A Direct Comparison of the Traditional Method and a New Approach in Determining 220 Transfer Lengths in Prestressed Concrete Railroad Ties

2013 ◽  
Author(s):  
Weixin Zhao ◽  
B. Terry Beck ◽  
Robert J. Peterman ◽  
Robert Murphy ◽  
Chih-Hang John Wu ◽  
...  
Keyword(s):  
Prestressed Concrete ◽  
The United States ◽  
Laser Speckle ◽  
Surface Strain ◽  
Transfer Length ◽  
New Approach ◽  
Average Maximum ◽  
Concrete Mix ◽  
Strain Data ◽  
Length Determination

This paper presents the detailed analysis of surface strain data obtained at six prestressed concrete tie plants in the United States. These data were obtained by the authors by conducting a total of 220 transfer length measurements on prestressed concrete railroad ties with different concrete-mix designs and reinforcement variations. The surface strain profiles of the railroad ties were obtained using the traditional Whittemore gage, as well as a rapid non-contact technology, called laser-speckle imaging (LSI), that was previously developed by the authors. The LSI technique achieved a microstrain resolution comparable to that was obtained using mechanical gauge technology. The measured surface strain profiles were then analyzed by both the 95% AMS (95% Average Maximum Strain) method, and the ZL (Zhao-Lee) method that was recently proposed by the authors. The ZL method is an unbiased statistical method that provides a more accurate and reliable transfer length determination. A direct comparison between the 95% AMS method and the ZL method was achieved by applying both methods to determine the 220 railroad tie transfer length values. The comparison confirmed the bias of the 95% AMS method in estimating transfer length value, as predicated by theoretical analysis.

Reliable Transfer Length Assessment for Real-Time Monitoring of Railroad Crosstie Production

2014 ◽  
Author(s):  
Weixin Zhao ◽  
B. Terry Beck ◽  
Robert J. Peterman ◽  
Chih-Hang John Wu ◽  
Naga Narendra B. Bodapati ◽  
...  
Keyword(s):  
Real Time ◽  
Prestressed Concrete ◽  
Laser Speckle ◽  
Surface Strain ◽  
Experimental Comparison ◽  
Profile Measurement ◽  
Transfer Length ◽  
General Validity ◽  
Strain Profile ◽  
Average Maximum

Automated in-plant diagnostic testing of prestressed concrete railroad crossties is now within reach due to recent progress in robust surface strain measurement techniques. The newly developed non-contact Laser Speckle Imaging (LSI) technique has been shown to provide rapid and accurate surface strain profile measurement, which is a key requirement for rapid transfer length assessment. Accurate determination of transfer length is critical for maintaining continuous production quality in the modern manufacture of prestressed concrete railroad crossties. Conventional assessment of transfer length generally presumes the underlying existence of a bilinear prestressing force distribution and a corresponding bilinear surface strain profile. Furthermore, it is well-known that this bilinear profile is smoothed due to the effects of finite gauge length during the process of measuring surface strain. In addition, recent extensive crosstie measurements in concrete railroad tie plants have shown significant departures from this simple bilinear profile, which bring to question the general validity and reliability of the traditional 95% AMS (95% Average Maximum Strain) method. Deviations from the simple bilinear profile shape were shown to be partially due to the non-prismatic shape of typical concrete railroad ties. In addition, extensive comparisons between predicted and measured surface strain profiles on numerous crossties suggest that the underlying strain distribution for crossties is best represented by an exponential strain profile, with an asymptotic approach to the fully-developed compressive strain. This is in contrast with extensive testing of prisms with fixed cross-section and fixed prestressing wire eccentricity, for which the surface strain appears to be best represented by the simple bilinear strain profile. Clearly, departures from non-prismatic behavior have added complexity to transfer length measurement. If accurate and reliable measurements of this important quality control parameter are to be realized, these issues of transfer length uncertainty need to be addressed. This paper provides an experimental comparison of several possible alternative transfer length assessment procedures, in an attempt to answer important uncertainty questions which need to be addressed if rapid real-time transfer length is to be achieved. It is shown that in spite of considerable differences in the transfer length processing methods, and significant departures from prismatic behavior, the averaged results are in many cases consistent with the simple bilinear underlying strain profile assumption. Bias in the measurement of crosstie transfer length due to non-prismatic behavior will also be investigated in this paper.

