Consideration of ITZ's strength prediction method of concrete under positive temperature curing and negative temperature freezing

Abstract

The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing considering ITZ belongs to the field of concrete strength prediction. In order to solve the problem that the current concrete strength prediction model does not pay attention to the ITZ region of concrete, thereby the problem of low concrete strength prediction accuracy exists. The corresponding multi-scale composition structure is established according to the prediction object, and the cement paste damage control phase is determined; if the prediction object is cement paste, the hydration degree of cement paste is calculated; if the prediction object is mortar or concrete, the water-cement ratio distribution in the prediction object is determined, and the hydration degree of each sub-ITZ layer and the base paste is calculated; if negative temperature is experienced, the ice content needs to be calculated, then the cement paste phase division is carried out, the volume fraction of each inclusion phase in the RVE at each scale in the prediction object is obtained, the homogenization elastic modulus of the prediction object is calculated, and then the strength of the prediction object is calculated.

Description

Technical Field

[0001] This invention belongs to the field of concrete strength prediction, specifically relating to a method for predicting the strength of concrete under positive temperature curing and negative temperature freezing, taking into account ITZ. Background Technology

[0002] Concrete strength is a crucial indicator for ensuring the safety and quality of engineering projects. Predicting concrete strength not only saves on the labor and economic costs of testing but also allows for a quick and effective determination of the concrete's long-term strength, providing guidance for mix design.

[0003] In concrete, the significant size difference between aggregate and cement particles leads to the wall effect and the interfacial transition zone (ITZ). As a "binder" between aggregate and matrix paste, the ITZ significantly affects the elastic modulus and strength of concrete. For concrete at positive temperatures, the locally higher water-cement ratio in the ITZ weakens its performance compared to matrix paste, reducing the bond between aggregate and paste and negatively impacting overall concrete strength. Conversely, for concrete at negative temperatures, the ice content within the ITZ after freezing is significantly higher than in the matrix paste, greatly enhancing the bond between aggregate and paste and significantly boosting concrete strength. Therefore, the role of the ITZ in concrete strength prediction cannot be ignored. However, current concrete strength prediction models do not consider the ITZ region, thus reducing the accuracy of strength predictions. Summary of the Invention

[0004] The present invention aims to address the problem that current concrete strength prediction models do not take into account the ITZ region of concrete, resulting in low accuracy in concrete strength prediction.

[0005] A method for predicting the strength of concrete under positive temperature curing and negative temperature freezing, considering ITZ, includes the following steps:

[0006] Step A: Establish a cement-based material information database; establish the corresponding multi-scale composition structure according to the predicted object, and determine the destructive control phase of cement paste; when the predicted object is mortar or concrete, execute step B; when the predicted object is cement paste, directly execute step C.

[0007] The cement paste failure control phase is the inclusion phase used to determine whether the cement paste has failed.

[0008] Step B: For cases where the object to be predicted is mortar or concrete, determine the water-cement ratio distribution within the object; then proceed to step C.

[0009] Step C: If the object to be predicted is cement paste, calculate the degree of hydration of the cement paste; if the object to be predicted is mortar or concrete, calculate the degree of hydration of each sub-ITZ layer and the matrix paste; the sub-ITZ layer refers to the ITZ layer being subdivided into multiple shell-like sub-ITZ layers of the same thickness that surround the aggregate.

[0010] Then determine whether the object being predicted has experienced a negative temperature. If it has, proceed to step D; otherwise, proceed directly to step E.

[0011] Step D: For the case where the predicted object experiences a negative temperature, if the predicted object is cement paste, calculate the ice content of the cement paste; if the predicted object is mortar or concrete, calculate the ice content of each sub-ITZ layer and the matrix paste of the predicted object; then proceed to step E.

[0012] Step E: Perform phase separation of cement paste to obtain the volume fraction of each inclusion phase inside the RVE at each scale in the cement paste;

[0013] Step F: Calculate the homogenized elastic modulus of the object to be predicted;

[0014] Step G: Calculate the intensity of the predicted object:

[0015] When the object of prediction is cement paste, calculate the load when the cement paste is in a critical failure state. This load is the strength of the cement paste.

[0016] When the predicted object is mortar or concrete, first calculate the external load when each undamaged sub-ITZ layer and the matrix paste are in the critical failure state. If the predicted object is lightweight aggregate concrete, it is also necessary to calculate the external load when the aggregate phase is in the critical failure state. The critical failure area corresponding to the minimum value of the external load of all predicted objects is judged as failure, and the minimum load is recorded as the instantaneous strength. Determine whether the failed area is the matrix paste of the predicted object. If not, reduce the mechanical properties of the failed area and return to step F. If the matrix paste of the predicted object is determined to be failed, the maximum value of all instantaneous strengths is the strength of the predicted object.

[0017] Furthermore, the process of establishing the multi-scale compositional structure described in step A includes the following steps:

[0018] Based on the characteristic length of the inclusion phases in concrete, the concrete is divided into several scales, each scale containing only one or more inclusion phases with basically the same characteristic length; a representative volume unit, i.e., RVE, is established at each scale to represent the distribution characteristics of the inclusion phases.

[0019] Furthermore, the process of determining the control phase for cement paste degradation in step A includes the following steps:

[0020] First, based on the established multi-scale composition structure, the scale of the cement paste destruction control phase is selected. The cement paste destruction control phase must be located in the cement paste, and its scale must be smaller than the cement paste scale. At the scale of the cement paste destruction control phase, the inclusion phase with the smallest elastic modulus other than the porous phase is selected as the cement paste destruction control phase. The cement paste destruction control phase cannot be composed of pores alone and should contain at least one solid hydration product.

[0021] Furthermore, the cement-based material information database mentioned in step A includes the mechanical properties and shapes of the aggregate phase, cement clinker phase, all hydration product phases, aqueous phase and gas phase, as well as the yield stress of the cement paste failure control phase and the yield stress of various aggregates.

[0022] The yield stress of the failure-controlling phase of the cement paste is taken as an empirical value or determined by experiment; the process of determining it by experiment includes the following steps:

[0023] A batch of cement pastes with different water-cement ratios were formed and cured under standard curing conditions for different ages. The strength of these cement pastes was tested. The strength of cement paste with a specified water-cement ratio after curing for a specified age was calculated, and the relationship function between the strength and the yield stress of the cement paste failure control phase was obtained. In this relationship function, the calculated strength was set to be equal to the test strength of the cement paste with the corresponding water-cement ratio and curing age, and the yield stress of the cement paste failure control phase under this condition was calculated. The same calculation operation was performed for cement pastes with all water-cement ratios and curing ages to obtain the yield stress of the cement paste failure control phase under multiple water-cement ratios and curing ages. Outliers were removed, and the average value of these yield stresses was taken as the yield stress of the cement paste failure control phase.

[0024] Furthermore, step B, the process of determining the water-cement ratio distribution in the target object, includes the following steps:

[0025] First, determine the ITZ width and calculate the width of each sub-ITZ layer. Calculate the total ITZ integral, the volume fraction of each sub-ITZ layer, and the volume fraction of the matrix paste based on the aggregate particle size distribution and aggregate volume fraction. Then, calculate the water-cement ratio distribution function on the aggregate surface to obtain the water-cement ratio of the matrix paste and the average water-cement ratio of each sub-ITZ layer.

[0026] Furthermore, the process of calculating the ice content of cement paste or calculating the ice content of each sub-ITZ layer and matrix paste of the predicted object in step D is achieved through a paste ice content calculation model; the specific process of establishing the paste ice content calculation model includes the following steps:

[0027] First, a pore size distribution model of the slurry is established. Then, the distribution of water in pores of different sizes is determined, as well as the relationship between freezing temperature and critical freezing pore size.

[0028] Water in pores smaller than the critical freezing diameter does not freeze; water in pores with a diameter greater than or equal to the critical freezing diameter freezes completely, and the water is replaced by an equal volume of ice. The total volume fraction of water is equal to the volume fraction of ice.

[0029] Furthermore, the process of phase separation of cement paste described in step E includes the following steps:

[0030] Calculate the volume fraction of each hydration product phase in the cement paste; combine the multi-scale composition structure, further subdivide the volume fraction of unhydrated cement clinker phase, the volume fraction of each hydration product phase, the volume fraction of gas phase and the volume fraction of liquid phase to each scale. If the object to be predicted has experienced a negative temperature, the ice volume fraction also needs to be subdivided to each scale to obtain the volume fraction of each inclusion phase inside the RVE at each scale in the cement paste.

[0031] Furthermore, in the process of phase division of cement paste described in step E, if the object to be predicted is mortar or concrete, the volume fraction of each inclusion phase inside the RVE at each scale in the cement paste is obtained for all sub-ITZ layer pastes and matrix pastes; thus, the volume fraction of inclusion phases in the RVE at all scales in the object to be predicted is obtained.

