Concrete structure construction period temperature deformation cracking risk prediction method

Abstract

The invention provides a concrete structure construction period temperature deformation cracking risk prediction method comprising the following steps: S1, temperature monitoring: after concrete structure pouring is completed, continuously monitoring a concrete internal and surface temperature development curve; s2, inverting thermal parameters, namely inverting an equivalent adiabatic temperature rise curve and an equivalent thermal diffusivity of concrete under a field condition and a convective heat transfer coefficient of the concrete under a template in-place condition by combining a monitoring temperature, an environment temperature and a heat preservation condition and utilizing a heat conduction equation inverse analysis method; s3, prediction in the form removal period and the later period: inputting the thermal parameters obtained by inversion into finite element software, performing temperature field prediction, switching a boundary condition to be a heat exchange coefficient after form removal, and simulating subsequent temperature evolution; s4, risk index calculation and grading: calculating a risk index, and quantifying or grading the cracking risk according to a set threshold value; s5, performing construction decision linkage; according to the method, the thermal parameters of the concrete can be quickly inverted by utilizing early field measured temperature data.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of mass concrete structure construction, and particularly relates to a concrete structure construction period temperature deformation cracking risk prediction method. BACKGROUND

[0002] In the early hardening process of mass concrete, the internal temperature is significantly increased due to the cement hydration heat release effect, and then the temperature stress is generated under the combined action of the temperature difference with the outside and the cooling rate in the cooling stage. When the temperature stress exceeds the early age tensile strength of the concrete, temperature cracks are prone to occur.

[0003] In engineering practice, the commonly used temperature control measures include layered pouring, heat preservation covering, delayed form removal, etc. However, the existing cracking risk assessment is mostly dependent on empirical formula or a single temperature difference threshold (such as the core-surface temperature difference not exceeding 25℃), which is difficult to accurately reflect the real thermal behavior of concrete under different mix proportions, construction environment and curing conditions. In addition, form removal is a mutation point of temperature boundary conditions, and if it is not predicted accurately, rapid cooling and temperature difference stress concentration will occur after form removal, thereby causing cracks.

[0004] Although the traditional finite element thermal-mechanical coupling analysis can predict the development of temperature and stress, the thermal parameters such as adiabatic temperature rise curve, thermal conductivity, specific heat capacity required for calculation are often derived from the laboratory or literature, which cannot reflect the actual conditions on site in real time, resulting in large prediction deviation.

[0005] Therefore, there is an urgent need for a method which can utilize early field measured temperature data, quickly invert the thermal parameters of concrete, and accurately predict the cracking risk during construction period combined with finite element simulation, to realize the transformation from "empirical judgment" to "data-driven prediction". SUMMARY

[0006] In view of the deficiencies in the prior art, the present application provides a method which can utilize early field measured temperature data, quickly invert the thermal parameters of concrete, and accurately predict the cracking risk during construction period combined with finite element simulation.

[0007] The technical scheme of the present application is as follows: a concrete structure construction period temperature deformation cracking risk prediction method, comprising the following steps:

[0008] S1, temperature monitoring, continuously monitoring the internal and surface temperature development curves of the concrete in the period of 0-t1 after the concrete structure is poured;

[0009] S2, thermal parameter inversion, combining the monitored temperature, environmental temperature and heat preservation conditions, using the inverse analysis method of heat conduction equation, inverting the equivalent adiabatic temperature rise curve, equivalent thermal diffusivity of the concrete under the site conditions, and the convective heat transfer coefficient under the template in-place condition;

[0010] S3, prediction in the post-curing period, input the thermal parameters obtained by inversion into the finite element software to predict the temperature field in the period of t1-t2, and switch the boundary condition to the heat transfer coefficient after the formwork is removed at t2, simulate the subsequent temperature evolution, wherein t2 is the time when the formwork is removed;

[0011] S4, risk index calculation and classification, calculate the risk index based on the prediction results, and quantify or classify the cracking risk according to the set threshold, the risk index including the maximum core-surface temperature difference, the peak value of the temperature cooling rate, the internal peak temperature and the stress-to-tensile strength ratio;

[0012] S5, construction decision linkage, if the risk is at a high level, an early warning is given and disposal suggestions are given.

