Mix design method for mixed recycled coarse aggregate concrete

Abstract

The present invention provides a mix design method suitable for mixed recycled coarse aggregate concrete, belonging to the field of recycled concrete. The main components of coarse aggregate used in mixed recycled coarse aggregate concrete are natural coarse aggregate, recycled concrete coarse aggregate, and recycled broken brick coarse aggregate. Ordinary concrete mix design methods are no longer applicable to mixed recycled coarse aggregate concrete. The design method first adjusts the mortar density to minimize the density difference between the mortar density and the density of each component of the coarse aggregate, thereby reducing the heterogeneity of the coarse aggregate distribution in the hardened concrete; then, the equal volume mortar replacement method and the minimum slurry theory are combined to calculate the amount of water, cement, coarse and fine aggregates. The equal volume mortar replacement method ensures that the total content of new and old mortars in the mixed recycled coarse aggregate concrete is equal to the mortar content in ordinary concrete. Combined with the minimum slurry theory, it ensures that the cement slurry content in the mixed recycled coarse aggregate concrete can reach the minimum slurry content required to meet the concrete performance.

Description

Technical Field

[0001] The invention relates to the field of recycled concrete, in particular to a mix ratio design method suitable for mixed recycled coarse aggregate concrete. Background Art

[0002] In China, with the rapid development of urbanization, a large number of low-rise buildings have been demolished, and the treatment of construction waste has become an urgent problem to be solved. At the same time, due to the rapid development of the construction industry in recent decades, natural aggregates have been over-exploited, and many regions are facing resource depletion. On-site treatment and reuse of construction waste can not only alleviate environmental pressure to a certain extent, but also reduce the transportation of waste and the use of natural aggregates, thereby bringing economic benefits.

[0003] Most low-rise buildings are made of concrete and sintered clay bricks as the main building materials. Due to the imperfect construction waste sorting system, the aggregates obtained by crushing and screening construction waste are mostly mixed recycled coarse aggregates with recycled concrete aggregates and broken brick aggregates as the main components. The main components of mixed recycled coarse aggregates are natural coarse aggregates, recycled concrete coarse aggregates, and recycled broken brick coarse aggregates. There are large differences in the density of the three aggregates. The natural coarse aggregate has the highest density, the recycled concrete coarse aggregate is the second, and the recycled broken brick coarse aggregate has the lowest density; the density difference between aggregates and mortar will cause the aggregates in the fresh concrete to sink or float, making the uneven distribution of aggregates in the concrete more obvious. In addition, a layer of old mortar is adhered to the surface of coarse aggregate in recycled concrete and recycled crushed bricks. Generally, when designing the mix ratio of recycled concrete, the old mortar adhered to the surface of recycled coarse aggregate is usually directly used as part of the coarse aggregate, resulting in the mortar content in the mixed recycled coarse aggregate concrete being higher than that in ordinary concrete, which has an adverse effect on the mechanical properties and durability of the concrete.

[0004] Mixed recycled coarse aggregate has complex composition, large quality dispersion, and defects such as high water absorption and low strength. The mix design method of ordinary concrete is no longer applicable to mixed recycled coarse aggregate concrete. In view of the characteristics of mixed recycled coarse aggregate, it is very necessary to formulate a mix design specification specifically suitable for mixed recycled coarse aggregate concrete, which has important application value for the application of mixed recycled coarse aggregate concrete in actual engineering. Summary of the invention

[0005] In order to overcome the disadvantage that the performance of mixed recycled coarse aggregate concrete is inferior to that of ordinary concrete, the present invention proposes a mix ratio design method suitable for mixed recycled coarse aggregate concrete, which can significantly improve the heterogeneity of coarse aggregate distribution in the mixed recycled coarse aggregate concrete, improve the compressive strength and elastic modulus, and reduce shrinkage.

[0006] The technical solution adopted by the present invention is: first, by adjusting the mortar density, the density difference between the mortar density and the density of each component of the coarse aggregate is minimized, thereby reducing the heterogeneity of the coarse aggregate distribution in the hardened concrete; then, the water consumption, cement consumption, coarse aggregate consumption and fine aggregate consumption are calculated by combining the equal volume mortar replacement method and the minimum paste theory. The equal volume mortar replacement method ensures that the total content of new and old mortars in the mixed recycled coarse aggregate concrete is equal to the mortar content in ordinary concrete. Combined with the minimum paste theory, it can be ensured that the cement paste content in the mixed recycled coarse aggregate concrete can reach the minimum paste content required to meet the concrete performance.

