AUTHOR=Dou Hui , Xu Huaxin , Xie Junjie 

TITLE=Long-term mechanical performance of high fluidity fiber reinforced concrete modified by metakaolin

JOURNAL=Frontiers in Materials

VOLUME=10

YEAR=2023

URL=https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2023.1162053

DOI=10.3389/fmats.2023.1162053

ISSN=2296-8016

ABSTRACT=<p>To clarify the long-term strength and toughness of metakaolin (MK) and steel fiber (SF) modified concrete with higher fluidity and water/binder ratio, a series of tests including slump tests, compression tests, splitting tests, digital image processing and Scanning Electron Microscope (SEM) tests were performed on MK-SF concrete cured for 7–360 days. Results reveal that the slump of fresh concrete decreased with an increase in the MK and SF replacement rates. Moreover, the impact of MK on the slump of steel fiber reinforced concrete (SFRC) was more pronounced when combined with a lower water/binder ratio, resulting in increased viscosity. At the pre-peak stress region of the strain-stress curve, the compressive strength <italic>f</italic><sub>c</sub>, tensile strength <italic>f</italic><sub>t</sub>, Young’s modulus <italic>E</italic><sub>c</sub>, elastic modulus <italic>E</italic><sub>0,</sub> and tensile strain at peak stress <italic>ε</italic><sub>t-max</sub> of high fluidity MK-SF concrete increased with increasing MK and SF admixing ratio, regardless of curing age. Notably, the coupling effects of MK and SF became more prominent after long-term curing. Without MK incorporation, the effects of SF and curing time on the above indices were relatively implicit. At the post-peak stress region of strain-stress curves, there existed a residual stage. The inclusion of MK significantly improved the long-term residual strength and strain of SFRC. Additionally, the toughness index <italic>M</italic><sub>c</sub>, which represents the total area of the compressive strain-stress curve containing both the pre-peak and post-peak regions, also exhibited substantial development with curing time, primarily attributed to the incorporation of MK and SF. The coupling of MK and SF led to a transformation of the concrete failure mode from brittle to ductile. Regression analysis reveals that a linear equation adequately described the long-term relationships of <italic>f</italic><sub>c</sub>-<italic>f</italic><sub>t</sub>, <italic>f</italic><sub>c</sub>-<italic>E</italic><sub>c</sub>, <italic>f</italic><sub>c</sub>-<italic>E</italic><sub>0</sub>, <italic>f</italic><sub>c</sub>-<italic>M</italic><sub>c,</sub> and <italic>f</italic><sub>c</sub>-<italic>ε</italic><sub>t-max</sub> in MK-modified SFRC. Based on the testing data, a relative strength or toughness index λ and a new generalized hyperbola model were proposed to predict the long-term mechanical behavior mentioned above. Through crack morphology and microstructure analysis, the distinct roles of MK and SF in the composite material were examined.</p>