Naga Narendra B. Bodapati

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

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

Development of a Standard Bond Test for Indented Prestressing Wires

An experimental testing program was conducted at Kansas State University (KSU) to test the bond characteristics of various 5.32 mm-diameter, Grade 270 low-relaxation steel wires used in prestressed concrete railroad ties. This un-tensioned pullout test could serve as a quality control test similar to the NASP (North American Strand Producers) Strand Bond Test that has been developed for pre-tensioned strands. A total of twelve (12) wires produced by six different steel manufacturers were used to develop the wire pullout test. All of the wires were tested in their “as-received” condition and have different indent geometries. It is generally accepted that indentations in the wire improve the bond between the steel and concrete. However, there are currently no commonly accepted quality control tests that accurately predict a wire’s bond characteristics in a pre-tensioned application.The un-tensioned pullout test developed is comparable to the NASP [Strand] Bond Test. The specimens consist of a 4 in. (100 mm) outer-diameter tube with a total length of 8 in. (200 mm) and a steel plate welded to the bottom. The 5.32 mm-diameter wire was centered in the tube and the sand-cement mortar was placed and allowed to cure. The flow of the mortar was measured for consistency and 2” × 2” (50 mm × 50 mm) mortar cubes were used to determine the compressive strength of the mortar. The specimens were tested when the compressive strength of the mortar was between 4500 and 5000 psi (31.0 MPa and 34.5 MPa). Each batch of mortar contained 12 pullout specimens; one with each wire type. Each wire was tested six times leading to a total of six batches and a total of 72 mortar specimens.During testing, the wires were loaded in force control at the bottom, while continuously monitoring and recording the movement (slip) of the wire with respect to the mortar at the opposite (top) end. The force verses end-slip data of the six tests for each wire type were numerically combined to obtain the average bond performance. These average results from the un-tensioned pullout tests were then compared to transfer length measurements from accompanying pre-tensioned concrete prisms. In general, the wire end slip measurements from the pullout tests were found to have good correlation with the measured transfer length. For all 12 wires, a coefficient of determination (R2) of 0.872 was found between the average pullout force (at 0.10-inch (2.54 mm) of wire free-end slip) and average transfer length measurements from the accompanying concrete prism tests. However, when only the indented wires were considered, the R2 increased to 0.913.© 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

Effect of Concrete Release Strength on the Development Length and Flexural Capacity of Members Made With Different Prestressing Strands

Load tests were conducted on pretensioned members made with five different strands (three 7-wire strands and two 3-wire strands) to determine the effect of concrete release strength on the development length and flexural capacity of members. Strands named generically SA, SC, SD, SE and SF and they were all indented except SA (no surface indentation). All strands had diameter of 3/8″ (9.52 mm) except SC which had diameter of 5/16″ (7.94 mm). Among all types of strands used in manufacturing of test prisms, SC and SF were 3-wire strands, while SA, SD and SE were 7-wire strands. A consistent concrete mixture was used for the manufacture of all test specimens, and the different release strengths were obtained by allowing the specimens to cure for different amounts of time prior to de-tensioning. For SA, SD, SE and SF strands, each prismatic specimen (prism) had a 5.5″ (139.7 mm) × 5.5″ (139.7 mm) square cross section with four strands arranged symmetrically. However, prisms made with SC strand had 4.5″ (114.3 mm) × 4.5″ (114.3 mm) square cross section with four strands arranged symmetrically. The prisms were identical except for the strand type and the compressive strength at the time of de-tensioning. All four strands were pulled and de-tensioned gradually when the concrete compressive strength reached 3500 (24.13 MPa), 4500 (31.03 MPa) and 6000 (41.37 MPa) psi. Precise de-tensioning strengths were ensured by testing 4-in.-diameter (101.6 mm) × 8-in.-long (203.2 mm) compression strength cylinders that were temperature match-cured.The prisms were loaded in 3-point-bending to determine the ultimate bond characteristics of each reinforcement type for the different concrete release strengths. A loading rate of 900 lb/min (4003 N/min) for 5.5″ (139.7 mm) × 5.5″ (139.7 mm) prisms was applied at mid-span and the maximum sustained moment was calculated for each. Same procedure with loading rate of 500 lb/min (2224 N/min) was applied to 4.5″ (114.3 mm) × 4.5″ (114.3 mm) prisms. Three 69-in.-long (175.26 cm) prisms, each having different concrete release strength, were tested with each of the 5 strand types. Two out of three testing prisms were tested at only one end and one was tested at its both ends. Thus, for each strand type and concrete release strength evaluated, a total of 4 tests were conducted for a total of 60 tests (5 strand types × 3 release strengths × 4 tested embedment lengths). Test results indicate that the concrete compressive strength at de-tensioning can have a direct impact on the ultimate flexural capacity of the members, and this has significant design implications for pretensioned concrete railroad ties. Results are discussed and recommendations made.Copyright