Determining Transfer Length in Pre-Tensioned Concrete Railroad Ties: Is a New Evaluation Method Needed?

2013 ◽  
Author(s):  
Weixin Zhao ◽  
B. Terry Beck ◽  
Robert J. Peterman ◽  
Chih-Hang John Wu ◽  
Grace Lee ◽  
...  
Keyword(s):  
Evaluation Method ◽  
Compressive Strain ◽  
Accurate Determination ◽  
Laser Speckle ◽  
Surface Strain ◽  
Bond Quality ◽  
Transfer Length ◽  
General Validity ◽  
Imaging Device ◽  
Average Maximum

The transfer length is perhaps the most significant KEY indicator of the bond quality between reinforcing wire/strand and concrete, and its measurement in pre-tensioned concrete railroad ties can enable concrete tie producers to identify problem ties before they are put into service. The 95% AMS method is the traditional method used to determine the transfer length from measurements of surface strain. The method generally presumes the underlying existence of a bilinear strain profile. During recent field trips to six concrete railroad tie plants, we conducted hundreds of transfer length measurements on concrete railroad cross-ties using a newly developed automated Laser Speckle Imaging device. It has been observed that many of the strain profiles depart significantly from this underlying bilinear profile, and bring to question the general validity and applicability of the 95% AMS (95% Average Maximum Strain) method. This paper discusses the difficulties with accurate determination of transfer length in various practical situations using the traditional 95% AMS method. Deviations of the strain profiles from the simple bilinear shape are shown to be partially due to the non-prismatic shape of typical concrete railroad ties. In addition, computational evidence suggests that the underlying strain distribution may be exponential in nature, with an asymptotic approach to the fully-developed compressive strain, potentially superimposed on the non-prismatic problem identified above. These departures are discussed along with proposed solutions to the basic problem of accurate transfer length assessment.

Effects of Cover, Compressive Strength, and Wire Type on Bond Performance in Prismatic Prestressed Concrete Members

2018 ◽  
Author(s):  
Adrijana Savic ◽  
B. Terry Beck ◽  
Aaron A. Robertson ◽  
Robert J. Peterman ◽  
Jeremiah Clark ◽  
...  
Keyword(s):  
Compressive Strength ◽  
Cross Sections ◽  
Prestressed Concrete ◽  
Test Procedure ◽  
Concrete Cover ◽  
Laser Speckle ◽  
Surface Strain ◽  
Qualification Test ◽  
Concrete Mix ◽  
In Series

The bond between wire and concrete is crucial for transferring the stresses between the two materials in a prestressed concrete member. Furthermore, bond can be affected by such variables as thickness of concrete cover, type of pre-stressing (typically indented) wire used, compressive (release) strength of the concrete, and concrete mix. This work presents current progress toward the development of a testing procedure to get a clear picture of how all these parameters can ruin the bond and result in splitting. The objective is to develop a qualification test procedure to proof-test new or existing combinations of pre-stressing wire and concrete mix to ensure a reliable result. This is particularly crucial in the concrete railroad crosstie industry, where incompatible conditions can result in cracking and even tie failure. The goal is to develop the capability to readily identify compatible wire/concrete designs “in-plant” before the ties are manufactured, thereby eliminating the likelihood that defectively manufactured ties will lead to in-track tie failures due to splitting. The tests presented here were conducted on pre-tensioned concrete prisms cast in metal frames. Three beams (prismatic members) with different cross sections were cast simultaneously in series. Four pre-stressing wires were symmetrically embedded into each concrete prism, resulting in a common wire spacing of 2.0 inches. The prisms were 59.5in long with square cross sections. The first prism was 3.5 × 3.5in with cover 0.75in, the second was 3.25 × 3.25in with cover 0.625in and the third prism in series was 3.0 × 3.0 in with cover 0.50in. All pre-stressing wires used in these initial tests were of 5.32 mm diameter and were of the same wire type (indent pattern) denoted by “WE”, which had a spiral-shaped geometry. This is one of several wire types that are the subject of the current splitting propensity investigation. Others wire types include variations of the classical chevron shape, and the extreme case of smooth wire with no indentions. The wires were initially tensioned to 7000 pounds (31.14 KN) and then gradually de-tensioned after reaching the desired compressive strength. The different compressive (release strength) strength levels tested included 3500 psi (24.13 MPa), 4500psi (31.03 MPa), 6000 psi (41.37 MPa) and 12000psi (82.74MPa). A consistent concrete mix with water-cement ratio 0.38 was used for all castings. Geometrical and mechanical properties of test prisms were representative of actual prestressed concrete crossties used in the railroad industry. Each prism provided a sample of eight different and approximately independent splitting tests of concrete cover (four wire cover tests on each end) for a given release strength. After de-tensioning, all cracks that appeared on the prisms were marked, and photographs of all prism end surfaces were taken to identify the cracking field. During the test procedure longitudinal surface strain profiles, along with live-end and dead-end transfer lengths, were also measured using an automated Laser-Speckle Imaging (LSI) system developed by the authors. Both quantitative and qualitative assessment of cracking behavior is presented as a function of cover and release strength. In addition to the identification of whether cracking took place at each wire end location, measurements of crack length and crack area are also presented for the given WE wire type. The influence of concrete cover and release strength are clearly indicated from these initial tests. The influence of indented wire type (indent geometry) will also be discussed in this paper, along with a presentation of some preliminary test results. This work represents a successful first step in the development of a qualification test for validating a given combination of wire type, concrete cover, and release strength to improve the reliability of concrete railroad crosstie manufacturing.

