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The durability of a cement-based material is mainly dependent on its permeability. Modifications of porosity, pore-structure and pore-connectivity could have significant impacts on permeability improvement, which eventually leads to more durable materials. One of the most efficient solutions in this regard is to use permeability reducing admixtures (PRA). Among these admixtures for those structures exposed to hydro-static pressure, crystalline waterproofing admixtures (CWA) have been serving in the construction industries for decades and according to ACI 212—chemical admixtures’ report, it has proven its capability in permeability reduction and durability-enhancement. However, there is substantial research being done on its durability properties at the macro level but very limited information available regarding its microstructural features and chemical characteristics at the micro level. Hence, this paper presents one of the first reported attempts to characterize microstructural and chemical elements of hydration products for cementitious composites with CWA called K, P and X using Scanning Electron Microscopy (SEM). Backscattered SEM images taken from a polished-section of one CWA type—K—admixture were analyzed in ImageJ to obtain paste matrix porosity, indicating a lower value for the CWA-K mixture. X-ray analysis and SEM micrographs of polished sections were examined to identify chemical compositions based on atomic ratio plots and brightness differences in backscatter-SEM images. To detect chemical elements and the nature of formed crystals, the fractured surfaces of three different CWA mixtures were examined. Cementitious composites with K admixture indicated needle-like crystal formation—though different from ettringite; X and P admixtures showed sulfur peaks in Energy Dispersive Spectrum (EDS) spectra, like ettringite. SEM images and X-ray analyses of mixtures incorporating Portland Limestone Cement (PLC) indicated lower-than-average porosity but showed different Si/Ca and Al/Ca atomic ratios.

This study aims to evaluate self-healing properties and recovered dynamic moduli of engineered polypropylene fiber reinforced concrete using non-destructive resonant frequency testing. Two types of polypropylene fibers (0.3% micro and 0.6% macro) and two curing conditions have been investigated: Water curing (at~25 Celsius) and air curing. The Impact Resonance Method (IRM) has been conducted in both transverse and longitudinal modes on concrete cylinders prior/post crack induction and post healing of cracks. Specimens were pre-cracked at 14 days, obtaining values of crack width in the range of 0.10-0.50 mm. Addition of polypropylene fibers improved the dynamic response of concrete post-cracking by maintaining a fraction of the original resonant frequency and elastic properties. Macro fibers showed better improvement in crack bridging while micro fiber showed a significant recovery of the elastic properties. The results also indicated that air-cured Polypropylene Fiber Reinforced Concrete (PFRC) cylinders produced~300 Hz lower resonant frequencies when compared to water-cured cylinders. The analyses showed that those specimens with micro fibers exhibited a higher recovery of dynamic elastic moduli.

Concrete is prone to cracking when subjected to tensile forces because of its low tensile strength. The cracking potential is an even bigger concern when concrete is relatively young and is in the plastic stage. Cracks induced early can grow with time because of drying shrinkage and with application of service loads. Concrete, which is otherwise impermeable, allows for free passage of moisture and other deleterious chemicals when it is cracked, leading to reduced durability of the material and, in many cases, reduced service life of the structure. The severity of some of these issues can be alleviated because of an inherent property of concrete known as “self-sealing.” As the name suggests, “self-sealing” allows for the cracks (of limited width) to be sealed on their own over a period of time. However, currently there is no standard test technique to quantify this property of concrete and other cement-based materials, such as mortar. An innovative and straightforward technique was developed by the authors and is presented in this paper. In this technique, a standard crack is induced in concrete cylinders using a standard crack-inducing jig (SCIJ). The specimens are then inserted in special rubber sleeves and this assembly is then subjected to a constant water head. The reduction in flow through the cracked specimen is measured at a given time to compare the performance of different specimens. This technique can also be used to compare the performance of concrete mixes modified using various admixtures. This paper describes this innovative test technique and includes sample test results to explain the analysis process proposed by the authors.