Researchers in the Civil Engineering Departments at Covenant University and Delta State University have presented modifications to the content of concrete, facilitating easier compliance and workability. This contributes to a more sustainable world for applications in the built environment. The team’s results are expanded upon here, putting them into context in terms of concrete performance and the role that rheology plays in maintaining it.
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A Compliant Material Turning Solid over Time
Concrete is a remarkable material. Composed of a mix of aggregates and paste, its ubiquity can be explained by the contrasting properties it exhibits from when it is freshly mixed as cement and water to when it hardens.
Initially malleable with plastic-like qualities via hydration, the paste gains strength, hardening into a robust mass, often of impenetrable material. It is a property known as “workability” and it is key in the successful exploitation of concrete. The property also allows it to be manipulated and conform to new shapes.
Contrary to almost all other things, concrete gets stronger as it gets older as a slow hydration rate continues years beyond the initial life cycle. The robustness in hardened concrete together with its compressive strength creates durability, fire, and impact resistance in addition to acoustic insulation properties that see its extensive adoption in the building industry. In tall constructions, high-performance concrete can significantly reduce the steel content of a column needed for reinforcement, making its use highly cost-efficient.
Composed of Portland-Limestone cement and water, fundamental to concrete and its performance is the ingredient mix or proportioning where the character of the concrete is determined by the quality of the paste formed and its strength is dependent on the ratio of water to cement. The paste coats each stone particle and the surfaces of the fine and coarse aggregates. The size and nature of the aggregates used are typically dictated by the purpose (and thickness) of the final concrete product. Typically, 60-75% of the total volume of a concrete product will be comprised of these aggregates.
Designing the Mix
Designing the correct mix is critical in producing high-quality concrete that possesses the desired properties for any given purpose. It is a balance between the fresh concrete’s workability and the necessary strength and durability of the finished product.
The water-cement ratio is the weight of the mixing water divided by the weight of the cement. A mixture with a paucity of paste tends to produce porous and rough concrete while too much can leave the material susceptible to cracking.
Proper placement, consolidation, and curing drives the need for low water-cement ratios but not at the expense of the fresh concrete’s workability. This is where the research work on supplementary cementitious materials being undertaken by Oyebisi and colleagues at the Civil Engineering Department at Covenant University, Nigeria, in collaboration with peers at Delta State University, is significant.
Rheology is Fundamental
To ascertain an understanding of a concrete’s workability (and ensuring performance) requires a knowledge of its rheology.
Rheology can be considered the flow behavior of the concrete. To ensure an excellent surface finish, appropriate mix design, or material without segregation requires balancing of the rheological characteristics of the concrete.
Oyebisi et al. have been examining the rheological properties and optimizing them for concrete modified with high silica and alumina content. It has changed the cementitious material ratios in what is known as EMDPs or experimental mix design properties.
Its results demonstrate how critical flow properties enable workability or consistency before setting, which is known as “slump”. Generally, the greater the slump the more fluidity is exhibited by the concrete. However, this team found that optimizing is possible with the addition of high silica and alumina oxides. This comes under the collective term of supplementary cementing materials (SCMs).
A Nigerian Discarded Waste Product
New concrete constructions drive the need for new properties. What makes the team’s work so exciting is that they have incorporated the ash from the recycling of cashew nutshells (estimated generation of which in Nigeria even in 2017 was ca. 100 million metric tons per annum). This SCM satisfies two agendas, namely contributing to environmental preservation and sustainable development while also being a mineral admixture naturally enhancing concrete quality.
The team used slump values as a response factor to obtain optimized rheological properties for concrete flow. Generating data from empirical works, statistical analysis, and regression modeling (software, Minitab 17) was undertaken to make cashew nutshell-infiltrated concrete workability predictions in the built environment. This allowed concrete trial mixes to be conducted without any compromise of the concrete under applied loading conditions that would be experienced practically.
Increasing and Decreasing Slump
Concrete has various grades, including “C” grade measured using cubic concrete testing. This is where batch samples are tested for suitable strength. For example, a concrete demonstrating a compressive strength of 20 newtons gets designated as “C 20”. Using concrete mix designs with grades between C 25-40 and utilizing cashew nutshell ash at 5, 10, and 15 wt.% of cement, the group sought target strengths of 25-40 MPa at 28 days.
Various design factors were explored and the results have proved encouraging. Chiefly among them is the fact that slump increases and decreases with increasing water/cementitious materials and cashew nutshells ash/Portland-Limestone cement ratios respectively.
In quantitative terms, the scientists determined that as concrete grades increased from C 25-C 40 ca., an 18-49% decrease in a slump was optimum in maintaining good flow properties. A comparison with a mix containing only Portland-Limestone cement revealed that there was a 13-33%, 10-34%, and 8-33% slump decrease with escalating ash content from 5-15% for C 25, C 30, and C 40 concrete grades, respectively.
These results point towards effective concrete performance using what was once a waste product. With more experimentation, other waste products and possibilities are likely to see use in concrete applications.
References and Further Reading
Oyebisi, S. O., et al. (2021) Optimizing the Rheology of Concrete modified with High Silica and Alumina Precursor for Normal Workability in the Built Environment. IOP Conference Series: Materials Science and Engineering, DOI:10.1088/1757-899X/1107/1/012211