There are over 35 million concrete crossties (sleepers) installed in track in North America, with approximately 750,000 to 1,500,000 additional new ones installed annually. These components have been the focus of much research over recent years, which is resulting in significant advancements in their design and use. New solutions are addressing rail seat deterioration, abrasion, splitting, and other issues that have been associated with past derailments or have required major intervention. Moreover, concrete crosstie design is moving toward a performance-based approach, which is more efficient than the traditional prescription-based methods with generous safety factors. Smart crossties are also emerging, which go far beyond the traditional concepts of crosstie application. This paper presents an overall picture of the state of the art of concrete crossties in North America, linking fundamentals, industry challenges, design approaches, recent developments, and trends for the future.

The importance of the indent geometry of prestressing wire used in the manufacture of prestressed concrete railroad ties is well known. In particular, it has been shown to have a major affect on the magnitude of the transfer length. Important parameters affecting transfer length include indent depth, indent volume and indent sidewall angle, with indent volume providing a major influence. Previous experimental evidence has revealed that the larger the indent volume, the shorter the transfer length. Furthermore, for full load bearing capacity, it is critical that the transfer length not exceed the distance to the rail seat. As a result, transfer length represents a key diagnostic parameter for evaluating the load bearing capability of prestressed concrete railroad ties, with potential for use as a quality control parameter. Adequate transfer length alone, however, is not sufficient to guarantee tie performance. The geometry of the prestressing wire indents also plays a major role in the formation of cracking, which is of particular importance to the manufacture of concrete ties intended for high-speed rail applications. Insufficient consideration of cracking and de-bonding of prestressing wires associated with ties in service can result in severe splitting and complete tie failure. It should also be noted that while the wire specifications in standard ASTM A881 are intended to promote quality prestressed railroad tie behavior, they do not address either the detailed causes of cracking and splitting, or the specific indent features that are responsible. Although indent geometry was known to influence both transfer length and splitting propensity, specific quantitative relationships between indent features and tie performance had not been identified. Accurate quantitative measurements of indent geometry on a statistically significant basis were simply not yet available. Recently, however, a high- resolution automated non-contact optical wire indent scanning system was developed for rapidly characterizing all relevant indent parameters including indent depth, indent width, indent sidewall angle, indent pitch, and indent volume. This paper presents, for the first time, development of reliable quantitative relationships between the relevant indent parameters and the tie performance indicators with a high degree of correlation. In addition, measurements using the new automated profiling system have revealed a new indent parameter that, on its own, shows direct correlation with transfer length. The physical concept behind this new parameter, referred to as the "Bite Per Length (BPL)," and its measured characteristics, will be presented along with its demonstrated experimental relationship to transfer length.

Research is conducted to estimate uncertainty in concrete monoblock tie flexural behavior through Monte Carlo simulation. A parameterized Moment-Curvature based computational program was developed for design and analysis of the prestressed concrete monoblock tie flexural behavior. For Monte Carlo simulation, a parameter sweep was performed to sample the expected range of input quantities. The parameter sweep program generates independent parameter combination input files, and the quantities of interest (QOIs) are computed by the Moment-Curvature based computational program. The QOIs are computed at governing locations, including: 1) moment at first crack, 2) moment at crack propagation to outer layer of reinforcement, 3) deflection at rail-center and 4) crack height at AREMA recommended design force. Several simulation input parameters were randomly varied including 1) concrete compressive strength at 28 days, 2) initial concrete strength, 3) modulus of rupture, 4) prestress level, and 5) wire position. The results of the Monte Carlo simulation allow the estimation of the probabilistic prediction of tie performance. The prediction could be used to check primary tie design before proceeding to experimental tests for design approval. This approach could be used to analyze existing tie behavior, in track, for risk analysis.

