Dr. Santhosh B S, Professor, Department of Civil Engineering, JSS Academy of Technical Education, Bengaluru
Assessing the Workability of Concrete
Concrete is a heterogeneous composite complex material, and its hardened property is influenced by its fresh property. Concrete today has transformed into an advanced type with new and innovative ingredients added - either singly or in combination. The traditional four ingredient mix of cement + water + fine aggregate + coarse aggregate mix-design is a thing of the past . Basically, the modifications with the addition of new ingredients like mineral and chemical admixtures were meant to improve the quality of concrete in the backdrop of rapidly growing concrete construction and demand for new properties (both in fresh and hardened state).
Whatever may be the advancements in concrete making, the basic normal concrete remains the benchmark for any comparison, and workability remains a very critical property of concrete. Workability of fresh concrete can directly affect many properties, including the strength, durability, and appearance, besides the cost. Even to this day, the workability of concrete is “that property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, placed, consolidated, and finished.”
Workability is measured even to this day by empirical tests, in particular, the simplistic slump test, in spite of its limitations, drawbacks, and misleading results. Most of the tests that are developed are empirical in nature attempting to simulate placement condition in the field by measuring a single value like time or distance which can be the index of workability . In fact, the monitoring of workability in the field has become even more essential and critical for advanced concrete production like high performance concrete since the mixes are susceptible even for small changes in mix proportions, which is less significant in case of conventional concrete.
The empirical test is a single parameter test; a quantitative fundamental science approach is more reliable, suitable, and needed for characterizing fresh concrete with minimum of two parameters.
The three important practical reasons why it is highly desirable to understand the property of workability are: mix design, processes, and control. A better understanding of workability would result in greater efficiency, less frustration, better use of materials and consequently lower cost. Even today, subjective judgment of the results is the essence of most of the reliable empirical methods.
Importance of Fresh Concrete Rheology
Rheometer is a device to measure the rheological properties of fresh concrete and there are various rheometers available with each rheometer having its own advantages and limitations.
To understand the importance of fresh concrete rheology, one must look at the limitations of empirical tests, in particular, the slump test. In general, the workability results are quoted as a single parameter in terms of slump, compaction factor and so on. It is seen that these tests do not provide a unique characterization. To mention a few, for example, concrete with different composition may have same slump and may behave differently in each situation.
Looking further, it can be pointed out that the slump test is quite incapable of differentiating two concretes of very low workability (zero slump). More discussion on the limitations of slump test will be made in the subsequent sections and it is obvious that the slump test is incapable of characterizing the workability of concrete.
Similarly, the other empirical test like the compaction factor test does not provide more information than the slump test even though considered a dynamic test. Also, it is not commonly used at sites since the instrument is bulky. In general, single parameter empirical tests are ineffective in characterizing the fresh property of concrete and there is a need to adopt material science approach with at least two parameters and the rheological properties of fresh concrete plays a vital role in this direction.
Rheology is a well-established branch and widely applied to fresh concrete. The fresh concrete is assumed as a non-Newtonian Bingham fluid. It can be considered as a complex behaviour material, with concentrated suspension of aggregates in cement paste. To characterize such complex material by rheology is very difficult and the behaviour of fresh concrete cannot be predicted by any specific method. Therefore, a better understanding of the flow properties of concrete is needed to predict the flow of concrete. There are several attempts that have been made to predict the flow and relate fresh concrete properties to measurable quantities. In that, Ritchie for the first time applied the rheological application to the fresh concrete and divided the rheology of fresh concrete into three fundamental properties, namely: stability, compatibility, and mobility . The stability is represented in terms of bleeding and segregation. Compatibility is represented by relative density. Similarly, the mobility is represented in terms of internal friction angle, bonding force, and viscosity.
The application of the concept of rheology - a scientific study based on the flow and deformation of matter - to the workability of concrete has made progress in concrete industry. The rheology of concrete has emerged as a viable technique for characterizing the workability of fresh concrete to overcome the deficiencies of the empirical tests. It is re-emphasized that the widely used empirical test measure only one parameter which is ineffective in fully characterizing the concrete behaviour and rheological parameters can be more effective. Any attempt to study rheological properties of fresh concrete is very much essential and need of the hour to understand the behaviour of fresh concrete .
- Peter.J.M.Bartos “Workability of Special Fresh Concretes”, Proceedings of the International RILEM Workshop, University of Paisley, Scotland. March 2–3, 1993 E & FN Spon, Pages 2-6, 2003.
- Christopher Stanley, “The Use and Abuse of the Slump Test for Measuring the Workability of Concrete”, 36th Conference on Our World in Concrete & Structures, Singapore, August 14-16, 2011.
- Sergiy Shyshko, “Numerical simulation of the rheological behaviour of fresh concrete”, Ph.D. Thesis, TU Dresden, Ukraine, 2013.
- Chiara F. Ferraris, “Concrete Rheology: Knowledge and Challenges?”, 2nd International Symposium on Advances in Concrete through Science and Engineering11-13 September, Canada, Pages 141-149, 2006.
- Santhosh B S “Determining the Rheological Properties of Fresh Concrete using Concrete Shear Box” published in International Journal of Engineering and Advanced Technology (IJEAT), ISSN: 2249 – 8958, Volume 9, Issue-1, October 2019, pp 351-36.