A method for fully utilizing the old mortar on the surface of recycled coarse aggregate to improve the impact resistance of concrete

Abstract

The present invention discloses a method for fully utilizing the old mortar on the surface of recycled coarse aggregate to improve the impact resistance of concrete, belonging to the technical field of building materials. The method comprises the following steps: S1. Weakening treatment is carried out on the recycled coarse aggregate to obtain pretreated recycled coarse aggregate, and the elastic modulus of the old mortar on the surface of the pretreated recycled coarse aggregate is 15% - 80% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate; S2. A polyurea coating is obtained by reacting an isocyanate component and an amino compound in solvent oil, and then the polyurea coating is sprayed on the surface of the pretreated recycled coarse aggregate to obtain modified recycled coarse aggregate; S3. Recycled concrete is prepared by using the modified recycled coarse aggregate obtained in step S2. The present invention utilizes the characteristics of the brittle, low elastic modulus and low strength of the weak old mortar on the surface of the recycled coarse aggregate. Under explosion and impact loads, the old mortar breaks first to absorb the impact energy, ensuring the integrity of the main material and improving the blast and impact resistance of the recycled concrete.

Description

Technical Field

[0001] The present invention belongs to the technical field of building materials, and particularly relates to a method for improving the impact resistance of concrete by making full use of the old mortar on the surface of recycled coarse aggregate. Background Art

[0002] Crushing waste concrete into recycled coarse aggregate and applying it to new concrete structures has good environmental and economic effects. However, the surface of recycled coarse aggregate is inevitably adhered with old cement mortar, resulting in poor adhesion, high porosity and strong water absorption in the interfacial transition zone between recycled coarse aggregate and new mortar, thus significantly reducing the performance such as the compressive strength of recycled concrete.

[0003] At present, a large number of studies focus on the strengthening of the properties of recycled coarse aggregate, including physical, chemical and biological strengthening methods. The strengthening ideas of recycled coarse aggregate are mainly divided into two categories: 1) using technical means to remove or reduce the old mortar on the surface of recycled coarse aggregate; 2) improving the density of the interfacial transition zone between recycled coarse aggregate and old cement mortar through the filling of fine particles or chemical reactions, and enhancing the bonding force of the interfacial transition zone. The first strengthening idea includes: physical methods such as mechanical grinding method, heating grinding method, particle shaping, and chemical methods such as pickling. The second strengthening idea includes: mineral admixture modification method, carbonation strengthening method, microbial mineralization deposition and other methods. For example, Chinese Patent CN110436837A discloses a renewable concrete and its preparation method, and the modification method of the modified recycled aggregate adopted therein includes the following steps: mechanical activation, acidification treatment, filling treatment, and coating treatment. This invention removes the old mortar and edges and corners on the surface of concrete particles through mechanical activation, dissolves the hardened cement paste on the surface of concrete particles in acid solution through acidification treatment to strengthen the interfacial transition zone of recycled concrete, then enables nano-fillers to enter the gaps of acidified concrete particles through filling treatment to enhance the structural strength of the aggregate, and finally envelopes the recycled aggregate in a resin network to improve the adhesion between aggregates and the compressive strength of concrete.

[0004] At present, there are many studies on the strengthening methods of recycled coarse aggregate, with different principles, but most of them have cumbersome procedures, high costs, low profitability, low cost performance, and are difficult to be popularized and applied in engineering. Summary of the Invention

[0005] Aiming at the deficiencies of the above-mentioned existing technologies, one of the objectives of the present invention is to provide a method for fully utilizing the old mortar on the surface of recycled coarse aggregate to improve the impact resistance of concrete. Instead of achieving its large-scale application by strengthening recycled coarse aggregate, the present invention starts from the opposite aspect, selects recycled coarse aggregate with appropriate particle size, weakens it, and utilizes the characteristics of the weak old mortar on the surface of recycled coarse aggregate, such as large brittleness, low elastic modulus, and low strength. Under explosion and impact loads, the old mortar breaks first to absorb impact energy, ensuring the integrity of the main material and improving the blast and impact resistance of recycled concrete, thereby realizing the high-value-added and large-scale application of recycled coarse aggregate.