Performance of a Continuously Traversing 2-Camera Non-Contact Optical Strain Sensor for In-Plant Assessment of Prestressed Concrete Railroad Crosstie Transfer Length

2016 ◽  
Author(s):  
B. Terry Beck ◽  
Aaron A. Robertson ◽  
Robert J. Peterman ◽  
Chih-Hang John Wu
Keyword(s):  
Measurement System ◽  
Prestressed Concrete ◽  
Strain Measurement ◽  
Production Quality ◽  
Step Change ◽  
Surface Strain ◽  
Length Measurement ◽  
Transfer Length ◽  
Measurement Systems ◽  
New System

Accurate knowledge of transfer length has been shown to be crucial to the goal of maintaining continuous production quality in the modern manufacture of prestressed concrete railroad ties. Traditional manual laboratory methods, such as the conventional Whittemore method which requires the use of embedded reference points, are clearly not suitable for production operation or for use in reliable production quality-control. This paper presents the results of another advance in the development of automated transfer length measurement systems for practical in-plant operation. The new device offers a significant improvement over the previously successful automated Laser-Speckle Imaging (LSI) system developed by the authors. The earlier automated LSI strain measurement system has been modified to provide significantly improved optical resolution of longitudinal surface strain, with the ability to resolve longitudinal prestressed concrete crosstie surface strain without time-consuming special surface preparation. More importantly, the new system is also capable of making measurements of strain in a real-time “on-the-fly” manner over the entire distance range of interest on the tie associated with transfer length development. It features both a “jog” mode of operation, similar to its predecessor in which measurements of longitudinal surface strain are automatically captured in arbitrary spatial increments over the entire range of the computer-controlled traverse, and an “on-the-fly” mode in which measurements of longitudinal surface strain are captured without the need for stopping at each measurement location. This latter mode offers the potential of a much faster capture of the strain profile and should prove to be very beneficial for field testing and in-plant diagnostic applications. The performance of this new system is first demonstrated using a new calibrated step-wise uniform strain field setup which has been developed specifically for verification of this and other automated transfer length measurement systems. This verification system produces a calibrated step change in surface deflection, effectively subjecting the automated strain measurement system to an ideal step change in longitudinal strain for a given gauge length. In addition, the new automated system is demonstrated by conducting measurements of longitudinal surface strain on prestressed concrete crossties in a manufacturing plant. For this latter experimental in-plant testing, strain measurements using the new system are also compared directly with those from the recently introduced 6-camera transfer length measurement system, as well as with the traditional Whittemore gauge measurements. The agreement between these independent measurement systems is remarkable, and it is shown to even be possible to discern differences in strain profile and associated transfer length between adjacent crossties within a given casting bed. This new automated and high-resolution device should provide a very convenient and fast diagnostic tool for the manufacturer to quickly identify the need to modify production (e.g., concrete mix) if transfer length specifications fall out of desired range.