[0007] The components of the mixed recycled coarse aggregate include natural coarse aggregate, recycled concrete coarse aggregate and recycled crushed brick coarse aggregate.

[0008] The above-mentioned natural coarse aggregate refers to the coarse aggregate with a particle size not greater than 31 mm and conforming to the particle grading obtained by crushing and screening natural stone.

[0009] The above-mentioned recycled concrete coarse aggregate refers to the coarse aggregate obtained by crushing and screening concrete components in building demolition waste.

[0010] The above-mentioned recycled crushed brick coarse aggregate refers to the coarse aggregate obtained by crushing and screening sintered clay bricks in building demolition waste.

[0011] The calculation steps of the above mixed recycled coarse aggregate concrete mix design method are as follows:

[0012] Step 1: Determine the density of cement mortar for mixed recycled coarse aggregate concrete;

[0013] Step 2: Calculate the volume of ordinary concrete cement mortar;

[0014] Step 3: Determine the volume of fresh cement mortar in mixed recycled coarse aggregate concrete by combining the equal volume mortar replacement method;

[0015] Step 4: Calculate water usage Cement dosage Amount of coarse aggregate and fine aggregate;

[0016] Step 5: Using the minimum paste theory, calculate the minimum paste content required for mixed recycled coarse aggregate concrete based on the amount of coarse aggregate and fine aggregate, and further calculate the minimum cement amount And water consumption

[0017] Step 6: If The cement dosage is Water consumption like The cement dosage is Water consumption

[0018] The density calculation process of the mixed recycled coarse aggregate concrete cement mortar is as follows:

[0019] (1) Experimental determination of the volume ratio of natural coarse aggregate, recycled concrete coarse aggregate and recycled crushed brick coarse aggregate in mixed recycled coarse aggregate

[0020]

[0021] in, It is the volume of natural coarse aggregate in each cubic meter of mixed recycled coarse aggregate concrete; It is the volume of recycled concrete coarse aggregate in each cubic meter of mixed recycled coarse aggregate concrete; It is the volume of recycled crushed brick coarse aggregate in every cubic meter of mixed recycled coarse aggregate concrete.

[0022] (2) Calculation of mortar density

[0023]

[0024] Among them, ρ NA is the saturated surface dry density of natural coarse aggregate; ρ RCA is the saturated surface dry density of coarse aggregate in recycled concrete; ρ RBA It is the saturated dry density of coarse aggregate of recycled crushed bricks; It is the density of cement mortar in mixed recycled coarse aggregate concrete.

[0025] The calculation process of the volume of the above ordinary concrete cement mortar is as follows:

[0026] (1) Volume of concrete per cubic meter

[0027]

[0028] Among them, V NC is the volume of ordinary concrete; It is the amount of cement used per cubic meter of ordinary concrete; The amount of natural coarse aggregate per cubic meter of ordinary concrete; is the amount of fine aggregate per cubic meter of ordinary concrete; ρ c Cement density should be determined according to GB / T 208, or 2900kg / m 3 ~3100kg / m 3 ρ s The apparent density of fine aggregate should be determined in accordance with the current industry standard "Standard for Quality and Inspection Methods of Sand and Stone for Ordinary Concrete" JGJ 52; ρ w is the density of water, which can be 1000kg / m 3; α is the percentage of air content in concrete, which can be taken as 1 when no air-entraining admixture is used.

[0029] (2) Sand ratio of ordinary concrete

[0030]

[0031] In the formula, It is the sand ratio of ordinary concrete.

