A VVTM design method for recycled aggregates in waste concrete stabilized with brick powder cement

Abstract

The present invention discloses a VVTM design method for recycled aggregate from waste concrete stabilized with brick dust and cement. The design utilizes the specimen vertical vibration molding method (VVTM) and includes steps such as raw material testing, aggregate gradation design, determination of the optimal brick dust dosage, optimal moisture content and maximum dry density, and cement dosage. Practical data indicates that the strength of the cement-stabilized recycled aggregate from waste concrete obtained by the present invention meets the design requirements for road bases of various grades. It can replace traditional cement-stabilized crushed stone in semi-rigid road bases, transforming construction waste into valuable resources and contributing to the ecological, environmentally friendly, and orderly development of highway construction.

Description

Technical Field

[0001] The invention belongs to the technical field of road construction materials and relates to a VVTM design method for recycled aggregate of brick powder cement-stabilized waste concrete. Background Art

[0002] With the continuous advancement of industrialization and urbanization in my country, the construction industry has grown rapidly, and the amount of construction waste generated by new construction, renovation, and demolition projects has increased exponentially. It is estimated that the annual national production of construction waste exceeds 2 billion tons, but the resource utilization rate is less than 10%. At the same time, the contradiction between the sustainable, green, and high-quality development of road transportation and the shortage of sand and gravel materials is becoming increasingly prominent. Researching technologies for utilizing construction waste in highway projects and formulating corresponding regulations will help promote the application of construction waste in highway projects. This will not only reduce the large amount of land resources occupied by construction waste and the environmental pollution it causes, but also alleviate the increasingly prominent contradiction between the supply and demand of sand and gravel materials.

[0003] At present, there are still the following problems in the use of cement-stabilized waste recycled aggregates in highway projects: (1) The composition of construction waste is complex and variable, usually containing bricks, concrete, and tiles, and the proportions between them vary greatly, resulting in extremely unstable engineering practice quality; (2) Article 6.1.4 of JTG / T 2321-2021 "Technical Specifications for the Utilization of Construction Waste in Highway Engineering" stipulates that "the mixing rate of recycled aggregate in the base layer shall not exceed 50%, and the subbase layer shall not exceed 80%", and Article 6.1.6 states that "the largest size material (20-30mm) should be natural aggregate to form a higher strength skeleton." Obviously, the specification's consideration of limiting the amount of recycled aggregate in construction waste stems from the impact of the large variability of recycled aggregate composition on engineering quality. However, the large-scale application of recycled aggregates from waste concrete, which has stable quality, is inevitably affected. (3) The static pressure method is currently commonly used to prepare test specimens. However, the crushing value of recycled aggregates from waste concrete is large. The recycled coarse aggregate is easily crushed during the preparation of test specimens using the static pressure method, which does not match the actual site of the project. As a result, the prepared test specimens are difficult to reflect the actual project, and the adaptability of traditional design methods (test methods, mineral grading and strength standards) has not been effectively verified. (4) At present, the utilization rate of brick powder in engineering practice is not high, and JTG / T 2321-2021 does not involve the effect of brick powder on the road performance of the mixture. However, studies have found that brick powder has a high powder content and strong activity, which can improve the density and strength of recycled aggregates from cement-stabilized waste concrete and improve the problem of construction segregation. Summary of the Invention

[0004] The purpose of the present invention is to provide a VVTM (vertical vibration molding method) design method for recycled aggregate of waste concrete stabilized by adding brick dust and cement, so as to quickly and accurately determine the optimal brick dust content, optimal moisture content, maximum dry density and cement dosage of the recycled aggregate of waste concrete stabilized by adding brick dust and cement. The designed recycled aggregate of waste concrete stabilized by adding brick dust and cement has good road performance and can replace traditional cement-stabilized crushed stone for semi-rigid base of roads.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is: a VVTM design method for stabilizing waste concrete recycled aggregate by adding brick powder cement, which is specifically implemented according to the following steps:

[0006] Step 1: Raw material inspection: Select representative samples from the materials actually used in the project as raw materials for inspection. The representative samples include cement, recycled coarse aggregate, recycled fine aggregate and brick powder.

