A method of determining curing parameters for a precast concrete product

Abstract

The application discloses a method for determining curing parameters of precast concrete products, comprising: obtaining a relationship curve between concrete strength and curing age under laboratory standard curing conditions, and calculating a relationship graph between strength and maturity; based on hydration heat release amounts at multiple ages under different temperatures, using a nonlinear least square method to obtain a maximum heat release amount per unit cementitious material, a hydration activation energy and a pre-exponential factor; predicting a temperature development history, calculating a temperature difference between a center point of the precast concrete product and a protective layer, and a temperature difference between the protective layer and the environment; calculating the maturity of the concrete at different ages under the temperature history; based on the maturity theory of the same maturity and the same strength, according to the relationship graph between the strength and the maturity, the relationship between the strength of the precast concrete product and the age is obtained; and according to the predicted temperature of the precast concrete product, the temperature difference and the strength requirement, the curing system of the precast concrete product is determined. By reasonably arranging the parameters such as steam curing temperature and time, the energy consumption in the curing process of the precast concrete product is reduced, the social and economic benefits are increased, and the carbon reduction effect is obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cement concrete, in particular to a method for predicting temperature history, strength and temperature difference development and determining curing system of precast concrete products based on maturity theory. BACKGROUND

[0002] The present application mainly describes the selection of steam curing system of precast concrete products under different environmental temperatures in actual production. Precast concrete products have been widely used in infrastructure construction due to their fast production speed and controllable quality. However, in actual production, there are certain technical difficulties in predicting the strength development of concrete products and determining the steam curing system, especially under different external environmental temperature conditions. The adjustment of steam curing system has become a key factor to ensure the quality of precast concrete products.

[0003] In view of the above problems, the present application provides a method for predicting temperature history, strength and temperature difference development of concrete products and determining curing parameters of precast concrete products based on maturity theory. The method aims to accurately predict the temperature development history of precast concrete products through a scientific theoretical calculation model, calculate the strength and temperature difference development at different positions, and optimize the steam curing system accordingly, so as to realize crack control and resource and energy saving of concrete products. SUMMARY

[0004] Technical problem: The purpose of the present application is to solve the problems of difficult real-time measurement of concrete strength development at different positions of precast products and the need for adjustment of steam curing under different external environmental temperatures. A method for determining curing parameters of precast concrete products is proposed, which predicts temperature history, strength and temperature difference development based on maturity theory and determines curing parameters of precast concrete products.

[0005] Technical solution: The method for determining curing parameters of precast concrete products comprises the following steps:

[0006] Step one: prepare concrete test pieces according to the engineering mix proportion, and cure the concrete test pieces under standard conditions;

[0007] Step two: test the strength of the concrete test pieces at different ages, and calculate the maturity of the concrete test pieces at different ages;

[0008] Step three: fit the strength-maturity relationship curve under standard conditions according to the strength and maturity at different ages;

[0009] Step four: prepare cementitious material samples according to the engineering mix proportion, and test the heat release at multiple ages under three temperatures;

[0010] Step five: according to the heat release of different ages at different temperatures, the maximum heat release Q of unit cementing material is obtained by using nonlinear least squares fitting max , hydration reaction activation energy E a and pre-exponential factor A;

[0011] Step six: according to the fitting maximum heat release Q max , hydration reaction activation energy E a and pre-exponential factor A, the temperature development history of precast concrete product is predicted, and the maturity-time relationship is calculated;

[0012] Step seven: based on the strength-maturity relationship under standard conditions and the maturity-time relationship under the predicted temperature development history, the strength-time relationship under the predicted temperature development history is converted;

[0013] Step eight: according to the predicted temperature development history in step six, according to the predicted temperature development history of different positions of precast concrete product and the average environment temperature, the temperature difference between the center and the surface of precast product, the temperature difference between the surface and the environment and the strength development curve are calculated, and the temperature, time and other parameters in the curing system are adjusted and determined according to the standard (TB / T 3353-2014 "Railway tunnel reinforced concrete segment").

[0014] Wherein,

[0015] The standard condition of step one is that the temperature is 20℃±2℃, and the humidity is ≥95%.

[0016] The different ages of step two are 1d, 3d, 7d and 28d; the maturity calculation method formula is: M=(t-t0)T;

[0017] Wherein, M is the maturity, ℃·h or ℃·d; h is hour, d is day; T is the average temperature of concrete in time interval, ℃; t0 is the final setting time, h or d; t is the age of concrete.