An experimental testing of prestressed concrete monoblock ties was conducted at Kansas State University to collect loaddeflection data. The experimental results were used to validate the developed tie analysis and design computational tool. Present prestressed concrete monoblock tie design specification, American Railway Engineering and Maintenance -of-Way Association (AREMA), recommends flexural test method for critical locations (rail-seat and rail-center). As commonly known that rail-seat positive and rail-center negative are the governing cases in flexural design of monoblock ties. Considering the cross-section is generally reduced at center region of tie, the capacity that could be sustained is relatively smaller compared to rail-seat. Additionally, the center bound boundary condition induces large negative bending moment at center region of the tie. Thus, rail-center negative bending test was selected. A four-point bending testing setup followed AREAMA 2020 Chapter 30 Part 4 Section 4.9.1.6 [2]. In total 10 tests were conducted, ties selected for the test include three ties that were retired from service and seven virgin ties. The vertical deformation was measured, collecting by Keithley series 2750 data acquisition system. A 5-power magnifying glass was recommended to observed crack. Load control loads were applied until first crack, and the displacement control loads were applied. The load-deflection results at rail-center were used to compare with the estimation computed by developed computational tool. A total of 9 comparisons were made. There were seven virgin ties (CXT type) and two post-service ties (TypeF). To eliminate uncertainties resulting in discrepancy, it is desired that the program inputs follows as closely as possible to the validation benchmarks. Cross-section properties of CXT type tie used in the analysis program, and it was measured by previous researcher at K-state. Type-F tie cross-section were measured at each shape change point. The concrete compressive strength was determined by conducting experiments following ASTM C42/C42M 2020 standards. Additionally, the Young's modulus of elasticity was defined through best fitting to the elastic region of experimental load-deflection results at railcenter. Then, the load-deflection curves were computed and compared with experimental outcomes.

A study was conducted to estimate the flexural capacity of monoblock tie under practical load and various ballast support conditions through a numerical approach. The current recommended design approach is maximum allowable stresses by American Railway Engineering and Maintenance -of-Way Association (AREMA) [1]. The recommended design standard includes general design aspects (material, dimensions and loads) and structural strength and testing method for prestressed concrete monoblock tie. Considering the complexity of the tie behavior especially post-cracking and varying ballast support conditions, it is desired to capture tie performance under cyclic train traffic load. Furthermore, the effects of changes in tie geometry should be incorporated into tie design and analysis. Thus, a numerical approach based on Moment-Curvature (M- C) principle is employed for design and analysis of the flexural behavior of prestressed concrete monoblock ties. The computational tool is computing M-C curve for each slice of tie where the tie can be divided into minimum half inch slice. At this stage, the crack propagation can be observed on concrete strain diagram. Then repeating the same process of computing M-C curve along the tie, and the rotation can be determined at a particular moment. Finally, deflection of tie is calculated by using moment-area method, additionally changes in rail-seat center-to-center spacing can be defined. A verification and validation approach are employed to establish accuracy and reliability of the developed numerical program. In this paper, the program verification will be discussed. The developed code was compared with corresponding analytical solution or numerical solution at each intermediate step. There are three benchmarks used to verify the code, including hand solution, commercial finite element result, and existing tie analysis example. A small discrepancy was noted in the third case, however, the comparisons had good agreement overall.

The experimental tests which were performed in this paper evaluate the effect of the concrete mixture using granite as aggregate on the longitudinal splitting behavior in prestressed concrete members. Three prisms having different edge distances were cast concurrently having different release strengths of concrete 4500-psi (31-MPa) and 6000-psi (41M-Pa). Furthermore, four different wires type were used in these experiments to evaluate the influence of all variables on the bond between steel and concrete which is highly important for transferring the stresses between the two materials.

Prestressed concrete ties could develop end-splitting cracks along tendons due to lateral bursting stresses. The lateral bursting stresses can form due to Hoyer effect (change in diameter of the prestressing tendons due to Poisson’s ratio), the jacking force in the tendons, geometrical features and indent characteristics of the prestressing tendons. End-splitting cracks can occur immediately after de-tensioning procedure in some cases, but they also can be developed during the first weeks after de-tensioning procedure due to sustained lateral stresses exerted by the prestressing tendons. The ability of concrete to resist these bursting stresses without cracking is primarily the function of the thickness of concrete cover, the type of concrete mixture used and the maximum compressive strength of the concrete. Qualification test will be great tool for prestressed concrete tie manufacturers to identify tie designs that may be susceptible to end-splitting cracks. This test was formally adopted as section 4.2.4 in Chapter 30 of the 2021 AREMA Manual for Railway Engineering.

This research evaluates the influence of the different types of concrete mixture, using a shallow type of indentation of wire, having the different edge distance and compressive strength of concrete on splitting resistance in pretensioned concrete railroad sleepers. The investigated compressive strength of concrete was 4500psi. The research was experimental, and the part of this research was formally adapted in Arema StandardsforRailwayEngineering Chapter 30 section 4.2.4.

