A project report submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 1998 / Aggregate stiffness is known to influence the magnitude of creep of concrete. The purpose of this research project was to quantify the influence of aggregate stiffness on the measured and predicted long-term creep behaviour of plain concrete. Basic and total creep tests were conducted on concrete specimens of two different strength grades for each of three different commonly used South African aggregate types (quartzite, granite and andesite). In addition, elastic modulus tests Were conducted on cores of the aggregate types considered. The test results revealed that no correlation exists between the creep of concrete and the stiffness of the included aggregate. These results appear to be attributable to the stress-strain behaviour of the aggregate/paste interfacial zone, in the case of aggregates with an elastic modulus in excess of 70 GPa. The experimental basic and total creep values from this investigation were compared with those predicted for each mix at the corresponding ages by the BS 8110 (1985), ACI 209 (1992), AS 3600 (1988), CEB-FIP (1970), CEB-FIP (1978), CEB-FIP (1990) and the RILEM Model B3 (1995). This comparison indicated that the results predicted by each model vary widely and that no correlation exists between the magnitude of the aggregate stiffness and the creep strains predicted by each model. / MT2017

Sustainability and reduction of environmental impacts have continued to increase in importance in the concrete marketplace. Portland-limestone cement (PLC) has been shown to reduce total energy consumed and CO2 produced during the cement manufacturing process. This material may also have the ability to benefit concrete properties, such as compressive strength and time of set. Other concrete performance measures of potential interest evaluated in this study include durability and modulus of elasticity. In this dissertation PLC was evaluated for its ability to further increase concrete sustainability, while at the same time providing advantageous properties. This study’s focus was to show that PLC can improve concrete mixtures that are similar to commonly used ordinary portland cement (OPC) mixtures. PLC was also evaluated for its ability to increase the amount of total cement replacement (further increasing sustainability). Additionally PLC properties and concrete mixture combinations were evaluated in an attempt to clarify which PLC properties are crucial in performance benefits. Approximately 2000 concrete specimens were tested along with approximately 1000 cement paste specimens. This dissertation also includes an evaluation of PLC being used in a large scale construction and renovation project on a college football stadium. The scope of the dissertation included 12 cements from four manufacturing facilities that represent a large portion of the cement industry in the southeast US. Supplementary cementitious materials (SCMs), Class C fly ash, Class F fly ash, and slag cement, were also evaluated in single and dual SCM concrete mixtures at replacement rates up to 70%. Replacement rates of this magnitude are not being used in common practice but may become preferred in some conditions with PLC. Results indicated that PLC outperformed OPC in areas tested, in almost all cases at up to 50% replacement with single and dual SCMs. PLC also showed considerable advantages at 60% replacement but was often outperformed by OPC at 70% replacement. Aggregate type played a large role at 70% replacement. Elastic modulus, durability, and variability were all similar with PLC and OPC. Combinations of certain SCMs were more advantageous than others, and optimal SCM combinations changed depending on cement source.

The structure-property relationships of Ultra-High Performance Concrete (UHPC) were quantified using imaging techniques to characterize the multiscale hierarchical heterogeneities and the mechanical properties. Through image analysis the average size, percent area, nearest neighbor distance, and relative number density of each inclusion type was determined and then used to create Representative Volume Element (RVE) cubes for use in Finite Element (FE) analysis. Three different size scale RVEs at the mesoscale were found to best represent the material: the largest length scale (35 mm side length) included steel fibers, the middle length scale (0.54 mm side length) included large voids and silica sand grains, and the smallest length scale (0.04 mm side length) included small voids and unhydrated cement grains. By using three length scales of mesoscale FE modeling, the bridge of information to the macroscale cementitious material model is more physically based.

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