A method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete.

Abstract

This invention relates to the field of concrete testing technology, specifically a method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete. This invention involves subjecting marine sand concrete specimens to early carbonation curing for varying durations, followed by uniaxial compressive tests and carbonation depth measurements. This allows for the acquisition of mechanical property data, including the complete stress-strain curve, under different carbonation degrees, thereby addressing the problem of incomplete data due to insufficient carbonation time. The experimental method of this invention can be used to obtain the stress-strain curve of marine sand concrete under uniaxial compression, and based on this, establish a corresponding stress-strain constitutive model. This model can provide analytical basis for subsequent industrial and engineering applications and can be used to predict the mechanical response of carbonized cured marine sand concrete.

Description

Technical Field

[0001] This invention relates to the field of concrete testing technology, specifically a method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete. Background Technology

[0002] Carbon dioxide curing can improve the microstructure of concrete and significantly increase its early compressive strength. However, most existing studies have been conducted under short-term carbonation conditions of 4-24 hours. The insufficient carbonation duration results in limited CO2 sequestration by cement-based materials, and their carbon sequestration potential has not been fully realized.

[0003] On the other hand, the current concrete mechanics test standard takes ordinary silicate cement concrete as the object and does not take into account two special properties of carbonized sea sand concrete: First, the difference in the interfacial transition zone (ITZ): the interaction between the chloride film on the surface of sea sand and the carbonation products makes the ITZ performance significantly different from that of ordinary concrete; Second, the increased brittleness: the high content of calcium carbonate leads to the increase of material brittleness, and traditional displacement control loading is prone to instantaneous failure, making it difficult to obtain a stable descent segment.

[0004] A more prominent problem is that existing constitutive models are "unsuitable for application": First, the parameters of the classical concrete damage plasticity model are based on ordinary concrete test calibration. When directly applied to carbonized sea sand concrete, they will deviate significantly from the actual stress distribution, crack propagation and failure mode; Second, the current model has not yet established a quantitative relationship between parameters and key control variables or microstructural characteristics of carbonation curing.

[0005] In view of this, the lack of dedicated test data and reliable calculation models for carbonized sea sand concrete in the engineering field makes it difficult to incorporate into mainstream structural design software and standards, which seriously hinders the engineering and industrial application of the material.

[0006] Therefore, it is urgent to develop comprehensive mechanical property testing methods for carbonized marine sand concrete and to construct an elastoplastic damage constitutive model that can characterize its damage evolution. Summary of the Invention

[0007] To address the aforementioned technical problems, existing studies on carbonated marine sand concrete mostly focus on short-term carbonation conditions of 4-24 hours, neglecting the characteristics of carbonated marine sand concrete. Furthermore, existing constitutive models suffer from poor applicability and a lack of basis for engineering applications. The technical solution adopted by this invention to solve these problems is as follows: A method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete includes the following steps: S1. Prepare sea sand concrete specimens according to the preset mix proportion, and perform early carbonation curing on the specimens for different durations to obtain test specimens with different carbonation degrees. S2. Perform uniaxial compression test and carbonization depth test on the test piece respectively; wherein, the uniaxial compression test adopts displacement controlled loading method to obtain stress-strain full curve data including the descending segment; the carbonization depth test is used to determine the degree of carbonization of the test piece. S3. Based on the stress-strain curve, the constitutive model control parameters for characterizing the material damage evolution behavior are obtained by inversion, and the control parameters are quantitatively correlated with the corresponding carbonation depth to obtain the constitutive model of uniaxial compressive damage of carbonized cured marine sand concrete.

[0008] Furthermore, in some embodiments of the present invention, in step S3, damage variables are defined based on statistical distribution theory, a basic equation of the damage constitutive model containing control parameters is constructed, and the control parameters in the basic equation of the damage constitutive model are solved in reverse according to the geometric characteristics of the stress-strain full curve data.

[0009] Furthermore, in some embodiments of the present invention, the control parameters include parameter a and parameter b, and the damage evolution behavior is described by a Weibull distribution probability density function.

[0010] Furthermore, in some embodiments of the present invention, the geometric features extracted from the stress-strain full curve data include the coordinates of the peak point and the tangent modulus of the rising segment.

[0011] Furthermore, in some embodiments of the present invention, linear or nonlinear regression analysis is performed with the carbonization depth as the independent variable and the constitutive model control parameters a and b as the dependent variables, respectively.

[0012] Furthermore, in some embodiments of the present invention, step S2 further includes compressive strength, splitting tensile strength and flexural strength tests, and the obtained mechanical property data are used to verify the stress-strain curve.

[0013] Furthermore, in some embodiments of the present invention, the carbonization depth test employs the phenolphthalein indicator method as the carbonization depth of the specimen.

[0014] Furthermore, in some embodiments of the present invention, the method further includes performing a pore solution pH test on the test specimen, wherein the pH test value is used to assist in characterizing the degree of carbonization.

[0015] Furthermore, in some embodiments of the present invention, in step S3, thermogravimetric analysis or X-ray diffraction analysis is performed on specimens cured for different durations of carbonation; the amount of calcium carbonate generated is determined, and the filling effect of carbonation products on concrete pores and interface transition zones is analyzed, so as to correct the quantitative correlation between the constitutive model control parameters and the carbonation depth.

[0016] Furthermore, in some embodiments of the present invention, carbonation curing includes a pre-curing stage, a carbonation curing stage, and a subsequent curing stage. In the carbonation curing stage, the demolded sea sand concrete specimen is placed in a sealed carbonation chamber, and the CO2 concentration is controlled at ≥90%, the curing temperature is 20-30℃, and the relative humidity is 50-70%. The pre-curing stage is in-mold curing for no less than 12 hours, the carbonation curing time is between 0.5 and 7 days, and the subsequent curing is film covering and moisturizing curing for no less than 28 days.

[0017] The beneficial effects of this invention are as follows: 1. This invention provides a method for constructing a constitutive model of uniaxial compression damage in carbonized cured marine sand concrete. By subjecting marine sand concrete specimens to early carbonization curing for different durations, followed by uniaxial compression tests and carbonation depth tests, mechanical property data including the full stress-strain curve under different carbonization degrees are obtained, thereby improving the problem of incomplete data caused by insufficient carbonization time.

[0018] 2. The experimental method of this invention can be used to obtain the stress-strain curves of marine sand concrete under uniaxial compression, and to establish a corresponding stress-strain constitutive model based on these curves. This model can provide analytical basis for subsequent industrial and engineering applications, and can be used to predict the mechanical response of carbonation-cured marine sand concrete. Attached Figure Description

[0019] Figure 1 This is a schematic diagram showing the carbonation depth of sea sand concrete after carbonation curing in an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram showing the pH values ​​of samples with different water-cement ratios after 3 days of carbonization curing in an embodiment of the present invention.

[0021] Figure 3 The pH values ​​of the 0.43 water-cement ratio sample in the embodiments of the present invention after different carbonization curing times are shown.

[0022] Figure 4 The images show the XRD patterns of carbon-cured and standard-cured sea sand concrete in the embodiments of the present invention.

[0023] Figure 5 The images show the XRD patterns of sea sand concrete under different carbonation curing times in the embodiments of the present invention.

[0024] Figure 6 These are the TG-DTG curves of carbon-cured and standard-cured sea sand concrete in the embodiments of the present invention.

[0025] Figure 7 The TG-DTG curves of sea sand concrete with different carbonation curing times in the embodiments of the present invention are shown.

[0026] Figure 8A schematic diagram of the microstructure of standard-cured sea sand concrete.

[0027] Figure 9 This is a schematic diagram of the microstructure of carbonized cured sea sand concrete in an embodiment of the present invention.

[0028] Figure 10 A schematic diagram of the interface transition zone of standard-cured marine sand concrete.

[0029] Figure 11 This is a schematic diagram of the interface transition zone of carbonized cured sea sand concrete in an embodiment of the present invention.

[0030] Figure 12 This is the stress-strain curve of carbonized cured sea sand concrete in an embodiment of the present invention.

[0031] Figure 13 This is a schematic diagram showing the relationship between the cubic compressive strength and the axial compressive strength in an embodiment of the present invention.

[0032] Figure 14 This is a schematic diagram illustrating the relationship between the peak strain relative coefficient and the carbonization depth in an embodiment of the present invention.

[0033] Figure 15 This is a schematic diagram illustrating the variation of parameter a with carbonization depth in an embodiment of the present invention.

[0034] Figure 16 This is a schematic diagram illustrating the variation of parameter b with carbonization depth in an embodiment of the present invention.

[0035] Figure 17 This is a schematic diagram comparing the constitutive model fitting results with the experimental results in an embodiment of the present invention. Detailed Implementation

[0036] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0037] A method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete includes the following steps: S1. Prepare sea sand concrete specimens according to the preset mix proportion, and perform early carbonation curing on the specimens for different durations to obtain test specimens with different carbonation degrees. S2. Perform uniaxial compression test and carbonization depth test on the test piece respectively; wherein, the uniaxial compression test adopts displacement controlled loading method to obtain stress-strain full curve data including the descending segment; the carbonization depth test is used to determine the degree of carbonization of the test piece. S3. Based on the stress-strain curve, the constitutive model control parameters for characterizing the material damage evolution behavior are obtained by inversion, and the control parameters are quantitatively correlated with the corresponding carbonation depth to obtain the constitutive model of uniaxial compressive damage of carbonized cured marine sand concrete.