[0032] Furthermore, in step G, during the calculation of the intensity of the predicted object,

[0033] If the object of prediction is cement paste, it is first necessary to establish the relationship between the cement paste load and the maximum stress of the cement paste failure control phase. Then, the maximum stress of the cement paste failure control phase is made equal to the yield stress of the failure control phase, so as to obtain the load when the cement paste is in the critical failure state.

[0034] If the object being predicted is mortar or concrete, the specific process for strength calculation includes the following steps:

[0035] G2.1 Establish the relationship between the external load of the predicted object and the maximum stress of the cement paste failure control phase in each sub-ITZ layer, and establish the relationship between the external load of the predicted object and the maximum stress of the cement paste failure control phase in the matrix paste; if the predicted object is lightweight aggregate concrete, also establish the relationship between the external load of the concrete and the maximum stress of the aggregate phase.

[0036] G2.2. Set the maximum stress of the cement paste failure control phase to equal its yield stress to obtain the predicted external load of each sub-ITZ layer when it is in the critical failure state, and obtain the predicted external load of the matrix paste when it is in the critical failure state. If the predicted object is lightweight aggregate concrete, set the maximum stress of the aggregate phase to equal the yield stress of the phase to obtain the concrete external load when the aggregate phase is in the critical failure state.

[0037] G2.3. Compare the external loads of all predicted objects obtained in G2.2, and determine the critical failure zone corresponding to the minimum value as failure. The minimum load is recorded as the instantaneous intensity.

[0038] G2.4. Determine whether the damaged area in step G2.3 is the matrix paste of the predicted object. If not, reduce the mechanical properties of the damaged area by multiplying the effective elastic modulus of the damaged area by the corresponding reduction factor. Use the reduced mechanical properties of the damaged area and the mechanical properties of the remaining non-damaged phases to return to step F and obtain the homogenized elastic modulus of the damaged predicted object. For the undamaged areas within the predicted object, execute steps G2.1 to G2.3. The already damaged areas are not included in the calculation of instantaneous strength. Obtain the instantaneous strength of the predicted object when the new area in step G2.3 is damaged. Determine again whether the newly damaged area is the matrix paste. If not, reduce the mechanical properties of the newly damaged area as well and return to step F to calculate the instantaneous strength of the new damaged area and the corresponding predicted object until the newly damaged area is the matrix paste. If the damaged area in G2.3 is the matrix paste, the predicted object is determined to have failed, and the maximum value among all instantaneous strengths is the strength of the predicted object.

[0039] Furthermore, the reduction factor in the mechanical property reduction process of the damaged area is obtained through the following steps:

[0040] The reduction factor corresponding to the mechanical properties of the damaged area of ​​concrete or mortar is determined according to the reduction factor of mechanical properties at the failure of the sub-ITZ layer. It is obtained by back-calculating through strength tests of concrete or mortar: First, a batch of concrete or mortar with different mix proportions is formed and cured for different ages under standard curing conditions, and the strength of these concrete or mortars is tested; the strength of concrete or mortar with each mix proportion at different curing ages is calculated; the relationship function between the strength of concrete or mortar and the reduction factor of mechanical properties of the sub-ITZ layer is obtained; the calculated strength is set equal to the test strength of concrete or mortar with the corresponding mix proportion and curing age, and the reduction factor of mechanical properties of the sub-ITZ layer in the concrete or mortar under this condition is calculated; the above calculation operation is performed for all mix proportions and curing ages of concrete or mortar to obtain the reduction factor of mechanical properties of the sub-ITZ layer under multiple mix proportions and curing ages; outliers are removed, and the average value of these reduction factors is taken as the reduction factor of mechanical properties of the sub-ITZ layer.

[0041] When the target of the prediction is lightweight aggregate concrete, the failure of the aggregate phase must be considered. It is necessary to determine the mechanical property reduction factor at which the aggregate phase fails. This reduction factor is based on the known reduction factor of the sub-ITZ layer. The sub-ITZ reduction factor can be obtained through the above steps. Then, the reduction factor of the aggregate phase failure is derived by back-calculating the strength test results of the lightweight aggregate concrete: A batch of lightweight aggregate concrete with different mix proportions is formed and cured to maturity under standard curing conditions. The strength of these concretes is then tested. The strength of lightweight aggregate concrete with each mix proportion is calculated separately. The relationship function between the strength and the reduction factor of the lightweight aggregate phase is obtained. The calculated strength is set equal to the test strength of the corresponding mix proportion concrete, and the reduction factor of the aggregate phase mechanical properties in that mix proportion concrete is calculated. The above calculation operation is performed for all mix proportions of lightweight aggregate concrete to obtain multiple reduction factors of the lightweight aggregate phase mechanical properties. Outliers are removed, and the average value of these reduction factors is taken as the reduction factor of the aggregate phase mechanical properties.

[0042] Beneficial effects:

[0043] The purpose of this invention is to provide a method for predicting concrete strength, taking into account the effect of locally high water-cement ratio in the internal temperature zone (ITZ) and the uneven distribution of the water-cement ratio in concrete, thereby further improving the accuracy of concrete strength prediction. This invention has the following beneficial effects on the prior art:

[0044] 1. This invention can predict the development of concrete under positive temperature curing as it ages, taking into account the weakening effect of the weak ITZ on the elastic modulus and strength, and also taking into account the uneven distribution of water-cement ratio in the ITZ, thus improving the prediction accuracy of elastic modulus and strength.

[0045] 2. This invention can calculate the elastic modulus and strength of concrete subjected to freezing at negative temperatures as pre-curing time and freezing temperature develop, taking into account the gain effect of the ice-enhanced ITZ on the elastic modulus and strength.

[0046] 3. This invention can reveal the failure process of concrete during loading. It can be used not only to calculate the failure process of ordinary concrete, but also to calculate the failure process of lightweight aggregate concrete. For lightweight aggregate concrete, the failure sequence of aggregate and matrix paste can be obtained through the method of this invention, providing theoretical support for the improvement of the mix proportion of lightweight aggregate concrete.

[0047] 4. This invention fully considers the influence of concrete raw materials, mix proportions and curing conditions, and has a wide range of applications.

[0048] 5. This invention can calculate the strength of concrete under various stress states and has a wide range of applications. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of a concrete strength prediction calculation process.

[0050] Figure 2 The steps for multi-scale composition and homogenization of concrete in Examples 1 and 2 are described.

[0051] Figure 3 The curves show the development of the dynamic elastic modulus of positive-temperature concrete as predicted and measured in Example 1.

[0052] Figure 4 The curves show the predicted and measured uniaxial compressive strength development of positive-temperature concrete in Example 1.

[0053] Figure 5 This describes the failure process of cement paste when positive temperature concrete is subjected to uniaxial compressive stress in Example 1.

[0054] Figure 6 This illustrates the reinforcing effect of ice on the negative temperature concrete ITZ and the matrix paste in Example 2.

[0055] Figure 7 The curves show the predicted and measured dynamic elastic modulus development of negative-temperature concrete in Example 2.

[0056] Figure 8 The curves show the predicted and measured uniaxial compressive strength development of negative-temperature concrete in Example 2.

[0057] Figure 9 This describes the failure process of cement paste when concrete is subjected to uniaxial compressive stress at a freezing temperature of -20℃ in Example 2. Detailed Implementation

[0058] To address the aforementioned problems in the prior art, this invention provides a concrete strength prediction model. This model not only considers the transitional properties of the intermediate temperature zone (ITZ), but also the weakening effect of a weak ITZ on the strength of positively heated concrete and the beneficial effect of an ice-enhanced ITZ on the strength of negatively heated frozen concrete. This improves the accuracy of strength prediction and reveals the internal failure process of concrete under load. The strength prediction of this invention is applicable to cement paste, mortar, and concrete. The invention will be further described below with reference to specific embodiments.

[0059] Combination Figure 1 This implementation method is described below.

[0060] This embodiment is a method for predicting the strength of concrete under positive temperature curing and negative temperature freezing, considering ITZ, and includes the following steps:

[0061] Step A: Establish a cement-based material information database; determine whether the predicted object is cement paste, mortar, or concrete, establish the corresponding multi-scale composition structure, and identify the cement paste failure control phase; the cement paste failure control phase is an inclusion phase that can determine whether the cement paste has failed. Once the stress on the cement paste failure control phase reaches its yield stress, the cement paste can be considered to have failed.

[0062] The process of establishing a multi-scale composition structure is as follows: Based on the characteristic length of the inclusion phases, the prediction object is divided into several scales, with each scale containing only one or more inclusion phases of basically the same characteristic length. A representative volumetric unit (RVE) representing the distribution characteristics of the inclusion phases is then established at each scale.

