多尺度混杂纤维增强珊瑚混凝土的单轴压缩性能

Uniaxial Compressive Behavior of Multi-Scale Hybrid Fiber Reinforced Coral Concrete

珊瑚混凝土是进行岛礁开发建设的重要建筑材料之一,多尺度混杂纤维掺入可从多尺度改善珊瑚混凝土的受力性能,从而拓展其应用范围。以碳纤维(CFs)和玄武岩纤维(BFs)作为微丝纤维、以塑钢纤维(PSFs)作为粗纤维制备了混杂纤维增强珊瑚混凝土(HFRCC),通过单轴压缩试验研究了其受力性能。结果表明:HFRCC的单轴受压全过程可分为线弹性、裂缝稳定发展、裂缝不稳定发展和峰后破坏4个阶段,微丝纤维主要在峰前裂缝发展阶段发挥作用,而粗纤维主要在峰后阶段发挥作用。综合考虑强度和韧性指标,当CFs、BFs体积掺量分别为0.15%,PSFs掺量为7 kg/m^(3)时,HFRCC表现出更优异的性能;与基准组相比,峰值应力、峰值应变和峰后压缩韧性分别提高了6.22%、38.54%和116.44%。基于CEB-FIP和过镇海的本构模型建立了适用于HFRCC的修正本构模型,模型计算曲线与实测曲线吻合较好。通过修正本构模型探讨了HFRCC的损伤演化过程,发现CFs、BFs和PSFs混杂掺入可以协同延缓珊瑚混凝土的损伤进程。

Introduction Using coral concrete made from coral aggregates instead of conventional coarse and fine aggregates and seawater instead of freshwater for island reef construction can reduce costs and shorten the construction period for the development of the marine economy.However,coral concrete has some challenges such as high brittleness,poor toughness,and limited durability,thus restricting its application in engineering.Previous studies confirm that incorporating fibers can effectively enhance the mechanical properties of concrete.Therefore,in coral concrete,some microfibers with high elastic modulus are considered to reduce the microcracks,and some macro-fibers with high fracture toughness and elongation are considered to bridge the macrocracks,enhancing its mechanical performance in multiple scales.Carbon fibers(CFs)have the maximum elasticity modulus among commonly used micro-fibers.However,replacing carbon fibers with more economical and relatively high-modulus basalt fibers(BFs)is an effective approach.In this paper,CFs and BFs were selected as microfibers.High-elongation plastic steel fibers(PSFs)were also selected as macro-fibers instead of the steel fibers most commonly used in concrete.The CFs,BFs,and PSFs were incorporated into coral concrete.The mechanical properties of the hybrid fibers-reinforced coral concrete(HFRCC)under axial compression were investigated,and the corresponding constitutive model was proposed.Methods Cement used was ordinary Portland cement P·O 42.5,adhering to the code GB175.A coarse aggregate used was a crushed coral with a continuous gradation of 5 mm to 20 mm and a tube compressive strength of 3.1 MPa.A fine aggregate used was a coral sand with a fineness modulus of 3.0.Seawater was taken from Beibu Gulf,Guangxi,China.The admixtures include a polycarboxylate superplasticizer,a hydroxypropyl methyl cellulose(HPMC),and an antifoaming agent.The study involved sixteen mix proportions for HFRCCs with different dosages of CFs,BFs and PSFs.A uniaxial compression test was conducted on prismatic specimens with the dimensions of 100 mm×100 mm×300 mm by a model RMT-201 electro-hydraulic servo testing machine(1500 k N).To measure the axial deformation of specimens,two LVDTs were placed in the middle of the specimen.Simultaneously,during the loading,a digital image correlation(DIC)system was arranged at the front and rear positions of the specimen to record complete deformations throughout the specimen failure.The failure process,failure modes,and stress-strain curves of the specimens were determined via uniaxial compression tests.Key parameters such as peak stress,peak strain,residual stress,and ultimate strain were extracted from the stress-strain curves.The strain field and failure process were analyzed by digital image correlation(DIC)system.In addition,a modified constitutive model suitable for HFRCC was also established based on the experimental data.Results and discussion The uniaxial compression process of HFRCC can be divided into four stages,i.e.,linear elasticity,stable crack development,unstable crack propagation,and post-peak failure.Microfibers predominantly affect the crack development before the peak stage,while macro-fibers play a crucial role in the post-peak stage.In the post-peak failure stage,a combination of DIC system obtained horizontal strain distribution cloud maps reveals that HFRCC with micro-fibers exhibits a more uniform strain distribution,compared to HFRCC without fibers or with only macro-fibers.However,in HFRCC with macro-fibers,the main through crack has a width of approximately 1 mm,which is smaller than that of HFRCC without fibers or with only micro-fibers(approximately 1.5 mm).The incorporation of high-modulus microfibers and high-fracture toughness macro-fibers into coral concrete enhances its mechanical properties in multiple scales.This is attributed to a ability of micro-fibers at low dosages to form a randomly oriented fiber network in the concrete matrix.This network bridges the micro-cracks,suppresses the extension of initial cracks,and transfers stress across micro-cracks to reduce stress concentration as well as enhances the performance of the interface transition zone.However,most of them break or pull out after macro-crack formation due to the high modulus and low fracture toughness of micro-fibers.Also,macro-fibers with a higher fracture toughness can continue to bridge macro-cracks and transfer stress between them even after the formation of macro-cracks.For the both strength and toughness,HFRCC exhibits a superior performance when the volume fractions of CFs and BFs are 0.15%each,and PSFs dosage is 7 kg/m^(3).Compared to the coral concrete without fibers,the peak stress,peak strain,and post-peak compressive toughness of HFRCC are increased by 6.22%,38.54%and 116.44%,respectively.A modified constitutive model suitable for HFRCC is proposed based on the CEB-FIP model and the Guo Zhenhai model,and the calculated data of the model fit well with the measured results.The damage evolution process of HFRCC investigated by the modified constitutive model reveals that the incorporation of CFs,BFs,and PSFs can synergistically delay the damage progression of coral concrete.Conclusions The uniaxial compression process of HFRCCs could be divided into four stages,i.e.,linear elasticity,stable crack development,unstable crack propagation,and post-peak failure.Microfibers played a crucial role in expanding microcracks during the first three stages,showing beneficial effects on the peak stress,peak strain,and pre-peak compressive toughness.Macro-fibers acted as bridging elements for macro-cracks in the peak failure stage,demonstrating a more significant impact on the residual stress,ultimate strain,and post-peak compressive toughness.Each type of fiber had an optimal dosage level,and the reasonable dosage of fibers could generate positive synergistic effects.When only micro-fibers were added,BFs contributed more to the increase in peak stress rather than CFs,but their improvement in peak strain and compressive toughness was slightly lower than that of CFs.This indicated that for engineering applications with lower toughness requirements,it could be feasible to replace CFs entirely with more cost-effective BFs.The calculated data of the modified constitutive model aligned well with the experimental results.This work investigated the damage evolution process of HFRCC,revealing that the incorporation of CFs,BFs,and PSFs could collaboratively delay the damage progression of coral concrete.