[0006] To achieve the above objective, the specific technical solution of the present invention is as follows:

[0007] A method for fully utilizing the old mortar on the surface of recycled coarse aggregate to improve the impact resistance of concrete, comprising the following steps:

[0008] S1. Weakening treatment of the recycled coarse aggregate, and the specific operation is as follows: soaking the recycled coarse aggregate in a weak acid aqueous solution and / or putting the recycled coarse aggregate into a rotating cylinder with a toothed ring thorn on the inner surface for rotation to obtain pretreated recycled coarse aggregate; the elastic modulus of the old mortar on the surface of the pretreated recycled coarse aggregate is 15% - 80% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate;

[0009] S2. Reacting an isocyanate component and an amino compound in solvent oil to obtain a polyurea coating, and then spraying the polyurea coating on the surface of the pretreated recycled coarse aggregate to form a polyurea coating on the surface of the pretreated recycled coarse aggregate to obtain modified recycled coarse aggregate;

[0010] S3. Using the modified recycled coarse aggregate obtained in step S2 to prepare recycled concrete.

[0011] The present invention pre - treats recycled coarse aggregates by pickling with weak acid to reduce the density of the old mortar paste and the strength of the old mortar, so as to ensure that it preferentially fractures under high - strain - rate loading; and / or, by putting the recycled coarse aggregates into a rotating cylinder with tooth - ring thorns on its inner surface and then performing high - speed rolling, the recycled coarse aggregates generate stress concentration by hitting the tooth - ring thorns in the rotating cylinder, causing the old mortar paste to crack and produce micro - cracks. During application, the micro - cracks in the old mortar paste rapidly expand under high - strain loading to consume impact energy. In summary, through the weakening treatment of soaking in weak acid aqueous solution and / or pre - cracking by the rotating cylinder, the elastic modulus of the old mortar on the surface of the obtained pre - treated recycled coarse aggregates is 15% - 80% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregates. The old mortar on the surface of the modified recycled coarse aggregates fractures first under high - strain loading to absorb impact energy and preserve the integrity of the main material; finally, a polyurea coating is sprayed on the surface of the recycled coarse aggregates after the weakening treatment. Polyurea is a kind of high - molecular elastic polymer, which can absorb and dissipate a large amount of energy due to the breakage of hydrogen bonds between its hard segments and soft segments under high - strain - rate loading, thus showing excellent anti - explosion and anti - impact properties. Spraying a polyurea coating on the surface of the recycled coarse aggregates, when the shock wave band reaches the interface of the old mortar - polyurea coating, due to the huge difference in anti - impact strength between the two, the old mortar cannot bear the high - strain - rate load transmitted by the polyurea coating, so it fractures to absorb energy, while preserving the integrity of the recycled coarse aggregate body and improving the anti - explosion and anti - impact properties of recycled concrete.

[0012] Preferably, in step S1, the content of the old mortar on the surface of the recycled coarse aggregates is 18%wt - 30%wt. When the content of the old mortar on the surface of the recycled coarse aggregates is too low, the impact energy dissipation is too low, and the shock wave passes through the old mortar layer and reaches the coarse aggregate layer, unable to achieve the improvement of anti - impact performance; when the content of the old mortar on the surface of the recycled coarse aggregates is too high, the cracks in the old mortar layer penetrate each other, forming through - cracks in the recycled concrete, unable to ensure the integrity of the main material.

[0013] Preferably, the particle size of the recycled coarse aggregates is 25 - 40 mm. To ensure the residual amount of the old mortar on the surface of the recycled coarse aggregates, the particle size of the recycled coarse aggregates should not be too small. Therefore, in the present invention, recycled coarse aggregates with a particle size of 25 - 40 mm are selected.

