Chih-Hang John Wu

Prestressing Steel Reinforcement Wire Bond Index Number

The purpose of this research project is to develop a mathematical model that predicts the bond strength of a prestressing steel reinforcement wire given the known geometrical features of the wire. The geometrical features of the reinforcement wire were measured by a precision non-contact profilometer. With this mathematical model, prestressing reinforcement wires can now be analyzed for their bond strength without destructive testing. This mathematical model has the potential to serve as a quality control assessment in reinforcement wire production. In addition this mathematical model will provide insight into which reinforcement wires provide the greatest bond strength and which combinations of geometrical features of the reinforcement wire are responsible for providing the bond strength.A precision non-contact profilometer has been developed to measure the important geometrical features of the reinforcement wire. The profilometer is capable of sub-micron resolution measurements to provide an extremely high quality three-dimensional rendering of the reinforcement wire surface profile. From this detailed profile data it is then possible to extract all of the relevant geometrical features of the reinforcement wire.A mathematical model has been created by testing a variety of different reinforcement wires available in the market. By correlating the transfer length of concrete prisms made with the reinforcement wires to various geometrical features, several different levels of mathematical correlation complexity have been investigated. The current empirical correlation models under development are first order and combine three to four unique geometrical features of the reinforcement wire which then act as predictors of the concrete prism transfer length.The resulting mathematical model relating the wire geometrical features to transfer length is referred to as the Bond Index Number (BIN). The BIN is shown to provide a numerical measure of the bond strength of prestressing steel reinforcement wire, without the need for performing destructive tests with the reinforcement wire.Copyright

Prestressing Steel Reinforcement Wire Measurement Protocol

The purpose of this paper is to propose new measurement guidelines for pre-stressing steel reinforcement wire indent geometries. The current guidelines for measuring pre-stressing steel reinforcement wire indent geometries are within ASTM A881M-10. These measurement guidelines provide instructions on measuring indent depth, indent side wall angle, and indent orientation. However, since the creation ASTM A881M-10 new measurements have been presented that serve as better predictors of wire performance than the current measurement requirements. The new measure guidelines presented in this research have been shown to have superior correlation to transfer length and the pull out force of pre-stressing steel reinforcement wires used in concrete railroad ties.These measurement guidelines are intended to more completely quantify the surface profile of pre-stressing steel reinforcement wires and do so in a manner that adheres to the geometric dimensioning and tolerancing standards of ASME Y14.5-2009. The measurement guidelines presented in this research use the concept of minimal zone on a variety of different measurements. This includes measurements such as indent volume, indent surface area, and indent edge wall surface area. These new measurements are shown through a variety of statistical models to be strong predictors of transfer length and pull out force for the given reinforcement wire.By presenting these new measurement procedures that define the surface geometry of reinforcement wire indent geometry in greater detail, suppliers can present more complete information of their wire type to consumers. Likewise, consumers will be able to more fully define the design requirements that are needed for their pre-stressed concrete railroad ties. The overall impact of the proposed changes will be the improvement of the quality control of pre-stressing steel reinforcement wires and the extended lifespan and durability of pre-stressed concrete railroad ties.Copyright

Automated real-time search and analysis algorithms for a non-contact 3D profiling system

The purpose of this research is to develop a new means of identifying and extracting geometrical feature statistics from a non-contact precision-measurement 3D profilometer. Autonomous algorithms have been developed to search through large-scale Cartesian point clouds to identify and extract geometrical features. These algorithms are developed with the intent of providing real-time production quality control of cold-rolled steel wires. The steel wires in question are prestressing steel reinforcement wires for concrete members. The geometry of the wire is critical in the performance of the overall concrete structure. For this research a custom 3D non-contact profilometry system has been developed that utilizes laser displacement sensors for submicron resolution surface profiling. Optimizations in the control and sensory system allow for data points to be collected at up to an approximate 400,000 points per second. In order to achieve geometrical feature extraction and tolerancing with this large volume of data, the algorithms employed are optimized for parsing large data quantities. The methods used provide a unique means of maintaining high resolution data of the surface profiles while keeping algorithm running times within practical bounds for industrial application. By a combination of regional sampling, iterative search, spatial filtering, frequency filtering, spatial clustering, and template matching a robust feature identification method has been developed. These algorithms provide an autonomous means of verifying tolerances in geometrical features. The key method of identifying the features is through a combination of downhill simplex and geometrical feature templates. By performing downhill simplex through several procedural programming layers of different search and filtering techniques, very specific geometrical features can be identified within the point cloud and analyzed for proper tolerancing. Being able to perform this quality control in real time provides significant opportunities in cost savings in both equipment protection and waste minimization.

