Method for microwave steam combined curing of ultra-high performance concrete test piece

Abstract

This invention discloses a method for microwave-steam combined curing of ultra-high performance concrete specimens, relating to the field of ultra-high performance concrete curing technology. The method includes: molding the ultra-high performance concrete specimen, covering it with a film, and allowing it to stand until it reaches a preset initial strength; after demolding, placing it in a microwave-steam combined curing environment, first introducing steam to a first humidity range, and then curing with low-frequency microwaves within a first temperature range for a first duration; then increasing the steam to a second humidity range, increasing the microwave frequency and power, and using intermittent radiation within a second temperature range to enhance curing for a second duration; finally, stopping the steam, controlling the cooling rate, and supplementing with low-power intermittent microwaves to bring the specimen to room temperature. This invention employs a gradient microwave-steam curing scheme, dividing the curing process into four stages: natural pre-curing, low-frequency microwave pre-excitation, high-frequency microwave enhanced excitation, and microwave passivation cooling, achieving a shorter curing time and improved compressive strength under normal pressure.

Description

Technical Field

[0001] This invention relates to the field of ultra-high performance concrete curing technology, and in particular to a method for microwave steam combined curing of ultra-high performance concrete specimens. Background Technology

[0002] Ultra-high performance concrete (UHPC) is a new type of cement-based composite material with ultra-high strength, high toughness, and high durability, playing an important role in long-span bridges, prefabricated buildings, and special protective engineering. The curing method is one of the core factors determining the final performance of UHPC. Currently, conventional UHPC curing methods include natural curing, atmospheric pressure steam curing, and high-temperature and high-pressure curing. These methods suffer from problems such as long curing times, expensive equipment, high energy consumption, and a tendency to cause temperature stress cracks in UHPC specimens. Summary of the Invention

[0003] The purpose of this invention is to provide a method for microwave steam curing of ultra-high performance concrete specimens, aiming to solve or improve at least one of the above-mentioned technical problems.

[0004] To achieve the above objectives, the present invention provides the following solution: A method for microwave-steam combined curing of ultra-high performance concrete specimens includes: Step 1: After the ultra-high performance concrete specimen is formed, cover the specimen and the mold with waterproof plastic, and place the specimen and the mold together in a standard environment until the specimen reaches the preset initial strength. Step 2: Place the demolded specimen in a microwave-steam combined curing environment. First, introduce steam to the first relative humidity range, and then use low-frequency microwaves for irradiation curing. Control the temperature within the first temperature range and set the curing time to the first duration. Step 3: Increase the steam flow rate to raise the relative humidity of the environment to the second relative humidity range, and at the same time increase the microwave frequency and power to high frequency microwave parameters. Use intermittent radiation to strengthen the curing of the specimen, control the temperature within the second temperature range, and set the curing time to the second duration. Step 4: Stop the steam supply, reduce the ambient temperature according to the set cooling rate, and at the same time use low-power intermittent microwave radiation treatment until the specimen temperature drops to room temperature.

[0005] Optionally, the initial strength in step 1 is 5–15 MPa; the standard environmental conditions are a temperature of 20 ± 2 °C and a relative humidity of not less than 98%.

[0006] Optionally, the first relative humidity range in step 2 is 70%–85%; the frequency of the low-frequency microwave is 550–750 MHz, and the radiation power density is 1.5–3.2 W / cm².3 The first temperature range is 50℃~65℃; the first duration is 8~12h.

[0007] Optionally, the low-frequency microwave radiation in step 2 is symmetrically directed towards each other along the four sides of the ultra-high performance concrete specimen.

[0008] Optionally, the second relative humidity range in step 3 is not less than 98%; the frequency of the high-frequency microwave is 750–1050 MHz, and the radiation power density is 5.0–8.2 W / cm². 3 The second temperature range is 60℃~85℃; the second duration is 18~24h.

[0009] Optionally, the intermittent radiation method in step 3 is as follows: each microwave radiation session lasts 20 to 30 minutes, and there is a 10 to 20 minute pause after each radiation session; the microwave radiation direction is symmetrical and opposite to each other along the four sides of the ultra-high performance concrete specimen.

[0010] Optionally, the cooling rate in step 4 is 8–15 °C / h; the low-power intermittent microwave radiation uses a frequency of 550–750 MHz and a radiation power density of 0.8–1.5 W / cm². 3 The microwave radiation lasts for 10 to 15 minutes each time, followed by an 8 to 20 minute pause after each radiation session. This process is repeated until the cooling process is complete.

