PSI - Issue 11

Iara Silva et al. / Procedia Structural Integrity 11 (2018) 130–137 Iara Silva et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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3. Manufacturing and inspection The studied sleepers were fabricated through the long- line system, which is used for large sleepers’ production with an average duration of 24 hours, depending on the curing mode. In this system, the formworks are very long, more than 150 meters in length, and the prestressed wires pass through them completely, and are pre-tensioned by equipment located at the ends of the formwork. These concrete sleepers have a pre-assembled fastening system, for which the inserts are incorporated into the built-in components or partially embedded components. The function of the inserts is to enable the mounting of the railing fastening assemblies, supporting plates and other sleepers’ accessories. In this case, the inserts are nodular cast iron fastening anchors to secure the rail to the sleeper. After the placement of the fastening anchors in the formwork, the prestressed wires are tensioned, and then the pouring, vibration and normal curing of the concrete occurs. The concrete used in the sleeper’s manufacture is prepared in its own concrete plant. The curing and protection process is started as soon as possible after the concrete is densified in the formwork, where care is taken to avoid concrete dehydration and direct exposure to the sun and wind using plastic tarpaulins to cover it. After the curing time of the concrete, once the compressive strength exceeds 30 MPa, the pretension can be transferred to the concrete by cutting the wires at the ends of the formwork and the sleepers are stripped. During manufacturing, quality control is performed, where the materials conditions, the concrete strength, the dimensions and resistance of the sleeper and the fastenings positioning are checked. The inspection is a mandatory procedure to validate the sleepers, where they eliminate all those that have imperfections and/or are outside the criteria specified in standard and according to the technical specification of the client. 4. Cracking During the manufacture of concrete sleepers, defects related to the raw materials of the concrete or to the actual fabrication (tensioning and concreting) may arise. The presence of these defects, detected during the inspection phase, may justify the disposal and subsequent destruction of the piece. Among the most frequent defects, which may arise during production and cause sleepers’ rejection, are cracks. These, in general, are not related to structural causes, are stable, and therefore do not directly compromise the structural function of the sleeper. On the other hand, if untreated, these same cracks can provide passageways for aggressive agents to enter and, over time, cause prestress loss, compromising the good performance and durability of the piece, including loss of functionality on the track, or making the piece susceptible to the appearance of other pathologies, like rebar corrosion, for example. Therefore, it was concluded that the stable cracks resulting from the manufacturing process of the sleepers did not justify the immediate discarding, and that there is a possibility of recovery and application of these pieces in certain parts of the railroad. However, the criteria for such a procedure should be analyzed, studied and formalized and a practical and effective method for recovering these cracks should be proposed. 5. Crystallization Technology Due to the practicality, cost-effectiveness and durability, the treatment based on painting crystallization technology was chosen. This technology concerns a chemical treatment applied on the concrete surface as a paint for recovery, protection and waterproofing of existing structures. The product, when reacted with water, forms insoluble crystals that promote sealing of the cracks and reduce the passage of water and aggressive agents by penetrating deeply into the pores and capillaries of the concrete. 6. Methods and procedures In the present work, a methodology was adopted based on the following steps, namely: characterization and identification of cracks, establishment of criteria for recovery or disposal of sleepers, recovery and verification procedure through mechanical tests and final inspection.