[0014] Preferably, in step S1, the mass concentration of the weak acid aqueous solution is 0.3% - 1.0%, and the soaking time is 0.5 - 2 h.

[0015] When the mass concentration of the weak acid aqueous solution and the soaking time are within the defined range of the present invention, the density of the old mortar paste can be reduced, and at the same time, it can be ensured that pickling does not cause the old mortar to fall off, so that the elastic modulus of the old mortar on the surface of the pre - treated recycled coarse aggregates is 15% - 80% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregates.

[0016] Preferably, the pH value of the weak acid aqueous solution is 4.5 to 6.0.

[0017] Preferably, the weak acid aqueous solution is an aqueous solution of at least one of carbonic acid, nitrous acid, acetic acid, hypochlorous acid, and hydrofluoric acid.

[0018] Preferably, the recycled coarse aggregate is put into a rotating cylinder with a toothed ring thorn on the inner surface, and then rotated at 150 - 500 rpm for 20 - 40 minutes.

[0019] When the rotation speed and time of the rotating cylinder are within the defined range of the present invention, the old mortar paste can be cracked to generate microcracks, so that the elastic modulus of the old mortar on the surface of the pretreated recycled coarse aggregate is 15% - 80% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate.

[0020] Preferably, the rotating cylinder includes an inner rotating cylinder, an intermediate filter screen, and an outer cylinder shell. The toothed ring thorns are arranged on the inner surface of the inner rotating cylinder and are evenly distributed along the circumferential direction of the inner rotating cylinder. The cross-section of the toothed ring thorns is triangular. When the rotating cylinder rotates, the recycled coarse aggregate impacts with the tips of the toothed ring thorns.

[0021] More preferably, the aperture of the intermediate filter screen is 15 - 20 mm. To ensure that the tiny old mortar particles falling off are discharged, while the recycled coarse aggregate remains in the cylinder.

[0022] Preferably, during the preparation processes of step S2 and step S3, it shall not be irradiated by ultraviolet rays. The polyurea coating will age after being irradiated by ultraviolet rays, resulting in a reduction in energy consumption capacity.

[0023] Preferably, in step S3, the casting of the recycled concrete is completed within 5 hours after spraying the polyurea coating.

[0024] Preferably, the isocyanate component includes at least one of aromatic isocyanate, aliphatic isocyanate, and polyisocyanate, the amino compound includes at least one of diamine, polyamine, and amino-terminated polyether, and the solvent oil includes at least one of toluene, naphthenic base oil, and No. 150 solvent oil.

[0025] Preferably, the molar ratio of the isocyanate component to the amino compound is (1.2:1) - (0.95:1.05).

[0026] Preferably, the thickness of the polyurea coating is 3 - 5 mm.

[0027] Preferably, the preparation method of the polyurea coating includes the following steps:

[0028] P1. Weigh the isocyanate component, amino compound, and solvent oil and put them into a glass reaction kettle, heat up to 90 - 105 °C, remove water under vacuum, and react for 30 - 60 minutes;

[0029] P2. Lower the temperature in the reactor to 60 °C, add water and then react for 10 - 20 min to remove the unreacted isocyanate component.

[0030] P3. Raise the temperature in the reactor to 90 °C again, add additives, evacuate to remove air bubbles, and stir at a speed of 100 - 300 rpm for 1 - 5 min to obtain the polyurea coating.

[0031] More preferably, the additives include at least one of defoamer, catalyst, water remover, dispersant, and curing agent.

[0032] Preferably, the recycled coarse aggregate is obtained by crushing waste concrete with a strength of 25 MPa - 40 Mpa.

[0033] Preferably, the strength grade of the old mortar on the surface of the recycled coarse aggregate is M5 - M15, and the mud content of the recycled coarse aggregate is less than 4%wt.

