A preparation method of a centrifugal enhanced prestressed solid square pile

Abstract

The present application relates to the field of prefabricated concrete pile production, and specifically discloses a preparation method of a centrifugal reinforced prestressed solid square pile, comprising: mold pretreatment and cage reinforcement assembly, primary high-strength concrete distribution, mold closing and prestress tensioning, five-stage gradient centrifugal forming, residual slurry and impurity discharge, pile core high-strength concrete grouting and densification, and high-pressure steam curing. The present application uses gradient centrifugation to compact the outer layer of the pile body concrete layer by layer and uniformly arrange the aggregate, eliminates the soft interlayer of the pile core by discharging the residual slurry, and then fills the internal voids by secondary grouting of the pile core and low-speed centrifugal compaction, forming a full-face homogeneous structure with dense outer layer and high-strength core. Finally, the high-pressure steam curing strengthens the overall performance. The present application effectively solves the technical defects of uneven cross-section density and high core porosity caused by the traditional vibration process, significantly improves the compressive strength, bending performance and structural stability of the pile body, and is suitable for industrialized mass production.

Description

Technical Field

[0001] This invention belongs to the field of precast concrete pile production, specifically a method for preparing centrifugally reinforced prestressed solid square piles. Background Technology

[0002] Prestressed solid square piles are precast concrete vertical load-bearing components widely used in building pile foundations, municipal foundations, and transportation engineering. Compared with prestressed pipe piles, solid square piles have advantages such as better overall pile body integrity, high end face bearing capacity, large bending stiffness, and superior pull-out and settlement resistance. They are particularly suitable for complex working conditions such as heavy-load pile foundations for high-rise buildings, soft soil foundation treatment, bridge pier foundations, and heavy industrial plants. The existing production process for prestressed solid square piles is usually as follows: first, clean the mold and assemble the prestressed steel cage; then, pour C80 grade high-strength concrete in one go; use an immersion vibrator for manual compaction or an attached vibrator for external compaction; then close the mold, tension the prestressed steel bars, cure with atmospheric pressure steam, and then remove the mold to obtain the finished pile body.

[0003] The core flaw of the existing preparation method is that, in the traditional one-time vibratory compaction process, high-strength concrete is inherently limited by the vibration process. The effective range of the immersion vibrator is limited, and areas far from the vibration point (such as the four corners of the pile and the middle of a long pile) are difficult to achieve effective compaction. At the same time, the bottom and corners of the pile are prone to over-vibration due to concentrated vibration energy, leading to the settling and segregation of coarse aggregate. Meanwhile, the top and blind areas of vibration are under-vibrated, resulting in insufficient air bubbles and grout accumulation. This causes a severe uneven distribution of concrete density on the same cross-section of the pile—aggregate concentration and mortar shortage in over-vibrated areas, while under-vibrated areas have more grout and less aggregate and higher porosity, forming an anisotropic and heterogeneous structure. This cross-sectional inhomogeneity defect, caused by the inherent limitations of the vibration compaction process, directly results in inconsistent elastic deformation and plastic response between the dense and loose areas of the pile body when subjected to external loads. This increases the risk of local stress concentration and premature failure, weakens the overall bearing capacity and long-term service reliability of the pile body, and makes it difficult to meet the design requirements of modern super high-rise buildings and heavy industrial pile foundations for highly homogeneous pile structures. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing centrifugally reinforced prestressed solid square piles, thereby solving the problems in the prior art where uneven distribution of concrete aggregate in the pile cross-section, density stratification, and anisotropic heterogeneity caused by the vibration compaction process result in localized weakness in the pile's mechanical properties and insufficient overall bearing stability.

[0005] This invention provides a method for preparing centrifugally reinforced prestressed solid square piles, comprising the following steps:

[0006] 1) Mold pretreatment and cage reinforcement assembly: Provide a solid square pile forming mold with an openable sealing plate at the tail end. Clean the mold, apply release agent, and hoist and position the prestressed cage reinforcement inside the mold.

[0007] 2) Initial concrete placement: Place the foundation concrete into the mold after the reinforcement cage is assembled, with the placement amount being 75% to 80% of the volume of the finished pile body concrete;

[0008] 3) Mold closing and prestressing tensioning: The mold is closed and locked, and the prestressed cage reinforcement is tensioned and locked;

[0009] 4) Gradient centrifugal molding: The mold is hoisted into the centrifuge and centrifuged using a gradient speed change process including low speed, medium speed, high speed, medium speed and low speed, so that the concrete is layered and dense and excess paste is expelled.

