Post-tensioning frequently solves design and construction challenges that other construction methods simply cannot. It is achieved by casting concrete into pre-assembled forms in combination with rebar and steel cable (strand) reinforcement. These cables are tensioned (stretched) to approximately three-quarters of their ultimate strength. Once the concrete cures to its required strength, the tensioning is released. The steel cables reacting to the release, transfer the tensile stresses into the concrete, rendering an even stronger structural component. When it comes to posting tensioning, there is often a dilemma on whether to go for a bonded or unbonded post-tensioning system.
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In post-tensioning, there are two different techniques; bonded and unbonded. Due to these techniques – used in tendons/strands – more flexible and fast construction is possible.
Bonded post-tensioning system
Unbonded Post-Tensioning typically consists of single (mono) strands or threaded bars that remain unbonded to the surrounding concrete giving them the freedom to move locally relative to the structural member. The strands in unbonded mono strand systems are coated with specially formulated grease, with an outer layer of seamless plastic extruded in one continuous operation to protect against corrosion. It is e typically used in new construction for elevated slabs, slabs-on-grade, beams and transfer girders, joists, shear walls and mat foundations. Light and flexible, unbonded mono strand can be easily and rapidly installed – providing an economical solution.
Bonded Post-Tensioning comprises tendons from one to multiple strands (multistrand) or bars. For bonded systems, the prestressing steel is encased in a corrugated metal or plastic duct. After the tendon is stressed, cementitious grout is injected into the duct to bond it to the surrounding concrete. Besides, the grout creates an alkaline environment which provides corrosion protection for the prestressing steel. Bonded multi-strand systems, while used extensively in new construction of bridges and transportation structures, can be and have been successfully applied to commercial building structures. When these multi-strand systems are used for large structural elements such as beams and transfer girders, design advantages include increased span lengths and load carrying capacity and reduced deflection.
Bonded vs Unbonded post-tensioning
The PT reinforcement requirement for the bonded system is comparatively more than the unbonded system. This can be attributed to the losses in friction. The friction coefficient for bonded tendons is more than unbonded tendons, resulting in the loss of effective stress in the tendons which ultimately results in the loss of effective prestress force in the section. Hence the number of tendons required for a bonded PT system as compared to an unbonded PT system is more for the same prestress force.
Unbonded post-tensioning system
The Non-PT reinforcement requirement for bonded PT systems than unbonded PT systems comes out to be more, comparatively. But this is attributed to the fact that for the bonded system the minimum amount of Non-PT reinforcement as stipulated by code is 0.12% of the section. Therefore, the bars considered are through and no curtailment is done. But for an unbonded PT system the Non-PT reinforcement, as given by the software, is a curtailed one, wherein the bars are either top or bottom reinforcement. A comparison between the two is given below.
Hybrid Model with both Unbonded and Bonded Post-Tensioning System
Bonded and unbonded systems can be mixed within a structure. The unbonded post-tensioned systems can be used in typical levels, while the bonded post-tensioning systems can be specified for the transfer girders on different levels to provide optimum crack and deflection control features essential for transfer girders required to carry the loads from the multistory structure.
Conclusion
It is estimated that post-tensioning hollow unit walls that carry small gravity loads will be the most economical and popular method of application. Presumably, a bonded system will give higher strength and certainly a more ductile system than that of an unbonded, laterally restrained system. But with the evolution of technology and the growing complexity of project structures hybrid model too is gaining momentum.
Bonded Prestressed Strand(PC Strand) for Post-Tensioning System
BRIEF INTRODUCTION
Bonded PC Strand is mainly used for the reinforcement of prestressed concrete structures, such as large-span railway and road bridges, crane beams,anchorages and multi-storey industrial buildings etc. We offer PC Strand which confirm to the technical standards such as GB/T , GB/T , ASTM A-416, BS , JIS G or the standards agreed by both of the customers and us.
SPECIFICATIONS
Low Relaxation PC Strand
Tensile Strength: MPa
Dia: 9.3,9.5,12.7,15.2,15.7,17.8,21.8mm
Prestressed Concrete Steel Strand
Descriptions:prestressed concrete steel wire strand
Structure: 1x7 wires
Raw Material: SWRH 77B, 82B steel wire rod
Standards: ASTM A416, BS, GB/T or equivalent
Categories: round plain, spiral ribs, indented, galvanized, PE coated.
Grade: 270K, Tensile Strength: MPa
We offer PC Strand which confirm to the technical standards such as GB/T , GB/T , ASTM A-416, BS , JIS G or the standards agreed by both of the customers and us.
PC strand can also be supplied in various forms for pre- or post-tensioning applications, with plain or galvanized material ranging from 3 wires, 7 wires, 19 wire and compacted construction.
