دانلود رایگان مقاله اثرات عرض ترک، محتوای سرباره و تهویه قلیایی در بهبود کامپوزیت سیمانی مهندسی

Abstract

Engineered cementitious composite (ECC) is a unique group of fiber-reinforced strain-hardening cementitious composites exhibiting crack self-healing. Due to the absence of coarse aggregates in the ECC mix design, high amount of supplementary cementitious materials (SCM) are generally used to reduce the cement content. The inclusion of slag not only changes the chemical compositionss of the matrix but also alters the crack width of ECC. Both may influence the autogenous healing potential of the slag-based ECC. This paper systematically investigates the influence of the individual factor, i.e. slag content, crack width, and environmental alkalinity, on the autogenous healing efficiency of ECC. Specifically, single-cracked ECC specimens with different slag content and crack width were conditioned under water/dry or NaOH/dry cycles. The autogenous healing performance was evaluated based on crack width reduction, resonant frequency recovery and microstructure analysis. The results show that autogenous healing is determined by a couple effect of physical properties (crack width), chemical compositionss (slag content), and environmental conditions (conditioning alkalinity). At a given slag content and certain alkalinity, there exists a maximum allowable crack width for complete healing, beyond which only partial or no healing would happen. The dominant healing product for the water/dry conditioning is CaCO3 while the NaOH/dry cycles promote slag hydration and results in the formation of Csingle bondSsingle bondH and CaCO3 as main healing products. It is concluded that CaCO3 precipitation is more effective to engage autogenous healing than the formation of Csingle bondSsingle bondH. The concept to associate allowable crack width and slag content is proposed, which would guides ingredients selection and component tailoring to engage robust autogenous healing in ECC in the future.

4. Conclusions

This paper studied the effects of slag content, crack width, and conditioning alkalinity on the autogenous healing behavior of slagbased ECC. Single-cracked ECC specimens with three different slag contents (cement replacement ratio at 0%, 30%, and 60%) and crack width up to 300 mm were conditioned under water/dry or NaOH(pH ¼ 13)/dry cycles. The healing performance was evaluated based on crack width reduction, resonant frequency recovery, and SEM/EDX analysis. Following conclusions can be made in current study.  ECC autogenous healing is determined by a coupled effect of physical properties (crack width), chemical compositions (slag content), and environmental conditions (conditioning alkalinity). At a given slag content and certain alkalinity, there exists a maximum allowable crack width for complete healing, beyond which only partial or no healing would happen.  The addition of slag up to 30% enables complete healing of larger crack (86 mm and 80 mm for water/dry and NaOH/dry conditioning cycles). After that, the maximum allowable crack width to engage complete healing reduces with slag content. Water/ dray cycles caused more crack width reduction compared to NaOH/dry cycles. Optimum slag addition, smaller crack width and water/dry conditioning cycle favor self-healing in ECC.  CaCO3 is the dominant healing products for samples subjected to the water/dry cycle while a mixture of CaCO3 and CeSeH represents the major healing products for samples subjected to the NaOH/dry cycle.  NaOH/dry conditioning promotes slag hydration but suppresses the formation of CaCO3 precipitates in cracks as healing products. CaCO3 precipitation through carbonation is a more effective mean to engage self-healing than the formation of CeSeH through alkali-activated slag hydration.  In the future, a complete design chart for ECC autogenous healing should consider the coupled effect completely, i.e. including the effect of physical properties, chemical compositions, and environmental conditions, as the concept illustrated in this paper.