TECHNICAL FIELD

[0001] The present invention relates to a novel beam construction for structural works, and particularly to a system for constructing prestressed concrete beams using Basalt Fibre Reinforced Polymer (BFRP) bar.

BACKGROUND

[0002] Prestressed concrete (PC or PSC) is increasingly used for the construction of large scale infrastructure projects required for the development of the society. PSC can be classified as pre-tensioned or post-tensioned based on the methodology adopted to prestress the concrete members. In post-tensioning, the strands are stretched after concrete has gained sufficient strength with the usage of portable hydraulic jacks. Whereas, in the case of the pre-tensioning method, the strands are stretched before the casting of the concrete and released after the concrete has gained enough strength. Pre-tensioning eliminates the usage of additional transfer equipment’s and pre-tensioning is more efficient and economical than post-tensioning.

[0003] The pre-tensioning process consists of tensioning the high-tensile steel tendons (either single wire or seven-wire strands) in the prestressing bed before the casting of concrete elements. The tensile forces in the strands are transferred as equivalent compression to the concrete elements, upon release of strands after hardening of concrete. Thus, concrete is brought into a state of nearly permanent compression. This known prior art of transferring prestress using end abutments is known as Long-Line method or Hoyer’s method. [0004] Due to the pre-compression, while loading, tensile stresses are eliminated, or a very small amount of tensile stresses are generated at the lower surface of the PSC beam, and accordingly, no cracks are formed in a PSC member. Even if the tensile stresses are generated on the lower surface of the beam, no cracks are formed if the tensile stresses does not exceed the flexural tensile strength of concrete. Therefore, such a PSC beam is widely used in civil engineering applications. For example, a bridge of short and medium spans is commonly constructed with a PSC beam, long-span bridges which are constructed with steel-reinforced concrete can be constructed with the PSC beam, and in terms of buildings, PSC is typically used in beams and slabs.

[0005] The Long-Line or the Hoyer’s method is typically used to cast, and pre-tension PSC beams have several limitations. Firstly, a large span is required to caste several beams at the same time. Such large span requirement leads to the Hoyer’s method feasible only for mass production and makes it uneconomical for small scale projects. Secondly, due to the requirement of the end blocks (also known as bulkheads or abutments), the Hoyer’s method is difficult to be replicated at the site. Furthermore, the conventional PSC beam with prestressing steel element has certain limitations, particularly the durability of the PSC beam due to the corrosion of the steel prestressing strand.

[0006] The conventional construction method of the PSC beam has disadvantages of development of tensile cracks in concrete beam due to the loss in prestressing force caused by corrosion in the steel strand. The concern for durability and prolonged service life of PSC structures is rapidly gaining its importance with the American Concrete Institute (ACI) releasing guidelines in the year 2004 to replace steel strands with composite bars in PSC members.

[0007] Fibre-reinforced polymer (FRP) composite bars are manufactured from artificial polymer materials such as basalt, glass, aramid and carbon and can be adopted for prestressing of concrete members. Due to their non-metallic and non-corrosive nature, the artificial polymer materials are potential alternatives to reduce the corrosion of prestressing elements in PSC members. However, due to the complex nature of FRP material, its behaviour is categorically different from steel prestressing strands and involves more complicated modes of response.

[0008] FRP bars have different surface and mechanical characteristics than steel strands, and therefore, the same system utilised for stressing steel strands cannot be utilised for stressing FRP bars. Thus, there exists a disparity and incompleteness of a generic methodology to fabricate PSC beams with FRP bars that stem from the lack of one unifying system for casting and prestressing PSC beams with FRP as the prestressing element. Therefore, even after the successful demonstration of FRP in a few field applications, the knowledge of FRP bars as a prestressing element is very primitive and its application in PSC members remains in a developmental stage.

