Method for determining minimum air entraining agent mixing amount of early frozen concrete based on pore absorption frost heaving

Abstract

The invention discloses a method for determining the minimum air entraining agent mixing amount of early frozen concrete based on pore absorption frost heaving, belongs to the field of concrete winter construction, and aims to effectively evaluate the freezing damage risk of concrete, optimize the air entraining agent mixing amount, reduce the construction cost and improve the anti-freezing performance of concrete. According to the method, the capillary index and the initial gas content in the concrete at the specified pre-curing age and the deformation index of the concrete in the negative temperature environment are obtained respectively, and the water-gas ratio and the pore absorption efficiency value are calculated by combining the capillary index, the initial gas content and the deformation index; and obtaining the minimum air entraining agent mixing amount required by the concrete to reach the anti-freezing state according to the quantitative relationship between the water-air ratio and the air hole absorption efficiency.

Description

Technical Field:

[0001] The present invention belongs to the field of winter concrete construction, and specifically relates to a method for determining the minimum air-entraining agent dosage of early-frosted concrete based on bubble absorption of frost heave. Background Art:

[0002] Due to the high water content and fragile structure of early-age concrete, during winter construction, when early-age concrete is not fully hardened, it is exposed to a frozen environment, with low strength and is extremely vulnerable to frost damage, which will seriously affect the safety and durability of the project. In a negative temperature environment, when the water in the pores of concrete freezes, it will produce a volume expansion of up to 9%, squeezing the unfrozen water to migrate to the surrounding pores and releasing a huge frost heave stress. However, when the pores cannot fully absorb the squeezed unfrozen water, hydrostatic pressure will be formed in the pores. Once the hydrostatic pressure exceeds the tensile strength of the pore wall, cracks will be generated. Therefore, the frost damage of concrete is the result of the combined action of water freezing and expansion and the ability of pores to absorb frost heave.

[0003] To mitigate the frost damage of concrete, an air-entraining agent can be added to optimize the pore structure of concrete, shorten the water migration path, and improve the ability of pores to absorb frost heave. However, an excessive amount of air-entraining agent will increase costs and reduce the strength of concrete. Since existing technical methods do not quantify the ability of pores to absorb frost heave, the dosage of the air-entraining agent depends on the strength retention rate test of concrete after being frozen. This method not only requires a large amount of time, curing costs, and labor costs, etc., but also cannot fully consider the influence of concrete mix ratio and pre-curing time, and is difficult to be widely applied. Therefore, there is an urgent need to establish a scientific method for determining the minimum air-entraining agent dosage. Summary of the Invention:

[0004] To solve the problems mentioned in the above background art, the purpose of the present invention is to provide a method for determining the minimum air-entraining agent dosage of early-frosted concrete based on bubble absorption of frost heave.

[0005] A method for determining the minimum air-entraining agent dosage of early-frosted concrete based on pore absorption of frost heave. The method for determining the minimum air-entraining agent dosage of early-frosted concrete is a process of obtaining the capillary index, initial air content in concrete at a specified pre-curing age, and the deformation index of concrete in a negative temperature environment, and then calculating the water-vapor ratio and the pore absorption efficiency value by combining the capillary index, initial air content, and deformation index, and obtaining the minimum air-entraining agent dosage required for the concrete to reach the frost-resistant state through the quantitative relationship between the water-vapor ratio and the pore absorption efficiency.

[0006] Preferably, obtaining the capillary index in concrete at a specified pre-curing age includes obtaining the capillary water volume W c and the capillary pore volume ΔV, and the obtaining process includes the following steps:

[0007] A1. Calculate the overall hydration degree ξ(t) of the cement paste using the Parrot&Killoh model:

[0008] ξ(t) = ξ t,i m i , where i = C3S, C2S, C3A, C4AF,

[0009] In the above formula, ξ(t) represents the overall hydration degree of the cement paste at time t, i represents the mineral components of the cement clinker as C3S, C2S, C3A, and C4AF, ξ t,i represents the hydration degree of phase i at time t, and m i is the mass fraction of phase i in the cement;

[0010]

[0011] In the above formula, S and S0 are the actual specific surface area and reference specific surface area of the cement respectively, S0 = 385 m 2 / kg, R 1,i , R 2,i and R 3,i represent the reaction rates of the hydration reaction of phase i clinker controlled by the nucleation and growth stage, diffusion process, and ion dissolution and transport process respectively, and β w / c,i , β RH and β T,i represent the effects of water-cement ratio, relative humidity, and temperature on the hydration rate of phase i respectively;

[0012]

[0013] R 2,i = K 2,i (1 - ξ t,i ) 2 / 3 / [1 - (1 - ξ t,i ) 1 / 3 ;

[0014]

[0015] β w / c,i = [1 + 3.333(H i ·w / c - ξ t,i )] 4 ;

[0016] β RH = [(RH - 0.55) / 0.45] 4 ;

[0017] β T,i = exp[-E a,i / R(1 / T - 1 / T0)];

[0018] In the above formula, w / c represents the water-cement ratio, RH represents the relative humidity, R represents the universal gas constant, T and T0 are the actual environmental temperature and the reference temperature, and T0 = 293K;

[0019] A2. After obtaining the overall hydration degree ξ(t) of the cement paste, the Powers hydration model is further used to calculate the capillary water content W c and the capillary pore content ΔV in the concrete:

[0020]

[0021] f air,cp (t) = 1 - f clin,cp (t) - f hyd,cp (t) - f w,cp (t);

[0022]

[0023] W c = f w,cp (t) × f cp ;

[0024] ΔV = f air,cp (t) × f cp ;

[0025] In the above formula, f clin,cp (t), f hyd,cp (t), f w,cp (t) and f air,cp (t) respectively represent the volume fractions of unhydrated clinker, hydration products, capillary water, and capillary pores caused by chemical shrinkage in the cement paste at time t; ρ clin , ρ w and ρ hyd respectively represent the densities of cement clinker, capillary water, and hydration products; f cp represents the volume fraction of the cement paste in the concrete; m w , m c , m ca and m fa are respectively the masses of water, cement, coarse aggregate, and fine aggregate in 1 cubic meter of concrete, ρ w , ρ c , ρ ca and ρ fa are respectively the densities of water, cement, coarse aggregate, and fine aggregate, V A is the initial air content of the concrete; when VA = 0, the capillary water and capillary pore contents of the concrete at the specified pre-curing age are denoted as W C_VA0 and ΔV VA0 .