[0010] 5) Excess slurry discharge: After centrifugation, open the tail end of the mold to discharge the excess slurry and impurities separated by centrifugation, and clean the cavity of the pile core.

[0011] 6) Secondary grouting and compaction of pile core: High-strength concrete is pumped into the cavity of the pile core, and secondary centrifugal compaction is carried out after the material is delivered.

[0012] 7) High-pressure steam curing: The pile mold after secondary compaction is sent into the high-pressure steam curing facility for steam curing. After steam curing is completed, the mold is removed to obtain the finished product.

[0013] Preferably, the gradient speed change process in step 4) includes the following steps in sequence:

[0014] The first low-speed stage is 60-75 r / min, running for 3-4 minutes;

[0015] The first medium-speed stage is 220-250 r / min, running for 2 minutes;

[0016] During the high-speed phase, the rotation speed is 400-430 r / min, and the operation lasts for 3-4 minutes.

[0017] The second medium-speed stage is 220-250 r / min, running for 2 minutes;

[0018] The second low-speed stage is at a speed of 60-75 r / min, running until the machine stops smoothly.

[0019] Preferably, the high-strength concrete in step 6) is C80 grade concrete, the pumping pressure is 0.8 to 1.2 MPa, the rotation speed of the secondary centrifugal compaction is 50 to 65 r / min, and the compaction time is 2 to 3 min.

[0020] Preferably, the process parameters for high-pressure steam curing in step 7) are: steam curing pressure 0.8-1.0 MPa, steam curing temperature 170-185℃, and constant temperature steam curing time 8-12h.

[0021] Preferably, the foundation concrete in step 2) is C80 grade concrete.

[0022] Preferably, the tensioning in step 3) is performed using a graded tensioning method, with the tensioning stress controlled according to the design value, and the load is held and locked after tensioning.

[0023] Preferably, in step 5), after opening the tail end of the mold, the step further includes cleaning the laitance adhering to the inner wall of the pile core cavity.

[0024] Preferably, in step 4), gradient acceleration and deceleration control is used throughout the centrifugation process to avoid sudden stops and starts.

[0025] Preferably, in step 6), the high-strength concrete adopts a low heat of hydration and high density mix ratio.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The five-stage gradient centrifugal process uses a variable speed curve of low speed, medium speed, high speed, medium speed, and low speed to adjust the centrifugal force step by step, so that the concrete is compacted layer by layer from the outside to the inside along the pile body section and the aggregate is evenly distributed. This effectively avoids the layering defect of dense bottom and loose top caused by natural settlement of aggregate in the traditional vibration process, and achieves a high degree of uniformity of density of the entire pile body section. It completely eliminates the problem of anisotropic non-homogeneity, and makes the mechanical properties of all parts of the pile body balanced, without any local weak links.

[0028] The specially designed residual slurry discharge and pile core cleaning process after centrifugation thoroughly removes excess cement slurry, free water, and fine impurities separated by centrifugation. It also eliminates the weak interlayer formed by residual slurry on the inner wall of the pile core cavity in traditional processes, providing a clean and regular bonding interface for subsequent secondary grouting and ensuring a strong bond and integrity between the outer concrete layer and the core grouting layer.

[0029] By combining high-pressure grouting of the pile core with secondary low-speed centrifugation, high-strength concrete is pumped into the pile core cavity and then centrifuged twice to compact it. This ensures that the pile core concrete is fully filled, uniformly compacted, and tightly bonded to the outer foundation concrete. This completely solves the inherent defects of loose core and high porosity in solid square piles, forming a full-section homogeneous structure with a dense and wear-resistant outer layer and a high-strength and dense core. This significantly improves the pile's compressive strength, bending resistance, and impact resistance.