[0009] BFRP bar is a newly developed composite bar manufactured using organic basalt continuous fibre roving’s and has high resistance to UV exposure, low thermal and electrical conductivity and high resistance to chemical attack. BFRP has a great potential as a composite material in areas having an abundant supply of basalt rocks. However, due to the lack of research available on BFRP prestressed members, BFRP has been excluded from the ACI report, which proposes guidelines for prestressing concrete structures using composite bars.

[0010] The present invention aims to fabricate prestressed concrete (PSC) members pre-tensioned using BFRP bars.

OBJECT OF THE INVENTION

[0011] The primary object of the present disclosure is to provide a system for constructing pre-tensioned prestressed concrete beams using BFRP bars as a prestressing element in concrete. [0012] Another object of the present disclosure is to provide a method or procedure for constructing and pre-tensioning concrete beams with BFRP bars, using the system disclosed.

SUMMARY

[0013] One aspect of the present disclosure is a system for manufacturing composite pre-tensioned concrete (PSC) beams using a Basalt Fibre Reinforced Polymer (BFRP) bar. In the system, the prestressing force is induced by the BFRP bar. The system comprises a T-Frame having two Indian standards medium weight beams (ISMB) of overall depth 450 mm (5 and 12), a spandrel beam (13) with a built-up section comprising of two rolled steel sections of ISMB with overall depth of 150 mm and top and bottom plates of 8 mm thickness, two steel formworks (6) placed beside the T-frame to cast concrete beams, two BFRP bars of length 5 metres each (1) placed inside each of the two steel formworks (6) and locked at both ends using wedge anchors (2 and 9), and a hydraulic jack (3) to stretch and release BFRP bars and transfer prestress force to the concrete beams.

[0014] Another aspect of the present disclosure is a method for constructing two small scale composite prestressed concrete members each with a single concentric BFRP bar. The method comprises the steps of providing a T-frame having a back supporting beam (12) and a mid- supporting beam (5) connected to each other, providing a spandrel beam (13) with a built-up section comprising of two rolled steel sections of ISMB- 150 and two steel plates, providing two steel formworks (6) placed beside the T-frame to cast the concrete beams. The method further comprises the steps of positioning at least one BFRP bar placed inside each of the two steel formworks followed by simultaneous pre-tensioning of the BFRP bars, pouring of concrete in the formworks to cast the concrete beams and curing the concrete in the two steel formworks and releasing the prestress in the BFRP bar of each concrete beam to transfer the prestress to concrete beams.

BRIEF DESCRIPTION OF THE DRAWINGS [0015] The detailed description of the figures has been described below with the reference to the accompanying figures.

[0016] Figure 1 illustrates a principle mechanism utilised to design the system.

[0017] Figures 2a - 2d show the top-view, side-elevation, front-elevation and back-elevation of the system fabricated in accordance with an embodiment of the present disclosure.

[0018] Figures 3a and 3b show the assembly and meshing of the finite element model for the finite element analysis, respectively.

[0019] Figure 4a shows the locations of the displacement applied on the finite element model of the frame.

[0020] Figure 4b shows the boundary conditions applied on the plane of symmetry of the finite element model.

[0021] Figure 5 shows the deformation contours of the back supporting beam and spandrel beam of the system based on the finite element analysis.

[0022] Figure 6a shows the schematic layout of the instrumentation for the pilot study.

[0023] Figures 6b and 6c shows the location and nomenclature for the strain gauges installed on the BFRP bars for the pilot study.

[0024] Figure 7 shows a Table 1 reporting the load, strain and slip history from the test data recorded during the manufacture of two PSC beams fabricated using the system disclosed.

