Experiment And Research On Microstructure And SHPB Of Ready-mixed Shrinkage-compensating Concrete

The dry-mixed materials, which are pre-mixed by compounding the sand, gravel and water at a certain mass ratio and used as shotcrete, can effectively reduce the dust concentration at the time of sprayed concrete construction, improve the construction efficiency, and promote the specialization and industrialization of concrete materials supply. However, restricted by procedure and costs of producing the dry-mixed materials, the sand in the dry-mixed materials does not reach the full-dry state, and often contains a small amount of water. Then, pre-hydration reactions between the cement and water occur, so that the structure of concrete that is made of dry-mixed materials (ready-mixed concrete) is loose and the internal porosity will be increased. The problems of poor durability and prone to cracking are caused when the concrete is applied in the engineering. Therefore, the ready-mixed shrinkage-compensating concrete is proposed by adding the expansive agent to the dry-mixed materials in order to improve the above-mentioned problems.Combined with the project "Study on microstructure and mechanical properties of ready-mixed shrinkage-compensating shotcrete" of the Doctoral Fund of Priority Development Fields by the Ministry of Education, theoretical analysis and indoor experiment on microstructure and impact resistance of the pre-mixed shrinkage compensating concrete samples, with respect to different moisture contents of 0.5%, 1.0%,1.5% and 2.0%, different storage periods of 3d,7d, 10d,15d,20d and 30d, and different expansive agent dosages of 0%,4%,6%,8% and 10%, were respectively carried out. The microstructure testing included the scanning electron microscope test (SEM), the X-ray diffraction test (XRD) and the porosity test, and the impact resistance testing included the SHPB dynamic compressive mechanical properties test. The main conclusions can be drawn as follows.(1) The expansive agent can effectively promote the growth of the needlelike ettringite crystals (AFt). With the increasing expansive agent dosage, the amount of the needlelike ettringite crystals (AFt) is obviously increased. Besides, the needlelike ettringite crystals (AFt) constantly grow in the holes and micro cracks within the concrete and gradually lap together to form a mutually staggered network structure. The network structure runs through the flocculation and mesh shaped C-S-H gel, leading to gradually compacted internal structure of the concrete. When the expansive agent dosage is 6%, the concrete structure is relatively compacted. However, when the expansive agent dosage is larger than 6%, apart from the dense gel continuum and initial cracks in the concrete, several new micro cracks are induced due to large expansion performances. Moreover, with the increasing expansive agent dosage, the size of these new cracks also increases gradually.(2) For an identical storage period of the dry-mixed materials, with increasing moisture content, obvious changes happen to the concrete. The number of holes in the concrete increase and the hole size increases slightly, leading to decreasing density of the concrete. When the moisture content is 0.5%, the density of concrete is better.(3) Sand moisture content of 0.5% and 1.0%, storage period respectively within 30d and 15d, little influence on the results of the change storage period to the micro structure of ready-mixed shrinkage-compensating concrete. Storage period has little influence on the microstructure of ready-mixed shrinkage-compensating concrete when the sand moisture content are 1.5% and 2.0% and storage period within 10d and 7d respectively.(4) In a certain diffraction angle range of XRD atlas for the pre-mixed cement slurry with 5 kinds of expansive agent dosages, the characteristic peaks of multiple phase such as CaCO3, C2S, C3S, CH, AFt, SiO2 and trace C-S-H are concentrated, and hydration products of the cement slurry with various expansive agent dosages are basically the same. The strength of the cement slurry increases gradually when the expansive agent dosage increases from 0% to 6%. And the strength of the cement slurry decreases gradually when the expansive agent dosage increases from 6% to 10%. The strength of cement slurry with the expansive agent dosage of 6% is larger than that for the cement slurry with other 4 kinds of expansive agent dosages. So, the expansive agent dosage of 6% is the most suitable.(5) When the expansive agent dosage is 0%-6%, the porosity of concrete decreases gradually with the increasing expansive agent dosage. However, when the expansive agent dosage is 6%-10%, the porosity of concrete increases gradually with the increasing expansive agent dosage, with the minimum porosity of concrete obtained with the expansive agent dosage of 6%. When the moisture content of the pre-mixed materials is larger than 1.0%, the moisture content has a significant influence on the porosity of the pre-mixed shrinkage compensating concrete. When the moisture content of the pre-mixed materials is lower than 1.0%, the storage period has little effect on the porosity of the concrete, but when the moisture content of the pre-mixed materials is larger than 1.0%, the effect of storage period on the porosity is obvious.(6) With the increase of moisture content and the extension of storage period, the dynamic compressive strength of ready-mixed shrinkage-compensating concrete was decreases. When the moisture content is 0.5%, the decrease of strength of storage period of 3 days slower, strength decline rapidly between 3 to 7 days, when the moisture content is 1.0% and the moisture content is 1.5%, the decrease of strength of storage period of seven days faster, lower slower between 7 to 10 days.