Production and testing methods for prefabricated ultra-high strength concrete foundations of transmission towers

Abstract

The present invention provides a production method and testing method for a prefabricated ultra-high-strength concrete foundation for a transmission tower, belonging to the field of power construction technology. The production method comprises: placing a steel reinforcement skeleton tied according to preset requirements into a steel mold; pouring a concrete slurry prepared according to preset requirements into the steel mold; vibrating the poured concrete slurry; curing until a prefabricated ultra-high-strength concrete foundation is obtained; and testing the obtained prefabricated ultra-high-strength concrete foundation. The prefabricated ultra-high-strength concrete foundation for a transmission tower provided by the present invention can be produced into several smaller-sized foundation components, flexibly adapting to the assembly of a wider range of tower foundation types and suitable for more complex and corrosive environments. It features high strength and low weight, fast and convenient construction, and significant social and environmental benefits.

Description

Technical Field

[0001] The present invention belongs to the technical field of electric power construction, and in particular relates to a production method and a test method for a prefabricated ultra-high strength concrete foundation of a transmission tower. Background Art

[0002] Precast concrete foundations for transmission towers are manufactured using scientific calculations and standard production processes to meet specified operating conditions. These foundations are primarily used in construction sites where raw material acquisition is difficult or space is insufficient. However, with the evolving nature of society, young people are reluctant to join the construction industry, resulting in a shortage of construction workers. Furthermore, driven by technological advancements, replacing manual labor with mechanization has become a fundamental trend in today's engineering landscape. Precast concrete foundations are widely used due to their advantages, such as workshop-based mechanized production, reduced on-site labor costs, and improved project efficiency.

[0003] The precast concrete foundations of transmission towers are standardized and manufactured in workshops. Strict control is exercised over material selection, production equipment, processing techniques, and maintenance methods, significantly improving product quality and reducing construction pollution. Modular design, standardized processes, factory-based production, and efficient on-site application effectively reduce construction time and costs.

[0004] However, the existing precast concrete foundation has obvious disadvantages: (1) The steel formwork size specifications are limited. The steel formwork size specifications are fixed and cannot be flexibly adapted to all concrete foundation sizes.

[0005] (2) It is not suitable for the production of large-sized precast concrete foundations. Due to the limitations of production equipment and transportation, the size of precast concrete foundations should not be too large, and they cannot support towers with large foundation forces.

[0006] (3) Poor corrosion resistance. It cannot effectively inhibit foundation corrosion in corrosive areas. The main corrosion is chloride ion and sulfate ion corrosion. Summary of the Invention

[0007] The embodiments of the present invention provide a production method and a test method for a precast ultra-high-strength concrete foundation of a transmission tower, aiming to solve the problems of poor bearing capacity and poor corrosion resistance of the precast concrete foundation.

[0008] In a first aspect, to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: to provide a production method of a prefabricated ultra-high strength concrete foundation of a transmission tower, the production method comprising:

[0009] The steel bar skeleton tied according to preset requirements is placed into the steel formwork;

[0010] Concrete slurry prepared according to preset requirements is poured into the steel mold;

[0011] Vibrating the poured concrete slurry;

[0012] Curing until a precast ultra-high strength concrete foundation is obtained;

[0013] The prepared prefabricated ultra-high strength concrete foundation was tested.

[0014] In conjunction with the first aspect, in a possible implementation, the prefabrication of the steel frame includes:

[0015] The main reinforcement shall be made of stress-relieving ribbed steel bars with equal diameter, diameter ≥12mm, and clear distance ≥60mm. The main reinforcement shall be straightened and cut to a fixed length by a straightening machine. The requirements are as follows: no obvious bending visible to the naked eye; main reinforcement length error <2mm; strength loss <4% of the design value of tensile strength;

[0016] The main reinforcement is configured throughout the entire length, and the main reinforcement is connected by double-sided lap welding, with a weld length of ≥140mm, and the welds of adjacent main reinforcements are not in the same plane;

[0017] The ends of the main reinforcements that are configured throughout the length after welding use the forging heads of the steel bar forging machine, and the diameter of the forging heads is 1.8 times the diameter of the steel bar;

[0018] The upper part of the main reinforcement is evenly arranged vertically along the main column. After the lower part enters the expanded bottom part for ≥80mm, the lower part of the main reinforcement is parallel to the side of the expanded bottom part at the first bending part. The second bending part at the end of the main reinforcement is bent outward to form a 180° hook. The length of the straight section at the end of the hook is not less than 5 times the diameter of the main reinforcement. The bending is performed using a steel bar bending machine, and no welding is allowed at the bending part.

[0019] Stirrups are made of low-carbon cold-drawn round steel bars of equal diameter, with a diameter not less than 0.25 times the diameter of the main bars. They are formed on a skeleton forming machine, with the straight sections of the main bars straightened and tightened, and then annular inner stirrups are tied, followed by welding of spiral outer stirrups and annular outer stirrups. Annular inner stirrups and fully welded annular outer stirrups are tied at the upper end of the main bars, the first bend, and 80mm from the second bend. The annular inner stirrups and annular outer stirrups are staggered by a vertical distance of 4-5mm to ensure that the inner and outer stirrups are not in the same plane. In addition, annular inner stirrups are fully welded every 1000mm or less. Spiral outer stirrups are fully welded from the upper end of the main bars to the first bend, and annular outer stirrups are fully welded between the first bend and the second bend. The spacing between the annular outer stirrups is not more than 100mm.