[0011] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects: This invention discloses a method for microwave-steam combined curing of ultra-high performance concrete specimens. The method includes molding the ultra-high performance concrete specimen, covering it with a film, and allowing it to stand until it reaches a preset initial strength; after demolding, placing it in a microwave-steam combined curing environment, first introducing steam to a first humidity range, and then curing with low-frequency microwaves within a first temperature range for a first duration; then increasing the steam to a second humidity range, increasing the microwave frequency and power, and using intermittent radiation within a second temperature range to enhance curing for a second duration; finally, stopping the steam, controlling the cooling rate, and supplementing with low-power intermittent microwaves to bring the specimen to room temperature. This invention employs a gradient microwave-steam curing scheme, dividing the curing process into four stages: natural pre-curing, low-frequency microwave pre-excitation, high-frequency microwave enhanced excitation, and microwave passivation cooling, achieving a shortened curing time and improved compressive strength under normal pressure. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 This is a flowchart of the method for microwave steam curing of ultra-high performance concrete specimens according to the present invention. Detailed Implementation

[0014] 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.

[0015] The purpose of this invention is to provide a method for microwave steam curing of ultra-high performance concrete specimens, aiming to solve or improve at least one of the above-mentioned technical problems.

[0016] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0017] like Figure 1 As shown, this invention provides a method for microwave-steam combined curing of ultra-high performance concrete specimens, comprising: Step 1: After the ultra-high performance concrete specimen is formed, cover the specimen and mold with waterproof plastic, and place the specimen and mold together under standard environmental conditions until the specimen reaches the preset initial strength.

[0018] As a further implementation, this step includes: After the UHPC specimen is molded, the mold is covered with waterproof plastic, and the specimen and mold are placed together in a standard curing environment for static curing. When the compressive strength of the UHPC specimen reaches 5–15 MPa, the specimen is demolded. This step, as the natural curing stage, is the first stage of microwave-steam combined curing of ultra-high performance concrete specimens. Its main purpose is to allow the UHPC specimen to acquire initial strength so that it can withstand the vapor pressure generated by microwave heating of the internal moisture during subsequent microwave-steam combined curing, preventing the specimen from bursting due to rapid evaporation of internal moisture under microwave action. It also prevents the specimen from cracking due to insufficient hydration caused by excessively high initial strength, which would hinder microwave energy penetration and external water vapor infiltration, leading to a decrease in microwave excitation effect and affecting specimen strength. The standard environmental conditions are a temperature of 20±2℃ and a relative humidity of not less than 98%.

[0019] Step 2: Place the demolded specimen in a microwave-steam combined curing environment. First, introduce steam to the first relative humidity range, then use low-frequency microwaves for irradiation curing, control the temperature within the first temperature range, and set the curing time to the first duration.

[0020] As a further implementation, this step includes: After the natural curing stage, the demolded UHPC specimens were placed in a microwave-steam combined curing environment. Steam was introduced first, and when the relative humidity reached 70% to 85%, low-frequency microwave radiation was turned on to irradiate and cure the UHPC specimens. The temperature was controlled at 50℃ to 65℃, and the curing time was 8 to 12 hours.

[0021] In the low-frequency microwave excitation curing stage, a combination of low-frequency microwaves and steam is used to cure the UHPC specimens. The 550–750 MHz low-frequency microwaves used in this invention can effectively penetrate cement-based materials, achieving uniform heating of the UHPC specimens both internally and externally, thus solving the problem of uneven heating in existing microwave curing methods. The 1.5–3.2 W / cm² frequency used... 3Microwave radiation power density can uniformly raise the internal temperature of UHPC specimens to 30–45 °C, effectively promoting cement hydration. The non-thermal effects of microwave radiation can disrupt the agglomeration of hydration products on the surfaces of cement and mineral admixtures, stimulating the hydration activity of cementitious materials and promoting the uniform formation of hydration products. This avoids the increase in structural defects caused by excessively rapid localized hydration within the UHPC specimens during subsequent high-temperature curing, thus improving specimen strength. Simultaneously, the 70%–85% relative humidity selected in this stage avoids the problem of excessive microwave absorption by steam in high relative humidity environments, leading to a significant decrease in microwave energy utilization; it also avoids the problem of rapid water loss in UHPC specimens under microwave action causing the hydration reaction to terminate when relative humidity is too low, achieving a balance between microwave radiation excitation and ambient humidity. The low-frequency microwave radiation direction is symmetrical and opposite to each other along the four sides of the ultra-high performance concrete specimen.

[0022] Step 3: Increase the steam flow to raise the relative humidity of the environment to the second relative humidity range, and at the same time increase the microwave frequency and power to high-frequency microwave parameters. Use intermittent radiation to strengthen the curing of the specimen, control the temperature within the second temperature range, and set the curing time to the second duration.

[0023] As a further implementation, this step includes: After the low-frequency microwave pre-excitation curing stage is completed, the steam flow rate is increased in the sealed curing environment, the relative humidity is controlled at no less than 98%, the microwave frequency and power are increased, and the temperature is controlled at 60 ℃~85 ℃. Radiation-enhanced curing is carried out on the UHPC specimens during curing, and the curing time is 18~24 h.