FEATURES
Bonded PC Strand is High tensile strength, low relaxation, stable modulus of elasticity, stress-relieved, firm connection with concrete, low stress, stable construction, good combination with steel reinforced concrete; saving material, reducing distortion and construction weight, increasing the abrasion resistance, water resistance, stiffness.
BS /3- Standard
Type
Nominal Diameter
Tensile Strength
Unit Weight
Min yield strength kN
Min breaking strength kN
Min Elongation Lo>600m
h Value No more than
mm
Mpa
g/m
%
Standard 7 wire
15.2
197
232
3.5 (l≥500mm)
≤2..5%
15.2
221
260
Super 7 wire
12.9
785
158
186
15.7
225
265
15.7
237
279
Compact 7 wire
12.7
890
178
209
15.2
255
300
18
323
380
ASTM A416
The company is the world’s best PC Strand Bonded supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.
Standard
Grade
Nominal Diameter
Tolerance
Nominal section area
Mass per m (kg/)
Min breaking load
Min Elongation Lo>610m
h Value No more than
(In mm
(mm)
(mm2)
%
%
250KSI [Mpa]
0.375 9.53
±0.40
51.61
405
89
3.5
2..5
0.438 11.11
69.68
548
120.1
0.500 12.7
92.9
730
160.1
0.600 15.24
139.35
240.2
270KSI [Mpa]
0.375 9.53
±0.65 ±0.15
54.84
432
102.3
0.438 11.11
74.19
582
137.9
0.500 12.7
98.71
775
183.7
0.600 15.24
140
260.7
APPLICATIONS
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Bonded PC Strand
are mainly used for prestressed steel structure, a variety of rail and road bridges, factory plant, high-rise building, prestressed construction and a variety of building foundation columns and the transformation of reinforcement, the city's overpass, Shell safety of nuclear power stations, television tower, water towers, concrete silo, sewage treatment, anchoring the rock, water conservancy and hydropower project, the standard pre-stressed concrete structures, poles, water pipes, crane beam, the level of large plates, hollow Floor, and so on.
PACKAGE AND SHIPPING
Bonded PC Strand
The full-scale BH girder bridge prototype constructed for the test had a length of 35 m and a deck depth of 2.5 m ( Figure 4 ). The anchorages were disposed at the top and bottom of the cross-section and near the connections between the segments. To observe the overall state of the prestressing force in the bridge, one OSPC strand was installed at each anchorage. Considering the profile of the tendon involving straight and curved parts, each sensor was set to have three to seven sensing points. Figure 4 depicts the shape and dimensions of the BH girder bridge applying the OSPC strand and indicates the locations of the anchors (A, B, C). Figure 5 shows the position of the OSPC strand in the cross-section for each of the anchors, A, B, and C, of Figure 4 , as well as the numbering of the tendons. Table 1 provides the details of the tendons one, two, three, and four, like the profile and length, the number of sensing points, and the prestressing method.
The so-prestressed BH girder bridge prototype was finally completed by injecting grout and placing the top concrete. The performance of the completed full-scale BH girder bridge was verified via static test through four-point loading and dynamic test using an actuator. Figure 9 presents the placing of top concrete and the completed bridge in place for testing.
In addition, the anchor set length that is the distance over which the difference in the prestressing force due to the slip of the wedge at the removal of the hydraulic jack (step-7) and the maximum tensioning force (step-6) had an effect that was seen to be longer than 20 m in Figure 8 . This length was significantly longer than the value of 13.9 m obtained by calculation for the anchor set length. This calculated value is usually adopted in studies related to the anchor set length because of the difficulty to acquire actual data all along the tendon in the PSC structure. Note that KICT is currently developing an OSPC strand with distributed sensing to improve the present OSPC strand with a finite number of sensing points. The future application of this new OSPC strand to PSC structures will make it possible to compute the anchor set length more accurately.
Besides, the loss of prestressing occurring during the removal of the hydraulic jack after the tensioning process could also be measured. This loss is called the immediate loss and is arranged per anchor in Table 2 . Assuming a loss of 5 mm due to the slip of the wedge in the anchor, the immediate loss that may occur in the anchors runs between 2.3% and 3.9%. However, the values of the immediate loss in Table 2 fell slightly beyond this range. This greater immediate loss can be explained by some imprecision in the relative arrangement of the wedge and strands and in the fixation of the wedge of the hydraulic jack. Nevertheless, these higher values for the loss are acceptable considering that the loss of the prestress force generally ranged between 15% and 30% by accounting for the immediate loss as well as the loss according to time.
