PSI - Issue 11

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

131

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

2

piece rupture (Bastos and Pinheiro (2007)). However, the development of the prestressed concrete, allowed a new stage in the design of the concrete sleepers. Through extensive experiments and investigations in several railroads of the world it was possible to arrive at the current models: safe, resistant and economic (Bastos (1999)). In the railway structure, sleepers perform a very important role, since they are responsible for receiving and transmitting to the ballast the stresses and tensions caused by the trains traffic, providing support to the rails, ensuring their fastening and maintaining constant the distance between them (gauge). (Brina (1988)). At the same time, they are subject to cyclic loading throughout their entire life, taking the materials that constitute them to an intense process of fatigue (Bastos and Pinheiro (2007)). These functions require the sleeper applied to the track to be intact, free of defects that may compromise its structural capacity and durability. However, it is possible that during the manufacture and handling of these concrete pieces defects occur, due to the process for the densifying by means of vibration, thermal retraction, retraction by drying along the curing or deformation of the pieces, among other causes. Some of these defects can be repaired and the sleeper can be used normally or reclassified for use in railroads with less traffic and less speed, avoiding the rejection of the piece, as long as they do not involve structural reasons, that can cause increase of the tensions in the prestressing rebar and certainly compromise the resistance of the sleeper, causing loss of its structural function. Among the non-structural defects of the prestressed concrete sleepers that can be repaired are corners and edges breaks, misaligned fastening anchors and cracks. The cracks in concrete sleepers should receive special attention because they can cause loss of prestress, entrance of aggressive agents and damage in relation to the structural function, compromising the good performance of the piece, including loss of functionality in the railroad. However, the recovery of this type of defect is conditioned by some factors such as correct identification of the cause, cracking activity, which refers to thickness variation, and depth (Ripper and Souza (1998)). The treatment is usually simpler in the case of superficial and stable cracks. Currently there are many methods of recovering cracks in concrete structures, among the most common are the techniques of injection and sealing of cracks and another technique known for its easy application and high durability, painting crystallization. This technique consists of a chemical treatment for recovery, protection and waterproofing, applied to the surface of the concrete as a paint, which is able to form crystals in the visible depth inside the cracks, as shown by Cappellesso (2016). Thus, the present article aims to present a proposal for the recovery of cracked concrete sleepers, using the paint crystallization technique, and to describe all stages of the process, including the manufacture of the studied sleepers, the cracking defects issue, the criteria used to justify the validation of the sleepers’ repair or rejection, the treatment application, the validation and reclassification tests. 2. Concrete Sleeper As explained by Bastos (1999), concrete sleepers are classified according to their external shape and characteristics. As for the shape, the main sleepers manufactured today are the bi-block or mixed types, which combines reinforced concrete and prestressed concrete monoblock, recommended for railways that demand high loads per axle, as in the case of Heavy Haul Railroads. The prestressed concrete monoblocks are monolithic pieces that have longitudinally prestressed ropes or cables, because of the increase of their resistant capacity, and generally have a slender section in their central part. They are usually pre-tensioned, although some post-tensioned models are also produced. As for the concrete used for the manufacture of sleepers, it should be designed to have high strength and durability, low porosity and good waterproofing conditions. These features are obtained through the use of additions and special additives, low water/cement ratio and adequate optimization of materials, respecting the normative requirements in relation to the minimum consumption of cement, which should be 350 kg/m³ of concrete and characteristic strength of the concrete (fck) not less than 45 MPa (NBR 11709 (2015)).