Understanding the significance of aggregate composition, aggregate pore structure and paste void characteristics on the properties of cementitious systems

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Melugiri Shankaramurthy, Bharath
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Wang, Kejin
Taylor, Peter
Williams, Christopher
Beresnev, Igor
Buss, Ashley
Sturgill, Roy
Committee Member
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Civil, Construction, and Environmental Engineering
This research (summarized in chapters 2−5) was mainly focused on understanding by and large the influence of pore and void structure on the properties of cementitious systems (mortars and concretes). The major emphasis was on cementitious systems of higher performance (High Performance Concrete-HPC and Ultra-High Performance Concrete-UHPC) that are more relevant to the present day research and practice (for field application and installation). The main objectives of this research program were to (1) characterize high porosity aggregates and concrete systems containing them, (2) investigate the effect of aggregate composition and pore characteristics on transport properties of concrete, (3) understand the mechanism of water absorption due to composite pore structure, (4) study the influence of depressurized- or low pressure mixing (or vacuum mixing) on the voids structure and associated properties in cementitious mortars. Significant efforts have been made to shed light on the role of porous natural aggregates (normal weight- and not light weight-), which by far was not researched to a greater detail. Despite aggregate constituting 60−70% of concrete (by volume), the importance of their physical (for e.g., pore structure and features) and chemical properties (e.g., composition, mineralogy) for the transport characteristics of composite concrete are often overlooked. As a result, at first, related research pertaining to this topic (which are extremely rare nonetheless) were identified and critically studied to understand the major knowledge gaps. Following this, an in-depth literature review was conducted regarding the choice of test methods for evaluating the permeability of cementitious systems. Based on this, relevant test methods were carefully selected to capture the required information to greater details. Accordingly, three (out of the four) studies (Chapters 2, 3 and 4) in this research program are focused on the key roles of aggregate and their properties in the context of HPC whereas Chapter 5 aimed at understanding the effects of low pressure mixing (termed vacuum mixing) on the voids structure modification and properties of different cementitious mortars (normal strength and high performance mortars, and UHPC). Specifically, Chapter 2 discusses the relevance of aggregate composition to the development of concrete resistivity (an electrical indication of concrete durability). Using limestone and dolostone aggregates, this study highlights the importance of aggregate contribution to the changes in the aggregate-paste micromorphology and potential modifications in the concrete pore solution chemistry. For the first time, the possible mechanisms leading to such modifications are theorized based on a detailed microstructural investigation using thin section petrographic analysis and scanning electron microscopy. The research outcomes are particularly important as existing concrete surface resistivity specifications (established by various state and federal agencies) do not consider any contributions from aggregate but rather highlights the contribution only from the paste composition for the concrete resistivity development. Based on the obtained results, requirement for a future study is emphasized. Chapter 3 attempts, for the first time, to explain the influence of aggregate physical pore system features in terms of their porosity, pore size distribution, pore shapes, pore connectivity and pore network complexity on the pore structure and moisture ingress of concretes. Key pore features that could negatively impact the concrete durability are elucidated by separately analyzing the pore structure of aggregates, paste, and concretes using image analysis of thin section photomicrographs and mercury intrusion porosimetry. The moisture transport in composite concrete structure is investigated using water sorption experiments and the outcomes are phenomenologically illustrated based on the analyzed pore structure parameters. Furthermore, additional key durability indicators such as formation factor and intrinsic permeability of the different components/phases are predicted based on experimentally obtained results in this study. Recognizing the importance and relevance of the aggregate pore structure on the performance of the cementitious composites (as highlighted in Chapters 2 and 3), Chapter 4 explores the possibility of utilizing the aggregate pore index test for potential indication of coarse aggregate permeability. This is done through testing of aggregates/rock cores at various scales (as coarse aggregate mass/smaller rock plug samples/larger rock cores) and interlinking the obtained system properties (pore indices, pore sizes and sorption indices). In the absence of current test methods for measuring the aggregate permeability, the obtained results from this study seem promising to bridge this knowledge gap for extending the use of pore index test beyond its current capability to provide further insights into coarse aggregate pore network and their permeability. Chapter 5 is focused on air voids entrapment in UHPC, a major issue that could be a roadblock for the field-application of UHPC (in key applications such as bridge deck overlays, construction of joints etc.,). To this end, vacuum mixing technique was considered as a potential solution for resolving such issues to a significant extent. A comprehensive experimental program was designed for this purpose to study the benefits of this modified mixing procedure on the mechanical performance and durability of UHPC. Normal strength and high performance mortars were also considered for comparison. Possibility to engineer air-entrained vacuum mixed cementitious mortars are discussed in the context of stability of air voids/bubbles in the complex microstructure of cementitious composites.
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