[0020] A steel ring mesh structure is set up at the bottom of the foundation. The outer ring steel bars are stress-relief ribbed steel bars, and the inner ring steel bars are cross-staggered steel bars. The longest staggered steel bars in the middle are stress-relief ribbed steel bars. The other steel bars are low-carbon cold-drawn round steel bars. The diameter of the stress-relief ribbed steel bars is ≥12mm and the diameter of the low-carbon cold-drawn round steel bars is not less than 0.25 times the diameter of the stress-relief ribbed steel bars. The intersections of the steel bars should be fully welded.

[0021] The 180° hook at the end of the upper main reinforcement hooks the outer ring reinforcement and ties it together to increase the overall structural strength of the reinforcement skeleton;

[0022] Among them, the design value of tensile strength of stress-relieved ribbed steel bars is ≥1040Mpa; the design value of tensile strength of low-carbon cold-drawn round steel bars is ≥510Mpa.

[0023] In conjunction with the first aspect, in a possible implementation, preparing the concrete slurry includes:

[0024] Raw materials: including 130-150 parts of cement, 130-150 parts of quartz sand, 50-60 parts of crushed stone, 15-16 parts of water reducer, 50-55 parts of active admixture, 25-35 parts of toughening and reinforcing admixture, and the rest is clean neutral water. The water-binder ratio is 0.18;

[0025] Ingredients: weigh cement, quartz sand, crushed stone, water reducer, active admixture, toughening and reinforcing admixture, and water according to the mix ratio;

[0026] Mixing: Use a mixer to mix cement, quartz sand, gravel and active admixtures together and mix for 1 to 2 minutes; add the first water and water-reducing agent aqueous solution, with the first water volume and the water-reducing agent aqueous solution using a total of 2 / 3 water, and mix for 1 to 2 minutes; add toughening and reinforcing admixtures and part or all of the remaining water and mix for 2 to 3 minutes. The remaining water volume can be flexibly controlled according to the workability index; among which, the workability index: the slump is controlled at 7 to 11 cm according to the ambient temperature.

[0027] In conjunction with the first aspect, in a possible implementation, the concrete slurry vibration process includes:

[0028] Use a high-frequency electric insert vibrator to vibrate the concrete slurry in the steel mold;

[0029] Quickly insert the high-frequency electric insert vibrator, start vibrating from the bottom of the steel mold, and vibrate at the same position for 30~40s; move the high-frequency electric insert vibrator 40~50cm, and continue vibrating for 30~40s; move the high-frequency electric insert vibrator 40~50cm and continue vibrating; the principle of moving the high-frequency electric insert vibrator is to move horizontally first and then upward, and the high-frequency electric insert vibrator should be moved slowly until all the concrete slurry is vibrated; reinsert the high-frequency electric insert vibrator into the bottom of the concrete and repeat the above operation, repeat the vibration until the concrete no longer sinks obviously and no obvious bubbles appear, and the vibration is completed.

[0030] In conjunction with the first aspect, in a possible implementation, curing until a prefabricated ultra-high strength concrete foundation is obtained includes:

[0031] Atmospheric pressure steam curing:

[0032] Place the vibrated precast concrete foundation in the steam curing tank, seal the steel formwork ends, and perform normal pressure steam curing;

[0033] Warming up period: room temperature rises to 85℃;

[0034] Heating time: controlled within 1.5~2h according to the initial temperature;

[0035] Constant temperature period: 85℃;

[0036] Constant temperature time: 2h;

[0037] Cooling period: 85℃ to 45℃;

[0038] Cooling time: 0.5h;

[0039] Maintenance room maintenance:

[0040] After demoulding, the product is placed in a concrete high-temperature curing room for curing. The temperature is maintained at 40-42°C and the humidity is above 90%. The curing time is 2-4 days. This will promote the chemical reaction of substances in the concrete to form crystals, increase the density of the concrete's internal structure, improve the concrete's strength, and accelerate the hardening of the concrete.

[0041] Natural maintenance:

[0042] The precast concrete foundation needs to be naturally cured for 7 days after leaving the curing room. The upper end should be sealed and water should be sprinkled on the foundation every 5 to 6 hours. The watering should be even to ensure that there are no leaking parts of the foundation to obtain a precast ultra-high strength concrete foundation.

[0043] In a second aspect, an embodiment of the present invention provides a test method for a prefabricated ultra-high strength concrete foundation. The test method for the prefabricated ultra-high strength concrete foundation of a transmission tower prepared based on the production method includes: a compressive strength test, a splitting tensile strength test, and a corrosion test;

[0044] The corrosion test includes a chloride ion penetration resistance test and a sulfate ion resistance test.

[0045] In conjunction with the second aspect, in one possible implementation, the compressive strength test includes:

[0046] During the same period, 150mm×150mm×150mm cubic specimens were cured, with a total of 1 group and 3 specimens in each group;

[0047] Place the specimen on the testing machine for testing. During the test, the load should be applied continuously and evenly at a loading rate of 27 kN / s.