[0063] When the object of prediction is mortar or concrete, the gradient properties of the ITZ around the aggregate in the multi-scale composition structure need to be considered, i.e., the water-cement ratio of the ITZ decreases with increasing distance from the aggregate surface. Here, the ITZ is regarded as a multi-layered shell structure, which can be subdivided into multiple shell-like sub-ITZ layers of equal thickness surrounding the aggregate, with approximately uniform distribution of cement paste within each sub-ITZ layer. The number of sub-ITZ layers can be determined based on the computer's processing power; the more sub-ITZ layers, the more accurate the calculation results, but the longer the calculation time. The matrix paste is a non-ITZ cement paste far from the aggregate surface, and its water-cement ratio does not change with distance from the aggregate surface. Both the sub-ITZ layers around the aggregate and the matrix paste are cement pastes within the object of prediction. These pastes have the same composition and structure, but the volume fraction of each inclusion phase differs due to the different water-cement ratios.

[0064] The process for determining the control phase of cement paste failure is as follows: First, based on the established multi-scale composition structure, the scale of the control phase is selected. The control phase must be located within the cement paste, and its scale must be smaller than the cement paste scale. This scale is the smallest scale for the stress calculation of inclusions in the subsequent prediction object, affecting the accuracy of the strength calculation. Second, at this scale, the inclusion phase with the smallest elastic modulus besides the porous phase is selected as the control phase of cement paste failure. The control phase of cement paste failure cannot consist solely of pores; it should contain at least one solid hydration product.

[0065] The cement-based material information database includes the mechanical properties and shapes of the aggregate phase, cement clinker phase, all hydration product phases, aqueous phase, and gas phase. The mechanical properties include Poisson's ratio, elastic modulus, shear modulus, and bulk modulus. The shape can be represented by the aspect ratio. In addition, the information database should also include the cement paste failure control phase and the yield stress of various aggregates.

[0066] When the predicted object is mortar or concrete, proceed to step B; when the predicted object is cement paste, proceed directly to step C.

[0067] Step B: For cases where the object of prediction is mortar or concrete, determine the water-cement ratio distribution within the object. The specific steps are as follows:

[0068] B1. Determine the ITZ width of the prediction object and calculate the width of each sub-ITZ layer;

[0069] B2. Calculate the total ITZ integral, the volume fraction of each sub-ITZ layer, and the volume fraction of the matrix paste based on the aggregate particle size distribution and aggregate volume fraction.

[0070] B3. Calculate the water-cement ratio of the paste at different locations from the aggregate surface in the ITZ, i.e., the distribution function of the water-cement ratio on the aggregate surface, and calculate the water-cement ratio of the matrix paste.

[0071] B4. Calculate the average water-cement ratio for each sub-ITZ layer.

[0072] The ITZ width in step B1 can be determined by empirical values ​​in the prior art, and B2 and B3 can be determined by empirical formulas in the prior art.

[0073] Step C: If the object to be predicted is cement paste, calculate the degree of hydration of the cement paste; if the object to be predicted is mortar or concrete, calculate the degree of hydration of each sub-ITZ layer and the matrix paste.

[0074] This process is achieved through the establishment of a cement paste hydration model, which can be established using existing technology. By inputting the water-cement ratio, cement composition, positive temperature curing age, positive temperature curing temperature, and positive temperature curing humidity into the cement hydration model, the total hydration degree of the cement paste and the hydration degree of each clinker phase in the paste can be obtained. From the calculation results of the hydration model, the volume fractions of the hydrated and unhydrated portions of each clinker mineral phase, the volume fractions of consumed and unconsumed water, the total volume fraction of hydration products, and the total porosity integral can be calculated using existing technology.

[0075] Step D: If the object being predicted experiences a sub-zero temperature, calculate the ice content of the object or the ice content of the slurry in each sub-ITZ layer and the matrix. This process is achieved through the established slurry ice content calculation model. The specific steps for establishing the slurry ice content calculation model are as follows:

[0076] D1. Establish a pore size distribution model for the slurry;

[0077] D2. Assuming that water is preferentially distributed in smaller pores, determine the distribution of water in pores of different sizes.

[0078] D3. Determine the relationship between freezing temperature and critical freezing aperture;

[0079] D4. Water in pores smaller than the critical freezing diameter does not freeze; water in pores with a diameter greater than or equal to the critical freezing diameter freezes completely, and this water is replaced by an equal volume of ice. The total volume fraction of this water is equal to the volume fraction of ice.

[0080] Steps D1 and D3 can be determined using existing technology.

[0081] Step E: Establish a method for phase division of cement paste. The specific steps are as follows:

[0082] E1. Calculate the volume fraction of each hydration product phase in the cement paste;

[0083] E2. Combining the multi-scale composition structure, the volume fraction of unhydrated cement clinker phase, the volume fraction of each hydration product phase, the volume fraction of gas phase and the volume fraction of liquid phase are further divided into each scale. If the object to be predicted experiences a negative temperature, the ice volume fraction also needs to be divided into each scale to obtain the volume fraction of each inclusion phase inside the RVE at each scale in the cement paste.

[0084] If the object to be predicted is mortar or concrete, then for all sub-ITZ layer slurries and matrix pastes, steps C, D, E1, and E2 are executed respectively to obtain the RVE composition of all cement pastes in the object to be predicted at various scales. Since the volume fraction of the sub-ITZ layer and the volume fraction of the matrix paste have been obtained in step B, and the volume fraction of the aggregate phase is known, the volume fraction of the inclusion phase of the RVE at all scales in the object to be predicted can be obtained.

[0085] Steps E1 and E2 can be obtained through existing simulation methods and empirical formulas.

[0086] Step F: Calculate the homogenized elastic modulus of the object to be predicted.

[0087] Using the homogenization calculation method, the mechanical properties and shapes of each phase provided in the cement-based material information database established in step A are used, combined with the inclusion phase volume fraction of the RVE of the predicted object at all scales obtained in step E. The homogenized mechanical properties of the RVE at each scale are calculated in sequence from the smallest scale to the largest scale in the multi-scale composition structure, and finally the homogenized elastic modulus of the predicted object is obtained.

[0088] The homogenization calculation method can be obtained through existing technologies.

[0089] Step G: Calculate the intensity of the predicted object. The specific calculation steps are as follows:

[0090] G1. When the object of prediction is cement paste, establish the relationship between the cement paste load and the maximum stress of the cement paste failure control phase; set the maximum stress of the cement paste failure control phase equal to the yield stress of the failure control phase, and obtain the load when the cement paste is in the critical failure state. This load is the strength of the cement paste.

[0091] The relationship between the load on cement paste and the stress on the failure-controlling phase of cement paste can be determined based on the relationship between the average stress of the RVE and the maximum stress on a specific inclusion phase within it, as described in existing technologies, and can be achieved in various ways. This embodiment provides the following implementation method:

[0092] (a) The process of establishing the relationship between cement paste load and the maximum stress of the cement paste failure control phase is as follows: Existing technology has given the relationship between the average stress of RVE and the maximum stress of a certain specified inclusion phase inside it, as shown in Equation (1); the RVE related parameters in Equation (1) are... and Σ RVE Replace with cement paste related parameters and Σ cp And specify the inclusion phase related parameters μ, and Replace with cement paste failure control phase-related parameter μ control , and The relationship between the cement paste load and the force on the cement paste failure control phase can be obtained, as shown in equation (2).

[0093]

[0094]

[0095] In the formula, σ dev and To specify the maximum value of the stress peaks in all orientations for the inclusion phase and the cement paste failure control phase, θ and Euler angles, representing direction; μ and μ control The shear modulus for specifying the inclusion phase and the cement paste failure control phase; and for Volume fraction of the specified inclusion phase and the destructive control phase of the cement paste in the upward orientation; and for The shear modulus of the specified inclusion phase and the cement paste failure control phase in the specified direction; Σ RVE and Σ cp The average stress on the RVE and cement paste, i.e., the external load; and For RVE and cement paste, the homogenization stiffness tensor is given.

[0096] (b) Another process for establishing the relationship between the cement paste load and the maximum stress of the cement paste failure control phase is as follows: Apply a load to the entire cement paste. Based on the RVE at each scale obtained in step F and the mechanical properties of each inclusion phase in these RVEs, obtain the average stress and average strain of each RVE and each inclusion phase in the RVE in the order from large scale to small scale in the multi-scale composition structure, until the RVE where the cement paste failure control phase is located is calculated. The above process can obtain the relationship between the cement paste load and the average stress of the RVE where the cement paste failure control phase is located. Combine this with equation (1) to obtain the relationship between the cement paste load and the maximum stress of the cement paste failure control phase.