Utilization of High Resolution 3D Optical Scanning of Crossties to Assess Cross-Sectional Parameters and the Effects of Long-Term Abrasion and Wear

2016 ◽  
Author(s):  
B. Terry Beck ◽  
Aaron A. Robertson ◽  
Naga Narendra B. Bodapati ◽  
Robert J. Peterman ◽  
Chih-Hang John Wu ◽  
...  
Keyword(s):  
Prestressed Concrete ◽  
Surface Strain ◽  
Scanning System ◽  
Cad Model ◽  
Surface Geometry ◽  
Transfer Length ◽  
Cross Sectional ◽  
Optical Scanning ◽  
3D Cad

Accurate unbiased assessment of transfer length for prestressed concrete railroad ties requires detailed knowledge of the longitudinal variation of geometrical cross-section parameters responsible for establishing the resulting surface strain profile. This is because the complex cross-sectional shape produces a non-uniform strain plateau region, which makes the accurate evaluation of transfer length more difficult. In particular, human judgment of a “plateau region” for assessment of the average maximum strain becomes subject to large uncertainty, and clearly this procedure cannot be used in any type of automated in-plant transfer length diagnostics. The important geometrical tie parameters include the cross-sectional area, centroid, moment of inertia, and the eccentricity of the prestressing wires. If a CAD drawing is available, this information can be digitally extracted from the CAD model representation of the crosstie. In fact, this digital extraction has been done and has already been in use for some time in assessing transfer length for one of the common crosstie manufacturer designs. However, current research efforts are investigating the characteristics of existing crossties which have been in track for many years, for which CAD drawings of the original designs are unlikely to be available. The objective of the current research is to develop a comprehensive understanding of the material characteristics that have caused splitting failures in prestressed concrete railroad ties, and those characteristics that have resulted in ties that have performed well after many years in track. As part of this effort, a three-dimensional (3D) Optical Scanning System is being used to accurately scan and quantify the surface geometry of previously manufactured ties that have been in service, so as to produce an accurate 3D CAD model for later analysis associated with the above long-term research objectives. For the initial phase of this work, a sample from the CXT crossties of known geometrical characteristics that were subjected to representative long-term loading at the TTCI Facility in Pueblo Colorado, was scanned so as to accurately map out detailed 3D tie surface geometry. These ties were cast using the same concrete materials but with different prestressing wires, and were all subjected to the same extreme in-track loading for a period of several years. A commercially-available 3D Laser-Based Optical Scanning System, having a maximum spatial resolution of approximately 0.1mm, was used to perform the surface scanning operations presented in this paper. The CXT tie provides a useful initial evaluation of the accuracy and general feature capture capability of the scanning procedure, since a 3D CAD model for this tie has been provided by the manufacturer. A detailed qualitative and quantitative analysis is presented which compares the 3D CXT CAD model geometry with the 3D geometry of the experimentally scanned ties. Illustrations as to how this 3D technique can reveal such features as abrasion and wear, along with the longitudinal variation of the above mentioned cross-section parameters associated with longitudinal surface strain and transfer length assessment, are included in this paper.

Transfer Length Characterization of Entire Crosstie Plant Casting Bed Using Continuously Traversing Dual-Camera Non-Contact Optical Strain Sensors

2017 ◽  
Author(s):  
B. Terry Beck ◽  
Aaron A. Robertson ◽  
Robert J. Peterman ◽  
Kyle A. Riding ◽  
John Wu
Keyword(s):  
Quality Control ◽  
Real Time ◽  
Prestressed Concrete ◽  
Manufacturing Process ◽  
Control Parameter ◽  
Strain Sensors ◽  
Surface Strain ◽  
Manufacturing Plant ◽  
Transfer Length ◽  
Optical Strain

