A NEW CORROSION TREATMENT FOR PRESTRESSED REBARS: THE DIRECT INJECTION OF A CORROSION INHIBITOR BY AN ULTRASONIC PUMP

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1 A NEW CORROSION TREATMENT FOR PRESTRESSED REBARS: THE DIRECT INJECTION OF A CORROSION INHIBITOR BY AN ULTRASONIC PUMP E. Cailleux & V. Pollet CSTC-WTCB-BBRI Avenue Pierre Holoffe 21 B-1342 Limelette Belgium emmanuel.cailleux@bbri.be valerie.pollet@bbri.be P. M. Dubois PMD Rue les culots 37 B-1421 Ophain-Bois-Seigneur-Isaac Belgium pmd@skynet.be D. Michaux ATEAV 15 allée Courbet Livry Gargan France ATEAV@wanadoo.fr KEYWORDS: prestressed rebars, chloride, corrosion inhibitor, ultrasonic pump ABSTRACT Corrosion mechanisms induced by chloride contamination and injection defects are the main causes of degradation of the prestressed concrete structures. Recently, a new technique has been developed for the treatment of the prestressed rebars. It is based on the injection of a corrosion inhibitor by an ultrasonic pump directly into the grout by means of holes drilled through the concrete covering the duct. At the end of the injection, an additional grout is also introduced in the duct in order to fill initial injection defects. This technique seems very attractive as it may ensure the contact between the inhibitor and the rebar. In order to evaluate its efficiency and its durability, several tests were performed during a research program. In a first step, the propagation of the inhibitor solution in the grout was evaluated. Several representative compositions were tested and two injection techniques were compared: the ultrasonic technique and a second one using only an injection pressure. In a second step, the efficiency against corrosion was evaluated. Concrete samples containing tendons inserted in metallic ducts were cast. These ducts were filled by a grout contaminated with several chloride concentrations. After the injection of the corrosion inhibitor, several analyses were performed over one year: the corrosion activity was recorded, the rebars were inspected by sample autopsies and the presence of the corrosion inhibitor was sought. INTRODUCTION One of the major sources of failure of the existing prestressed concrete structures corresponds to a chloride contamination inducing the corrosion of the prestressed tendons. These chlorides can be provided by several sources such as deicing salts or seawaters. To initiate the corrosion reaction, they migrate through the concrete cover up to the steel. Their diffusion speed is a function of the concrete permeability and compacity (Calgaro & Lacroix, 1997) but more generally, they reach the tendons by the "weak points" of the structure such as bearing, expansive joints, conception defects or microcracks. The corrosion mechanisms caused by chloride contamination generate pitting corrosion processes inducing localized attacks which may promote brittle failure of the tendons when pit size and sharpness are such to allow fast crack propagation (Bertolini & Raharinaivo, 2007). In the case of post-tensioned structures, the tendons are located in a duct filled with a cement grout. For such structures, an insufficient grouting of the duct (injection defects) is often encountered. The steel is then not protected by the alkalinity of the grout. Such defects can lead to the development of corrosion processes and can be responsible for serious problems. Moreover, in the case of chloride contamination, the corrosion mechanisms can be enhanced by these voids in the ducts (Mallett, 1996). Concerning both the treatment and the repair of post-tensioning structures affected by corrosion processes due to chloride contamination, few solutions are actually available. Indeed, despite the actual knowledge on durability and safety of reinforced concrete structures, the treatment of such alteration process is still problematical. The conventional repair techniques developed for reinforced concrete cannot be applied to prestressed structures because prestressing does not allow full removal (Bertolini & Raharinaivo, 2007).