[0097] G2. When the object of the prediction is mortar or concrete, the strength calculation steps are as follows:

[0098] G2.1 Establish the relationship between the external load of the predicted object and the maximum stress of the cement paste failure control phase in each sub-ITZ layer, and establish the relationship between the external load of the predicted object and the maximum stress of the cement paste failure control phase in the matrix paste; if the predicted object is lightweight aggregate concrete, it is also necessary to establish the relationship between the external load of the concrete and the maximum stress of the aggregate phase.

[0099] The relationship between the external load on an object and the stress on its internal constituent phases can also be determined based on the relationship between the RVE average stress and the maximum stress on a specified inclusion phase within the object, as described in existing technologies. This can be achieved in various ways. This embodiment provides the following implementation method:

[0100] (a) The process of establishing the relationship between the external load of the predicted object and the maximum stress of the cement paste failure control phase in each sub-ITZ layer or matrix paste is as follows: Following the process of establishing the relationship between the cement paste load and the maximum stress of the cement paste failure control phase, the relevant parameters of the RVE element in equation (1) are... and Σ RVE Replace with the parameters related to the predicted object, and specify the correlation parameters μ of the mixed phase. and By replacing the parameters with those related to the failure control phase of cement paste in the sub-ITZ layer or matrix paste, the relationship between the external load on the predicted object and the stress on the failure control phase of cement paste in each sub-ITZ layer or matrix paste can be obtained.

[0101] (b) Another process for establishing the relationship between the external load on the predicted object and the maximum stress of the cement paste failure control phase in each sub-ITZ layer or matrix paste is as follows: Apply a load to the predicted object. Based on the RVE at each scale and the mechanical properties of each inclusion phase in these RVEs obtained in step F, obtain the average stress and average strain of the RVE at each scale and each inclusion phase in the RVE in the order from large scale to small scale in the multi-scale composition structure, until the average stress of the sub-ITZ layer or matrix paste to be calculated is reached. The relationship between the average stress of the sub-ITZ layer or matrix paste and its internal cement paste failure control phase has been given in G1. By combining the relationship between the external load on the predicted object and the average stress of the cement paste, and the relationship between the cement paste load and the maximum stress of the cement paste failure control phase, the relationship between the external load on the predicted object and the stress of the cement paste failure control phase in each sub-ITZ layer or matrix paste can be obtained.

[0102] Similarly,

[0103] (a) The process of establishing the relationship between the external load of the predicted object and the maximum stress of the aggregate phase is as follows: similar to the process of establishing the relationship between the cement paste load and the maximum stress of the cement paste failure control phase, the relevant parameters of the RVE element in equation (1) are used. and Σ RVE Replace with the parameters related to the predicted object, and specify the correlation parameters μ of the mixed phase. and By replacing the parameters with aggregate phase-related parameters, the relationship between the external load on the predicted object and the stress on the aggregate phase can be obtained.

[0104] (b) Another process for establishing the relationship between the external load of the predicted object and the maximum stress of the aggregate phase is as follows: Apply a load to the entire predicted object. Based on the RVE at each scale obtained in step F and the mechanical properties of each inclusion phase in these RVEs, obtain the average stress and average strain of each RVE and each inclusion phase in the RVE at each scale in the order from large scale to small scale in the multi-scale composition structure, until the calculation reaches the RVE where the aggregate phase is located. The above process can obtain the relationship between the load of the predicted object and the average stress of the RVE where the aggregate phase is located. The relationship between the average stress of the RVE where the aggregate phase is located and the maximum value of the stress peak of the aggregate phase in each orientation has been given by equation (1). By combining the relationship between the load of the predicted object and the average stress of the RVE where the aggregate phase is located and the relationship between the average stress of the RVE where the aggregate phase is located and the maximum stress of the aggregate phase, the relationship between the load of the predicted object and the stress of the aggregate phase can be obtained.

[0105] G2.2. Set the maximum stress of the cement paste failure control phase to equal its yield stress to obtain the predicted external load of each sub-ITZ layer when it is in the critical failure state, and obtain the predicted external load of the matrix paste when it is in the critical failure state. If the predicted object is lightweight aggregate concrete, the maximum stress of the aggregate phase should also be set to equal the yield stress of that phase to obtain the concrete external load when the aggregate phase is in the critical failure state.

[0106] G2.3. Compare the external loads of all predicted objects obtained in G2.2, and determine the critical failure zone corresponding to the minimum value as failure. The minimum load is recorded as the instantaneous intensity.

[0107] The instantaneous intensity in this step represents the load on the predicted object when an undamaged area of ​​the predicted object is destroyed during the loading process.

[0108] G2.4. Determine if the damaged area in step G2.3 is the matrix paste of the predicted object. If not, reduce the mechanical properties of the damaged area by multiplying the effective elastic modulus of the damaged area by the corresponding reduction factor. Use the reduced mechanical properties of the damaged area and the mechanical properties of the remaining non-damaged phases to return to step F and obtain the homogenized elastic modulus of the damaged predicted object. For undamaged areas within the predicted object, execute steps G2.1 to G2.3. Damaged areas are not included in the instantaneous strength calculation. Obtain the instantaneous strength of the predicted object when the new area in step G2.3 is damaged. Determine again if the newly damaged area is the matrix paste. If not, reduce the mechanical properties of the newly damaged area as well. Return to step F again according to the aforementioned process and execute steps F and G2.1 to G2.3 to calculate the new damaged area and the corresponding instantaneous strength of the predicted object. Repeat the loop until the newly damaged area is the matrix paste. If the damaged area in G2.3 is the matrix paste, the predicted object is determined to have failed, and the maximum value among all instantaneous strengths is the strength of the predicted object.

[0109] In step G2.4, the cyclic process represents the gradual failure of the slurry region or aggregate phase of the predicted object during loading, and the maximum instantaneous strength represents the historical peak value of the load experienced by the predicted object during loading.

[0110] Follow steps A through G Figure 1 This is the procedure for calculating the strength of concrete. If the object being tested is cured at a positive temperature, the ice content in step D is recorded as 0. If the object being tested has experienced freezing at a negative temperature, step C calculates the degree of hydration of the object during the pre-curing stage, and step D calculates the ice content based on the freezing temperature of the object.

[0111] In the strength calculation program established in this invention, the mix proportion, cement composition, and positive temperature curing conditions of the predicted object are input. If the predicted object is mortar or concrete, the aggregate type and its particle size distribution must also be input to obtain the strength of the predicted object under positive temperature curing. When the predicted object undergoes a freezing process at negative temperatures, in addition to the above input parameters, the freezing temperature of the predicted object must also be input into the strength calculation program to obtain the negative temperature strength of the predicted object. When calculating the strength of mortar or concrete, two essential input parameters are required: the yield stress of the cement paste failure control phase in step A and the reduction factor of the mechanical properties of the failure zone in step G.

[0112] When there is relevant research on the selected cement paste failure control phase, its yield stress can be taken as an empirical value. When there is no relevant research on the yield stress of the selected cement paste failure control phase, this value needs to be determined experimentally. Theoretically, once the cement paste failure control phase is determined, its yield stress is constant and unaffected by raw materials and curing conditions. A batch of cement pastes with different water-cement ratios is formed, and these cement pastes are cured for different ages under standard curing conditions. The strength of these cement pastes is then tested. To ensure the yield stress of the obtained cement paste failure control phase has universality, the water-cement ratio range of the tested cement pastes should cover 0.3–0.5, and the cement paste strength data should include both early-age and mature-age strengths. To avoid the influence of size effects, the strength of the cement paste should be converted to the strength of a standard concrete specimen. Using the above steps for calculating the strength of cement paste, the strength of cement paste with a specified water-cement ratio after curing for a specified age is calculated, obtaining the relationship function between strength and the yield stress of the cement paste failure control phase. In this relational function, the calculated strength is equal to the test strength of the cement paste at the corresponding water-cement ratio and curing age, and the yield stress of the failure control phase of the cement paste under this condition is calculated in reverse. The same calculation operation is performed for cement pastes of all water-cement ratios and curing ages to obtain the yield stress of the failure control phase of the cement paste under multiple water-cement ratios and curing ages. Outliers are removed, and the average of these yield stresses is taken as the yield stress of the failure control phase of the cement paste.