[0034] Another object of the present invention is to provide recycled concrete prepared by the above method, which comprises the following components in parts by weight: 750 - 920 parts of modified recycled coarse aggregate, 120 - 800 parts of fine aggregate, 150 - 225 parts of cement, 130 - 195 parts of fly ash, 140 - 310 parts of water, and 3 - 20 parts of admixture.

[0035] Another object of the present invention is to provide the application of the recycled concrete prepared by the above method in an explosion - resistant and impact - resistant protection structure.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] (1) Instead of strengthening the recycled coarse aggregate to improve the performance of recycled concrete, the present invention realizes the improvement of the explosion - resistant and impact - resistant performance of recycled concrete by actively sacrificing the weak link (old mortar) of the recycled coarse aggregate.

[0038] (2) The prior art improvements on recycled concrete are all achieved by strengthening the recycled coarse aggregate, which have problems such as cumbersome procedures and high costs, and are difficult to be popularized and applied in engineering. However, the present invention directly utilizes the weak link of the recycled coarse aggregate, without the need to modify the recycled coarse aggregate. The technical path is simple, easy to implement, and has high cost - effectiveness, and can realize the high - value utilization of a large amount of construction waste concrete.

[0039] (3) By spraying polyurea elastomer on the surface of recycled coarse aggregate, the present invention forms an energy dissipation difference at the interface between the old mortar of the recycled coarse aggregate and the polyurea coating. The old mortar cannot bear the high strain rate load transmitted by the polyurea coating and thus breaks to absorb energy, while preserving the integrity of the recycled coarse aggregate itself. This method realizes the directional crushing of microscopic materials by artificially creating an energy dissipation capacity difference, making the recycled concrete material have obvious designability.

[0040] (4) The present invention can significantly improve the blast resistance and impact resistance of recycled concrete, enabling it to have a wide range of application scenarios, realizing the large-scale application of recycled concrete, and having significant ecological and economic effects. Brief Description of the Drawings

[0041] Figure 1 It is a schematic diagram of the rotary drum adopted by the method of the present invention;

[0042] Among them, 1, rotary drum; 2, driving gear; 3, base; 4, motor; 5, crawler; 6, support; 7, outer cylinder shell; 8, intermediate filter screen; 9, inner rotary drum; 10, tooth ring thorn. Specific Embodiments

[0043] The technical solutions of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work fall within the protection scope of the present invention.

[0044] The method for fully utilizing the old mortar on the surface of recycled coarse aggregate to improve the impact resistance of concrete of the present invention includes the following steps:

[0045] S1. Weakening treatment is carried out on the recycled coarse aggregate, and the specific operation is as follows: soaking the recycled coarse aggregate in a weak acid aqueous solution and / or putting the recycled coarse aggregate into a rotary drum with tooth ring thorns on the inner surface for rotation to obtain pretreated recycled coarse aggregate; the elastic modulus of the old mortar on the surface of the pretreated recycled coarse aggregate is 15% - 80% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate;

[0046] S2. A polyurea coating is obtained by reacting an isocyanate component and an amino compound in solvent oil, and then the polyurea coating is sprayed on the surface of the pretreated recycled coarse aggregate to form a polyurea coating on the surface of the pretreated recycled coarse aggregate, obtaining modified recycled coarse aggregate;

[0047] S3. Recycled concrete is prepared by using the modified recycled coarse aggregate obtained in step S2.

[0048] The content of the old mortar on the surface of the recycled coarse aggregate is 18%wt to 30%wt. For example, it can be 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, etc.

[0049] The strength grade of the old mortar on the surface of the recycled coarse aggregate is M5 to M15. For example, it can be M5, M7.5, M10, M15, etc.

[0050] The mud content of the recycled coarse aggregate is less than 4.0%. For example, it can be 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, etc.

[0051] The weak acid aqueous solution is an aqueous solution of at least one of carbonic acid, nitrous acid, acetic acid, hypochlorous acid, and hydrofluoric acid.

