Super-mass steel box concrete and mix proportion design method

Abstract

The invention relates to the technical field of self-compacting concrete, in particular to super-mass steel box concrete and a mix proportion design method. The method comprises the following steps: step 1, determining an initial mix proportion D0 = (W10, W20, W30, W40, W50, W60) T according to performance parameters of concrete; step 2, adjusting the design range of each component of the concrete by taking the initial mix proportion as a benchmark to form a to-be-selected mix proportion set D; 3, concrete mixing is conducted according to all the to-be-selected mixing ratios, and adiabatic temperature rise calculation, shrinkage testing, compressive strength testing and self-compacting performance testing are conducted; and 4, selecting an optimal mix proportion D * by adopting an optimal mix proportion decision model. The concrete has multiple performance objectives of self-compaction, low shrinkage, low temperature rise and high crack resistance at the same time, is suitable for design of the concrete mix proportion in a bridge tower steel box of an extra-large bridge, and improves the durability of the bridge.

Description

Technical Field

[0001] The present invention relates to the technical field of self-compacting concrete, and in particular to an over-volume steel box concrete and a mix ratio design method. Background Art

[0002] With the development of composite structure technology, steel-box concrete structures are increasingly used in the towers of cable-supported bridges. Their cross-sections can be designed into various shapes such as square, rectangular or polygonal, and they have stress advantages such as high bearing capacity, high rigidity, toughness and good seismic performance. With the continuous breakthroughs in bridge spans, steel-box concrete bridge towers are also developing towards larger scales; currently, the cross-sectional dimensions of steel-box concrete bridge towers are over 10m×10m.

[0003] The excessive concrete volume inside the box causes a high adiabatic temperature rise, while the surface steel box formwork has a high thermal conductivity and dissipates heat quickly. This results in a large temperature difference between the inside and outside of the concrete, poor crack resistance, and a high risk of temperature stress and premature cracking caused by hydration heat. A common temperature control measure used in engineering projects is to pre-embed a large number of cooling water pipes within the large concrete volume, using circulating water to remove internal heat and achieve internal cooling. Extensive project investigations and testing have shown that: ① The cooling water pipes have a limited cooling effect, only 2-8°C. Furthermore, the low cooling water temperature and high concrete temperature create large local temperature differences, which can easily cause numerous microcracks around the cooling water pipes, affecting the structural performance. ② During the later stages of construction, 80% of the cooling water pipe grouting is difficult to achieve denseness, becoming a weak link affecting the durability of the concrete. ③ The installation of multiple layers of cooling water pipes is complex, the water flow control process is complex, and the labor intensity is high. Compared to the passive cooling method of cooling water pipes, a more scientific and reasonable technical approach is to reduce the hydration heat, achieve active cooling by manipulating the properties of concrete materials, and eliminate cooling water pipes.

[0004] Unlike conventional large-volume concrete structures, the application of self-compacting concrete has gradually expanded to include complex, ultra-high-rise and ultra-large structures, posing new challenges to the functional requirements of self-compacting concrete. Steel-concrete boxes are composite structures, and the density of the concrete within the box is crucial for influencing synergistic loads. When concrete is poured loosely or shrinks excessively, debonding and voids between the steel box and the concrete can occur, not only reducing the restraining effect of the steel-concrete box and affecting the synergistic load behavior of the composite section, but also causing the concrete to act as an external load on the steel structure, losing its support for the external steel box and further reducing the load-bearing capacity of the steel-concrete box structure. This further raises the requirements for concrete: it must be able to flow densely and completely fill the external steel box; at the same time, the shrinkage must be controllable and the bonding performance with the steel wall interface must be good, so that the steel box and concrete can always be tightly constrained during the construction and operation phases. Concrete needs to be multifunctional: self-compacting, low shrinkage, low temperature rise, and high crack resistance; however, for the super-volume steel box composite structure, the volume is large, low shrinkage is difficult, low temperature rise is difficult, and high crack resistance is difficult; the self-compacting concrete slurry material is excessive, the shrinkage itself is relatively large, the temperature rise is relatively high, and cracks are extremely difficult to control, which is significantly different from the multiple performance objectives. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the existing technology of super-volume steel box concrete, such as large amount of adhesive used, easy shrinkage, high hydration heat, poor crack resistance, as well as poor cooling effect of the passive cooling technology of cooling water pipes in the existing steel box concrete, many microcracks around the pipes, loose grouting inside the pipes, and affecting the durability of the steel-concrete structure, and to provide a super-volume steel box concrete and a mix ratio design method.

[0006] In a first aspect, the present invention provides a method for designing a mix ratio of an oversized steel box concrete, comprising the following steps:

[0007] Step 1: According to the performance parameters of concrete, determine the initial mix ratio D0 = (W 10 , W 20 , W 30 , W 40 , W 50 , W 60 ) T ;W 10 Indicates the mass of cement, W 20 Indicates the mass of fly ash, W 30 Indicates the mass of the expander, W 40 Indicates the mass of sand, W 50 Indicates the mass of the stone, W 60 Indicates the quality of water;

[0008] Step 2: Based on the initial mix ratio, adjust the design range of each component of the concrete to form a candidate mix ratio set D;

[0009] Step 3: concrete mixing is performed for all the candidate mix ratio sets D, and adiabatic temperature rise calculation, shrinkage test, compressive strength test, and self-compacting performance test are performed;

[0010] Step 4: Use the optimal mix ratio decision model to select the optimal mix ratio D * .