[0026] Preferably, obtaining the initial air content in the concrete includes: presetting a plurality of low air-entraining agent dosages and testing the initial air content V of the fresh concrete at these dosages. A Among them, the dosage of the low air-entraining agent refers to a dosage not higher than the common dosage range of the selected air-entraining agent.

[0027] Among them, the initial air content can be tested by the pressure method in Standard GB / T50080.

[0028] Preferably, obtaining the deformation index of the concrete in a negative temperature environment includes: testing the apparent deformation V of the concrete in a negative temperature environment C_T and the thermal deformation V T . Among them, the process of testing the apparent deformation V of the concrete in a negative temperature environment C_T is to conduct tests through a negative temperature concrete apparent deformation testing device and obtain data, and calculate the apparent deformation according to the following formula:

[0029] V C_T =(H - D f ) 3 / (H - D0) 3 -1

[0030] In the above formula, V C_T represents the apparent deformation of the concrete after cooling to temperature T, H represents the height from the laser sensor to the vertex of the cone; D represents the distance from the laser sensor to the concrete surface; H - D represents the height of the concrete; the subscripts 0 and f correspond to the concrete before and after freezing respectively.

[0031] Preferably, the calculation process of calculating the water-vapor ratio and the pore absorption efficiency value by combining the capillary index, the initial air content, and the deformation index is as follows:

[0032] B1. Calculate the pore absorption frost heave deformation V ab_air ,

[0033] V ab_air =V E -V T -V C_Tf ;

[0034] In the above formula, V E represents the capillary water frost heave deformation volume, which is 0.09W C ; V C_Tf represents the apparent deformation value when reaching the preset negative temperature T f ; V T represents the thermal deformation, where V T =α C ΔT, α C =V C_5℃ / 15. In the above formula, α CAnd ΔT represent the volumetric thermal expansion coefficient and temperature difference of the cement-based material respectively. ΔT is calculated from the temperature difference between 20°C and the preset negative temperature T f The average volumetric thermal expansion coefficient α between 20°C and the preset negative temperature T of the concrete is approximately represented by using the volumetric thermal expansion coefficient of the concrete in the temperature range of 20 - 5°C f between; C ;

[0035] B2. Calculate the water-air ratio W C / A of the concrete before freezing,

[0036] W C / A = W C / (V A +ΔV);

[0037] B3. And the pore absorption efficiency coefficient γ of the concrete after freezing,

[0038] γ = V ab_air / (V A +ΔV);

[0039] Wherein, the water-air ratio W C / A refers to the ratio of the capillary water volume W c in the concrete to the total volume A of all pores. A is the sum of the initial air content and the capillary pore volume.

[0040] Preferably, the process of obtaining the minimum air-entraining agent dosage required for the concrete to reach the frost-resistant state through the quantitative relationship between the water-air ratio and the pore absorption efficiency includes:

[0041] Plot the γ-W c / A relationship diagram and fit the γ-W c / A damage area;

[0042] Determine the critical water-air ratio (W c / A) according to the intersection point of the data points and the non-damage line in the γ-W c / A relationship diagram; cr ;

[0043] Calculate the initial air content required for the concrete to reach the critical frost-resistant state according to the critical water-air ratio (W c / A), the capillary water volume fraction W cr under the condition of 0 initial air content and the capillary pore volume fraction ΔV C_VA0 of the concrete at the specified pre-curing age, and select the corresponding air-entraining agent dosage according to the type of air-entraining agent. VA0 ;

[0044] Preferably, the reasoning process of the non-damage line includes:

[0045] When the concrete reaches the non-damaged state, V ab_air =V E =0.09W C , stomatal absorption efficiency coefficient γ and water-gas ratio W C / A satisfies γ=0.09W C / A.

[0046] Preferably, the process of obtaining the minimum amount of air entraining agent required for concrete to achieve frost resistance through the quantitative relationship between the water-gas ratio and the pore absorption efficiency further comprises: c / A relationship determines the risk of concrete damage by freezing. c / A data point is located on the non-damage line, it is determined that the concrete has not suffered frost damage. c / A data point is below the non-damage line, which indicates that the concrete is at risk of freezing damage. The moisture migration process of the concrete after freezing is divided into three stages:

[0047] S1, when the water vapor ratio W C When / A is high, the pores absorb frost heave deformation V ab_air and the stomatal absorption efficiency coefficient γ are both C / A decreases and increases, then the moisture migration of concrete after freezing is in the stage of pore spacing coefficient control;

[0048] S2, when the water vapor ratio W C When / A decreases to a certain value, the pores absorb frost heave deformation V ab_air With W C / A decreases and the stomatal absorption efficiency coefficient γ increases with W C / A decreases and shows a decreasing trend, then the moisture migration of concrete after freezing is in a transition stage;

[0049] S3, when the water vapor ratio W C When / A is lower than the critical value, the pores absorb frost heave deformation V ab_air Reaching the capillary water frost heave deformation volume V E value, the pore absorption efficiency coefficient γ is in a state of steep decrease, and the moisture migration of concrete after freezing is in the stage of air content control.

[0050] Preferably, the γ-W c The intersection of the data point in the relationship diagram with the non-damage line determines the critical water-gas ratio (W c / A) cr include:

[0051] C1. When the γ-W c / A relationship diagram has obvious transition stage, and the data of transition stage are fitted;

[0052] C2. When there is no obvious transition stage shown in the γ-W c / A relationship diagram, data points in the damaged area are added or the data in the air void spacing coefficient control stage is directly fitted according to the required accuracy or workload requirements. Among them, the air-entraining agent dosage corresponding to the added data points in the damaged area should not be less than the maximum value in the air void spacing coefficient control stage but less than the minimum value in the air content control stage;

[0053] C3. According to the situation of adding data points in the damaged area, determine again whether there is a transition stage. If there is, execute step C1; if not, execute C2 again;

[0054] C4. The water-vapor ratio corresponding to the intersection point of the fitting curve and the non-damaged line is the critical water-vapor ratio (W c / A) cr , and the critical water-vapor ratio (W c / A) cr corresponds to the critical frost-resistant state of the cement-based material.