[0038] This invention provides a method for constructing a constitutive model of uniaxial compressive damage in carbonized sea sand concrete. By subjecting sea sand concrete specimens to early carbonization curing for different durations, followed by uniaxial compressive tests and carbonation depth tests, mechanical property data including the full stress-strain curve is obtained under different carbonization degrees, thereby improving the problem of incomplete data caused by insufficient carbonization time.

[0039] To address the increased brittleness and potential for sudden failure under conventional loading methods in carbonation-cured marine sand concrete due to the formation of calcium carbonate internally, this invention employs a high-precision loading method with displacement control to obtain a complete stress-strain curve including a stable descent phase. This supports the assessment of the entire material failure process and provides a data foundation for subsequent modeling.

[0040] Specifically, existing concrete constitutive models are mostly calibrated based on ordinary concrete tests, which can easily lead to prediction errors when directly applied to carbonated sea sand concrete. This invention obtains damage control parameters by inverting the stress-strain curve data and establishes a quantitative relationship between these parameters and the carbonation depth of concrete. This allows the model to reflect the strength changes and damage development patterns of the material under compression, based on the actual degree of carbonation, thereby improving the applicability and prediction accuracy of the model.

[0041] Furthermore, as a preferred embodiment of the present invention and not a limitation thereof, the experimental method and the constitutive model constructed in this invention can be used in engineering practice. This constitutive model can be embedded in general-purpose finite element analysis software such as ABAQUS and ANSYS, providing a calculation basis for the analysis and design of carbonized marine sand concrete structures, and helping to promote the widespread application of this material in engineering.

[0042] Furthermore, in some embodiments of the present invention, in step S3, damage variables are defined based on statistical distribution theory, and a basic equation of the damage constitutive model containing undetermined parameters is constructed. Based on the geometric characteristics of the stress-strain full curve data, the undetermined parameters in the basic equation of the damage constitutive model are solved in reverse.

[0043] Furthermore, as a preferred embodiment of the invention and not a limitation thereof, the invention describes the damage behavior of carbonized sea sand concrete based on the Weibull distribution in statistical distribution theory. Considering the characteristics of this material as a heterogeneous quasi-brittle material and the randomness of its internal microcrack and defect distribution, a damage variable is defined using statistical distribution theory, correlating the probability of micro-element failure with macroscopic damage. This model can not only fit the stress-strain curve macroscopically but also help to understand the evolution of internal damage accumulation, propagation, and eventual failure of concrete under carbonation curing at a mesoscale.

[0044] Specifically, this invention utilizes the geometric features of the stress-strain curve to invert and solve for the undetermined parameters in the model. These geometric features include boundary conditions such as peak stress, peak strain, and initial elastic modulus. The undetermined parameters mainly include parameters a and b. This inversion method allows the theoretical curve to pass through the key feature points of the experimental curve, thereby improving the agreement between the model curve and measured data in the rising and falling segments, and enhancing the accuracy of describing the behavior of carbonized marine sand concrete in the brittle softening segment.

[0045] As mentioned earlier, by establishing fundamental equations containing undetermined parameters, this invention describes the damage evolution process as a change in mathematical parameters. This parametric form enables the model to adapt to the mechanical behavior of marine sand concrete under different water-cement ratios and carbonation depths. After obtaining basic geometric characteristic data through conventional experiments, engineers can determine the model parameters and then conduct structural nonlinear analysis, facilitating the engineering application of this material in numerical simulations.

[0046] Furthermore, in some embodiments of the present invention, the constitutive model control parameters include shape parameter a and scale parameter b, and the damage evolution behavior is described by the Weibull distribution probability density function.

[0047] Furthermore, as a preferred embodiment of the invention and not a limitation thereof, the invention employs a Weibull distribution probability density function to describe the damage evolution process of carbonized cured marine sand concrete. This method is based on the fact that concrete, as a multiphase heterogeneous material, has a random distribution of the strength of its internal micro-defects, which can approximately follow a statistical distribution law. By associating the probability of micro-element failure with macroscopic damage variables, the model formally reflects the statistical characteristics of damage accumulation in the material during stress.

[0048] Specifically, the model introduces a shape parameter 'a' and a scale parameter 'b' to characterize the shape of the stress-strain curve. Parameter 'a' is related to the brittle characteristics of the material and the concentration of damage development, and can be used to reflect the steepening of the post-peak softening segment caused by calcium carbonate formation after carbonation curing. Parameter 'b' is related to the macroscopic strength level of the material and affects the location of the peak stress.

[0049] As mentioned earlier, the constructed damage constitutive model is based on the Weibull distribution, and its parameters can be calibrated using experimental data. In this embodiment, uniaxial compression tests were conducted on sea sand concrete specimens with different water-cement ratios and carbonation curing times to obtain the full stress-strain curves, from which parameters a and b were derived. The fitting results show that the correlation coefficient R between the model-calculated curves and the measured data is [value missing]. 2 With a value not lower than 0.954, it can reasonably reproduce the elastic response of the rising segment and the strain softening behavior after the peak, and is suitable for describing the mechanical response characteristics of carbonized sea sand concrete under high brittle conditions.

[0050] Furthermore, in some embodiments of the present invention, the geometric boundary conditions of the stress-strain full curve data are extracted, and the geometric boundary conditions include the coordinates of the peak point and the tangent modulus of the rising segment.

[0051] In this invention, the coordinates of the peak points of the stress-strain curve correspond to the peak stress and peak strain of the specimen, and the tangent modulus of the rising segment is used to characterize the initial elastic modulus of the material. During modeling, the above-mentioned geometric features are introduced as constraints into the parameter inversion process of the constitutive model, so that the theoretical curve is consistent with the experimental data in terms of peak point position and initial slope, thereby reducing the systematic deviation of the model in key mechanical indicators such as strength and stiffness.

[0052] As mentioned earlier, since the shape parameters in the damage constitutive model cannot be obtained through direct measurement, this invention extracts several geometric features, including peak stress, peak strain, and initial elastic modulus, from the measured stress-strain curve, establishes corresponding boundary equations, and transforms the parameter identification problem into a mathematical solution problem under given constraints. This method helps improve the stability of parameter solutions and simplifies the calculation process.

[0053] Specifically, carbonation curing alters the microstructure of marine sand concrete, leading to an increase in elastic modulus and a decrease in peak strain. This invention uses tangent modulus and peak strain as primary constraints, enabling the final model to reflect the influence of different carbonation durations on the material's elastoplastic behavior, thus providing a more realistic calculation basis for structural deformation analysis.

[0054] Furthermore, in some embodiments of the present invention, linear or nonlinear regression analysis is performed with the carbonization depth as the independent variable and the constitutive model control parameters a and b as the dependent variables, respectively.

[0055] As mentioned earlier, existing model parameters are typically set for specific working conditions, and if curing conditions change, they need to be recalibrated through mechanical tests. This invention establishes a quantitative functional relationship between carbonation depth and model control parameters through regression analysis, enabling the model to continuously reflect the changing trend of the mechanical behavior of marine sand concrete according to the degree of carbonation, thus enhancing the model's adaptability to different curing conditions.

[0056] Specifically, based on the aforementioned regression relationship, in engineering practice, the carbonation depth can be measured and substituted into the regression equation to directly calculate the corresponding model parameters a and b, thereby predicting the uniaxial compressive stress-strain curve of concrete under this state. This method can, to some extent, replace complex full-process uniaxial compression tests, providing a simpler analytical approach for evaluating the mechanical properties of carbonated marine sand concrete structures in practical engineering.

[0057] Existing damage model parameters are often difficult to correlate directly with changes in the microstructure of materials. This invention uses carbonization depth as a comprehensive indicator reflecting the degree of CO2 diffusion, pore filling, and calcium carbonate formation. Through linear or nonlinear regression, this invention correlates carbonization depth with constitutive model parameters, establishing a correspondence between changes in model parameters and the degree of carbonization reaction within the material. This helps to anticipate the impact on mechanical properties during the material design stage by adjusting carbonization curing parameters.

[0058] Furthermore, in some embodiments of the present invention, step S2 further includes compressive strength, splitting tensile strength and flexural strength tests, and the obtained mechanical property data are used to verify the stress-strain curve.

[0059] As mentioned above, uniaxial compressive stress Obtaining the full strain curve places high demands on the stiffness and loading control of the testing machine, making it susceptible to equipment disturbances. This invention uses standardized compressive strength, splitting tensile strength, and flexural strength tests as references, verifying the results by comparing the peak stress measured in a uniaxial compression test with the standard compressive strength. By cross-checking multiple sets of mechanical data, the potential for random errors or systematic biases in a single test can be reduced, thereby improving the reliability of the stress-strain full curve data.