Noncontact Inspection Method to Determine the Transfer Length in Pretensioned Concrete Railroad Ties

The traditional experimental method to determine the transfer length in pretensioned concrete members consists of measuring concrete surface strains before and after detensioning with a mechanical strain gauge. This method is prone to significant human error and inaccuracies. In addition, because it is a time-consuming and tedious process, transfer lengths are seldom if ever measured on a production basis for product quality assurance. A rapid noncontact method for determining transfer lengths in pretensioned concrete railroad ties has been developed. The new method uses laser-speckle patterns that are generated and digitally recorded at various points along the pretensioned concrete member. A prototype was fabricated as a portable self-contained unit for field testing, which incorporates a unique modular design concept that has several preferable features. These include flexible adjustment of the gauge length, easy upgradability to automatic operation, robustness, and higher accuracy. A laser-speckle strain sensor was applied to the transfer length measurements of typical pretensioned concrete railroad ties in a railroad tie plant. These prestressed concrete tie members are expected to withstand repeated axle loadings of 290 kN, totaling 250 million gross tons annually and occurring at speeds in excess of 110 km/h. The technique achieved a microstrain resolution comparable to what could be obtained using mechanical gauge technology. Surface strain distributions were measured on both ends of 12 ties, and their associated transfer lengths were subsequently extracted. The measurements of the transfer length using the laser-speckle strain sensor were unprecedented because it was the first time that the laser-speckle technique had been applied to pretensioned concrete inspection, and particularly for use in transfer length measurements of concrete railroad ties. It was also demonstrated that the technique was able to withstand the harsh manufacturing environment, making transfer length measurements possible on a production basis for the first time.

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

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.© 2013 ASME

Influence of Indented Wire Geometry and Concrete Parameters on the Transfer Length in Prestressed Concrete Crossties

A study was conducted to determine the variation in the transfer length of prestressed concrete railroad ties with different indented wire geometries and different concrete properties, including slump and release strength. The study included 12 different reinforcement wire types that are used in concrete railroad ties worldwide.This paper presents the results from transfer length measurements on 96 pretensioned concrete members that were cast in the laboratory. In order to replicate the wire-to-concrete proportions commonly used in prestressed concrete railroad ties, small (3 1/2″ (88.9 mm) × 3 1/2″ (88.9 mm)) prestressed concrete prisms were fabricated and each contained four 5.32-mm-diameter indented wires. A special jacking arrangement was used to ensure that each of the wires was tensioned to the same jacking force.The wires were initially tensioned to 7000 pounds (31.14 kN) each, and the transfer of prestress force into the members was accomplished by a gradual release method replicating the one used in most prestressed concrete crosstie manufacturing plants. The study consisted of two phases. In the first phase, 36 concrete prisms were cast to investigate the effect of different wire indent geometry in a 6-inch (152.4mm) slump concrete mix with 4500 psi (31.03 MPa) release strength.In the second phase, a total of 60 prisms were used to investigate the effect of 4 different concrete parameters with a select group of 5 indented wire types. The second phase included concrete release strengths of 3500 psi (24.13 MPa) and 6000 psi (41.37 MPa), and concrete consistencies (slumps) of 3 (76.2) and 9 inches (228.6 mm).The results have shown that there is a significant variation in transfer lengths for the different indented wires at the same release strength. Additionally, the results show that the transfer lengths decreased significantly with modest increases in the concrete release strength. However, there was no correlation observed between transfer lengths and different concrete slumps for mixes having the same water-to-cementitious (w/c) ratio. For each concrete pour, the splitting tensile strength and modulus of elasticity were measured at the time of prestress transfer. All wire indents were measured according to ASTM A-881 [1] and the results of both phases are presented.Copyright

Effect of Prestressing Wire Indentation Type on the Development Length and Flexural Capacity of Pretensioned Concrete Crossties

Load tests were conducted on pretensioned concrete prisms cast with 13 different 5.32-mm-diameter prestressing wire types that are used in the manufacture of pretensioned concrete railroad ties worldwide. The tests were specifically designed to evaluate the development length and bonding performance of these different reinforcements. The prestressing wires were denoted “WA” through “WM” and indentation types included smooth, spiral, chevron, diamond, and 2-dot and 4-dot. Four wires were embedded into each concrete prism, which had a 3.5″ (88.9 mm) × 3.5″ (88.9 mm) square cross section. The wires were initially tensioned to 7000 pounds (31.14 KN) 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 tested in 3-point-bending at different spans to obtain estimations of the development length of each type of reinforcement. Two identical 69-in.-long (175.26 cm) prisms were load tested, at both ends, for each reinforcement type evaluated. First prisms were tested at 20-in. (50.8 cm) from one end and 13-in. (33.02 cm) from the other end, whereas the second prisms were loaded at 16.5-in. (41.9 cm) from one end and 9.5-in. (24.13 cm) from the other end. Thus, a total of 52 load tests (13 wire types × 4 tests each) were conducted in this study.During each test, a concentrate load with the rate of 300 lb/min (1334 N/min) was applied at mid-span until failure occurred, and values of load, mid-span deflection, and wire end-slip were continuously monitored and recorded. Plots of load-vs-deflection were then compared for prisms with each wire type and span, and the maximum sustained moment was also calculated for each test. The load tests revealed that there is a very large difference in the development length of the different wire types currently used in the manufacture of pretensioned concrete railroad ties. The results imply that there would also likely be large differences in the reserve capacity (beyond first cracking) for pretensioned concrete crossties fabricated with these different reinforcements.Copyright