[0055] Preferably, the process of calculating the initial air content required for the concrete to reach the critical frost-resistant state according to the critical water-vapor ratio (W c / A) cr , the capillary water volume fraction W C_VA0 of the concrete under the condition of 0 initial air content at the specified pre-curing age, and the capillary pore volume fraction ΔV VA0 is as follows:

[0056]

[0057] where W C_VA0 and ΔV VA0 are the capillary water volume and capillary pore volume content when the initial air content of the concrete is 0, W C_VA0 = W C / (1 - V A ), ΔV VA0 = ΔV / (1 - V A ). Compared with the prior art, the beneficial effects of the present invention are as follows:

[0058] First, the present invention can evaluate the moisture migration state of the cement paste and further evaluate the magnitude of the risk of frost damage to the concrete.

[0059] Second, the method proposed by the present invention is applicable to mortars and concretes with different air-entraining agent dosages and has universality.

[0060] Third, compared with the traditional method relying on strength testing, the method provided by the present invention not only improves the prediction accuracy of the minimum air-entraining agent dosage but also reduces the test amount and saves the time required for strength curing.

[0061] IV. After a large number of tests, a series of critical water-vapor ratios of concrete can be obtained. Based on this, the lowest critical water-vapor ratio with universality is obtained. According to this critical water-vapor ratio, the admixture dosage of air-entraining agent for frost-resistant cement-based materials with universality can be further obtained. Description of the Drawings:

[0062] For ease of explanation, the present invention will be described in detail by the following specific embodiments and the accompanying drawings.

[0063] Figure 1 Schematic diagram of the deformation test device used in the present invention;

[0064] Figure 2 Mechanism diagram of the influence of the increase in air-entraining agent dosage on the evolution of γ;

[0065] Figure 3 For a typical γ-W C / A relationship diagram;

[0066] Figure 4 Variation curves of capillary water and capillary pores of cement paste with curing age in the examples;

[0067] Figure 5 Determination diagram of the critical water-vapor ratio of concrete in the examples;

[0068] Figure 6 Strength verification diagram of the minimum air-entraining agent dosage of concrete in the examples.

[0069] In the figure, 1 - horizontal base; 2 - vertical column; 3 - horizontal bracket; 4 - bottom box; 5 - upper cover; 6 - laser sensor; 7 - computer; 8 - temperature recorder. Specific Embodiments:

[0070] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be described below by specific examples shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the present invention.

[0071] Here, it should also be noted that in order to avoid obscuring the present invention due to unnecessary details, only the structures and / or processing steps closely related to the solution according to the present invention are shown in the drawings, while other details less related to the present invention are omitted.

[0072] Example 1: As Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 and Figure 6As shown in the figure, the following technical solution is adopted in this specific implementation manner: A method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on air pore absorption of frost heave, which is characterized in that: the method for determining the minimum air-entraining agent dosage of early frost-damaged concrete is to separately obtain the capillary index, initial air content in the concrete at a specified pre-curing age, and the deformation index of the concrete in a negative temperature environment, and then calculate the water-vapor ratio and the air pore absorption efficiency value by combining the capillary index, initial air content, and deformation index. The process of obtaining the minimum air-entraining agent dosage required for the concrete to reach the frost-resistant state is obtained through the quantitative relationship between the water-vapor ratio and the air pore absorption efficiency.

[0073] In the process of determining the minimum air-entraining agent dosage of early frost-damaged concrete, obtaining the capillary index in the concrete at a specified pre-curing age may include but is not limited to obtaining the capillary water volume and the capillary air pore volume. For the initial air content, it may be obtained but is not limited to by presetting multiple low air-entraining agent dosages and testing the initial air content of the fresh concrete at different air-entraining agent dosages. Among them, the type of air-entraining agent is not limited, and it may be but is not limited to rosin-based, synthetic, saponin-based, nitrate-based, etc. The deformation index may be but is not limited to the apparent deformation, temperature deformation, volume deformation, etc. of the concrete at different temperatures. This embodiment does not limit the method of obtaining the deformation index. Based on the above data, the water-vapor ratio and the air pore absorption efficiency value can be calculated, and the minimum air-entraining agent dosage required for the concrete to reach the frost-resistant state is obtained through the quantitative relationship between the water-vapor ratio and the air pore absorption efficiency. Among them, the quantitative relationship between the water-vapor ratio and the air pore absorption efficiency can be expressed by drawing a relationship diagram, forming a relationship table, etc.

[0074] Embodiment 2: This embodiment is a further limitation of the above embodiment. This embodiment only provides a preferred implementation manner. Obtaining the capillary index in the concrete at a specified pre-curing age includes obtaining the capillary water volume W c and the capillary air pore volume ΔV. The obtaining process includes the following steps:

[0075] A1. Use the Parrot&Killoh model to calculate the overall hydration degree ξ(t) of the cement paste:

[0076] ξ(t) = ξ t,i m i , i = C3S, C2S, C3A, C4AF,

[0077] In the above formula, ξ(t) represents the overall hydration degree of the cement paste at time t, i represents the cement clinker mineral composition as C3S, C2S, C3A, and C4AF, ξ t,i represents the hydration degree of phase i at time t, and m i is the mass fraction of phase i in the cement;

[0078]

[0079] In the above formula, S and S0 are the actual specific surface area and the reference specific surface area of cement respectively, and S0 = 385 m 2 / kg, R 1,i , R 2,i and R 3,i represent the reaction rates of the hydration reaction of the i-phase clinker controlled by the nucleation growth stage, the diffusion process, and the ion dissolution and transport process respectively. β w / c,i , β RH and β T,i represent the effects of the water-cement ratio, relative humidity, and temperature on the hydration rate of the i-phase respectively;

[0080]

[0081] R 2,i = K 2,i (1 - ξ t,i ) 2 / 3 / [1 - (1 - ξ t,i ) 1 / 3 ;

[0082]

[0083] β w / c,i = [1 + 3.333(H i ·w / c - ξ t,i )] 4 ;

