Rehabilitation of Prestressed Reinforced Concrete Structures Using an Ultrasonic Injection Process

Pierre-Marie DUBOIS, Daniel MICHAUX

PMD-ATEAV Systems sprl - Prestressed concrete repair

Rue les Culots 37

B-1421 Ophain-Bois-Seigneur-Isaac, Belgium

Control Institute: CSTC-WTCB-BBRI (Belgian Building Research Institute)Avenue Pierre Holoffe 21, B-1342 Limelette, BelgiumSupervisor: V. Pollet & E. Cailleux

Laboratoire des Matériaux de Construction Université de Liège – ArGEnCo – GeMMeSart Tilman – Bâtiment B52 B-4000 Liège 1, Belgium

AbstractThe civil engineering process stabilizes in the long term the corrosion of prestressed concrete cables. The process principal idea is to saturate the protective grout of cables - irrespective of the cables condition - with an inhibiting solution conveyed by a vibrating ultrasonic power pump. The process is slightly intrusive.

A map of the injection faults within the existing cement grout can be established. Additional cement grout can be injected where needed.

The process was applied to a viaduct for the first time in 1994.

The effectiveness of the treatment was assessed in 2012 when the entire viaduct was demolished.

The process is regarded as a long-lasting and durable treatment.

KeywordsPre-stressed concrete, corrosion, rehabilitation process, power ultrasound, ultrasonic transducer, mapping of cement grout injection fault

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1 Treating the Corrosion of Adherent Prestressed Cables

1.1 The Origin of the Process

Often older types of construction were built with prestress using adherent prestressed cables. When cables become corroded the treatment can be challenging in different ways. The cables are embedded within the inner concrete structure of the beam. They are barely

accessible. Since they are threaded within a metallic sheath, no electrochemical protective process can be

applied. Therefore the authors imagined how to open up another way. First an appropriate corrosion

inhibitor saturates the existing protective cement grout, then additional cement grout is further injected where injection faults are discovered.

The rehabilitation process of prestressed cables was devised and applied in 1994 as a test case to a stretch of a five span VIPP type viaduct in Luxembourg (Lultzhausen close to the high head dam of Esch-sur-Sûre).

1.2 General Principle

The unique idea of the process is to take advantage of ultrasonic waves energetic properties in order to permeate the cables surroundings with the inhibiting solution.

The effect of ultrasound onto the inhibiting solution is to build up a cavitational condition in vapor phase.

The energy released by the cavitation effectively clears the pores in the grout at the prestressed cable interface site and the inhibiting solution has direct access to the prestress steel to be protected.

The now open path is the very same path that the corrosive agents (water, oxygen, chlorides) have used over the decades of the construction lifecycle.

The main difference is that ultrasound makes it possible to cover the paths in a few hours. The cavitational condition in the vapor phase is generated by an alternating ultrasonic power

pump. It expands and contracts generating alternating overpressures and underpressures at an ultrasonic

frequency of approximately 23000 Hz (Figure 1). Consequently the impregnation of the cable surroundings by the inhibiting solution occurs under

low pressure.

Fig. 1 General operating principle of the ultrasonic pump

The selected inhibiting solution is a calcium nitrite solution whose effectiveness - with or without chlorides present - is proven by several studies and practical findings over thirty years.

Calcium nitrite transforms permeable ferrous oxyde into impermeable ferric oxyde which restores the natural passivation coating of the steel embedded in sound concrete, causing no secondary effect to the steel behavior.

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1.3 Operating Procedure

The treatment consists of three stages:

Stage #1: Installing the injectors

The prestressed cables must be precisely located. The concrete envelope is drilled up to the steel level. The bore holes are drilled at regular intervals of 50 cm along the prestress sheath (Figure 2-1). An injection nozzle is fitted into each bore hole.

Fig. 2-1 After boring and installing the injection nozzles

Fig. 2-2 Injecting the inhibiting solution using an ultrasonic transducer

Stage #2: Injecting the corrosion inhibitor

In the second stage, the corrosion inhibitor is injected into each nozzle using a specific ultrasonic pump (Figure 2-2).

