Method and apparatus for casting prefabricated prestressed concrete products

Abstract

A method and an apparatus for casting prefabricated prestressed concrete products with a substantially horizontal slipform casting process, in which method reinforcement strands are stressed in a bundle on a casting bed before the slipform casting is started, wherein the expected behavior of at least one measurable variable affecting the strand stressing process during the strand stressing process is predetermined, and the behavior of the at least one measurable variable is measured and compared to its predetermined expected behavior during the strand stressing process.

Claims

1. A method for casting prefabricated prestressed concrete products with a substantially horizontal slipform casting process, the method comprises: stressing reinforcement strands in a bundle on a casting bed before the slipform casting process is started, predetermining an expected behavior of at least one measurable variable affecting the stressing of the reinforcement strands, during the stressing of the reinforcement strands, comprising predetermining how the at least one measurable variable will evolve and change during the stressing of the reinforcement strands, measuring a behavior of the at least one measurable variable during the stressing of the reinforcement strands, comprising measuring how the at least one measurable variable evolves and changes during the stressing of the reinforcement strands, comparing the measured behavior of the at least one measurable variable to the predetermined expected behavior of the at least one measurable variable during the stressing of the reinforcement strands, and ending the stressing of the reinforcement strands if the measured behaviour deviates from the predetermined expected behaviour by more than a predetermined amount during the stressing of the reinforcement strands.

2. The method according to claim 1, wherein the at least one measurable variable comprises a force exerted during the stressing of the reinforcement strands, an elongation of the reinforcement strands, or any other measurable variable which can be used to determine either the force or the elongation, or a combination thereof.

3. The method according to claim 1, wherein in predetermining the expected behavior of the at least one measurable variable, amount and type of the reinforcement strands in the bundle are used.

4. The method according to claim 1, further comprising controlling the stressing of the reinforcement strands with an automatic control system implementing predetermination of the expected behavior of the at least one measurable variable, or the measuring of the behavior of the at least one measurable variable during the stressing of the reinforcement strands, which automatic control system is a part of a production control system of a manufacturing facility of the prefabricated prestressed concrete products.

5. The method according to claim 4, wherein the automatic control system issues an alert and ends the stressing of the reinforcement strands, releases stress affecting the reinforcement strands in the bundle, or both, when the measured behavior of the at least one measurable variable deviates from the predetermined expected behavior of the at least one measurable variable by more than the predetermined amount during the stressing of the reinforcement strands.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Next the invention, in its embodiments, is discussed in greater detail in the sense of example and with reference to accompanying drawings, where

(2) FIG. 1 shows schematically a layout of a manufacturing facility for prefabrication of prestressed concrete products in accordance with an embodiment of the present invention,

(3) FIG. 2 shows schematically a bundle stressing device of an embodiment of the present invention, and

(4) FIG. 3 shows schematically one principle for following and controlling stressing process based on Hooke's law as a graph.

BRIEF DESCRIPTION OF SPECIFIC EMBODIMENTS

(5) FIG. 1 shows schematically a layout of a manufacturing facility 1 for prefabrication of prestressed concrete products, in which an embodiment of the present invention is used.

(6) The manufacturing facility 1 shown in FIG. 1 comprises a plurality of slipform casting beds 2, a plurality of transfer beds 3 for moving cut hollow-core concrete products to a storage area, a storage area 4, bridge cranes 5, 6, 7 for lifting and transferring cast concrete products and casting equipment, and a bundle stressing device 8.

(7) When new casting process is to be started, the casting beds 2 are generally first cleaned and oiled, after which reinforcements strands are pulled on the lengths of the casting beds and the ends of the reinforcement strands located on the same casting bed are fixed to a strand pulling plate to form a reinforcement strand bundle. The strand pulling plate is connected after fixing of the reinforcement strands to a bundle stressing device 8.

(8) Next the stressing process is started, where the bundle stressing machine 8 starts to stress the reinforcement strands by pulling the strand pulling plate with hydraulic cylinders. The start of the stressing process is generally accompanied with warning lights and sounds to inform the personnel to stay clear of the area. Once the stressing of the reinforcement strands is carried out to the required stress with the bundle stressing device 8, the strand pulling plate is fixed to a mechanical fixing structure 9 located at the end of each casting bed 2, after which the strand pulling plate is detached from the bundle stressing device, so that the bundle stressing device can be moved along transverse rails to the end of another casting bed for a new stressing process.

(9) After the ends of the reinforcement strands are fixed to the strand pulling plate and the strand pulling plate is connected to the bundle stressing device 8, the strand pulling plate may be pulled a short distance before starting the actual stressing of the reinforcement strands. This short pull, which can be 20 cm for example, will remove slack from the reinforcement strands and reduce the risk of uneven stressing of the reinforcement strands.

(10) After the stressing process is done, the slipform casting process is started by lifting a slipform casting machine on the casting bed and over the reinforcement strands, and by transferring concrete mass to the mass container of the slipform casting machine.

(11) FIG. 2 shows schematically a bundle stressing device 8 of an embodiment of the present invention.

(12) The bundle stressing device 8 comprises two hydraulic cylinders 10 used for pulling the strand bundle plate (not shown) connected to the shafts 11 of the hydraulic cylinders. The operator of the bundle stressing device is located behind protective cover 12.