[0084] β RH = [(RH - 0.55) / 0.45] 4 ;

[0085] β T,i = exp[-E a,i / R(1 / T - 1 / T0)];

[0086] In the above formula, w / c represents the water-cement ratio, RH represents the relative humidity, R represents the universal gas constant, T and T0 are the actual environmental temperature and the reference temperature respectively, and T0 = 293 K;

[0087] A2. After obtaining the overall hydration degree ξ(t) of the cement paste, the Powers hydration model is used to further calculate the capillary water content W c and the capillary pore content ΔV:

[0088]

[0089] f air,cp (t) = 1 - f clin,cp (t) - f hyd,cp (t) - f w,cp (t);

[0090]

[0091] W c =f w,cp (t)×f cp ;

[0092] ΔV=f air,cp (t)×f cp ;

[0093] In the above formula, f clin,cp (t), f hyd,cp (t), f w,cp (t) and f air,cp (t) respectively represent the volume fractions of unhydrated clinker, hydration products, capillary water, and capillary pores caused by chemical shrinkage in the cement paste at time t; ρ clin , ρ w and ρ hyd respectively represent the densities of cement clinker, capillary water, and hydration products; f cp represents the volume fraction of the cement paste in the concrete; m w , m c , m ca and m fa are respectively the masses of water, cement, coarse aggregate, and fine aggregate in 1 cubic meter of concrete, and ρ w , ρ c , ρ ca and ρ fa are respectively the densities of water, cement, coarse aggregate, and fine aggregate, and V A is the initial air content of the concrete; when VA = 0, the contents of capillary water and capillary pores in the concrete at the specified pre-curing age are denoted as W C_VA0 and ΔV VA0 . Among them, the parameters related to i used in the above formula can be seen in Table 1:

[0094] Table 1 Parameters required for the Parrot&Killoh model calculation

[0095]

[0096]

[0097] Example 3: This example is a further limitation of the above example. This example only provides a preferred implementation method. Obtaining the initial air content of the concrete includes: presetting multiple low air-entraining agent dosages, and testing the initial air content V A of the fresh concrete at these dosages. Among them, the dosage of the low air-entraining agent refers to not higher than the common dosage range of the selected air-entraining agent, and the initial air content V AThe test can be but is not limited to being carried out in accordance with the specification of GB / T 50080.

[0098] Example 4: This example further limits the above examples. This example only provides a preferred implementation manner. Obtaining the deformation index of concrete in a negative temperature environment includes: testing the apparent deformation V of concrete in a negative temperature environment C_T and the thermal deformation V T .

[0099] Specifically, this example provides a preferred implementation manner. Among them, the apparent deformation is the deformation value directly observed in the test. During the process of testing the apparent deformation and thermal deformation of concrete in a negative temperature environment, it involves the acquisition of relevant data by the apparent deformation test device. The apparent deformation test device is jointly composed of a deformation test system and a temperature test system. The deformation test system is composed of a horizontal base 1, a vertical column 2, a horizontal bracket 3, a bottom box 4, an upper cover 5, a laser sensor 6, and a computer 7. Among them, the vertical column 2 is arranged on the horizontal base 1, the bottom box 4 is placed on the horizontal base 1, the bottom box 4 is a double-layer cavity structure, the outer layer is a cylinder, and the inner layer is an inverted cone. The apex angle of the cone is 60°. Fresh concrete can be poured inside the conical shell of the bottom box 4. The upper part of the bottom box 4 is equipped with an upper cover 5. The upper cover 5 is a ring-shaped double-layer cavity structure. To achieve temperature control of the concrete, two interfaces are respectively arranged on the side surface of the bottom box 4 and the side surface of the upper cover 5, both of which are connected to a hollow pipe, serving as the inlet and outlet of the circulating liquid. Among them, the circulating liquid can be but is not limited to selecting a liquid with a freezing point lower than the freezing temperature and a boiling point higher than 40°C, such as ethylene glycol, etc. The temperature of the bottom box 4 and the upper cover 5 is controlled by the circulating liquid, thereby controlling the temperature of the concrete. The laser sensor 6 is fixed on the vertical column 2 through the horizontal bracket 3, and the laser can reach the top surface of the concrete through the hole in the center of the upper cover 5. The laser sensor 6 is connected to the computer 7 to record the laser transmission distance and calculate the apparent deformation according to the following formula. The temperature test system is composed of a thermocouple and a temperature recorder 8. When pouring concrete into the bottom box 4, one end of the thermocouple is embedded in the center position of the concrete, and the other end is connected to the temperature recorder 8.

[0100] V C_T =(H - D f ) 3 / (H - D0) 3 - 1

[0101] In the above formula, V C-T represents the apparent deformation of the concrete after cooling to temperature T, H represents the height from the laser sensor 6 to the cone vertex; D represents the distance from the laser sensor 6 to the concrete surface; H - D represents the height of the concrete; the subscripts 0 and f respectively correspond to the concrete before and after freezing.

[0102] In this embodiment, when testing the apparent deformation of concrete, the temperature of the circulating liquid can be first adjusted to 20°C, and the circulating liquid is circulated in the bottom box body 4 and the upper cover 5 until the temperature inside the conical shell is stabilized at 20°C. Subsequently, the freshly mixed concrete is poured into the conical shell of the bottom box body 4, the thermocouple is embedded at the center of the concrete, and after the concrete is cured to the specified time, the temperature of the circulating liquid is immediately adjusted to the preset negative temperature, and at the same time, the laser sensor 6 and the temperature recorder 8 are turned on to record the apparent deformation and temperature deformation of the concrete. During the above process, except for the pouring process of the cement-based material, the upper cover 5 should always be covered on the bottom box body 4 to maintain the internal temperature of the conical shell.

[0103] In this embodiment, since the moisture in the concrete is mainly composed of capillary water and gel water, and the freezing temperature of the gel water is lower than -50°C, considering that the temperature in winter in China is generally not lower than -50°C. Therefore, the present invention only considers the case of capillary water being frozen. For safety reasons, the air-entraining agent dosage in the present invention can enable the concrete to resist the frost heave damage caused by all capillary water being frozen. Since the pores of early-age concrete are relatively coarse and are extremely vulnerable to freezing, for energy conservation, the maximum negative temperature of -15°C can be preset as the temperature at which all capillary water in the concrete is frozen. Therefore, a negative temperature between -15 and -50°C can be preset to test the apparent deformation and temperature change of the concrete after being frozen at the specified pre-curing time and negative temperature. When the concrete is cooled to 5°C and reaches the preset negative temperature T f the apparent deformation values are denoted as V C_5℃ and V C_Tf .