[0013] Further, in S1, at least four temperature sensors are arranged along the thickness direction of the key section of the concrete structure to monitor the internal temperature development curve of the concrete, temperature sensors are arranged at the temperature measuring points on the surface of the concrete and the outer surface of the formwork to monitor the surface temperature development curve of the concrete, and the ambient temperature is recorded.

[0014] Further, in S1, four temperature sensors are arranged along the thickness direction of the key section of the concrete structure to monitor the internal temperature development curve of the concrete, the four temperature sensors are arranged at the positions of 2-5 cm below the surface of the concrete, 1 / 4 thickness, 1 / 2 thickness and 3 / 4 thickness, respectively.

[0015] Further, the sampling interval of the temperature sensor is 10-60 min; 24h≤t1≤48h.

[0016] Further, in S2, the following heat conduction control equation is established:

[0017]

[0018] Wherein, ρ is the density, c(t) is the variable specific heat capacity, k is the thermal conductivity of the material, t is the time, q(t) is the derivative of the adiabatic temperature rise curve ΔT ad (t), and ΔT ad (t) is expressed by a three-parameter model as follows:

[0019]

[0020] Wherein, ΔT ∞ is the limit adiabatic temperature rise, τ is the characteristic time constant, and β is the shape parameter;

[0021] The time-varying equivalent thermal diffusivity is treated as a piecewise constant;

[0022] Wherein, k(t) is the time-varying thermal conductivity;

[0023] The boundary heat exchange coefficient when the template is in place is h form .

[0024] Further, in the S2, the least square or Bayesian inversion algorithm is used to minimize the error between the simulated temperature and the measured temperature, so as to obtain the field measured value of the thermal parameter.

[0025] Further, in the S3, the boundary condition is switched to the heat exchange coefficient h bare after the template is removed at the t2 time, the scene analysis is carried out for different boundary heat exchange modulation factors, and the internal and surface temperature distributions at each time are calculated.

[0026] Further, in the S4, the following indexes are extracted from the prediction results:

[0027] The maximum core-surface temperature difference ΔT core–surf,max ;

[0028] The temperature cooling rate peak |dT / dt| max ;

[0029] The internal peak temperature T max ;

[0030] The stress and tensile strength ratio R = σ t / f t (t).

[0031] Further, in the S4, the cracking risk is quantified or graded according to a preset threshold value:

[0032] ΔT core–surf,max ≤15℃ is determined as low risk;

[0033] 15℃<ΔT core–surf,max ≤22℃ is determined as medium risk;

[0034] ΔT core–surf,max >22℃ is determined as high risk;

[0035] T max >70℃ is determined as high risk;

[0036] R>0.85 is determined as high risk.

[0037] Further, the treatment suggestion includes delaying the template removal, increasing the insulation thickness, local slow cooling, and segmented template removal.

[0038] The beneficial effects of the present application are:

[0039] (1) The risk prediction method in the present application uses field early data to inversely calculate thermal parameters in real time, avoiding pure dependence on laboratory data;

[0040] (2) Prediction accuracy, the finite element prediction after inversion can more accurately reflect the actual maintenance and weather conditions;

[0041] (3) Multi-index comprehensive criterion, not only depending on the core-surface temperature difference, but also considering the cooling rate, peak temperature and stress-strength ratio, etc.;

[0042] (4) Direct service for construction decision, the prediction result can be used for formulating the form removal scheme and heat preservation adjustment strategy, and reducing the cracking risk. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The flow chart of the concrete structure construction period temperature deformation cracking risk prediction method in the application. DETAILED DESCRIPTION

[0044] Various exemplary embodiments of the present application will now be described in detail with reference to the accompanying drawings. The description of the exemplary embodiments is merely illustrative in nature and is in no way intended to limit the application, its application, or its uses, except as provided by the appended claims. The application can be embodied in many different forms without departing from the spirit or essential characteristics thereof. These embodiments are provided so that this disclosure will be thorough and complete, and fully convey the scope of the application to those skilled in the art. It should be noted that the relative arrangement of components and steps, the numerical expressions, and the numerical values set forth in these embodiments are to be interpreted as merely exemplary, unless otherwise specifically stated, and not as limitations of the application.