Transfer length has been identified as a key diagnostic parameter for evaluating the load bearing capability of prestressed concrete railroad crossties. Furthermore, it has been proposed for use as a valuable quality control parameter. However, until quite recently the capability to easily and accurately measure transfer length has been limited primarily to a laboratory setting. This is especially true for measurements made in the harsh environment of a tie manufacturing plant. The development of portable non-contact optical strain sensors has opened the door to rapid in-plant transfer length measurement. The measurement capability of these devices has been repeatedly demonstrated not only in the laboratory, but more importantly also through actual testing at multiple tie manufacturing plants. The latest version of the automated Laser-Speckle Imaging (LSI) system developed by the authors offers improved optical resolution of longitudinal surface strain, with the ability to resolve longitudinal prestressed concrete crosstie surface strain without time-consuming special surface preparation. The new system is also capable of making measurements of strain in a real-time “on-the-fly” manner over the entire distance range of interest on the tie associated with transfer length development. This faster capability to capture the strain profile with high resolution makes this new technology very beneficial for field testing and in-plant diagnostics applications. It has been demonstrated to be capable of resolving minor differences in longitudinal surface strain profiles associated with ties even in adjacent cavities. As a logical next step toward eventual implementation of transfer length as a quality control parameter, it is important to evaluate the expected variation of transfer length during the tie manufacturing process. This paper presents the results of extensive in-plant assessment of transfer length in an attempt to characterize experimentally the in-plant manufacturing variations that can occur in practice. To the best of the authors’ knowledge, this is the first time extensive real-time measurements to this extent have been attempted in an actual tie manufacturing plant with the expressed purpose of statistically characterizing the variations in transfer length that take place over an entire casting bed. A sampling of transfer lengths from well over 50 ties was determined during the manufacturing process (corresponding to over 100 transfer length measurements). The sampled tie measurement locations were distributed at different “form” locations along the casting bed, and included samplings of ties from several different “cavities” within a given form. The entire bed was 45 forms in length, each form having 6 tie cavities, for a total bed size of 270 ties. The statistical distribution of overall transfer length measurement results is presented, along with what may be typical variations in strain profile and resulting transfer length as a result of variations that took place in the manufacturing process. The overall range of transfer length observed, along with an investigation of possible bias due to position within the casting bed, and apparent variations of transfer length within a given form, are identified and discussed.

Long-Term End-Slip Measurements and Corresponding Transfer Lengths in Pretensioned Concrete Railroad Ties Fabricated With 15 Different Reinforcements

2015 ◽  
Author(s):  
Robert J. Peterman ◽  
Naga Narendra B. Bodapati ◽  
B. Terry Beck ◽  
Chih-Hang John Wu
Keyword(s):  
Surface Strain ◽  
Manufacturing Plant ◽  
Transfer Length ◽  
Strain Measurements ◽  
Pretensioned Concrete ◽  
Concrete Mix ◽  
Different Types ◽  
Low Humidity ◽  
Wire Strand

Fifteen different reinforcements that are widely employed in manufacturing of railroad ties worldwide were selected for the study presented in this paper. Selected reinforcements include; 12 number of 5.32 mm diameter wires, two 3/8-inch diameter 7 wire strands, and one 5/16-inch diameter 3 wire strand. Twelve wire reinforcements are differed by surface indent geometries with one wire being smooth surfaced profile. Strand reinforcements consisted one smooth and one indented 7 wire strand, and one smooth 3 wire strand. All reinforcements were stored in low-humidity environment to avoid rust. Later, pre-tensioned concrete railroad ties were fabricated at a tie manufacturing plant with the selected 15 different reinforcements in January 2013. Same concrete mix proportions were used during the fabrication of the ties with these 15 reinforcement types. Reinforcement end-slips were measured for each concrete tie at every reinforcement location during August 2014 (after one and half years). Simultaneously, transfer length measurements were measured on all these ties through surface strain measurements. Detailed analysis of the measured end-slips for the ties fabricated with 15 reinforcements is presented. This analysis includes the variation of end-slip measurements at different locations in the cross-section. Variation in end-slip measurements for different types of reinforcements is also discussed. Transfer lengths are compared with end slip measurements and an equation to predict transfer lengths from long term end-slip values is presented.

