale, NLB, Indescor and others. The commercially available equipment operates typically at a pressure range about 103 to 207 MPa (15,000 to 30,000 psi) and water consumption about 56 to 230 l/min (15 to 60 gpm). Most of the units are designed for floor operation, with computer controlled parameters. There are few models adapted for vertical and overhead work including hand held lances for hard to reach areas. The production unit consists of: high pressure pump unit powered by a diesel or electric motor (several hundred HP), and a robot which can operate up to a few hundred meters away from the pump.

When traditional jackhammers are used, they are 7kg (15 lbs) units for concrete partial removals, and heavier ones allowed only in the case of complete demolition of the structure. The 7 kg of mass limit was established because of excessive crack propagation outside of the repair area.

2.1 Hydrodemolition versus other methods

Any technique of restoration of concrete involves the removal of the degraded, poor-quality concrete. During this operation it is essential to:
- remove all traces of spalled, delaminated, and poor concrete;
- avoid damage to reinforcement;
- avoid damage to still good-quality concrete;
- ensure good bonding to concrete repair material..

From this standpoint there are some benefits of hydrodemolition (HD) versus draw-backs of other methods (Ref. 1, 2, 3) as shown in Table 1.

3. PARTIES INVOLVED IN A TYPICAL CONCRETE REHABILITATION PROJECT

Generally any concrete rehabilitation project involves the following parties:
- owner
- consultant
- contractor
- public
- legal authorities, represented by by-laws, regulations etc.

Each of the parties has its own specific approach and motivations, and the consensus reached is often far away of what could be seen as optimum by researchers developing the hydro-demolition technology.

The owner: usually it is a government (roads, bridges) or private real-estate corporations (garages, parking). In both cases the ownership of the object is likely to be with the same owner for a long time. This would imply long-term financial policy and sensitivity to methods providing the longest possible lifetime of their properties. From this point of view hydrodemolition is a better option than jackhammers since it imposes no secondary destructive effects onto the sound structure and the reinforcing grid, and removes effectively contaminants (chlorides) from the concrete and reinforcing steel. That kind of surface preparation provides for a longer period before the next repair is required.

However, the dominating practice of today is to call the tenders for the job, and to select the lowest bid. The owner need not have a technical knowledge relating to variety of methods available for contractors. His concern is to have the job done at the price, which is acceptable for him. And even if he knows that something more is available except jackhammers the total time and range of commercial use of the hydrodemolition technology is not long enough to practically prove its superiority in terms of between repairs time.

The owner, therefore, tends to prefer - although indirectly and unknowingly - jackhammers as the less expensive alternative.

Consultant is the owner's representative hired for professional preparation and management of the project. His position requires balancing between financial restrictions imposed by the owner and technical capabilities of contractors, It is, however, his decision of which method of performing each given task shall be employed. Contractors usually shall comply with terms of the project specification set up by the consultant. Consultant's (engineer's) awareness of all aspects of the hydrodemolition, its benefits as well as limitations, is therefore essential for promoting the technology.

Contractor is the next to the consultant in his position to choose the technology and, in practice, his voice can even dominate. A number of contracting businesses operates on contract-to-contract basis. They rent the equipment and hire the people depending on the project they already have won. From their point of view each project has to pay for itself. Capital cost of the hydrodemolition equipment is high and it capitalizes after approximately ton thousand square meters (one hundred thousand square feet) of the surface demolished. If one wants to reach that point within, say, three years, he would have to operate the machine 8 hrs a day for about 200 days a year. For a small- or even medium-size contractor such an equipment utilization is rather a dream, especially in present economic conditions. In contrary conventional jackhammers with all what is necessary to run them, are inexpensive.

Another point here is qualification of the operator for hydrodemolition. Since the equipment is expensive it would be unwise to hire casual workers to operate it. The owner of such an equipment should rather keep the operator on a permanent position. This means he has to pay him high salary even if there is no contract pending. On the other hand people for operating jackhammers need to have simple training only and can be hired for each contract solely.