2 Moreover, several difficulties limit the possibility of applying the electrochemical protection methods such as cathodic protection that are successfully used for reinforced concrete. First of all, the application of the electrochemical methods requires a continuous electrolytic path from the anode to the protected steel surface. This makes it difficult (or impossible) to use these techniques to protect the prestressing steel inside the ducts of the post-tensioned structures. Moreover, the lowering of the steel potential caused by the applied current may promote the risk of hydrogen formation and the hydrogen embrittlement of the prestressing steel. A safe lower limit of -900mV/SCE is usually considered to prevent the risk of hydrogen induced stress corrosion cracking (HI-SCC) (NBN EN12696, 2000). Nevertheless, in the case of metallic ducts, the potential of the enclosed tendons cannot be accurately known because of the possible random contacts between tendon and duct (Bertolini & Raharinaivo, 2007). The control of the electrochemical treatment by the potential measurement is then difficult in this case. At the beginning of the nineties, a new technique of treatment of prestressed steel was developed for applications on post-tensioned structures. This method was the result of a collaboration between two companies : PMD and ATEAV. The technique is based on the two main following steps: - the injection of a corrosion inhibitor solution directly into the grout by the use of a special ultrasonic pump. The solution is injected by means of holes drilled through the concrete covering the duct. - the introduction of an additional grout in the duct in order to fill the initial injection defects. This method seems very attractive as it may ensure the contact between the inhibitor and the rebar. In order to evaluate its efficiency and its durability, a research project was carried out in the Belgian Building Research Institute (BBRI). In a first step, the injection of the corrosion inhibitor by the ultrasonic pump was compared with a traditional injection pressure. The injection into several representative grout compositions was tested. In a second step, the efficiency and the durability against corrosion were evaluated. Concrete samples containing tendons inserted in metallic ducts were cast. These ducts were filled by a grout contaminated with several chloride concentrations. The injection of the corrosion inhibitor was then performed and sample autopsies were carried out after two months and one year. DESCRIPTION OF THE INJECTION TECHNIQUE BY ULTRASONIC PUMP The treatment is based on the use of an alternative pump working at a high frequency and corresponding to a power ultrasonic transducer for which the sonotrode is confined in a compressive chamber (Fig. 1). The pump dilates and contracts at an ultrasonic frequency by inducing series of pressure and suction allowing the progress of the inhibitor solution inside the grout microstructure. Fig. 1 Piezo electric transducer. Fig. 2 Technical description of the treatment of prestressed tendons by the injection of a corrosion inhibitor solution by the ultrasonic pump.

3 This treatment is applied in the final step of the concrete restoration after the repair of the spalling by the conventional techniques. The treatment is applied in three steps (Fig. 2): - The first step corresponds to the drilling of the concrete cover, the duct and the grout up to the tendons. This step is carried out after the accurate location of the prestressed ducts inside the structure. These drillings are performed along the duct with regular spacing of about 50cm. They correspond to either injection points or migration checking points. These points alternate regularly along the zone of treatment. - In a second step, the injection of the solution of corrosion inhibitor by the ultrasonic pump is performed. The migration of the solution inside the grout is checked from the drills performed before and after each injection point. In fact, when the solution appears, a colour change of the grout due to its moistening can be observed. During the treatment, the migration time of the solution between each injection point is also recorded. This mapping of the migration times gives information about the compacity of the grout: a long migration time will correspond to a very compact grout when a very short one will allow detecting the presence of voids and injection defects (Fig. 3). - Finally, in a third step, a new grout based on cement is introduced by using the previous drillings into the zones where voids and injection defects were detected. For this step a conventional technique of injection by pressure is applied. At the end, the drills are filled up with a repair mortar. Fig. 3 Example of mapping of the migration times of the solution of corrosion inhibitor. The inhibitor used for the treatment is a nitrite based solution which is actually commercially available. Numerous studies have already demonstrated the efficiency of this corrosion inhibitor in the case of chloride corrosion (Elsener B. & Cigna R., 2003). Numerous tests performed on mortar samples and concrete in situ applications have also shown the capacity of the nitrite of both delaying the onset of corrosion and reducing the corrosion reactions when they have already begun (Berke N. S. & Hicks M. C., 2004; Trepanier et al, 2001; Montes et al, 2004). Since the nineties, the treatment has been applied on several prestressed bridges in Belgium (viaduct of Huccorgne, Viaduct of Landelies, Bridge of Clabecq, ). The first application was performed in 1994 on the Lultzhausen Bridge in the Grand Duchy of Luxembourg. After more than 10 years, the treated prestressed concrete of this structure shows no sign of degradation.