[0045] The "first", "second", and similar words used in the application do not represent any order, number or importance, but are only used to distinguish different parts. "Include" or "contain" and similar words mean that the elements before the word cover the elements listed after the word, and do not exclude the possibility of also covering other elements. "Up", "down", "left", "right" and the like are only used to represent relative positional relationship, and when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0046] As shown in Figure 1 , a concrete structure construction period temperature deformation cracking risk prediction method is disclosed, comprising the following steps:

[0047] S1, temperature monitoring, after the pouring of the concrete structure is completed, the internal and surface temperature development curves of the concrete in the period of 0-t1 are continuously monitored;

[0048] S2, thermal parameter inversion, combining the monitored temperature, environmental temperature and heat preservation conditions, using the inverse analysis method of heat conduction equation, the equivalent adiabatic temperature rise curve of the concrete under the site conditions, the equivalent thermal diffusivity and the convective heat transfer coefficient under the template in-place conditions are inverted;

[0049] S3, in the post-curing period, the thermal parameters obtained by inversion are input into the finite element software to predict the temperature field in the period of t1-t2, and the boundary condition is switched to the heat transfer coefficient after formwork removal at t2, and the subsequent temperature evolution is simulated, where t2 is the formwork removal time;

[0050] S4, risk index calculation and classification, based on the prediction results, the risk index is calculated, and the cracking risk is quantified or classified according to the set threshold, the risk index includes the maximum core-surface temperature difference, the peak value of temperature cooling rate, the internal peak temperature and the stress-to-tensile strength ratio;

[0051] S5, construction decision linkage, if the risk is at a high level, an early warning is given and disposal suggestions are given.

[0052] In some embodiments, in S1, at least four temperature sensors are arranged along the thickness direction of the key section of the concrete structure for monitoring the internal temperature development curve of the concrete, temperature sensors are arranged on the surface of the concrete and the outer surface of the formwork for monitoring the surface temperature development curve of the concrete, and the ambient temperature is recorded; Specifically, the temperature sensor is a buried thermocouple, an optical fiber thermometer or a wireless temperature acquisition module.

[0053] In some embodiments, in S1, four temperature sensors are arranged along the thickness direction of the key section of the concrete structure for monitoring the internal temperature development curve of the concrete, and the four temperature sensors are arranged at the positions of 2-5 cm below the surface of the concrete, 1 / 4 thickness, 1 / 2 thickness and 3 / 4 thickness, respectively.

[0054] In some embodiments, the sampling interval of the temperature sensor is 10-60 min; 24h≤t1≤48h.

[0055] In some embodiments, in S2, the following heat conduction control equation is established:

[0056]

[0057] Where, ρ is the density, c(t) is the variable specific heat capacity, k is the thermal conductivity of the material, t is the time, q(t) is the derivative of the adiabatic temperature rise curve ΔT ad (t), and ΔT ad (t) is represented by a three-parameter model as follows:

[0058]

[0059] Where, ΔT ∞ is the limiting adiabatic temperature rise, τ is the characteristic time constant, and β is the shape parameter;

[0060] The time-varying equivalent thermal diffusivity is treated as a piecewise constant;

[0061] where k(t) is the time-varying thermal conductivity;

[0062] The boundary heat exchange coefficient of the template in place is h form .

[0063] In some embodiments, in S2, a least square or Bayesian inversion algorithm is used to minimize the error between the simulated temperature and the measured temperature, to obtain the field measured value of the thermal parameter.

[0064] In some embodiments, in S3, the boundary condition is switched to the heat exchange coefficient h bare after the template is removed at t2, and scenario analysis is performed for different boundary heat exchange modulation factors, to calculate the internal and surface temperature distribution at each time; wherein the boundary heat exchange modulation factors include different insulation measures, wind speed, humidity, etc.

[0065] In some embodiments, in S4, the following indicators are extracted from the prediction results:

[0066] The maximum core-surface temperature difference ΔT core–surf,max ;

[0067] The peak temperature cooling rate |dT / dt| max ;

[0068] The internal peak temperature T max ;

[0069] The stress-to-tensile strength ratio R = σ t / f t (t).