[0060] Specifically, a single compression test has limited relevance to the brittle characteristics of a material. This invention, by adding splitting tensile strength and flexural strength tests that are sensitive to crack propagation, can indirectly quantify the degree of brittleness changes caused by carbonation curing. By using auxiliary mechanical data to corroborate the relatively steep softening segment in the stress-strain curve, the physical reality of the parameter changes describing damage evolution in the model can be verified. This allows the constructed constitutive model to not only achieve numerical fitting but also reflect the actual mechanical properties of carbonized marine sand concrete, which exhibits improved compressive strength and reduced toughness.

[0061] Furthermore, as a preferred embodiment of the present invention and not a limitation thereof, by simultaneously acquiring multiple mechanical indicators such as compressive strength, splitting tensile strength, and flexural strength, a comprehensive understanding of the performance of carbonized marine sand concrete under different stress states can be obtained. Especially when the carbonization depth is large, if the auxiliary mechanical indicators show a downward trend, it can prompt designers to pay attention to the changes in the material's tensile or flexural properties and take them into account in the structural design. This multi-indicator testing method helps to reduce the performance dimensions that may be overlooked when relying solely on a uniaxial compression constitutive model for structural analysis.

[0062] Furthermore, in some embodiments of the present invention, the carbonization depth test employs the phenolphthalein indicator method as the carbonization depth of the specimen.

[0063] Furthermore, as a preferred embodiment of the invention and not a limitation thereof, the present invention employs the phenolphthalein indicator method to determine the carbonation depth of concrete specimens. Specifically, a phenolphthalein solution is sprayed onto the split fracture surface of the specimen. Utilizing its characteristic of turning purple-red in an alkaline environment and colorless in a neutral or weakly acidic environment, it distinguishes between uncarbonated and carbonated areas, thereby determining the location of the carbonation front. This method is simple to operate, requires conventional equipment, and is suitable for determining the carbonation depth of batches of specimens.

[0064] Specifically, the measured carbonation depth, as an indicator characterizing the degree of carbonation in the material, is used as an independent variable in subsequent regression analysis. This indicator indirectly reflects the degree of calcium hydroxide conversion to calcium carbonate within the concrete, transforming it into a quantifiable and repeatable geometric parameter. This parameter is used to establish the functional relationship between the constitutive model's control parameters and carbonation curing conditions, providing input for the model's application under different carbonation states.

[0065] Furthermore, in some embodiments of the present invention, the method further includes performing a pore solution pH test on the test specimen, wherein the pH test value is used to assist in characterizing the degree of carbonization.

[0066] As mentioned earlier, the phenolphthalein indicator method determines the carbonization area through color changes, and its color change threshold corresponds to a pH value of approximately 9.0. Therefore, it can only reflect areas with pH values ​​below this threshold and is a semi-quantitative method. To overcome the limitations of this method, this invention adds a pore solution pH test. By directly measuring the alkalinity values ​​at different locations inside the concrete, more detailed chemical state information can be obtained. The test results can be used to assist in verifying the carbonization depth determined by the phenolphthalein method, reducing uncertainties caused by visual interpretation or local ionic interference.

[0067] Specifically, because the carbonation reaction diffuses gradually within concrete, the pH value typically forms a gradient distribution from the surface to the interior. By sampling and measuring the pH value at different depths, the trend of alkalinity changes with depth can be obtained, reflecting the transition characteristics between fully carbonized, partially carbonized, and uncarbonized zones. This chemical characterization method provides auxiliary evidence for analyzing the relationship between the degree of carbonation and macroscopic mechanical properties, and helps to understand the performance evolution of materials under carbonation curing conditions.

[0068] Furthermore, in some embodiments of the present invention, in step S3, thermogravimetric analysis or X-ray diffraction analysis is performed on specimens cured for different durations of carbonation; the amount of calcium carbonate generated is determined, and the filling effect of carbonation products on concrete pores and interface transition zones is analyzed, so as to correct the quantitative correlation between the constitutive model control parameters and the carbonation depth.

[0069] Furthermore, as a preferred embodiment of the invention and not a limitation thereof, the invention combines thermogravimetric analysis (TGA) and X-ray diffraction (XRD) to quantitatively determine the amount of calcium carbonate produced by carbonation, thereby reflecting the degree of carbonation reaction at the microscopic level and providing an analytical basis for the material origin of changes in concrete strength and increased brittleness. This allows the determination of constitutive model parameters to be supported by microstructural data, in addition to mathematical fitting.

[0070] Specifically, given the heterogeneity of concrete, the internal pore structure and reaction degree may differ even at the same carbonation depth. This invention utilizes the amount of calcium carbonate generated and its filling effect on the microstructure as a basis for correcting the parameter correlation established based on carbonation depth. This multi-scale correction method helps reduce potential biases when judging solely based on macroscopic carbonation depth, allowing model parameters to more closely approximate the actual internal state of the material.

[0071] Furthermore, the interfacial transition zone (ITZ) of marine sand concrete is typically quite weak. By analyzing the distribution and filling of carbonation products in the pores and ITZ region, the influence mechanism of carbonation curing on material stiffness, peak strength, and peak strain can be further understood. The relevant microscopic analysis results were used to support the setting of the damage evolution law in the constitutive model, making it more suitable for describing the mechanical response characteristics of carbonated marine sand concrete.

[0072] Furthermore, in some embodiments of the present invention, carbonation curing includes a pre-curing stage, a carbonation curing stage, and a subsequent curing stage. In the carbonation curing stage, the demolded sea sand concrete specimen is placed in a sealed carbonation chamber, and the CO2 concentration is controlled at ≥90%, the curing temperature is 20-30℃, and the relative humidity is 50-70%. The pre-curing stage is in-mold curing for no less than 12 hours, the carbonation curing time is between 0.5 and 7 days, and the subsequent curing is film covering and moisturizing curing for no less than 28 days.

[0073] Furthermore, as a preferred embodiment of the invention and not a limitation thereof, the relative humidity of the environment is controlled within the range of 50%-70% during the carbonization curing stage. This humidity range helps maintain an appropriate amount of liquid phase in the pores, avoiding both excessive humidity leading to pore water saturation and hindering CO2 diffusion, and insufficient humidity causing insufficient water for the reaction, thereby affecting ion migration and the carbonization reaction process. Simultaneously, the CO2 in the carbonization chamber... 22 The concentration should be no less than 90%, and the temperature should be maintained at 20-30℃ to provide relatively stable reaction conditions, promote the penetration of CO2 into the pores of concrete and its reaction with components such as calcium hydroxide and CSH gel.

[0074] Specifically, compared to the short-term carbonation treatment of 4-24 hours commonly found in existing studies, this invention extends the carbonation curing time to 0.5-7 days. Under these conditions, CO2 can diffuse more fully into the interior of the specimen, increasing the amount of calcium carbonate generated per unit mass of slurry. The generated calcium carbonate partially fills the pores and interfacial transition zone (ITZ), helping to reduce porosity and improve the compressive strength and surface hardness of the specimen in the early stages.

[0075] As mentioned above, this invention employs a three-stage curing system: first, in-mold pre-curing for no less than 12 hours is carried out to allow the specimens to acquire initial strength to withstand the shrinkage stress that may occur during subsequent carbonation; then, the aforementioned carbonation curing is performed; after carbonation, a film-coating and moisture-retaining method is used to continue curing for 28 days or longer. This subsequent curing measure helps to replenish moisture, reduce the risk of drying shrinkage cracking, and provide conditions for the continued hydration of incompletely hydrated cement particles, thereby alleviating to some extent the hydration inhibition that may be caused by carbonation product encapsulation and supporting the continuous development of later strength.

[0076] It should be noted that, based on the above-mentioned overall design concept, this invention relates to the study of the effects of carbonation curing on the microstructure and compressive strength of marine sand concrete. Specifically, it includes: using X-ray diffraction (XRD) to analyze the changes in the phase composition of cement hydration products in marine sand concrete under long-term CO2 exposure; assessing the effect of carbonation on the internal alkalinity of concrete by measuring the pH value of the pore solution; using thermogravimetric analysis (TGA) to determine the amount of calcium carbonate generated to indirectly reflect the degree of CO2 absorption; and combining mechanical property testing to analyze the effect of carbonation curing on compressive strength.

[0077] Based on this, this invention proposes a test method for the mechanical properties of carbonized cured marine sand concrete and establishes a stress-strain relationship model under uniaxial compression conditions. This model defines damage variables based on Weibull statistical distribution theory, determines model parameters by inverting the measured stress-strain curves, and correlates the parameters with the carbonation depth to form a damage constitutive model that can characterize the compressive behavior of carbonized marine sand concrete. Experimental results show that the calculated curves of this model and the measured data exhibit good consistency in both the rising and falling segments.

[0078] Example 1 Specifically, this embodiment describes the preparation and curing methods for concrete specimens.

[0079] Preparation of sea sand concrete: The raw materials for sea sand concrete include cement, fly ash, fine aggregate, coarse aggregate, and water.