4 COMPARISON OF THE INJECTION TECHNIQUES (1) Sample and test descriptions The tests were performed on metallic ducts of 45mm in diameter and 30cm in length. They were filled by several kinds of grouts representative of those used in post-tensioned structures. Injection heads of 12cm in length were fixed on the ducts by an araldite resin (Fig. 4). A part of these heads was also included in the grout over a length of 4cm. Four compositions of grout listed in table 1 were tested. They differ mainly by their sand content and their water/cement (W/C) ratio. The total porosities of the grouts were evaluated by mercury intrusion. Values between 20% and 25% were obtained and densities ranging from around 1600kg/m³ to 1730kg/m³ were measured. Two injection techniques were tested: the conventional method by pressure and the new technique by ultrasonic pump (Fig. 5). For each technique and grout composition, the inhibitor solution was injected in two samples. The migration of the solution was evaluated by visual observation of the grout after sample autopsy. Moreover, in order to evaluate the quantity of solution injected in the grout, the samples were weighed before and after the injection. Fig. 4 Duct equipped with an injection head. Fig. 5 Injection of the inhibitor solution in the duct by the ultrasonic pump and the pressure technique. Table 1 Composition of the tested grouts. Grout Composition 1, Composition 2, with Composition 3, without sand sand without sand Superstresscem Cement (kg) Water (l) Ready mixed Intraplast Z (kg) grout, the (plasticising admixture) composition is not Sand (kg) specified W/C ratio 0,40 0,40 0,34 Density (kg/m 3 ) Total porosity (%) (mercury intrusion) ,5 22,5 (2) Results Fig. 6 and 7 show the grout after the injection and the opening of the sample. The darker zones correspond to a moitening of the grout by the inhibitor solution during the injection. It should therefore indicate the main migration way of the solution. These zones are different for the two injection techniques. For the conventional method by pressure, the solution seems to be mainly located on the sides of the grout (fig. 6).

5 Moreover, during the injection, it was observed that the solution emerged at the opposite end of the sample after few minutes at the interface between the grout and the metallic duct. For the injection by the ultrasonic pump, Fig. 7 shows that all the grout is damp and not only its sides. This seems to indicate that the solution migrated over all the volume of the grout, including the sample core. For three of the grout compositions, the weighing performed before and after the tests show an increase between 10 and 70% of the quantities of inhibitor solution injected by the ultrasonic pump in comparison with the quantities measured for the injection by pressure (table 2). Only for the grout with the higher density, the quantities of solution injected by pressure were higher than those obtained with the ultrasonic pump. Fig. 6 Injection of the grout by the conventional pressure technique. Fig. 7 Injection of the grout by the ultrasonic pump. Table 2 Quantities of injected inhibitor solution as a function of the grout and the injection technique. Grout Test Solution injected Solution injected by Evolution of the quantity of injected by pressure (g) ultrasonic pump (g) solution between the two techniques (%) Composition (1) without sand ,40 13,25 14,29 15, , ,66 Composition (2) with sand ,00 14,05 12,06 14, ,22 +3,53 Composition (3)without sand ,80 15,40 21,06 17, ,74 +18,19 Superstresscem 1 22,40 19,02-14, ,45 16,09-33,96 EVALUATION OF THE EFFICIENCY AND THE DURABILITY OF THE TREATMENT (1) Sample and test descriptions For these tests, the samples were obtained in several steps. First of all, metallic ducts of 45mm in diameter and 80cm in length were filled with a grout corresponding to the composition number 2 of the table 1: grout with sand. Five tendons with a yield strength (Rp 0.2 ) of 1823MPa and an ultimate tensile strength of 1928MPa were placed at the centre of the duct. In order to initiate a corrosion of the tendons, chlorides were added in the grouts. Three chloride concentrations were considered: 1%, 2% and 3% by weight of cement. Four samples were cast for each chloride concentration. Samples without chlorides were also cast in order to be used as references. The ducts were then placed outdoor in order to initiate the corrosion reactions. During three years, the

