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Hydrodemolition for bridge repair (Reprint from Nordisk Betong no. 2-3:1988). BACKGROUND
The pneumatic hammer is the most widespread and perhaps most widely used equipment for mechanical chipping in concrete. In recent times, however, there has been a decline in the use of this equipment, probable reasons being that new alternatives have emerged and the negative effects of the mechanical hammer on the operator. Vibration injuries in fingers and arms are very common. Moreover, how well the result of the chipping turns out depends to a very great extent on the skin of the operator and his feeling for both the equipment and the concrete. With regard to mechanical chipping, the following negative factors may also be mentioned:
Dust formation and a high noise level are also of major significance, since both dust and noise cause discomfort and irritation to the surroundings. Abrasive removal takes place mainly by milling, usually with a rotary disc coated with carborundum grit or industrially manufactured diamonds. Sand blasting is another alternative used for cleaning of concrete surfaces. Abrasive removal has the following disadvantages:
Damaged concrete can also be blasted away by means of very small, closely spaced explosive charges. In connection with bridge repairs, the use of this method has largely been restricted to removal of parapet beams. See Molin [1].
HISTORY OF WATER-JET CUTTING Water-jet cutting has been utilized for many years in different application areas. The method, in fact, was already being used right at the beginning of the 20th century for cleaning machine parts that were difficult to reach. See Lohse [2]. in those days, the maximum water pressure used was 7 Mpa. With the more modern pumps and better materials now available, the application area has been extended. Nowadays, water Jets are used for such purposes as cutting in wood (puzzle making lass fibre, fabrics, leather, plastic, rubber, etc. The technique has also been employed in Sweden for jet Injection of soil to facilitate pile driving. It has also been used for cleaning of soda recovery boilers, etc., in industry. See Oinert [3]. Major research efforts are currently being pursued with the goal of enabling water-jet cutting to be used in rock cutting and in tunnelling. See Pasche [4]. With abrasive additives, it is even possible to cut steel with a water jet. The latest advances in the field of water-jet cutting were presented at an international symposium in England in 1986 [5]. The use of water at very high pressure for concrete works in the form of drilling, cutting or hole-making is a technique that has been well known for many years. The new type of water-jet cutting described below is concerned in the first instance with surface removal of concrete, i.e. hydrodemolition of damaged concrete from undamaged concrete which is left intact. THE
MECHANISM OF HYDRODEMOLITION
It
is important to remember this mode of action, whereby it can be assumed
that the removal depth increases if the concrete is cracked and laminated,
i.e. is of low homogeneity. A further consequence of the removal mechanism
described is that the removal depth increases if the strength of the concrete
is low, i.e. at higher water-cement ratios. This provides an explanation
of how hydrodemolition enables selective concrete removal, partly in view
of the degree of damage and partly in view of the strength level. HYDRODEMOLITION
OF TEST SLABS The
experiments conducted with hydrodemolition of test slabs, see Fig. 5,
showed that selective removal of concrete of lower quality is possible.
It should nevertheless be observed that the selectivity is limited to
a zone (S.) of size 50 - 100 mm around the mean removal depth (D), see
Fig. 6. This circumstance should be observed when hydrodemolition work
is being planned. These observations apply to the firms and equipment
mentioned above for horizontal surface removal. For removal of concrete
to a lesser extent, other types of water jet equipment, such as rotary
nozzles and hand-held lances are available.
EXPERIENCE FROM HYDRODEMOLITION OF BRIDGE DECK SLABS
Another condition of great importance to be remembered is that the result of hydrodemolition appears to be repeatable, i.e. removal of the damaged concrete leads to the same result regardless of the operator, provided the hydrodemolition equipment is correctly calibrated. When hand-held mechanical impact hammers are used the removal limit is decided, as previously mentioned, by each individual operator. The use of water-jet cutting implies in this context that a good concrete of uniform and high quality is left behind, regardless of the personnel. In all cases, the hydrodemolition must be post checked by observation of the surface and sounding with a hammer to determine whether or not an acceptable result has been obtained. If complete grains of aggregate loosen in large amounts when checking by sounding, a satisfactory result has not been attained. Considering that almost no micro cracks occur in the concrete and that greater adhesion to the underlying surface is thereby obtained after hydrodemolition, the Swedish National Road Administration [9] has stated that bridge deck slabs with a secondary bearing effect can be cast without sectioning. A further justification for this is the increased roughness in the bonding surface accomplished by the water-jet cutting. According to [9] this circumstance is further utilized in that no shear dowelling is required if the damaged concrete has been removed by hydrodemolition, albeit provided that the slab only has a secondary load-bearing function. Hydrodemolition does not only imply good effects on the bonding properties. When casting the concrete it is necessary for the compaction to be carried out with the greatest possible thoroughness as the surface being concreted against has considerably greater roughness than when demolition is done with mechanical