[0011] The technical solution of this invention addresses the problems of large-volume steel box concrete, such as high adhesive consumption, easy shrinkage, high hydration heat, and poor crack resistance. Furthermore, existing passive cooling technology for cooling water pipes has poor cooling effects, numerous microcracks around the pipes, loose grouting inside the pipes, and impacts on structural durability. By manipulating the properties of concrete materials, this approach reduces hydration heat, achieves active cooling, and eliminates cooling water pipes. By conducting mechanical property tests, performance tests, adiabatic temperature rise calculations, shrinkage tests, and density measurements, and based on strength constraints, with self-compacting, low shrinkage, low temperature rise, and high crack resistance as optimization goals, an optimal mix ratio decision model was proposed, ultimately resulting in a recommended mix ratio suitable for pylons of extra-large bridges. This improves bridge durability.

[0012] For the optimal mix ratio decision model, selecting a reasonable initial mix ratio can avoid extensive mix ratio adjustments and related calculations and testing, accelerating the search for the optimal mix ratio. Furthermore, determining a reasonable initial mix ratio can prevent the optimal mix ratio decision model from falling into a local optimum during execution and can assist in finding the global optimal mix ratio.

[0013] The selection of the initial mix ratio should be oriented towards low temperature rise, low shrinkage, self-compacting and other properties as much as possible. It should be comprehensively based on the "Standards for Construction of Large Volume Concrete", "Technical Specifications for the Application of Shrinkage Compensating Concrete", "Technical Specifications for the Application of Self-compacting Concrete", and determined based on initial trial mixing tests. The requirements are as follows:

[0014] In step 1, the initial mix ratio is determined as follows: first determine the cement type and fly ash ratio range based on low temperature rise performance: W 20 / (W 10+ W 20 ); According to the low shrinkage performance, determine the water-binder ratio range, water consumption range and expansion agent dosage range, the water-binder ratio refers to: W 60 / (W 10+ W 20 ); According to the self-compacting performance, determine the stone particle size range and sand ratio range: W 40 / (W 40+ W 50 );

[0015] In order to achieve low temperature rise performance, low heat general purpose Portland cement is selected, and the 3d hydration heat is required to be ≤250kJ / kg and the 7d hydration heat is required to be ≤280kJ / kg. In addition, the cement dosage should be reduced and the replacement rate of fly ash for cement should be increased to reduce the heat release of concrete hydration. The total mass of cement and fly ash (i.e. W 10+ W 20 ) should be less than 400kg, and the fly ash ratio (ie W 20 / (W 10+ W 20 ))It is advisable to design it to be 40% to 50%.

[0016] In order to achieve low shrinkage performance, the required water consumption W 60 It should not be greater than 160kg, and the water-binder ratio (i.e. W 60 / (W 10+ W 20 )) should not be greater than 0.45; and, the amount of expansion agent should be 20 to 50 kg.

[0017] In order to achieve self-compacting properties, the maximum particle size of the stone should not be greater than 20 mm, and the sand ratio (i.e. W 40 / (W 40+ W 50 )) should be 38% to 45%.

[0018] Preferably, the cement is P·O 42.5 general-purpose Portland cement, and the fly ash is Class F Grade I ash.

[0019] According to the performance guidance range, 3-5 groups of concrete mixes with different proportions were tested, and the slump, expansion and 7d compressive strength performance tests of the above 3-5 groups of concrete slurries with different proportions were carried out. cu,k , f cu,k On the premise of the standard value of concrete design strength, a set of mix proportions with the largest slump and expansion is selected as the initial mix proportion.

[0020] Preferably, in step 2, the dosage of each raw material is designed separately, such as:

[0021] Cement: In the base dosage W 10 On the basis of the above, the amount of cement was increased or decreased by 5kg / grade to form W 10 -20, W 10 -15, W 10 -10, W 10 -5, W 10 、W 10 +5, W 10 +10, W 10 +15, W 10 +20 waiting for selection.

[0022] Fly ash: The proportion of fly ash in the benchmark mix (i.e. W 20 / (W 10+ W 20 )) on the basis of the fly ash dosage, the fly ash dosage is changed in 3-4 steps with the fly ash dosage of ±3% / level.

[0023] Water: Keep the base water-binder ratio unchanged, and adjust the amount of cementitious materials in the mix ratio to be selected (i.e. W 10+ W 20 ), determine the water consumption.

[0024] Expansion agent: Based on the expansion agent dosage in the benchmark mix ratio, the expansion agent dosage can be changed in 3-4 steps at a rate of ±10kg / step.

[0025] Sand and stone: Keep the design density of concrete unchanged, and use the design density minus the mass of cement, fly ash, water, and expansion agent to get the total weight of sand and stone. 40 / (W 40+ W 50 ), the sand rate was changed in 3-4 steps of ±2% / grade to obtain the sand quality and stone quality respectively.

[0026] Based on the design range of each component mentioned above, several groups of candidate mix ratio sets D are formed through permutations and combinations.

[0027] Preferably, in step 3, the adiabatic temperature rise calculation is performed with reference to the "Standard for Construction of Large Volume Concrete", the shrinkage test is performed with reference to the "Technical Specification for Application of Shrinkage Compensating Concrete", the compressive strength test is performed with reference to the "Code for Design of Concrete Structures", and the self-compacting performance test is performed with reference to the "Technical Specification for Application of Self-compacting Concrete".

[0028] Further preferably, in step 4, the design of low shrinkage, low temperature rise and self-compacting concrete mix ratio can be regarded as a multi-objective optimization problem, and the optimal mix ratio is sought under the constraint condition of satisfying basic mechanical properties, so that the concrete can achieve the comprehensive performance of low shrinkage, low temperature rise and self-compacting.