[0080] The cement used in this embodiment is P·O42.5 grade ordinary Portland cement produced by Conch Cement Plant. Its chemical composition is 19.50% SiO2, 57.57% CaO, 1.21% MgO, 3.08% Fe2O3, 6.45% Al2O3, 2.01% SO3, and 1.35% K2O. The properties of this cement meet the material technical requirements of "General Portland Cement" (GB 175-2007).

[0081] This embodiment uses Class II fly ash with a moisture content of 0.4% and a density of 2.1 g / cm³. 3 The chemical composition of fly ash is 36.8% Al2O3, 45.1% SiO2, 1.2% SO3, and 0.02% Cl. - It contains 4.5% CaO, which meets the standard of "Fly Ash for Cement and Concrete" (GB 1596-2005).

[0082] The fine aggregate in this embodiment is only sea sand from the waters surrounding Shenzhen, Guangdong, which contains a small amount of impurities such as shells and seaweed. The chloride ion concentration in the sea sand was measured to be 0.028%, higher than that of river sand (0.004%), and the shell content was 0.63%. Sieve analysis showed that the fineness moduli of river sand and sea sand were 2.23 and 1.92, respectively, indicating that sea sand contains finer particles than river sand.

[0083] In this embodiment, the coarse aggregate is Jingyangshan crushed stone with a particle size of 5-20mm and an apparent density of 2.65g / cm³. 3 The bulk density is 1.45 g / cm³. 3 .

[0084] This embodiment uses tap water for concrete mixing and test block curing.

[0085] This invention, considering the commonly used water-cement ratio range in precast concrete plants, selects water-cement ratios of 0.38, 0.43, and 0.48. Fly ash has the advantages of low cost and reduced carbon emissions; therefore, this invention uniformly adopts a 25% admixture. Calculations are performed according to the "Specification for Mix Proportioning Design of Ordinary Concrete" (JGJ 55-2011), resulting in... The specific concrete mix design is shown in Table 1.

[0086] Table 1. Concrete Mix Proportions

[0087] The specimen dimensions are based on concrete samples of the following form: 1. Test indicators: compressive strength, splitting tensile strength, actual dimensions (mm) 3 ): 100×100×100, Number of specimens: 204×2=408.

[0088] 2. Test indicators: carbonization depth, flexural strength, actual dimensions (mm) 3 ): 100×100×400, Number of specimens: 9+54=63.

[0089] 3. Test parameters: Uniaxial compression test, actual dimensions (mm) 3 ): 100×100×300, Number of specimens: 45.

[0090] The specimen preparation process was carried out in accordance with the "Standard for Test Methods of Mechanical Properties of Ordinary Concrete" (GB / T 50081-2002). The preparation process of sea sand concrete is as follows: 1) Remove impurities from the sea sand, and then weigh the raw materials such as sea sand, cement, fly ash, and gravel (5-20mm in diameter) according to the concrete mix proportion; 2) First, pour the coarse and fine aggregates into the mixer and mix them. After one minute, add the cement and fly ash. Finally, while mixing, pour water evenly into the mixer. Observe the state of the concrete during the process. Discharge the concrete after it meets the requirements. 3) Pour the mixed concrete into the mold, compact it on a vibrating table, scrape off the excess concrete with a scraper, and smooth the surface of the concrete. After the specimen has initially formed, cover the concrete surface with plastic wrap to prevent moisture evaporation and let it cure for 24 hours.

[0091] Carbonization maintenance system The carbonization test equipment uses the HYT-CCT-2A carbonization chamber produced by Beijing Hangjian Huaye Technology Development Co., Ltd., which can automatically adjust according to the set temperature, humidity, carbon dioxide concentration, and carbonization time.

[0092] The carbonation curing parameters used in this embodiment are: CO2 concentration 95%, curing temperature 25℃, relative humidity 60%, and atmospheric pressure. Unlike existing technologies, this embodiment changes the early carbonation curing time of the concrete, setting it to 0.5 days, 1 day, 3 days, and 7 days respectively, and sets a standard curing as a control group. The carbonation curing regime is mainly divided into three parts: after demolding following 24 hours of in-mold curing, the pre-curing stage, the carbonation curing stage, and the subsequent curing stage.

[0093] The pre-curing stage is conducted in an outdoor ventilated environment with a fan to accelerate drying. The curing time is 12 hours, and the purpose of curing is to remove some of the pore water.

[0094] The curing conditions during the carbonization curing stage are 95% CO2 concentration, 25℃, and 60% RH, with curing times of 0.5d, 1d, 3d, and 7d, respectively. The purpose of the curing is carbon fixation and performance enhancement.

[0095] The subsequent curing stage is subject to standard curing conditions, with a curing time of 28 days. The purpose of curing is to allow the cement to continue hydrating.

[0096] Due to the large number of test blocks used in this embodiment, outdoor film covering and moisture retention curing were adopted in the subsequent standard curing stage. The carbonized concrete was covered and wrapped with geotextile, and the test blocks were watered regularly, with watering no less than twice a day. After 28 days, watering should be done at least once a day until the curing age reaches 56 days.

[0097] Example 2 Based on Example 1, Example 2 also has the following implementation method: Specifically, this embodiment describes the main testing method.

[0098] (1) Depth of carbonization The carbonation depth of carbonated sea sand concrete was measured using a 1% phenolphthalein solution. Short beam specimens measuring 100mm × 100mm × 400mm were used in the experiment, with both ends sealed with epoxy resin to ensure that the carbonation process occurred only within the designated area. To ensure measurement accuracy, each short beam was divided into four equal segments to allow for testing of carbonation depth at four different carbonation durations, thus minimizing measurement errors caused by the heterogeneity of the concrete material itself.

[0099] The testing method is as follows: M1. Place the specimens in a high-concentration carbonization chamber for curing. Specifically, the curing times are set to 0.5d, 1d, 3d and 7d respectively. M2. Phenolphthalein solution was sprayed onto the cross-section of the specimen, and the color change was observed. Specifically, after each curing cycle, a section of the specimen was split in half, dust and debris on the test surface were removed, and phenolphthalein solution was evenly sprayed onto the cross-section. After standing for 30 seconds, the carbonation status of the cross-section was observed. The carbonized area of ​​the concrete was colorless, while the uncarbonized area turned purple.

[0100] M3. Carbonization depth measurement: Measure the carbonization depth at regular intervals, and take the average value of all measurement points as the final result. Specifically, carbonization depth measurement should be performed at a point every 10 mm, and the average value of all measurement points should be taken as the final result. Measure the carbonization time of 3 specimens for each group, and take the average value as the final carbonization depth of that group of specimens.

[0101] (2) pH value The pH value of concrete was measured using a PHS-3C digital display benchtop pH meter. First, concrete specimens cured to the target age were dried for 24 hours. Then, a pulverizer was used to collect a 2mm layer of powder from the surface to the interior. This powder was then passed through a 200-mesh sieve and used for the concrete pH value test.

[0102] The testing method is as follows: N1. Prepare a solution from the treated powder, shake it, and let it stand. Specifically, during the test, mix the treated powder with a solid-liquid ratio of 1:20 (5g of concrete powder to 95mL of distilled water), pour the solution into an Erlenmeyer flask, shake it for 5 minutes, and let it stand for 24 hours to ensure that the solution is fully dissolved and reaches a stable state.

[0103] N2. After settling, filter the solution using filter paper to obtain a clear filtrate and measure the pH value. Specifically, before measurement, perform three-point calibration using a standard buffer solution to ensure the accuracy of the instrument. Then, measure the pH value using a pH meter. Test each concrete sample three times and take the average value.

[0104] (3) Mechanical property test According to the "Standard for Test Methods of Mechanical Properties of Ordinary Concrete" (GB / T50081-2002), compressive, splitting tensile, and flexural strength tests were conducted on sea sand concrete with different water-cement ratios and different carbonation curing times. The test time points were the end of carbonation curing for each group, as well as 28 days and 56 days of age. After the concrete specimens were cured to the specified age according to the above curing regime, three specimens were selected from each group, and the arithmetic mean of the strengths of the three specimens was taken as the final strength.

[0105] 1) Compressive strength This embodiment uses a TYA-2000 electro-hydraulic pressure testing machine to determine the compressive strength of sea sand concrete, with a maximum pressure of 2000 kN. For the cube compressive strength test, the geometric center of the specimen should be aligned with the center of compression. During the test, the loading rate is controlled by adjusting the oil supply valve, ensuring continuous and uniform loading. The loading rate is 0.5 MPa / s–0.8 MPa / s. When the specimen is nearing failure, the machine's throttle is stopped, and the failure load is recorded.

[0106] 2) Splitting tensile strength The splitting tensile strength was still tested using a TYA-2000 electro-hydraulic pressure testing machine. The specimen was placed in the splitting fixture, with one arc-shaped pad and one wooden strip placed between the upper and lower pressure plates. The pads and strips were aligned with the center lines of the upper and lower surfaces of the specimen and perpendicular to the top surface during molding. The load was applied continuously and uniformly, with the loading rate controlled between 0.05 MPa / s and 0.08 MPa / s. When the specimen was nearing failure, the machine throttle was stopped and the failure load was recorded.