The progress of the inhibitor solution within the sheath is monitored at the bore holes on both sides of the nozzle hole.

Upon emergence of the solution at the neighboring holes, injection is stopped and the pump is carried over to the next hole.

During the treatment the progress of the inhibitor from one hole to the next is timed. A mapping of the migration timings is obtained revealing details about the quality of the grout

filling or packing. A long migration time indicates a compact grout with few faults. On the contrary, a very short

migration time detects a zone with poor filling (Figure 3).

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Fig. 3 Mapping the migration times of the inhibiting solution. The arrows mark the injection points. Example: Beam #4 of span #1 of the Lultzhausen viaduct was constructed of 11 cables, Type 12, diameter 7 mm

This mapping quantifies and localizes how serious the injection faults are, visualized in Figure 4 for a Type 12 cable of 7 mm diameter.

> 120min > 60 min <6 0 min < 30 min < 15 minFig. 4 Illustrates the injection faults (Cable Type 12, diameter 7 mm)

Stage #3: Filling the grout faults

After the injection of the inhibiting solution has been completed, and based upon the mapping obtained earlier, the nozzles are reused to inject a very fluid Portland cement-based microgrout.

In the case of a complete empty gap of long range within the injection site, usual grout is injected.

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1.4 Validating the Process

The process was applied for the first time in 1994 as a test case to one stretch of the Lultzhausen viaduct in the Grand Duchy of Luxembourg.

The Belgian Scientific Center for Construction Engineering (Belgian Building Research Institute – BBRI) validated the process between 2000 and 2008 [1].

A detailed communication was published in the SETRA paper #59 of November 2008 [2]. Several communications were presented at the Structural Faults and Repair Congress in

Edinburgh in June 2008 and at the "Le pont" Symposium in Toulouse in 2005 [4] and 2008 [5]. The last and rare opportunity to validate the process occurred when the Lultzhausen viaduct was

replaced, a decision made because of concerns about the stability of several beams in spans left untreated in 1994.

The work was demolished in April 2012. The span treated in 1994 merited a special soft demolishing procedure. Scaffolding supported the span which was then cut into sections. The cutting of the sections was visually inspected in detail. One section was selected to undergo tests in the lab. The test was completed in 2012. It showed beyond doubt that the process was effective and long-lasting. The findings of the analysis are commented here below.

2 Destructive Analysis of the First Practical Application of the Process in 1994

2.1 Description of the Construction Work Treated in 1994

The earliest application of the process dates back to 1994. It was applied to a stretch of the Lultzhausen viaduct close to the great dam of Esch-sur-Sûre in Luxembourg. (Figures 5-1 and 5-2)

Fig. 5-1 Locating the 1994 treated stretch of the Lultzhausen viaduct

Fig. 5-2 View from below the deck

The work was constructed around 1960. It was a structure of 5 independent spans made of prefabricated prestress beams (VIPP). The size of the stretches was 35 meters. Each beam contained 11 prestress cables made of 12 threads of 7 mm diameter. (Figure 6)

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Figure 6 Five stretches of 35 m made of 4 prestressed VIPP beams

The design had not provided for any protective water sealing of the deck. Moreover the protection flaw was leading to put the work out of service. The work site is in a feedwater catchment area. In winter it was not allowed to spread salt to melt black ice. The test assessments revealed little or no chloride pollution.

2.2 Pathology of the Prestress Highlighted in 1990

Investigations performed in 1990 revealed that many cracks were present along the prestressed cable pathways with water leaks and limy exudations (Figures 7 et 8).

Many cables were poorly or not injected, showing corroded condition sometimes advanced, but still unbroken with no reduction in bearing capacity.

Fig. 7 1990 Inspection - Cracks along the prestress path

Fig. 8 1990 Inspection - Cracks along the prestress path and limy exudations

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2.3 Corrosion Treatment of the Prestressed Cables in 1994

The inhibitor used in 1994 was sodium nitrite. Its inhibiting action is identical to the calcium nitrite action preferred nowadays, in order to prevent any risk of alkali-aggregate reaction within the concrete environment, surrounding the treated cables.

