VVTM design method of brick powder-doped cement stabilized waste concrete recycled aggregate

Abstract

The invention discloses a VVTM design method of brick powder doped cement stabilized waste concrete recycled aggregate, which adopts a test piece vertical vibration molding method (VVTM) design, and comprises the steps of raw material inspection, aggregate gradation design, optimal brick powder doping amount determination, optimal moisture content and maximum dry density determination, cement dosage determination and the like. Practice data shows that the strength of the cement-stabilized waste concrete recycled aggregate designed by the invention can meet the design requirements of various grades of road bases, and the cement-stabilized waste concrete recycled aggregate can replace traditional cement-stabilized macadam to be used for a road semi-rigid base, so that the construction waste is turned into wealth, and ecological environmental protection and cyclic and ordered development of road construction are assisted.

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 stabilized waste concrete mixed with brick powder cement. Background Art

[0002] With the continuous advancement of industrialization and urbanization in my country, the construction industry has developed rapidly, and the construction waste generated by the construction, renovation and demolition of projects has increased exponentially. It is estimated that the annual output of construction waste in the country 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 traffic and the shortage of sand and gravel materials is becoming increasingly prominent. Researching the technology of using construction waste in highway projects and formulating corresponding regulations will help promote the application of construction waste in highway projects, which can not only reduce the problems of construction waste occupying a large amount of land resources and environmental pollution, 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 with cement-stabilized waste recycled aggregates in highway projects: (1) The composition of construction waste is complex and varied, 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 blending 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 made of natural aggregate to form a higher strength skeleton". Obviously, the consideration of the specification on the limit on the amount of recycled aggregate in construction waste is due to the impact of the large variability of recycled aggregate composition on project quality. However, the large-scale application of recycled aggregates from waste concrete with stable quality is inevitably affected; (3) At present, the static pressure method is generally used to prepare test specimens, but the crushing value of recycled aggregates from waste concrete is large. The recycled coarse aggregate is easy to crush during the preparation of test specimens by 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 impact of brick powder on the road performance of the mixture. However, the study found that brick powder has a high powder content and strong activity, which can improve the density and strength of recycled aggregates of 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 powder cement, so as to quickly and accurately determine the optimal brick powder content, optimal moisture content, maximum dry density and cement dosage of the recycled aggregate of waste concrete stabilized by adding brick powder cement. The designed recycled aggregate of waste concrete stabilized by adding brick powder 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 recycled aggregate of waste concrete by adding brick powder cement, which is specifically implemented according to the following steps:

[0006] Step 1, inspecting raw materials: 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 according to the aggregate screening results and gradation design requirements, wherein the aggregates are recycled coarse aggregates and recycled fine aggregates;

[0008] Step 3, determine the best 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 specimen is formed by the specimen vertical vibration molding method and its 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 best 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 7d 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, the brick powder is obtained by crushing and screening waste clay bricks, the particle size of the recycled coarse aggregate is 4.75mm~37.5mm, and the nominal maximum particle size of the recycled fine aggregate and brick powder is ≤4.75mm.

[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 material 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 operating frequency 30 Hz ± 1 Hz, static eccentric moment 0.215 kN·m ± 0.005 kN·m, upper mass 120 kg ± 1 kg, lower mass 180 kg ± 1 kg, and vibration time 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 a mass of m k × i of water, mix well to get 1 part of wet mixture, w i 6% to 12%;

[0017] Step 4.2, using the specimen vertical vibration molding method to mold 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-water 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: di —Dry density of the sample at the i-th test, g / cm 3 ;

[0022] m di —mass of the sample in 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 recycled aggregate of waste concrete stabilized with brick powder cement 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 as follows: 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 "waste to 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] Embodiments of the present invention are described in detail below, examples of which 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 only used to explain the present invention, and cannot be understood as limiting the present invention.

[0031] In the description of the present invention, unless otherwise clearly defined, terms such as setting, installing, connecting, etc. 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 the present invention for stabilizing recycled aggregate of waste concrete by adding brick powder cement is characterized by being implemented specifically according to the following steps:

[0034] Step 1, inspecting raw materials: 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 relevant provisions of the current "General Portland Cement" (GB 175). The technical indicators of the recycled aggregate used are shown in Table 1, which complies with the relevant provisions of 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 grading design requirements are shown in Table 3.