[0029] Specifically, the optimal mix ratio model of the concrete formula is:

[0030]

[0031] Where D is the optimization variable - concrete mix ratio; W1 is the mass of cement per cubic meter of concrete, W2 is the mass of fly ash per cubic meter of concrete, W3 is the mass of expansion agent per cubic meter of concrete, W4 is the mass of sand per cubic meter of concrete, W5 is the mass of aggregate per cubic meter of concrete, and W6 is the mass of water per cubic meter of concrete.

[0032] Target f1 represents the pursuit of self-compacting performance, where T L Indicates slump, K z Indicates the expansion degree, T 500 represents the time when the expansion reaches 500 mm, U represents the filling height of the U-shaped box; the target f2 represents the pursuit of low temperature rise performance, T max represents the peak adiabatic temperature rise; target f3 represents the pursuit of low shrinkage performance, Δ is the free shrinkage rate in a closed environment; target f4 represents the best choice for the comprehensive performance of self-compacting, low shrinkage and low temperature rise.

[0033] Preferably, in step 4, due to the slump, expansion, T 500 The performance parameters such as the filling height of the U-shaped box, shrinkage deformation, and adiabatic temperature rise belong to data of different scales and numerical ranges. They are normalized and mapped to the range of [0, 1] for performance comparison. Among all the candidate mix proportions that meet the strength constraint conditions, the group with the smallest f4 is selected as the optimal low-temperature rise, low-shrinkage, self-compacting concrete mix proportion D * .

[0034] In the technical solution of this application, the concrete working performance is expressed by slump H, expansion SF and expansion time T. 500 The concrete compressive strength is measured using the 28d cube compressive strength test. Based on the service environment of steel-concrete boxes, the shrinkage deformation of concrete is tested using the closed environment shrinkage rate test. The density of concrete components is tested using ultrasonic nondestructive testing.

[0035] Slump, spread, T 500 The units and magnitudes of metrics like the filling height of a U-shaped box vary significantly, making direct comparison impossible. Max-min normalization is used to linearly map the data to a specified range (usually [0, 1]) to eliminate differences in characteristic dimensions. This transforms dimensional expressions into dimensionless scalars, facilitating mathematical operations across multiple performance metrics.

[0036] The specific steps are:

[0037] 1) Calculate extreme values: Calculate x for each feature separately min and x max ,

[0038] 2) Convert all data points according to the following formula:

[0039]

[0040] x min and x max are the minimum and maximum values ​​of the original data, respectively.

[0041] Preferably, for multi-objective decision-making problems, due to the high degree of coupling between multiple objectives, the optimal solution is generally selected based on subjective experience. To eliminate the influence of subjective judgment, the technical solution of this application sets an objective function f4 to comprehensively achieve self-compacting performance, low temperature rise performance, and low shrinkage performance; the smaller f4, the optimal overall performance of the mix ratio.

[0042] Further preferably, among all the candidate mix proportions that meet the strength constraint conditions, the group with the smallest f4 is selected as the optimal low temperature rise, low shrinkage, self-compacting concrete mix proportion D * .

[0043] A concrete having a mix ratio obtained according to the above-mentioned super-volume steel box concrete mix ratio design method.

[0044] The concrete formula includes the following raw materials in parts by weight: cement 200±5kg, fly ash 200±5kg, expansion agent 30±5kg, sand 822±5kg, stone 1047±5kg, and water 151±5kg.

[0045] A bridge tower structure comprises a steel box in which the above-mentioned concrete is poured.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The technical solution of the present invention addresses the problems of large-volume steel box concrete, such as high adhesive consumption, easy shrinkage, high hydration heat, and poor crack resistance. It also addresses the problems of existing passive cooling technology for cooling water pipes, such as poor cooling effect, numerous microcracks around the pipes, loose grouting inside the pipes, and impact on structural durability. A method for designing the mix proportions of large-volume steel box concrete is proposed. The initial mix proportion is determined by comprehensively considering the design requirements of basic single concrete specifications, achieving preliminary self-compacting, low shrinkage, low temperature rise, and high crack resistance. Using this initial mix proportion as a benchmark, fine-tuning of various material proportions is carried out, which helps accelerate the optimal mix proportion decision-making process and obtain the global optimal mix proportion. This method achieves the goal of reducing hydration heat, achieving active cooling, and eliminating cooling temperature-controlled water pipes through the regulation of concrete material properties. Ultimately, a recommended mix proportion suitable for bridge towers of extra-large bridges is obtained, significantly improving the durability of the bridge.

[0048] The mix design method of the present invention is more scientific than other concrete admixture ratio design methods, and can quickly obtain the optimal mix ratio for multiple optimization targets. The existing mix design method is a single mix design for self-compacting concrete, large volume concrete, and low shrinkage concrete. It is easy to achieve a single goal, but when the performance requirements of self-compacting, low temperature rise, and low shrinkage appear at the same time, the existing single methods cannot be comprehensively achieved. The mix design method provided by the present invention can comprehensively achieve the best self-compacting, low shrinkage, low temperature rise, and high crack resistance on the basis of meeting the concrete strength. The concrete under this optimal mix ratio has various performance parameters that are within the design requirements. It meets the stringent concrete performance requirements in the steel box of the bridge tower of a super-large bridge. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a structural diagram of the super-massive steel box concrete structure of the present invention.

[0050] Figure 2 It is a flow chart of the concrete mix design method of the present invention.

[0051] Figure 3 This is a state diagram of concrete with the best mix ratio of the present invention after mixing.

[0052] Figure 4 This is the steel box concrete hydration heat analysis model of the present invention.