This is obvious that the contractor will opt for a less expensive equipment, unless he specializes in demolition and anticipates full load for the machine over the next few years.

In some cases the equipment is offered for a rent. However, as contractors indicate this, the renting rate is set at such a level, that it can hardly compete with jackhammers.

Public: In the case of apartment buildings, which are occupied at all times, tenants would exercise some pressure on the contractor to reduce noise, vibration and dust propagation throughout their building. It was reported that the tenants even on the 15th floor were still complaining about unbearable effects of jackhammering in the adjoining parking garage structure.

Would the public be fully aware that hydrodemolition is much less noisy and nuisance than jackhammers, there is no doubt it would insist that contractors use it.

By-laws: Many, if not all, of the cities have their own by-laws regulating noise levels during and after business hours. Since most of the potential projects such as underground parking garages are located in the downtown core and business areas, this poses limitations on the hours of work and equipment used. In the office towers this could be controlled by working after business hours, at night. In some cases, such as hotels, even during night operation the noise level have to be reduced, since disruptions to the business activities cannot be tolerated. Those noise level regulations would prefer hydrodemolition, over jackhammers.

In an otherwise attractive area of restoration of bridges severe restrictions as to waste water disposal might reduce interest in hydrodemolition. Environmental regulations that water disposed into the natural streams or municipal sewer systems must not contain chloride, solid particles (and so on) charge contractors with an additional burden and cost of compliance with those requirements. In order to do so one has to collect all the water, at least separate the solids, and dispose both components separately. Since at the time being no standard commercially available equipment exist for this purpose, contractors considering using of hydrodemolition have to solve the problem individually. However, time restrictions - common in bidding process - as well as lack of experience and, therefore, uncertainty as to the results, would direct them toward well known jackhammers rather than towards hydrodemolition.

4. EXAMPLES OF HYDRODEMOLITION IN OTTAWA

In the Ottawa-Carleton area two concrete rehabilitation projects can be looked at as typical examples of consultants, contractors and public attitude.

Bank Street Canal Bridge

In the summer of 1991 the Regional Municipality of Ottawa Carleton called a contract for the rehabilitation of the Bank Street Canal bridge, crossing over the Rideau Canal (Ref. 4). The bridge is the major link across the Rideau Canal and is designated as a heritage structure. The bridge itself is a six-span, closed spandrel, concrete arch structure, and has been in continuous service since it was built in 1911-1912 (see Fig.1 and 2). The bridge was a typical example of where the hydrodemolition should be employed and, indeed, it was recommended by the consultant that concrete removals to the arches be done using this technology. However, many local contractors did not like the idea, claiming lack of experience, difficult equipment adaptation for overhead and vertical works, and most of all, environmental restrictions related to waste water disposal. The specifications had to be revised to allow for jackhammers as well. And the contractor who specified this traditional method won.

One interesting aspect of this project may be highlighted. The bridge crosses areas, which are under three different jurisdictions. Two of them are responsible for roads over and under the bridge, and one for the section crossing the canal. The agencies commented that it would be allowable to work on the bridge deck over road but not over the water course, unless the water be collected and filtered. The Ministry of Natural Resources and Parks Canada will not permit construction activities, which will cause chlorine, ammonia, debris or other contamination enter water and which may be detrimental to marine life or quality of water.

Underground garages in high-rise buildings

On another occasion, one of the authors was retained as a technical advisor for the rehabilitation of the underground parking garage. After a few weeks of struggling with noise, dust and poor workmanship, especially on soffit repairs, he convinced the owner to retain a subcontractor for all concrete removals with water-jets. The owner listened and never looked back. The noise from the pump unit now was reaching only the third floor, versus vibrations felt before throughout the whole building. Since the high pressure pump unit was located outside of the building and in front of its balconies some additional noiseshielding was used. It was very simple to construct, by placing plywood sheets around and on top of the high pressure pump unit to deflect sound. By experimenting for a while with this not so sophisticated arrangement, the contractor was able to satisfy everyone. The owner did not have any major reservations as to the water intake (from a fire hydrant standpipe) and disposal (Ref. 5).