[0032] (3) Volume of cement mortar per cubic meter of concrete

[0033]

[0034] The calculation process of the volume of fresh cement mortar in the above mixed recycled coarse aggregate concrete is as follows:

[0035] (1) Test to determine the content of old mortar adhering to the surface of recycled concrete coarse aggregate and recycled crushed brick coarse aggregate

[0036]

[0037]

[0038] Among them, OMC RCA OMC is the content of old mortar adhering to the surface of coarse aggregate in recycled concrete; RBA The content of old mortar adhering to the surface of coarse aggregate of recycled broken bricks; m RCA It is the absolute dry mass of coarse aggregate of recycled concrete before removing old mortar; m ONA It is the absolute dry mass of the components with particle size greater than 4.75 mm in the coarse aggregate of recycled concrete after removing the old mortar; m RBA It is the absolute dry mass of coarse aggregate of recycled broken bricks before removing old mortar; m OBA It is the absolute dry mass of the components with particle size greater than 4.75mm in the coarse aggregate of recycled broken bricks after removing the old mortar.

[0039] (2) Volume of natural aggregate, recycled concrete coarse aggregate, and recycled crushed brick coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete

[0040]

[0041]

[0042]

[0043]

[0044] in, is the volume of coarse aggregate in ordinary concrete; ρ ONAThe saturated surface dry density of coarse aggregate in recycled concrete without adhesive mortar; ρ OBA Saturated surface dry density of coarse aggregate of recycled crushed bricks for removing adhesion mortar.

[0045] (3) Volume of old mortar per cubic meter of mixed recycled coarse aggregate concrete

[0046]

[0047] in, It is the volume of old mortar in each cubic meter of mixed recycled coarse aggregate concrete.

[0048] (4) Volume of fresh mortar per cubic meter of mixed recycled coarse aggregate concrete

[0049]

[0050] in, It is the volume of old mortar in each cubic meter of mixed recycled coarse aggregate concrete.

[0051] (5) Amount of each component per cubic meter of mixed recycled coarse aggregate concrete

[0052]

[0053]

[0054]

[0055]

[0056]

[0057]

[0058] in, The amount of water used per cubic meter of mixed recycled coarse aggregate concrete; The cement consumption per cubic meter of mixed recycled coarse aggregate concrete; The amount of natural coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of recycled concrete coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of recycled crushed brick coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of natural coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; It is the moisture content of natural coarse aggregate in its natural state; is the water absorption rate of natural coarse aggregate in saturated surface dry state; It is the moisture content of recycled concrete coarse aggregate in its natural state; It is the water absorption rate of saturated surface dry state of coarse aggregate in recycled concrete; It is the moisture content of recycled crushed brick coarse aggregate in its natural state; It is the water absorption rate of coarse aggregate of recycled crushed bricks in saturated surface dry state.

[0059] The calculation process of the minimum water consumption and minimum cement consumption of the above mixed recycled coarse aggregate concrete is as follows:

[0060] (1) Aggregate accumulation

[0061]

[0062]

[0063]

[0064]

[0065]

[0066]

[0067] y S +y G =1

[0068] Among them, y S and G are the volume fractions of fine aggregate and coarse aggregate, respectively; k D k is the factor that determines the ratio of coarse and fine aggregate diameters; s is a statistical factor; x is the ratio between the accumulation volume of fine aggregate and the void volume of coarse aggregate; x0 = 0.4753, k0 = 0.3881; φ ST ,φ GT are the characteristic packing degrees of fine aggregate and coarse aggregate, respectively; ρ ST , ρ GT is the packing density of fine aggregate and coarse aggregate; ρ S , ρ G are the particle densities of fine aggregate and coarse aggregate, respectively; d ST , d GT They are the characteristic particle sizes of fine aggregate and coarse aggregate respectively. The particle size of aggregate when the sieve residue is 36.8% can be taken as the characteristic particle size.

[0069] (2) Void volume per cubic meter of aggregate PO

[0070]

[0071] (3) Total surface area of ​​aggregate per unit volume S

[0072]

[0073]

[0074]

[0075] Among them, n S , n G are the theoretical amounts of fine aggregate and coarse aggregate, respectively.

[0076] (4) Total volume of cement paste required per unit volume of aggregate V P

[0077] V P =V PO +APT·S

[0078] Among them, APT is the thickness of cement paste wrapped on the surface of aggregate. When APT = 5μm, the working performance requirements of concrete can be met.

[0079] (5) Minimum total cement paste volume V required per unit volume of mixed recycled coarse aggregate concrete P '

[0080]

[0081] (6) Minimum cement content per unit volume of mixed recycled coarse aggregate concrete and minimum water consumption

[0082]

[0083]

[0084] in, It is the water-cement ratio of mixed recycled coarse aggregate concrete.