[0107] 3) Flexural strength The flexural strength test equipment was a DNS-300 electronic universal testing machine. The flexural strength test was carried out using a four-point loading method. The specimen support and loading head were made of hardened steel cylinders. The bottom support spacing was 300mm. The upper part used a flexural strength test device that could simultaneously apply two identical loads to the three-quarters span of the specimen. A uniform and continuous loading rate was maintained. The loading speed was 0.100mm / min. The failure load was recorded after the specimen broke.

[0108] 4) Uniaxial compressive strength test According to the "Standard for Test Methods of Physical and Mechanical Properties of Concrete" (GB / T 50081-2019), uniaxial compressive strength tests were conducted on concrete using prism specimens with side lengths of 100mm × 100mm × 300mm. Three specimens were selected for each group. All specimens were transferred to standard curing after carbonation curing and tested at 56 days. This embodiment uses the WAW-1000 microcomputer-controlled electro-hydraulic servo universal testing machine manufactured by Shanghai Hualong Testing Instruments Co., Ltd., with a maximum and minimum loading rate of 1mm / s and 0.0001mm / s, respectively, enabling high-precision displacement control loading. The displacement control measurement accuracy of the testing machine is 0.001mm, and the load accuracy is 0.001KN. Furthermore, the machine has high rigidity, effectively controlling and measuring the softening section of the stress-strain curve. The displacement sensor is model YHD-20, with a range of ±5mm and a measurement accuracy of 400... με / mm. The pressure signal was acquired by the TDS-530 dynamic data acquisition instrument and recorded by the DHDAS dynamic signal acquisition and analysis system.

[0109] (4) Microstructure test 1) Scanning electron microscopy (SEM-BSE) experiment The microstructure of each phase in carbonized cured marine sand concrete was observed using a Tescan Mira4 scanning electron microscope (TEEM) in secondary electron scanning (SE) mode. Before testing, the sample was fixed to a copper disk using conductive tape and then sputtered with gold to enhance conductivity and improve imaging quality. The treated sample was then placed inside the instrument chamber and a vacuum was applied to observe its microstructure.

[0110] 2) X-ray diffraction analysis (XRD) The experimental instrument used in this embodiment was a MiniFlex 600 X-ray diffractometer manufactured by Rigaku Corporation of Japan. During the experiment, a copper target was selected as the X-ray source, with a tube voltage of 40 kV, a tube current of 15 mA, and a diffraction angle (2θ) range of 5°–55°. Before testing, the sample to be tested was pressed onto a special glass sample slide for the diffractometer and placed in a sample holder for testing. After the test, the diffraction data was analyzed using Jade 9 phase analysis software to analyze the phase composition and evolution of carbonized cured marine sand concrete.

[0111] 3) Thermogravimetric analysis (TGA) The testing instrument used was a TGA / DSC2 thermogravimetric simultaneous differential thermal analyzer manufactured by Mettler Toledo, USA, with a sensitivity of 0.1 μg. During the experiment, 20 mg of fine powder sample was weighed and placed in the testing device. Nitrogen was used as the protective gas, and the temperature was scanned at a rate of 10 °C / min, from 30 °C to 900 °C. The thermogravimetric curves (TG) and differential thermogravimetric curves (DTG) of the sample mass as a function of temperature were finally obtained. Based on these results, the thermal decomposition behavior of various hydration products in marine sand concrete was analyzed, and the CO2 absorption of marine sand concrete was calculated by examining the decomposition characteristics of CaCO3 within a specific temperature range.

[0112] Example 3 Based on Example 2, Example 3 also has the following implementation method: Specifically, this embodiment analyzes the influence of carbonation curing on the strength of marine sand concrete and its microscopic mechanism.

[0113] In this embodiment, the porosity and hydration of the concrete matrix are changed by adjusting the water-cement ratio parameter, thereby controlling the carbon dioxide transport rate and carbonation depth inside the concrete; at the same time, the degree of carbonation reaction and the final strength index of the concrete are adjusted by setting different carbonation curing times.

[0114] Specifically, this embodiment selects marine sand concrete specimens with different water-cement ratios and carbonation curing times as test objects, and measures their carbonation depth, pH value, compressive strength, splitting tensile strength, and flexural strength to obtain data on the evolution of these performance indicators with changes in process parameters. Furthermore, combined with microstructure characterization methods, the mechanism by which carbonation curing enhances the macroscopic mechanical properties of marine sand concrete is verified.

[0115] 2.1 The variation law of concrete carbonation depth and pH value (1) Depth of carbonization like Figure 1 This study demonstrates the trend of carbonation depth in marine sand concrete after carbonation curing, as a function of curing time. With prolonged curing time, the carbonation depth of the marine sand concrete gradually increases, but the rate of increase slows down. During the carbonation process, CO2 can easily penetrate the concrete and react with cement hydration products in the early stages of carbonation. However, in the later stages, due to the formation of a surface carbonized layer, the CO2 diffusion path becomes longer, leading to a gradual slowdown in the carbonation rate.

[0116] As the water-cement ratio increases, the porosity of concrete increases, leading to a faster CO2 diffusion rate and thus promoting deeper carbonation. Taking 7 days of carbonation curing as an example, the carbonation depth of concrete with water-cement ratios of 0.38 and 0.48 differs by nearly double. Similarly, for concrete with a water-cement ratio of 0.48, the carbonation depth increases from 8.06 mm at 0.5 days to 20.16 mm at 7 days; the extended carbonation curing time also significantly increases the carbonation depth. The combined effect of a high water-cement ratio and a long carbonation curing time significantly increases the carbonation depth of concrete, clearly demonstrating that the water-cement ratio and carbonation curing time are key factors influencing carbonation depth.

[0117] (2) pH value The pH value of the sea sand concrete specimens was tested layer by layer using a pH meter after carbonation curing and at 56 days. Figure 2 and Figure 3 The effects of water-cement ratio and carbonation curing time on pH test results were demonstrated. The pH value of the carbonized specimens gradually increased from the outside to the inside and eventually stabilized. The distribution could be clearly divided into three regions: the low pH zone on the surface of the concrete corresponds to the fully carbonized zone of the carbonized concrete, the rising zone in the middle corresponds to the partially carbonized zone, and the stable zone at the end corresponds to the uncarbonized zone.

[0118] Figure 2The pH values ​​of samples with different water-cement ratios after 3 days of carbonation curing are shown in (a) at the end of carbonation curing and (b) at 56 days. After 3 days of carbonation curing, the higher the water-cement ratio, the lower the overall pH value of the concrete specimens and the deeper the fully carbonized zone. At the 56-day stage, specimens with different water-cement ratios showed different degrees of pH recovery. After a long period of standard curing, the pH value of the surface layer of the 0.38 water-cement ratio specimen rose from 10.6 to 11.2, while the other two groups of water-cement ratio specimens showed weaker recovery ability. This phenomenon is consistent with the recovery pattern of the cross-sectional color of the carbonation depth.

[0119] Figure 3 The pH values ​​of samples with a water-cement ratio of 0.43 after different carbonation curing times are shown in (a) at the end of carbonation curing and (b) at 56 days. When the water-cement ratio remains constant, with the extension of early carbonation curing time, the depth of the fully carbonized zone of specimens after 0.5 days, 1 day, 3 days, and 7 days of carbonation curing is approximately 5 mm, 8 mm, 12 mm, and 16 mm, respectively. Due to the increased consumption of alkaline substances such as Ca(OH)2, the depth of the fully carbonized zone on the surface continuously increases. In addition, the average pH value of the concrete also continuously decreases, and is lower than the average value of standard-cured specimens. From the results at 56 days, the pH value of the concrete surface has rebounded somewhat compared with the end of carbonation, which is obvious in specimens after 0.5 days and 1 day of carbonation curing, but it still does not reach the pH value of standard curing.

[0120] 2.2 The Influence of Carbonation Curing on the Strength of Sea Sand Concrete (1) Variation law of concrete compressive strength Table 2. Effect of carbonation curing time on the compressive strength of sea sand concrete

[0121] Table 2 shows the changes in compressive strength of sea sand concrete with different carbonation curing times. The compressive strength of sea sand concrete continuously increases with age, while the increase in water-cement ratio leads to a decrease in compressive strength. The experimental results show that, regardless of the water-cement ratio or carbonation curing time, the early compressive strength of carbon-cured specimens is generally significantly higher than that of standard-cured specimens, while the effect on 28-day and 56-day curing times is relatively small. The main reason for this is that the large amount of calcium carbonate generated in the early stage of carbonation curing can effectively fill the pores, improve the density of the concrete, and thus enhance the early strength. In the later stages, as the hydration products gradually mature, the additional contribution of carbonation is relatively weakened.