[0052] The molar ratio of the isocyanate component to the amino compound is (1.2:1) to (0.95:1.05).

[0053] The isocyanate component includes at least one of aromatic isocyanates, aliphatic isocyanates, and polyisocyanates. The amino compound includes at least one of diamines, polyamines, and amino-terminated polyethers. The solvent oil includes at least one of toluene, naphthenic base oil, and No. 150 solvent oil.

[0054] In the following examples and comparative examples, the original compressive strength of the waste concrete is 26.9 MPa (obtained by actual measurement). The content of the old mortar on the surface of the recycled coarse aggregate is 18%wt to 30%wt; the fine aggregate is natural river sand with a particle size of 0 - 5 mm and a fineness modulus of 2.4; the cement is Huaxin 42.5 ordinary Portland cement; the fly ash is grade I fly ash produced by Wuhan Yangluo Power Plant; the admixture is a polycarboxylate-based high-range water reducer with a water reduction rate of 22 - 28%.

[0055] As Figure 1As shown in the figure, the self-made rotating cylinder used in the following examples and comparative examples includes a rotating cylinder 1. The rotating cylinder 1 is installed on a base 3 through a support 6. A driving gear 2 and a motor 4 are also provided on the base 3. A crawler 5 is connected between the rotating cylinder 1 and the driving gear 2. The motor 4 drives the driving gear 2 to rotate, and the driving gear 2 then drives the rotating cylinder 1 to roll at high speed through the crawler 5. The rotating cylinder 1 includes three parts: an inner rotating cylinder 9, an intermediate filter screen 8, and an outer cylinder shell 7. Tooth ring thorns 10 are arranged circumferentially on the inner surface of the inner rotating cylinder 9 every 30°, and a row of tooth ring thorns 10 is arranged axially along the inner rotating cylinder 9 every 20 mm. The aperture of the intermediate filter screen is 15 mm - 20 mm. When the motor 4 is started, the recycled coarse aggregate inside it is driven to roll by the circumferential rotation of the rotating cylinder 1. The recycled coarse aggregate generates stress concentration by hitting the tooth ring thorns 10 inside the rotating cylinder 1, causing the old mortar paste to crack and generate microcracks.

[0056] Example 1

[0057] This example provides a method for fully utilizing the old mortar on the surface of recycled coarse aggregate to improve the impact resistance of concrete, including the following steps:

[0058] S1. The waste concrete is broken into concrete blocks with a size of about 100 mm × 100 mm × 100 mm by a manual pneumatic pick, and then put into the feeding port of a jaw crusher. After mechanical crushing, recycled coarse aggregate with a particle size of 25 - 40 mm is selected for standby;

[0059] S2. The recycled coarse aggregate obtained in step S1 is immersed in a hypochlorous acid solution for 1.5 hours. The mass fraction of hypochlorous acid in the hypochlorous acid solution is 0.45%, and the pH value is 5.2. After fishing out, washing with water and draining the water for standby;

[0060] S3. The recycled coarse aggregate obtained in step S2 is put into a self-made rotating cylinder. The rotating speed of the rotating cylinder is 150 revolutions per minute, and it rotates for 20 minutes to obtain pretreated recycled coarse aggregate; the elastic modulus of the old mortar on the surface of the obtained pretreated recycled coarse aggregate is 50.9% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate;

[0061] S4. Put 100 g of isophorone diisocyanate (IPDI) (mass fraction 90%), 89 g of polyoxypropylene diamine (mass fraction 98%, average molecular weight 230), and 20 g of xylene into a glass reaction kettle, heat up to 95 ± 5 °C, remove water under vacuum, and react for 30 minutes; then lower the temperature in the reaction kettle to 60 °C, add 20 ml of water, react for 15 minutes to remove the unreacted isocyanate; then raise the temperature in the reaction kettle to 90 °C again, add the silicone defoamer TEGO (mass fraction 0.1%) and the organobismuth catalyst E20, evacuate to remove bubbles, stir for 1 minute, and the stirring speed is 200 revolutions per minute to obtain a polyurea coating; then use an air gun to spray the obtained polyurea coating onto the surface of the recycled coarse aggregate to form a polyurea coating with a thickness of 3 mm to obtain modified recycled coarse aggregate;