[0048] The compressive strength of concrete cube is calculated according to the following formula (1):

[0049] (1)

[0050] Where: —Compressive strength of concrete cube specimen, unit: MPa;

[0051] F—specimen failure load, unit N;

[0052] A—pressure-bearing area of the specimen, unit: mm 2 ;

[0053] The compressive strength value is taken as the average value of the three specimens tested as the compressive strength value of the group of specimens, and the calculation is accurate to 0.1MPa; when the difference between the maximum and minimum compressive strength values of the specimens and the median is greater than 15% of the median, the median is taken as the compressive strength value of the group of specimens; when the difference between the maximum and minimum compressive strength values of the specimens and the median is greater than 15%, the test results are invalid.

[0054] In conjunction with the second aspect, in a possible implementation, the splitting tensile strength test includes:

[0055] During the same period, 150mm×150mm×150mm cubic specimens were cured, with a total of 1 group and 3 specimens in each group;

[0056] Place the specimen on the tensile testing machine and load it evenly at a rate of 0.1 MPa / s until the specimen breaks and the load begins to decrease. When it drops to 100 kN, close the oil supply valve, open the oil return valve, and record the data.

[0057] The compressive strength of concrete cube is calculated according to formula (2):

[0058] (2)

[0059] Where: —Concrete splitting tensile strength, unit: MPa;

[0060] F—specimen failure load, unit N;

[0061] A—specify the area of the splitting surface, unit: mm 2 ;

[0062] The tensile strength value is the average value of the three specimens tested as the tensile strength value of the group of specimens, and the calculation is accurate to 0.1MPa; when the difference between the maximum and minimum values of the specimens and the median is greater than 15% of the median, the median is taken as the tensile strength value of the group of specimens; when the difference between the maximum and minimum values of the specimens and the median is greater than 15%, the test results are invalid.

[0063] In conjunction with the second aspect, in one possible implementation, the chloride ion penetration resistance test includes:

[0064] Specimen selection: Cylindrical specimens with a diameter of 100±1mm and a height of 50±2mm are used. There is one group of specimens, and one group includes multiple specimens. The test is carried out in a room at 20~25℃.

[0065] Before the experiment, the specimens were vacuum-filled with water, and excess water was wiped off after vacuum-filling.

[0066] Clamp the saturated specimen in the test tank. After checking for permeability, inject NaCl solution and NaOH solution into both sides of the specimen. The copper mesh in the test tank where NaCl solution is injected should be connected to the negative pole of the power supply, while the copper mesh in the test tank where NaOH solution is injected should be connected to the positive pole of the power supply.

[0067] Turn on the power of the chloride ion permeation meter and apply a DC constant voltage of 60±0.1V to the two copper meshes, and record the initial current reading I0; record the current value every 5 minutes at the beginning, and when the current value does not change much, record the current value I0 every 10 minutes. t When the current changes very little, the current value should be recorded every 30 minutes until the power is on for 6 hours;

[0068] After the test, drain the test solution in time and rinse the test tank with cold boiled water and detergent for more than 60 seconds, then rinse with distilled water and blow dry with a hair dryer on the cold air setting;

[0069] The total electric flux of each specimen is calculated using formula (3);

[0070] (3)

[0071] Where: ——The total electric flux C passing through the test piece with a diameter of

[0072] I0——initial current A, accurate to 0.001A;

[0073] I t ——Current A at time t (min), accurate to 0.001A;

[0074] According to the Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete (GB / T 50082-2009), the total electric flux of the specimen is converted into the electric flux value of a specimen with a diameter of 95 mm according to formula (4);

[0075] × (4)

[0076] Where: ——Electric flux C passing through a test piece with a diameter of 95 mm;

[0077] ——Through the diameter mm, the electric flux C of the specimen;

[0078] For one group, the arithmetic mean of the electric flux of three test pieces is taken as the measured electric flux value of the test pieces in that group;

[0079] When the difference between a certain electric flux value and the median value exceeds 15% of the median value, the arithmetic mean of the electric flux values of the remaining two specimens shall be taken as the test result value of the group of specimens;

[0080] When the difference between two electric flux values and the median value exceeds 15% of the median value, the median value should be taken as the measured value of the electric flux test result for the group of specimens.

[0081] In conjunction with the second aspect, in one possible implementation, the sulfate ion resistance test includes:

[0082] During the same period, 100mm×100mm×100mm specimens were cured and divided into 4 groups, with 3 pieces in each group. They were immersed in clean water and 8% Na2SO4 solution for 30 days and 90 days respectively. The liquid was not less than 10mm above the upper surface of the specimens. After reaching the specified age, the concrete compressive strength of the specimens was measured and the corrosion resistance coefficient K value was calculated.