[0085] The beneficial effects of the present invention are as follows: the mix ratio design method of the mixed recycled coarse aggregate concrete proposed in the present invention can significantly improve the heterogeneity of the coarse aggregate distribution in the mixed recycled coarse aggregate concrete, improve the compressive strength and elastic modulus of the concrete, and reduce the shrinkage of the concrete. The improvement of the performance of the mixed recycled coarse aggregate concrete further promotes its promotion and application in practical engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0086] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0087] Figure 1 This is the graph showing the change of shrinkage value over time;

[0088] Figure 2The present invention is a flow chart of the mix design method applicable to mixed recycled coarse aggregate concrete. DETAILED DESCRIPTION

[0089] In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with examples. It should be understood that the specific implementation described herein is only used to explain the present invention and is not used to limit the present invention.

[0090] The present application provides a mix design method suitable for mixed recycled coarse aggregate concrete, the method comprising determining the amount of water, cement, coarse aggregate and fine aggregate in the mix design of mixed recycled coarse aggregate concrete, and is characterized in that the method firstly adjusts the mortar density to minimize the density difference between the mortar density and the density of each component of the coarse aggregate, thereby reducing the heterogeneity of the coarse aggregate distribution in the hardened concrete; then combines the equal volume mortar replacement method and the minimum paste theory to calculate the amount of water, cement, coarse aggregate and fine aggregate, the equal volume mortar replacement method ensures that the total content of new and old mortar in the mixed recycled coarse aggregate concrete is equal to the mortar content in ordinary concrete, and combines the minimum paste theory to ensure that the cement paste content in the mixed recycled coarse aggregate concrete can reach the minimum paste content required to meet the concrete performance. The components of the above-mentioned mixed recycled coarse aggregate include natural coarse aggregate, recycled concrete coarse aggregate, and recycled crushed brick coarse aggregate; the natural coarse aggregate refers to the coarse aggregate with a particle size not greater than 31 mm obtained by crushing and screening natural stone and conforming to the particle grading; the recycled concrete coarse aggregate refers to the coarse aggregate obtained by crushing and screening concrete components in building demolition waste; the recycled crushed brick coarse aggregate refers to the coarse aggregate obtained by crushing and screening sintered clay bricks in building demolition waste.

[0091] The calculation steps of the mixed recycled coarse aggregate concrete mix design method are as follows:

[0092] Step 1: Determine the density of mixed recycled coarse aggregate concrete cement mortar; the density calculation process of the mixed recycled coarse aggregate concrete cement mortar is as follows:

[0093] Step 1.1: Test and determine the volume ratio of natural coarse aggregate, recycled concrete coarse aggregate and recycled crushed brick coarse aggregate in mixed recycled coarse aggregate

[0094]

[0095] in, It is the volume of natural coarse aggregate in each cubic meter of mixed recycled coarse aggregate concrete; It is the volume of recycled concrete coarse aggregate in each cubic meter of mixed recycled coarse aggregate concrete; It is the volume of recycled crushed brick coarse aggregate in every cubic meter of mixed recycled coarse aggregate concrete.

[0096] Step 1.2, calculate the density of mortar

[0097]

[0098] Among them, ρ NA is the saturated surface dry density of natural coarse aggregate; ρ RCA is the saturated surface dry density of coarse aggregate in recycled concrete; ρ RBA It is the saturated dry density of coarse aggregate of recycled crushed bricks; It is the density of cement mortar in mixed recycled coarse aggregate concrete.

[0099] Step 2: Calculate the volume of ordinary concrete cement mortar; the calculation process of the volume of ordinary concrete cement mortar is as follows:

[0100] Step 2.1, Volume of concrete per cubic meter

[0101]

[0102] Among them, V NC is the volume of ordinary concrete; It is the amount of cement used per cubic meter of ordinary concrete; The amount of natural coarse aggregate per cubic meter of ordinary concrete; is the amount of fine aggregate per cubic meter of ordinary concrete; ρ c Cement density should be determined according to GB / T 208, or 2900kg / m 3 ~3100kg / m 3 ρ s The apparent density of fine aggregate should be determined in accordance with the current industry standard "Standard for Quality and Inspection Methods of Sand and Stone for Ordinary Concrete" JGJ 52; ρ w is the density of water, which can be 1000kg / m 3 ; α is the percentage of air content in concrete, which can be taken as 1 when no air-entraining admixture is used.