[0007] Step 2, aggregate gradation design: preliminarily determine the proportion of aggregates of various specifications based on the aggregate screening results and gradation design requirements. The aggregates are recycled coarse aggregate and recycled fine aggregate;

[0008] Step 3, determine the optimal brick powder dosage: clay brick powder is selected as the brick powder, and its particle size is less than 4.75mm. Brick powder is weighed according to different proportions of 5%, 10%, 15% or 20% of the total mass of the mixture, and is added to the mixture instead of recycled fine aggregate. The specimens are formed by the vertical vibration molding method and their 7d unconfined compressive strength is tested. The relationship curve between the representative value of the 7d unconfined compressive strength and the brick powder dosage is drawn. The brick powder dosage corresponding to the highest strength point is the optimal brick powder dosage, and finally the proportion of aggregates of various specifications is determined;

[0009] Step 4, determine the maximum dry density and optimum moisture content: use the specimen vertical vibration molding method to determine the maximum dry density and optimum moisture content of the mixture with different cement dosages. The base cement dosage is 3.0%, 3.5%, 4.0% or 4.5%, and the subbase cement dosage should be 2.5%, 3.0%, 3.5% or 4.0%.

[0010] Step 5, determine the cement dosage: use the specimen vertical vibration molding method to form specimens of mixtures with different cement dosages, test the unconfined compressive strength of the mixture after standard curing for 7 days, draw a curve of the relationship between the representative value of the 7-day unconfined compressive strength and cement dosage, and determine the cement dosage according to the strength design standard.

[0011] As a preferred technical solution of the present invention, in step 1, the recycled coarse aggregate and the recycled fine aggregate are obtained by crushing and screening waste concrete, and the brick powder is obtained by crushing and screening waste clay bricks. The particle size of the recycled coarse aggregate is 4.75 mm to 37.5 mm, and the nominal maximum particle size of the recycled fine aggregate and brick powder is ≤4.75 mm.

[0012] As a preferred technical solution of the present invention, in step 1, the crushing value of the recycled coarse aggregate is ≤35%, and the equivalent of the recycled fine aggregate and brick silt is ≥40%.

[0013] As a preferred technical solution of the present invention, in step 2, the mineral gradation should be a skeleton dense gradation, wherein the amount of recycled coarse aggregate with a particle size of 9.5 mm or more should be ≥55%.

[0014] As a preferred technical solution of the present invention, in step 3, the diameter of the molded specimen is 150 mm and the height is 150 mm. The test parameters of the vertical vibration molding method of the specimen are an operating frequency of 30 Hz ± 1 Hz, a static eccentric moment of 0.215 kN·m ± 0.005 kN·m, an upper mass of 120 kg ± 1 kg, an lower mass of 180 kg ± 1 kg, and a vibration time of 120 s.

[0015] As a preferred technical solution of the present invention, in step 4, the method for determining the maximum dry density and the optimal moisture content is:

[0016] Step 4.1: Take 1 part of cement and aggregate and mix them evenly to obtain dry mixture m k , add mass m k ×w i of water, mix evenly to obtain 1 part of wet mixture, w i 6% to 12%;

[0017] Step 4.2: Use the vertical vibration molding method to form a cylindrical specimen with a diameter of 150 mm and a height of 120 mm, and calculate the dry density and moisture content of the specimen;

[0018] Step 4.3: After obtaining 3 to 5 groups of valid data, draw a dry density-moisture content relationship curve with the mixing moisture content as the horizontal axis and the dry density as the vertical axis. The horizontal and vertical axes of the peak of the curve are the optimal moisture content and the maximum dry density, respectively.

[0019] The dry density of the sample should be calculated according to formula (1):

[0020]

[0021] Where: ρ di —Dry density of the sample at the i-th test, g / cm 3 ;

[0022] m di —mass of the sample at the i-th test, unit: g;

[0023] h i —Height of the sample during the i-th test, unit: cm;

[0024] w i —Moisture content of the sample during the i-th test, unit: %.

[0025] As a preferred technical solution of the present invention, in step 5, the strength design standard of the recycled aggregate of the brick powder cement stabilized waste concrete is specifically: the 7d unconfined compressive strength of the base layer is ≥7.0MPa, and the 7d unconfined compressive strength of the subbase layer is ≥6.5MPa.