[0104] Example 5: This example is a further limitation of the above example. This example only provides a preferred implementation manner. Considering that the apparent deformation is jointly composed of temperature deformation, capillary water frost heave deformation, and air pore absorption frost heave deformation, the calculation process of the water-gas ratio and the air pore absorption efficiency value calculated in combination with the capillary index, the initial air content, and the deformation index is as follows:

[0105] B1. Calculate the air pore absorption frost heave deformation V ab_air ,

[0106] V ab_air =V E -V T -V C_Tf ;

[0107] In the above formula, V E represents the capillary water frost heave deformation volume, which is 0.09W C ; V C_Tf represents the apparent deformation value when reaching the preset negative temperature T f ; V TRepresents the temperature deformation, where V T = α C ΔT, α C = V C_5℃ / 15. In the above formula, α C and ΔT respectively represent the volumetric thermal expansion coefficient and the temperature difference of the cement-based material. ΔT is calculated from the temperature difference between 20°C and the preset negative temperature T f . The volumetric thermal expansion coefficient of concrete in the temperature range of 20 - 5°C is used to approximately represent the average volumetric thermal expansion coefficient α f between 20°C and the preset negative temperature T C ;

[0108] B2. Calculate the water-vapor ratio W C / A of the concrete before freezing,

[0109] W C / A = W C / (V A +ΔV);

[0110] B3. And the pore absorption efficiency coefficient γ of the concrete after freezing,

[0111] γ = V ab_air / (V A +ΔV);

[0112] Among them, the water-vapor ratio W C / A refers to the ratio of the capillary water volume W c in the concrete to the total volume A of all pores. A is the sum of the initial air content and the capillary pore volume. The pore absorption efficiency coefficient refers to the ratio of the volume of the squeezed water frozen after entering the pores to the total pore volume.

[0113] Example Six: This example is a further limitation of the above example. This example only provides a preferred implementation manner. The process of obtaining the minimum air-entraining agent dosage required for the concrete to reach the frost-resistant state through the quantitative relationship between the water-vapor ratio and the pore absorption efficiency includes:

[0114] Draw the γ - W c / A relationship diagram and fit the γ - W c / A damage area;

[0115] Determine the critical water-vapor ratio (W c / A) c according to the intersection point of the data points and the non-damage line in the γ - W cr / A relationship diagram;

[0116] According to the critical water-vapor ratio (W c / A) cr, the capillary water volume W of concrete under the initial air content condition of 0 at the specified pre-curing age C_VA0 and the capillary pore volume ΔV VA0 , calculate the initial air content required for the concrete to reach the critical frost-resistant state, and select the corresponding air-entraining agent dosage according to the type of air-entraining agent.

[0117] Example 7: This example is a further limitation of the above example. This example only provides a preferred implementation manner. The reasoning process of the non-damage line includes:

[0118] When the concrete reaches the non-damage state, V ab_air = V E = 0.09W C , the pore absorption efficiency coefficient γ and the water-vapor ratio W C / A satisfy γ = 0.09W C / A.

[0119] Example 8: This example is a further limitation of the above example. This example only provides a preferred implementation manner. The process of obtaining the minimum air-entraining agent dosage required for the concrete to reach the frost-resistant state through the quantitative relationship between the water-vapor ratio and the pore absorption efficiency also includes: According to the γ-W c / A relationship, determine the frost damage risk of the concrete. When the γ-W c / A data point is located on the non-damage line, it is determined that the concrete has not suffered frost damage. At this time, the water migration after the concrete is frozen is controlled by the air content. When the γ-W c / A data point is located below the non-damage line, it is determined that the concrete has a risk of frost damage. As the air-entraining agent dosage increases, the initial air content V A of the concrete increases, resulting in a decrease in W C / A. The pore absorption of frost heave deformation V ab_air and the pore absorption efficiency coefficient γ are both affected. According to the development trends of V ab_air and γ, the development process of γ and its corresponding water migration process after the cement-based material is frozen can be divided into three stages, as shown in Figure 2 and Figure 3 , Figure 2 where a represents the maximum value of the pore absorption efficiency coefficient γ, b represents the critical frost-resistant state, Figure 3 in which the P line represents the non-damage line, the L line is the critical water-vapor ratio line, the M area represents the safe water-vapor ratio, and the N area represents the water-vapor ratio with a risk of damage;

[0120] In the following steps, the S1 stage is the air pore spacing coefficient control stage, the control factor is the air pore spacing coefficient, the S2 is the transition stage, the control factors are the air pore spacing coefficient and the air content, and the S3 is the air content control stage. The control factor in this stage is the air content;

[0121] S1. When the water-vapor ratio W C / A is high, both the frost heave deformation V ab_air absorbed by the air voids and the air void absorption efficiency coefficient γ increase with the decrease of W C / A. Then, the moisture migration in concrete after freezing is in the stage controlled by the air void spacing coefficient. This corresponds to concrete with poor pore structure. The large air void spacing coefficient is the main obstacle for the squeezed moisture in the frozen pores to migrate to the air voids. At this time, increasing the air-entraining agent dosage will lead to a significant decrease in the air void spacing coefficient and an increase in the frost heave volume absorbed by the air voids. Correspondingly, γ also increases. The concrete in this stage faces a high risk of frost damage.

[0122] S2. When the water-vapor ratio W C / A decreases to a certain value, the frost heave deformation V ab_air absorbed by the air voids increases with the decrease of W C / A, and the air void absorption efficiency coefficient γ shows a decreasing trend with the decrease of W C / A. Then, the moisture migration in concrete after freezing is in the transition stage. With the further increase of the air-entraining agent dosage, the number of air voids gradually tends to be saturated, and the decreasing trend of the air void spacing coefficient slows down. Its promotion effect on moisture migration is limited. At this time, the contribution of the increase in the air content V A to the frost heave absorption volume cannot be ignored. However, since the increase in the air content V A exceeds the increase in the absorbed frost heave deformation, γ begins to decrease. Therefore, the moisture migration process in the frozen concrete in this stage is jointly controlled by the air void spacing coefficient and the air content, and the concrete still has a risk of frost damage.