[0103] Step 2.2, sand ratio of ordinary concrete

[0104]

[0105] In the formula, is the sand ratio of ordinary concrete;

[0106] Step 2.3, volume of cement mortar per cubic meter of concrete

[0107]

[0108] Step 3: Determine the volume of fresh cement mortar in the mixed recycled coarse aggregate concrete by combining the equal volume mortar replacement method; the calculation process of the volume of fresh cement mortar in the mixed recycled coarse aggregate concrete is as follows:

[0109] Step 3.1: Test and determine the content of old mortar adhering to the surface of recycled concrete coarse aggregate and recycled crushed brick coarse aggregate

[0110]

[0111]

[0112] Among them, OMC RCA OMC is the content of old mortar adhering to the surface of coarse aggregate in recycled concrete; RBA The content of old mortar adhering to the surface of coarse aggregate of recycled broken bricks; m RCA The absolute dry mass of coarse aggregate in recycled concrete before removing old mortar; m ONA It is the absolute dry mass of the coarse aggregate of recycled concrete with a particle size greater than 4.75 mm after removing the old mortar; m RBA The absolute dry mass of coarse aggregate of recycled broken bricks before removing old mortar; m OBA It is the absolute dry mass of the components with particle size greater than 4.75mm in the coarse aggregate of recycled broken bricks after removing the old mortar.

[0113] Step 3.2: Volume of natural aggregate, recycled concrete coarse aggregate, and recycled crushed brick coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete

[0114]

[0115]

[0116]

[0117]

[0118] in, is the volume of coarse aggregate in ordinary concrete; ρ ONA The saturated surface dry density of coarse aggregate in recycled concrete without adhesive mortar; ρ OBA Saturated surface dry density of coarse aggregate of recycled crushed bricks to remove adhesive mortar;

[0119] Step 3.3, volume of old mortar per cubic meter of mixed recycled coarse aggregate concrete

[0120]

[0121] in, It is the volume of old mortar in each cubic meter of mixed recycled coarse aggregate concrete;

[0122] Step 3.4: Volume of fresh mortar per cubic meter of mixed recycled coarse aggregate concrete

[0123]

[0124] in, It is the volume of old mortar in each cubic meter of mixed recycled coarse aggregate concrete;

[0125] Step 3.5, dosage of each component per cubic meter of mixed recycled coarse aggregate concrete

[0126]

[0127]

[0128]

[0129]

[0130]

[0131]

[0132] in, The amount of water used per cubic meter of mixed recycled coarse aggregate concrete; The cement consumption per cubic meter of mixed recycled coarse aggregate concrete; The amount of natural coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of recycled concrete coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of recycled crushed brick coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of natural coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; It is the moisture content of natural coarse aggregate in its natural state; is the water absorption rate of natural coarse aggregate in saturated surface dry state; It is the moisture content of recycled concrete coarse aggregate in its natural state; It is the water absorption rate of saturated surface dry state of coarse aggregate in recycled concrete; It is the moisture content of recycled crushed brick coarse aggregate in its natural state; It is the water absorption rate of coarse aggregate of recycled crushed bricks in saturated surface dry state.

[0133] Step 4: Calculate water usage Cement dosage The amount of coarse aggregate and fine aggregate; the calculation process of the minimum water consumption and minimum cement consumption of the mixed recycled coarse aggregate concrete is as follows:

[0134] Step 4.1, Aggregate Packing Degree

[0135]

[0136]

[0137]

[0138]

[0139]

[0140]

[0141] y S +y G =1

[0142] Among them, y S and G are the volume fractions of fine aggregate and coarse aggregate, respectively; k D k is the factor that determines the ratio of coarse and fine aggregate diameters; s is a statistical factor; x is the ratio between the accumulation volume of fine aggregate and the void volume of coarse aggregate; x0 = 0.4753, k0 = 0.3881; φ ST ,φ GT are the characteristic packing degrees of fine aggregate and coarse aggregate, respectively; ρ ST , ρ GT is the packing density of fine aggregate and coarse aggregate; ρ S , ρ G are the particle densities of fine aggregate and coarse aggregate, respectively; d ST , d GT They are the characteristic particle sizes of fine aggregate and coarse aggregate respectively. The particle size of aggregate when the remainder is 36.8% can be taken as the characteristic particle size. ;

