Repair methods, reinforced concrete

This chapter deals with the strategies for repair of reinforced-concrete structures damaged by carbonation or chloride-induced corrosion of the reinforcement, and outlines the most common methods. Details of conventional repair and electrochemical maintenance methods are reported in Chapters 19 and 20. The repair of prestressed concrete structures is outside the scope of this book. 

The adhesion or bond of latex-modified mortars to reinforcing bars is found to be very important for their use in repair work for damaged reinforced concrete structures. Figure 4.54l l illustrates the bond strength of the latex-modified mortars to the reinforcing bars, determined under direct tension by pull-out method. In general, the bond strength of the latex-... 

Kikukawa K, Mutoh K, Ohya H, Ohyama Y, Tanaka H, Watanabe K, Flexural reinforcement of concrete floor slabs by carbon fiber textiles. Composite Interfaces, 5(5), 469-478, 1998. Katsumata H, Yagi K, Applications of retrofit method with carbon fiber for existing reinforced concrete structures, Obayashi Corp. and Mitsubishi Kasei Corp. Presented at 22nd joint UJNR panel meeting on Repair and Retrofit of Existing Reinforced Concrete Structures, National Institute for standards and Testing (NIST), Gaithersburg, 1990. 

Avril S, Ferrier E, Vautrin A, Hamelin P and Surrel Y (2004), A full-field optical method for the experimental analysis of reinforced concrete beams repaired with composites . Compos Part A - Appl Sci, 35(7-8), 873-884. 

In many cases, the repair of a structure may be limited to casting a new external layer of highly impermeable material with properties carefully adjusted to those of the substrate. In other cases, however, it is essential that new material takes part in the load bearing. Then, active and durable connections are to be anticipated to ensure transfer of load on to new material. For example, increasing the load-carrying capacity of reinforced concrete beams may be realized by different methods ... 

As described in detail in the RILEM 124 SRC recommendation (RILEM, 1994), the basic repair methods for carbonation induced corrosion consist in repassivation of the steel by a mortar layer or by local repair. The procedure for patch repair is described in the published guidelines (e.g. Bentur et al., 1997). Limitation of the concrete moisture content is another repair principle, its aim is dry out the concrete, e.g. when additional facade elements are mounted to reduce energy consumption. When carbonated concrete is not completely removed in the vicinity of the reinforcement, corrosion occurs after repair when sufficient moisture is present (Schiessl and Breit, 1996). 

Mietz, J., Isecke, B., Jonas B. and Zwiener, F. (1994) Electrochemical realkalisation as repair method for reinforced concrete structures (in German), Berlin Bundesanstalt fur Materialpriifung (BAM). 

Curing needs close supervision both in terms of method and duration. Clearance to strike the form work must be recorded as must subsequent inspection, survey, repair, if any, and markings of the finished reinforced concrete structure. 

Figure 18.4 shows the methods used for repair of carbonated structures, which are based on repassivation of steel, limitation of moisture content of concrete and coating of the reinforcement. 

The anodic process can be stopped by applying a coating to the reinforcement that acts as a physical barrier between the steel and the repair mortar. For this purpose only organic coatings, preferably epoxy based, should be used. Protection is entirely based on the barrier between the reinforcement and the mortar, and passivation of steel cannot be achieved because contact with alkaline repair material is prevented. This method should be used to protect depassivated areas of the reinforcement only as a last resort, i. e. when other techniques are not applicable and only for small specific applications [1,4]. It may be used, for instance, when the thickness of the concrete cover is very low and it is impossible to increase it to the proper level, so that the repair material cannot provide durable protection to the embedded steel. 

Weyers, R.E., Prowell, B.D., Sprinkel, M.M. and Vorster, M. (1993). Concrete Bridge Protection, Repair, and Rehabilitation Relative to Reinforcement Corrosion A Methods Application Manual. SHRP Reports. SHRP-S-327. 

The appUcation of so called wandering corrosion inhibitors is among the modem methods of the damaged structure repair. The sodium monofluorophosphate is used for this purpose [USA patent no 5071579], as well as the potassium monofluorophosphate, patented in Europe by Domtar Inc. The clean dry surface of concrete is covered by the monofluorophosphate water solution or in the form of gel the phosphate ions infiltrate the concrete and reconstmct the passive film on reinforcement. 

EN 1542 Products and systems for the protection and repair of concrete structures. Test methods. Measurement of bond strength by pull-off. EN 1544 Products and systems for the protection and repair of concrete structures. Test methods. Determination of creep under sustained tensile load for synthetic resin products (PC) for the anchoring of reinforcing bars. 

The reasons described above lead to a conclusion that the interface may be the weakest element in the repair work, and as such, a source of future destruction. That is why there are many practical methods available to reinforce the interface and to ensure its high quality. Among the various known measures, special interface layers of the Portland cement paste, neat or reinforced with thin grids of steel or textile wires may be mentioned. Careful preparation of the surface of old concrete is always needed by cleaning and moistening. Intermediary layers may be prepared with increased Portland cement content, admixtures of polymeric materials or silica fume, or composed with epoxy resins. 

Electrochemical methods, such as cathodic protection and chloride extraction, can be used as a part of a repair strategy. Cathodic protection techniques, described above, provide alkalinity. The impressed current transports alkalies to the reinforcing bar and allows alkalinity to be retained. The chloride extraction system removes chloride ions (Cl ) from the concrete electro-chemically, and does not allow the breaking of passive layers. In yet another process, called re-alkalization, alkaline metal ions penetrate concrete from an external source of a suitable electrode to re-alkaline the concrete and regenerate the hydroxyl ions. 

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