Experimental Investigation of the Influence of Surface Contaminants on the Transfer Length of Smooth and Indented Prestressing Reinforcements Used in the Manufacture of Concrete Railroad Ties

2015 ◽  
Author(s):  
B. Terry Beck ◽  
Robert J. Peterman ◽  
Chih-Hang John Wu ◽  
Steve Mattson
Keyword(s):  
Real Time ◽  
Compressive Strain ◽  
Surface Strain ◽  
Strain Level ◽  
Lubricating Oil ◽  
Recent Attempt ◽  
Transfer Length ◽  
Worst Case ◽  
Surface Contaminants ◽  
Average Maximum

It has been hypothesized that surface contaminants, such as lubricants on prestressing wires or strands, influence the resulting transfer length. However, until recently, the extent of this possible influence has only been speculation, as has been the relative influence on wire in comparison to strand. With the recent development of the ability to rapidly assess transfer length using new non-contact optical methods, it is now possible to explore hypothetical scenarios such as this with nearly real-time capability in the manufacturing plant. This paper presents a recent attempt to determine the effect of lubricating oil on the transfer length of ties, by conducting nearly real-time in-plant transfer length measurements using a newly developed prototype multi-camera non-contact transfer length measurement system. The testing was conducted on prismatic concrete turnout ties manufactured at the Nortrak plant in Cheyenne, Wyoming. Two different types of turnout ties were investigated, one containing indented 5.32-mm-diameter wire reinforcement and the other containing 3/8-in.-diameter 7-wire strand. These ties were located near the end of the casting bed. Prior to casting, one end of the form was sprayed with a generic lubricant, literally saturating the prestressing wires or strands. The ties were then cast and de-tensioned following the normal manufacturing process. This clearly represented a highly worst-case scenario for the influence of surface contaminants. Measurements were made using the new multi-camera system, providing a detailed profile of surface strain over several feet along each end of the last three ties in the casting beds (one for strand and one for wire) — the last tie being the one subjected to the application of oil prior to casting. Hence, the influence of oil application on adjacent ties was also revealed by these tests. For the tie end with strand reinforcement subjected to oil soaking, the maximum compressive strain only reached about 400 microstrain, far below the nominal average maximum strain level of approximately 1000 microstrain. In fact, the associated transfer length for the oil-soaked end could not be definitively measured because the strain level never achieved the plateau level of strain. In contrast, the tie end with oil-soaked indented wire exhibited a significant increase in transfer length; however, the transfer length remained well below the distance to the rail seat. From these worst-case tests, one can conclude that smooth strand is potentially highly influenced by lubricating oils, whereas the influence on indented wire is likely small by comparison.

Development of a 5-Camera Transfer Length Measurement System for Real-Time Monitoring of Railroad Crosstie Production

2013 ◽  
Author(s):  
Weixin Zhao ◽  
B. Terry Beck ◽  
Robert J. Peterman ◽  
Chih-Hang John Wu
Keyword(s):  
Real Time ◽  
Measurement System ◽  
Laser Speckle ◽  
Surface Strain ◽  
Length Measurement ◽  
Plant Production ◽  
Automated System ◽  
Production Operation ◽  
Transfer Length ◽  
Strain Measurements

Knowledge of transfer length during production is critical for maintaining continuous production quality in the modern manufacture of prestressed concrete railroad crossties. Traditional laboratory methods for measuring transfer length, using manual instruments such as a Whittemore mechanical gauge or surface mounted resistance-type strain gauge, are simply not suitable for production operation. They are too time-consuming to implement, require extensive surface preparation, and can also require special operator training to provide accurate and reliable surface strain profiles from which the transfer length can be determined in a post-processing manner. In contrast with earlier manual methods, the newly developed non-contact Laser Speckle Imaging (LSI) technique has been shown to be capable of providing rapid and accurate surface strain measurement and consequently also rapid transfer length assessment. This system has recently been automated and combined with the new Zhao-Lee (ZL) least-squares strain profile fitting technique for quickly and reliably processing surface strain data. The automated system and processing procedure have been shown to provide an improved assessment of transfer length, unhampered by human intervention and subsequent potential human judgment bias. This paper presents recent progress toward the development of a 5-camera non-contact transfer length measurement system that is capable of continuous monitoring of railroad crossties in a production plant. This is made possible using an optimized version of the previously successful LSI system, which minimizes the number of surface strain measurements required to achieve reliable transfer length assessment. Experimental results and analysis will be presented for the latest multi-camera prototype concept for this new system design, demonstrating that only a few discrete surface strain measurements are required to achieve accurate and reliable transfer length assessment. Thus, for the first time it is now possible to envision practical real-time quality control monitoring of railroad crossties during an in-plant production operation.

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