6 evolution of the corrosion activity was recorded by sensors embedded in the grout (Fig. 8). These sensors are constituted of a metallic wire winded around a plastic cylinder. They record the evolutions of electrical resistance induced by a section reduction of the metallic wires caused by corrosion (Fig. 8). These variations are considered representative of the evolutions of the corrosion activity. The results showed a higher increase of the electrical resistance, and therefore of the corrosion activity, for the samples containing the most important chloride concentrations. For the grout with 3% of chlorides, several ruptures of the wires were observed due to the pitting corrosion. Fig. 8 Evolution of the corrosion activity of the tendons. In a second step, concrete covering of dimensions 15x15x80cm³ were cast around the metallic ducts in order to proceed to the injection of the inhibitor solution in a way close to the in situ conditions (Fig.9 and 10). For each chloride concentration, one of the samples was not injected in order to be used as a reference during the autopsies. These autopsies were performed two months and one year after treatment. The characterisations analysis consisted in a visual comparison of the corrosion state of the tendons and a detection of the inhibitor. Between the autopsies, the concrete samples were kept outdoor in order to maintain a corrosion activity of the tendons. Fig. 9 Concrete covering cast around the metallic duct before the inhibitor injection. Fig. 10 Injection of the inhibitor solution by the ultrasonic pump. The migration of the inhibitor solution in the grout can be observed at the sample extremity.

7 (2) Sample autopsy after 2 months Two months after treatment, the first autopsies were performed and half of the samples were fractured. The concrete covering and the ducts were removed in order to inspect the tendons. The Fig. 11 compares the corrosion state of the steel between the reference samples and the treated ones. The following observations can then be noticed: - In the case of the chloride concentrations of 1% and 2%, a very clear reduction of the corroded areas was observed on the rebars after treatment. With 1% of chlorides, nearly no corrosion can be detected after injection. - For the chloride concentration of 3%, corrosion layers can be observed both on the treated and the non-treated tendons. However, the treated samples might present a low corrosion state. The presence of the inhibitor was evaluated by analytical test strips (Merckoquant ) which were applied directly at the surface of the tendons. The nitrite concentration was evaluated semi quantitatively by visual comparison of the reaction zone of the test strip with the field of a colour scale. The results showed a significant colour change for the strip applied on the surface of the treated samples indicating that the rebars were recovered by the nitrites after the treatment. With treatment Without treatment Chloride content (%) 0% 1% 2% 3% Fig. 11 Comparison of the treated and the non-treated rebars, two months after the injection of the inhibitor by the ultrasonic pump.

8 (3) Sample autopsy after 1 year Fig. 12 presents the surface of the tendons one year after treatment. For these tests, the second part of the samples was fractured in order to inspect the rebars. From these pictures, it can be observed that: - For the treated sample containing a chloride concentration of 1%, no corrosion was detected at the surface of the steel while the development of corrosion oxides can be noticed on the reference sample which was not treated. - For the chloride contents of 2% and 3%, the corrosion layers can be observed both for the samples treated and non-treated. With treatment Without treatment Chloride content (%) 0% 1% 2% 3% Fig. 12 Comparison of the corrosion state of the treated and the non-treated rebars, one year after the injection of the inhibitor by the ultrasonic pump.