[0083] The method for producing prefabricated ultra-high-strength concrete foundations for transmission towers provided by this invention offers the following advantages over existing technologies: by improving the crack resistance, load-bearing capacity, and corrosion resistance of prefabricated concrete components, it allows for the production of several smaller-sized foundation components, flexibly adapting to the assembly of a wider range of tower foundations and suitable for use in more complex and corrosive environments. Furthermore, the foundation's weight is reduced by approximately 30%, easing the difficulty and workload of transportation and lifting, and reducing construction costs. The prefabricated ultra-high-strength concrete foundation produced by this invention boasts high strength and low weight, enabling quick and convenient construction, and significant social and environmental benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0084] Figure 1 A schematic structural diagram of a prefabricated ultra-high-strength concrete foundation for a transmission tower provided in an embodiment of the present invention;

[0085] Figure 2 A schematic diagram of the steel mesh structure at the bottom of the prefabricated ultra-high strength concrete foundation of a transmission tower provided in an embodiment of the present invention;

[0086] Figure 3 for Figure 1 Provided is a top-down structural diagram of the prefabricated ultra-high-strength concrete foundation of a transmission tower;

[0087] Description of reference numerals:

[0088] 1. Stress-relieving ribbed steel bars; 2. Low-carbon cold-drawn round steel bars. DETAILED DESCRIPTION

[0089] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0090] Please also refer to Figures 1 to 3 The production method of the prefabricated ultra-high strength concrete foundation of the transmission tower provided by the present invention is now described. The production method of the prefabricated ultra-high strength concrete foundation of the transmission tower comprises:

[0091] Step 1: Place the steel bar skeleton tied according to preset requirements into the steel mold;

[0092] Step 2: pour the concrete slurry prepared according to the preset requirements into the steel mold;

[0093] Step 3, vibrating the poured concrete slurry;

[0094] Step 4: Curing until a prefabricated ultra-high strength concrete foundation is obtained;

[0095] Step 5: Test the prepared prefabricated ultra-high strength concrete foundation.

[0096] The prefabricated ultra-high-strength concrete foundation provided in this embodiment replaces the original prefabricated concrete foundation, which can improve the crack resistance of the components, the bearing capacity and corrosion resistance of the foundation. The preparation of several smaller-sized foundations can adapt to more types of pole towers and can be applied to more complex corrosive environments. At the same time, the foundation will reduce its own weight by about 30%, reducing the difficulty and engineering workload of transportation and lifting, saving project costs, and making construction quick and convenient, with significant social and environmental benefits.

[0097] The present invention implements the preparation of prefabricated ultra-high-strength concrete foundations through meticulous care from material selection, production process preparation to test method control, as follows:

[0098] (1) Selection of raw materials

[0099] 1. Rebar: stress-relieved ribbed steel bar, design tensile strength ≥ 1040Mpa; low-carbon cold-drawn round steel bar, design tensile strength ≥ 510Mpa.

[0100] 2. Concrete:

[0101] (1) Cement: 140 parts of 52.5 grade ordinary Portland cement;

[0102] (2) Sand: quartz sand, 140 parts;

[0103] (3) Stone: diabase crushed stone, 60 parts;

[0104] (4) Water reducer: high-efficiency carboxylic acid series water reducer, 16 parts;

[0105] (5) Active admixtures: 55 parts of microsilica powder and 45 parts of fly ash;

[0106] (6) Toughening and strengthening admixture: steel fiber with an aspect ratio of 60-90, 30 parts;

[0107] (7) Water: clean neutral water with a water-to-binder ratio of 0.18.

[0108] (2) Steel skeleton forming

[0109] (1) The main reinforcement shall be made of stress-relieving ribbed steel bars with equal diameter, diameter ≥12mm, and clear distance ≥60mm. The main reinforcement shall be straightened and cut to a fixed length by a straightening machine. The requirements are as follows: no obvious bending visible to the naked eye; the error in the length of the steel bar shall be less than 2mm; and the strength loss shall be less than 4% of the design value of the tensile strength.

[0110] (2) The main reinforcement is arranged throughout the entire length, and the main reinforcement is connected by double-sided lap welding. The weld length is ≥140mm, and the welds of adjacent main reinforcements are not in the same plane.

[0111] (3) After welding, the ends of the main reinforcement that are arranged throughout the length use the steel bar heading machine's heading head, and the heading head diameter is 1.8 times the diameter of the steel bar.

[0112] (4) The upper part of the main reinforcement is evenly arranged vertically along the main column. After the lower part enters the expanded bottom part ≥80mm, it is bent at a certain angle (the first bending part) so that the lower part of the main reinforcement is parallel to the side of the expanded bottom part. The end of the main reinforcement is bent outward to form a 180° hook (the second bending part). The length of the straight section at the end of the hook should not be less than 5 times the diameter of the main reinforcement. The steel bar bending machine is used for bending, and no welding is allowed at the bending part.

[0113] (5) Stirrups are made of low-carbon cold-drawn round steel bars of equal diameter, with a diameter not less than 0.25 times the diameter of the main bars. They are formed on a skeleton forming machine, the straight section of the main bars is straightened and tightened, and the ring-type inner stirrups are tied. Then, the spiral outer stirrups and the ring-type outer stirrups are welded. The ring-type inner stirrups and the fully welded ring-type outer stirrups are tied at the upper end of the main bars, the first bend, and 80 mm away from the second bend. The inner and outer stirrups are staggered by a vertical distance of 4 to 5 mm to ensure that the inner and outer stirrups are not in the same plane.

[0114] In addition, a ring-type internal stirrup is fully welded every 1000mm, a spiral external stirrup is fully welded from the upper end of the main reinforcement to the first bending part, and a ring-type external stirrup is fully welded between the first bending part and the second bending part. The spacing between the external stirrups is not more than 100mm.