[0053] Figure 5 This is the temperature change curve of the steel box concrete over time of the present invention.

[0054] Figure 6 This is a curve diagram of the temperature difference between the inside and outside of the steel box concrete of the present invention.

[0055] Figure 7 This is the temperature field distribution of the steel box concrete of the present invention.

[0056] Figure 8 (a) shows the steel box concrete σ of the present invention. z Stress field distribution diagram.

[0057] Figure 8(b) shows the steel box concrete σ of the present invention. x Stress field distribution diagram.

[0058] Figure 8(c) shows the steel box concrete σ of the present invention. y Stress field distribution diagram.

[0059] Figure 9 This is a diagram showing the arrangement of temperature sensor measuring points in the steel box concrete structure of the present invention.

[0060] Figure 10 This is the measured temperature change curve over time in the steel box concrete structure of the present invention.

[0061] Markings in the figure:

[0062] 1-lower bridge tower, 2-tower base, 3-steel box concrete structure, 4-first partition, 6-second partition, 7-middle chamber, 8-side chamber. DETAILED DESCRIPTION

[0063] The present invention will be further described in detail below with reference to specific embodiments. However, this should not be construed as limiting the scope of the present invention to the following embodiments, as all technologies implemented based on the present invention fall within the scope of the present invention.

[0064] Example 1

[0065] This example discloses a mix design method for oversized steel box concrete. This mix design method was implemented in a steel box concrete project at the base of a pylon for a major river bridge. Through mechanical property testing, workability testing, adiabatic temperature rise calculations, shrinkage testing, and density testing, multiple performance indicators for the experimental mix were determined. With strength as the primary constraint and self-compacting, low shrinkage, low temperature rise, and high crack resistance as optimization goals, an optimal mix decision model was developed to obtain a recommended mix for the steel box concrete for the major river bridge.

[0066] Project Overview:

[0067] like Figure 1 Shown is a structural diagram of the bridge's tower base. The bridge is a single-tower, double-cable-plane cable-stayed bridge with a span arrangement of 45m + 185m + 238m + 45m. The towers are shaped steel towers with twisted surfaces, standing 173.3m tall. Upon completion, they will be the world's tallest inclined bridge towers. The "V-leg" section at the tower base is the primary load-bearing part. To improve bearing capacity, C40 concrete was poured into this section, forming a steel-concrete box composite structure. The steel-concrete box at the tower base has a cross-sectional dimension of 8.5 x 20.0m, with a single "V-leg" containing 3,055 cubic meters of concrete, exhibiting a "super-voluminous" character. Concrete was poured in four layers, aligning with the cross-sectional cavity structure.

[0068] The mix ratio design scheme of this application includes the following steps: Figure 2 Flowchart understanding:

[0069] Step 1: Determine the initial mix ratio based on the performance parameters of the concrete. In this embodiment, D0=(W 10 , W 20 , W 30 , W 40 , W 50 , W 60 ) T ;

[0070] W10 Indicates the mass of cement, W 20 Indicates the mass of fly ash, W 30 Indicates the mass of the expander, W 40 Indicates the mass of sand, W 50 Indicates the mass of the stone, W 60 Indicates the quality of water.

[0071] Concrete slurry needs to meet multiple performance indicators: in this super-large steel box concrete structure, it needs to meet the requirements of self-compacting, low shrinkage, low temperature rise, and high crack resistance.

[0072] In order to achieve the design of no cooling water pipe in the super-volume steel box, P·O 42.5 general-purpose Portland cement with low hydration heat is selected. Its 3d hydration heat is ≤250kJ / kg, and 7d hydration heat is ≤280kJ / kg. The other technical indicators are: standard consistency water consumption is 27.4%, specific surface area is ≥300m 2 / kg, initial setting time 196 minutes, final setting time 295 minutes, qualified volume stability, 3d flexural strength 6.3MPa, 3d compressive strength 31.3MPa.

[0073] To reduce the heat released by the adhesive during hydration, fly ash was selected to partially replace the cement. Class F, Grade I fly ash was selected. To ensure self-compacting properties, the coarse aggregate particle size was ≤20 mm, with a continuously graded particle size range of 5 to 20 mm. The crushing value was 6.4%, the mud content was 0.4%, and the needle-like content was 3.9%. The fine aggregate had a fineness modulus of 2.8, a stone powder content of 0.6%, and a mud content of 0.4%.

[0074] The admixture is a polycarboxylic acid-based high-performance water-reducing agent with a water-reducing rate of 28%.

[0075] The expansion agent used is type II calcium sulfoaluminate-calcium oxide concrete expansion agent, which has a 7-day limited expansion rate in water of 0.064% and a 21-day limited expansion rate in air of -0.002%.

[0076] More specifically, in step 1, the initial mix ratio is determined as follows:

[0077] Step 11: Based on the low temperature rise performance, first determine the cement type and the fly ash ratio range: W 20 / (W 10+ W 20 );

[0078] According to the low shrinkage performance, the water-binder ratio range is determined, and the water-binder ratio refers to: W 60 / (W 10+ W 20 ); water usage range, and expansion agent usage range;

[0079] According to the self-compacting performance, determine the stone particle size range and sand ratio range: W 40 / (W 40+ W 50 );

[0080] Step 12: Conduct trial mixing of 3-5 groups of concrete mixes with different mix ratios according to the performance guidance range of step 11, and conduct the following performance tests on the concrete slurries with different mix ratios: slump test, expansion test and 7d compressive strength. cu,k (f cu,k Under the premise of the concrete design strength standard value), the group of mix ratios with the largest slump and expansion is selected as the initial mix ratio. In this embodiment, preferably, 5 groups of concrete mix ratios are designed as shown in Table 1:

[0081] Table 1 shows the four groups of concrete mix ratio data initially proposed according to the above method.