[0070] In some embodiments, in S4, the cracking risk is quantified or classified according to a preset threshold:

[0071] ΔT core–surf,max ≤ 15℃ is determined as low risk;

[0072] 15℃ < ΔT core–surf,max ≤ 22℃ is determined as medium risk;

[0073] ΔT core–surf,max > 22℃ is determined as high risk;

[0074] T max > 70℃ is determined as high risk;

[0075] R > 0.85 is determined as high risk.

[0076] In some embodiments, the treatment suggestion includes delaying the form removal, increasing the insulation thickness, local slow cooling, segmented form removal; specifically, when the predicted risk is medium or high, the system outputs the treatment suggestion, including delaying the form removal, increasing the insulation thickness, segmented form removal, covering the outer surface with slow cooling or spray cooling, and evaluating the improvement effect on the temperature difference and the cooling rate.

[0077] As a more specific embodiment of the concrete structure construction period temperature deformation cracking risk prediction method.

[0078] Taking the basement outer wall of a super high-rise building as an example, the wall thickness is 1.0 m, the height is 4.2 m, the length is 12 m, the concrete design strength grade is C40, the mix ratio is water-cement ratio 0.38, cement 340 kg / m 3 , fly ash 60 kg / m 3 , maximum aggregate size 25 mm, slump 200 mm, and mixture temperature 24℃. Double-layer wood formwork is provided on the outer side of the outer wall and covered with insulation cotton felt, and the formwork is planned to be removed at 72h after pouring. The external temperature during construction is 15-25℃, the diurnal temperature difference is 10℃, and the relative humidity is 60-75%.

[0079] The concrete structure construction period temperature deformation cracking risk prediction method comprises the following steps:

[0080] S1, temperature monitoring, four embedded temperature sensors are arranged in the wall thickness direction, the four embedded temperature sensors are respectively 5cm, 25cm, 50cm and 75cm away from the outer surface, in addition to the ambient temperature of the outer surface of the formwork, the sampling interval is 15min, and the monitoring is continued to 48h after pouring (t1=48h);

[0081] S2, thermal parameter inversion, using the multi-point temperature curve of 0-48h, a three-parameter adiabatic temperature rise model is used to fit to obtain:

[0082] ΔT ∞ =43.5℃, τ=18.2h, β=1.28;

[0083] The equivalent thermal diffusivity α(t) is 7.5×10 -7 m 2 / s in 0-24h, and 8.2×10 7 m 2 / s in 24-48h; the convective heat transfer coefficient h form of the formwork in place is 4.2W / (m 2 ·K);

[0084] S3, prediction during and after form removal, input the above parameters into the finite element model, set t2=72h form removal, the outer surface heat transfer coefficient h bare after form removal is 11.5W / (m 2• K), the external meteorological conditions are input according to the measured curve, and the temperature distribution from t1 to t3 (t 3 = 168h) is predicted;

[0085] The prediction results show that:

[0086] At t2, the core temperature is 55.2℃, the surface temperature is 43.1℃, and the maximum core-surface temperature difference ΔT core–surf,max = 12.1℃;

[0087] The peak cooling rate of the core temperature within the first 8h after demolding is |dT / dt| max = 0.94℃ / h;

[0088] The internal peak temperature T max occurs at 38h, which is 56.0℃;

[0089] The early-age stress and tensile strength ratio R = 0.62;

[0090] S4, risk index calculation and classification, according to the set risk classification standard (ΔT core–surf,max ≤ 15℃ and R ≤ 0.65 is low risk), the prediction result is determined as low risk; if a cold wave occurs outside, leading to h bare increasing to 18W / (m 2 · K), the model predicts that ΔT core–surf,max will rise to 20.5℃, R will increase to 0.81, and the risk level will change to medium;

[0091] S5, construction decision linkage, low risk can be demolded according to the plan at 72h, medium risk suggests delaying demolding for 24h and increasing the external insulation layer.

[0092] Implementation effect: after demolding according to the prediction scheme on site, no visible temperature cracks appear; through infrared thermal imaging and surface observation, no abnormal cooling area is found. Compared with the experience demolding of the same structure in the past, the cooling rate control is more gentle, and the temperature difference peak value is reduced by about 5℃.

[0093] Another embodiment of the method for predicting the cracking risk of temperature deformation of concrete structures during construction.