[0018] The strength-maturity relationship curve under standard conditions is obtained by fitting in step three, which is obtained by using nonlinear curve fitting according to the strength and maturity of different ages obtained by testing and calculating in step two;

[0019] The three temperatures of step four are 20℃, 40℃ and 60℃, and the different ages are 1d, 3d and 7d;

[0020] The nonlinear least squares fitting formula of step five is: The fitting accuracy is greater than 0.9;

[0021] Where q(t,T) is the heat released per unit mass of cementitious material at temperature T after age t, kJ / kg; Q max is the total heat released by complete hydration of unit mass of cementitious material, kJ / kg; K T is the hydration exothermic rate constant, 1 / h; A is the pre-exponential factor; E a is the hydration activation energy, kJ / mol; R is the universal gas constant, kJ / (K·mol); T is the average temperature of the concrete during the time interval, K.

[0022] The step 6 predicts the temperature development history of precast concrete products, wherein the temperature field control equation is:

[0023] Where λ is the thermal conductivity, kJ / (m·h·℃); c p is the specific heat capacity, kJ / kg·℃; ρ is the density, kg / m 3 ; mq is the rate at which energy is generated per unit volume of concrete, kJ / (m 3 ·h); m is the mass of cementitious material per unit volume of concrete, kg / m 3 , q is the heat of hydration released per unit mass of cementitious material per unit time, kJ / (kg·h); T is the temperature; t is the age; x, y, z are the positions of the characteristic points in the precast concrete product.

[0024] The step seven of calculating the strength-time relationship diagram under the predicted temperature is based on the strength-maturity relationship under standard conditions and the maturity-time relationship under the predicted temperature development history, thereby converting the strength-time relationship diagram under the predicted temperature development history.

[0025] In step eight, based on the predicted temperature development history in step six, the predicted temperature development history at different locations of the precast concrete product and the average ambient temperature, the temperature difference between the center and surface of the precast product, the temperature difference between the surface and the environment, and the strength development curve are calculated. In accordance with the requirements of standard TB / T 3353-2014 "Reinforced Concrete Segments for Railway Tunnels", parameters such as temperature and time in the maintenance system are adjusted and determined.

[0026] Beneficial Effects: This invention measures and calculates the relationship between concrete strength and maturity under standard curing conditions. Based on maturity theory, it predicts the concrete strength and temperature differential at a specific age under a temperature history. Based on the strength and temperature differential requirements for precast concrete products specified in the standard (TB / T 3353-2014, "Reinforced Concrete Segments for Railway Tunnels"), it determines parameters such as curing temperature and time for precast concrete products at different ambient temperatures. Based on the accurately predicted concrete strength and temperature differential, this invention determines a curing system, such as steam curing temperature and time, for precast concrete products. This system significantly increases economic benefits, achieves carbon reduction, prevents cracking in concrete products, and improves quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A flow chart for predicting concrete strength and temperature difference and determining a curing system according to the present invention;

[0028] Figure 2 This is the relationship curve between concrete strength and maturity under standard conditions;

[0029] Figure 3 The results of constant temperature calorimetry of cementitious materials at different temperatures;

[0030] Figure 4 is the temperature prediction point of the precast concrete segment; Figure 4 (a) is the overall diagram of the segment. Figure 4 (b) is the midline cross-section of the segment, which includes: segment center point 1, center surface point 2, and longitudinal end point 3;

[0031] Figure 5 Temperature prediction results for precast concrete products at different steam curing temperatures and durations;

[0032] Figure 6 This is a temperature difference curve diagram of precast concrete products under different steam curing temperatures and durations;

[0033] Figure 7 Predict the strength curves of precast concrete products at different steam curing temperatures and times. DETAILED DESCRIPTION

[0034] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] Example 1: This example takes the ambient temperature of 5°C, the mold entry temperature of 20°C, the constant temperature steam curing temperature of the precast concrete pipe segment of 40°C, and the constant temperature curing duration of 1 hour as an example.

[0036] Example 2: This example takes the ambient temperature of 5°C, the mold entry temperature of 20°C, the constant temperature steam curing temperature of the precast concrete pipe segment of 40°C, and the constant temperature curing duration of 2 hours as an example.

[0037] Example 3: This embodiment takes the ambient temperature as 5°C, the mold entry temperature as 20°C, the constant temperature steam curing temperature of the precast concrete pipe segment as 40°C, and the constant temperature curing duration as 3 hours as an example.