9 After one year, the presence of nitrite at the surface of the tendons was also evaluated by analytical test strips (Merckoquant ). As after two months, the colour change of the strip corresponding to a significant concentration of nitrite at the surface of the steel indicated that the inhibitor is still present (Fig. 13). With treatment Without treatment Chloride content (%) 0% 1% 2% 3% Fig. 13 Colour change of the analytical test strip indicating the presence of nitrite on the tendon surfaces after one year. CONCLUSION The objective of the technique developed by PMD and ATEAV is the treatment of the prestressed tendons used in the post-tensioned structure and affected by corrosion mechanisms induced by chloride contamination. This technique is mainly based on the injection of a corrosion inhibitor solution directly in the grout by a specific ultrasonic pump. At the end of the inhibitor treatment, the process is also completed by the introduction of a new grout in the voids of the duct. Several tests were performed at the BBRI in order to evaluate this new treatment. Firstly, the injection technique with the ultrasonic pump was compared with the conventional injection method by pressure. Several grout compositions representative of those used in the post-tensioned structure were tested. The results showed that the migration of the inhibitor solution is mainly located in the side of the grout when it is injected by pressure, while it can migrate over all the bulk of the grout including its core when injected by the ultrasonic pump. Concerning the quantities of injected inhibitor, it was observed that the use of the ultrasonic pump conducted to an increase between 10% and 70% of the weigh of the injected solution for three of the tested grouts by comparison with the method by pressure. For the grout with the higher density, this increase of injected solution was not noticed. The efficiency and the durability of the treatment were evaluated by injecting the inhibitor solution in concrete specimens casting around a metallic duct containing five tendons embedded in a grout contaminated with chlorides with concentrations of 1%, 2% and 3% by weight of cement. The autopsies of the samples performed after two months and one year after treatment revealed the absence of corrosion products on the surface of the tendons of the treated specimen containing 1% of chloride while corrosion layers were clearly observed on the non-treated samples. For the other chloride concentrations, corrosion layers were notice at the surface of the steel for both the treated and the non treated specimens. These results allow reaching the conclusion that the treatment consisting in the injection of a corrosion inhibitor by an ultrasonic pump can significantly limit the corrosion activity of the tendons in the case of chloride concentration below 1%.

10 ACKNOWLEDGMENTS The authors would like to thank the Walloon region for its financial support through this research project performed as part of the repair guidance actually carried out in the BBRI. We also wish to thank the compagnies PMD and ATEAV for their help and all the persons who have participated to this program. REFERENCES Berke N. S., Hicks M. C. (2004), Predicting long-term durability of steel reinforced concrete with calcium nitrite corrosion inhibitor, Cement and concrete composites, vol. 26, pp Bertolini L., Raharinaivo A. (2007), On the corrosion of prestressing steel, COST Action 534, New materials, systems, methods and concepts for durable prestressed steel structures, Final report, Part IV, Electrochemical maintenance and repair methods. Calgaro J. A., Lacroix R. (1997), Maintenance et réparation des ponts, 666 p. Elsener B., Cigna R. (2003), Mixed in inhibtors, COST Action 521, Corrosion of steel in reinforced concrete structures, Final report, Part 1 Preventative measures. Mallett G. P., Repair of concrete briges, State of the art review, Transport Research Laboratory, 1996, 194 p. Montes P., Bremmer T. W., Lister D. H. (2004), Influence of calcium nitrite inhibitor and crack width on corrosion of steel in high performance concrete subjected to a simulated marine environment, Cement and concrete composites, vol. 26, pp NBN EN 12696, Cathodic protection of steel in concrete, 2000, 42p. Trépanier S. M., Hope B. B., Hansson C. M. (2001), Corrosion inhibitors in concrete part III, Effect on time to chloride-induced corrosion initiation and subsequent corrosion rates of steel in mortar, Cement and concrete research, vol. 31, pp

11 FIGURES 12 and 13 COLORED

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