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 (B?er 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.
In this paper, mechanical tests for compressive stress and strain of putty-based composite rubber strips along with waterproofing performance tests at the interface between putty-based composite rubber strips and concrete are conducted. These tests investigate influence of strip compressive force and the joint stretching value on waterproofing of sealing rubber strips. It attempts to establish a design model and proposed control target for mechanical and waterproofing properties of this new type of rubber strip. Waterproofing test of a full-scale tunnel joint is carried out. The research work of this paper provides a scientific reference for the engineering application and design of composite sealing rubber strips with putty for underground post-tensioned precast concrete structures.
The putty-based composite silicone weather stripping extruded rubber strip was made up of Ethylene-Propylene-Diene Monomer polymer rubber (EPDM foam strip) and the external composite layer (putty paste) of high viscosity reactive polymer cement (butyl rubber). The primary reason for use of the composite was to take advantage of the external putty-based material’s properties of viscosity and superplasticity, which can heal mesoscopic cracks and defects on the surface of concrete structures to improve interface waterproof ability. Cross section dimensions and picture of the rubber strip are shown in Fig. 1.
Figure 2 displays the lateral confinement loading test device, which is composed of two parts: convex shape of the upper part and concave shape of the lower part. The inner and outer diameter of the annular groove was 170 mm and 220 mm, respectively. The upper part has protrusion that squeezes the strip, and the annular groove is set at the lower part of the device with an annular rubber strip installed in it (see Fig. 2c). The length of the EPDM foam rubber in the elastic state has 640 mm, and the compression area is 15,315 mm2. Quasi-stress control was selected for the tests.
Experimental results of the putty-based composite strip under lateral confinement for compressive stress and displacement are provided in Fig. 7. At the early stage of loading, the compressive stress of the composite strip gradually increased with the displacement. It was observed that the displacement dramatically increased and at the later stage of the loading when the load reached at 112.36 kN primarily maintained at 11 mm. At end of the test, the rubber strip was not crushed and the internal EPDM foam rubber after unloading almost recovered to its original shape. The maximum displacement of the putty-based composite rubber strip under lateral confinement was approximately 11 mm, which was brought on by the squeezing of the inner hole of the composite strip raising the internal pore of the EPDM rubber. The instantaneous elastic recovery during the unloading process was 85% of total deformation. The residual deformation of the composite rubber strip was gradually recovered to its original state with time. Eventually, the rubber strip was not damaged. The deformation recovery of the inner elastic material to its original shape can partly drive unrecoverable external putty material.
In the loading process, the two end faces of the rubber sealing strip weatherstrip for window were partially extruded upon loading, since they were not restrained at the end face (see Fig. 8). When maximum deformation was reached, the upper and lower parts of the concrete were in contact with each other.
There was an inflection point in the curves of the rubber strip in the two different grooves, as presented in Fig. 10. Before the point, the internal pore and middle hole of the sealing strip were not tightly compressed signifying that the compression strain increased gradually with compressive stress. Moreover, the relationship between compressive stress and compression strain for the two different sizes of groove were almost the same before the inflection point. At the inflection point, the central hole of sealing strip and the pore of foam rubber were completely compressed. The whole strip was so dense that the compressive force increased sharply with the compression strain. The compressive stresses of the sealing strip in the two different grooves at the inflection points were almost the same and their corresponding compression strain differed by roughly 20%.
In the early stage of compression, the compression moduli of the Waterproof Rubber Seal Strip Gasket were almost the same under the two groove constraints. In the later stage of loading, the compression stress of 6 mm depth of groove was greater than that of 4 mm depth under the same compression strain, and the compressive strain of the 4 mm depth of groove was greater than that of the 6 mm depth of groove under the same stress. This was mainly attributed to the difference in the constraint degree of the groove to the strip at the later stage of loading. In the final stage, the two compression interfaces of the 6 mm depth specimens were close in contact with each other. The remaining space at the joint was rather small, and there was no compression space. However, there was still a large space between the two interfaces of the 4 mm deep specimens. This was mainly due to the sum of the strip deformation and groove depth limit. The bilinear outsourced line was taken as an approximate stress-deformation relation model as shown in Eq
The tests started with two culverts gradually assembled in place. After initial post-tensioning, dial gauges were installed inside the culverts to measure joint space variations in the process of post-tensioning. Simultaneously, the strains on the post-tensioning steel bars were recorded. The water injection pump and water pressure gauges at the lower part of the water injection hole of the box culvert were installed. After the steel bars were set in the duct, the conductor was run through the perforated sheet. The sheet was tightly attached to the concrete surface and bolts fastened. Table 2 provides experimental results of the post-tensioning process. The upper and lower prestressed steel bars were tensioned at the same time, otherwise the friction resulted in the vertical location due to the friction at the bottom so that the two tendons were employed. Upon completion of a post-tensioning cycle, the gap change and steel strain were measured. The bolts were then fastened. The maximum tension force was 180 kN. During the post-tensioning process, strain of steel bars varied linearly, indicating that the post-tensioned steel bars were in the elastic state with strain close to the theoretical value.
In order to investigate the waterproofing performance of the putty-based composite rubber, mechanical behavior tests using rubber strip and waterproofing performance tests of the interface between the strip and precast concrete were performed.
- Created: 22-10-21
- Last Login: 22-10-21