[0038] Example 4: This embodiment takes the ambient temperature as 5°C, the mold entry temperature as 20°C, the constant temperature steam curing temperature of the precast concrete pipe segment as 40°C, and the constant temperature curing duration as 4 hours as an example.

[0039] Example 5: This embodiment takes the ambient temperature as 5°C, the mold entry temperature as 20°C, the constant temperature steam curing temperature of the precast concrete pipe segment as 45°C, and the constant temperature curing duration as 1 hour as an example.

[0040] Example 6: This embodiment takes the ambient temperature as 5°C, the mold entry temperature as 20°C, the constant temperature steam curing temperature of the precast concrete pipe segment as 45°C, and the constant temperature curing duration as 2 hours as an example.

[0041] Example 7: This embodiment takes the ambient temperature as 5°C, the mold entry temperature as 20°C, the constant temperature steam curing temperature of the precast concrete pipe segment as 45°C, and the constant temperature curing duration as 3 hours as an example.

[0042] Example 8: This example takes the ambient temperature of 5°C, the mold entry temperature of 20°C, the constant temperature steam curing temperature of the precast concrete pipe segment of 45°C, and the constant temperature curing duration of 4 hours as an example.

[0043] Step 001: Prepare multiple concrete specimens according to the engineering mix ratio, and cure the concrete specimens under standard conditions of temperature 20℃±2℃ and humidity ≥95%. Figure 1 shown.

[0044] Table 1 Mixing ratio (kg / m 3 )

[0045]

[0046] Step 002: Test the strength of the concrete specimens at 1d, 3d, 7d and 28d, and calculate the maturity of the concrete specimens at 1d, 3d, 7d and 28d according to the formula M=(t-t0)T; where M is the maturity, ℃·h or ℃·d; h is hours, d is days; T is the average temperature of the concrete during the time interval, ℃; t0 is the final setting time, h or d; and t is the age of the concrete.

[0047] Step 003: According to the relationship between strength and maturity at 1 day, 3 days, 7 days and 28 days, the strength-maturity relationship curve under standard conditions is fitted;

[0048] Step 004: Prepare cementitious material samples according to the engineering proportions and test the heat release at 20°C, 40°C and 60°C for 1 day, 3 days and 7 days;

[0049] Step 005: Based on the heat release at 20℃, 40℃ and 60℃ for 1d, 3d and 7d respectively, use the formula The maximum heat release Q per unit of cementitious material is obtained by fitting using the nonlinear least squares method. max , hydration reaction activation energy E aand the pre-exponential factor A, the fitting accuracy is greater than 0.9. Among them, Q(t, T) is the heat release of unit mass of cementitious materials at temperature T after age t, kJ / kg; Q max is the total heat release of unit mass of cementitious materials after complete hydration, kJ / kg; K T is the hydration heat release rate constant, 1 / h; A is the pre-exponential factor; E a is the hydration activation energy, kJ / mol; R is the gas universal constant, kJ / (K·mol);

[0050] Step 006: Based on the strength-maturity relationship under standard conditions and the maturity-time relationship under the predicted temperature development history, the strength-time relationship diagram under the predicted temperature development history is converted, wherein the temperature field control equation is Among them, λ is the thermal conductivity, kJ / (m·h·℃); c p is the specific heat capacity, kJ / kg·℃; ρ is the density, kg / m 3 ; mq is the energy generation rate of unit volume of concrete, kJ / (m 3 ·h); m is the mass of cementitious materials per unit volume of concrete, kg / m 3 , q is the hydration heat released per unit mass of cementitious materials per unit time, kJ / (kg·h); T is the temperature, ℃.

[0051] Step 007: According to the predicted temperature development history in step six, according to the predicted temperature development history and the average ambient temperature at different positions of the prefabricated concrete product, the center and surface temperature difference of the prefabricated product, the surface and ambient temperature difference and the strength development curve are calculated, and according to the requirements of the standard (TB / T 3353-2014 “Railway Tunnel Reinforced Concrete Segment”), the temperature, time and other parameters in the curing system are adjusted and determined.

[0052] According to the embodiments and the drawings, when the ambient temperature is low, the constant temperature steam curing time should be appropriately extended to ensure that the segment has sufficient demolding strength and factory strength; when the ambient temperature is high, the constant temperature steam curing time can be appropriately shortened to reduce the large hydration temperature and temperature difference of the segment and ensure the overall performance of the segment.