[0113] The reduction factor for the mechanical properties of the damaged region in step G2 can still be obtained through experimental back-calculation. The specific steps are as follows:

[0114] The reduction factor for mechanical properties at failure of the sub-ITZ layer can be derived by back-calculating the strength of ordinary concrete. Ordinary concrete has a high elastic modulus of aggregate, which typically does not fail under load. A batch of ordinary concrete with different mix proportions is formed, cured at different ages under standard curing conditions, and the strength of these concretes is tested. To simplify the test, the concrete strength test can be replaced with a mortar strength test. The elastic modulus of the aggregate should be higher than 50 GPa to ensure that the aggregate does not fail during the strength test. To make the obtained reduction factor for mechanical properties of the sub-ITZ layer more universal, the water-cement ratio of the tested concrete or mortar should cover the range of 0.35–0.6, and the strength data should include both early-age and mature-age strengths. To avoid the influence of size effects, the concrete or mortar strength should be converted to the strength of a standard concrete specimen. Using the strength calculation steps, the strength of each mix proportion of concrete or mortar at different curing ages is calculated. Since aggregates do not fail when concrete or mortar fails, step G2 of the calculation does not need to consider the concrete or mortar load when the aggregate phase is in a critical failure state, nor the mechanical property reduction factor after aggregate failure. Obtain the relationship function between the concrete or mortar strength and the mechanical property reduction factor of the sub-ITZ layer. Set the calculated strength equal to the test strength of the concrete or mortar at the corresponding mix proportion and curing age, and then calculate the mechanical property reduction factor of the sub-ITZ layer within the concrete or mortar under that condition. Perform the above calculation operation for all mix proportions and curing ages of concrete or mortar to obtain the mechanical property reduction factors of the sub-ITZ layer for multiple mix proportions and curing ages. Remove outliers and take the average of these reduction factors as the mechanical property reduction factor of the sub-ITZ layer.

[0115] When predicting the mechanical property reduction factor of aggregate phase failure in lightweight aggregate concrete, it is necessary to solve this factor. The determination of this factor requires the prior knowledge of the mechanical property reduction factor of the sub-ITZ layer, which can be obtained through the above steps. Then, the mechanical property reduction factor of aggregate phase failure can be derived by back-calculating the strength test results of the lightweight aggregate concrete. The failure of the aggregate phase must be considered in lightweight aggregate concrete. When the aggregate fails before the matrix paste, the matrix paste will bear enormous concentrated stress and fail rapidly. Therefore, for lightweight aggregate concrete, the failure of the matrix paste can still be used as a criterion for judging concrete failure. A batch of lightweight aggregate concrete with different mix proportions was formed and cured under standard curing conditions to maturity. The strength of these concretes was then tested. To ensure that the aggregate phase fails when the concrete fails, the elastic modulus of the matrix paste should be as high as possible. Therefore, the water-cement ratio of the concrete should be lower than 0.4, and the curing age should exceed 28 days. Furthermore, aggregates with low elastic modulus should be selected; the elastic modulus of the aggregate should not exceed 20 GPa. To avoid the influence of size effects, the concrete strength should be converted to the strength of a standard concrete specimen. Using the strength calculation steps, calculate the strength of lightweight aggregate concrete for each mix proportion. Obtain the relationship function between strength and the reduction factor of the lightweight aggregate phase's mechanical properties. Set the calculated strength equal to the test strength of the corresponding mix proportion concrete, and then calculate the reduction factor of the aggregate phase's mechanical properties for that mix proportion concrete. Perform the above calculation operation for all mix proportions of lightweight aggregate concrete to obtain multiple reduction factors of the lightweight aggregate phase's mechanical properties. Remove outliers and take the average of these reduction factors as the reduction factor of the aggregate phase's mechanical properties.

[0116] The cement paste, mortar, and concrete to which this invention is applicable are all isotropic materials and do not contain fibers.

[0117] The concrete strength that can be calculated by this invention includes compressive strength and tensile strength. The cement-based material information database established in step A should include the compressive elastic modulus and tensile elastic modulus of each component phase. Based on the compressive or tensile properties of the strength to be calculated, the corresponding homogenized compressive or tensile elastic modulus of the concrete is calculated in step F, and the corresponding concrete compressive or tensile strength is calculated in step G.

[0118] This invention can calculate the strength of concrete not only under uniaxial stress, but also under biaxial or triaxial stress. During the calculation, the concrete load tensor in step G needs to be set to the appropriate tensor form according to the strength stress state of the concrete.

[0119] This invention can calculate not only the strength development of concrete under positive temperature curing conditions, but also the strength development of concrete under negative temperature freezing conditions.

[0120] Example:

[0121] Example 1: This example uses ordinary concrete cured at positive temperature as an example to predict the uniaxial compressive strength. The water-cement ratio of the concrete used to verify this example is 0.5. The water, cement, fine aggregate, and coarse aggregate contents per cubic meter of concrete are 209 kg, 416 kg, 640 kg, and 1090 kg, respectively. The cement is the reference cement with a strength grade of 52.5. The chemical and mineral composition of the cement is shown in Table 1. The average particle size of the cement is 20 μm. The coarse aggregate is continuously graded basalt crushed stone with a particle size range of 5 mm to 20 mm, where the mass ratio of small stones (5 mm to 10 mm) to large stones (10 mm to 20 mm) is 4:6. The fine aggregate is river sand with a maximum particle size of 4.75 mm and a fineness modulus of 2.7. The dynamic elastic modulus of the aggregate is 65 GPa, and the Poisson's ratio is 0.2. The concrete is a cylinder with a diameter of 100 mm and a height of 200 mm. After casting, a plastic film is placed over the surface of the specimen to prevent moisture loss. The specimen is then demolded after curing in a standard curing room for one day. The demolded specimen is sealed and placed in a standard curing room for further curing at different ages before testing the dynamic modulus of elasticity and compressive strength of the concrete.

[0122] Table 1. Chemical and mineral composition of cement (wt%)

[0123]

[0124] As a specific example, the multi-scale composition structure of concrete consists of Figure 2 As shown. The concrete is composed of coarse aggregate, multiple sub-ITZ layers of paste, and mortar sequentially coated; the mortar is composed of fine aggregate, multiple sub-ITZ layers of paste, and a matrix paste sequentially coated; both the sub-ITZ layers of paste and the cement paste are composed of hydrated cement particles, water, ice, and gas from large capillaries, and additionally introduced gas; the hydrated cement particles are sequentially coated with unhydrated cement clinker, an internal hydration product layer, an external CSH layer, and a micron-sized non-gel hydration product layer; the unhydrated clinker particles are composed of a mixture of unhydrated clinker phases; the internal hydration product layer is composed of frozen and unfrozen high-density CSH gel (HD CSH) and various nano-sized non-gel hydration product phases; the external CSH layer is composed of frozen and unfrozen low-density CSH gel (LD CSH) and water, ice, and gas from small capillaries; the micron-sized non-gel hydration product layer is composed of a mixture of various micron-sized non-gel hydration product phases; the unfrozen HD and LD... CSH is composed of CSH gel particles and gel water in different proportions, while frozen HD and LD CSH are composed of CSH gel particles, gel water, and ice in different proportions. For concrete cured at positive temperature, the volume of ice is 0 at all scales, and the volume of frozen HD and LD CSH is also 0.

[0125] LD CSH gel, with a relatively high volume fraction and low elastic modulus, was used as the failure control phase for cement paste. The mechanical properties and shapes of various minerals in concrete can be obtained from existing literature, as shown in Table 2. Preliminary experiments and calculations showed that the Von Mises yield stress of LD CSH is 105 MPa. Since the concrete is ordinary concrete with a high aggregate elastic modulus, the failure of the aggregate phase is not considered in the strength calculation; therefore, the yield stress of the aggregate does not need to be input.

[0126] Table 2 Mechanical and shape parameters of the constituent phases in concrete

[0127]

[0128]

[0129] Note: * indicates that the parameter is calculated from the previous scale. ** in aggregate indicates that the parameter depends on the type of aggregate used.

