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5 days ago Multimedia Battambang   4 views

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Location: Battambang
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The use of doubled strips of rubber dam material for drains has become general in this clinic, especially for wounds that do not require irrigation. These have been found superior to tubing and the ordinary type of cigaret wicks because they are nonirritating to the wound and the capacity of the drain is increased as a result of the capillarity derived from the two opposed surfaces of the rubber dam material. On removal, these drains leave very little deformity in the tissue and they do not tend to plug the wound, as do wick or tube drains frequently.

This type of material has the fault of stretching considerably and of sometimes breaking, a portion being left within the wound. To obviate this we have strengthened strips of rubber dam material by running a seam down the center with a sewing machine. This makes the rubber nonelastic and adds sufficiently to its tensile

The proper assembly of underground precast concrete structures is often critical in the construction of underground structures. In particular, interfacial waterproofing between precast concrete segments is a key factor influencing use, safety, and life span. Current practice is to incorporate waterproofing rubber strips in the design. During the installation process, compressive stress is applied to the strip by post-tensioning to achieve performance. For this paper, lateral constraint compression tests were carried out on composite rubber seal strips that utilize putty. Special waterproofing and sealing test devices were designed to investigate corresponding relationships between water pressure and compressive stress (or strain). A relationship between water resistance pressure and compression stress and strain of the putty-based composite rubber strip was proposed based on the series tests and the control target of the minimum compression strain of the putty composite rubber strip was then suggested. Finally, full-scale waterproofing tests on tunnel joints were conducted. The experimental results provide a scientific reference for the engineering application and design of composite sealing rubber strips putty for underground post-tensioned precast concrete structures.

Waterproofing is typically a key design goal for underground precast concrete structures (Ossai 2017). For modern tunnel structures, segments often require casting of high performance concrete with very low permeability (DAUB 2013). Therefore, the primary possible leakage point considered is the segmental joint (Yurkevich 1995; Lee and Ge 2001; Henn 2010; Wang et al. 2011; Wu et al. 2014; Fang et al. 2015; Soltani et al. 2018). For tunnel lining, one of the most significant factors impacting the overall behavior and structure response was the existence of the segmental joints for precast concrete units (Wood 1975; Koyama 2003). Due to the underground environment, repair after the leakage in the structure is very difficult. In general, design service life for underground structures ranges from 75 to 100 years. Structures within urban underground tunnel networks tend to deform due to the long-term dynamic load and impacts associated with surrounding buildings. Under working conditions, the largest deformation was frequently observed and entered into failure state at the joint (Böer et al. 2014; Huang et al. 2015; Hong et al. 2016). Therefore, waterproofing materials need to accommodate structural deformation.

In present concrete construction, elastic rubber strips in sealing and waterproofing joints of assembling segments have been commonly used. For underground concrete structures, standard design for sealing joints uses Ethylene-Propylene-Diene Monomer (EPDM) polymer rubber strips arranged circumferentially on the end faces of the segment (Ding et al. 2017). Putty-based composite rubber strips have great viscosity and elasticity, which can compensate to a certain degree for the adverse effect of the interface defects at joints. To evaluate the waterproofing ability at joints, special attention is directed to the sealant behavior of the EPDM sealing strips. A time-dependent constitutive model is proposed to assess the long-term waterproof ability of EPDM rubber used in segmental joints (Shi et al. 2015). At present, there are few requirements for rubber strips in the design specifications, and there is limited understanding of the relationship between applied forces and waterproofing performance. In addition, precast concrete structures incorporate a groove at the joint interface for the rubber strip positioning (Hu et al. 2009). The type of groove at the joint interface can limit extruded profile epdm rubber seal strip lateral deformation, which can increase pressure on the strip and improve waterproofing ability. The various types of grooves offer different degrees of constraint. Therefore, mechanical properties of rubber strips along with groove design at precast concrete structure joints are key elements in waterproof design. The joint open width is also regarded as a key performance indicator, since it is the weakest part of the shield segmental lining (Liao et al. 2008; Zhang et al. 2015). As the weakest and vulnerable point in the segmental lining, joints have been investigated in experiments (Ding et al. 2013; Liu et al. 2015; Kiani et al. 2016), numerical analyses (Ding et al. 2004; Teachavorasinskun and Chub-uppakarn 2010) and case studies (Jun 2011; Basnet and Panthi 2018). Testing apparatus was designed to accurately monitor water leakage pressure of segmental joints under various combinations of opening and offsets (Ding et al. 2017). Molins and Arnau (2011) presented an in situ load test and 3D numerical simulation on a full-scale segmental lining for the Barcelona metro line. According to a case study in Shanghai, Huang et al. (2017) perceived that longitudinal joints of the metro tunnel have large open widths and lose waterproofing when disrupted by unexpected surcharges.