[0062] S5. Use the modified recycled coarse aggregate obtained in step S4 to prepare recycled concrete. The prepared recycled concrete contains the following components in parts by weight: 810 parts of modified recycled coarse aggregate, 370 parts of fine aggregate, 185 parts of cement, 160 parts of fly ash, 245 parts of water, and 12 parts of admixture;

[0063] S6. Pour the recycled concrete prepared in step S5 into a cylindrical specimen with dimensions of Φ100 mm × 50 mm, cure it in a standard curing room for 28 days, and then conduct a single-shot impact dynamic compression test. The specific test process is as follows: Use a grinding machine to polish the surfaces of both ends of the specimen with high precision until the average flatness of the loading surface is less than 0.03 mm. Use an SHPB tester to conduct a single-shot impact dynamic compression test on the recycled concrete specimen. Anti-friction sheets are pasted on the loading surfaces at both ends of the specimen. Set the impact air pressure of the emitter to seven levels: 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, and 0.7 MPa. Each group of tests contains 3 cylindrical specimens.

[0064] In this example, all preparation processes were not irradiated with ultraviolet light, and the pouring of recycled concrete was completed within 2 hours after the spraying of the polyurea coating.

[0065] Example 2

[0066] The method of this example is basically the same as that of Example 1, except that the thickness of the polyurea coating in step S4 is 4 mm.

[0067] Example 3

[0068] The method of this example is basically the same as that of Example 1, except that the thickness of the polyurea coating in step S4 is 5 mm.

[0069] Example 4

[0070] The method of this embodiment is basically the same as that of Embodiment 1. The difference is that in this embodiment, step S2 is omitted, and the elastic modulus of the old mortar on the surface of the pretreated recycled coarse aggregate obtained is 77.6% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate.

[0071] Embodiment 5

[0072] The method of this embodiment is basically the same as that of Embodiment 1. The difference is that in this embodiment, step S3 is omitted, and the elastic modulus of the old mortar on the surface of the pretreated recycled coarse aggregate obtained is 69.8% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate.

[0073] Embodiment 6

[0074] The method of this embodiment is basically the same as that of Embodiment 1. The difference is that step S2 is as follows: The recycled coarse aggregate obtained in step S1 is immersed in a hypochlorous acid solution for 0.5 hours. The mass fraction of hypochlorous acid in the hypochlorous acid solution is 1.0%, and the pH value is 4.7. After fishing out, washing with water and draining the water for standby; step S3 is as follows: The recycled coarse aggregate obtained in step S2 is put into a self-made rotating cylinder. The rotation speed of the rotating cylinder is 500 revolutions per minute, and it rotates for 40 minutes. The elastic modulus of the old mortar on the surface of the obtained pretreated recycled coarse aggregate is 20.3% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate.

[0075] Comparative Example 1

[0076] The method of this comparative example is basically the same as that of Embodiment 1. The difference is that steps S2 to S4 are omitted; that is, in this comparative example, the recycled coarse aggregate with a particle size of 25 - 40 mm obtained in step S1 is directly used to prepare recycled concrete.

[0077] Comparative Example 2

[0078] The method of this comparative example is basically the same as that of Embodiment 1. The difference is that the modified recycled coarse aggregate obtained in step S4 is irradiated under ultraviolet light for 4 hours and then the recycled concrete is poured.