The important characteristic in the creation of longitudinal splitting cracks in pretensioned concrete members has found to be the geometry of the pre-stressing wire indents. Longitudinal splitting along prestressing tendons can result in severe splitting of prestressed member in the field under loading over time. The research evaluated the influence of wire type indentation on the longitudinal splitting in prestressed concrete members fabricated with different concrete mixtures and different compressive strength of concrete. A key objective was to find the best type of wire to avoid failures in the field. A study was conducted at Kansas State University to understand the effect of wire type on the longitudinal splitting between prestressing steel and prestressed concrete. Three different types of wires will be presented in this paper denoted as “WB”, “WF” and “WQ”. The wires have different parameters which include indent depth, indent width, indent sidewall angle, indent pitch and indent volume.

This research was part of a larger project, “Quantifying the Effect of Prestressing Steel and Concrete Variables on the Transfer Length in Pretensioned Concrete Crossties,” conducted by Kansas State University (KSU) and sponsored by the Federal Railroad Administration (FRA). This report highlights advances in methods to determine the bond-slip relationships of different prestressing steel wire types. These relationships can predict wire performance in concrete ties and are useful for modeling tie designs. The full technical report can be downloaded from the K-State Research Exchange.

Prestressed concrete ties could develop end-splitting cracks along tendons due to lateral bursting stresses. The lateral bursting stresses can form due to Hoyer effect (change in diameter of the prestressing tendons due to Poisson’s ratio, the jacking force in the tendons, geometrical features, and indent characteristics of the prestressing tendons. End-splitting cracks can occur immediately after de-tensioning procedure in some cases, but they also can be developed during the first weeks after de-tensioning procedure due to sustained lateral stresses exerted by the prestressing tendons. The ability of concrete to resist these bursting stresses without cracking is primarily the function of the thickness of concrete cover, the type of concrete mixture used and the maximum compressive strength of the concrete. The test purpose was to identify tie designs that may be susceptible to end-splitting cracks. The Qualification test will be great tool to identify tie designs that have ability to form end-splitting cracks. The System Qualification Test involves six pre-tensioned concrete prisms with the same prestressing tendons and concrete mixture that is used in the concrete ties, except that the edge distance for the prisms is reduced by approximately 25 percent. If this reduction in edge distance results in longitudinal splitting cracks along the prestressing tendons, then the system (tie design and material selection) may be susceptible to concrete end-splitting cracks. In this case, changes to the design and/or material selection should be made prior to mass production of ties.

This research was focused on evaluating the influence of the different variables that affectsplitting in prestressed concrete members, particularly prestressed concrete ties. These include the thickness of concrete cover, release strength of concrete, and the type of wire indentation. Different combinations of these variables can affect splitting. This research was focused on finding the best possible material characteristics to avoid potential failures in the field. The objective of this research was to set the minimum value of the thickness of concrete cover, for different wire types used in manufacturing having given types of aggregate for concrete mixture. The importance of this research was to identify the best materials and the best system (concrete, wire, cover) designs before the production of prestressed concrete ties. Splitting test results presented in this research were focusedon three different thicknesses of concrete cover, three different concrete mixes, a 4500psi concrete release strength, and seven different indented wire types.

In this study, the effect of concrete mix design on end-splitting cracks in prestressed concrete members was examined. The concrete material needs to be resistant to splitting-crack growth at the strengths at which the pretensioned member is detensioned. Mixtures with variable coarse aggregate volume, angularity, and water-to-cementitious ratio were designed and Fracture toughness tests on 4 three-point bending concrete specimens for each mix were carried out. Next, pretensioned prisms with three different reinforcement edge distances 0.750, 0.625 and 0.500 in. (19 mm, 16 mm and 13 mm) were fabricated and detensioned when the concrete compressive strength reached 4500 psi (31 MPa). Splitting cracks were quantitatively evaluated by direct measurement. The results indicated a strong correlation between crack length measurements and fracture toughness. Additionally, a linear regression model was developed by which splitting-crack growth in prestressed cross-sections with varied edge-distance thickness can be predicted. The findings from this study could lead to a significant improvement in splitting crack resistance in pretensioned concrete members where the high amount of transverse stress is often introduced at low compressive strengths. (C) 2020 Elsevier Ltd. All rights reserved.