[0123] S3. When the water-vapor ratio W C / A is lower than the critical value, the frost heave volume generated by the capillary water freezing can be completely absorbed by the nearby air voids. The frost heave deformation V ab_air absorbed by the air voids reaches the value of the frost heave deformation volume V E of the capillary water. The moisture migration in concrete after freezing is in the stage controlled by the air content. With the continuous increase of the air-entraining agent dosage, the frost heave deformation V ab_air absorbed by the air voids decreases with the frost heave deformation volume V E of the capillary water. At the same time, the total air content increases. The superposition effect of the two makes the decreasing trend of γ more significant. The cement-based materials in this stage will not have a risk of frost damage.

[0124] For concrete with a specific mix ratio and pre-curing time, as the air-entraining agent dosage increases, the S1 stage and the S2 stage do not always coexist. The reason for the non-existence of the S1 stage is that the blank-group concrete may exhibit a better pore structure due to dense filling and a long pre-curing time. At this time, increasing the air-entraining agent dosage cannot significantly reduce the air void spacing factor, and the water migration of the blank-group concrete has entered the transition stage when it is frozen. The reason for the non-existence of the S2 stage is that the pore structure of the blank-group concrete is poor when it is frozen, and a large number of pores are required to reduce its air void spacing factor. When its air void spacing factor is reduced to a level where it is not the main obstacle to water migration, its air content is already sufficient to accommodate all the water expansion volume during freezing. Therefore, there is no transition stage.

[0125] Example Nine: This example further defines the above examples. This example only provides a preferred implementation manner. The critical water-vapor ratio (W c / A) is determined based on the data points at the intersection of the γ-W c / A relationship diagram and the non-damage line, including: cr including:

[0126] C1. When there is an obvious transition stage in the γ-W c / A relationship diagram of the cement-based material, fit the data of the transition stage;

[0127] C2. When there is no obvious transition stage shown in the γ-W c / A relationship diagram, then select to increase the data points in the damage area or directly fit the data of the air void spacing factor control stage according to the required accuracy or workload requirements. Among them, the air-entraining agent dosage corresponding to the increased data points in the damage area should not be less than the maximum value in the air void spacing factor control stage, but less than the minimum value in the air content control stage;

[0128] C3. According to the situation of increasing the data points in the damage area, determine again whether there is a transition stage. If it exists, execute step C1. If it does not exist, execute C2 again;

[0129] C4. The water-vapor ratio corresponding to the intersection of the fitting curve and the non-damage line is the critical water-vapor ratio (W c / A), and the critical water-vapor ratio (W cr / A) c / A) cr corresponds to the critical frost-resistant state of the cement-based material;

[0130] C5. The critical water-vapor ratio (W c / A) crIt is converted into the critical saturation through the following formula. The critical saturation is a more commonly used index, which can be converted into the critical saturation as an index, and it has a more intuitive physical meaning, facilitating comparison between different studies and engineering applications:

[0131]

[0132] Example Ten: This example further limits the above examples. This example only provides a preferred implementation manner. The process of calculating the initial air content required for concrete to reach the critical frost-resistant state according to the critical water-vapor ratio (W c / A) cr , the capillary water volume fraction W C_VA0 and the capillary pore volume fraction ΔV VA0 of concrete under the condition of the initial air content of 0 at a specified pre-curing age is as follows:

[0133]

[0134] Example Eleven: This example provides a method for determining the dosage of air-entraining agent required for concrete to reach the critical frost-resistant state when pre-cured at 20°C for 24 hours. This example only provides a specific implementation manner, and the method provided by the present invention is not limited to only the situation described in this example. In this example, the main component of the selected air-entraining agent is triterpenoid saponin. The mix proportion and composition phase density of the blank group concrete are shown in Table 2, and the cement composition is shown in Tables 3 and 4:

[0135] Table 2 Mix Proportion and Composition Phase Density of Blank Group Concrete (kg / m 3 )

[0136]

[0137] Table 3 Chemical Composition of Cement

[0138]

[0139]

[0140] Table 4 Mineral Composition of Cement

[0141]

[0142] Among them, the change curves of the capillary water volume content and capillary pore volume content of the cement paste with the pre-curing age are shown in Figure 4 . At 24 hours, the water content of the cement paste is 42.5%, and the capillary pore content is 2.34%. When the initial air content is 0, the capillary water content W C_VA0 and the capillary pore content ΔV VA0 of the concrete are 15.28% and 0.84% respectively.

[0143] The preset air-entraining agent dosage is Test the initial air content V of the concrete A , which are 2.6%, 3.7%, 5.1% and 6.3% respectively. Then, according to the aggregate volume fraction and initial air content of the concrete, the capillary water content Wc of the pre-cured concrete for 24 h can be calculated to be 14.88%, 14.71%, 14.50% and 14.31% respectively, and the capillary pore volume ΔV are 0.82%, 0.81%, 0.80% and 0.79% respectively; the temperature at which the capillary water in the preset concrete is completely frozen is -17.5°C. Test the apparent deformation and temperature curve of the concrete during the cooling process. After the concrete is cooled to -17.5°C, the apparent deformation values V of the concrete under the four air-entraining agent dosages at the preset negative temperature c_-17.5℃ are stabilized at 0.132%, 0.014%, 0.012% and -0.039% respectively; the deformations of the concrete in the temperature range of 20~5°C are 13.72×10 -6 , 15.32×10 -6 , 18.52×10 -6 and 19.21×10 -6 °C respectively, and their corresponding thermal deformations are 0.0514%, 0.0575%, 0.0695% and 0.0720% respectively. The expansion volume V E after the capillary water is completely frozen is 1.34%, 1.32%, 1.31% and 1.29% respectively; then the air void absorption of frost heave deformation V ab_air is 1.157%, 1.249%, 1.229% and 1.257% respectively.