[0130] The width of the ITZ can be taken from empirical values ​​in the literature or from the median of the cement particle size distribution. In this embodiment, the width of the ITZ is taken as the average particle size of the cement particles, which is 20 μm. The ITZ is divided into 10 equally spaced sub-ITZ layers, each with a width of 2 μm. The total volume of the ITZ is the product of the width of the ITZ and the total surface area of ​​the aggregate. The total volume fraction of the ITZ is the ratio of the total volume of the ITZ to the volume of the concrete. The volume fraction of each sub-ITZ layer is 1 / 10 of the total ITZ integral. The volume fraction of the matrix paste is the difference between the total paste integral and the total ITZ integral. The empirical formulas for the water-cement ratio of the ITZ and the water-cement ratio distribution of the matrix paste are adopted as formula (3):

[0131]

[0132] In the formula, h is the distance from the aggregate surface, and w / c is the water-cement ratio of the paste at a distance h from the aggregate surface. When 0 <h<h ITZ When w / c is the water-cement ratio at ITZ at a distance h from the aggregate surface, when h > h ITZ At that time, w / c is the water-cement ratio of the matrix paste, and h ITZ ρ is the total width of ITZ; cem and ρ w These are the densities of cement and water, respectively; f cem (h) represents the initial volume fraction of cement particles at a distance h from the aggregate surface, which can be obtained using an empirical formula in this embodiment. cem (h):

[0133]

[0134]

[0135] In the formula, w0 / c is the overall water-cement ratio of the concrete; f cem,bulk The initial volume fraction of cement particles in the matrix paste can be calculated based on the principle that the total volume of cement particles equals the sum of the ITZ and the volume of cement particles in the matrix paste, as shown in equation (5). cem,bulk The only unknown in equation (5):

[0136] V cem =V cem,ITZ +V cem,bulk (5)

[0137] In the formula, V cem V represents the total volume of cement particles in the specimen. cem =(1-f agg ) / (1+ρ cem / ρ w ·w0 / c), f agg V represents the aggregate volume fraction. cem,ITZ The total volume of cement particles in all ITZs, i.e., the volume fraction function of cement particles for each aggregate, is f. cem (h) Perform volume integration within the ITZ, iterate through all aggregates, calculate and sum them. r agg Let r be the aggregate radius. agg,max and r agg,min These are the maximum and minimum radii of the aggregate, dV. agg (r agg ) / dr agg This is the aggregate particle size distribution curve; V cem,bulk V represents the volume of cement particles in the matrix paste. cem,bulk =f cem,bulk (1-f agg -f ITZ ), f ITZ This is the volume of ITZ.

[0138] Each sub-ITZ layer is considered as a material with a uniform water-cement ratio, and its water-cement ratio is the average water-cement ratio of that sub-ITZ layer. The empirical model for ITZ water-cement ratio distribution used in this embodiment assumes that the water-cement ratio of the paste closest to the aggregate surface is close to infinity, which would significantly reduce the final elastic modulus of the concrete. To improve the accuracy of elastic modulus prediction, the average water-cement ratio of the sub-ITZ layer closest to the aggregate surface is replaced by the average water-cement ratio within a region of 1μm to 2μm from the aggregate surface. This treatment improves the homogenized elastic modulus of the sub-ITZ layer closest to the aggregate surface, which can be seen as compensation for the enrichment effect of Ca(OH)2 on the aggregate surface.

[0139] The hydration model of cement paste adopts the Parrot & Killoh hydration model. Since the concrete is positive temperature concrete, its ice content is set to 0. During the phase division process, the volume of unhydrated clinker phase, total liquid phase volume, total gas phase volume, and volume of each hydration product phase can be solved by the GEMS platform. When dividing the above phases into RVE elements at various scales, empirical formulas can be used for calculation. When dividing the HD and LDC-SH gel volumes, the calculation formulas for the gel and gel pore volume fractions are shown in equations (6) to (8). The volume fraction division formulas for nano- and micro-scale non-gel hydration products are shown in equations (9) to (10). The volume division formulas for gel pores, small capillaries, and large capillaries are shown in equations (11) to (17). Thus, the volume fractions of all inclusion phases in the concrete can be obtained.

[0140] f hdcsh =f gs (1-M r ) / 0.76 (6)

[0141] f ldcsh =f gs M r / 0.63 (7)

[0142] M r =min[1,(3.017w / c-1.347)α+0.538] (8)

[0143] f nch =0.25f hdcsh (9)

[0144] f mch =f ch -f nch (10)

[0145]

[0146]

[0147]

[0148]

[0149]

[0150] φ3=φ cp -φ2 (16)

[0151]

[0152] In the formula, f hdcsh f is the volume fraction of HD CSH gel in cement paste; gsM represents the total volume fraction of gel particles in the cement paste, calculated using GEMS. r f represents the mass fraction of gel particles in LD CSH relative to the total gel particles; ldcsh is the LD CSH gel volume fraction in the cement paste; w / c is the water-cement ratio of the cement paste to be calculated; α is the degree of hydration of the cement paste, calculated using the Parrot & Killoh hydration model; f nch f represents the volume fraction of nanoscale non-gel hydration products in cement paste; mch f represents the volume fraction of micron-sized non-gel hydration products in cement paste; ch This represents the total volume fraction of non-gel hydration products in the cement paste, calculated using GEMS. This represents the volume fraction of HD CSH gel pores in the cement paste. φ1 represents the volume fraction of LD CSH gel pores in the cement paste; φ2 represents the volume fraction of small capillary pores; φ ohl It is the volume fraction of small capillaries in the outer hydration product layer, that is, the ratio of the volume fraction of small capillaries to the volume fraction of the outer hydration product layer. The outer hydration product layer is composed of the volume fraction of the outer CSH layer and micron-sized non-gel hydration products. ψ represents the initial volume fraction of small capillaries within the external hydration product layer, i.e., the initial ratio of the volume fraction of small capillaries to the volume fraction of the external hydration product layer; here, it is taken as 0.63. f φ represents the percentage of the initial capillary volume occupied by the solid phase, i.e., the percentage of the total volume fraction of all hydration products in the outer hydration product layer relative to the total volume fraction of these hydration products and the capillary; cp φ3 represents the total volume fraction of capillaries; φ3 represents the volume fraction of large capillaries.

[0153] When calculating the homogenized compressive modulus of elasticity of concrete, homogenization begins from the smallest scale. When the RVE is composed of mixed inclusion phases, the self-consistent method is used for calculation; when the RVE is covered by inclusion phases, the generalized self-consistent method is used for homogenization calculation. Finally, the elastic modulus of the RVE element at each scale and the homogenized elastic modulus of elasticity of the concrete as a whole can be obtained. Figure 2 The algorithm used for homogenization calculation at each scale is marked, where SC represents the self-consistent method and GSC represents the generalized self-consistent method.

[0154] When performing strength calculations, only the stress on the paste within the concrete can be considered, namely, the individual ITZ layers of the matrix paste, coarse aggregate, and fine aggregate. Since the concrete strength in this embodiment is the uniaxial compressive strength, the load on the concrete can be expressed as: In the formula Σ con For concrete loads, compression is defined as negative, Σ 11The stress component of the concrete in the 11 direction is the uniaxial compressive stress in the uniaxial compression test. The relationship between the maximum stress of the failure control phase LD CSH of each sub-ITZ layer grout and the uniaxial compressive stress on the concrete is shown in Equation (18).

[0155]

[0156] In the formula, The maximum value of the LD CSH stress peak in all orientations; μ ld The shear modulus of LD CSH; for Volume fraction of LD CSH gel facing upwards; This refers to the stress on the external CSH layer when the concrete is subjected to a unit compressive stress. It is related to the position of the cement paste in the concrete (a certain sub-ITZ layer or matrix paste), and can be obtained by stress distribution calculation based on the RVE at each scale and the mechanical properties of each inclusion phase in these RVEs; For the homogenized stiffness tensor of the external CSH layer; for The shear modulus of LD CSH gel in the direction of rotation.

[0157] For the matrix paste, each coarse aggregate sub-ITZ layer, and each fine aggregate sub-ITZ layer paste, the stress on the LD CSH is set equal to its yield stress, and the instantaneous strength development of the concrete during the gradual failure of these pastes is calculated. Based on previous tests and calculations, the mechanical property reduction factor for the failed sub-ITZ layer is taken as 0.5. After the matrix paste fails, the maximum value of all instantaneous strengths is taken as the uniaxial compressive strength of the concrete.

[0158] Figure 3 To compare the predicted and measured dynamic elastic modulus of concrete, the predicted dynamic elastic modulus was obtained by converting the homogeneous elastic modulus of concrete. The specific conversion method is to divide the homogeneous elastic modulus of concrete by 0.83. Figure 4 To compare the predicted and measured concrete strength, Figure 5 The deterioration process of the internal paste of concrete after 1 day (a) and 28 days (b) of curing.

[0159] Figure 3 and Figure 4 This demonstrates that the algorithm of the present invention can accurately evaluate the dynamic elastic modulus and strength development. Figure 5In the diagram, the dashed box represents the ITZ (Integrated Layer Zone) around the aggregate. Both coarse and fine aggregates are surrounded by 10 ITZ layers, each 2 μm wide. The outermost ITZ layer is in contact with the matrix paste. The thick solid box represents the ITZ region within the concrete that fails first, and the arrows indicate the ITZs that fail sequentially. When the arrow points to the matrix paste, the concrete fails, and the calculation ends. Figure 5 This indicates that most of the ITZ (Intense Iron Zone) fails before the matrix cement paste fails. This corresponds to the phenomenon observed in the experiment where cracks propagated along the periphery of the aggregate. However, not every location of the ITZ will fail before the matrix cement paste. If the concrete curing age is very short, the innermost sub-ITZ layer, due to its low elastic modulus, can hardly bear stress during the process and is therefore difficult to fail. As the curing age increases, the area of ​​ITZ failure expands. This is because the elastic modulus of the ITZ gradually increases, allowing it to begin bearing some of the force. After curing for more than one day, concrete failure almost always begins in the ITZ closest to the aggregate surface, as the elastic modulus is weakest there.