[0122] (2) Variation law of splitting tensile strength of concrete Table 3. Effect of carbonation curing time on splitting tensile strength of sea sand concrete

[0123] Table 3 shows that the splitting tensile strength of both carbon-cured and standard-cured specimens increases with age. The water-cement ratio has a smaller impact on splitting tensile strength compared to compressive strength, but it still exhibits the pattern that a lower water-cement ratio results in a higher splitting tensile strength. Furthermore, carbonation curing significantly enhances the splitting tensile strength of sea sand concrete, and this increase becomes more pronounced with prolonged carbonation curing time. Early carbonation curing affects the 28-day splitting tensile strength of concrete. However, at 56 days, the splitting tensile strength of carbon-cured specimens decreased to varying degrees compared to 28 days, while that of standard-cured specimens continued to increase, resulting in carbon-cured specimens generally exhibiting lower strength than standard-cured specimens. This is because early carbonation curing, while increasing the density of concrete, also reduces the internal moisture transport capacity, thus limiting further hydration of cement particles and affecting the final strength development.

[0124] (3) Variation law of concrete flexural strength Table 4. Comparison of flexural strength of carbon-cured concrete after 7 days

[0125] Table 5. Effect of carbonation curing time on the 28-day flexural strength of sea sand concrete

[0126] Tables 4 and 5 show the comparison of flexural strength of carbon-cured concrete after 7 days and the variation of flexural strength of marine sand concrete at 28 days under different carbonation curing times, respectively. The flexural strength of the samples after 7 days of carbonation curing was generally higher than that of samples that were subsequently cured to 28 days, indicating that carbonation curing had a certain enhancing effect on flexural strength in the early stages. However, with the subsequent 21 days of standard curing, the flexural strength of the samples decreased. Specifically, the strength of the 0.38 water-cement ratio sample decreased from 5.04 MPa to 4.85 MPa, and the strength of the 0.48 water-cement ratio sample decreased from 3.48 MPa to 3.33 MPa. This suggests that in the early stages of carbonation curing, calcium carbonate deposition may fill the pores, temporarily improving the local strength of the sample and thus enhancing flexural performance. However, during subsequent curing, due to the inhibitory effect of carbonation curing on hydration, the internal microstructure of the sample may change, such as carbonation shrinkage and microcrack propagation, leading to a decrease in flexural strength.

[0127] 2.3 Microscopic Mechanism Analysis of Carbonation Curing of Sea Sand Concrete During carbonation curing, high-concentration CO2 gas diffuses through the pores and microcracks within the concrete, reacting chemically with alkaline substances in the concrete with the participation of pore water to generate carbonate products. This reaction alters the phase composition, microstructure, interfacial properties, and material density of the concrete. The continuous accumulation of these microstructural changes ultimately manifests as changes in the macroscopic mechanical properties of the concrete. This embodiment analyzes the microstructural characteristics of carbonated sea sand concrete to establish the correlation mechanism between these characteristics and macroscopic properties. (1) Phase composition (XRD) This embodiment uses standard-cured sea sand concrete as a baseline to analyze the carbonation mechanism of carbon-cured concrete. The XRD patterns of carbon-cured sea sand concrete with different water-cement ratios and curing times are shown below. Figure 4 and Figure 5 As shown in the figure. Comparing the phase changes of carbon-cured and standard-cured concrete, the peak values ​​in the spectra show that the main phases of standard-cured sea sand concrete include Ca(OH)2, ettringite (AFt), SiO2, and a small amount of CaCO3. The presence of CaCO3 may be related to the natural carbonization process. After carbonization curing, the Ca(OH)2 peak in the sample disappears, indicating that the carbonization reaction of the surface concrete is relatively complete, and Ca(OH)2 has been almost completely converted into CaCO3. This results in a much higher CaCO3 content in the carbon-cured concrete compared to the standard-cured sample, which is the main reason for its faster early strength growth. In addition, the calcium silicate minerals contained in the cement clinker in the concrete undergo a carbonization reaction under the action of CO2, generating CaCO3 and SiO2, which increases the corresponding peak value of SiO2. At the same time, the characteristic peak of AFt in the carbon-cured sample is significantly reduced or even disappears. This is because AFt decomposes in the CO2 environment to form gypsum and CaCO3. The above reaction process can be described by the following chemical equation:

[0128]

[0129]

[0130] according to Figure 5As the carbonation curing time increases, Ca(OH)₂ gradually reacts with CO₂ to transform into CaCO₃, deepening the carbonation degree of the concrete. The relative content of Ca(OH)₂ decreases, while the relative content of CaCO₃ increases significantly. The highest CaCO₃ peak value was observed in the 7-day carbonation-cured sample, indicating that extending the carbonation curing time improves the degree of concrete carbonation. Combined with mechanical property analysis, with prolonged carbonation curing time, a denser carbonized layer forms on the concrete surface, effectively filling pores, improving early strength, and also helping to inhibit the further diffusion of subsequent corrosion products, thus enhancing the durability of the concrete. Therefore, appropriately extending the carbonation curing time has a positive effect on optimizing the early strength and durability of marine sand concrete.

[0131] (2) Thermogravimetric analysis (TGA) 1)Phase composition Figure 6 The TG-DTG curves of sea sand concrete under carbonization curing for 7 days and standard curing for 7 days are shown, and the effects of the two curing methods on thermal decomposition characteristics are compared and analyzed. Figure 7 Furthermore, the TG-DTG curves of sea sand concrete with a water-cement ratio of 0.43 under different carbonation curing times are given, which shows the influence of carbonation curing time on the evolution of concrete hydration products.

[0132] contrast Figure 6 In the DTG curves of standard-cured and carbonized-cured sea sand concrete, the decomposition peak positions of CaCO3 show a significant change in the carbonized-cured samples. The peak value of CaCO3 in the standard-cured samples appears around 680℃, while in the carbonized-cured samples it appears around 720℃, showing a shift towards higher temperatures. This phenomenon may be related to two factors: firstly, the increased CaCO3 content in carbonized-cured samples compared to standard-cured samples causes a lag in its decomposition rate relative to the rate of temperature increase; secondly, compared to the CaCO3 in the standard-cured samples, the higher degree of crystallinity of the carbonates formed during carbonization is more structurally stable, requiring a relatively higher temperature for thermal decomposition, thus leading to the shift of the decomposition peak towards higher temperatures. Furthermore, the decomposition peaks of CSH and AFt in the carbonized-cured samples are significantly weakened compared to the standard-cured samples. This indicates that CSH and AFt also undergo a certain degree of reaction during carbonization, partially converting into carbonate products, resulting in a decrease in their content.

[0133] from Figure 7It can be seen that with the extension of carbonation time, the degree of carbonation of concrete gradually deepens, leading to an increasing trend in CaCO3 content. Regarding the effect of different carbonation curing times on CaCO3 content, the variation in CaCO3 content among short-term carbonized (0.5d to 3d) samples is relatively small. Specifically, the CaCO3 content of the 1-day carbonized sample is lower than that of the 0.5-day carbonized sample, and the content difference between the 0.5-day and 3-day carbonized samples is not significant. This phenomenon is related to the sampling location and the characteristics of the carbonation reaction. The carbonation process is an expansion process from the surface to the interior. In a relatively short time, the CaCO3 content in the surface area may have already stabilized. Therefore, even with extended carbonation time, the increase in total CaCO3 content is relatively limited because the degree of surface carbonation is already close to saturation.

[0134] 2) Carbon sequestration The carbon sequestration test samples for sea sand concrete were taken from the surface carbonized zone of split-face concrete specimens cured by carbonation. Within the temperature range of 600°C to 900°C, the TGA curve typically shows a significant mass loss peak, corresponding to the decomposition of CaCO3. Based on the mass loss of the sample within this temperature range, the CO2 absorption per unit mass of sea sand concrete paste material can be calculated using the following formula:

[0135] In the formula, CO2 absorption is the carbon fixation amount (%) of the unit mass of paste material in carbonized curing concrete. and The values ​​are the mass (mg) of the test sample at 600℃ and 900℃, respectively.

[0136] Table 6. CO2 absorption of sea sand concrete (%)

[0137] Based on the thermogravimetric analysis (TGA) results, the CO2 absorption of sea sand concrete is shown in Table 6. The extension of early carbonation curing time plays a crucial role in CO2 absorption, mainly due to two key factors: (1) increased carbonation depth allows CO2 to penetrate deeper into the cement paste; (2) improved carbonation reaction degree leads to the conversion of more Ca(OH)2 into CaCO3 per unit mass of cement paste. These results indicate that extending the early carbonation curing time can result in higher CO2 absorption in concrete.

[0138] (3) Microscopic morphology (SEM-BSE) This embodiment uses scanning electron microscopy (SEM) and backscattered electron imaging (BSE) to compare and analyze the microstructure of cement paste and interfacial transition zone in standard-cured and carbonized-cured marine sand concrete samples. Typical SEM and BSE image results are shown below. Figures 8 to 11 As shown.