[0079] Comparative Example 3

[0080] The method of this comparative example is basically the same as that of Embodiment 1. The difference is that in step S2, the mass fraction of hypochlorous acid in the hypochlorous acid solution is 1.5%, the pH value is 3.2, and the soaking time is 4 hours; the elastic modulus of the old mortar on the surface of the obtained pretreated recycled coarse aggregate is 9.06% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate.

[0081] Comparative Example 4

[0082] The method of this comparative example is basically the same as that of Example 1, except that in step S3, the rotation speed of the self-made rotating cylinder is 500 revolutions per minute and it rotates for 60 minutes. The elastic modulus of the old mortar on the surface of the pretreated recycled coarse aggregate obtained is 11.2% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate.

[0083] Comparative Example 5

[0084] The method of this comparative example is basically the same as that of Example 1, except that steps S2 and S3 are omitted and the recycled coarse aggregate is not pretreated.

[0085] Comparative Example 6

[0086] The method of this comparative example is basically the same as that of Example 1, except that in step S1, recycled coarse aggregates with a particle size of 16 - 31 mm are selected for standby.

[0087] The dynamic stress-strain curves of the recycled concrete specimens of Examples 1 - 6 and Comparative Examples 1 - 6 are sorted out and analyzed, and the load strain rate is calculated according to the following method :

[0088] The input-end load is ;

[0089] The output-end load is ;

[0090] The average stress of the specimen is ;

[0091] The displacement at both ends of the specimen is ;

[0092] ;

[0093] The average strain of the specimen is ;

[0094] The average strain rate is ;

[0095] Based on the assumption of uniformity: ;

[0096] The average strain rate of the specimen is calculated as ;

[0097] In the above formula, E, A 0 , C 0 are the elastic modulus, cross-sectional area and stress pulse velocity of the rod of the SHPB tester; A s , L s are the cross-sectional area and thickness of the specimen; ε i (t), ε r (t), ε t(t) is the strain signals of the incident wave, reflected wave and transmitted wave in the test. The test results are shown in Table 1.

[0098] Table 1 Dynamic compression mechanical property indexes of recycled concrete specimens

[0099]

[0100] It can be seen from the test results in Table 1 that the dynamic compression peak stress of the recycled concrete specimens obtained by the method of the present invention under the impact air pressure of 0.5 MPa is 100.51 - 249.13 Mpa, which is increased by 100.25% - 396.37% compared with the recycled concrete specimens in Comparative Example 1 (the dynamic compression peak stress is 50.19 MPa), indicating the effectiveness of the method of the present invention.

[0101] Comparing Examples 1 - 3, it can be known that with the increase of the thickness of the polyurea coating, the impact resistance of the prepared recycled concrete is significantly improved, and the thickness of the polyurea coating can be designed according to the expected target of the material.

[0102] By comparing Example 1 with Examples 4 - 5, it can be known that by using weak acid immersion or rotary drum pre - cracking weakening treatment alone, the impact resistance of the recycled concrete specimens is weakened.

[0103] Compared with Example 1, the modified recycled coarse aggregate in Comparative Example 2 is irradiated by ultraviolet rays, which causes the aging of the polyurea coating and reduces the energy - dissipation capacity. The dynamic compression peak stress under the impact air pressure of 0.5 MPa is reduced from 122.09 Mpa to 89.18 Mpa.

[0104] Compared with Example 1, in Comparative Example 3, the recycled coarse aggregate is immersed in a strong acid solution, and in Comparative Example 4, the rotary drum runs at a high speed for a long time, causing the old mortar on the surface of the recycled coarse aggregate to peel off, and the residual amount of the mortar is significantly reduced. There is not enough old mortar to resist the shock wave under the impact load, resulting in the reduction of the impact resistance of the recycled concrete.

[0105] Compared with Example 1, in Comparative Example 5, the old mortar on the surface of the recycled coarse aggregate is not subjected to weakening treatment, resulting in the reduction of the energy - dissipation difference at the old mortar - polyurea coating interface, and significantly weakening the impact resistance of the recycled concrete specimens.