[0115] (6) A steel ring mesh structure is set at the bottom of the foundation. The outer ring uses stress-relieving ribbed steel bars, and the inner ring uses a cross-staggered steel bar structure. The longest staggered steel bar in the middle uses stress-relieving ribbed steel bars, and the other steel bars use low-carbon cold-drawn round steel bars. The diameter of the stress-relieving ribbed steel bars is ≥12mm, and the diameter of the low-carbon cold-drawn round steel bars is not less than 0.25 times the diameter of the stress-relieving ribbed steel bars. Full welding is used at the intersection of the steel bars.

[0116] (7) The upper main reinforcement is bent 180 degrees to hook the outer ring reinforcement and tied together to increase the overall structural strength of the reinforcement skeleton.

[0117] (3) Concrete production

[0118] (1) Weigh cement, sand, stone, water reducer, microsilica powder, fly ash, steel fiber and water according to the mix ratio.

[0119] (2) Mixing: Use a mixer to mix cement, sand, stone, microsilica powder and fly ash together for 1 to 2 minutes; add the first water and water-reducing agent aqueous solution, the first water volume and the water-reducing agent aqueous solution water volume are 2 / 3 water in total, and mix for 1 to 2 minutes; add steel fiber and part or all of the remaining water and mix for 2 to 3 minutes. The remaining water volume can be flexibly controlled according to the workability index.

[0120] Workability index: The slump is controlled at 7~11cm according to the ambient temperature.

[0121] (4) Concrete pouring

[0122] Fix the steel skeleton at the exact position inside the steel membrane, and use a pump to pump and feed the material into the steel mold.

[0123] (5) Concrete vibration

[0124] Use a high-frequency electric insertion vibrator to vibrate the concrete inside the steel membrane.

[0125] Quickly insert the vibrator and start vibrating from the bottom of the steel membrane. Vibrate at the same position for 30-40 seconds, move the vibrator 40-50 cm, continue vibrating for 30-40 seconds, and then move the vibrator 40-50 cm. The principle of moving the vibrator is to move horizontally first and then upward. Move the vibrator slowly until all the concrete is vibrated. Reinsert the vibrator at the bottom of the concrete and repeat the above steps. Repeat the vibration until the concrete no longer sinks significantly and no obvious bubbles appear. The vibration is complete.

[0126] (6) Maintenance

[0127] (1) Normal pressure steam curing

[0128] To accelerate the hardening of concrete and shorten production time, atmospheric pressure steam curing is used. The steel mold is placed in the steam curing tank, the ends of the steel mold are sealed, and the concrete is steam cured using the following process.

[0129] Warming up period: room temperature 85℃;

[0130] Heating time: controlled within 1.5~2 hours according to the initial temperature;

[0131] Constant temperature period: 85℃;

[0132] Constant temperature time: 2 hours;

[0133] Cooling period: 85℃45℃;

[0134] Cooling time: 0.5 hours.

[0135] (2) Maintenance in the maintenance room

[0136] After demoulding, the product is placed in a concrete high-temperature curing room for curing. The foundation is placed on upper and lower supports. The temperature is maintained at 40~42℃, the humidity RH is above 90%, and the curing time is 3 days. Its function is to promote the chemical reaction between the microsilica powder, fly ash and calcium dioxide in the concrete to form crystals, increase the density of the internal structure of the concrete, improve the strength of the concrete, and accelerate the hardening of the concrete.

[0137] (3) Natural maintenance

[0138] The prefabricated foundation needs to be naturally cured for 7 days after leaving the curing room. The upper end should be sealed and water should be sprinkled on the foundation every 5 to 6 hours. The watering should be even to ensure that there are no leaking parts of the foundation.

[0139] The present invention conducts strength tests on the prepared prefabricated high-strength concrete foundation, including compressive strength test, splitting tensile strength test and corrosion test. The details are as follows:

[0140] (1) Compressive strength test

[0141] (1) Test method

[0142] The test method was carried out in accordance with the Standard for Test Methods for Mechanical Properties of Ordinary Concrete (GB / T 50081-2019). The compressive strength test used 150mm×150mm×150mm cubic specimens, each cured simultaneously, in one group of three. The press was a WHY-5000 microcomputer-controlled electro-hydraulic servo expansion ring testing machine.

[0143] The steps for the cube compressive strength test are as follows:

[0144] a) Place the specimen on the lower platen of the testing machine. The pressure-bearing surface of the specimen should be perpendicular to the top surface during molding. The center of the specimen should be aligned with the center of the lower platen of the testing machine. Start the testing machine. When the upper platen approaches the specimen or steel backing plate, adjust the ball seat to ensure balanced contact.

[0145] b) During the test, the load should be applied continuously and evenly at a loading rate of 27 kN / s.

[0146] c) When the specimen approaches failure and begins to deform rapidly, stop adjusting the throttle of the testing machine until failure occurs. Then record the failure load.

[0147] The compressive strength of concrete cube is calculated according to the following formula:

[0148]

[0149] Where: —Compressive strength of concrete cube specimen (MPa);

[0150] F—specimen failure load (N);

[0151] A—Test piece pressure bearing area (mm 2 ).

[0152] The compressive strength value is the average value of the three test specimens, calculated to the nearest 0.1 MPa. If the difference between the maximum or minimum value and the median value is greater than 15%, the median value is used as the compressive strength value for the group of specimens. If the difference between the maximum and minimum values and the median value is greater than 15%, the test results are invalid.