The prestressing of the BH girder bridge was executed by tensioning the tendons sequentially with respect to their number, as given in Figure 5 and Table 1 . The process was performed in six steps by introducing prestress in tendons one to three of up to kN and, then, tensioning tendon four up to kN. The calculation provided an average prestressing force of 177.8 kN in the strands of tendons one to three and of 175.0 kN in the strands of tendon four. The prestressing force measured by the OSPC strand installed in each tendon was 173.1 kN for tendon one, 164.0 kN for tendon two, 165.7 kN for tendon three, and 168.5 kN for tendon four. These values were slightly lower than the target prestressing force and about 92.3% to 97.4% to the target values. However, if prestressing is conducted simultaneously using multiple hydraulic jacks for several strands, the resulting prestressing force for each individual strand would naturally exhibit a slight difference. This situation was reported in by Chandoga and Jaroševič [ 15 ] through actual measurements. Moreover, Cho et al. [ 16 ] analyzed the distribution of the prestressing force in the strands of the tendon and revealed that this distribution was normal with a variation of about ±4% around the mean value. Accordingly, even if the prestressing force measured by the OSPC strand is slightly lower than the target value, the measurement can be assumed to be correct.
2.3. Test and Measurement of Full-Scale BH Girder Bridge
For the measurement of the dynamic properties, accelerometers and displacement sensors were disposed vertically at the center on the bottom of each segment. As presented in Table 3 , the values measured by the FBG sensor at mid-length of the OSPC strand in tendon one were used. Consequently, the accelerometers measured the vertical vibration, and the OSPC strand measured the transversal vibration. Figure 10 illustrates an installed accelerometer and the actuators used to load the bridge.
The forced vibration test using the actuators applied loading with gradual increase of the exciting frequency from 1 Hz to 5 Hz, and the corresponding responses of the bridge were observed. The observation revealed that the largest amplitude of vibration occurred around 4 Hz. As shown in Figure 11 , resonance happened between 4.2 Hz and 4.3 Hz in both data from the accelerometers and FBG sensor.
In order to analyze the frequency contents of the vibration test data from the accelerometers and FBG sensor, the Welch method [ 17 ] was adopted to estimate the averaged periodograms of the overlapping segments in the signals, and these averaged periodograms were used to perform the analysis of the two different data in the frequency domain. Since the resonant frequency occurred below 30 Hz for the bridge, the sampling frequency was set to 100 Hz to secure a sufficient margin. The frequency band computed by the Welch method fell within the Nyquist frequency between 0 and 50 Hz. The Hanning function was used as the window function needed to conduct the segmental averaging of Welch and minimize the leakage effect. The window function involved a total of data, and the segments were generated by moving the window according to the time sequence. Fast Fourier transform (FFT) was executed for each segment to compute the power in the frequency domain, which was averaged to provide the final value. Figure 12 plots the natural frequencies of the accelerometer data and FBG sensor data obtained by this method. The computed natural frequency of the first mode appears to be identical for both types of data. This result indicates that the FBG sensor embedded in the OSPC strand that was arranged transversally in the bridge could accurately measure the acceleration of the structure.
The dynamic analysis of the bridge structure was performed. To that goal, a three-dimensional model was established and reflected the track structure to achieve precise analysis. Two-node beam elements were used for the girder and cross beams, and four-node shell elements were adopted for the deck. The natural frequency analysis was executed using an in-house program developed at KICT in . Figure 13 depicts the analytic model and the first-mode shape of the structure [ 13 ].
From the analysis, a natural frequency of 4. Hz was computed for the first mode, which is slightly smaller by about 0. Hz compared to the frequency of 4. Hz obtained from the test. This small difference can be credited to the boundary conditions, which assumed simple supports (hinge and roller) in the analysis, whereas the prototype was supported actually by two rollers at its ends. Moreover, some differences in the material properties adopted in the analysis might also possibly have caused this discrepancy.
The data above are those obtained by vibrating the structure sufficiently using the actuator and allowed clear identification of the natural frequency. In general, data related to the longitudinal direction along the girder in bridge structures present poor sensitivity compared to the vertical acceleration response of the bridge, which made them improper for identifying the natural frequency. However, Lee et al. [ 18 ] determined the impact factor of an actual in-service railway bridge structure based upon data measured by electrical resistance gages installed longitudinally in the girder near a FBG sensor during the crossing of train convoys. Moreover, prior to the test presented in this study, the OSPC strand was already installed inside the girder of a road bridge with a span length of 60 m and in operation. Figure 14 shows the layout and a photograph of this bridge together with its natural frequencies extracted from dynamic acceleration data acquired during the crossing of a truck.
Several months after the completion of the bridge shown in Figure 14 , the wavelength variation in the structure was measured by the OSPC strand under the crossing of a truck. Figure 15 plots the dynamic data obtained during the measurement and the identified natural frequency.
The wavelength variation plotted in Figure 15 a was obtained by removing the Direct Current (DC) content from the raw data and applying a high pass filter. The natural frequency in Figure 15 b was computed by FFT. The natural frequency obtained from the OSPC data differed slightly from that of the acceleration response in Figure 14 c, but this difference was probably due to the effects of the time elapsed between the two measurements like temperature and change in the state of the bridge in operation.