Bond Index Numbers of Prestressed Concrete Reinforcement Wires and Their Relationships to Transfer Lengths and Pull-Out Forces

The purpose of this research is to establish mathematical models that predicts the bond strength of a reinforcement wire in prestressed concrete members, given the known geometrical features of the wire. A total of nineteen geometrical features of the reinforcement wire were measured and extracted by a precision non-contact profilometer. With these mathematical models, prestressing reinforcement wires can now be analyzed for their bond strength without destructive testing. These mathematical models, based upon a large collection of empirical data via prestressing reinforcement wires from various wire manufacturers in US and Europe, have the potential to serve as quality assessment tools in reinforcement wire and prestressed concrete member production. Most of these models are very simple and easy to implement in practice, which could 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.Our various empirical models have shown that the indent side-wall angle, which is suggested by the ASTM-A881/A881M, may not be the only significant geometrical feature correlated to the transfer length and bond strengths. On the contrary, features such as the indent surface area, indent width, indent edge surface area, indent volume, and release strengths do have significant correlations with the ultimate transfer lengths of the prestressed concrete members. Extensive experiments and testing performed at the Structures Laboratory in Kansas State University, as well as field tests at Transportation Technology Center, Inc. (TTCI) and one Prestressed Concrete Railroad Tie manufacturing facility, have been used to confirm the model predictions.In addition, our experimental results suggest that the maximum pull out force in the un-tensioned pullout testing has significant correlation with the ultimate transfer length. This finding could provide reinforcement wire manufactures with a quality assurance tool for testing their wires prior to the production.The resultant 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. We believe that with the BIN and the maximal pull-out forces from the un-tensioned pull-out tests, one can have better insight into the optimal reinforcement wire design by testing the performance of wires before they are put into production lines.Copyright

Comparison of Transfer Lengths in Pretensioned Concrete Railroad Ties Subjected to Different Magnitudes of Rail Loads

In order to quantify the effect of different reinforcement types on transfer lengths, an extensive study was conducted with the selected group of twelve different reinforcement types. These reinforcements are extensively used to produce concrete railroad ties across the world. These employed twelve (12) different types are of 5.32 mm diameter wires with different surface indent geometries. A research team from Kansas state university visited a PCI certified concrete tie manufacturing plant during January 2013. During the plant visit, four (4) concrete railroad ties were cast for each reinforcement type for a total of 48 ties. Considerable part of the study conducted at the plant was previously published by the authors. However for effective understanding, brief explanation of the tie manufacturing process will be presented in this paper. Strain measuring points were mounted on the bottom surface of a concrete railroad tie during the casting process. Proper measures were taken to safeguard these strain measuring points during loading. Transfer lengths were calculated using these mounted strain measuring points. Transfer length measurements were calculated at the plant, immediately after the application of prestressing forces to the concrete ties. After the casting process, two ties for each reinforcement type were stored at plant location for approximately one year and the remaining two ties (companion ties) for the each reinforcement types were shipped and stored at Kansas state university. Transfer length measurements were again calculated at this stage for all 48 ties. Ties stored at plant location were later subjected to cumulative in-track railroad loading of 85 million gross tons over six (6) months period of time. Whereas, the companion ties stored at Kansas state university were not subjected to any loading. Transfer lengths are calculated and compared at this stage and presented [4] in the past.Ties which were already subjected to 85 million gross tons were further loaded to cumulative total of 236.3 million gross tons and the companion ties stored at Kansas State University were not subjected to any loading. Transfer lengths for the ties (twenty four) that were subjected 263.3 million gross tons were calculated and presented in this paper with detailed explanation. Transfer length behavior under different magnitudes of loading is also presented along with the discussion.Copyright