[0144] Next, calculate the water-vapor ratio W C / A and the air void absorption efficiency coefficient γ, and draw the γ-W c / A relationship diagram, as shown in Figure 5 ; the γ value of the concrete with the air-entraining agent dosage of is lower than that of the concrete with , and it is below the non-damage line, indicating that the concrete is in the transition stage under these two dosages and there is a risk of frost damage; the data points corresponding to the concrete with the air-entraining agent dosages of and are located on the non-damage line, and there is no risk of frost damage to the concrete under this dosage; the abscissa of the intersection point of the data fitting line of the concrete with the air-entraining agent dosages of and and the non-damage line is 2.77. According to the formula The initial air void content required for the concrete to reach the frost resistance critical state is 4.47%; according to the properties of the air-entraining agent selected in this embodiment, when reaching this initial air content, the required AEA dosage is

[0145] To verify the air-entraining agent dosage selected in this embodiment, a strength test was conducted in this embodiment. The air-entraining agent dosages for the formed positive-temperature strength specimens, negative-temperature strength specimens, and temperature specimens with the selected optimal air-entraining agent dosage were and the sizes of all of them were 100mm×100mm×100mm. Thermocouples were embedded inside the temperature specimens to record the temperature of the concrete; the positive-temperature strength specimens were cured under standard curing conditions until 28 days. After the negative-temperature strength specimens and temperature specimens were cured under standard curing conditions for 24h, they were placed in an environment of -17.5°C. Through the temperature monitoring of the specimens, it can be seen that after 3h, the temperature of the concrete decreased to the ambient temperature. The negative-temperature strength specimens were transferred to an environment of 20°C and continued to be cured until 28d. The strengths of the positive-temperature and negative-temperature strength specimens at 28d were respectively tested. The obtained strength data are shown in Figure 6 . When the air-entraining agent dosage is lower than , the strength retention rates of the frozen concrete are all lower than 95%. When the air-entraining agent dosage reaches , its strength retention rates all exceed 95%. This proves the accuracy of the prediction of the air-entraining agent dosage in the present invention.

[0146] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on pore absorption of frost heave, characterized in that: The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete is a process in which, after obtaining the capillary index, initial air content, and deformation index of concrete under a specified pre-curing age, the water-vapor ratio and air void absorption efficiency value are calculated by combining the capillary index, initial air content, and deformation index, and the minimum air-entraining agent dosage required for the concrete to reach the frost-resistant state is obtained through the quantitative relationship between the water-vapor ratio and air void absorption efficiency.

2. The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on air pore absorption of frost heave according to claim 1, wherein: Obtaining the capillary index in concrete at a specified pre-curing age includes obtaining the capillary water volume W c and the capillary pore volume ΔV. The obtaining process includes the following steps: A1. Calculate the overall hydration degree ξ(t) of the cement paste using the Parrot&Killoh model: ξ(t) = ξ t,i m i , i = C3S, C2S, C3A, C4AF, In the above formula, ξ(t) represents the overall hydration degree of the cement paste at time t, i represents the mineral components of the cement clinker as C3S, C2S, C3A, and C4AF, and ξ t,i represents the hydration degree of the i-phase at time t, and m i is the mass fraction of the i-phase in the cement; In the above formula, S and S0 are the actual specific surface area and reference specific surface area of cement respectively, and S0 = 385 m 2 / kg, R 1,i 、R 2,i and R 3,i represent the reaction rates of the hydration reaction of i-phase clinker controlled by the nucleation growth stage, diffusion process, and ion dissolution transport process respectively. β w / c,i 、β RH and β T,i represent the effects of water-cement ratio, relative humidity, and temperature on the hydration rate of i-phase respectively; R 2,i = K 2,i (1 - ξ t,i ) 2 / 3 / [1 - (1 - ξ t,i ) 1 / 3 ; β w / c,i = [1 + 3.333(H i ·w / c - ξ t,i )] 4 ; β RH = [(RH - 0.55) / 0.45] 4 ; β T,i = exp[-E a,i / R(1 / T - 1 / T0)]; In the above formula, w / c represents the water-cement ratio, RH represents the relative humidity, R represents the universal gas constant, T and T0 are the actual ambient temperature and reference temperature, and T0 = 293K; After obtaining the overall hydration degree ξ(t) of the cement paste, the Powers hydration model is further used to calculate the capillary water content W c and the capillary pore content ΔV inside the concrete: f air,cp f(t) = 1 - f clin,cp f(t) - f hyd,cp f(t) - f w,cp f(t); W c = f w,cp (t) × f cp ; ΔV = f air,cp (t) × f cp ; In the above formula, f clin,cp (t), f hyd,cp (t), f w,cp (t) and f air,cp (t) represent the volume fractions of unhydrated clinker, hydration products, capillary water, and capillary pores caused by chemical shrinkage in the cement paste at time t, respectively; ρ clin , ρ w and ρ hyd represent the densities of cement clinker, capillary water, and hydration products, respectively; f cp represents the volume fraction of the cement paste in the concrete; m w , m c , m ca and m fa are the masses of water, cement, coarse aggregate, and fine aggregate in 1 cubic meter of concrete, respectively; ρ w , ρ c , ρ ca and ρ fa are the densities of water, cement, coarse aggregate, and fine aggregate, respectively; V A is the initial air content of the concrete. When VA = 0, the contents of capillary water and capillary pores in the concrete at the specified pre-curing age are denoted as W C_VA0 and ΔV VA0 .

3. The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on pore absorption of frost heave according to claim 2, wherein: Obtaining the initial air content in concrete includes: presetting a plurality of low air-entraining agent dosages, and testing the initial air content V of the freshly mixed concrete at these dosages A , wherein the dosage of the low air-entraining agent refers to a dosage not higher than the common dosage range of the selected air-entraining agent; Among them, the initial air content can be tested using the pressure method in Specification GB / T50080.