[0053] In summary, the present invention discloses a method for predicting temperature history, strength, and temperature difference development and determining a curing system for precast concrete products based on maturity theory. The method comprises: preparing concrete specimens according to a mix ratio and curing them under standard conditions to different ages, establishing a relationship between concrete strength and maturity under standard conditions; preparing cementitious material samples according to the mix ratio, measuring heat release at different temperatures and ages, and fitting to calculate the maximum heat release per unit of cementitious material, hydration reaction activation energy, and pre-exponential factor; predicting the temperature development history of precast concrete products; calculating the maturity of precast concrete products under the predicted temperature history; calculating the strength and temperature difference of precast concrete products under the predicted temperature history based on the relationship between strength and maturity under standard conditions; and determining parameters such as steam curing temperature and time based on the requirements for predicted strength and temperature difference of precast concrete products in the standard (TB / T 3353-2014 "Reinforced Concrete Segments for Railway Tunnels").

[0054] It should be understood that the application of the present invention is not limited to the above examples. For ordinary technicians in this field, they can make improvements or changes based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A method for determining curing parameters of precast concrete products, characterized in that: The following steps are involved: Step 1: Prepare concrete specimens according to the engineering mix ratio and cure the concrete specimens under standard conditions; Step 2: Test the strength of concrete specimens at different ages and calculate the maturity of concrete specimens at different ages; Step 3: Based on the strength and maturity of different ages, fit the strength-maturity relationship curve under standard conditions. According to the strength and maturity of different ages tested and calculated in step 2, use nonlinear curve fitting to obtain the strength-maturity relationship curve under standard conditions; Step 4: Prepare cementitious material samples using the raw materials of precast concrete products according to the engineering mix ratio and test the heat release at three temperatures and multiple ages; Step 5: Based on the heat release at different temperatures and at multiple ages, the maximum heat release per unit cementitious material Q is obtained by fitting using the nonlinear least squares method. max , hydration reaction activation energy E a and pre-exponential factor A; The nonlinear least squares fitting formula is: The fitting accuracy is greater than 0.9; Where q(t,T) is the heat released per unit mass of cementitious material at temperature T after age t, kJ / kg; Q max is the total heat released by complete hydration of unit mass of cementitious material, kJ / kg; K T is the hydration exothermic rate constant, 1 / h; A is the pre-exponential factor; E a is the hydration activation energy, kJ / mol; R is the universal gas constant, kJ / (K·mol); T is the average temperature of the concrete during the time interval, K; Step 6: According to the fitting maximum heat release Q max , hydration reaction activation energy E a and pre-exponential factor A, predict the temperature development history of precast concrete products and calculate the maturity-time relationship diagram; Among them, the temperature field control equation is: Where λ is the thermal conductivity, kJ / (m·h·℃); c p is the specific heat capacity, kJ / kg·℃; ρ is the density, kg / m 3 ; mq is the rate at which energy is generated per unit volume of concrete, kJ / (m 3 ·h); m is the mass of cementitious material per unit volume of concrete, kg / m 3 , q is the heat of hydration released per unit mass of cementitious material per unit time, kJ / (kg·h); T is the temperature; t is the age; x, y, z are the positions of the characteristic points in the precast concrete product; Step 7: Based on the strength-maturity relationship under standard conditions, calculate the strength-time relationship diagram under the predicted temperature. Based on the strength-maturity relationship under standard conditions and the maturity-time relationship under the predicted temperature development history, the strength-time relationship diagram under the predicted temperature development history is converted; Step 8: Based on the predicted temperature development history in step 6, and according to the predicted temperature development history at different locations of the precast concrete product and the average ambient temperature, calculate the temperature difference between the center and surface of the precast product, the temperature difference between the surface and the environment, and the strength development curve. In accordance with the requirements of standard TB / T 3353-2014 "Reinforced Concrete Segments for Railway Tunnels", adjust and determine the temperature and time parameters in the maintenance system.

2. The method for determining curing parameters of precast concrete products according to claim 1, characterized in that: The standard conditions described in step 1 are a temperature of 20°C ± 2°C and a humidity of ≥ 95%.

3. The method for determining curing parameters of precast concrete products according to claim 1, wherein: The different ages mentioned in step 2 are 1 day, 3 days, 7 days and 28 days; the maturity calculation formula is: M = (t-t0)T; Where M is maturity, °C·h or °C·d; h is hours, d is days; T is the average temperature of concrete during the time interval, °C; t0 is the final setting time, h or d; and t is the age of concrete.

4. The method for determining curing parameters of precast concrete products according to claim 1, wherein: The three temperatures in step 4 are 20° C., 40° C. and 60° C., and the different ages are 1 day, 3 days and 7 days.

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