[0082]

[0083] Table 2 shows the working performance and mechanical properties test results of the four groups of concrete mix proportions mentioned above

[0084]

[0085] According to the data in Table 2, the optimal mix ratio is: 200 kg cement, 200 kg fly ash, 20 kg expansion agent, 752 kg sand, 1127 kg stone, and 151 kg water.

[0086] Step 2: Based on the initial mix ratio, adjust the design range of each component of the concrete to form a candidate mix ratio set D;

[0087] More specifically, in step 2, the components of the concrete are designed as follows:

[0088] Cement: In the base dosage W 10 On the basis of the above, the amount of cement was increased or decreased by 5kg / grade to form W 10 -20, W 10 -15, W 10 -10, W 10 -5, W 10 、W 10 +5, W 10 +10, W 10 +15, W 10 +20 quantity to be selected;

[0089] Fly ash: Based on the fly ash proportion in the benchmark mix ratio, the fly ash dosage can be varied in 3-4 steps at a rate of ±3% per level.

[0090] Water: Keep the base water-binder ratio unchanged, and adjust the amount of cementitious materials in the mix ratio to be selected, i.e. W 10+ W 20 , determine water consumption;

[0091] Expansion agent: Based on the expansion agent dosage in the benchmark mix ratio, the expansion agent dosage can be changed in 3-4 steps at a rate of ±10kg / step;

[0092] Sand and stone: Keeping the designed bulk density of concrete unchanged, the total weight of sand and stone is obtained by subtracting the mass of cement, fly ash, water, and expansion agent from the designed bulk density. Based on the initial sand ratio in the mix, change the sand ratio in 3-4 steps of ±2% / grade to obtain the sand mass and stone mass respectively.

[0093] According to the above raw material design values, a number of candidate mix ratio sets D are formed. In this embodiment, a total of M groups of concrete mix ratios are included; as shown in Table 3:

[0094] Table 3 shows a further detailed summary of concrete mix ratio data

[0095]

[0096] Note: Since there is a lot of data in the mix ratio set D, this example only shows part of the data to clearly illustrate the method.

[0097] Step 3: Mix concrete for all candidate mix ratios, and perform adiabatic temperature rise calculation, shrinkage test, compressive strength test, and self-compacting performance test; the self-compacting performance includes slump, expansion, T 500 , U-shaped box filling height. Specifically, adiabatic temperature rise calculations were conducted in accordance with the "Massive Concrete Construction Standard," shrinkage testing was conducted in accordance with the "Technical Specifications for the Application of Shrinkage-Compensating Concrete," compressive strength testing was conducted in accordance with the "Concrete Structure Design Code," and self-compacting performance testing was conducted in accordance with the "Technical Specifications for the Application of Self-Compacting Concrete." Each mix ratio was cured under standard curing conditions. The test results for various properties of each mix ratio are shown in Table 4.

[0098] Table 4 shows the test results of various properties of concrete with different mix ratios in Group M

[0099]

[0100] Step 4: Use the optimal mix ratio decision model to select the optimal mix ratio D * .

[0101] Specifically, the mathematical model for optimal mix ratio decision-making is:

[0102]

[0103] In the above formula (1), D is the set of all candidate mix proportions, W1~W6 are the mass of cement, fly ash, expansion agent, sand, aggregate and water per cubic meter of concrete respectively; target f1 is the self-compacting performance target, T L is the slump, K z is the expansion, T 500 is the time when the expansion reaches 500mm, U is the filling height of the U-shaped box; target f2 is the low temperature rise performance target, T max is the peak value of adiabatic temperature rise; target f3 is the low shrinkage performance target, Δ is the concrete shrinkage rate; target f4 is the comprehensive performance target of self-compacting, low shrinkage and low temperature rise; g1 is the constraint to ensure the mechanical properties of concrete. In order to stably achieve the design strength, the measured concrete strength f is required. c Greater than 1.1 times the standard value of compressive strength f cu,k .

[0104] Due to the slump, expansion, T 500 The performance parameters such as the filling height of the U-shaped box, shrinkage deformation, and adiabatic temperature rise belong to data of different scales and numerical ranges. They are normalized and mapped to the range of [0,1] for performance comparison. Among all the candidate mix proportions that meet the strength constraint conditions, the group with the smallest f4 is selected as the optimal low-temperature rise, low-shrinkage, self-compacting concrete mix proportion D * The various performance indicators of each mix ratio and the dimensionless data after normalization are shown in Table 5.

[0105] Table 5 Normalized results of various performances

[0106]

[0107]

[0108] According to the optimal mix ratio decision-making approach of the present invention, a comparison of the comprehensive performance of each group is shown in Table 5. Under the premise of meeting the strength constraint conditions, group 15 has the smallest f4, showing the comprehensive performance advantages of self-compacting, low shrinkage, and low temperature rise, and was selected as the optimal mix ratio.

[0109] Table 6 summarizes the multi-objective performance of the M group mix ratio

[0110]

[0111]

[0112] The mixing state of the concrete with the best mix ratio (Group 15) is as follows: Figure 3 As shown:

[0113] Density test:

[0114] To verify the self-compacting and low-shrinkage properties of the optimal mix ratio, six steel-concrete box components were fabricated and subjected to density testing. Table 7 shows the ultrasonic density test results. The average sound velocity measured along the long sides was 4.753 km / s, and the average sound velocity measured along the short sides was 4.258 km / s. The sound velocity fluctuation was less than 0.3 km / s, indicating that the internal concrete filling was dense and uniform. Using this optimal mix ratio facilitates the combined effect between the steel box and concrete.

