Significance of transition zones on physical and mechanical properties of Portland cement mortar
Abstract
In this research the mechanical properties of the transition zone between cement paste and aggregate, and their relationships to microstructure are investigated. The mechanical properties studied include dynamic modulus of elasticity (Ed), dynamic shear modulus (Gd), logarithmic decrement of damping $(\delta),$ flexural tensile strength (MOR), and compressive strength. In addition, the effects of changing the water-to-cementitious material ratio by mass (W/C), aggregate type, volume fraction of aggregate (Va), and silica fume substitution, on these properties were also studied. Since no specific techniques are available to measure the mechanical properties of the transition zone, an alternate way to study these properties was developed and used in this research. Microstructural aspects of the transition zone were studied by using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). MIP was used to measure the volume and size distribution of pores in the specimens. SEM in the backscattered mode was used to analyze pores and crack patterns in the specimens. In addition, energy dispersive X-ray spectrometry (EDS) was applied to study the chemical compositions of aggregate, bulk paste, and transition zone. Loss-on-Ignition (LOI) method was used to compare the degree of hydration of specimens. A criterion based on water quantity and the specific surface area of aggregate by mass (SA) in the specimen was developed to eliminate biased data from the analysis process. This criterion was used to detect mixing and compaction problems that may have resulted in erroneous values of mechanical properties of specimen. In order to realize the compaction condition of the fresh mixture, an index of compaction (called gross porosity), was introduced. The three-phase model of Hashin-Shtrikman bounds was employed, tested, and validated with the experimental data from this research. A modification of this model linked the theory of Hashin-Shtrikman bounds to the results of this research on dynamic moduli of the transition zone. A form of optimal water content is recommended. This optimal water content may be used for a mixture to gain its possibly highest moduli, strengths, and density. Thus, the rule of the optimal water content may potentially be applied to optimize the mixture design for conventional concrete and high-strength concrete.
Degree
Ph.D.
Advisors
Cohen, Purdue University.
Subject Area
Civil engineering
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