[0026] The beneficial effects of the present invention are: the recycled aggregate of waste concrete stabilized with brick powder and cement obtained by the present invention can replace traditional cement-stabilized crushed stone for semi-rigid road base, meet the design requirements of road bases of various grades, realize the "transformation of waste into treasure" of construction waste, and help the ecological, environmental protection and orderly development of highway construction, and have good market promotion and application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0028] Figure 1 This is a relationship diagram between the representative value of the 7d unconfined compressive strength and the brick powder dosage in Example 1 of the present invention.

[0029] Figure 2 This is a relationship diagram between the representative value of the 7d unconfined compressive strength and cement dosage in Example 1 of the present invention. DETAILED DESCRIPTION

[0030] The following describes embodiments of the present invention in detail. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended only to explain the present invention and are not to be construed as limiting the present invention.

[0031] In the description of the present invention, unless otherwise clearly defined, terms such as setting, installing, and connecting should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in the present invention based on the specific content of the technical solution.

[0032] Example 1

[0033] The VVTM design method of recycled aggregate of waste concrete stabilized by adding brick dust cement of the present invention is characterized by being implemented specifically according to the following steps:

[0034] Step 1: Raw material inspection: Select representative samples from the materials actually used in the project as raw materials for inspection. The representative samples include cement, recycled coarse aggregate, recycled fine aggregate and brick powder.

[0035] The strength grade of the cement used is 42.5, which complies with the current "General Portland Cement" (GB 175). The technical indicators of the recycled aggregate used are shown in Table 1 and comply with the current "Technical Specifications for the Utilization of Construction Waste in Highway Engineering" (JTG / T2321-2021).

[0036] Table 1 Technical indicators of recycled aggregate

[0037]

[0038] Step 2, aggregate gradation design: Preliminarily determine the proportion of aggregates of various specifications based on the aggregate screening results and gradation design requirements. The aggregates are recycled coarse aggregate and recycled fine aggregate.

[0039] The aggregate screening results are shown in Table 2, and the ore gradation design requirements are shown in Table 3.

[0040] Table 2 Aggregate screening results

[0041]

[0042] Table 3 Gradation design

[0043] Sieve hole size (mm) 37.5 31.5 19.0 9.5 4.75 2.36 0.6 0.075 Quality pass rate (%) 100 88～100 58～70 38～46 26～34 20～28 8～16 3～6

[0044] According to the aggregate screening results and grading design requirements, the ratio of aggregates of various specifications is determined to be: 9.5~31.5mm:4.75~9.5mm:0~4.75mm=55:10:35.

[0045] Step 3, determine the optimal brick powder dosage: select clay brick powder with a particle size of less than 4.75mm for brick powder, weigh brick powder according to different proportions of 5%, 10%, 15% or 20% of the total mass of the mixture, and add it to the mixture instead of recycled fine aggregate. Use the specimen vertical vibration molding method to form the specimen and test its 7d unconfined compressive strength. Draw a relationship curve between the representative value of the 7d unconfined compressive strength and the brick powder dosage. The brick powder dosage corresponding to the highest strength point is the optimal brick powder dosage, and finally determine the proportion of aggregates of various specifications.

[0046] like Figure 1 As shown in the figure, when the brick powder content is 10%, the 7d unconfined compressive strength of the mixture is the largest. The optimal brick powder content is determined to be 10%. At this time, the proportion of aggregates of various specifications in the mixture is: 9.5-31.5mm waste concrete recycled aggregate: 4.75-9.5mm waste concrete recycled aggregate: 0-4.75mm waste concrete recycled aggregate: 0-4.75mm brick powder = 55:10:25:10.

[0047] Step 4, determine the maximum dry density and optimal moisture content: Use the specimen vertical vibration molding method to determine the maximum dry density and optimal moisture content of the mixture with different cement dosages. The base cement dosage is 3.0%, 3.5%, 4.0%, or 4.5%, and the sub-base cement dosage is preferably 2.5%, 3.0%, 3.5%, or 4.0%. Specifically:

[0048] ① Before the test, place aggregates of various specifications in an oven and dry them to constant weight. Prepare 5 to 6 samples of aggregates of various specifications after drying according to the required proportions. The mass of each sample is m k 4800g~5500g, prepare 5~6 parts of cement, the mass of each part of cement is m k ×P s , P s For preset cement dosage.