[0160] Example 2: This example uses concrete subjected to freezing at sub-zero temperatures as an example to predict its uniaxial compressive strength. The raw materials, mix proportions, and pre-curing temperature and humidity conditions of the concrete used for verification are the same as those in Example 1. After curing for one day, the concrete is placed in a -20℃ environment for freezing, and the dynamic modulus of elasticity, strength, and temperature of the concrete during the cooling process are tested.

[0161] In this embodiment, it is necessary to calculate the volume fraction of ice generated during the phase transition of concrete when it freezes. First, a cement paste pore size distribution model needs to be established. The cement paste pore size distribution model used in this embodiment is shown in equation (19).

[0162]

[0163] In the formula, φ(r) is the pore volume fraction with a radius smaller than r; φ i B represents the volume fraction of the i-th type of pores, where i = 1, 2, and 3, representing gel pores, small capillaries, and large capillaries, respectively; i When the pore size is represented by a logarithmic coordinate, the pore size corresponds to the peak point of the pore size distribution curve of the i-th type of pore, where B1 = 1.5 nm, B2 = 9.5 nm, and B3 = 100 nm.

[0164] When determining the moisture distribution in the cement paste, it is assumed that the gel pores are always saturated. This embodiment does not consider extreme drying conditions due to gel dehydration. Therefore, the volume fraction of gel water is equal to the volume fraction of gel pores, and the total volume fraction of gel pores is the sum of the volume fractions of HD CSH gel pores calculated by equation (11) and LD CSH gel pores calculated by equation (12). For capillaries, it is assumed that moisture preferentially fills the smaller pores and is not affected by the type of large or small capillaries. The total volume fraction of capillary water can be obtained by subtracting the volume fraction of gel water from the total volume fraction of water in the cement paste calculated by GEMS.

[0165] The relationship between freezing temperature and critical freezing pore size is given by the Gibbs-Thomson equation. The critical freezing pore size should take into account the thickness of the unfrozen water layer, as shown in equation (20). When the pore size is lower than the critical freezing pore size, the pores are not frozen. However, when the pore size is greater than the critical freezing pore size, the pores are frozen. At this time, if there is water in the pores larger than the critical freezing pore size, this water will freeze into ice. The volume fraction of ice is the same as the volume fraction of water in the pores larger than the critical freezing pore size. It is assumed that when cement paste freezes, LD CSH freezes preferentially over HD CSH.

[0166]

[0167] In the formula, r eq Critical frost-resistant pore size considering the thickness of the unfrozen aquifer; k s Let k be the pore shape coefficient. For cylindrical pores, k s =2;T m T is the melting temperature of large volumes of ice. m =273K; γ iw For the ice-water interface energy, γ iw =29×10 -3 J / m 2 ;ΔT f The temperature difference between 0℃ and the freezing temperature; ρ i ρ is the density of ice. i =920kg / m 3 ;ΔH f Let H be the entropy of ice melting. f =333×10 3 J / kg; δ is the thickness of the unfrozen aquifer, δ=0.8×10 -9 m.

[0168] In this example, the multi-scale composition structure of concrete still adopts... Figure 2The structure is shown. The failure control phase of the slurry is unfrozen LD CSH gel, with a yield stress equal to that of the LD CSH gel in Example 1, which is 105 MPa. The mechanical property reduction factor of the ITZ at concrete failure is the same as that in Example 1, which is 0.5. Figure 6 The calculation shows the reinforcing effect of ice on ITZ and matrix paste of concrete during freezing. Figure 7 This is to predict and measure the growth rate of the dynamic elastic modulus of concrete relative to the positive temperature dynamic elastic modulus. Figure 8 This refers to the rate of increase of concrete strength relative to its positive temperature strength, both predicted and measured. Figure 9 This refers to the internal grout failure process of concrete under pressure at a temperature of -20℃.

[0169] exist Figure 6 In the figure, since the development of the elastic modulus of the ITZ at a distance of 10μm to 20μm from the aggregate surface is almost the same, only the change in the elastic modulus of the ITZ at 18μm to 20μm is shown. The elastic modulus of both the ITZ and the matrix paste is significantly increased in the temperature range of 0℃ to -10℃. As the temperature continues to decrease, the increasing trend of the elastic modulus gradually weakens. The closer the sub-ITZ layer is to the aggregate surface, the greater the contribution of ice to its elastic modulus.

[0170] from Figure 7 and Figure 8 As can be seen, the algorithm proposed in this invention also has good predictive effects on the elastic modulus and strength of negative-temperature concrete. ITZ has a high water-cement ratio, and after freezing, it produces more ice, which significantly increases the elastic modulus and strength of ITZ. Nevertheless, Figure 9 The results show that the ITZ (intermediate zone) remains the most vulnerable area to failure when concrete is subjected to compressive stress. Therefore, the ITZ is a key area that deserves attention when improving the performance of concrete, whether it is positive-temperature concrete or negative-temperature concrete that has been used in cold regions for many years.

[0171] Examples 1 and 2 demonstrate that the method of the present invention has a good predictive effect on the strength of concrete cured at positive temperature and subjected to freezing at negative temperature.

[0172] This invention may have other embodiments. Without departing from the spirit and essence of this invention, those skilled in the art can make various corresponding changes and modifications according to this invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A method for predicting the strength of concrete under positive temperature curing and negative temperature freezing, considering ITZ, characterized in that: Includes the following steps: Step A: Establish a cement-based material information database; establish the corresponding multi-scale composition structure according to the predicted object, and determine the destructive control phase of cement paste; when the predicted object is mortar or concrete, execute step B; when the predicted object is cement paste, directly execute step C. The cement paste failure control phase is the inclusion phase used to determine whether the cement paste has failed. Step B: For cases where the object to be predicted is mortar or concrete, determine the water-cement ratio distribution within the object; then proceed to step C. Step C: If the object to be predicted is cement paste, calculate the degree of hydration of the cement paste; if the object to be predicted is mortar or concrete, calculate the degree of hydration of each sub-ITZ layer and the matrix paste; the sub-ITZ layer refers to the ITZ layer being subdivided into multiple shell-like sub-ITZ layers of the same thickness that surround the aggregate. Then determine whether the object to be predicted has been frozen, that is, whether it has experienced sub-zero temperatures. If it has experienced sub-zero temperatures, proceed to step D; otherwise, proceed directly to step E. Step D: For the case where the predicted object experiences a negative temperature, if the predicted object is cement paste, calculate the ice content of the cement paste; if the predicted object is mortar or concrete, calculate the ice content of each sub-ITZ layer and the matrix paste of the predicted object; then proceed to step E. Step E: Perform phase division of cement paste to obtain the volume fraction of each inclusion phase inside the RVE at each scale in the predicted object; Step F: Calculate the homogenized elastic modulus of the object to be predicted; Step G: Calculate the intensity of the predicted object: When the object of prediction is cement paste, calculate the load when the cement paste is in a critical failure state. This load is the strength of the cement paste. When the predicted object is mortar or concrete, first calculate the external load when each undamaged sub-ITZ layer and the matrix paste are in the critical failure state. If the predicted object is lightweight aggregate concrete, it is also necessary to calculate the external load when the aggregate phase is in the critical failure state. The critical failure area corresponding to the minimum value of the external load of all predicted objects is judged as failure, and the minimum load is recorded as the instantaneous strength. Determine whether the failed area is the matrix paste of the predicted object. If not, reduce the mechanical properties of the failed area and return to step F. If the matrix paste of the predicted object is determined to be failed, the maximum value of all instantaneous strengths is the strength of the predicted object.

2. The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing considering ITZ as described in claim 1, characterized in that: Step A describes the process of establishing a multi-scale compositional structure, which includes the following steps: Based on the characteristic length of the inclusion phases in concrete, the concrete is divided into several scales, each scale containing only one or more inclusion phases with basically the same characteristic length; a representative volume unit, i.e., RVE, is established at each scale to represent the distribution characteristics of the inclusion phases.

