Mechanical properties of self consolidating concrete blended with high volumes of fly ash and slag
Graphical abstract
Highlights
► Evaluate high volumes of fly ash and slag used in self consolidating concrete (SCC). ► Binary and ternary mixes evaluated. ► Chemical composition of entire binder can be used to predict compressive strength. ► Compressive strength, elastic modulus, and creep similar to conventional SCC at later ages.
Introduction
Due to its design versatility, availability and cost efficiency, concrete continues to play a dominant role in the construction industry. However, the production of Portland cement, a primary component of typical concrete mixes, is known to have a serious impact on the environment. For every ton of cement produced, approximately a ton of CO2 is emitted. According to the European Cement Association, 3 billion tons of cement were consumed worldwide in 2009 [1]. Thus, the carbon footprint associated with cement, and therefore conventional concrete, production is high.
Increasing the use of supplementary cementitious materials (SCMs) in concrete is an obvious and necessary step to reduce carbon emissions [2], [3]. SCMs, such as silica fume, fly ash and slag, are often waste materials from industrial processes. These waste by-products possess hydraulic and/or pozzolanic properties and, when used at optimal levels, enhance fresh state properties, mechanical performance and composite durability. Inclusion of SCMs in concrete lessens the environmental impact of concrete in several ways, in that it: (1) reduces Portland cement consumption and thereby production, (2) can reduce the amount of inert filler (typically sand in conventional concrete) required and (3) uses waste materials that would otherwise be land filled.
However, concretes containing SCMs tend to have slower strength development especially at high cement replacement rates, since the Portland cement reaction (hydraulic) is much faster than the SCM reaction (primarily pozzolanic) [4]. The pozzolanic reaction can only take place if there is available calcium hydroxide, a by-product of the hydraulic reaction. Therefore, a careful balance between the cement and SCM volumes is needed to ensure both reactions contribute to the strength. Achieving this balance requires a quantification of all the salient binder materials and their role in both hydraulic and pozzolanic reactions. A further challenge to using large volumes of SCMs is the inherent variability of the waste materials. Significant variation in performance is seen, a function of the source and type of SCM.
A significant amount of research has focused on optimizing the fresh and hardened state properties of cement-based composites in which Portland cement is partially replaced with SCMs. Numerous authors have proposed the use of efficiency factors to classify the effectiveness of SCMs in enhancing compressive strength [5], [6]. Most studies have involved the use of efficiency factors for evaluating optimal dosages for maximizing compressive strength in fly ash systems, although slag systems have also been considered; in these studies efficiency factors are used to quantify this optimization. Although the research has directly supported the use of SCMs in concrete construction today, there are limitations in use, in particular with respect to replacement percentages. In the literature, typical replacement levels are between 20% and 50% and rarely exceed 60% [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Fig. 1 shows the distribution of replacement percentages in the literature gathered by the authors; less than 10% of the research programs have considered replacement percentages exceeding 60%. To the authors knowledge, efficiency factors have not been yet been considered for self consolidating concrete (SCC).
Furthermore, most research programs have solely investigated compressive strength neglecting other engineering properties. A few studies have considered the effects of SCMs on the elastic modulus [14], [16], [24], shrinkage [10], [14], [16], [17], [18], [25], or creep strain [10], [14], [17], [24], [25]. For the lower replacement levels investigated, the elastic modulus is smaller at early ages when SCMs are included, but approaches or exceeds the control at later ages. No clear trends are seen for shrinkage and creep with performance varying based on SCM type and the experimental methods used.
Section snippets
Research impetus
More sustainable cement-based systems can be developed through the use of SCMs to minimize the Portland cement used, thereby decreasing embedded carbon content. The present study aims at optimizing the replacement percentage of SCMs in SCC with the objective of achieving comparable long-term properties. Specific application of these materials is in composite structural components, although other applications are possible.
The type of composite structural component considered here consists of a
Research objective
This research is investigating the immediate and long-term mechanical properties of high SCM SCC mixes with fly ash and slag. Engineering properties of SCC at replacement levels higher than what are reported in literature are investigated. Two SCMs were used: one fly ash and one slag. A total of 16 mixes with 60%, 80% and 90% cement replacement were prepared: one control mix, six binary mixes and nine ternary mixes (cement and two SCMs). The ternary mixes were made with three different ratios
Materials and experimental procedure
An experimental program was developed to investigate the mechanical properties of binary and ternary concretes blended with cement, slag and/or fly ash. The work builds on prior research by the authors on binary concrete [26], [27], [28]. That work considered a wide range of replacement percentages and was isolated to study of the compressive strength. In contrast, this study focuses on high replacement percentages with a broader range of mechanical properties studied.
The prior study identified
Results
The following sections present the measured results for the mechanical property tests. The results are summarized in tables, and presented in figures when appropriate. More detailed presentation and analysis of the data are available in [27].
Analytical expressions for compressive strength
Predicting the compressive strength of SCM-rich concretes is a challenge that must be overcome to implement these materials in actual construction. Few rules or expressions are available to proportion the SCM and the cement to result in a reliable mix design. The data developed as part of this research program were combined with prior research results to develop these expressions to estimate the compressive strength based on the time-dependent response of the control mix and the chemical
Conclusion
Data are lacking to describe the effect of high SCM replacement on the engineering properties of self-consolidating concrete. These materials typically exhibit slow strength gain due to the dominant pozzolanic reaction and should be used in appropriate applications. This work is motivated by the use of high SCM concrete in composite applications in which an outer steel shell serves to carry initial loads as the concrete develops strength. However, the data are useful for other applications in
Acknowledgements
This research was partly funded by the Transportation Northwest, a Regional University Transportation Center (UTC). The authors gratefully acknowledge this support.
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