[0024] The intermittent high-frequency microwave enhanced excitation curing stage involves curing UHPC specimens using a combination of high-frequency microwaves and steam. This stage uses 750–1050 MHz and 5.0–8.2 W / cm². 3 Intermittent irradiation of UHPC specimens with high-frequency microwaves serves two purposes. Firstly, it avoids the problems caused by excessively high internal temperatures, large temperature differences between the inside and outside of the specimen, and vapor pressure accumulation resulting from continuous high-power microwave irradiation. Secondly, it utilizes the non-thermal effects of high-frequency microwaves to further promote the pozzolanic reaction of mineral admixtures such as silica fume and fly ash, accelerating the formation of cementitious materials such as CSH and improving strength. The intermittent irradiation method involves each microwave irradiation session lasting 20–30 minutes, followed by a 10–20 minute pause after each session. The microwave irradiation direction is symmetrical and opposite to each other along the four sides of the ultra-high performance concrete specimen.

[0025] Step 4: Stop the steam supply, reduce the ambient temperature according to the set cooling rate, and at the same time use low-power intermittent microwave radiation treatment until the specimen temperature drops to room temperature.

[0026] As a further implementation, this step includes: After the high-frequency microwave enhanced excitation curing stage is completed, the steam supply is stopped, and the curing environment temperature is gradually reduced at a rate of 8–15 °C / h. Simultaneously, low-power high-frequency microwaves are used to intermittently excite the UHPC specimens with microwave radiation until the temperature in the curing environment drops to room temperature, thus completing the microwave-steam combined curing of the UHPC specimens.

[0027] According to claim 1, the maintenance method is characterized in that, during the intermittent microwave passivation cooling stage, the low-frequency microwave frequency is 550–750 MHz, and the microwave radiation power density is 0.8–1.5 W / cm². 3 Each radiation session lasts 10 to 15 minutes, and after each radiation session, the duration is tentatively set at 8 to 20 minutes until cooling is complete.

[0028] The intermittent microwave excitation and cooling stage is mainly used to avoid rapid cooling of UHPC specimens. At the same time, low-power microwaves continue to gently excite the remaining unhydrated mineral admixtures inside the specimen, further promoting the formation of cementitious materials, reducing the matrix structure of UHPC specimens, and improving strength and durability.

[0029] Based on the above technical solution, the following embodiments are provided.

[0030] Step 1: Natural Maintenance Stage Ultra-high performance concrete (UHPC) specimens (e.g., 100 mm × 100 mm × 100 mm cube specimens) were prepared according to the designed mix proportions. After molding, the entire specimen and mold were immediately covered with a waterproof plastic film to prevent moisture evaporation. The covered specimens and molds were then placed in a standard curing chamber for static curing. The conditions in the standard curing chamber were controlled as follows: temperature 20 ± 2 ℃, relative humidity not less than 98%.

[0031] The settling time is approximately 24 hours. Testing showed that the compressive strength of the UHPC specimen reached 8 MPa at this time. After reaching the preset initial strength, the specimen was carefully demolded from the mold, ready for the next curing stage. This stage ensures the specimen has sufficient strength to withstand the vapor pressure generated internally during subsequent microwave heating, while preventing excessively high initial strength from hindering microwave penetration and moisture infiltration.

[0032] Step 2: Low-frequency microwave excitation curing stage The demolded UHPC specimens were placed in a microwave-steam curing device. First, water vapor was introduced into the sealed curing chamber, and humidity was monitored in real time using a humidity sensor. When the relative humidity inside the chamber reached 75%, the low-frequency microwave radiation source was turned on.

[0033] Microwave parameters: frequency 650 MHz, radiated power density 2.5 W / cm² 3 .

[0034] Radiation method: radiate symmetrically from all sides of the specimen to ensure uniform heating of the specimen.

[0035] Temperature control: The surface and center temperature of the specimen are controlled at 58 ℃ using a temperature sensor.

[0036] Maintenance duration: 10 hours of continuous maintenance.

[0037] During this stage, low-frequency microwaves achieved uniform heating inside and outside the specimen, effectively promoting the cement hydration reaction. At the same time, the non-thermal effect of microwaves stimulated the activity of cementitious materials, laying a good foundation for subsequent high-temperature curing.

[0038] Step 3: Intermittent high-frequency microwave enhanced excitation curing stage After the low-frequency microwave pre-excitation curing is completed, the intensive curing stage begins immediately.

[0039] Ambient humidity: Increase the steam flow rate to raise the relative humidity in the curing chamber to no less than 98%.

[0040] Microwave parameters: The microwave frequency is increased to 900 MHz, and the power density is increased to 6.5 W / cm². 3 .