[0153] (2) Test results

[0154] When loaded to 75% to 85% of the maximum load, a noticeable cracking sound was heard within the concrete specimens of the precast ultra-high-strength concrete foundation, which was being cured simultaneously. Before failure, the cracking sound persisted for a long time, and cracks appeared on the side surfaces of the specimens. After the cracks formed, the steel fibers bridging the cracks began to work, delaying their propagation. Furthermore, the extraction of the steel fibers from the concrete matrix consumes a significant amount of deformation energy, leading to a noisy and tearing sound before failure. During failure, fragments burst, followed by a loud bang and eventual failure, but the specimen remained largely intact. After compression failure, the specimen cracked without breaking apart, and remained intact, essentially maintaining its parallelepiped shape.

[0155] Test data table 1

[0156]

[0157] The compressive strength of the concrete specimens cured at the same time as the prefabricated ultra-high-strength concrete foundation is 103.51MPa, reaching the C100 concrete grade.

[0158] (2) Splitting tensile strength test

[0159] (1) Test method

[0160] The test method was carried out in accordance with the Standard for Test Methods for Mechanical Properties of Ordinary Concrete (GB / T 50081-2019). The tensile strength test used prefabricated ultra-high-strength concrete foundations, cured concurrently with the foundation, with three cubic specimens in each group (150 mm × 150 mm × 150 mm). The tensile testing machine used was a digital display universal materials testing machine.

[0161] The steps for the cube tensile strength test are as follows:

[0162] a) Place the specimen in the fixture, start the machine, adjust the upper pressure plate to close to the upper pressure surface of the pad, close the protective net, and prepare to apply the load.

[0163] b) Adjust the digital display so that the instrument is loaded evenly at a rate of 0.1 MPa / s until the specimen breaks and the load begins to decrease. When it drops to 100 kN, close the oil supply valve and open the oil return valve. Record the data

[0164] The compressive strength of concrete cube is calculated according to the formula:

[0165]

[0166] Where: —Concrete splitting tensile strength (MPa);

[0167] F—specimen failure load (N);

[0168] A—specimen splitting surface area (mm 2 ).

[0169] The tensile strength value is the average value of the three test specimens, calculated to the nearest 0.1 MPa. If the difference between the maximum or minimum value and the median value is greater than 15%, the median value is used as the tensile strength value for the group of specimens. If the difference between the maximum and minimum values and the median value is greater than 15%, the test results are invalid.

[0170] (2) Test results

[0171] Initially, the applied load gradually increased, and a crack distribution approximately in an inverted triangle formed in the middle of the top of the specimen, near the pad. Subsequently, the crack continued to extend and expand downward along the middle of the specimen. During this process, the fiber bridge connected the concrete matrix on both sides of the crack, slowing the rate of crack development. Finally, as the load continued to increase, a "bang bang" fracture sound was heard from the concrete specimen, forming a main crack that expanded downward, losing its bearing capacity and would not directly collapse into two pieces.

[0172] Test data table 2

[0173]

[0174] The tensile strength of the concrete specimens cured at the same time as the prefabricated ultra-high-strength concrete foundation is 8.1MPa, which has better ductility than ordinary concrete.

[0175] (3) Corrosion test

[0176] 1. Chloride ion penetration resistance test

[0177] (1) Test method

[0178] The test method is carried out in accordance with the electric flux method in the Standard for Test Methods for Long-term Performance and Durability of Ordinary Concrete GB / T50082-2009. The test uses cylindrical specimens with a diameter of (100±1) mm and a height of (50±2) mm. There are 1 group of specimens, with 3 specimens in each group.

[0179] The electric flux test utilizes the NEL-VJH intelligent concrete vacuum saturator and the NEL-PEU concrete electric flux meter. First, a cylindrical concrete specimen is vacuum-saturated with water. After removing excess water, the saturated specimen is mounted on a fixture and inspected for impermeability. NaCl solution (negative) and NaOH solution (positive) are then injected into the fixture's two terminals. The chloride ion permeability meter and the positive and negative terminals of the fixture are then connected to the test computer. The electric flux flowing through the specimen is measured over a six-hour period using NEL Lec test software to investigate the concrete's resistance to chloride ion penetration.

[0180] The specific test steps are as follows:

[0181] a) The electric flux test should be conducted using cylindrical specimens with a diameter of (100 ± 1) mm and a height of (50 ± 2) mm. The test should be conducted indoors at a temperature of (20-25)°C. First, the specimens, cured to the specified age, should be exposed to air until the surface is dry. The cylindrical sides of the specimens should then be coated with a silicone or resin sealant, and any holes in the coating should be filled.

[0182] b) Before the electric flux test, the specimen should be saturated with water under vacuum. First, place the specimen in a vacuum container. Then, start the vacuum pump and reduce the absolute pressure in the container to 1-5 kPa within 5 minutes. Maintain this vacuum for 3 hours. Then, while the vacuum pump is still running, inject enough distilled or deionized water to submerge the specimen. After the specimen has been immersed for 1 hour, return it to normal pressure and continue immersion for (18 ± 2) hours.

[0183] c) After vacuum saturation, remove the specimen from the water and wipe away any excess moisture. Maintain the relative humidity of the test environment above 95%. Install the specimen in the test chamber, using screws to clamp the two chambers and the specimen, which is fitted with vulcanized rubber pads. After the specimen is installed, check the seal between the specimen and the chamber using distilled water or other effective methods.