[0143] Step 4.2, void volume V per cubic meter of aggregate PO

[0144]

[0145] Step 4.3, total surface area per unit volume of aggregate S

[0146]

[0147]

[0148]

[0149] Among them, n S , n Gare the theoretical quantities of fine aggregate and coarse aggregate, respectively;

[0150] Step 4.4: The total volume of cement paste required per unit volume of aggregate V P

[0151] V P =V PO +APT·S

[0152] Among them, APT is the thickness of cement paste wrapped on the surface of aggregate; when APT = 5μm, the working performance requirements of concrete can be met.

[0153] Step 4.5: Minimum total cement paste volume V′ required per unit volume of mixed recycled coarse aggregate concrete P

[0154]

[0155] Step 4.6, Minimum cement content per unit volume of mixed recycled coarse aggregate concrete and minimum water consumption

[0156]

[0157]

[0158] in, It is the water-cement ratio of mixed recycled coarse aggregate concrete.

[0159] Step 5: Using the minimum paste theory, calculate the minimum paste content required for mixed recycled coarse aggregate concrete based on the amount of coarse aggregate and fine aggregate, and further calculate the minimum cement amount and minimum water consumption

[0160] Step 6: If The cement dosage is Water consumption like The cement dosage is Water consumption

[0161] The mixed recycled coarse aggregate concrete mix design method proposed in the present invention can significantly improve the heterogeneity of coarse aggregate distribution in the mixed recycled coarse aggregate concrete, improve the compressive strength and elastic modulus of concrete, and reduce concrete shrinkage. The improvement of the performance of mixed recycled coarse aggregate concrete further promotes its promotion and application in practical engineering.

[0162] Example 1

[0163] A total of six groups of concrete were cast in the experiment, including two groups of ordinary concrete (NC0.5, NC0.65), in which all the coarse aggregates used in the ordinary concrete were natural coarse aggregates, and four groups of mixed recycled coarse aggregate concrete (MRC0.5C, MRC0.65C, MRC0.5N, MRC0.65N), in which the volume proportions of natural coarse aggregate, recycled concrete coarse aggregate, and recycled broken brick coarse aggregate in the coarse aggregate used in the mixed recycled coarse aggregate concrete were 0.5, 0.25, and 0.25, respectively. The mix proportion design of MRC0.5C and MRC0.65C adopts the ordinary concrete mix proportion calculation method, and the mix proportion design of MRC0.5N and MRC0.65N adopts the mix proportion calculation method for mixed recycled coarse aggregate concrete proposed in the present invention. The mix proportions of the six groups of concrete are shown in Table 1. The MRC0.5N of the concrete calculated by the mix proportion calculation design method proposed in the present invention is and 310kg / m 3 and 320kg / m 3 Therefore, the cement consumption and water consumption are 320kg / m 3 and 160kg / m 3 ; Concrete MRC0.65 239kg / m 3 and 271kg / m 3 Therefore, the cement consumption and water consumption are 271kg / m 3 and 176kg / m 3 .

[0164] Table 2 Concrete mix ratio

[0165]

[0166] Note: NA-ordinary coarse aggregate, RCA-recycled concrete coarse aggregate, RBA-crushed brick coarse aggregate

[0167] The cube compressive strength and elastic modulus of the six groups of concrete were measured at 28 days of age (as shown in Table 1). Compared with MRC0.5C and MRC0.65C, the cube compressive strength of MRC0.5N and MRC0.65N increased by 5% and 4%, respectively, and the elastic modulus increased by 22.8% and 33.5%, respectively.

[0168] The inhomogeneity index VI can be used to reflect the homogeneity of the distribution of coarse aggregate in concrete. The larger the VI, the more uneven the distribution of coarse aggregate. The VI values ​​of MRC0.5N, MRC0.65N, MRC0.5C, and MRC0.65C are 5.95%, 10.68%, 14.45%, and 39.12%, respectively. The test results show that the mix design method proposed in the present invention can significantly improve the inhomogeneity of the distribution of mixed recycled coarse aggregate concrete.