[0106] Compared with Example 1, in Comparative Example 6, the particle size of the recycled coarse aggregate is too small, and the residual amount of the old mortar on the surface of the recycled coarse aggregate is low. There is not enough old mortar to resist the shock wave under the impact load, resulting in the reduction of the impact resistance of the recycled concrete.

[0107] In summary, the method for improving the impact resistance of recycled concrete by using the old mortar on the surface of recycled coarse aggregate in the present invention first weakens the recycled coarse aggregate through soaking in weak acid aqueous solution and / or pre-cracking in a rotating drum, and then sprays polyurea elastomer on the surface of the recycled coarse aggregate to artificially create a difference in the energy dissipation capacity between the old mortar and the polyurea coating interface. Under high-speed shock waves, the weak link (old mortar) of the recycled coarse aggregate actively ruptures and sacrifices to absorb and consume energy, achieving a significant improvement in the impact resistance of recycled concrete.

[0108] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for making full use of old mortar on the surface of recycled coarse aggregate to improve the impact resistance of concrete, characterized in that: The following steps are involved: S1. Weakening the recycled coarse aggregate, specifically, the steps are as follows: soaking the recycled coarse aggregate in a weak acid aqueous solution, and / or, placing the recycled coarse aggregate in a rotating drum having a toothed ring thorn on the inner surface for rotation, to obtain pre-treated recycled coarse aggregate; the elastic modulus of the old mortar on the surface of the pre-treated recycled coarse aggregate is 15% to 80% of the elastic modulus of the old mortar on the surface of the original recycled coarse aggregate; S2. The isocyanate component and the amino compound are reacted in solvent oil to obtain a polyurea coating, and then the polyurea coating is sprayed on the surface of the pretreated recycled coarse aggregate to form a polyurea coating on the surface of the pretreated recycled coarse aggregate to obtain a modified recycled coarse aggregate; S3. Preparing recycled concrete using the modified recycled coarse aggregate obtained in step S2.

2. The method according to claim 1, characterized in that In step S1, the content of old mortar on the surface of the recycled coarse aggregate is 18%wt~30%wt.

3. The method according to claim 1, characterized in that In step S1, the mass concentration of the weak acid aqueous solution is 0.3% to 1.0%, and the soaking time is 0.5 to 2 hours.

4. The method according to claim 1, characterized in that: Put the recycled coarse aggregate into a rotating drum with toothed ring thorns on the inner surface, and then rotate it at 150-500 rpm for 20-40 minutes.

5. The method according to claim 1, characterized in that: The rotating cylinder comprises an inner rotating cylinder, a middle filter screen and an outer cylinder shell. The toothed ring thorns are arranged on the inner surface of the inner rotating cylinder and are evenly distributed along the circumferential direction of the inner rotating cylinder. The cross section of the toothed ring thorns is triangular.

6. The method according to claim 1, characterized in that The preparation process of step S2 and step S3 must not be irradiated by ultraviolet rays.

7. The method according to claim 1, characterized in that In step S3, the pouring of the recycled concrete is completed within 5 hours after the polyurea coating is sprayed.

8. The method according to claim 1, characterized in that The isocyanate component includes at least one of aromatic isocyanate, aliphatic isocyanate and polyisocyanate; the amino compound includes at least one of polyamine and amino-terminated polyether; and the solvent oil includes at least one of toluene, cycloalkyl base oil and No. 150 solvent oil.

9. The recycled concrete prepared by the method according to any one of claims 1 to 8, characterized in that: The invention comprises the following components in parts by weight: 750-920 parts of modified recycled coarse aggregate, 120-800 parts of fine aggregate, 150-225 parts of cement, 130-195 parts of fly ash, 140-310 parts of water and 3-20 parts of admixture.

10. Use of the recycled concrete prepared by the method according to any one of claims 1 to 8 in anti-riot and anti-impact protective structures.

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