4. The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on pore absorption of frost heave according to any one of claims 1 to 3, characterized in that: The deformation indexes of concrete under negative temperature environment include: testing the apparent deformation V of concrete under negative temperature environment C_T and the thermal deformation V T , wherein, the process of obtaining the apparent deformation V of the tested concrete under negative temperature environment C_T is to conduct tests through a negative temperature concrete apparent deformation test device and obtain data, and calculate the apparent deformation according to the following formula: V C_T = (H - D f ) 3 / (H - D0) 3 - 1 In the above formula, V C_T represents the apparent deformation of the concrete after it cools down to temperature T; H represents the height from the sensor to the apex of the cone; D represents the distance from the sensor to the concrete surface; H - D represents the height of the concrete; the subscripts 0 and f correspond to the concrete before and after freezing respectively.

5. The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on pore absorption of frost heave according to claim 4, wherein: The calculation process for calculating the water-vapor ratio and air void absorption efficiency value by combining the capillary index, initial air content, and deformation index is as follows: B1. Calculate the frost heaving deformation V absorbed by the pores ab_air , V ab_air = V E -V T -V C_Tf ; In the above formula, V E represents the volume of capillary water frost heaving deformation, which is 0.09W C ; V C_Tf represents the apparent deformation value at the preset negative temperature T f ; V T represents the temperature deformation, where V T =α C ΔT, α C =V C_5℃ / 15. In the above formula, α C and ΔT respectively represent the volume thermal expansion coefficient and temperature difference of the cement-based material. ΔT is calculated from the temperature difference T f between 20°C and the preset negative temperature. The volume thermal expansion coefficient of concrete in the temperature range of 20 - 5°C is used to approximately represent the average volume thermal expansion coefficient α f of concrete in the range from 20 to the preset negative temperature T C ; B2. Calculate the water-vapor ratio W of the concrete before freezing C / A, W C / A = W C / (V A + ΔV); B3. And the air void absorption efficiency coefficient γ after the concrete is frozen, γ = V ab_air / (V A + ΔV); Among them, the water-vapor ratio W C / A refers to the volume of capillary water W c in concrete, which is the ratio of the volume of capillary water to the total volume of all pores A. A is the sum of the initial air content and the volume of capillary pores.

6. The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on pore absorption of frost heave according to claim 5, characterized in that: The process of obtaining the minimum air-entraining agent dosage required for the concrete to reach the frost-resistant state through the quantitative relationship between the water-vapor ratio and air void absorption efficiency includes: Plot the γ-W c / A relationship diagram and fit the γ-W c / A damage area; According to the data points in the γ-W c / A relationship diagram and the intersection points of the non-damage line to determine the critical water-gas ratio (W c / A) cr ; According to the critical water-vapor ratio (W c / A) cr and the capillary water volume fraction W C_VA0 and the capillary pore volume fraction ΔV VA0 of concrete under the conditions of 0 initial air content at a specified pre-curing age, calculate the initial air content required for the concrete to reach the critical frost-resistant state, and select the corresponding air-entraining agent dosage according to the type of air-entraining agent.

7. A method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on pore absorption of frost heave according to claim 6, characterized in that: The reasoning process of the non-damage line includes: When the concrete reaches the non-damaged state, V ab_air = V E = 0.09W C , the pore absorption efficiency coefficient γ and the water-vapor ratio W C / A satisfy γ = 0.09W C / A.

8. The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on pore absorption of frost heave according to claim 7, wherein: The process of obtaining the minimum air-entraining agent dosage required for concrete to reach the frost-resistant state through the quantitative relationship between the water-vapor ratio and the air pore absorption efficiency further includes: According to the γ-W c / A relationship, determine the frost damage risk of the concrete. When the γ-W c / A data point is located on the non-damage line, it is determined that the concrete has not suffered frost damage. When the γ-W c / A data point is located below the non-damage line, it is determined that the concrete has the risk of frost damage. Among them, the moisture migration process of the concrete after being frozen is divided into three stages: S1. When the water-gas ratio W C / A is high, both the frost heave deformation V ab_air absorbed by the pores and the pore absorption efficiency coefficient γ increase with the decrease of W C / A, and the moisture migration in the concrete after freezing is in the stage controlled by the air void spacing factor; S2. When the water-gas ratio W C / A decreases to a certain value, the air pores absorb the frost heaving deformation V ab_air increases as W C / A decreases, and the air pore absorption efficiency coefficient γ shows a decreasing trend as W C / A decreases, then the moisture migration in the concrete after freezing is in the transition stage; S3, when the water vapor ratio W C When / A is lower than the critical value, the pores absorb frost heave deformation V ab_air Reaching the capillary water frost heave deformation volume V E value, the pore absorption efficiency coefficient γ is in a state of steep decrease, and the moisture migration of concrete after freezing is in the stage of air content control.

9. The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on pore absorption of frost heave according to claim 8, characterized in that: The critical water-gas ratio (W c / A) is determined based on the data points in the γ-W c / A relationship diagram and the intersection points with the non-damage line cr and includes: C1. When there is an obvious transition stage in the γ-W / A relationship diagram of the cement-based material, fit the data of the transition stage; c ​ C2. When there is no obvious transition stage shown in the γ-W c / A relationship diagram, data points in the damaged area are added or the data in the air void spacing coefficient control stage is directly fitted according to the required accuracy or workload requirements. Among them, the air-entraining agent dosage corresponding to the added data points in the damaged area should not be less than the maximum value in the air void spacing coefficient control stage but less than the minimum value in the air content control stage; C3. According to the situation of adding data points in the damaged area, determine again whether there is a transition stage. If there is, execute step C1. If not, execute C2 again; The water-vapor ratio corresponding to the intersection point of the fitting curve and the non-damage line is the critical water-vapor ratio (W c / A) cr , and the critical water-vapor ratio (W c / A) cr corresponds to the critical frost-resistant state of the cement-based material.

10. The method for determining the minimum air-entraining agent dosage of early frost-damaged concrete based on air pore absorption of frost heave according to claim 9, wherein: The process of calculating the initial air content required for concrete to reach the critical frost-resistant state according to the critical water-vapor ratio (W c / A) cr and the initial air content condition, the capillary water volume fraction W C_VA0 and the capillary pore volume fraction ΔV VA0 of concrete at a specified pre-curing age is as follows: Where, W C_VA0 and ΔV VA0 are the capillary water volume and the capillary pore volume content when the initial air content of the concrete is 0, W C_VA0 = W C / (1 - V A ), ΔV VA0 = ΔV / (1 - V A ).

Images