DETAILED DESCRIPTION

[0025] A system and a method for constructing prestressed concrete (PSC) beams using Basalt Fibre Reinforced Polymer (BFRP) bars as a prestressing element in concrete are disclosed. The system is capable of constructing and prestressing the PSC beam with a single concentric BFRP bar without the need of end blocks. The present system aims to enhance the potential use of the newly developed BFRP bars as a prestressing element to exploit its excellent longitudinal tensile strength for prestressing applications. [0026] The system disclosed is designed as a self-equilibrating steel frame, and the forces acting on each part of the framework is balanced by formulating a box-shaped configuration. Referring to Figure 1, illustrated is a principle mechanism utilised to design the system. A force equilibrium is ensured by balancing the forces Fl applied by the hydraulic jack and the resisting force F2 offered by the wedge anchors. The second condition necessary to achieve equilibrium is that the net external torque on the system must be equal to zero. The second condition is achieved by maintaining the same height of the centre of gravity for each part of the system and thereby ensuring that the torque is balanced between the forces Fl and F2.

[0027] Based on the design principle illustrated in Figure 1, a schematic of the system fabricated in accordance with an embodiment of the present disclosure is illustrated in Figures 2a - 2d. Figures 2a - 2d shows the top-view, side- elevation, front-elevation and back-elevation of the system, respectively.

[0028] The prestress is transferred after the concrete has gained strength more than 70% of the peak characteristic compressive strength. The release of prestressing force in the bars is facilitated by releasing the locking arrangement in the spandrel beam followed by the unlocking of the locknut on the ram of the hydraulic jack. The system comprises a back supporting beam (12), and a mid supporting beam (5) fabricated using ISMB-450 rolled sections and connected in a T-shape and forming a T-frame (Figure 2a), a spandrel beam (13) comprising a built-up section is fabricated using ISMB-150 rolled sections and 8 mm thick rectangular steel plates. Two adjustable mechanisms are provided in the system. The spandrel beam (13) is supported on the first adjustable mechanism having four adjustable bolts (15). The first adjustable mechanism comprises two bolts in the front of the spandrel beam (13) and two bolts at the back of the spandrel beam (13). The first adjustable mechanism is installed to adjust the height of the spandrel beam with respect to the back supporting beam (12) (Figure 2b). [0029] A plurality of rubber rollers with a locking arrangement (16) are provided at the bottom of the adjustable bolts of the spandrel beam (13) (Figure 2b) to reduce the contact friction and to allow the free movement of the spandrel beam during the pre-tensioning operation. The system further comprises two steel formworks (6) fabricated and placed beside the T-frame to cast the concrete beams. Both the formworks are attached with a second adjustable mechanism (7) to align/adjust the height of the formwork with reference to the back supporting beam (12) and the spandrel beam (13). The second adjustable mechanism (7) comprises four adjustable bolts, two bolts in the front end of each formwork and two bolts at the back end of each formwork. The system further comprises at least one Basalt Fibre Reinforced Polymer (BFRP) bar (1) placed inside each of the two steel formworks (6) and a hydraulic jack (3) placed between the spandrel beam (13) and mid supporting beam (5). The hydraulic jack (3) has a piston which is operated using a manually operated hydraulic pump. A plurality of vertical stiffener angles (or angle sections) (4 and 8) are added to the mid- supporting beam (5), and a plurality of additional stiffener plates (10 and 14) are added to the back supporting beam (12) and spandrel beam (13) to avoid any damage to the system during the tensioning operation.

[0030] A method for fabricating the PSC beams with a composite bar using the system disclosed. The various steps involved in the method are explained below;

a. Before the casting of concrete, 5-metres-long BFRP bars (1) are placed inside each of the two steel formworks (6). The bars are passed through a plurality of holes of diameter 15 mm, drilled in the back supporting beam (12), and the spandrel beam (13) and the BFRP bars (1) are locked at the two ends using mechanical wedge anchors (2 and 9), as shown in Figure 2a and Figure 2b. The anchors grip the BFRP bars and prevent their slippage during the tensioning operation. b. The BFRP bars in each of the two steel formworks (6) are pre tensioned simultaneously using the hydraulic jack (3) by displacing the spandrel beam (13) by a pre-determined magnitude of 29.65 mm. The linear potentiometers (LP3 and LP4) are monitored to ensure that the system has maintained equilibrium and both BFRP bars are stretched equally. The rubber rollers (16) facilitate the frictionless movement in the displacement of the spandrel beam during pre-tensioning. The allowable maximum initial prestress limit for carbon and aramid FRP bars reported in Section 3.7 of ACI 440.4R report, is 65% and 50% of the peak rupture strength of the FRP bar, respectively. Since no such guidelines are reported for BFRP bars, the maximum initial prestress is selected to be same as AFRP (50% of the peak rupture strength). This is because the Young’s modulus and the peak rupture strain for BFRP bars are similar to AFRP bars.