[0030] This invention also uses a high-pressure constant-temperature steam curing process to accelerate the hydration reaction of concrete under high temperature and high pressure, promote the full hydration of cement mineral components, generate a denser hydration product structure, significantly improve the overall strength index and structural stability of the pile, and at the same time ensure low dispersion and high consistency of pile strength. Attached Figure Description

[0031] Figure 1 This is a flowchart illustrating the preparation method of the centrifugally reinforced prestressed solid square pile of the present invention. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Example 1: This example provides a method for preparing centrifugally reinforced prestressed solid square piles, such as... Figure 1 As shown, the preparation of a C80 grade prestressed solid square pile with a side length of 400mm and a pile length of 12m includes the following steps:

[0034] 1) Mold pretreatment and cage reinforcement assembly

[0035] Select a special forming mold for irregular square piles that matches the specifications of the solid square piles to be prepared. The mold consists of an upper mold and a lower mold, and the inner cavity has a square cross section.

[0036] First, a high-pressure water gun and a special scraper are used to thoroughly clean the residual concrete blocks, slag, and dust inside the mold cavity, ensuring that the mold cavity is clean, flat, and free of debris.

[0037] Then, using a spraying device, a high-performance water-based release agent is evenly sprayed onto all surfaces of the mold cavity, with the coating amount controlled at 15-20 g / m². 2 Ensure even coating, without accumulation or missed areas.

[0038] Open the upper mold and hoist the pre-processed and inspected prestressed cage reinforcement into the lower mold using a crane. Adjust the position of the cage reinforcement to center it and fix it with a special positioning clamp to ensure that the spacing between the cage reinforcement and the four walls of the mold is uniform. The protective layer thickness is controlled at 35mm. The center line of the cage reinforcement in the length direction coincides with the center line of the mold in the length direction, with a deviation of no more than ±2mm. Only after confirming that there is no offset or sticking to the mold can the next process be carried out.

[0039] 2) Initial placement of high-strength concrete

[0040] C80 grade prestressed concrete was used as the outer foundation concrete.

[0041] The concrete mix proportions by weight are as follows: 480 parts of P·II 52.5 grade silicate cement, 650 parts of medium-coarse sand (fineness modulus 2.8), 1150 parts of crushed stone (particle size 5-25mm, parent rock strength 135MPa), 8.5 parts of high-efficiency water-reducing agent (polycarboxylate system, water reduction rate ≥30%), 140 parts of water, 80 parts of slag powder (S95 grade), and 60 parts of fly ash (II grade F).

[0042] The concrete mixing time should be no less than 120 seconds, and the slump at the discharge point should be controlled at 60-80 mm.

[0043] A walking feeder is used to uniformly and continuously place the high-strength concrete into the mold along the length of the mold. The thickness of the concrete is controlled to be 1 / 2 to 2 / 3 of the mold depth, and the total amount of concrete is calculated as 75% to 80% of the volume of the finished pile body concrete.

[0044] The concrete is laid from one end of the mold to the other, with the speed controlled at 0.3 to 0.5 m / min to ensure uniform spreading of the concrete and avoid local accumulation, hollow areas, or segregation.

[0045] After the material is laid, use a vibrating scraper to smooth it along the top edge of the mold to make the concrete surface flat.

[0046] 3) Mold closing and prestressing tensioning

[0047] After the fabric is finished, the upper mold is hoisted above the lower mold, aligned with the mold closing positioning pin, and then slowly lowered to completely close the upper and lower molds.

[0048] Specialized locking bolts and clips are used to tighten the mold in all directions, alternating from the middle to both ends, with the tightening torque controlled between 180 and 220 N·m.

[0049] Inspect the joints of the mold to ensure a tight seal and no risk of grout leakage.

[0050] Tensioning is performed on the prestressed steel cage.

[0051] The prestressed steel bars are low-relaxation spiral channel steel bars for prestressed concrete with a nominal diameter of 10.7 mm and a standard value of 1420 MPa for tensile strength.

[0052] The tensioning equipment uses a through-hole type jack. Before tensioning, the jack and oil pressure gauge are calibrated.

[0053] Tensioning is controlled according to the design tension control stress of 994 MPa (i.e., 0.7 times the standard value of tensile strength), and a staged tensioning method is adopted:

[0054] The first stage of tensioning is carried out to 50% of the control stress, and the load is held for 2 minutes.

[0055] The second stage of tensioning is carried out to 80% of the control stress and held for 1 minute.

[0056] The third stage of tensioning is carried out to 100% of the control stress, and the anchor is locked after holding the load for 3 minutes.

