| In order to improve the shrinkage cracking of road concrete and improve the durability of pavement,lightweight aggregate internal curing concrete has received extensive attention in recent years.Based on its unique porous characteristics,lightweight fine aggregate can release the water absorbed by aggregate during pre-wetting during cement hydration,improve the hydration environment of cementitious materials,and optimize the microstructure of matrix,thus effectively reducing the shrinkage and early cracking of matrix.However,due to the low strength of lightweight aggregate,its influence on the strength and toughness of concrete is still controversial.The incorporation of hybrid fibers into the matrix is considered to be an effective method for strengthening and toughening modification of concrete,but the existing research focuses on traditional fibers such as steel-polypropylene and carbon-polypropylene.Compared with steel and carbon fiber,basalt fiber also has high elastic modulus,good chemical stability and low price.Because the density is similar to that of mortar,it can overcome the shortcomings of traditional steel fiber.In view of this,this paper takes basalt-polypropylene hybrid fiber lightweight aggregate concrete(BPLC)as the research object.Based on the internal curing of lightweight aggregate and the strengthening and toughening effect of basalt-polypropylene hybrid fiber,the multi-level optimization of slurry microstructure is realized to improve the performance of road concrete.The effects of various factors on the mechanical properties,microstructure,hydration product composition and meso-pore structure of concrete were clarified by changing the factors such as lightweight aggregate content,basalt-polypropylene fiber content,fiber aspect ratio and mineral admixture content,and the synergistic mechanism of lightweight aggregate,basalt-polypropylene hybrid fiber and mineral admixture was revealed.The results show that :(1)The introduction of pre-wetted lightweight fine aggregate can significantly improve the flexural strength of the matrix,but lead to a significant decrease in compressive strength.Compared with the concrete without lightweight aggregate,the flexural tensile strength of the concrete with 40%lightweight aggregate increased by 14.07%,the splitting tensile strength increased by 4.95%,and the compressive strength decreased by 19.39%.The porous characteristics of lightweight and fine aggregate lead to a significant reduction in its own strength,especially easy to become a weak area when the matrix is destroyed by compression.Therefore,large amount of lightweight and fine aggregate has a significant negative reduction effect on the compressive strength of concrete.(2)After the introduction of basalt-polypropylene hybrid fiber,the concrete changed from brittle failure to ductile failure,and the compressive strength,splitting tensile strength,flexural strength and flexural toughness of the matrix were greatly improved.Compared with ordinary concrete,when the content of lightweight fine aggregate is 20%,the total content of basalt-polypropylene hybrid fiber is0.20% and the ratio of basalt fiber to polypropylene fiber is 3:1,the flexural performance of concrete is optimal,and the flexural tensile strength is increased by 39.26%.(3)The introduction of mineral admixtures can significantly improve the macroscopic mechanical properties of BPLC.When the total content of mineral admixtures is 25% and the ratio of fly ash: slag: metakaolin is 2:2:1,the strength and toughness of BPLC are obviously improved.The compressive strength,splitting tensile strength and flexural strength are increased by 19.64%,6.54%and 19.26% respectively.(4)From the results of thermogravimetric analysis and microstructure analysis,it can be seen that the internal curing water released by pre-wetting lightweight fine aggregate can promote the formation of hydrated calcium silicate gel,significantly improve the hydration degree of cement,optimize the interfacial transition zone between aggregate and cement mortar,improve the cementation between basalt-polypropylene hybrid fiber and matrix,enhance the bridging crack effect of basalt-polypropylene hybrid fiber,improve the integrity of matrix,and weaken the negative effect of low strength of lightweight fine aggregate on concrete performance.Therefore,there is a mutual’positive hybrid effect’ between pre-wetted lightweight fine aggregate and basalt-polypropylene hybrid fiber.(5)The results of industrial CT scanning show that under low fiber content,the mixture of prewetted lightweight fine aggregate and basalt-polypropylene fiber can significantly reduce the porosity and average pore size of concrete and optimize the pore size distribution of concrete.When the content of lightweight aggregate and basalt-polypropylene fiber is 20% and 0.15%,0.05%respectively,the porosity and average pore size of concrete reach the lowest,which is 42.47% and30.65% lower than that of ordinary concrete.After adding mineral admixture into BPLC,the pore structure of concrete is further optimized.When the mineral admixture content is 10% of fly ash,10%of slag and 5% of metakaolin,the porosity and average pore size decrease by 16.22% and 11.36%.(6)Based on fractal dimension and sphericity,the pore characteristics such as pore distribution characteristics and pore complexity of lightweight aggregate,basalt-polypropylene hybrid fiber and mineral admixture are clarified,and the mathematical relationship function between BPLC pore characteristics and macroscopic mechanical properties is constructed.Through the quantitative characterization of the relationship between pore structure characteristics and mechanical properties of concrete,it is confirmed that the internal pores of lightweight fine aggregate have a significant reduction effect on the compressive strength of BPLC,while the effect of internal curing of lightweight fine aggregate on the flexural strength is more significant.(7)The three-dimensional distribution of basalt-polypropylene hybrid fiber in concrete was extracted.By introducing fiber orientation angle and fiber orientation coefficient,the basaltpolypropylene hybrid fibers in BPLC were statistically analyzed to clarify the relationship between basalt-polypropylene hybrid fiber and the mechanical properties of BPLC.The results show that the basalt-polypropylene fiber is characterized by three-dimensional random distribution,and there is a good quadratic function relationship between fiber orientation coefficient and flexural tensile strength. |
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