| "Carbon Peaking" and "Carbon Neutrality" are the strategic goals proposed by China.Intelligent manufacturing is the development plan proposed by China’s "14th Five Year Plan".Green and Intelligent are the two wings of high-quality development of the construction industry.In this paper,a kind of carbon fiber reinforced green concrete(CFRGC)used in the field of 3D printing was designed.The mechanical properties,durability,pore characteristics and acoustic emission(AE)characteristics of the 3D printing samples and mold casting samples of CFRGC were studied.The research results are as follows:(1)Through the trial test of optimizing mix proportion,a kind of CFRGC with slag Portland cement,silica fume and fly ash as binder was designed.The binder content of this CFRGC is 645kg/m3,and cement clinker accounts for about 38-61%of the cementitious material.The printability of the CFRGC was verified by extrudability test and buildability test.(2)The flexural strength test and compressive strength test were carried out on the 3D printing samples and mold casting samples of CFRGC under 5 different mix proportions.The results show that,with 2.5%(mass percentage of binder)of MCF content,the flexural strength and compressive strength of the 3D printing samples are the highest,which means that the milled carbon fiber(MCF)plays a reinforcing materials role in CFRGC.Compared with the mold casting process,the 3D printing process enhances the flexural strength of the samples,but weakens the compressive strength.When the content of MCF is 2.5%,the flexural strength of 3D printing samples is 61.25%higher than that of mold casting samples.(3)The chloride ion penetration resistance test,frost resistance test and carbonization test were carried out on the 3D printing samples and mold casting samples of CFRGC under three different mix proportions.The results show that the CFRGC samples have strong chloride ion penetration resistance and normal carbonation resistance.The frost resistance of 3D printing samples is weak.The 3D printing samples all fail due to the cracking occurring within the printing layers before freeze-thaw cycles less than 125 times.(4)By applying X-ray computed tomography(X-ray CT)and mercury intrusion porosimetry(MIP),the characteristics of full-size pores in the range from"nanometer gel pores" to "centimeter printed voids" were studied for 3D printing samples and mold castings samples of CFRGC under three different mix proportions.The CT scanning images were processed and the 2D pore feature information was extracted by using MATLAB self programming.3D pore characteristics were obtained after CFRGC samples and pores in the sample were reconstructed by applying Avizo software.The results show that the 3D printing process can compact the CFRGC and reduce the number of pores in the printing filament,but the total porosity of the sample does not decrease due to the large voids generated by the 3D printing process.An irregularity ratio was proposed to characterize the irregular shape of pores.The results show that the high irregularity ratio of pores and the connectivity between 3D printing layers in samples lead to the stress concentration phenomenon which will decrease the compressive strength during compression.The most probable diameter in micro pores was found,and the results show that the gel pores with a pore size of 7-11nm are the most developed.(5)The AE technology was used to collect the AE signal generated by the crack failure in the specimen during the three-point bending test of the 3D printed samples and mold casting samples with five different mix proportions.The AE characteristic information was studied and analyzed.The results showed that when the crack failure occurred in the specimen,the frequency of the AE signal generated by the specimen with more carbon fiber content was lower,as well as the frequency of AE signal generated by 3D printing process samples with the same content of MCF.A method was proposed to characterize the internal crack development mode of the specimen.The results show that the linear fitting R2 value of the AE counts(cumulative)curve and the MARSE(cumulative)curve varying with the loading time can characterize the internal crack development mode of the specimen.When R2 value tends to 1,it indicates that the internal crack development of the specimen is orderly and uniform.A quantitative analysis model for quantifying the specimen manufacturing process(3D printing,mold casting)to solve the relationship between the flexural strength and the specimen manufacturing process,as well as the content of MCF was proposed.The results show that the model is suitable for finding out the relationship between the proportion of AE signals with Peak Frequency in the range of 0～50kHz,the proportion of AE signals with Centroid Frequency in the range of 0～100kHz,the linear fitting R2 value of AE counts(cumulative)curve,the linear fitting R2 value of MARSE(cumulative)curve,the proportion of AE signals with Time Duration in the range of 0～2000μs et al.,a total of five AE statistical characteristic parameters and the specimen manufacturing process,as well as the amount of MCF.The first four AE statistical characteristic parameters have a positive correlation with the flexural strength,while the last one has a negative correlation with the flexural strength.That is to say,under the action of external force,the internal cracks of the test samples with MCF under the 3D printing process tend to develop evenly,and generate more acoustic emission signals with low Peak Frequency and low Centroid Frequency,and generate less transient AE signals with short duration.The research results of this paper have important reference significance for the promotion of 3D printing green concrete and the real-time structural health monitoring of 3D printing concrete building components using AE technology in the future. |
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