[0057] During the tensioning process, the tension force is the primary factor and the elongation value is the secondary factor for dual control. The deviation between the measured elongation value and the theoretical elongation value is controlled within ±5%.

[0058] 4) Five-segment gradient centrifugal molding

[0059] After the pile mold is tensioned and locked, it is hoisted onto the centrifuge workbench and a special clamp is used to lock and fix the two ends and the middle of the mold to ensure that the mold does not move during the centrifugation process.

[0060] Start the centrifuge and complete the initial centrifugation process using the following five-stage gradient speed change process:

[0061] First stage: Run at a low speed of 65r / min for 4 minutes to allow the concrete inside the mold to be evenly spread under low centrifugal force, fully filling the four corners and irregular cavities of the mold, and eliminating dead corners in the concrete distribution.

[0062] Second stage: Run at a medium speed of 230r / min for 2 minutes to allow the concrete to be initially compacted and evenly adhered to the formwork wall under the action of medium centrifugal force, while expelling most of the free air bubbles trapped inside the concrete.

[0063] The third stage: running at a high speed of 410 r / min for 3 minutes, under the action of high-speed centrifugal force, excess slurry, free water and fine impurities in the concrete quickly migrate outward and precipitate, and the outer layer of concrete of the pile body achieves high-density dehydration and shaping.

[0064] Fourth stage: Run at a medium speed of 230r / min for 2 minutes to slowly release centrifugal stress and avoid internal stress cracking or delamination of the formed concrete caused by sudden stopping after high-speed operation.

[0065] Fifth stage: Run at a low speed of 65 r / min until the centrifuge stops completely, with a smooth deceleration throughout the process.

[0066] During the centrifugation process described above, the acceleration / deceleration times of the centrifuge from low to medium speed, from medium to high speed, and from high to medium and from medium to low speed were all controlled within 30–45 seconds, with no sudden stops or starts throughout the process. For a square-section mold, the four corners are subjected to the superposition of centrifugal forces from two orthogonal directions in the centrifugal force field, resulting in a more significant actual compaction effect in the corner areas. Combined with sufficient material distribution during the low-speed stage, this ensures that the corner concrete also achieves a good compaction effect.

[0067] 5) Discharge of residual slurry and impurities

[0068] After the centrifuge has completely stopped, let the pile mold stand for 3 minutes to allow the grout inside the pile to stabilize and separate into layers.

[0069] Open the plugging plate at the tail end of the mold, slowly tilt the pile mold, so that the excess cement slurry, free water and fine loose impurities that have been centrifuged in the pile core cavity can be completely poured out from the tail end under the action of gravity. The slurry discharge time should be controlled within 2 to 3 minutes.

[0070] After the grouting is completed, a special long-handled scraper is used in conjunction with a high-pressure water gun to penetrate into the cavity of the pile core and thoroughly clean the loose attachments such as laitance and soft interlayers on the inner wall of the cavity. After cleaning, the inner wall of the cavity is dried with compressed air to ensure that the cavity is clean, regular and free of any attached weak layers. After cleaning, a visual inspection should be conducted to ensure that there is no obvious laitance residue on the inner wall of the cavity.

[0071] 6) The C80 high-strength concrete grouting in the pile core is dense.

[0072] Keep the mold closed and locked, and use high-pressure concrete pumping equipment to extend the pumping pipe into the bottom of the pile core cavity.

[0073] The pumping pipe is made of 40mm diameter hard steel pipe, and the pipe opening is about 50mm from the bottom of the cavity.

[0074] The pumped C80 high-strength special concrete adopts a low heat of hydration and high density mix proportion, which is as follows by weight: 500 parts of P·II52.5 grade silicate cement, 620 parts of medium and coarse sand (fineness modulus 2.6), 1120 parts of crushed stone (particle size 5-20mm, parent rock strength 140MPa), 9.0 parts of high-efficiency water-reducing agent (polycarboxylate), 135 parts of water, 30 parts of silica fume, and 70 parts of slag powder (S95 grade).

[0075] The slump of this concrete mix is ​​controlled at 160–200 mm (under pumping conditions), and the hydration heat temperature rise over 3 hours should not exceed 25°C.