Pretensioned concrete prisms made with five different prestressing strand types (four 7-wire strands and one 3-wire strand) were load tested to failure to understand the effect of strand indentation types on the development length and bonding performance of these different reinforcements. The prestressing strands were denoted SA, SB, SD, SE and SF. SA was a smooth strand while the other four were indented strands. All strands utilized in manufacturing ofprisms had diameter of 3/8″ (9.52 mm). Among all types of strands, SF was the only 3-wire strand and the remaining strands were all 7-wire strands. For all types of strands, four straight strands were embedded into each concrete prism, which had a 5.5″ (139.7 mm) × 5.5″ (139.7 mm) square cross section. The strands were tensioned to 75 percent of ultimate tensile strength of strands and gradually de-tensioned when the concrete compressive strength reached 4500 psi (31.03 Mpa). A consistent concrete mixture with type III cement, water-cement ratio of 0.32 and a 6-in. slump was used for all prisms. Prisms were load tested in 3-point-bending at different embedment lengths to obtain estimations of the development length of each type of strand. Two out of three identical 69-in.-long (175.26 cm) prisms were load tested at one end and one was tested at both ends for each reinforcement type evaluated. First prisms were tested at 28-in. (71.12 cm) from the end, while second prisms were tested at 20-in. (33.02 cm) from the end. Third prisms were loaded at 16.5-in. (41.9 cm) from one end and 13-in. (33.02 cm) from the other end. Thus, a total of 20 load tests (5 strand types × 4 tests each) were conducted in this study. During each test, a concentrated load with the rate of 900 lb/min (4003 N/min) was applied at mid-span until failure occurred. Values of load, mid-span deflection, and strand endslip were continuously monitored and recorded during each test. Plots of load-vs-deflection were then compared for prisms with each strand type and span, and the maximum sustained moment was also calculated for each test. The load tests revealed that there is a large difference in the development length of the strands based on their indentation type.

Splitting cracks in pre-tensioned concrete members often occur at the early stages after detensioning. To prevent these splitting failures, concrete members need to be resistant to crack growth at the strengths in which the beam is detensioned. In this paper, the effect of different concrete parameters including aggregate shape and content, water-to-cementitious (w/c) ratio, fly ash, paste and air void content on the crack resistance of concrete used in concrete railroad ties were investigated using Two-Parameter Model (TPM). For each mixture evaluated, twelve three-point prisms were tested at 27.5, 41.3 and 55.1 MPa concrete compressive strengths (4000, 6000 and 8000 psi) to determine the effect of concrete compressive strength on crack growth potential. In addition, splitting tensile tests were conducted on three samples at each compressive strength for all mixtures. Finally, the results were analyzed and the function of utilized method was discussed. The results show that increasing angularity, aggregate size and volume, and decreasing w/c ratio improve fracture toughness by 30% whereas changing paste, fly ash and air void content negligibly influence fracture toughness. However, all improving factors were seen to be most effective at low strengths. The obtained fracture toughness values from this study could lead to a significant improvement of splitting crack resistance in pre-stressed concrete members where high amount of transverse stress is often introduced at low compressive strengths. (C) 2019 Elsevier Ltd. All rights reserved.