Modeling the Behavior of Pre-Stressed Concrete Railroad Ties

Early failure of pre-stressed concrete railroad ties in the field is a costly occurrence with modern ties. A key predictor of the performance of a pre-stressed concrete cross tie is the transfer length. Assuring that the transfer length is less than the position of the rail seat is necessary to establish the full pre-stressing force at the load point of the concrete tie.Models have been developed based upon empirical data to predict the transfer length of concrete members given key design parameters. Given the release strength and design geometry of the reinforcement steel, accurate predictions can be made as to what the anticipated transfer length will be. The geometry of the indented profile in pre-stressing steel has been found to be critical for minimizing the fracture propensity of the concrete member and reducing the overall transfer length. Edge wall angles of the reinforcement wire indents have been shown within this study to have a critical influence on the fracture propensity of the concrete medium. Steel produced with too shallow or too steep indent edge wall angles generate excessive internal forces rupturing the concrete.By modeling the behavior of the transfer length in concrete members, the design and production tolerances can be better controlled increasing the life expectancy of concrete ties. This results in decreased costs for the rail infrastructure and greater uptime of tracks utilizing pre-stressed concrete railroad ties. By improving the overall design of concrete members and by improving the quality control tests used during production a longer lasting and lower cost product may be achieved.Copyright

Effect of Concrete Properties on Transfer Lengths in Concrete Rail-Road Ties

This paper presents findings from a current research project titled “Quantifying the Effect of Prestressing Steel and Concrete Variables on the Transfer Length in Pretensioned Concrete Crossties” that is funded by the Federal Railroad Administration (FRA). Specifically, the paper focuses on the effect of concrete properties on the resulting transfer lengths. These properties include concrete consistency (slump), compressive strength at the time of prestress transfer, the water-to-cementitious (W/C) ratio, the aggregate type, and the use of a viscosity-modifying admixture (VMA). Pre-tensioned concrete prisms were cast in the laboratory and transfer lengths were determined from surface strain measurements that were obtained prior-to and immediately after prestress transfer (de-tensioning). The concrete compressive strength at de-tensioning was determined using cylindrical concrete test specimens that were match-cured to the temperature of the pre-tensioned concrete members. The release strengths investigated were 3500 psi (24.13 Mpa), 4500 psi (31.03 Mpa), and 6000 psi (41.37 Mpa). The effect of concrete consistency on the transfer length was evaluated by varying the slump between 3″ (76.20 mm) and 9″ (228.60 mm) while maintaining release strength of 4500 psi (31.03 Mpa) and a W/C ratio of 0.32. The effect of W/C ratio on transfer length was evaluated by maintaining release strength of 4500 ±220 psi (31.03 ± 1.52 Mpa) and a slump of 6 ± 1/2″ (152.40 ± 12.7 mm) while varying the W/C between 0.27 and 0.42. These values represent the extreme values used in the North American concrete tie industry that were noted by the authors during research that was conducted in 2010–2011[15]. Results for each parameter type will be compared and discussed in this paper. Transfer length results obtained during earlier work [4] conducted by the authors at a W/C ratio of 0.32 will be compared. Finally, results will be presented from transfer length measurements that were obtained on identical-sized prisms that were manufactured with concrete mixtures that used different aggregate sources and also the use of a viscosity-modifying admixture.Work presented in this paper was conducted on two wire samples with generic labels [WG] and [WH]. Laboratory prism specimens of size 3 ½″ (88.9 mm) × 3 ½″ (88.9 mm) each with 4 wires were cast to measure transfer lengths. These proportions were chosen to replicate the original concrete crosstie wire-to-concrete proportions. Potential usage of these prisms in estimating transfer lengths was validated in another phase [5] and is not discussed here.Essential information obtained from results allowed researchers to discuss attributed influence of each concrete property on transfer length. The research knowledge acquired from this study will give proper insight about transfer length and will be helpful to manufacture a better product by adjusting concrete mix design.Copyright

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

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.Copyright