In view of such results, installing semi-permanently the OSPC strand in the railway BH girder bridge appears to offer a fair and sufficient solution enabling to observe and evaluate the change in the prestressing force as well as the natural frequencies of the structure without other accelerometers nor electrical resistance gages. These features will allow effective maintenance since it is possible to detect any abnormal change of the structure.
Furthermore, the flexural performance of the BH girder bridge was evaluated through the four-point bending test, as shown in Figure 16
Figure 17 shows the comparison of the strain measured during the four-point bending test by the optical sensor located at the center of tendon one among the OSPC strands and by the electrical resistance strain gage bonded on the closest reinforcement in the cross-section. Linearly varying strains were measured by both types of sensor at early loading until the plastic zone. Beyond a value of about με, the strain provided by the electric resistance gage showed a steep increase, which indicated yielding of the steel reinforcement arranged at the bottom of the girder bridge. On the other hand, the OSPC strand continued to behave normally and measure the strain even after the yield of the reinforcement and until the final loading stage at approximately kN. A deflection of about 350 mm was finally observed at mid-span.Figure 18 shows a plot of the change of the prestressing force measured by the OSPC strand in each tendon of the BH girder bridge at each stage from the start of tensioning during the fabrication of the specimen to the end of the bending test. The prestressing force was seen to undergo change since early prestress, and the elastic shortening could be clearly distinguished by the OSPC strand in the next prestressing stage. The partial increase of the prestressing force caused by the increase of the load following the placing of the top deck could also be observed. The large changes in the prestressing force due to the four-point bending are also clearly indicated in the graphs. Consequently, the OSPC strand could reliably measure the change of the prestressing force at major locations of the structure from construction to failure.
Considering its material characteristics, the optical sensor presents the disadvantage of being difficult to install without specific protection in civil structures for which steel or concrete are usually as construction materials. However, the embedment of the optical sensor in the strand endows it with semi-permanent durability in such an environment and will allow accurately measuring the change in the prestressing force all along the lifetime of the structure. For civil structures, such a possibility enables the OSPC strand to provide reliable and accurate data compared to the conventional electrical resistance gage and will allow more efficient maintenance of the structure.
PC Strand, the prestressed concrete steel strand, is a twisted steel cable composed of 2, 3, 7 or 19 high strength steel wires and is stress-relieved (stabilized) for prestressed concrete or similar purposes.
According to the number of steel wire in a strand: 2 wire strand, 3 wire strand, 7 wire steel strand and 19 wire steel strand.
According to the surface morphology can be divided into smooth steel strand, scoring strand, mold pulling strand (compact), coated epoxy resin steel strand.
They can also be classified by diameter, or intensity level, or standard.
In the description and list of the table we often see, there are 15-7Φ5, 12-7Φ5, 9-7Φ5 and other specifications of the prestressed steel strand.
To 15-7Φ5, for example, 5 said a single diameter 5.0mm of steel, 7Φ5 said seven of the steel wire to form a strand, and 15 that the diameter of each strand of 15mm, the total meaning is "one The beam consists of 7 strands of diameter 15 mm (each having a total diameter of about 15.24 mm, a dimensional deviation +0.40 -0.20; a diameter of about 5.0 mm per filament).
The general sectional area is calculated according to 140mm ^ 2. The theoretical breaking value is 140 * = 260.4kN, which can withstand the tension of 156.24-169.26kN according to the prestressing standard of 60% -65%.
Using high-carbon steel wire rod, after surface treatment of cold drawn into steel wire, and then by the strand structure will be a number of steel wire stranded into shares, and then through the elimination of stress from the stabilization process.
In order to extend the durability, the wire can have metal or non-metallic coating or coating, such as galvanized, epoxy resin coating. In order to increase the bond strength of the concrete, the surface can have nicks and so on.
The prestressed strands of the mold are twisted to form a mold compression process, the structure is more compact, and the surface layer is more suitable for the anchor.
Production of unbonded prestressed steel strand (unbonded steel strand) using ordinary prestressed steel wire, coated with oil or paraffin after the package of high-density polyethylene (HDPE) into.
The main characteristic of the prestressed steel strand is high strength and relaxation performance is good, the other when the more straight. Common tensile strength level of MPa, as well as ,,,, MPa and the like intensity level. The yield strength of this steel is also higher.
Founded in , Tianjin Chunpeng Prestressed Concrete Strand Co., Ltd is located in Tianjin.Our company is dedicated to providing you with low relaxation prestressing strand, unbonded PC strand, PC wire and matched corrugated pipe And anchorage products.
For more Prestressed Steel Strandinformation, please contact us. We will provide professional answers.