[0169] After 24 hours of pouring and demoulding, six groups of test blocks with a size of 100mm×100mm×515mm were placed in a constant temperature and humidity room to observe their shrinkage. As the number of days increases, the shrinkage changes of the six groups of concrete test blocks are as follows: Figure 1 As shown. Figure 1 It can be seen that with the increase of time, the shrinkage values ​​of MRC0.5N and MRC0.65N are significantly smaller than those of MRC0.5C and MRC0.65C.

[0170] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A mix design method for mixed recycled coarse aggregate concrete, the method comprising determining the amount of water, cement, coarse aggregate, and fine aggregate in the mix design of mixed recycled coarse aggregate concrete, characterized in that: The method first adjusts the mortar density to minimize the density difference between the mortar density and the density of each component of the coarse aggregate, thereby reducing the heterogeneity of the coarse aggregate distribution in the hardened concrete; then, the water consumption, cement consumption, coarse aggregate consumption, and fine aggregate consumption are calculated by combining the equal volume mortar replacement method and the minimum paste theory. The equal volume mortar replacement method ensures that the total content of new and old mortars in the mixed recycled coarse aggregate concrete is equal to the mortar content in the ordinary concrete, and the minimum paste theory can ensure that the cement paste content in the mixed recycled coarse aggregate concrete can reach the minimum paste content required to meet the concrete performance; The calculation steps of the mixed recycled coarse aggregate concrete mix design method are as follows: Step 1: Determine the density of cement mortar for mixed recycled coarse aggregate concrete; Step 2: Calculate the volume of ordinary concrete cement mortar; Step 3: Determine the volume of fresh cement mortar in mixed recycled coarse aggregate concrete by combining the equal volume mortar replacement method; Step 4: Calculate water usage Cement dosage Amount of coarse aggregate and fine aggregate; Step 5: Using the minimum paste theory, calculate the minimum paste content required for mixed recycled coarse aggregate concrete based on the amount of coarse aggregate and fine aggregate, and further calculate the minimum cement amount and minimum water consumption Step 6: If The cement dosage is Water consumption like The cement dosage is Water consumption The density calculation process of the mixed recycled coarse aggregate concrete cement mortar is as follows: Step 1.1: Test and determine the volume ratio of natural coarse aggregate, recycled concrete coarse aggregate and recycled crushed brick coarse aggregate in mixed recycled coarse aggregate in, It is the volume of natural coarse aggregate in each cubic meter of mixed recycled coarse aggregate concrete; It is the volume of recycled concrete coarse aggregate in each cubic meter of mixed recycled coarse aggregate concrete; It is the volume of recycled crushed brick coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; Step 1.2, calculate the density of mortar Among them, ρ NA is the saturated surface dry density of natural coarse aggregate; ρ RCA is the saturated surface dry density of coarse aggregate in recycled concrete; ρ RBA It is the saturated dry density of coarse aggregate of recycled crushed bricks; is the density of cement mortar in mixed recycled coarse aggregate concrete; The volume calculation process of ordinary concrete cement mortar is as follows: Step 2.1, Volume of concrete per cubic meter Among them, V NC is the volume of the ordinary concrete cement mortar; It is the amount of cement used per cubic meter of ordinary concrete; The amount of natural coarse aggregate per cubic meter of ordinary concrete; is the amount of fine aggregate per cubic meter of ordinary concrete; ρ c is the cement density; ρ s is the apparent density of fine aggregate; ρ w is the density of water; α is the percentage of air content in concrete; Step 2.2, sand ratio of ordinary concrete In the formula, is the sand ratio of ordinary concrete; Step 2.3, volume of cement mortar per cubic meter of concrete The volume calculation process of fresh cement mortar in the mixed recycled coarse aggregate concrete is as follows: Step 3.1: Test and determine the content of old mortar adhering to the surface of recycled concrete coarse aggregate and recycled crushed brick coarse aggregate Among them, OMC RCA OMC is the content of old mortar adhering to the surface of coarse aggregate in recycled concrete; RBA The content of old mortar adhering to the surface of coarse aggregate of recycled broken bricks; m RCA The absolute dry mass of coarse aggregate in recycled concrete before removing old mortar; m ONA It is the absolute dry mass of the coarse aggregate of recycled concrete with a particle size greater than 4.75 mm after removing the old mortar; m RBA