[0041] Radiation method: Intermittent radiation is used, specifically: each microwave radiation session lasts 25 minutes, followed by a 15-minute pause, and this cycle is repeated.

[0042] Temperature control: The specimen temperature is controlled at 72 ℃.

[0043] Maintenance duration: The total effective maintenance time is 22 hours (including interval time).

[0044] During this stage, the combination of high-frequency microwaves and a high-temperature, high-humidity environment significantly accelerated the pozzolanic reaction of mineral admixtures such as silica fume and fly ash, promoting the formation of high-density CSH gel. Intermittent radiation effectively avoided excessively high internal temperatures and vapor pressure buildup in the specimens, preventing the generation of structural defects.

[0045] Step 4: Intermittent microwave excitation and cooling stage After the intensive curing phase is completed, stop introducing steam. Start the cooling process and simultaneously use low-power microwaves for auxiliary treatment.

[0046] Cooling rate: The temperature inside the curing chamber is gradually reduced at a rate of 10 ℃ / h.

[0047] Microwave parameters: Low-power high-frequency microwaves are used, with a frequency of 850 MHz and a radiated power density of 1.2 W / cm². 3 .

[0048] Radiation method: intermittent radiation, each radiation session lasting 12 minutes, followed by a 15-minute pause, and repeated in a cycle.

[0049] Termination condition: Continue the above operation until the temperature inside the curing chamber and the surface temperature of the specimen naturally drop to room temperature (approximately 25°C).

[0050] This stage avoids temperature stress cracks caused by rapid cooling of the specimen. At the same time, the low-power microwave continues to gently excite the unhydrated particles remaining inside the specimen, further promoting the formation of cementitious materials and making the specimen matrix more compact, ultimately obtaining high-strength, high-durability ultra-high performance concrete specimens.

[0051] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0052] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for microwave-steam combined curing of ultra-high performance concrete specimens, characterized in that, include: Step 1: After the ultra-high performance concrete specimen is formed, cover the specimen and the mold with waterproof plastic, and place the specimen and the mold together in a standard environment until the specimen reaches the preset initial strength. Step 2: Place the demolded specimen in a microwave-steam combined curing environment. First, introduce steam to the first relative humidity range, and then use low-frequency microwaves for irradiation curing. Control the temperature within the first temperature range and set the curing time to the first duration. Step 3: Increase the steam flow rate to raise the relative humidity of the environment to the second relative humidity range, and at the same time increase the microwave frequency and power to high frequency microwave parameters. Use intermittent radiation to strengthen the curing of the specimen, control the temperature within the second temperature range, and set the curing time to the second duration. Step 4: Stop the steam supply, reduce the ambient temperature according to the set cooling rate, and at the same time use low-power intermittent microwave radiation treatment until the specimen temperature drops to room temperature.

2. The method for microwave-steam combined curing of ultra-high performance concrete specimens according to claim 1, characterized in that, The initial strength in step 1 is 5–15 MPa; the standard environmental conditions are a temperature of 20 ± 2 °C and a relative humidity of not less than 98%.

3. The method for microwave-steam combined curing of ultra-high performance concrete specimens according to claim 1, characterized in that, The first relative humidity range in step 2 is 70%–85%; the frequency of the low-frequency microwave is 550–750 MHz, and the radiation power density is 1.5–3.2 W / cm². 3 The first temperature range is 50℃~65℃; the first duration is 8~12h.

4. The method for microwave-steam combined curing of ultra-high performance concrete specimens according to claim 1, characterized in that, The low-frequency microwave radiation in step 2 is symmetrically directed towards each other along the four sides of the ultra-high performance concrete specimen.

5. The method for microwave-steam combined curing of ultra-high performance concrete specimens according to claim 1, characterized in that, The second relative humidity range in step 3 is not lower than 98%; the frequency of the high-frequency microwave is 750–1050 MHz, and the radiation power density is 5.0–8.2 W / cm². 3 The second temperature range is 60℃~85℃; the second duration is 18~24h.

6. The method for microwave-steam combined curing of ultra-high performance concrete specimens according to claim 1, characterized in that, The intermittent radiation method in step 3 is as follows: each microwave radiation session lasts 20 to 30 minutes, and there is a 10 to 20 minute pause after each radiation session; the microwave radiation direction is symmetrical and opposite to each other along the four sides of the ultra-high performance concrete specimen.

7. The method for microwave-steam combined curing of ultra-high performance concrete specimens according to claim 1, characterized in that, The cooling rate in step 4 is 8–15 °C / h; the low-power intermittent microwave radiation uses a frequency of 550–750 MHz and a radiation power density of 0.8–1.5 W / cm². 3 The microwave radiation lasts for 10 to 15 minutes each time, followed by an 8 to 20 minute pause after each radiation session. This process is repeated until the cooling process is complete.

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