[0076] Start the pumping equipment, set the pumping pressure to 1.0 MPa, and fill the pile core with C80 high-strength concrete at a uniform rate from bottom to top, controlling the pumping speed between 0.5 and 0.8 m / s. 3 / min, so that the air in the cavity is completely discharged from the exhaust port at the tail end.

[0077] When a uniform flow of concrete slurry is observed from the tail end vent hole, stop pumping, pull out the pumping pipe, and immediately cover and lock the tail end plugging mold.

[0078] Start the centrifuge and run it at a low speed of 60 r / min for 2 minutes to further compact the C80 concrete injected into the pile core using slight centrifugal force. It should be noted that at this time, although the outer foundation concrete has been centrifuged and dehydrated, it is still in a plastic state before initial setting and has not yet undergone high-pressure steam curing. Therefore, it has good chemical affinity and physical interpenetration conditions with the fresh concrete pumped into the pile core. Under the action of low-speed centrifugal force, the two layers of concrete permeate and interlock at the interface, eventually fusing into a whole structure.

[0079] After compaction is completed, stop the machine and check that there is no leakage of grout at both ends of the pile formwork.

[0080] 7) High-pressure steam curing intensity enhancement

[0081] The pile molds that have undergone secondary compaction are hoisted and moved to a closed high-pressure steam curing tank, and placed stably on the support frame inside the steam curing tank, ensuring that there is a gap of not less than 50mm between the pile molds to allow for steam circulation.

[0082] Close the sealed door of the steam curing tank, start the steam generation system, and perform high-pressure steam curing according to the following procedures:

[0083] Heating phase: The temperature in the steam curing tank is raised from room temperature to 175℃ at a heating rate of 20-25℃ / h, while the pressure is raised to 0.9MPa simultaneously. The heating time is about 6-7 hours.

[0084] Constant temperature and pressure stage: Maintain the pressure in the steam curing tank at 0.9 MPa (gauge pressure) and the temperature at 175℃ for 10 hours.

[0085] The entire steam curing process uses an automatic control system to monitor temperature and pressure in real time, with temperature fluctuations controlled within ±3℃ and pressure fluctuations controlled within ±0.02MPa.

[0086] Cooling and depressurization stage: Stop steam supply and adopt natural cooling method, with the cooling rate controlled at 15-20℃ / h. When the temperature in the pool drops below 60℃, slowly open the exhaust valve to release pressure to atmospheric pressure, with the depressurization rate controlled at 0.05-0.08MPa / h. The entire cooling and depressurization process shall not be less than 6 hours.

[0087] After steam curing is completed, open the steam curing tank door, lift out the pile mold, and remove it after it cools naturally to room temperature.

[0088] When dismantling the formwork, first remove the locking bolts and clips, then use special lifting equipment to lift the upper formwork and remove the finished pile from the lower formwork.

[0089] Repair the finished pile body, remove the cement slurry adhering to the surface of the end plate and the prestressed steel anchor head, and mark the pile body according to the design requirements.

[0090] Upon inspection, the finished pile body has a smooth and dense appearance, without honeycomb pitting, circumferential or longitudinal cracks, and a flat end face.

[0091] The centrifugally reinforced prestressed solid square pile prepared in this embodiment was tested and found that the compressive strength of the pile concrete reached the C80 standard, the density of each section of the pile was uniform, there were no internal pores or voids, and all mechanical properties met the design requirements.

[0092] Example 2: This example provides a method for preparing a centrifugally reinforced prestressed solid square pile, which prepares a C80 grade prestressed solid square pile with a side length of 450mm and a pile length of 15m. The difference between this example and Example 1 is that the specific process parameters of each process have been adjusted.

[0093] 1) The operation method of mold pretreatment and cage reinforcement assembly is the same as in Example 1, but the thickness of the cage reinforcement protective layer is controlled to be 40mm.

[0094] 2) The initial high-strength concrete placement uses the same C80 grade prestressed concrete mix ratio as in Example 1. The placement amount is controlled at 78% of the finished concrete volume, and vibration placement is used to assist the concrete in filling the corners of the mold.

[0095] 3) In the mold closing and prestressing tensioning steps, the prestressing steel bars are prestressed concrete low-relaxation spiral channel steel bars with a nominal diameter of 12.6mm. The designed tensioning control stress is also 994MPa, and the graded tensioning method is the same as in Example 1.