[0184] d) After checking the sealing between the specimen and the specimen groove, a NaCl solution with a mass concentration of 3.0% and a NaOH solution with a molar concentration of 0.3 mol / L should be injected into the test grooves on both sides of the specimen respectively. The copper mesh in the test groove injected with NaCl solution should be connected to the negative pole of the power supply, and the copper mesh in the test groove injected with NaOH solution should be connected to the positive pole of the power supply.

[0185] e) After properly connecting the power cord, maintain the test tank full of solution while turning on the power supply. Apply a constant voltage of (60±0.1) V DC to the two copper meshes, and record the initial current reading, I0. Initially, record the current value every 5 minutes. If the current value does not change much, record it every 10 minutes. If the current change is very small, record it every 30 minutes until the power is on for 6 hours.

[0186] f) When using a test device that automatically collects data, the interval for recording current can be set to 5-10 minutes. The current measurement value should be accurate to ±0.5 mA. The temperature of the solution in the test tank should be monitored simultaneously during the test.

[0187] g) After the test, the test solution should be drained out promptly, and the test tank should be rinsed with cold boiled water and detergent for more than 60 seconds, then rinsed with distilled water and blown dry with a hair dryer on the cold air setting.

[0188] The total electric flux of each specimen is calculated using the following simplified formula:

[0189]

[0190] Where: ——Total electric flux through the test piece with a diameter of φ 100mm (C);

[0191] I0——initial current (A), accurate to 0.001A;

[0192] I t ——Current (A) at time t (min), accurate to 0.001A;

[0193] According to the "Standard for Test Methods for Long-term Performance and Durability of Ordinary Concrete" GB / T 50082-2009, the electric flux of the specimen is converted into the electric flux value of a specimen with a diameter of 95 mm according to the formula.

[0194] ×

[0195] Where: —Electric flux through a test piece with a diameter of 95 mm (C);

[0196] ——Through the diameter (mm) Electric flux of the test piece (C);

[0197] ——The actual diameter of the specimen (mm). =100mm;

[0198] For each group of specimens, the arithmetic mean of the electric flux values of the three specimens should be taken as the test value for that group. When the difference between a single electric flux value and the median exceeds 15% of the median, the arithmetic mean of the electric flux values of the remaining two specimens should be taken as the test result for that group. When the difference between two measured values and the median exceeds 15% of the median, the median should be taken as the test result for that group.

[0199] (2) Test results

[0200] The electric flux test data are shown in Table 3;

[0201] Table 3 Electric flux test data

[0202]

[0203] As can be seen from the table, the measured electric flux values of the two groups of specimens are 37.9C and 46.4C respectively. According to the evaluation criteria in ASTM C1202, a standard test method for concrete resistance to chloride ion penetration developed by the American Society for Testing and Materials, an electric flux of less than 100C falls within the range of negligible chloride ion permeability, indicating that the concrete specimens cured at the same time as the ultra-high-strength concrete poles have very excellent resistance to chloride ion penetration, which can ensure the normal use of the ultra-high-strength concrete poles during their designed service life.

[0204] 2. Sulfate ion resistance test

[0205] During the same period, 100mm×100mm×100mm specimens were cured in four groups of three, each immersed in clean water and an 8% Na2SO4 solution for 30 and 90 days, respectively. The liquid should be at least 10mm above the specimen surface. After reaching the specified age, the concrete compressive strength was measured and the corrosion resistance coefficient K was calculated.

[0206] Corrosion resistance coefficient K value = compressive strength in solution R2 / compressive strength in clean water R1

[0207] Table 4

[0208]

[0209] It can be seen from the experimental values in Table 4 that the prefabricated ultra-high-strength concrete foundation has better resistance to sulfate ions.

[0210] In the above embodiments, the description of each embodiment has its own focus. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant description of other embodiments.