This paper is a continuation of a previous study conducted at Kansas State University [8]. This paper demonstrates the influence of the thickness of concrete cover, compressive strength of concrete and the type of wire indentation on bond performance between steel and concrete in pre-stressed concrete ties using a consistent concrete mixture. A key objective of this research is to find the best parameters for pre-stressed concrete ties to prevent them from splitting/cracking in the field. This is very important for pre-stressed manufacturers, and especially for the railroad crosstie industry, so as to avoid failures in the field. The goal is to develop a qualification test with the capability to identify the compatible combinations of wire type and concrete mix before the ties are manufactured.A study took place at Kansas State University to understand and quantify the influence of variables such as the thickness of concrete cover, type of indents, and the compressive release strength on the bond behavior between steel and concrete. For the experimental testing three prisms with different cross sections were cast at the same time in series. Four pre-stressing wires were symmetrically embedded into each concrete prism and the spacing between wires was 2.0 inches. All prisms had the same length of 59.5in with square cross section. With the thickness of concrete cover of 3/4" the first prism had a 3.5x3.5in square cross section, the second prism had a 5/8%" thickness of concrete cover and 3.25x3.25in square cross section and the third prism had a 1/2" thickness of concrete cover and a 3.0x3.0in square cross section. All pre-stressing wires which were used in these tests had a 5.32mm diameter and were of different wire types. The indent pattern variations of the wire types included spiral, classical chevron shape, and the extreme case of smooth wire with no indentations. 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 4500 psi (31.03 MPa) and 6000 psi (41.37 MPa). For this study, a consistent concrete mixture with 0.32 water-cement ratio was used for all prisms, except for prisms casted with WE wire. For these prisms a water-cement ratio of 0.38 was used. Prisms had almost identical geometrical and mechanical properties as prestressed concrete ties which are manufactured in the railroad industry.Each prism provided a sample of eight different independent splitting tests of concrete cover (four wire cover tests on each end) for a given release strength. All cracks which appeared after de-tensioning were observed and measured to identify the cracking field, and all sides of the prisms on the live and dead end were marked for identification. For all prisms, longitudinal strain profiles on the live end and dead end were measured along with the values of transfer lengths. The strain profiles were taken using an automated Laser-Speckle Imaging (LSI) system. All results, representing quantitative and qualitative assessment of cracking behavior, are given in this paper as a function of thickness of concrete cover and release strength of concrete. For each sample prism, crack length and crack width were measured, and crack area was calculated as a simple function of crack length and crack width. In the case where spalling occurred, the crack width used was arbitrary set at 0.2in. These tests reveal the influence of thickness of concrete cover, the indented wire type and the release strength of concrete on the bond between steel and concrete. This work represents a successful first step in the development of a qualification test to ensure adequate splitting resistance in pre-tensioned concrete railroad ties.

It is well-known that the geometrical characteristics of the indents on prestressing wire used in the manufacture of prestressed concrete railroad ties affect the magnitude of the transfer length. In particular, it has been shown that such parameters as indent depth, indent volume and indent sidewall angle all affect transfer length, with indent volume being a major influence. Previous research has shown that the larger the indent volume, the shorter the transfer length.For full load bearing capacity, it is important that the transfer length not exceed the distance to the rail seat. Consequently, 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.Ongoing research, as well as previously published research results, also indicates that the geometry of the prestressing wire indents plays a major role in the formation of cracking. This is particularly important in the manufacture of concrete ties intended for high speed rail applications. Cracking and de bonding of prestressing wires associated with ties in service can result in severe splitting and complete tie failure. It is therefore not sufficient to guarantee a safe transfer length alone, without consideration of the cracking propensity. The wire specifications in standard ASTM A881 are intended to promote quality prestressed railroad tie behavior; however, the detailed causes of cracking and splitting, and the specific indent features that are responsible, are not well-known from a quantitative perspective.Until recently, inspection of prestressing wire indent properties consisted of sampling a few indents from a small segment of wire, providing very limited statistical information on wire indent properties. To address this deficiency, a high-resolution automated non-contact optical wire indent scanning system has been developed for completely and rapidly characterizing all relevant indent geometrical parameters. The system is capable of measuring large segments of wire to yield statistically significant samples of all relevant indent parameters including indent depth, indent width, indent sidewall angle, indent pitch, and indent volume. The current state-of-the-art in this system development, along with some new insights based on recent indent scanning results, will be presented. This system represents a valuable tool to aid in identifying the key indent geometrical features related to cracking. The overall goal is to quickly assess critical indent parameters, so as to ensure high-quality bond and eliminate in-track tie splitting failures.

This paper introduces a general methodology for determining the uncertainty of the solution to implicit systems of equations. Equation systems of this type arise from many practical applications, including the analysis of pipe networks, and in the implementation of complex numerical (finite difference or finite element) solution algorithms. The procedure is applicable to either linear or nonlinear equation systems, and does not require any specific algorithm for solution to the equation system itself. A general sensitivity matrix is constructed from an implicit sensitivity analysis of the equation system. This overall sensitivity matrix is expressed in terms of input and output sensitivity matrices, which represent the sensitivity of the equation system to changes in the independent (parameters) and dependent (calculated) variables, respectively. A vector representing the root-mean-square (RMS) uncertainty of the solution variables in the equation system is then given as a function of the given uncertainty in the input parameters. Two specific examples are presented to illustrate the practical application of the technique: (1) An example from fluid mechanics evaluating the uncertainty in solutions to a pipe network problem and (2) an example evaluating the uncertainty of a thermistor calibration and measurement problem.