[0094] Taking a bridge pier cap as an example, the cap has a plan size of 12.0m x 9.0m, a thickness of 3.0m, a concrete design strength grade of C50, a mix ratio of water-cement ratio 0.36, cement 380kg / m 3 , and slag powder 80kg / m 3, aggregate maximum particle size 40 mm, slump 220 mm, and mold temperature 18℃. The pouring was conducted in the late spring season, with daytime temperature 22-28℃, nighttime temperature 14-18℃, and relative humidity 50-65% at the construction site. The pile cap was exposed around the periphery, and a double-layer thermal blanket and wet burlap were laid on the top after pouring, and the formwork was retained until the formwork was removed.

[0095] The method for predicting the risk of temperature deformation cracking of a concrete structure during construction comprises the following steps:

[0096] S1, temperature monitoring, two embedded thermocouples are arranged in the thickness direction (top, middle, and bottom) of the pile cap, one measuring point is arranged at 5 cm from the top, 1 / 4, 1 / 2, and 3 / 4 of the thickness, and one surface and ambient temperature measuring point is additionally arranged, the sampling interval is 20 min, and the monitoring is performed from the end of pouring to 60 h (t1=60 h);

[0097] S2, thermal parameter inversion, the multi-point temperature curve from 0 to 60 h is used to obtain the following parameters by inversion:

[0098] ΔT ∞ =48.7℃, τ=21.5h, β=1.35;

[0099] The equivalent thermal diffusivity α(t) is 6.9×10 -7 m 2 / s within 0-30 h, and 7.4×10 -7 m 2 / s within 30-60 h; the convective heat transfer coefficient h form =3.8W / (m 2 ·K) of the formwork in place;

[0100] S3, prediction during and after formwork removal, the parameters are input into the finite element model, t2=96 h is set as the formwork removal time (at this time, the formwork around the pile cap is removed, and the thermal blanket on the top is retained), the outside heat transfer coefficient h bare =9.0W / (m 2 ·K) after the formwork removal, the top heat transfer coefficient h top =4.5W / (m 2 ·K) is maintained due to the thermal insulation, and the temperature evolution from t1 to t3 (t3=192 h) is predicted;

[0101] The prediction results show that:

[0102] The core temperature is 62.8℃, the surface temperature is 50.4℃, the core-surface temperature difference ΔT core–surf,max =12.4℃ at T2 time;

[0103] The cooling rate peak |dT / dt| max =0.82℃ / h within the first 12 h after the formwork removal;

[0104] The peak temperature Tmax At 48h, 64.1℃;

[0105] Thermal stress-strength ratio R max = 0.68;

[0106] S4, risk index calculation and grading, ΔT core–surf,max and R max are in the low-risk zone, but T max is higher (> 60℃ and close to 65℃), considering that the air temperature will drop later, the exposed area of the pile cap is large, and the model scenario analysis shows that if the formwork is removed 24h in advance, R max will rise to 0.89, which is high-risk;

[0107] S5, construction decision linkage, high-risk suggestion to maintain the original 96h formwork removal, and continue to keep warm for 48h after formwork removal.

[0108] Implementation effect: After the scheme is executed, no visible cracks appear on the pile cap during the monitoring period; the temperature reduction process on the top surface is smooth, the peak cooling rate is controlled below 0.85℃ / h, and the maximum core-surface temperature difference is lower than 14℃. Compared with the same type of engineering in the past, the cracking rate is significantly reduced.

[0109] So far, the embodiments of the present application have been described in detail. In order to avoid obscuring the concept of the present application, some details known in the art are not described. Those skilled in the art can fully understand how to implement the technical solutions disclosed herein according to the above description.