[0211] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for producing a prefabricated ultra-high strength concrete foundation for a transmission tower, characterized in that: The production method comprises: The steel bar skeleton tied according to preset requirements is placed into the steel formwork; Concrete slurry prepared according to preset requirements is poured into the steel mold; Vibrating the poured concrete slurry; Curing until a precast ultra-high strength concrete foundation is obtained; Conducting tests on the prepared prefabricated ultra-high strength concrete foundation; The prefabricated forming of the steel frame includes: The main reinforcement shall be made of stress-relieving ribbed steel bars with equal diameter, diameter ≥12mm, and clear distance ≥60mm. The main reinforcement shall be straightened and cut to a fixed length by a straightening machine. The requirements are as follows: no obvious bending visible to the naked eye; main reinforcement length error <2mm; strength loss <4% of the design value of tensile strength; The main reinforcement is configured throughout the entire length, and the main reinforcement is connected by double-sided lap welding, with a weld length of ≥140mm, and the welds of adjacent main reinforcements are not in the same plane; The ends of the main reinforcements that are configured throughout the length after welding use the forging heads of the steel bar forging machine, and the diameter of the forging heads is 1.8 times the diameter of the steel bar; The upper part of the main reinforcement is evenly arranged vertically along the main column. After the lower part enters the expanded bottom part for ≥80mm, the lower part of the main reinforcement is parallel to the side of the expanded bottom part at the first bending part. The second bending part at the end of the main reinforcement is bent outward to form a 180° hook. The length of the straight section at the end of the hook is not less than 5 times the diameter of the main reinforcement. The bending is performed using a steel bar bending machine, and no welding is allowed at the bending part. Stirrups are made of low-carbon cold-drawn round steel bars of equal diameter, with a diameter not less than 0.25 times the diameter of the main bars. They are formed on a skeleton forming machine, with the straight sections of the main bars straightened and tightened, and then annular inner stirrups are tied, followed by welding of spiral outer stirrups and annular outer stirrups. Annular inner stirrups and fully welded annular outer stirrups are tied at the upper end of the main bars, the first bend, and 80mm from the second bend. The annular inner stirrups and annular outer stirrups are staggered by a vertical distance of 4-5mm to ensure that the inner and outer stirrups are not in the same plane. In addition, annular inner stirrups are fully welded every 1000mm or less. Spiral outer stirrups are fully welded from the upper end of the main bars to the first bend, and annular outer stirrups are fully welded between the first bend and the second bend. The spacing between the annular outer stirrups is not more than 100mm. A steel ring mesh structure is set up at the bottom of the foundation. The outer ring steel bars are stress-relief ribbed steel bars, and the inner ring steel bars are cross-staggered steel bars. The longest staggered steel bars in the middle are stress-relief ribbed steel bars. The other steel bars are low-carbon cold-drawn round steel bars. The diameter of the stress-relief ribbed steel bars is ≥12mm and the diameter of the low-carbon cold-drawn round steel bars is not less than 0.25 times the diameter of the stress-relief ribbed steel bars. The intersections of the steel bars should be fully welded. The 180° hook at the end of the upper main reinforcement hooks the outer ring reinforcement and ties it together to increase the overall structural strength of the reinforcement skeleton; Among them, the design value of tensile strength of stress-relieved ribbed steel bars is ≥1040Mpa; the design value of tensile strength of low-carbon cold-drawn round steel bars is ≥510Mpa.

2. The method for producing a prefabricated ultra-high strength concrete foundation for a transmission tower according to claim 1, wherein: The preparation of the concrete slurry includes: Raw materials: including 130-150 parts of cement, 130-150 parts of quartz sand, 50-60 parts of crushed stone, 15-16 parts of water reducer, 50-55 parts of active admixture, 25-35 parts of toughening and reinforcing admixture, and the rest is clean neutral water. The water-binder ratio is 0.18; Ingredients: weigh cement, quartz sand, crushed stone, water reducer, active admixture, toughening and reinforcing admixture, and water according to the mix ratio; Mixing: Use a mixer to mix cement, quartz sand, gravel and active admixtures together and mix for 1 to 2 minutes; add the first water and water-reducing agent aqueous solution, with the first water volume and the water-reducing agent aqueous solution using a total of 2 / 3 water, and mix for 1 to 2 minutes; add toughening and reinforcing admixtures and part or all of the remaining water and mix for 2 to 3 minutes. The remaining water volume can be flexibly controlled according to the workability index; among which, the workability index: the slump is controlled at 7 to 11 cm according to the ambient temperature.

3. The method for producing a prefabricated ultra-high strength concrete foundation for a transmission tower according to claim 2, wherein: The concrete slurry vibration process includes: Use a high-frequency electric insert vibrator to vibrate the concrete slurry in the steel mold; Quickly insert the high-frequency electric insert vibrator, start vibrating from the bottom of the steel mold, and vibrate at the same position for 30~40s; move the high-frequency electric insert vibrator 40~50cm, and continue vibrating for 30~40s; move the high-frequency electric insert vibrator 40~50cm and continue vibrating; the principle of moving the high-frequency electric insert vibrator is to move horizontally first and then upward, and the high-frequency electric insert vibrator should be moved slowly until all the concrete slurry is vibrated; reinsert the high-frequency electric insert vibrator into the bottom of the concrete and repeat the above operation, repeat the vibration until the concrete no longer sinks obviously and no obvious bubbles appear, and the vibration is completed.

4. The method for producing a prefabricated ultra-high strength concrete foundation for a transmission tower according to claim 1, wherein: The curing until the prefabricated ultra-high strength concrete foundation is obtained comprises: Atmospheric pressure steam curing: Place the vibrated precast concrete foundation in the steam curing tank, seal the steel formwork ends, and perform normal pressure steam curing; Warming up period: room temperature rises to 85℃; Heating time: controlled within 1.5~2h according to the initial temperature; Constant temperature period: 85℃; Constant temperature time: 2h; Cooling period: 85℃ to 45℃; Cooling time: 0.5h; Maintenance room maintenance: After demoulding, the product is placed in a concrete high-temperature curing room for curing. The temperature is maintained at 40-42°C and the humidity is above 90%. The curing time is 2-4 days. This will promote the chemical reaction of substances in the concrete to form crystals, increase the density of the concrete's internal structure, improve the concrete's strength, and accelerate the hardening of the concrete. Natural maintenance: The precast concrete foundation needs to be naturally cured for 7 days after leaving the curing room. The upper end should be sealed and water should be sprinkled on the foundation every 5 to 6 hours. The watering should be even to ensure that there are no leaking parts of the foundation to obtain a precast ultra-high strength concrete foundation.

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