[0040] Table 2 Aggregate screening results

[0041]

[0042] Table 3 Gradation design

[0043] Screen 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 proportion of aggregates of various specifications is determined as: 9.5~31.5mm:4.75~9.5mm:0~4.75mm=55:10:35.

[0045] Step 3, determine the best brick powder dosage: clay brick powder is selected as 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 added into the mixture instead of recycled fine aggregate. The specimens are formed by vertical vibration molding method and their 7d unconfined compressive strength is tested. The relationship curve between the representative value of 7d unconfined compressive strength and brick powder dosage is drawn. The brick powder dosage corresponding to the highest strength point is the best brick powder dosage, and finally the proportion of aggregates of various specifications is determined.

[0046] like Figure 1 As shown, 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 recycled aggregate from waste concrete: 4.75-9.5mm recycled aggregate from waste concrete: 0-4.75mm recycled aggregate from waste concrete: 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 subbase cement dosage is preferably 2.5%, 3.0%, 3.5% or 4.0%. Specifically:

[0048] ① Before the test, various specifications of aggregates are placed in an oven and dried to constant weight. After drying, various specifications of aggregates are prepared into 5 to 6 samples according to the required proportion of the mix ratio. The mass of each sample is m k 4800g~5500g, prepare 5~6 portions of cement, each portion of cement has a mass of m k ×P s , P s To 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 get 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, load the evenly 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 loaded (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 material to be pressed. 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 demoulding device and weigh it, recorded as m di Use a vernier caliper to measure the sample height 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 m shall be appropriately increased or decreased according to the sample height. s , re-prepare the sample.

[0052] ⑤ With mixing moisture content as the horizontal axis and 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 maximum dry density, respectively.

[0053] The maximum dry density and optimum moisture content of recycled aggregates 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 7d unconfined compressive strength and cement dosage, and determine the cement dosage according to the strength design standard.

[0057] The 7d unconfined compressive strength test results of cement stabilized waste concrete recycled aggregate are shown in Table 5. is the average compressive strength, C V is the coefficient of variation, R c0.95 It is the representative value of compressive strength. The strength test is carried out 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 and cement dosage of cement stabilized waste concrete is shown in 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 above description shows and describes several preferred embodiments of the invention, but as mentioned above, it should be understood that the invention is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the invention concept described herein through the above teachings or the technology or knowledge of the relevant field. Changes and modifications made by those skilled in the art should be within the scope of protection of the claims attached to the invention without departing from the spirit and scope of the invention.

Claims

1. A VVTM design method for recycled aggregates of waste concrete stabilized by mixing brick powder cement, characterized in that: Follow the steps below to implement it: Step 1, inspecting raw materials: 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 according to the aggregate screening results and gradation design requirements, wherein the aggregates are recycled coarse aggregates and recycled fine aggregates; Step 3, determine the best 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 specimen is formed by the specimen vertical vibration molding method and its 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 best brick powder dosage, and finally the proportion of aggregates of various specifications is determined; 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%. 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 7d unconfined compressive strength and cement dosage, and determine the cement dosage according to the strength design standard.

2. The VVTM design method for recycled aggregate stabilized by waste concrete mixed with brick powder cement according to claim 1, characterized in that: In step 1, the recycled coarse aggregate and 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.

3. The VVTM design method for recycled aggregate of waste concrete stabilized with brick powder 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 material 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%.

5. The VVTM design method for recycled aggregate stabilized by waste concrete mixed with brick dust cement according to claim 4 is 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: operating frequency 30 Hz ± 1 Hz, static eccentric moment 0.215 kN·m ± 0.005 kN·m, upper mass 120 kg ± 1 kg, lower mass 180 kg ± 1 kg, and vibration time 120 s.

6. The VVTM design method for recycled aggregate stabilized by waste concrete mixed with brick dust cement according to claim 5, characterized in that: In step 4, the maximum dry density and the optimum moisture content are determined by: Step 4.1, take 1 part of cement and aggregate and mix them evenly to obtain dry mixture m k , add a mass of m k × i of water, mix well to get 1 part of wet mixture, w i 6% to 12%; Step 4.2, using the specimen vertical vibration molding method to mold 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-water 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): Where: 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 —Moisture content of the sample during the i-th test, unit: %.

7. The VVTM design method for recycled aggregate stabilized by waste concrete mixed with brick dust cement according to claim 6, characterized in that: In step 5, the strength design standard of recycled aggregate of waste concrete stabilized with brick powder cement is as follows: 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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