3. The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing considering ITZ as described in claim 2, characterized in that: The process of determining the control phase for cement paste degradation in step A includes the following steps: First, based on the established multi-scale composition structure, the scale of the cement paste destruction control phase is selected. The cement paste destruction control phase must be located in the cement paste, and its scale must be smaller than the cement paste scale. At the scale of the cement paste destruction control phase, the inclusion phase with the smallest elastic modulus other than the porous phase is selected as the cement paste destruction control phase. The cement paste destruction control phase cannot be composed of pores alone and should contain at least one solid hydration product.

4. The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing considering ITZ as described in claim 3, characterized in that: The cement-based material information database mentioned in step A includes the mechanical properties and shapes of the aggregate phase, cement clinker phase, all hydration product phases, aqueous phase and gas phase, as well as the yield stress of the cement paste failure control phase and the yield stress of various aggregates. The yield stress of the cement paste failure control phase is taken as an empirical value or determined by experiment; the process of determining it by experiment includes the following steps: A batch of cement pastes with different water-cement ratios were formed and cured under standard curing conditions for different ages. The strength of these cement pastes was tested. The strength of cement paste with a specified water-cement ratio after curing for a specified age was calculated, and the relationship function between the strength and the yield stress of the cement paste failure control phase was obtained. In this relationship function, the calculated strength was set to be equal to the test strength of the cement paste with the corresponding water-cement ratio and curing age, and the yield stress of the cement paste failure control phase under this condition was calculated. The same calculation operation was performed for cement pastes with all water-cement ratios and curing ages to obtain the yield stress of the cement paste failure control phase under multiple water-cement ratios and curing ages. Outliers were removed, and the average value of these yield stresses was taken as the yield stress of the cement paste failure control phase.

5. A method for predicting the strength of concrete under positive temperature curing and negative temperature freezing, considering ITZ, as described in any one of claims 1 to 4, characterized in that: Step B, which describes the process of determining the water-cement ratio distribution in the target object, includes the following steps: First, determine the ITZ width and calculate the width of each sub-ITZ layer. Calculate the total ITZ integral, the volume fraction of each sub-ITZ layer, and the volume fraction of the matrix paste based on the aggregate particle size distribution and aggregate volume fraction. Then, calculate the water-cement ratio distribution function on the aggregate surface to obtain the water-cement ratio of the matrix paste and the average water-cement ratio of each sub-ITZ layer.

6. The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing considering ITZ according to claim 5, characterized in that: Step D, which involves calculating the ice content of cement paste or the ice content of each sub-ITZ layer and matrix paste of the predicted object, is achieved through a paste ice content calculation model. The specific process of establishing the paste ice content calculation model includes the following steps: First, a pore size distribution model of the slurry is established. Then, the distribution of water in pores of different sizes is determined, as well as the relationship between freezing temperature and critical freezing pore size. Water in pores smaller than the critical freezing diameter does not freeze; water in pores with a diameter greater than or equal to the critical freezing diameter freezes completely, and the water is replaced by an equal volume of ice. The total volume fraction of water is equal to the volume fraction of ice.

7. The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing considering ITZ as described in claim 6, characterized in that: Step E describes the process of phase separation in cement paste, which includes the following steps: Calculate the volume fraction of each hydration product phase in the cement paste; combine the multi-scale composition structure, further subdivide the volume fraction of unhydrated cement clinker phase, the volume fraction of each hydration product phase, the volume fraction of gas phase and the volume fraction of liquid phase to each scale. If the object to be predicted has experienced a negative temperature, the ice volume fraction also needs to be subdivided to each scale to obtain the volume fraction of each inclusion phase inside the RVE at each scale in the cement paste.

8. The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing, considering ITZ, as described in claim 7, is characterized in that: In step E, during the process of dividing the cement paste phases, if the object to be predicted is mortar or concrete, the volume fraction of each inclusion phase inside the RVE at each scale in the cement paste is obtained for all sub-ITZ layer pastes and matrix pastes; and then the volume fraction of the inclusion phase in the RVE at all scales in the object to be predicted is obtained.

9. The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing considering ITZ as described in claim 8, characterized in that: In step G, during the calculation of the intensity of the predicted object, If the object of prediction is cement paste, it is first necessary to establish the relationship between the cement paste load and the maximum stress of the cement paste failure control phase. Then, the maximum stress of the cement paste failure control phase is made equal to the yield stress of the failure control phase, so as to obtain the load when the cement paste is in the critical failure state. If the object being predicted is mortar or concrete, the specific process for strength calculation includes the following steps: G2.1 Establish the relationship between the external load of the predicted object and the maximum stress of the cement paste failure control phase in each sub-ITZ layer, and establish the relationship between the external load of the predicted object and the maximum stress of the cement paste failure control phase in the matrix paste; if the predicted object is lightweight aggregate concrete, also establish the relationship between the external load of the concrete and the maximum stress of the aggregate phase. G2.

2. Set the maximum stress of the cement paste failure control phase to equal its yield stress to obtain the predicted external load of each sub-ITZ layer when it is in the critical failure state, and obtain the predicted external load of the matrix paste when it is in the critical failure state. If the predicted object is lightweight aggregate concrete, set the maximum stress of the aggregate phase to equal the yield stress of the phase to obtain the concrete external load when the aggregate phase is in the critical failure state. G2.

3. Compare the external loads of all predicted objects obtained in G2.2, and determine the critical failure zone corresponding to the minimum value as failure. The minimum load is recorded as the instantaneous intensity. G2.

4. Determine whether the damaged area in step G2.3 is the matrix paste of the predicted object. If not, reduce the mechanical properties of the damaged area by multiplying the effective elastic modulus of the damaged area by the corresponding reduction factor. Use the reduced mechanical properties of the damaged area and the mechanical properties of the remaining non-damaged phases to return to step F and obtain the homogenized elastic modulus of the damaged predicted object. For the undamaged areas within the predicted object, execute steps G2.1 to G2.

3. The already damaged areas are not included in the calculation of instantaneous strength. Obtain the instantaneous strength of the predicted object when the new area in step G2.3 is damaged. Determine again whether the newly damaged area is the matrix paste. If not, reduce the mechanical properties of the newly damaged area as well and return to step F to calculate the instantaneous strength of the new damaged area and the corresponding predicted object until the newly damaged area is the matrix paste. If the damaged area in G2.3 is the matrix paste, the predicted object is determined to have failed, and the maximum value among all instantaneous strengths is the strength of the predicted object.

10. The method for predicting the strength of concrete under positive temperature curing and negative temperature freezing considering ITZ according to claim 9, characterized in that: The reduction factor during the reduction of mechanical properties in the damaged area is obtained through the following steps: The reduction factor corresponding to the mechanical properties of the damaged area of ​​concrete or mortar is determined according to the reduction factor of mechanical properties at the failure of the sub-ITZ layer. It is obtained by back-calculating through strength tests of concrete or mortar: First, a batch of concrete or mortar with different mix proportions is formed and cured for different ages under standard curing conditions, and the strength of these concrete or mortars is tested; the strength of concrete or mortar with each mix proportion at different curing ages is calculated; the relationship function between the strength of concrete or mortar and the reduction factor of mechanical properties of the sub-ITZ layer is obtained; the calculated strength is set equal to the test strength of concrete or mortar with the corresponding mix proportion and curing age, and the reduction factor of mechanical properties of the sub-ITZ layer in the concrete or mortar under this condition is calculated; the above calculation operation is performed for all mix proportions and curing ages of concrete or mortar to obtain the reduction factor of mechanical properties of the sub-ITZ layer under multiple mix proportions and curing ages; outliers are removed, and the average value of these reduction factors is taken as the reduction factor of mechanical properties of the sub-ITZ layer. When the predicted object is lightweight aggregate concrete, the failure of the aggregate phase must be considered. It is necessary to determine the mechanical property reduction factor when the aggregate phase fails. The mechanical property reduction factor for aggregate phase failure needs to be based on the known mechanical property reduction factor of the sub-ITZ layer. The mechanical property reduction factor of the sub-ITZ is obtained through the above steps. Then, the mechanical property reduction factor for aggregate phase failure is obtained by back-reasoning through the strength test of lightweight aggregate concrete: mold a batch of lightweight aggregate concrete with different mix proportions, cure them under standard curing conditions to maturity age, and test the strength of these concretes. Calculate the strength of lightweight aggregate concrete for each mix proportion; obtain the relationship function between strength and the reduction factor of mechanical properties of lightweight aggregate phase; Set the calculated strength equal to the test strength of the concrete with the corresponding mix proportion, and then calculate the reduction factor of the mechanical properties of the aggregate phase in the concrete with that mix proportion. Perform the above calculation operation for all mix proportions of lightweight aggregate concrete to obtain the reduction factor of the mechanical properties of the lightweight aggregate phase for multiple mix proportions. Remove outliers and take the average of these reduction factors as the reduction factor for aggregate phase mechanical properties.

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