Sliding planes of PTFE are commonly used because of their excellent tribological properties. However, especially in cases of high contact pressures, PTFE suffers from its comparatively poor mechanical properties. This paper presents a sliding construction developed within an innovative experimental test-setup to enable experimental investigation of large-scale concrete members subjected to punching shear. To fulfill the special demands of the new test-setup, greased, only 0.5 mm thin sheets of PTFE were used to minimize friction between the bearing construction and the test specimen. This highly effective sliding construction leads to a dynamic friction coefficient μ d,max between 0.0065 and 0.0035 while the static friction coefficient μ s remains below 0.0048. Simultaneously, compressive axial stresses of more than 60 MPa occur. The paper highlights major aspects of the sliding plane’s development and demonstrates its sliding abilities.
Polytetrafluoroethylene (PTFE) is one of the most frequently used solid sliding materials in the world. Due to its unique tribological properties, PTFE is nowadays established in a wide range of applications [
However, PTFE exhibits comparatively poor mechanical properties and a high rate of wear. One way to improve the rate of wear and the mechanical properties of PTFE is to incorporate filler materials such as glass fibers, bronze or carbon. For example, Khedkar, et al. [
In civil engineering earlier studies of Mark, et al. [
Nevertheless, PTFE is also used in case of substantially greater sliding paths. In order to preserve its sliding abilities, the thickness of the PTFE-sheets is then increased. For example, ordinary bridge bearings are equipped with PTFE-sheets of about 3 - 5 mm thickness (
In the remainder a sliding construction, fundamental for cost effective punching shear tests of large concrete slabs of practical relevance, is introduced, that complies rotational rigidity as well as sufficient shape stability. The basic concept of this new experimental test-setup is the application of the best practice in numerical investigations. By utilizing symmetry conditions, test loads, physical dimensions and self-weights are drastically reduced. As a result, considerably larger slabs can be tested with existing test infrastructure.
Prerequisite of this concept is the development of a symmetry bearing to fulfill the symmetry conditions at the symmetry lines of a punching test. Therefore, a bearing construction allowing almost frictionless sliding in vertical and horizontal directions as well as a rotational rigidity (almost no deformability) must be established.
Sliding planes of greased PTFE have proven to be suitable in this regard since they show excellent tribological properties even with high compressive axial stresses. Within the development process, PTFE-sheets of 0.5 mm thickness have proved to be particularly suitable (
The new experimental test-setup is developed in a step-by-step procedure. Important components like the sliding
plane or the back anchoring system are developed and optimized separately. To avoid unnecessary factors of influence, the sliding planes are developed strictly without concrete test members. However, certain demands are formulated to ensure the functionality of the sliding plane for subsequent use within the test-setup [
The requirements of the tribological system are:
・ Unidirectional vertical (up to 30 mm) and lateral (a few mm) movements.
・ Small friction coefficient (smaller than 1%) with simultaneous compressive axial stresses of more than 60 MPa.
・ Negligible inherent deformability: small rotations between sliding plate and bearing construction, no horizontal movements perpendicular to the bearing construction.
Polytetrafluoroethylene has a very complex tribological behavior. The relation between compressive axial stresses and the friction coefficient is reciprocal and depends on lots of variables. PTFE exhibits a low friction coefficient when it slides on a hard counter surface, especially stainless steel is commonly used. Additionally, the friction coefficient decreases with falling temperature, with low sliding speed and a low roughness of the counter surface [
Due to its chemical composition, PTFE has a strong tendency for creeping and yielding. PTFE (virgin) already starts to yield when the contact pressure reaches approx. 7 MPa [
Both lubricating grease and dimples have advantageous effects on the friction coefficient. It is well-established that the use of lubrication leads to a much lower friction coefficient. In the case of high pressure, however, the lubrication grease is squeezed out of the sliding plane. Consequently, the friction coefficient increases especially for higher slide paths. To prevent this, dimples are arranged to ensure a continuous greasing.
Usually lubricating grease for PTFE/Stainless Steel connections is based on silicone oil and lithium soap. In the presented work, the lubricating grease Syntheso 8002 is used. This lubricating grease, produced by KLUEBER, has been especially developed for plane bridge bearings [
With regard to bridge bearings the PTFE-sheets are equipped with dimples to ensure a continuous and uniformly greasing.
In the development process of the sliding plane the very low thickness of the PTFE-sheets posed a special challenge. Initial attempts with perforated PTFE-sheets led to an impairment of the sliding abilities. Much better results are achieved with a perforation of the double-faced adhesive tape. Due to high compressive axial stresses resulting from the pre-stressing process and the low thickness of the PTFE, the lubricating grease and the PTFE are pressed into the perforation. The occurring dimples prevent the lubrication grease of squeezing out.