[0115] Table 7 Density test results

[0116]

[0117]

[0118] The above steel box experiments show that the concrete mixed according to the optimal mix ratio meets the density requirements in multiple steel boxes, proving that the concrete with this mix ratio has excellent self-compacting properties in the steel box.

[0119] Example 2

[0120] This embodiment provides a super-volume steel box concrete structure, in which the concrete mix ratio is obtained according to the mix ratio design method of Example 1. The concrete slurry is mixed according to the mix ratio and poured into Figure 1 The permanent structure is formed in the steel shell box at the bottom of the tower in practice. No cooling water pipe temperature control system is installed inside this structure. Relying on the multi-performance characteristics of the concrete with the optimal mix ratio obtained in Example 1, the low temperature rise and high crack resistance can be achieved without the need for cooling water pipes.

[0121] Specifically, such as Figure 1 The figure shows the bridge's tower base structure. The tower base's steel box concrete is divided into four layers in elevation, each layer further divided into three chambers in cross-section, for a total of twelve chambers across the four layers. The bridge is a single-tower, double-cable-plane cable-stayed bridge with a span arrangement of 45m + 185m + 238m + 45m. The towers are shaped steel towers with twisted surfaces, standing 173.3m tall. Upon completion, they will be the world's tallest inclined bridge towers. The "V-leg" section at the tower base is the primary load-bearing part. To enhance its bearing capacity, C40 concrete was poured into this section, creating a steel-box-concrete composite structure. The tower base's steel box concrete section measures 8.5 x 20.0m, with a single "V-leg" containing 3,055 cubic meters of concrete, exhibiting a "super-voluminous" character. Concrete was poured in four layers, aligning with the cross-sectional chamber structure.

[0122] The steel box concrete structure 3 is arranged in the bridge tower 1 structure under the bridge. The steel box concrete structure 3 is located at the bottom of the lower bridge tower. The steel box concrete structure 3 includes a steel box shell and concrete poured into the inner cavity of the shell. No cooling water pipe is provided in the steel box shell. The concrete formula includes the following raw materials by weight per cubic meter: 190-200kg of cement, 190-200kg of fly ash, 20-40kg of expansion agent, 800-824kg of sand, 1018-1048kg of stone, and 143-158kg of water.

[0123] The steel box concrete structure 3 is embedded in the tower base 2 and is trapezoidal in shape. The transverse length of the steel box hybrid structure is in the range of 7-9m, the longitudinal length of the steel box concrete structure 3 is in the range of 10-18m, and the height of the steel box concrete structure 3 is in the range of 18-20m.

[0124] The steel box shell is provided with a plurality of first partitions 4 spaced apart from each other from bottom to top, and the first partitions 4 divide the inner cavity of the steel box shell into a plurality of first chambers. Each of the first chambers is provided with a plurality of second partitions 6 longitudinally along the transverse bridge direction, and the second partitions 6 divide the first chamber into three chamber units along the transverse bridge direction. The chamber unit located in the middle is the middle chamber 7, and the chamber units located on both sides of the middle chamber 7 are side chambers 8.

[0125] In order to verify the technical effect of the concrete of the present invention, the following verification tests were conducted:

[0126] Concrete hydration heat analysis

[0127] The FEA NX finite element program was used to conduct a hydration heat analysis of the steel box concrete at the bottom of the tower of the Tuojiang Bridge. The steel box concrete at the bottom of the tower was poured in four layers in different time periods to dissipate heat without cooling water pipes. The second layer had the largest concrete volume, with a single pouring of 984m 3 , so the pouring of the second layer of concrete is selected as the analysis object.

[0128] Among them, the mechanical parameters and thermal parameters of concrete are set as shown in Table 8, where E, α, and λ are the elastic modulus, linear expansion coefficient, and thermal conductivity of concrete, respectively.

[0129] Table 8 Concrete material properties

[0130]

[0131] like Figure 4The figure shows the hydration heat analysis model of steel box concrete. The concrete is established using 3D solid units, the mechanical boundary is the consolidation constraint on the bottom surface of the first layer of concrete, and the temperature boundary is the fixed temperature of 32°C on the bottom surface of the first layer of concrete. According to the actual bridge test, the concrete mold entry temperature is 25°C, and the average ambient temperature is 25°C; the adiabatic temperature rise calculated based on the material composition is 35.2°C, and m = 1.45 is obtained according to the specification. The convection coefficient β reflects the heat exchange properties between concrete and the environment, and is related to the wind speed at the bridge site, surface flatness, type and thickness of formwork and insulation materials. The larger the value, the faster the heat dissipation. The top surface of the second layer of concrete uses 5cm steel formwork and 2cm water storage, β = 14.5kJ / (m 2 ·h·℃); Due to the limitations of the structural shape and on-site construction conditions, no insulation measures were taken on the long side, and only 5cm steel formwork was provided, β=29.2kJ / (m 2 ·h·℃); the short side is the entrance for concrete pouring, which is covered with 1cm geotextile for insulation after pouring, β=5.0kJ / (m 2 ·h·℃).

[0132] like Figure 5 The figure shows the temperature change curve of the steel box concrete over time. The maximum temperature at the center of the second layer of concrete reached 59.0°C, with a temperature rise of 34°C after entering the mold. The maximum temperature at the surface reached 41.4°C, with a temperature rise of 16.4°C after entering the mold. Both adiabatic temperature rises are less than the 50°C allowable value specified in the specification. After reaching the maximum temperature, the cooling rate of the central concrete was 0.2°C / day, and the cooling rate of the surface concrete was 0.5°C / day, both less than the 2°C / day allowable value specified in the specification.