The microgrout used was Polyment Micropress microgrout based on Portland cement. This microgrout is no longer available. It has been replaced with another microgrout with similar properties.

The Figure 3 presented earlier shows the mapping of the injection faults in the 4th beam. The analysed beam section belongs to the right hand end. It illustrates how the disastrous condition of the injections, opening the path to corrosion build up.

2.4 Observations Made Between 1994 and 2012 to the Behavior of the Treated Span

As an example, the Figure 9 illustrates the filling of the visible cracks along the prestress path by the microgrout injected from within the cables. That filling constituted an excellent fissuring witness, should corrosion resume. No such problem occurred since the condition was kept immutable until the structure was demolished.

Fig. 9 Filling the cracks in the bottom section

2.5 Selecting and Sampling Elements and Samples to be Tested

The viaduct deck was demolished and replaced with a new deck in 2012. The first stretch treated in 1994 merited a soft demolishing procedure, as illustrated in Figures 10 and 11.

Fig. 10 Saw cutting demolition Fig. 11 Test element

Selecting the preserved beam section for analysis was conducted in order to keep sufficient longitudinal length of the treated prestressed cables as well as transverse prestressed cables left untreated in 1994.

The section at the end of beam # IV on the pile side was selected. Core samples were then taken onto the test element for analysis in the lab (Figures 12 and 13).

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Fig. 12 Taking core samples of longitudinal and transverse prestressed parts

Fig. 13 Core samples

2.6 Test Program on the Taken Samples

The test program was entrusted to the Laboratory of Construction Materials (Laboratoire des Matériaux de Construction) at the University of Liège, Belgium [6].

The program consisted of the following:

• Macroscopic description;

• Determining the chloride content in the grouts using potentiometer titration;

• Examining the polished sections in an electron microscope;

• Half-quantitative determination of the nitrite content using the colorimetric method with test strips;

• Determining the cement content of the grouts;

• Identifying minerals using XRD (X-ray diffraction).

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2.7 In Situ Observations after the Demolition in 2012

The visual examination of the sawed cuttings indicates that the microgrout reinjection is in excellent condition (Figure 14). The interface front between the older grout (dark) and the reinjected grout (white) is clearly marked.

Fig. 14 Visual examination of the cuttings

The retreat of the threads after the cuttings is almost the same for every cable. This finding confirms the excellent quality of the microgrout reinjection process.

It was feared that the action of the nitrites might alter the adherence of the prestressed steel and older/newer grout interface. No such problem occurred and the ULS (Ultimate Limit State) was not modified.

Therefore we can conclude that the waterproof barrier between the prestress steel and the outside environment was restored along the full cable length and that the input of newer corrosive agents was prevented (water, oxygen, chlorides).

2.8 In Lab Examinations

The chloride content measurements match to approximately 0.01 % of chlorides wrt the grout mass. Therefore no significant pollution due to chloride had occurred. Those measurements are coherent with the fact that the work is in a protected area where in principle no salt is spread in winter.

Determining the nitrite content using the colorimetric method with test strips to the treated cables of the longitudinal prestress and to the untreated cables of the transverse prestress, shows beyond doubt that the process is effective and long-lasting (Figure 15).

The visual examination showed that no further corrosion of the prestressed cables had resumed.

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Treated prestressed cable Untreated prestressed cable

Fig. 15 Treated and untreated prestressed cables

Even though the testing was not quantitative, it can be seen that after approximately 20 years, a significant amount of nitrites are still available in case new aggressive agents appear. The long-lasting effect of the treatment is practically unlimited.

It should be noted also there is no fear that the use of sodium nitrite leads to an alkali-aggregate reaction type within the concrete environment, even if some signs may have pointed that the concrete type is sensitive to internal disintegration phenomena.

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3 Process Action in the Presence of Chlorides

Most of the time the corrosion of prestressed cables is initiated by pollution involving chlorides. In the presence of chlorides, the corrosion velocity and severity level becomes considerably worse even if the pH in the cables environment stays around 11 or 12. In principle a pH of 11 or 12 inhibits corrosion when no chlorides are present.