[0096] 4) In the five-stage gradient centrifugal molding process, the centrifugal speed and time are adjusted as follows: low speed 70r / min for 4min → medium speed 240r / min for 2min → high speed 420r / min for 4min → medium speed 240r / min for 2min → low speed 70r / min for a smooth stop.

[0097] 5) The operation method for discharging residual slurry and impurities is the same as in Example 1.

[0098] 6) During the grouting and compaction of the C80 high-strength concrete in the pile core, the pumping pressure is set to 1.1 MPa, and the pumping speed is controlled between 0.6 and 0.9 m / s. 3 The secondary low-speed compaction speed is 65 r / min, and the compaction time is 3 min.

[0099] The mix proportions of the core grouting concrete are the same as in Example 1.

[0100] 7) In the high-pressure steam curing strength enhancement step, the steam curing pressure is controlled at 0.95MPa, the steam curing temperature is controlled at 180℃, and the constant temperature and pressure steam curing time is 11h.

[0101] The heating rate is controlled at 20℃ / h, and the cooling rate is controlled at 18℃ / h.

[0102] The finished pile body prepared in this embodiment is of good quality and is especially suitable for the preparation of large-size solid square piles. It has high molding accuracy, tight bonding between the core and the outer concrete layer, and excellent overall mechanical properties.

[0103] Example 3: This example provides a method for preparing a centrifugally reinforced prestressed solid square pile, which prepares a C80 grade prestressed solid square pile with a side length of 350mm and a pile length of 10m. The difference between this example and Example 1 is that the specific process parameters of each process have been adjusted.

[0104] 1) The operation method of mold pretreatment and cage reinforcement assembly is the same as in Example 1, and the thickness of the cage reinforcement protective layer is controlled at 35mm.

[0105] 2) The initial high-strength concrete placement ratio is the same as in Example 1, and the placement amount is controlled at 76% of the finished concrete volume.

[0106] 3) In the mold closing and prestressing tensioning steps, the prestressing steel bars are low-relaxation spiral channel steel bars for prestressed concrete with a nominal diameter of 9.0 mm. The designed tensioning control stress is 994 MPa, and the graded tensioning method is the same as in Example 1.

[0107] 4) In the five-stage gradient centrifugation molding process, the centrifugation speed and time are adjusted as follows: low speed 60r / min for 3min → medium speed 220r / min for 2min → high speed 400r / min for 3min → medium speed 220r / min for 2min → low speed 60r / min for a smooth stop.

[0108] 5) The operation method for discharging residual slurry and impurities is the same as in Example 1.

[0109] 6) During the grouting and compaction of the C80 high-strength concrete in the pile core, the pumping pressure is set to 0.8 MPa, and the pumping speed is controlled between 0.4 and 0.6 m / s. 3 The secondary low-speed compaction speed is 50 r / min, and the compaction time is 2 min.

[0110] 7) In the high-pressure steam curing strength enhancement step, the steam curing pressure is controlled at 0.8MPa, the steam curing temperature is controlled at 170℃, and the constant temperature and pressure steam curing time is 8h.

[0111] The finished pile body prepared in this embodiment meets the design requirements and is suitable for the efficient preparation of smaller solid square piles.

[0112] Example 4: This example provides a method for preparing a centrifugally reinforced prestressed solid square pile, which prepares a C80 grade prestressed solid square pile with a side length of 500mm and a pile length of 18m. The difference between this example and Example 1 is that the specific process parameters of each process have been adjusted.

[0113] 1) The operation method of mold pretreatment and cage reinforcement assembly is the same as in Example 1, and the thickness of the cage reinforcement protective layer is controlled to be 40mm.

[0114] 2) The initial high-strength concrete mix proportion is the same as in Example 1, and the amount of concrete mix is ​​controlled at 80% of the volume of the finished concrete.

[0115] 3) In the mold closing and prestressing tensioning steps, the prestressing steel bars are prestressed concrete low-relaxation spiral channel steel bars with a nominal diameter of 14.0 mm. The designed tensioning control stress is 994 MPa, and the graded tensioning method is the same as in Example 1.

[0116] 4) In the five-stage gradient centrifugal molding process, the centrifugal speed and time are adjusted as follows: low speed 75r / min for 4min → medium speed 250r / min for 2min → high speed 430r / min for 4min → medium speed 250r / min for 2min → low speed 75r / min for a smooth stop.