[0049] ② Prepare the sample according to the predetermined moisture content. Take 1 part of cement and dried aggregate and mix them evenly to obtain a dry mixture. Add a mass of m k ×(1+P s )×w i of water, mix well to obtain 1 part of wet mix. i It is 6% to 12%.

[0050] ③ Place the pad in the test mold and make the bottom flat, and place a round filter paper on the pad. Then, fill the well-mixed wet mixture into the test mold according to the quartering method, and tamp along the wall of the test mold for no less than 6 times, and record the mass of the wet mixture filled (m). s After fixing the test mold on the base of the vertical vibration compactor, lower the vibration hammer and make it contact with the compressed material. Set the vibration frequency to 30Hz and the vibration time to 120s to perform vibration compaction.

[0051] ④ After the vibration compaction is completed, take out the test mold, push out the sample with a demoulder and weigh it, record it as m di Use a vernier caliper to measure the height of the test sample in four cross-symmetrical directions and take the average value as the sample height h i If the sample height exceeds the requirement of 120mm±10.0mm, it shall be discarded and the number of mm shall be appropriately increased or decreased according to the sample height. s , re-prepare the sample.

[0052] ⑤ With the mixing moisture content as the horizontal axis and the dry density as the vertical axis, draw a dry density-moisture content relationship curve. The horizontal and vertical axes of the peak of the curve are the optimal moisture content and the maximum dry density, respectively.

[0053] The maximum dry density and optimum moisture content of recycled aggregate from cement-stabilized waste concrete are shown in Table 4.

[0054] Table 4 Maximum dry density and optimum moisture content of recycled aggregates of cement-stabilized waste concrete

[0055] Cement dosage (%) 3.0 3.5 4.0 4.5 Optimum moisture content (%) 10.2 10.5 10.6 10.7 <![CDATA[Maximum dry density (g / cm 3 )]]> 2.050 2.053 2.062 2.077

[0056] Step 5, determine the cement dosage: use the specimen vertical vibration molding method to form specimens of mixtures with different cement dosages, test the unconfined compressive strength of the mixture after standard curing for 7 days, draw a curve of the relationship between the representative value of the 7-day unconfined compressive strength and cement dosage, and determine the cement dosage according to the strength design standard.

[0057] The results of the 7d unconfined compressive strength test of recycled aggregates from cement-stabilized waste concrete are shown in Table 5. is the average compressive strength, C V is the coefficient of variation, R c0.95 The compressive strength is represented by the value shown in Table 1. Strength test is performed according to T 0805-1994 in JTG E51-2009.

[0058] Table 5 7d unconfined compressive strength and representative values

[0059]

[0060] According to the data in Table 5, the relationship between the strength of recycled aggregate in cement-stabilized waste concrete and cement dosage is shown in Table 5. Figure 2 .

[0061] Depend on Figure 2 It can be seen that the minimum cement dosage corresponding to the base design strength of 7.0MPa is 2.5%. Considering factors such as on-site mixing and construction technology, for the sake of safety, the recommended cement dosage is 4.0%.

[0062] The foregoing description shows and describes several preferred embodiments of the invention. However, as previously stated, it should be understood that the invention is not limited to the form disclosed herein and should not be construed as excluding other embodiments. Rather, the invention is applicable to various other combinations, modifications, and environments and is capable of modification within the scope of the inventive concept described herein, through the teachings above, or through techniques or knowledge in the relevant art. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the invention are intended to be within the scope of the appended claims.