[0133] like Figure 6 The figure shows the temperature difference curve of the steel box concrete. During the hydration heat release process of the super-volume steel box concrete, the temperature difference between the inside and outside is always less than the 25℃ allowable value in the specification, indicating that the internal cooling and external insulation measures are in place, which is conducive to reducing the temperature stress of the concrete. Figure 7 The figure shows the temperature field distribution when the center temperature of the second concrete layer reaches its highest point. The temperature of the second concrete layer transfers downward to the first concrete layer, and the temperature gradient distribution follows this trend. The maximum temperature occurs in the center of the second concrete layer, and the lowest temperature occurs at the corners of the structure surface.

[0134] The anti-cracking performance of super-volume steel box concrete is calculated according to the "Standard for Construction of Mass Concrete" (GB 50496-2018): σ≤f tk (1-e -γt ) / K

[0135] Where: σ is the normal stress of concrete in all directions; f tkis the standard value of concrete tensile strength, which is 2.40 MPa for C40 concrete; γ is the coefficient of concrete tensile strength increasing with age, which is 0.3; K is the anti-cracking safety factor, which is 1.15; thus, the concrete anti-cracking stress control value can be calculated to be 1.55 MPa.

[0136] Figures 8(a)-(c) show the stress field distribution at the moment when the temperature inside the concrete is the highest. The maximum tensile stress of the concrete is concentrated on the surface of the structure. In Figure 8(a), the maximum σ z =1.21MPa, Figure 8(b) Maximum σ x =1.48MPa, Figure 8(c) Maximum σ y =0.92MPa, which are all less than the concrete anti-cracking stress control value of 1.55MPa, indicating that the concrete anti-cracking performance meets the specification requirements in the absence of a cooling water pipe temperature control strategy.

[0137] Furthermore, in order to further verify the effectiveness of the temperature control strategy without cooling water pipes, the temperature of a real bridge was monitored during the pouring process of the second-layer steel box concrete of the bridge tower of a certain river bridge. Based on the temperature field distribution calculated by finite element method, temperature measurement points were arranged in the maximum and minimum temperature distribution areas. Figure 9 Shown is a schematic diagram of the layout of temperature sensor measurement points.

[0138] The temperature control targets of steel box concrete are: ① The temperature rise of the concrete casting body based on the mold entry temperature should not be greater than 50℃; ② The temperature difference between the inside and outside of the concrete casting body should not be greater than 25℃; ③ The cooling rate of the concrete casting body should not be greater than 2.0℃ / d; ④ The difference between the surface concrete temperature and the ambient temperature should not be greater than 20℃.

[0139] like Figure 10 The temperature curves for various parts of the steel box concrete over time are shown. The center point reaches a maximum temperature of 56.8°C at 45 hours. The temperature rise after mold placement is 31.8°C, and the subsequent cooling rate is 1.53°C / day, meeting the temperature control target. The maximum measured inside-outside temperature difference is 22.9°C, and the maximum temperature difference between the surface concrete and the ambient environment is 19.9°C, both meeting the temperature control target.

[0140] The technical solution of this application proposes a method for designing the mix ratio of super-volume steel box concrete. This method first determines the initial mix ratio, which comprehensively considers the design requirements of basic single concrete specifications and exhibits preliminary self-compacting, low shrinkage, low temperature rise, and high crack resistance. Using this initial mix ratio as a benchmark, fine-tuning of the various material ratios helps accelerate the optimal mix ratio decision-making process and achieve the global optimal mix ratio. This achieves the goal of reducing hydration heat, achieving active cooling, and eliminating cooling temperature control water pipes through the regulation of concrete material properties. Ultimately, a recommended mix ratio suitable for bridge towers of extra-large bridges is obtained, greatly improving the durability of the bridge.

[0141] The mix design method of the present invention is more scientific than other concrete admixture ratio design methods, and can quickly obtain the optimal mix ratio for multiple optimization targets. The existing mix design method is a single mix design for self-compacting concrete, large volume concrete, and low shrinkage concrete. It is easy to achieve a single goal, but when the performance requirements of self-compacting, low temperature rise, and low shrinkage appear at the same time, the existing single methods cannot be comprehensively achieved. The mix design method provided by the present invention can comprehensively achieve the best self-compacting, low shrinkage, low temperature rise, and high crack resistance on the basis of meeting the concrete strength. The concrete under this optimal mix ratio has various performance parameters that are within the design requirements. It meets the stringent concrete performance requirements in the steel box of the bridge tower of a super-large bridge.

[0142] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for designing the mix proportion of super-volume steel box concrete, characterized in that: The steps include: Step 1: According to the performance parameters of concrete, determine the initial mix ratio D0 = (W 10 , W 20 , W 30 , W 40 , W 50 , W 60 ) T ; W 10 Indicates the mass of cement, W 20 Indicates the mass of fly ash, W 30 Indicates the mass of the expander, W 40 Indicates the mass of sand, W 50 Indicates the mass of the stone, W 60 Indicates the quality of water; Step 2: Based on the initial mix ratio, adjust the design range of each component of the concrete to form a candidate mix ratio set D; Step 3: concrete mixing is performed for all the candidate mix ratio sets D, and adiabatic temperature rise calculation, shrinkage test, compressive strength test, and self-compacting performance test are performed; Step 4: Use the optimal mix ratio decision model to select the optimal mix ratio D * .