In this project the subcontractor retained was the one specializing in hydrodemolition for several years. As such and having his own equipment he was able to commence the work in time required, with minimum risk to general contractor.

5. RESEARCH BY QUESTIONNAIRES

In order to investigate what the actual state of awareness and knowledge about the hydrodemolition is among main parties dealing with concrete rehabilitation, a questionnaire research was performed.

Over 60 questionnaires were sent out to three groups: clients, consultants and manufacturers of the equipment, and contractors. At time of this publication 15 responders replied (three clients, four consultants, one manufacturer, and seven contractors), (Ref.6). Summary of the responses is shown in Table 2 as a comparison between the hydrodemolition and jackhammers.

The most trustworthy answers are those given by four contractors using the technology. In their opinion the noise level is definitely lower and the surface preparation is better with no opposing answers. However, the points for water and water/debris problems are distributed equally, what indicates the weak points of the technology. The weight of the water/debris disposal is even more clearly seen from the answers given by consultants, who indicate it unanimously. Some of them accent this point stating that extensive provisions of ponds and drainage control have been discouraging them from specifying the hydrodemolition.

6. SUMMARY

The aim of the performed survey was to identify the weak points and the areas, which need improvement. It was found that those who had any experience with HD were willing to use it again in the future.

However, they would like to see a better solution for waste water disposal, and a more affordable equipment. Also, the price of the equipment makes it available only to those contractors who are willing to specialize in HD, what in turn keeps the demand low.

Analysis of factors affecting the range of application of hydrodemolition technology indicates the following:

1. It is not the contractor alone who makes the decision to use either the hydrodemolition or jackhammers. Two parties most responsible for such a decision are consultant and contractor, but their move is biased - directly or indirectly - by the owner's position and by relating by-laws and regulations, such as noise and water control.

2. In order to break the barriers imposed by water-relating regulations development of portable water/debris collecting and clarifying systems is necessary.

3. Since at the present time one of the factors guiding the owners is the total cost of the restoration, it is critical to convince them of the long-term savings due to better quality of the job done, by use of hydrodemolition instead of by jackhammers (prolonged lifetime of the structure).

4. It is important, therefore, to emphasize in all marketing materials the long-term effects of using the hydrodemolition in place of jackhammers, namely avoidance of secondary damages to a sound structure by vibrations, and therefore, prolongation of between-repairs periods (relating research are strongly recommended).

5. Unless the hydrodemolition systems are available at lower price or on rental basis a few specializing subcontractors only could afford to buy it rather than a number of general contractors dealing with concrete rehabilitation.

ACKNOWLEDGMENTS

The authors wish to thank all the respondents for their contribution.

Special thanks go to Mr. S. T. Johnson of Jet Edge for his letter with valuable remarks on economical aspects of the application of HD technology.

REFERENCES

1. WJTA, Proceedings of the Conferences on Water Jet Cutting Technology.
2. Marketing materials relating to Hydrodemolition equipment.
3. Proceedings of the World of Concrete Conference, Atlanta, USA, 1992.
4. Tender documents for Bank Street Canal Bridge Rehabilitation. RMOC, Ottawa, 1992.
5. Remisz & Associates Engineering Ltd., "Report from the reconstruction of a parking garage", 1992.
6. Replies to questionnaires on Hydrodemolition, sent out by authors of the paper, 1993.

Table 1. Brief comparison of concrete demolition by use of water jets and conventional methods

Table 2. Summary of the questionnaire research on hydrodemolition vs. jackhammers

Sample photos:

Concrete removed from the slab, rebars cleaned from all scale, rust-very good surface preparation	Using a water jet on a lance to work onthe walls or soffit areas
Bank Street Bridge over Rideau Canal in Ottawa, Ontario around 1990, before rehabilitation contract	Water pump aggregate supplying high pressure water to the hydrodemolition robot inside the building, working on parking garage repairs
Hydrodemolition robot, remotely controlled, water supplied from the pump unit outside. Cutting nozzles are inside the covered box, seen on the right hand side photo. They move back and forth, and then the robot can move forward.

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