(13) The force exerted by the hydraulic cylinders 10 (F.sub.s) is determined, based on measured hydraulic pressure in the cylinders, the amount of hydraulic cylinders implementing the stressing process, and the cross-sectional hydraulic area of the hydraulic cylinders (surface area of the piston deducted with cross-sectional area of the piston shaft), for example. The distance pulled with the hydraulic cylinders is also measured, with distance sensors (not shown) for example. The distance pulled (L) is used to observe the behavior of the force during the pulling process in relation to the distance pulled. These measurements and the determination of the force exerted are advantageously carried out with an automatic control system (not shown) of the bundle stressing device 8, based on implementation of Hooke's law.

(14) In some embodiments each of the hydraulic cylinders 10 may be driven though separate valves, or with equalizing device, wherein the actual pressure within each of the hydraulic cylinders may vary. In these types of embodiments the mean value of the hydraulic cylinder pressures may be used in the determination of the force exerted by the hydraulic cylinders 10. The automatic control system of the bundle stressing device 8 of the invention may also compare the pressures of each of the hydraulic cylinders 10, and issue an alert if the pressure in one of the hydraulic cylinders deviates more than a preset maximum deviation value from the other.

(15) The automatic control system of the bundle stressing device 8 is advantageously connected to a production control system of the manufacturing facility, so that information about the amount and type of the reinforcement strands in a bundle can be provided to the automatic control system for the determination of the expected stress behavior of the reinforcement strand bundle during the stressing process.

(16) FIG. 3 shows schematically one principle for following and controlling stressing process based on Hooke's law as a graph.

(17) In FIG. 3, the force F.sub.s is the force exerted by hydraulic cylinders 10 of the bundle stressing device 8, and can be defined by equation:
F.sub.s=pA, where p=the pressure of the hydraulic fluid in cylinders, and A=cross-sectional area of the hydraulic fluid area of the cylinders (surface area of the piston deducted with cross-sectional area of the piston shaft and multiplied with the amount of cylinders)

(18) In FIG. 3, the elongation L is the obtained elongation of the reinforcement strand bundle during the stressing process, which can be defined by measuring the movement of the strand pulling plate connected to the strand stressing device during the stressing process. The elongation L can also be used to determining the force affecting the stressed reinforcement strands with equation:
F=(LAE)/L, where L=obtained elongation, A=combined cross-sectional area of the reinforcement strands in the bundle, E=modulus of elasticity of the reinforcement strands, and L=unstressed length of the reinforcement strands.

(19) As shown in FIG. 3 with a continuous line, the optimal bundle stressing process will create a straight line graph when measuring these two above mentioned variables during the strand stressing process. The angular coefficient of the optimal bundle stressing process in the graph of FIG. 3 corresponds to the elastic constant of Hooke's law, and can be predefined based on the type and amount of reinforcement strands in the bundle to be stressed. Thus the line presenting the optimal stressing process for a bundle can be predefined and used as a reference graph for the actual stressing process.

(20) Dashed line A in FIG. 3 shows an example of a graph for measured stressing process of a bundle, where at start there were some slack in some of the reinforcement strands and/or there was some sliding of at least some reinforcements strands in their fixing to the strand pulling plate, but the expected stressing process resumed during early stages. This is often acceptable tensioning process, if the following requirements are fulfilled: a) Deviation from the expected stressing process does not extend over maximum predefined length of the total elongation (L), preferably without the length of phase a. The set maximum may be 5% of the total elongation, for example. There may be also be predefined separate maximum values set for both the length of phase a and the combined length of phases b+c, where exceeding one of the two separate maximum values will lead to unacceptable tensioning process, for example. b) Required force F.sub.s is obtained at the end of the stressing process.

(21) In the stressing process of dashed line A, during phase a the slack from the reinforcement strands is removed which does not affect the measured force, during phase b the reinforcement strands starts to stress one by one, and during phase c all of the reinforcement strands stress according to the expected stressing process.

(22) Dashed line B in FIG. 3 shows an example of a graph for measured stressing process of a bundle, where the fixing of some of the reinforcement strands have failed, or there are too few reinforcement strands in the bundle.

(23) The comparison of predefined progression of the stressing process to the actual measured progression of the stressing process, as illustrated with reference to FIG. 3, allows for quick indication and thus reaction to problems in the stressing process.

(24) If the phase b extends over the predefined maximum or if the determined force does not reach predetermined force or force range, an alert is issued by the automatic control system of the bundle stressing device and/or release of the stressed reinforcement strands is required. Further, a predefined value is also set to the length of phases b+c, and if the required measured force is not achieved during this length, an alert is issued.

(25) Both ends of a slipform casting bed are often equipped with fixed strand combs which are used to maintain the reinforcement strands at their proper location during the stressing process of the strands and during the slipform casting. These strand combs may create friction during the stressing process of the strands, the effect of which can be taken into account in the stressing process by introducing corresponding coefficients into the calibration process of the strand pulling device, for example. The strand pulling devices are generally calibrated twice a year.

(26) The data obtained from the stressing process is also advantageously saved to the automatic control system, or to external database, so that correct stressing process and correct stressing of reinforcement strands can be checked and proved after casting of the prestressed product for each cast product.

(27) With an embodiment of the present invention is possible to know and guarantee that the stressing of the reinforcement strands is adequate, and that the differences of the stressing of separate reinforcement strands in the bundle is below a preset value (for example, the mentioned 5%).

(28) The specific exemplifying embodiments of the invention shown in figures and discussed above should not be construed as limiting. A person skilled in the art can amend and modify the embodiments in many evident ways within the scope of the attached claims. Thus the invention is not limited merely to the embodiments described above.