[0139] Depend on Figure 8 As can be seen, a large number of hydration products were observed in the standard-cured samples, including layered Ca(OH)₂, needle-like ettringite (Aft), and flocculent CSH gel, with the overall microstructure dominated by a layered structure. Unreacted cement particles remaining from the early stages of hydration, as well as fly ash glass microspheres encapsulated by the CSH gel network, were also clearly visible. These hydration products overlapped and formed numerous pores and cracks, resulting in a relatively loose overall spatial structure and relatively weak mechanical properties of the cement paste. In contrast, according to... Figure 9 As can be seen, the microstructure of the carbonation-cured samples underwent significant changes. The original layered Ca(OH)₂ and needle-like ettringite (Aft) largely disappeared, replaced by a large number of granular crystal particles, adhering to the surface of unhydrated cement particles or filling the pores, making the overall structure more compact. EDS energy dispersive spectroscopy analysis revealed that these crystal particles are mainly composed of O and Ca elements, indicating that they are CaCO₃. These calcium carbonate crystals are mainly calcite structures and exhibit different morphologies, such as rhombic, needle-like, and popcorn-like shapes.

[0140] from Figure 10 It can be seen that the ITZ region of the standard-cured specimens exhibits obvious porosity, cracks, and microcracks. The bond between the cement paste and coarse aggregate is relatively loose, and a large number of unreacted cement particles and pores are visible in the cement paste, which further reduces its density and mechanical properties. Figure 11 In the sample, almost no obvious cracks were observed, and the bond between the aggregate and the paste was very tight. This indicates that the early carbonation reaction effectively improved the defects at the ITZ (intermediate zone), strengthening the synergistic effect between the aggregate and the paste at the microscopic level. This is one of the main reasons for the strength increase of carbon-cured sea sand concrete after carbonation curing; the longer the early carbonation time, the greater the increase. However, at the same time, an overly dense interface structure may inhibit the dispersion and propagation of cracks, increasing the suddenness of fracture. The increased rigidity of concrete also reduces its plasticity, leading to increased brittle failure once cracks propagate. This also explains the decrease in the splitting tensile strength and flexural strength of concrete in the later stages.

[0141] Example 4 Based on Example 3, Example 4 also has the following implementation method: The constitutive relationship of carbonized cured marine sand concrete under uniaxial compression forms the theoretical basis for its structural design, bearing capacity calculation, and stress analysis. Carbonation significantly alters the macroscopic properties and microstructure of marine sand concrete. This embodiment establishes the constitutive relationship of carbonized cured marine sand concrete under uniaxial compression conditions through systematic experiments, thus providing a key theoretical basis for the practical application of this material in engineering structures.

[0142] 3.1 Study on stress-strain relationship of concrete (1) Destruction mode Table 7. Uniaxial compressive failure modes of concrete with a water-cement ratio of 0.38

[0143] Table 7 shows the typical failure modes of carbonized cured sea sand concrete under static loading conditions. In the initial loading stage, the specimens experienced relatively low compressive stress, with a linear relationship between stress and strain, and no obvious cracks on the surface. As the load continued to increase, the specimens entered the elastoplastic stage, with microcracks gradually appearing internally, but not yet extending to the surface, still exhibiting a certain degree of plasticity. With continued loading, cracks gradually developed. In carbonized cured specimens, visible micro-vertical cracks were observed on the surface before the peak stress, accompanied by a slight cracking sound, while in standard-cured specimens, visible cracks only appeared near the peak stress, after which the specimen's load-bearing capacity began to decrease. With further loading, small, short cracks began to appear along the direction of stress on the upper and lower parts of the specimen's sides. These discontinuous short cracks gradually connected, eventually forming one or more main cracks, most of which exhibited oblique shear failure characteristics.

[0144] The carbonization curing time significantly affects the failure mode. Before peak stress, specimens with shorter carbonization curing times exhibit a failure mode similar to those with standard curing, with visible cracks appearing only near the peak stress. Specimens with longer carbonization times show cracks before peak stress, particularly at the corners, and some specimens often show surface spalling, indicating that carbonization accelerates early crack development. After peak stress, the load-bearing capacity of specimens with longer carbonization curing times drops sharply, crack penetration increases, and cracks propagate rapidly, exhibiting overall sudden failure characteristics. The longer the carbonization curing time and the greater the carbonization depth, the more pronounced this brittle failure characteristic becomes.

[0145] (2) Stress-strain curve from Figure 12The curves shown reveal that the stress-strain curves of concrete cured for different durations of carbonation exhibit a similar overall trend to those of standard-cured specimens, roughly divided into a linear elastic stage, an elastoplastic stage, and a descending segment. In the initial loading stage, the concrete is in the elastic phase, and the curve is approximately linear. As the load continues to be applied, internal cracks begin to develop, and the curve exhibits significant nonlinear characteristics, rapidly progressing to a peak point. At the peak point, the specimen's load-bearing capacity reaches its maximum, and a short, gradual plateau appears. After the peak point, the curve enters a descending segment, with strain continuously increasing and stress gradually decreasing. During this descent, the curve's concavity and convexity change, indicating the gradual expansion of internal damage and eventual failure. Compared to standard-cured concrete, the peak point of the stress-strain curve for carbon-cured concrete shows a leftward shift and an upward trend, indicating improved strength but reduced ductility. Microscopic testing reveals that after early carbonation curing, the generated calcium carbonate makes the internal paste and interfacial transition zone of the concrete denser, increasing rigidity but simultaneously increasing brittleness.

[0146] Comparing the uniaxial compressive stress-strain curves of early-carbonated and late-carbonated marine sand concrete, it can be found that the stress-strain curve of marine sand concrete under early-carbonation curing conditions in this experiment shows a roughly similar trend. Regardless of whether it is early-carbonation or late-carbonation, the carbonation reaction increases the density of the concrete by generating CaCO3, resulting in the stress-strain curve exhibiting a steeper slope and peak stress in the rising segment, and a steeper curve in the falling segment.

[0147] (3) Analysis of characteristic parameters of axial compression of concrete 1) Peak stress Table 8. Peak stress of concrete

[0148] Based on the stress-strain curves of sea sand concrete, the peak axial compressive stresses of concrete with different water-cement ratios and different carbonation curing times are shown in Table 8. It can be seen that, except for the concrete with a water-cement ratio of 0.48, whose peak stress decreased after 3 and 7 days of carbonation curing compared to standard-cured concrete, carbonation curing generally has a certain effect on increasing the peak stress of sea sand concrete.

[0149] According to the "Code for Design of Concrete Structures" (GB50010-2010), the least squares method was used to linearly fit the compressive strength of concrete with different water-cement ratios. The conversion relationship between the cubic compressive strength and the axial compressive strength of concrete is as follows:

[0150] In the formula, This represents the axial compressive strength of concrete, corresponding to the peak stress. This represents the compressive strength of the concrete cube. The fitted value is compared with the experimental value, such as... Figure 13 As shown, the correlation coefficient R 2 =0.958, indicating that the model fits well and the measured value and the fitted value have a strong consistency. It can be seen that the conversion relationship of concrete with a water-cement ratio of 0.38 has the best fitting relationship with the curve, and the strength conversion factor of carbon-cured sea sand concrete is 0.728, which is smaller than the standard value. This difference reflects the special nature of carbonation curing on the strength growth law of concrete.

[0151] 2) Peak strain Table 9. Peak Strain of Concrete

[0152] Table 9 shows the peak strain under axial compression of concrete with different water-cement ratios and carbonation curing times. It can be seen that the peak strain of carbon-cured concrete is lower than that of standard-cured concrete. Early carbonation curing significantly lowers the peak strain of sea sand concrete compared to standard-cured specimens, and this decreasing trend becomes more pronounced with increasing carbonation curing time and water-cement ratio. For example, in specimens carbon-cured for 7 days, the peak strain of the three water-cement ratio groups of concrete decreased by 21.7%, 33.1%, and 43.6% respectively compared to standard-cured specimens.

[0153] The curves show a "fast at first, then slow" pattern in the rate of decrease of peak strain with carbonation curing time, consistent with the trend of change in concrete carbonation depth. As the carbonation depth increases, the relative coefficients of peak strain for all three water-cement ratios decrease significantly. Linear regression analysis of the relative coefficients of peak strain with carbonation depth using the least squares method is performed, as follows... Figure 14 As shown, the three groups of water-cement ratio specimens are all quite close to the fitted curve, with a correlation coefficient R0. 2 =0.921, and the relationship between its peak strain relative coefficient and carbonization depth is as follows:

[0154] In the formula, denoted as the peak strain relative coefficient, and d as the carbonization depth.

[0155] 3) Elastic modulus Table 10. Elastic Modulus of Concrete

[0156] The elastic modulus is defined as the secant modulus from the origin to 40% of the peak stress in the rising segment of the stress-strain curve. Table 10 shows the calculated elastic modulus of each group of concrete.