c. After the initial pre-tensioning operation is complete, the hydraulic jack is locked with the help of locknut available on the ram of the hydraulic jack to avoid any loss in prestress force. Once the system has stabilized, concrete is poured in the two steel formworks and allowed to cure until concrete has gained sufficient strength.

d. The hydraulic jack is released after the concrete has gained strength approximately greater than 70% of its peak characteristic compressive strength. Since the stretched BFRP bars are in its elastic state, the bars tend to regain its original cross-section which is prevented by the effective bond between the bars and concrete resulting in the transfer of prestress from the BFRP bars to concrete beams.

[0031] The framework of the proposed system was analysed by developing a three-dimensional finite element (FE) model for the system, and the behaviour of the system was accurately simulated using an FE tool ABAQUS. Figure 3 (3a - 3b) shows the three-dimensional finite element model and the mesh assembly for the system. The geometry and boundary conditions defined to simulate the half- scale FE model of the fabricated system is shown in Figure 4 (4a - 4b). Several parameters including, the size and location of the stiffeners to be used and the orientation of the sections were thoroughly investigated through the use of the FE model. Figure 5 shows the deformation contours of the back supporting beam (12) and the spandrel beam (13) of the system obtained through the FE analysis.

[0032] The modelling process meticulously simulated all the physical conditions, including the bar tensioning operation, stress transfer, and time- dependent behaviour of the concrete beams.

[0033] Based on the FE results, the system is strengthened by the addition of stiffeners (4, 8, 10 and 14) to the mid supporting beam (5), back supporting beam (12) and the spandrel beam (13) of the framework. The addition of stiffeners avoids any damage or development of the plastic strains in the system during the initial prestressing stage. Thus, through extensive FE investigation, it was ensured that the system has enough reserved capacity during the casting and prestressing operations.

[0034] A proof of concept of the working mechanism of the designed system was established from the FE study.

[0035] Further, a pilot study conducted to illustrate the fabrication of two PSC beams, according to the present disclosure is provided.

[0036] The developed system is utilised for the manufacture of two concrete beams prestressed with the use of BFRP bars at an initial prestress level of 50% of the ultimate strength of the bar to illustrate the application of the principles explained in the present disclosure.

[0037] The dimensions of the two concrete beams manufactured and prestressed were 3-meter-long, 0.1 -metre-wide and 0.2-meter-deep. The required initial prestress is achieved by the application of 30 kN force on the spandrel beam using a hydraulic jack. The 30 kN force generates an average displacement of 29.64 mm of the spandrel beam as measured by linear potentiometers (LP3 and LP4).

[0038] The loads, strain levels and anchor slippage of the two prestressing bars was accurately measured at the locations as shown in Figures 6a-6c. Figure 6a shows the schematic layout of the instrumentation for the pilot study, more precisely, the location of the linear potentiometers and load cells. Figures 6b and 6c show the location and nomenclature for the strain gauges installed on the BFRP bar for the pilot study. All dimensions in Figure 6b and Figure 6c are in mm. The data was acquired continuously during the entire procedure at an acquisition rate of 1 Hz with the help of HBM data logger MGCplus.