In order to determine the friction coefficient of the sliding construction, performance tests were carried out. The performance tests were conducted at the structural testing laboratory (KIB-KON) of the Ruhr-Universität Bochum. The main characteristics of the tests are summarized in
Parameter | Value | Unit |
---|---|---|
Stroke-controlled testing velocity | 0.5 | mm/min |
Pre-stressing forces | Up to 720 | kN |
Dead load of the construction | 0.32 | kN |
Max. displacement | Up to 30 | mm |
Contact surface (per interface) | 11,200 | mm² |
For the determination of the friction coefficient μ Equation (1) is used. In this equation the friction coefficient μ is defined as the ratio of vertical to horizontal loads. The dead load G of the construction, consisting of anchorage plates, spherical bearings (calottes), screw nuts (M33) and threaded rods (ISO-metric, M33), was determined by measuring the weight of all components before test. The horizontal load FN is generated by pre- stressing the anchorage plates against the bearing construction. FT, the tangential testing load, is introduced by hydraulic jacks.
In order to carry out the performance tests, two anchorage plates are prepared as described in section 2. Before the double-faced adhesive tapes and PTFE-sheets are applied, all essential surfaces are cleaned with industrial cleaner to ensure consistent conditions. Subsequently, the lubrication grease is added generous onto all four surfaces of the sliding plane by distributing it uniformly with a brush. Afterwards, all sliding components are arranged and aligned at the bearing construction as shown in
tened and overstretched by about +0.3‰. The overstretching is performed to compensate losses of the pre- stressing force due to retracting the hydraulic jack. In order to prevent torsions of the test specimen, the rear anchorage plate is held with a template during the pre-stressing process. After the pre-stressing process of the lower threaded rod has been completed, this procedure is repeated on the threaded rod at the upper end of the anchorage plate. The sequence of the pre-stressing process is chosen with regard to the pre-stressing procedure within the innovative test-setup for punching shear tests on concrete test members.
Subsequent to the pre-stressing process a time interval of exactly 10 minutes is used to set up the measuring equipment and to align the load application at the anchorage plate on the front side of the bearing element (
For the performance tests two symmetry bearing elements (X and Y) are available. The elements are made from a t = 80 mm steel front plate and six t = 40 mm steel panels welded to a comb-shaped construction. To fix the bearing construction, vertical rods are pre-stressed in the concrete floor of the testing laboratory. In
To assess the quality of the developed sliding plane an experimental test program with the final configuration of the sliding plane was carried out. Important parameters and main results of the tests are summarized in
The results of the performance tests are presented in
Name | Position | Orientation | P0,(t = 0) | P0,m | P0,min | μs | μd,max | Fs,max | Fd,max | |
---|---|---|---|---|---|---|---|---|---|---|
[kN] | [kN] | [kN] | [%] | [%] | [kN] | [kN] | ||||
GRV-Xi-20140612 | Xi | Punching shear | 650 | 644 | 640 | 0.43 | 0.41 | 2.48 | 2.32 | |
GRV-X1-20130605 | X1 | Punching shear | 652 | 642 | 634 | 0.43 | 0.53 | 2.48 | 3.08 | |
GRV-X2-20140605 | X2 | Punching shear | 655 | 648 | 643 | 0.38 | 0.41 | 2.17 | 2.34 | |
GRV-X3-20140604 | X3 | Punching shear | 646 | 625 | 633 | 0.26 | 0.46 | 1.36 | 2.56 | |
GRV-Xa-20140604 | Xa | Punching shear | 656 | 647 | 640 | 0.42 | 0.41 | 2.44 | 2.33 | |
GRV-2014-1-Xa-inverted | Xa | Inverted | 696 | 689 | 685 | 0.48 | 0.42 | 3.02 | 2.57 | |
GRV-2014-2-Xa-inverted | Xa | Inverted | 708 | 697 | 693 | 0.37 | 0.35 | 2.30 | 2.12 | |
GRV-2014-3-Xa-inverted | Xa | Inverted | 700 | 640 | 599 | 0.37 | 0.52 | 2.27 | 3.01 | |
GRV-2014-4-Xa-inverted | Xa | Inverted | 688 | 682 | 679 | 0.39 | 0.40 | 2.36 | 2.41 | |
GRV-Yi-20140523 | Yi | Punching shear | 695 | 649 | 609 | 0.10 | 0.58 | 0.38 | 3.44 | |
GRV-Yi-II-20140603 | Yi | Punching shear | 602 | 553 | 510 | 0.08 | 0.50 | 0.16 | 2.45 | |
GRV-Y1-20140526 | Y1 | Punching shear | 640 | 602 | 569 | 0.20 | 0.65 | 0.96 | 3.59 | |
GRV-Y1-II-20140526 | Y1 | Punching shear | 615 | 580 | 549 | 0.30 | 0.39 | 1.53 | 1.94 | |
GRV-Y2-20140522 | Y2 | Punching shear | 620 | 596 | 576 | 0.17 | 0.37 | 0.73 | 1.89 | |
GRV-Y2-II-20140527 | Y2 | Punching shear | 664 | 639 | 618 | 0.13 | 0.42 | 0.54 | 2.36 | |
GRV-Y3-II-20140528 | Y3 | Punching shear | 658 | 632 | 612 | 0.28 | 0.59 | 1.52 | 3.41 | |
GRV-Ya-II-20140602 | Ya | Punching shear | 651 | 625 | 609 | 0.22 | 0.57 | 1.11 | 3.24 | |
P0,(t = 0) = initial pre-stressing force; P0,m = mean pre-stressing force; P0,min = minimal pre-stressing force; μs = coefficient of static friction; μd = coefficient of dynamic friction; Fs = tangential test load corresponding to μs; Fd,max= tangential test load corresponding to μd,max.