A distinctive characteristic of the nitrites is to enable a corrosion inhibiting effect most notably effective when chlorides are present, provided that the quantity of nitrites is sufficiently available.

The application of this process ensures this important point because the process soaks to saturation the environment surrounding the prestressed cables.

It should be noted that the usage of nitrites in concrete was developed back in the 1970s as inhibiting agents in marine environments.

A convincing example to mention is the test application in 2005 to the Huccorgne viaduct on the Wallonia highway in Belgium, applied to particularly damaged VIPP beams.

Nine years later the repaired area that may possibly show exemplary evidence of cracks does not present any cracking nor any sign that corrosion has resumed (Figure 16).

A detailed intrusive test assessment confirms these findings.

Viaduct of HuccorgneWallonia Highway, Belgium

Type VIPP

Before treatment Present state

Construction date : Around 1968

Pathology : Corrosion and broken threads and serious injection faults and heavy local pollution of cables due to chlorides

Rehabilitation date : 2005

Rehabilitation : Treating two 30 m beams severely deteriorated containing 12 cables made of 12 threads of 8 mm

Fig. 16 Rehabilitation of the viaduct of Huccorgne

Since then, the Ministry of Public Works (Service Public de Wallonie - SPW) has had other structures repaired using the process method to the prestressed cables in several major works on the highways.

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4 Conclusions

The effectiveness and long-lasting effect of the process to stabilize the corrosion of the prestressed cables are widely confirmed as we benefit from approximately 20 years experience.

Therefore we regard here a reliable process that takes its place beside other high technology processes like the cathodic protection of reinforced concrete and other electrochemical processes.

It is important to note that the process does not require any maintenance phase and that it can be renewed or further completed if needed.

The process comes within the scope of durable development since it significantly extends the life of prestressed civil engineering structures endangered by corrosion.

It is important to further highlight how the application process bears little impact to highway traffic since it is usually implemented from the inner side of the bridge structure.

For more information, see www.PMD-ATEAV.com .

5 Acknowledgements

The development of the process owes a great deal to the active assistance of several departments at the Ministry of Public Works (Administration des Ponts et Chaussées) of the Grand Duchy of Luxembourg; of the Head of Works Expertise (Direction de l’expertise des ouvrages) at the Ministry of Public Works (SPW - Service Public de Wallonie, Belgium); of the Belgian Building Research Institute – BBRI, and of the Laboratory of Construction Materials (Laboratoire des Matériaux de Construction) at the University of Liège, Belgium.

The application works at the Lultzhausen viaduct were accomplished under the project management of the Ministry of Public Works (Administration des Ponts et Chaussées) on behalf of the Ministry of Durable Development and Infrastructures (Ministère du Développement Durable et des Infrastructures) of the Grand Duchy of Luxembourg.

References

[1] Centre Scientifique et Technique de la Construction (CSTC), Rapport DE 61079/bis avril 2001,

rapport DE 61079/2 octobre 2002, rapport DE 61079/3 novembre 2006, rapport DE 61079 27-11-

2006.

[2] Bulletin du Centre des Techniques d’Ouvrages d’Art, SETRA n°59, novembre 2008, pp.8-19

[3] Cailleux E., Pollet V., Dubois P-M., Michaux D, A new corrosion treatment for prestressed

rebars: the direct injection of a corrosion inhibitor by an ultrasonic pump, Structural Fault and

Repair, Edinburgh, 2008.

[4] Dubois P-M., Michaux D., Procédé de stabilisation de la corrosion de câbles de précontrainte,

Colloque Le Pont, Toulouse, 2005.

[5] Gilles P., Dubois P-M., Michaux D., Problématique des chlorures dans la postcontrainte -

Exemple de traitement de la corrosion par le procédé PMD-ATEAV au viaduc de Courrière

(Belgique), Colloque Le Pont, Toulouse, 2008.

[6] Laboratoire des Matériaux de Construction de l’Université de Liège (ArGEnCo), Rapport

GMC/12/027, 30 octobre 2012.

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