2. The method for designing the mix ratio of super-volume steel box concrete according to claim 1 is characterized in that: The cement is selected to be P·O 42.5 general-purpose Portland cement, and the fly ash is selected to be Class F Grade I ash.

3. The method for designing the mix proportion of super-volume steel box concrete according to claim 1, characterized in that: In step 1, the initial mix ratio is determined as follows: first determine the cement type and fly ash ratio range based on low temperature rise performance: W 20 / (W 10+ W 20 ); According to the low shrinkage performance, determine the water-binder ratio range, water consumption range and expansion agent dosage range, the water-binder ratio refers to: W 60 / (W 10+ W 20 ); According to the self-compacting performance, determine the stone particle size range and sand ratio range: W 40 / (W 40+ W 50 ); According to the performance guidance range, 3-5 groups of concrete mixes with different proportions were tested, and the slump, expansion and 7d compressive strength performance tests of the above 3-5 groups of concrete slurries with different proportions were carried out. cu,k , f cu,k On the premise of the standard value of concrete design strength, a set of mix proportions with the largest slump and expansion is selected as the initial mix proportion.

4. The method for designing the mix proportion of super-volume steel box concrete according to claim 3 is characterized in that: In step 2, the concrete components are designed as follows: Cement: In the base dosage W 10 On the basis of the above, the amount of cement was increased or decreased by 5kg / grade to form W 10 -20, W 10 -15, W 10 -10, W 10 -5, W 10 、W 10 +5, W 10 +10, W 10 +15, W 10 +20 quantity to be selected; Fly ash: Based on the fly ash proportion in the benchmark mix ratio, the fly ash dosage can be varied in 3-4 steps at a rate of ±3% per level. Water: Keep the base water-binder ratio unchanged, and adjust the amount of cementitious materials in the mix ratio to be selected (W 10+ W 20 ), determine water consumption; Expansion agent: Based on the expansion agent dosage in the benchmark mix ratio, the expansion agent dosage can be changed in 3-4 steps at a rate of ±10kg / step; Sand and stone: Keeping the designed bulk density of concrete unchanged, the total weight of sand and stone is obtained by subtracting the mass of cement, fly ash, water, and expansion agent from the designed bulk density. Based on the initial sand ratio in the mix, change the sand ratio in 3-4 steps of ±2% / grade to obtain the sand mass and stone mass respectively. According to the above raw material design values, several candidate mix ratio sets D are formed.

5. The method for designing the mix proportion of super-volume steel box concrete according to claim 1, characterized in that: In step 4, the mathematical model for optimal mix ratio decision is: <h2 style=";text-align:left;direction:ltr">find D=(W1,W2,W3,W4,W5,W6)<h2 style=";text-align:left;direction:ltr"> T obj. f1=min{-0.25×(T L (D)+K Z (D)-T 500 (D)+U(D))} f2=min{T max (D)} f3=min{Δ(D)} s.t. g1=f c (D)≥1.1f cu,k (1) In the above formula (1), D is the set of all candidate mix proportions, W1 is the mass of cement per cubic meter of concrete, W2 is the mass of fly ash per cubic meter of concrete, W3 is the mass of expansion agent per cubic meter of concrete, W4 is the mass of sand per cubic meter of concrete, W5 is the mass of aggregate per cubic meter of concrete, and W6 is the mass of water per cubic meter of concrete; Target f1 is the self-compacting performance target, T L is the slump, K z is the expansion, T 500 is the time when the expansion reaches 500mm, U is the filling height of the U-shaped box; target f2 is the low temperature rise performance target, T max is the peak value of adiabatic temperature rise; target f3 is the low shrinkage performance target, Δ is the concrete shrinkage rate; target f4 is the comprehensive performance target of self-compacting, low shrinkage and low temperature rise; g1 is the constraint to ensure the mechanical properties of concrete. In order to stably achieve the design strength, the measured concrete strength f is required. c Greater than 1.1 times the standard value of compressive strength f cu,k .

6. The method for designing the mix proportion of super-volume steel box concrete according to claim 5, characterized in that: In step 4, due to the slump, spread, T 500 The performance parameters such as the filling height of the U-shaped box, shrinkage deformation, and adiabatic temperature rise belong to data of different scales and numerical ranges. They are normalized and mapped to the range of [0, 1] for performance comparison. Among all the candidate mix proportions that meet the strength constraint conditions, the group with the smallest f4 is selected as the optimal low-temperature rise, low-shrinkage, self-compacting concrete mix proportion D * .

7. The method for designing the mix proportion of super-volume steel box concrete according to claim 1, characterized in that: In step 3, various performance tests are performed on several candidate concrete mix proportions: among them, the adiabatic temperature rise calculation is carried out in accordance with the "Standard for Construction of Large Volume Concrete", the shrinkage test is carried out in accordance with the "Technical Specification for the Application of Shrinkage Compensating Concrete", the compressive strength test is carried out in accordance with the "Code for Design of Concrete Structures", and the self-compacting performance test is carried out in accordance with the "Technical Specification for the Application of Self-compacting Concrete".

8. A concrete, characterized in that: The mix ratio is obtained according to the mix ratio design method for super-volume steel box concrete according to any one of claims 1 to 7.

9. The concrete according to claim 8, characterized in that The concrete formula includes the following raw materials in parts by weight: cement 200±5kg, fly ash 200±5kg, expansion agent 30±5kg, sand 822±5kg, stone 1047±5kg, and water 151±5kg.

10. A bridge tower structure, characterized in that: The invention comprises a steel box, wherein the concrete according to claim 9 is poured into the steel box.

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