[0039] The slippage of the prestressing elements at the anchorages is considered, and the axial load on the BFRP bar is modified using Equation 1, where, F is the modified load, F _pu is the load including slips, A is the cross- sectional area of the BFRP bar; D is the slippage of the bar from the anchors at the dead-end location, L is the original distance between the two anchors and E _pu is the Young’s modulus of the BFRP bar. Equation 1

[0040] Concrete was poured into the formworks two-hours after the stressing of the BFRP bars. The formworks were removed after 24 hours from the casting of the beams. Concrete was allowed to self-cure by covering the beams with polythene sheets. The prestress was maintained in the BFRP bars for approximately 225 hours (10 days). The time duration of 225 hours between the successive casting of the concrete beams and prestress transfer is deemed necessary for the concrete to gain sufficient strength before the prestress transfer. This time duration is dependent on the type of concrete and method of curing. However, regardless of the concrete type and curing methods, the de-tensioning should not be delayed as excessive relaxation of the prestressing element can negatively impact the effective prestress level. In the pilot study, an average relaxation loss of 3% has been measured after 225 hours based on the data recorded by the strain gauges (Sl to S6) installed on the BFRP bars.

[0041] Figures 7a - 7f show the load-history, slip-history, strain-history and the respective bar charts from the test data recorded during the manufacture of two PSC beams fabricated using the system. Figure 7a shows the load-history obtained from the load cells LC1, LC2 and LC3. Figure 8 shows a Table 1 reporting the load, strain and slip history from the test data recorded during the manufacture of two PSC beams fabricated using the system. The variation shows that there is a steep reduction in the magnitude of load between 100 hours and 200 hours. This variation is anticipated due to the increase in the slippage of the BFRP bar, as shown by LP2 (see Table 1 and Figure 7b), which starts at approximately 100 hours.

[0042] It can be observed from Table 1 that within the first forty-eight hours after casting, the strain gauges show fluctuations and do not show any specific pattern in the strain variation. The fluctuations could be due to the stiffening effect caused by the concrete hardening. Nonetheless, after approximately 114 hours when the concrete has gained sufficient strength and the impact of concrete shrinkage has reduced, a decrease in strain can be observed in all the strain gauge readings.

[0043] It can be observed from Table 1 that after the prestress release, all the load cell readings have reduced to zero. The strain gauges Sl and S3 on the BFRP bar Bl and S4 and S6 on the BFRP bar B2 reduced considerably.

[0044] The strain gauges S2 and S5 mounted on the BFRP bar Bl and B2, respectively, within the concrete region, maintained the strain level. This indicates an effective prestress transfer from the prestressing bars to concrete through bond action and strain compatibility and validates the proposed design of the manufacturing facility to cast and prestress concrete members using BFRP bars.

[0045] The system is capable of constructing two identical concrete beams simultaneously prestressed using a single concentric BFRP bar each, without the need of end blocks or buttresses. No specialised equipment’s are required for the application of the controlled tensioning and de-tensioning process other than the normal equipment’s typically utilised by the manufacturing plants. The system is a self-restraining-rig and the system is fabricated using rudimentary materials and equipment that are easily available, and the framework can be replicated in laboratories and factories. The framework can be used multiple times for repeated prestressing operations of concrete members.

[0046] Although the system is tested using BFRP bar, this does not limit its scope and the system and the method thereof, can be used in conjunction with any FRP or steel strand of equivalent diameter. As new FRP products are introduced by the industries, the present system can be used as a tool for the testing and certification of FRP bars in the laboratories before they are employed in real-life projects.

[0047] The above description along with the accompanying drawings is intended to disclose and describe the preferred embodiments of the invention in sufficient detail to enable those skilled in the art to practice the invention. It should not be interpreted as limiting the scope of the invention. Those skilled in the art to which the invention relates will appreciate that many variations of the exemplary implementations and other implementations exist within the scope of the claimed invention. Various changes in the form and detail may be made therein without departing from its spirit and scope. Similarly, various aspects of the present invention may be advantageously practiced by incorporating all features or certain sub-combinations of the features.