impact hammers. On those surfaces where the demolition depth is greater than about a centimetre, proper working of the concrete requires first vibration with a small rod vibrator and then surface vibration, as vibro-bridges or vibro-beams do not have sufficient depth effect. In the light of experience gained, the requirement for thorough cleaning prior to casting of the hydro demolished concrete surfaces must be stressed, since it has been found that the hydrodemolition residues often contain "cement sludge" which can adhere to the clean-demolished surface when the water evaporates. Unless this layer is removed prior to casting, the bond is jeopardized. To assure good adhesion, this layer should therefore be removed before casting takes place and while the hydrodemolition surface is still wet. On some of the bridge deck slabs on which the hydrodemolition technique has been used, the Swedish National Road Administration has carried out studies of capacities, costs, etc., for different kinds of hydrodemolition equipment - see [15], [16] and [17]. These studies have established the following: With the types of surface demolition equipment concerned, the capacity is of the order of magnitude of 10m2/h. This can be compared with manual demolition in which the capacity is normally between 0.2 and 0.5 m2/h and person. Hydrodemolition leads to a significant increase in capacity, which naturally increases even more when the degree of damage and thus the demolition depth exceeds the covering layer. The cost of bridge deck hydrodemolition, including establishment, chipping, electricity, water and flushing clean with a water jet is approx. SEK 400 - 1,200/m2 depending on the depth of the damage and the size of the demolished surface, which can be estimated as also equivalent to the cost of manual demolition (1986 prices). Other experience gained with regard to the benefits and disadvantages of removing damaged concrete on bridge deck slabs using the hydrodemolition technique are described in outline below.
Large
amounts of rubble. The hydrodemolition technique implies that large
amounts of rubble must be collected in a suitable manner. This problem
however is solved by using large industrial vacuum cleaners.
The
possibility of selectively removing concrete with the water-jet cutting
technique nevertheless presupposes that the water pressure and flow in
the nozzle and its motional speed in different directions is properly
calibrated. Moreover, it must be ensured that the necessary depth effect
is attained in relation to the depth of the damage. ZUSAMMENFASSUNG
Die
Möglichkeit der selektiven Beseitigung vonBeton mit der Waterjet-Technik
setzt jedoch voraus, daß der Wasserdruck und der Strahlstrom der
Düse wie auch deren Bewegungsgeschwindigkeit in verschiedenen Richtungen
richtig einkalibriert sind. Darüber hinaus ist dafür zu sorgen,
daß die im Verhaltnis zur Beschädigungstiefe erforderliche
Tiefenwirkung erzielt wird. RÉSUMÉ
Pour
cet enlèvement sélectif du béton, il faut toutefois
que le débit et la pression d'eau au niveau de la buse, ainsi que
la vitesse des déplacements de celle-ci, soient correctement étalonnés.
De plus, il faut veiller à obtenir la pénétration
que demande la profondeur du matériau endommagé. REFERENCES [1] Molin, C, 1983, "Försiktig sprängning av kantbalk. Fullskaleförsök på äldre betongbro". Cement- och betonginstitutet, Rapport 8368, Stockholm (in Swedish). [2] Lohse, U, 1929, "Versuch an einer Nassputzanlage". Die Giesserei 49,1929 (in German). [3] Öinert, A, 1983, "Högtrycksvattenstrålens möjligheter och begränsningar inom byggnadsindustrin", Byggforskningsrådets rapport R92:1983, Stockholm (in Swedish). [4] Pasche, E, 1984, "Wasserstrahlen im Tunnelvortrieb". Strasse- und Tiefbau 4/84, 1984 (in German). [5] BHRA, 1986, "8th International Symposium on Jet Cutting Technology", Durham, England, Sept. 9-11, 1986. [6] Summer, D, A & Reather R. J. 1982, "Comparative use of intermediate pressure water jets for slotting and removing concrete". 6th International Symposium on Jet Cutting Technology, April 1982. [7] Rehbinder, G. 1976, "Some aspects on the mechanism erosion of rock with high speed water jets". Third Intemational Symposium of Jet Cutting Technology, May 1976. [8] Ingvarsson, H & Skalin, H. "Erfarenheter av waterjet-tekniken vid broreparationer Sverige", rapport vid Nordiskt miniseminarium 1985 05 15, VTT, Helsinki (in Swedish). [9] Vägverket, 1985, "Repair of Concrete Bridges", Publ. No. TB 151 (English version). [10] Andersson, Y & Ingvarsson, H. "Vattenbilning av betongprovplattor 1985". Rapport 1986-02-21, Vägverkets brotekniksektion, Borlänge (in Swedish). [11] Andersson, Y & Ingvarsson, H. "Vattenbilning av betongprovplatta, februari 1986". Rapport 1986-04-15, Vägverkets brotekniksektion, Borlänge (in Swedish). [12] Eriksson, B & Ingvarsson, H. "Vattenbilning av betongprovplattor, april 1986". Rapport 1986-06-13, Vägverkets brotekniksektion, Borlänge (in Swedish). [13] Silfwerbrand, J,1984, "Betongytors råhet", Inst. för Byggnadsstatik, Kungl. Tekn. Högskolan, Stockholm, PM 1984-09 (in Swedish). [14] Silfwerbrand, J, 1984, "Samverkan mellan delvis nedbilad betongplatta och pågjutning, Balkförsök". Meddelande nr 142, Institutionen för Byggnadsstatistik, Kungl. Tekn. Högskolan, Stockholm (in Swedish). [15] Vägverket, 1985, "BDa-rapport 84602-32, Broreparationer - vattenbilning", 1985-01, Borlänge (in Swedish). [16] Vägverket, 1986, "BDa-nytt, Broreparationer - vattenbilning", maj 1986, Borlänge (in Swedish). [17] Vägverket, 1986, "BDa-nytt, Broreparationer - vattenbilning", sept 1986, Borlänge (in Swedish). AUTHORS Hans
Ingvarsson, Prof., Dr. Techn. Swedish
National Road Administration
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When
carrying out repairs to concrete structures, removal of the damaged concrete
constitutes a problem in itself. Several different methods can be used
to remove damaged concrete. Brief comments on these methods are given
below. 