[0117] 5) The operation method for discharging residual slurry and impurities is the same as in Example 1.

[0118] 6) During the grouting and compaction of the C80 high-strength concrete in the pile core, the pumping pressure is set to 1.2 MPa, and the pumping speed is controlled between 0.7 and 1.0 m / s. 3 The secondary low-speed compaction speed is 65 r / min, and the compaction time is 3 min.

[0119] 7) In the high-pressure steam curing strength enhancement step, the steam curing pressure is controlled at 1.0 MPa, the steam curing temperature is controlled at 185℃, and the constant temperature and pressure steam curing time is 12h.

[0120] The finished pile body prepared in this embodiment is of excellent quality and is suitable for the industrial production of large-diameter long piles.

[0121] Comparative Example 1: To verify the technical effect of the present invention, this comparative example provides a traditional method for preparing solid square piles using vibration molding. Using a mold of the same specifications as in Example 1 (400mm side length) and C80 grade concrete with the same mix proportions, the concrete is placed in a single pass, and then manually compacted at the four corners and center of the mold using an immersion vibrator. The compaction time is controlled at 3-5 minutes per point. After compaction, the mold is closed, and without centrifugal molding or secondary grouting, it is directly cured under normal pressure steam (curing temperature 80℃, constant temperature curing for 12 hours). After demolding, the finished pile body is obtained.

[0122] Comparative Example 2: To verify the technical effect of the secondary grouting and compaction step in this invention, this comparative example provides a method for preparing solid square piles by centrifugal molding without secondary grouting. The difference from Example 1 is that after the residual grout is discharged in step 5), the high-strength concrete grouting and compaction of the pile core in step 6) is not performed; instead, high-pressure steam curing in step 7) is performed directly. All other steps and parameters are the same as in Example 1.

[0123] Comparative test results

[0124] The solid square piles prepared in Examples 1-4 and Comparative Examples 1-2 were subjected to mechanical property testing under the same conditions. The test results are shown in the table below:

[0125] Testing items Example 1 Example 2 Example 3 Example 4 Comparative Example 1 (Traditional Vibration) Comparative Example 2 (without secondary grouting) Concrete compressive strength (MPa) 86.5 87.2 85.9 88.1 72.3 79.6 Uniformity of pile cross-section density Uniform, without layering Uniform, without layering Uniform, without layering Uniform, without layering There are obvious layers, with a dense bottom and a loose top. The outer layer is dense, while the core is loose. Internal defects (ultrasonic inspection) No cavities, no honeycombs No cavities, no honeycombs No cavities, no honeycombs No cavities, no honeycombs Multiple honeycomb and air pores The core contains a continuous loose layer Bending capacity (kN·m) 256 298 210 345 186 215 Cracking moment (kN·m) 168 195 138 225 112 135 Vertical compressive bearing capacity of a single pile (kN) 4280 4850 3520 5620 3560 3850 Coefficient of variation (compressive strength) 0.032 0.028 0.035 0.025 0.158 0.112

[0126] The above test results show that:

[0127] (1) The centrifugally reinforced prestressed solid square piles prepared in Examples 1 to 4 all achieved or exceeded the C80 grade design requirements for concrete compressive strength. Moreover, the strength differences and dispersion coefficients among the examples were small, indicating that the process of the present invention is stable and reliable and the product quality is consistent. Compared with the traditional vibration molding comparative example 1, the compressive strength was increased by about 15% to 22%.

[0128] (2) The pile sections prepared in Examples 1 to 4 have uniform density, and ultrasonic testing did not reveal internal voids, honeycomb, or other defects, indicating that the gradient centrifugation + secondary grouting combination process of the present invention completely eliminates the technical defects of cross-sectional stratification and core looseness. In Comparative Example 1, the traditional vibration process resulted in obvious cross-sectional stratification due to the natural settlement of aggregates; in Comparative Example 2, the core had a continuous loose layer because secondary grouting was not performed.

[0129] (3) The solid square piles prepared in each embodiment of the present invention are significantly superior to Comparative Examples 1 and 2 in terms of bending bearing capacity, cracking moment and vertical compressive bearing capacity. Taking Example 1 as an example, its bending bearing capacity is increased by about 37.6% compared with Comparative Example 1 and by about 19.1% compared with Comparative Example 2, which fully demonstrates the synergistic effect between the various processes of the present invention.