[0110] The above embodiments only express some implementation manners of the present application, which are described in detail and specifically, but should not be understood as a limitation on the scope of the patent. It should be pointed out that for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method of predicting the risk of cracking due to temperature deformation of a concrete structure during construction, characterized by, The method comprises the following steps: S1, temperature monitoring, continuously monitoring the internal and surface temperature development curves of the concrete in the period of 0-t1 after the pouring of the concrete structure is completed; S2, thermal parameter inversion, combining the monitored temperature, the environmental temperature and the heat preservation condition, using the inverse analysis method of heat conduction equation, inversely analyzing the equivalent adiabatic temperature rise curve, the equivalent thermal diffusivity of the concrete under the site condition and the convective heat transfer coefficient under the template in-place condition; S3, prediction in the form removal period and the later period, inputting the thermal parameters obtained by inversion into the finite element software, predicting the temperature field in the period of t1-t2, and switching the boundary condition to the heat transfer coefficient after form removal at the moment t2, simulating the subsequent temperature evolution, wherein the moment t2 is the moment of form removal; S4, risk index calculation and classification, calculating the risk index based on the prediction result, quantifying or classifying the cracking risk according to the set threshold, the risk index including the maximum value of the core-surface temperature difference, the peak value of the temperature cooling rate, the internal peak temperature and the stress-to-tensile strength ratio; S5, construction decision linkage, if the risk is in the high level, giving an early warning and giving a disposal suggestion.

2. The method of predicting the risk of cracking due to temperature-induced deformation of a concrete structure during construction according to claim 1, characterized in that: In the S1, at least four temperature sensors are arranged along the thickness direction of the key section of the concrete structure for monitoring the internal temperature development curve of the concrete, temperature sensors are arranged at the temperature measuring points of the surface of the concrete and the outer surface of the template for monitoring the surface temperature development curve of the concrete, and the environmental temperature is recorded.

3. The method of predicting the risk of cracking due to temperature-induced deformation of a concrete structure during construction according to claim 2, characterized in that: In the S1, four temperature sensors are arranged along the thickness direction of the key section of the concrete structure for monitoring the internal temperature development curve of the concrete, and the four temperature sensors are arranged at the positions of 2-5 cm below the surface of the concrete, 1 / 4 thickness, 1 / 2 thickness and 3 / 4 thickness, respectively.

4. The method of predicting the risk of cracking due to temperature-induced deformation of a concrete structure during construction according to claim 2, characterized in that: The sampling interval of the temperature sensor is 10-60 min; 24h≤t1≤48h.

5. The method of predicting the risk of temperature-induced cracking of a concrete structure during construction according to claim 1, characterized in that: In the S2, the following heat conduction control equation is established: where p is the density, c(t) is the variable specific heat capacity, k is the thermal conductivity of the material, t is time, q(t) is the derivative of the adiabatic temperature rise curve AT ad (t), and AT ad (t) is represented by a three-parameter model as follows: where ΔT ∞ is the limiting adiabatic temperature rise, τ is the characteristic time constant, and β is a shape parameter; Time-varying equivalent thermal diffusivity Segmentation constant processing; Wherein, k(t) is the time-varying thermal conductivity; The boundary heat transfer coefficient of the template when in place is h form .

6. The method of predicting the risk of cracking due to temperature-induced deformation of a concrete structure during construction according to claim 1, characterized in that: In the S2, the least square or Bayesian inversion algorithm is used to minimize the error between the simulated temperature and the measured temperature, and the site measured value of the thermal parameter is obtained.

7. The method of predicting temperature-induced cracking risk during construction of a concrete structure of claim 1, wherein: In the S3, the switching boundary condition is switched to the heat exchange coefficient h after the disassembly at the t2 moment bare The scenario analysis is performed for different boundary heat exchange modulation factors, and the internal and surface temperature distributions at each moment are calculated.

8. The method of predicting temperature-induced cracking risk of a concrete structure during construction according to claim 1, characterized in that: In the S4, the following indexes are extracted from the prediction result: Core - surface temperature difference maximum ΔT core–surf,max ; Temperature cooling rate peak |dT / dt| max ; Internal peak temperature T max ; Stress to tensile strength ratio R = σ t / f t (t).

9. The method of predicting the risk of cracking due to temperature induced deformation of a concrete structure during construction according to claim 8, wherein: In the S4, the cracking risk is quantified or classified according to the preset threshold: ΔT core–surf,max ≤ 15 °C is determined as low risk; 15°C < ΔT core–surf,max ≤ 22°C is determined as medium risk; ΔT core–surf,max > 22°C is determined as high risk; T max > 70°C is considered high risk; R>0.85 is determined as high risk.

10. The method of predicting the risk of temperature-induced cracking of a concrete structure during construction according to claim 1, characterized in that: The disposal suggestion includes delaying form removal, increasing heat preservation thickness, local slow cooling and segmented form removal.

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