Experience
from the use of water jet cutting, also known as hydrodemolition, as an
alternative to the methods mentioned above is presented below. The term
water jet is used to designate the technique that makes use of water jets
with extremely high pressure, up to about 400 Mpa. 
The
decisive factors for the results achieved by the water Jet in demolishing
concrete are the flow dynamics for the water jet and the properties of
the concrete. According to findings arrived at by, among others, Rehbinder
[7], the water jet works during the initial stage in such a manner that
an "inner" water pressure is built up in the material that is
hit by the jet. If this material is of a dense character, the treatment
effect will be minimal. If the material, in contrast, is porous, for instance
sandstone, it is broken up by this "inner" pressure. Rehbinder
[7] also asserts that decisive parameters for the demolition effect in
different materials are their permeability and grain size. In all probability
this also applies to concrete and consequently the permeability of the
neat cement paste, homogeneity and water-cement ratio, and the grades
of aggregate appear to be of great importance. 
Observations
made when using different kinds of hydrodemolition equipment for surface
removal of salt- and frost-damaged concrete in bridge deck slabs, partly
reported by Ingvarsson and Skalin [8], have shown that such equipment,
when used on the bridges concerned, indirectly appears to have adapted
the removal depth to the actual extent of damage. An example of such hydrodemolition
equipment is shown in Fig. 1. Furthermore, the use of such equipment for
hydrodemolition of special test slabs has shown that the strength affects
the degree of removal. See Fig. 2. This experience confirms the correctness
of the above-described hydrodemolition mechanism. 
The
test slabs were manufactured and the results of these experiments evaluated
in co-operation with the Dept. of Structural Mechanics and Engineering
at the Royal Institute of Technology in Stockholm and the Materials Testing
Laboratory of the Stockholm City Streets and Traffic Administration. With
regard to evaluation of the surface roughness of demolished concrete surfaces,
the method proposed by Silfwerbrand [13] and [14] has been used.
The
removal depths with associated number of passes observed on the test slabs
should also be representative of ordinarily damaged concrete in the field.
The reason for this is that the fundamental mechanism in hydrodemolition
of concrete is greater removal at higher permeability, which is either
caused by a greater degree of micro cracking or a higher water-cement
ratio in the cement paste (lower strength). Obviously, larger or smaller
removal depths than those observed in the experiments can be attained
in the field if the degree of damage differs from that simulated in the
test slabs (permeability corresponding to homogeneous K20 concrete). it
is thus of the utmost importance for, from case to case, the necessary
"calibration" of water pressure and flows etc. to be accurately
performed when using the equipment concerned. 
The
testing of the different kinds of equipment performed can be compared
with pretesting of concrete in which the potential possibilities of the
material, in this case the equipment, are investigated. Whether or not
this potential can be utilized to the full will depend from case to case
on the operation of the equipment, the calibration of pressures and flows,
etc., and subsequent checking of the bonding surface. The responsibility
for this rests with both contractors and clients. That the choice of water
pressure, for example, is of great importance is evident from Fig. 7 and
8, where the results obtained from demolition of test slabs are presented.
In
hydrodemolition work carried out on bridge deck slabs of concrete it has
been found that reinforcement that is uncovered on account of the high
water pressures, and the movements of the loosened masses on account thereof,
is cleaned from concrete residues and rust. Moreover, the reinforcement
remains undamaged, in contrast to demolition with hand-held mechanical
impact hammers which often cause damage to the reinforcement. Moreover,
no zip-fastener effect occurs, as the bond of the reinforcement to the
concrete is not damaged. This is otherwise a common phenomenon when the
chisel strikes the reinforcement in mechanical manuel demolition. After
hydrodemolition, the reinforcement has therefore been reusable, which
has enabled the continuity in the reinforcement to be retained despite
deep repairs. 