Measuring Yield Stress for Slump Correlation in Concrete and Cement Paste

Introduction

Concrete is the most widely used man-made material in the world when measured by tonnage. One of the most important characteristics of these materials is called workability. Workability is the ability of a fresh (plastic) concrete mixture to fill a specific form or mold completely with the desired work (vibration) without reducing concrete quality. Workability depends on water content, aggregate (shape and size distribution), cement content and age (hydration level), and can be modified by adding superplasticizing chemical admixtures or by increasing water content. However, excessive water causes segregation (water accumulation on the surface) and/or cracking of aggregates (when cement and aggregates begin to separate), and reduces concrete quality [1]. Workability can be measured with the "concrete slump test," a simple measure of the flexibility of a fresh concrete series according to ASTM C 143 [2] or EN12350-2 [3] test standards. Slump is normally measured by filling an "Abrams cone" with a sample from a new concrete series. The cone is placed on a flat, non-absorbent surface with the wide end down. It is then filled in three equal layers; each layer is compacted with a steel rod to consolidate the layer. When the cone is carefully lifted, the enclosed material subsides by a certain amount due to gravity. However, it has been shown in the past that rheological determination of yield stress is a quick and easy method to correlate with slump [4]. Like concrete, soft foods are often difficult to test on rotational rheometers using classical plate/plate or concentric cylinder geometries due to possible wall slip and excessive sample fragmentation during loading in narrow gaps. For this reason, vane geometries are recommended for these applications. When a vane rotor is completely immersed in the sample, the yield stress can be calculated according to Boger [5]: M is maximum torque and K is a vane parameter dependent on blade height (H) and diameter (D) as follows:

Figure 1. Thermo Scientific HAAKE Viscotester iQ Experimental Results and Discussion

As described earlier, rheological testing of concrete with vane rotors is recommended. In Figure 1, the new Thermo Scientific™ HAAKE™ Viscotester™ iQ shows the vane configuration. For the tests, standard Portland cement was mixed with water at typical concentrations and then fine gravel was added at three different concentrations by weight. See Table 1 for an overview of sample compositions.

Figure 2. Shear Stress versus Time after Mixing for Three Different Concrete Formulations at 25°C after 5 minutes

Samples were formulated by thoroughly mixing and tested after waiting 5 minutes. The test was conducted as follows. After the vane rotor was completely immersed in the sample, a constant rotational speed of Ω = 0.05 rpm was applied. The shear stress was then monitored as a function of measurement time. After the initially completely elastic response in the sample, the structure collapses and the shear stress drops again. The maximum value in shear stress corresponds to yield stress. Figure 2 shows the results for the three formulations 5 minutes after mixing at 25°C. As can be seen in Figure 2, the yield stress for pure cement paste is 375 Pa compared to 890 Pa with 75 g fine gravel and 1630 Pa with 125 g fine gravel. These easily determined yield stress values can now be converted to slump values (mm) determined by ASTM Abrams cone and a semi-empirical relationship determined by Hu et al. [6]: S is slump in mm, τ is the yield stress of the fluid and ρ is the density of the fluid.

Conclusion

The vane method on the Thermo Scientific HAAKE Viscotester iQ is a fast, simple and accurate approach to measure yield stress of cement paste and concrete. These values can be easily converted to slump values using semi-empirical relationships. Author: Jan Philip Plog Head of Rheology Applications Group Thermo Fisher Scientific Material Characterization Translator: Bahadır Aktaş Chemist Sales Representative Anamed Analitik ve Medikal Sistemler A.Ş.

References [1] See Wikipedia, Concrete, http://en.wikipedia/wiki/ Concrete (as of Jan. 13, 2014, 16:50 CET). [2] ASTM C 143 - Standard Test Method for Slump of Hydraulic-Cement Concrete. [3] EN 12350-2 - Testeing fresh concrete. Slump test [4] N. Roussel, Correlation between Yield Stress and Slump: Comparison between Numerical Simulations and Concrete Rheometers Results, Materials and Structures, May 2006, Volume39, Issue 4, pp 501- 509. [5] Dzuy NQ, Boger DV. 1985. Direct yield stress measure- ment with the vane method. J Rheol 29:335-47 [6] Chong Hu, François De Larrard, Odd E. Gjørv, Rheological testing and modelling of fresh high performance concrete, Materials and Structures, January/ February 1995, Volume 28, Issue 1, pp 1-7.