Claims

1. A VVTM design method for recycled aggregate stabilized with brick dust cement, characterized in that: Please follow the steps below to implement it: Step 1: Raw material inspection: Select representative samples from the materials actually used in the project as raw materials for inspection. The representative samples include cement, recycled coarse aggregate, recycled fine aggregate and brick powder. Step 2, aggregate gradation design: preliminarily determine the proportion of aggregates of various specifications based on the aggregate screening results and gradation design requirements. The aggregates are recycled coarse aggregate and recycled fine aggregate; Step 3: Determine the optimal brick powder dosage: Clay brick powder with a particle size of less than 4.75 mm is selected as the brick powder. Brick powder is weighed at different proportions of 5%, 10%, 15%, or 20% of the total mass of the mixture and added to the mixture instead of recycled fine aggregate. Specimens are formed using the vertical vibration molding method and their 7d unconfined compressive strength is tested. A curve is drawn showing the relationship between the representative value of the 7d unconfined compressive strength and the brick powder dosage. The brick powder dosage corresponding to the highest strength point is the optimal brick powder dosage, and the proportion of aggregates of various specifications is finally determined. Step 4, determine the maximum dry density and optimal moisture content: use the specimen vertical vibration molding method to determine the maximum dry density and optimal moisture content of the mixture with different cement dosages. The base cement dosage is 3.0%, 3.5%, 4.0%, or 4.5%, and the subbase cement dosage is preferably 2.5%, 3.0%, 3.5%, or 4.0%. The method for determining the maximum dry density and optimal moisture content is: Step 4.1: Take 1 part of cement and aggregate and mix them evenly to obtain dry mixture m k , add mass m k ×w i of water, mix evenly to obtain 1 part of wet mixture, w i 6%~12%; Step 4.2: Use the vertical vibration molding method to form a cylindrical specimen with a diameter of 150 mm and a height of 120 mm, and calculate the dry density and moisture content of the specimen; Step 4.3: After obtaining 3 to 5 groups of valid data, draw a dry density-moisture content relationship curve with the mixing moisture content as the horizontal axis and the dry density as the vertical axis. The horizontal and vertical axes of the peak of the curve are the optimal moisture content and the maximum dry density, respectively. The dry density of the sample should be calculated according to formula (1): (1) Where: ρ di —Dry density of the sample at the i-th test, g / cm 3 ; m di —mass of the sample at the i-th test, unit: g; h i —Height of the sample during the i-th test, unit: cm; w i —The moisture content of the sample during the i-th test, unit: %; Step 5, determine the cement dosage: Use the specimen vertical vibration molding method to form specimens of mixtures with different cement dosages. Test the unconfined compressive strength of the mixture after 7 days of standard curing. Draw a curve showing the relationship between the representative value of the 7-day unconfined compressive strength and the cement dosage. Determine the cement dosage according to the strength design standard.

2. The VVTM design method for recycled aggregate of waste concrete stabilized with brick dust cement according to claim 1, characterized in that: In step 1, the recycled coarse aggregate and the recycled fine aggregate are obtained by crushing and screening waste concrete, and the brick powder is obtained by crushing and screening waste clay bricks. The particle size of the recycled coarse aggregate is 4.75 mm to 37.5 mm, and the nominal maximum particle size of the recycled fine aggregate and the brick powder is ≤4.75 mm.

3. The VVTM design method for recycled aggregate of waste concrete stabilized with brick dust cement according to claim 2, characterized in that: In step 1, the crushing value of the recycled coarse aggregate is ≤35%, and the equivalent of the recycled fine aggregate and brick silt sand is ≥40%.

4. The VVTM design method for recycled aggregate of waste concrete stabilized with brick dust cement according to claim 3, characterized in that: In step 2, the mineral gradation should be a skeleton dense gradation, in which the amount of recycled coarse aggregate with a particle size of 9.5 mm or more should be ≥55%.

5. The VVTM design method for recycled aggregate of waste concrete stabilized with brick dust cement according to claim 4, characterized in that: In step 3, the diameter of the molded specimen is 150 mm and the height is 150 mm. The test parameters of the vertical vibration molding method of the specimen are an operating frequency of 30 Hz±1 Hz, a static eccentric moment of 0.215 kN·m±0.005 kN·m, an upper mass of 120 kg±1 kg, an lower mass of 180 kg±1 kg, and a vibration time of 120 s.

6. The VVTM design method for recycled aggregate of waste concrete stabilized with brick dust cement according to claim 5, characterized in that: In step 5, the strength design standard of the recycled aggregate of the brick dust cement stabilized waste concrete is specifically: the 7d unconfined compressive strength of the base layer is ≥7.0MPa, and the 7d unconfined compressive strength of the subbase layer is ≥6.5MPa.

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