The absolute dry mass of coarse aggregate of recycled broken bricks before removing old mortar; m OBA It is the absolute dry mass of the coarse aggregate of recycled broken bricks with a particle size greater than 4.75 mm after removing the old mortar; Step 3.2: Volume of natural aggregate, recycled concrete coarse aggregate, and recycled crushed brick coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete in, is the volume of coarse aggregate in ordinary concrete; ρ ONA The saturated surface dry density of coarse aggregate in recycled concrete without adhesive mortar; ρ OBA Saturated surface dry density of coarse aggregate of recycled crushed bricks to remove adhesive mortar; Step 3.3, volume of old mortar per cubic meter of mixed recycled coarse aggregate concrete in, It is the volume of old mortar in each cubic meter of mixed recycled coarse aggregate concrete; Step 3.4: Volume of fresh mortar per cubic meter of mixed recycled coarse aggregate concrete in, It is the volume of old mortar in each cubic meter of mixed recycled coarse aggregate concrete; Step 3.5, dosage of each component per cubic meter of mixed recycled coarse aggregate concrete in, The amount of water used per cubic meter of mixed recycled coarse aggregate concrete; The cement consumption per cubic meter of mixed recycled coarse aggregate concrete; The amount of natural coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of recycled concrete coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of recycled crushed brick coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; The amount of natural coarse aggregate per cubic meter of mixed recycled coarse aggregate concrete; It is the moisture content of natural coarse aggregate in its natural state; is the water absorption rate of natural coarse aggregate in saturated surface dry state; It is the moisture content of recycled concrete coarse aggregate in its natural state; It is the water absorption rate of saturated surface dry state of coarse aggregate in recycled concrete; It is the moisture content of recycled crushed brick coarse aggregate in its natural state; It is the water absorption rate of coarse aggregate of recycled broken bricks in saturated surface dry state; The calculation process of the minimum water consumption and minimum cement consumption of the mixed recycled coarse aggregate concrete is as follows: Step 4.1, Aggregate Packing Degree and S +y G =1 Among them, y S and G are the volume fractions of fine aggregate and coarse aggregate, respectively; k D k is the factor that determines the ratio of coarse and fine aggregate diameters; s is a statistical factor; x is the ratio between the accumulation volume of fine aggregate and the void volume of coarse aggregate; x0 = 0.4753, k0 = 0.3881; φ ST ,φ GT are the characteristic packing degrees of fine aggregate and coarse aggregate, respectively; ρ ST , ρ GT is the packing density of fine aggregate and coarse aggregate; ρ S , ρ G are the particle densities of fine aggregate and coarse aggregate, respectively; d ST , d GT are the characteristic particle sizes of fine aggregate and coarse aggregate, respectively; Step 4.2, void volume V per cubic meter of aggregate PO Step 4.3, total surface area per unit volume of aggregate S Among them, n S , n G are the theoretical quantities of fine aggregate and coarse aggregate, respectively; Step 4.4: The total volume of cement paste required per unit volume of aggregate V P V P =V PO +APT·S Among them, APT is the thickness of cement paste wrapped on the aggregate surface; Step 4.5: Minimum total cement paste volume V′ required per unit volume of mixed recycled coarse aggregate concrete P Step 4.6, Minimum cement content per unit volume of mixed recycled coarse aggregate concrete and minimum water consumption in, It is the water-cement ratio of mixed recycled coarse aggregate concrete.

2. The mix design method for mixed recycled coarse aggregate concrete according to claim 1, characterized in that: The components of the mixed recycled coarse aggregate include natural coarse aggregate, recycled concrete coarse aggregate and recycled crushed brick coarse aggregate.

3. The mix design method for mixed recycled coarse aggregate concrete according to claim 2, characterized in that: The natural coarse aggregate refers to natural stone obtained by crushing and screening with a particle size not greater than 31 mm.

4. The mix design method for mixed recycled coarse aggregate concrete according to claim 2, characterized in that: The recycled concrete coarse aggregate refers to the coarse aggregate obtained by crushing and screening concrete components in construction demolition waste.

5. The mix design method for mixed recycled coarse aggregate concrete according to claim 2, characterized in that: The recycled crushed brick coarse aggregate refers to the coarse aggregate obtained by crushing and screening sintered clay bricks in building demolition waste.

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