[0157] Overall, compared to standard-cured concrete, carbonation significantly increased the elastic modulus of all concrete groups, indicating that early carbonation effectively enhances the rigidity of concrete. Under short-term carbonation, the elastic modulus of each concrete group continuously increased; for example, the increases after 1 day of early carbonation for concrete with water-cement ratios of 0.38, 0.43, and 0.48 were 30.9%, 47.1%, and 50.2%, respectively. However, with further extension of carbonation time, the trends in elastic modulus changes for concrete with different water-cement ratios differed. The elastic modulus of concrete with water-cement ratios of 0.38 and 0.43 both showed a certain downward trend, with the trend being more pronounced for the 0.38 ratio. The elastic modulus of concrete with a water-cement ratio of 0.48 continued to increase, but the rate of increase gradually slowed. This is because high water-cement ratio concrete has larger pores, sufficient space for carbonation reaction, and a more significant continuous filling effect of carbonation products, thus making the impact of carbonation curing more pronounced.

[0158] 3.2 Constitutive Model of Concrete Damage (1) Model establishment and parameter solution Based on the principle of equivalent variation and statistical distribution theory, the ratio of the damaged micro-element to the total micro-element is defined as the damage variable, which is as follows:

[0159] In the formula, n is the number of destroyed infinitesimal elements, and N is the total number of infinitesimal elements.

[0160] The damage constitutive model of concrete is as follows:

[0161] In the formula, a and b are parameters in the Weibull distribution.

[0162] Based on the constitutive model and stress-strain curve boundary conditions described above, the constitutive model parameters a and b for different water-cement ratios were solved by measuring the physical quantities obtained from the uniaxial compression test of concrete. The calculation results are shown in Table 11 below.

[0163] Table 11. Results of concrete parameters a and b for different water-cement ratios

[0164] As the carbonation curing time increases, parameter a shows a decreasing trend, while parameter b continuously increases. The relationship between parameters a and b and the water-cement ratio and carbonation curing time is basically consistent with the measured stress-strain curves. Furthermore, fitting parameters a and b in Table 11 with the concrete carbonation depth yields their variation patterns and relationship equations with carbonation depth, all showing good correlation. Figure 15 and Figure 16As shown. Therefore, it can be concluded that in the constitutive model established in this study, parameters a and b can well reflect the stress-strain law of the uniaxial compression result of concrete.

[0165] (2) Damage constitutive model fitting results Figure 17 The fitting results of the damage constitutive model for carbon-cured marine sand concrete are presented. Most carbon-cured specimens exhibit higher elastic moduli and lower descent segments compared to standard-cured specimens. This indicates that carbonation improves the stiffness and reduces the brittleness of the marine sand concrete, resulting in an increase in peak strength. Comparison of the model curves and experimental results shows that the damage constitutive model established in this invention provides a good fit, with correlation coefficients exceeding 0.95, effectively describing the stress-strain curve characteristics of carbon-cured marine sand concrete.

[0166] Example 5 Example 5, based on the above examples, also has the following implementation method: This invention provides a standardized experimental method for accurately and comprehensively obtaining the basic mechanical parameters and uniaxial compressive behavior curves of carbonized marine sand concrete. It constructs a uniaxial compressive constitutive / stress-strain curve and a uniaxial compressive constitutive / model based on carbonized cured marine sand concrete.

[0167] Step 1: The statistical damage theory is as follows:

[0168] The interior of concrete is composed of countless tiny "micro-elements." The failure of these micro-elements does not occur simultaneously, but rather follows a Weibull distribution. Here, n represents the number of failed micro-elements, and N represents the total number of micro-elements. The formula ε describes how the number of failed micro-elements n increases according to a probability distribution as strain increases.

[0169] Step 2: Define damage variables:

[0170] The damage variable D is defined as the ratio of "damaged infinitesimal elements" to "total infinitesimal elements," with D=0 indicating no damage and D=1 indicating complete destruction. This was used to transform the statistical distribution into the degree of damage.

[0171] Step 3: Equivalent strain principle:

[0172] Where E0 is the initial elastic modulus (stiffness), and ε is the strain. The effective stress borne by the damaged material is equal to the stress generated when the load is applied to the remaining "undamaged area" (1-D).

[0173] Step 4: Damage constitutive model, where substituting D from step 2 into the formula of step 3 yields the final stress-strain constitutive equation. By determining parameters a and b, as well as the initial modulus E0, the entire process curve of concrete under compression, including the ascending and descending segments, can be obtained.

[0174]

[0175] in

[0176]

[0177] in, This indicates that parameter a is mainly related to peak strain (deformation capacity). This indicates that parameter b is related to the overall shape of the curve (the relationship between strength and modulus). Using the peak stress and peak strain measured experimentally, as well as the elastic modulus, the values ​​of a and b for each set of specimens are calculated.

[0178] like Figure 15 As shown, parameter a decreases linearly with increasing carbonation depth d. a is related to peak strain. A decrease in a leads to a smaller peak strain, thus explaining the poor ductility exhibited by concrete that becomes more brittle with deeper carbonation.

[0179] like Figure 16 As shown, parameter b increases linearly with increasing carbonization depth d. b controls the steepness of the descending segment of the curve. The larger b is, the more concentrated the Weibull distribution, and the more sudden the failure. This explains the phenomenon that carbonization leads to more pronounced brittle fracture characteristics.

[0180] like Figure 15 and Figure 16 As shown, parameters a and b exhibit a good correlation with carbonization depth, and their variation patterns with water-cement ratio and carbonization curing time are similar to those of the measured curves.

[0181] The experimental method of this invention is comprehensive, accurate, scientific and reasonable, and has good scalability and guidance. It successfully obtained the stress-strain curve of uniaxially compressed marine sand concrete and established a stress-strain constitutive model of uniaxially compressed marine sand concrete. This model lays the foundation for future industrial and engineering applications, and the model has certain predictive functions.

[0182] The constitutive model of uniaxially compressed marine sand concrete obtained by the experimental method of this invention can be directly imported into general finite element software such as ABAQUS and ANSYS, providing a reliable tool for structural analysis and design.

[0183] The above examples are merely illustrative of the technical content of the present invention to facilitate easier understanding by the reader, but do not imply that the implementation of the present invention is limited to these examples. Any technical extensions or re-creations made based on the present invention are protected by the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete, characterized in that: Includes the following steps, S1. Prepare sea sand concrete specimens according to the preset mix proportion, and perform early carbonation curing on the specimens for different durations to obtain test specimens with different carbonation degrees. S2. Perform uniaxial compression test and carbonization depth test on the test piece respectively; wherein, the uniaxial compression test adopts displacement controlled loading method to obtain stress-strain full curve data including the descending segment; the carbonization depth test is used to determine the degree of carbonization of the test piece. S3. Based on the stress-strain curve, the constitutive model control parameters for characterizing the material damage evolution behavior are obtained by inversion, and the control parameters are quantitatively correlated with the corresponding carbonation depth to obtain the constitutive model of uniaxial compressive damage of carbonized cured marine sand concrete.

2. The method according to claim 1, wherein the method is characterized by: In step S3, damage variables are defined based on statistical distribution theory, and a basic equation of the damage constitutive model containing control parameters is constructed. Based on the geometric characteristics of the stress-strain full curve data, the control parameters in the basic equation of the damage constitutive model are solved in reverse.

3. The method according to claim 2, wherein the method is characterized by: The control parameters include parameter a and parameter b, and the damage evolution behavior is described by the Weibull distribution probability density function.

4. The method according to claim 2, wherein the method is characterized by: The geometric features extracted from the stress-strain full curve data include the coordinates of the peak points and the tangent modulus of the rising segment.

5. The method according to claim 3, wherein the method is characterized by: Using the carbonization depth as the independent variable, and the constitutive model control parameters a and b as dependent variables, linear or nonlinear regression analysis is performed.

6. The method according to claim 1, wherein the method is characterized by: Step S2 also includes compressive strength, splitting tensile strength and flexural strength tests, and the obtained mechanical property data are used to verify the stress-strain curve.

7. The method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete according to claim 1, characterized in that: The carbonization depth test uses the phenolphthalein indicator method, and the measured value is used as the carbonization depth of the specimen.

8. The method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete according to claim 1, characterized in that: It also includes testing the pH value of the pore solution of the test specimen, and the pH value is used to help characterize the degree of carbonization.

9. The method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete according to claim 1, characterized in that: In step S3, thermogravimetric analysis or X-ray diffraction analysis is performed on specimens cured for different carbonation durations; the amount of calcium carbonate generated is determined, and the filling effect of carbonation products on concrete pores and interface transition zones is analyzed to correct the quantitative correlation between the control parameters of the constitutive model and the carbonation depth.

10. A method for constructing a constitutive model of uniaxial compressive damage in carbonized cured marine sand concrete according to any one of claims 1-9, characterized in that: Carbonation curing includes a pre-curing stage, a carbonation curing stage, and a subsequent curing stage. In the carbonation curing stage, the demolded sea sand concrete specimens are placed in a sealed carbonation chamber, with the CO2 concentration controlled at ≥90%, the curing temperature at 20-30℃, and the relative humidity at 50-70%. The pre-curing stage involves in-mold curing for no less than 12 hours, and the carbonation curing time is between 0.5 and 7 days. The subsequent curing involves covering the specimens with a film to retain moisture for at least 28 days.

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