less, the values of the coefficient of static friction μs ranges between 0.37% and 0.48% and the coefficient of dynamic friction μd,max varies from 0.26% to 0.52% (
should be mentioned that larger friction coefficients usually are a result of instabilities of the sliding sheets caused by alignment at the openings of the steel bearing or imprecise assembly.
In addition to the performance tests a stop-and-go test was carried out to investigate the influence of a not- uniformly movement of a test member (stick and slip effect). The stop-and-go test was performed with the test member GRV-2014-4-Xa-inverted (
In
To evaluate the functionality of the coupled system, a matching monitoring concept was elaborated. In order to determine the friction coefficient according to Equation (1), the test load, the vertical reaction force (globally) as well as the vertical displacement and the pre-stressing force of all six sliding elements (locally) are measured (
An exact calculation of the friction coefficient similar to the performance tests is unfeasible due to the fact, that each sliding plate exhibits different vertical displacements and pre-stressing forces. Nevertheless and in order to be able to assess the quality of the sliding system, an average friction coefficient is calculated. The calculation is based on the following values:
・ Average pre-stressing force when exceeding the point of static friction (separately for each sliding plate).
・ Maximal difference between the tangential test load FT and the reaction force FA when exceeding the point of static friction (uniformly distributed over all sliding plates).
・ Maximal vertical displacement of each sliding plate until failure occurs.
It is important to note that an analysis of the LDTs (
average pre-stressing force differs between 550 to 600 kN. A maximum sliding distance of 12.61 mm occurs. Based on the assumptions to calculate the friction coefficient, the results of the performance tests can be verified for the coupled sliding system. Moreover, all requirements of the tribological system (Section 2.1) are being fulfilled.
In this paper, a highly effective sliding plane of greased PTFE is presented. The sliding plane was developed within a new experimental test-setup to investigate the punching shear behavior of reinforced concrete slabs utilizing symmetry conditions. Due to the special requirements regarding the bearing construction, an innovative sliding construction had to be developed. As a result of the development process, a deformation resistant sliding plane is obtained that works highly effective even under high compressive axial stresses.
To assess the quality of the developed sliding plane, an experimental test program was carried out. The results of the test program show that the developed sliding plane exceeds the requirements to ensure the functionality of the sliding plane within the new test-setup. Based on the test results, the following conclusions can be drawn:
・ The presented sliding plane leads to a friction coefficient μ below 0.0065 with simultaneous compressive axial stresses of more than 60 MPa.
・ The friction coefficient μ tends to remain nearly constant over the sliding distance of about 30 mm.
・ Due to the low thickness of the PTFE-sheets, a negligible inherent deformability is achieved.
・ A not-uniformly movement of a test member has a negligible influence on the friction coefficient.
・ Tests with six coupled sliding plates lead to the conclusion, that the excellent sliding abilities can also be achieved by coupled sliding systems.
The financial support of the research project “Size-dependent punching shear failure of thick reinforced concrete slabs” by the German Research Foundation (DFG) is gratefully acknowledged. The authors like to express their gratitude to the staff of the structural testing laboratory of the Ruhr-Universität Bochum (http://www.rub.de/kib-kon) involved in this project.
Lennart Bocklenberg,Karsten Winkler,Peter Mark,Stefan Rybarz, (2016) Low Friction Sliding Planes of Greased PTFE for High Contact Pressures. Open Journal of Civil Engineering,06,105-116. doi: 10.4236/ojce.2016.62010