[0130] (4) The strength dispersion coefficients (0.025 to 0.035) of each embodiment of the present invention are much lower than those of Comparative Example 1 (0.158) and Comparative Example 2 (0.112), indicating that the mechanical properties of the pile body prepared by the process of the present invention are highly uniform, eliminating local weak links and greatly improving the safety of engineering service.

[0131] The embodiments of the present invention are given for the purposes of illustration and description. Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions and variations made by those skilled in the art to the above embodiments within the scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a centrifugally reinforced prestressed solid square pile, characterized in that, Includes the following steps: 1) Mold pretreatment and cage reinforcement assembly: Provide a solid square pile forming mold with an openable sealing plate at the tail end. Clean the mold, apply release agent, and hoist and position the prestressed cage reinforcement inside the mold. 2) Initial concrete placement: Place the foundation concrete into the mold after the reinforcement cage is assembled, with the placement amount being 75% to 80% of the volume of the finished pile body concrete; 3) Mold closing and prestressing tensioning: The mold is closed and locked, and the prestressed cage reinforcement is tensioned and locked; 4) Gradient centrifugal molding: The mold is hoisted into the centrifuge and centrifuged using a gradient speed change process including low speed, medium speed, high speed, medium speed and low speed, so that the concrete is layered and compacted and excess paste is expelled. 5) Excess slurry discharge: After centrifugation, open the tail end of the mold to discharge the excess slurry and impurities separated by centrifugation, and clean the cavity of the pile core. 6) Secondary grouting and compaction of pile core: High-strength concrete is pumped into the cavity of the pile core, and secondary centrifugal compaction is carried out after the material is delivered. 7) High-pressure steam curing: The pile mold after secondary compaction is sent into the high-pressure steam curing facility for steam curing. After steam curing is completed, the mold is removed to obtain the finished product.

2. The method for preparing a centrifugally reinforced prestressed solid square pile according to claim 1, characterized in that, Step 4) describes the gradient speed change process, which includes the following steps: The first low-speed stage is 60-75 r / min, running for 3-4 minutes; The first medium-speed stage is 220-250 r / min, running for 2 minutes; During the high-speed phase, the rotation speed is 400-430 r / min, and the operation lasts for 3-4 minutes. The second medium-speed stage is 220-250 r / min, running for 2 minutes; The second low-speed stage is at a speed of 60-75 r / min, running until the machine stops smoothly.

3. The method for preparing a centrifugally reinforced prestressed solid square pile according to claim 1, characterized in that, The high-strength concrete mentioned in step 6) is C80 grade concrete, the pumping pressure is 0.8 to 1.2 MPa, the rotation speed of the secondary centrifugal compaction is 50 to 65 r / min, and the compaction time is 2 to 3 min.

4. The method for preparing a centrifugally reinforced prestressed solid square pile according to claim 1, characterized in that, The process parameters for high-pressure steam curing described in step 7) are: steam curing pressure 0.8-1.0 MPa, steam curing temperature 170-185℃, and constant temperature steam curing time 8-12h.

5. The method for preparing a centrifugally reinforced prestressed solid square pile according to claim 1, characterized in that, The foundation concrete mentioned in step 2) is C80 grade concrete.

6. The method for preparing a centrifugally reinforced prestressed solid square pile according to claim 1, characterized in that, The tensioning described in step 3) adopts a graded tensioning method, the tensioning stress is controlled according to the design value, and the load is held and locked after tensioning.

7. The method for preparing a centrifugally reinforced prestressed solid square pile according to claim 1, characterized in that, In step 5), after opening the tail end of the mold, the step of cleaning the laitance adhering to the inner wall of the pile core cavity is also included.

8. The method for preparing a centrifugally reinforced prestressed solid square pile according to claim 1, characterized in that, In step 4), during the centrifugation process, gradient acceleration and deceleration control is used throughout to avoid sudden stops and starts.

9. The method for preparing a centrifugally reinforced prestressed solid square pile according to claim 1, characterized in that, In step 6), the high-strength concrete adopts a low heat of hydration and high density mix ratio.

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