High viscosity of fresh concrete has a big impact on its pump-ability, spray-ability, place-ability, and surface finishing. Nowadays, usage of excess cementitious and fines in concrete mixes is inevitable to make concrete pump-able. Since PolyCarboxylicEthers (PCEs) have been introduced, they shifted the limits of achievable concrete performance dramatically. Today, we can build higher, stronger, and more durable than ever before. Due to the outstanding water reduction and early strength performance of PCEs, the industry is enabled to lower W/Cm ratios, reduce cement contents, and utilize more supplementary cementitious material (SCM) in concrete mixes. However, these optimized concrete mixes are prone to increase concrete viscosity and stickiness of the concrete.
The workability of fresh concrete is usually characterized by its slump and slump flow values. However, workers often notice significant differences when handling concrete prepared according to different recipes, even if they exhibit very similar workability. Especially concrete with high SCM content that are treated with low viscosity concrete concept admixture are less sticky and can be placed with low effort or improve pumping efficiency. This behavior is quantified by comparing yield values, plastic viscosities and thixotropy and is characterized by using V-funnel and rheometer tests.
The advantages of low viscosity concrete concept technology are exemplified on mixes optimized for their sustainability and compared with conventional mix designs based on a life cycle assessment. Understanding the concept of low viscosity concrete in terms of rheology retention, can help concrete designers to pump the concrete in higher floors without any consequences of loss of workability due to higher pumping pressure and frictional losses.
What is concrete workability?
Concrete workability refers to how easily freshly mixed concrete can be placed, consolidated, and finished with minimum loss in homogeneity. Generally, the workability of concrete is determined by slump or flow measurements. Both these indicators of workability i.e., slump or flow, when achieved & demonstrated effectively, aid in concrete execution such as pumping, finishing and compaction. What effects this workability of concrete? (A.M.Neville, 2004):
- Water cement ratio
The purpose of this paper is to explore the plastic stage properties of concrete using state-of-the-art measurements of flow-based properties of concrete. A critical review of the tests available is given with special emphasis on tests for high performance concrete (HPC). Definitions of terms commonly used in the field and their link to material properties are provided by giving experimental data to differentiate between workability retention vs rheology retention.
Fundaments of fluid suspension
F/A = t = µ × ϒ ˙ ……... (1)
where µ = viscosity ϒ = shear rate = dv /dy (see Fig. 1)
t = shear stress = F/A
F = shear force
A = area of plane parallel to force.
The physical interpretation of this factor is that the yield stress is the stress needed to be applied to a material to initiate flow. For a liquid, the yield stress equal to the intersection point on the stress axis and the plastic viscosity is the slope of the shear stress-shear rate plot (see Fig. 2). A liquid that follows this linear curve is called a Bingham liquid. (Ferraris)
Concrete and rheology
Concrete is a heterogeneous material and made by various raw materials which have various physical properties such as specific gravity, texture, surface tension and size and shape. When it is combined in one mass, it is very difficult to predict performance in terms of rheology. To define flow behavior of concrete by linking it to various effects such as bleeding, sedimentation, and density, one must distinguish three properties: stability, compatibility, and mobility. Stability is linked to bleeding and segregation. Compatibility is equivalent to density, while mobility is linked to internal friction angle, bonding force, and viscosity. These descriptions link commonly used words with physical factors that can be measured. However, we believe that this is not enough. All these terms should be discarded in favor of physically measurable parameters. For instance, we could say that concrete has a higher viscosity, instead of referring to a lower workability.
Today, we are measuring workability retention by slump cone test and assuming that all the fluids property remain stable in a given workability window. We have observed concrete changes its viscosity with respect to time, and demonstrates by V funnel test. From fig 4, workability of concrete remains stable in-between 650 to 700 mm slump flow but viscosity in terms of V funnel value changes from 5 sec to 24 sec with respect to 5 mins, 60 mins, and 120 mins. This rheological impact creates more energy requirement of pumping, placing, and finishing of concrete.
So, our objective is to design low viscosity concrete to maintain not only workability retention as well as rheology retention for ease in concreting operations.
The yield stress and the plastic viscosity are measured by a vane able to rotate in a specific and proper designed container. The first parameter is proportional to the maximum torque detected when the vane constantly rotates at 0.025 rev/s The second parameter derives from the slope determination (according to specific calculations) of the interpolation line of six torques measured at six decreasing rotational speeds from 0.6 to 0.1 rev/s.
Slump-flow and T500 time for fluid concrete
The slump-flow and T500 time is a test to assess the flowability and the flow rate of high fluid/ SCC concrete in the absence of obstructions. It is based on the slump test described in EN 12350-2. The result is an indication of the filling ability of self-compacting concrete. The T500 time is also a measure of the speed of flow and hence the viscosity. The fresh concrete is poured into a cone as used for the EN 12350-2 slump test. When the cone is withdrawn upwards, the time from commencing upward movement of the cone to when the concrete has flowed to a diameter of 500 mm is measured; this is the T500 time. The largest diameter of the flow spread of the concrete and the diameter of the spread at right angles to it are then measured and the mean is the slump-flow. (EFNARC, 2005)
A high-rise construction was designed using M80 grade of high-performance concrete with w/cm ratio 0.20. Due to high supplementary cementitious replacement and very low total water content in the mix, it became very sticky or highly viscous. The site execution team struggled to pump concrete to the required heights, and faced issues with choking of pump and the very high pumping pressure held the possibility of the pipeline bursting. This could lead to loss of time and cost overruns.
It’s time to rethink the concept of modifying mix with low viscosity concrete and move the concrete design philosophy by workability retention to rheology retention.
|Grade||Cement (OPC 53)||Fly Ash||GGBS||M. Silica||Admixture MasterEase||w/b||Free water||20 mm||10 mm|
|Table 1: M95 concrete mix design.|
|Test Method||Parameters||Control concrete (As-is)||Low viscosity concrete (To -be)|
|ICAR Rheometer||Viscosity||70.8 Pa.s||36.1 Pa.s|
|Slump flow||Yield stress||700 mm||700 mm|
|V- Funnel||Viscosity||31 Sec||12 Sec|
|T500||Viscosity||5 Sec||2 Sec|
|Table 2: Fundamental fluid properties of concrete.|
It is evident that in order to cater to modern concreting requirements, we need to re-evaluate how we measure concrete’s performance. The use of admixtures in concrete, apart from contributing to water demand reduction and slump retention, needs to be adopted and utilized for rheological improvements to concrete.
A quantitative measure of rheological characterization of concrete can be obtained with rheometers, but in most of the cases the values (yield stress and plastic rheology) cannot be understood by all operators who work in the concrete production field.
Also, it is necessary to develop an easy tool and test methodology to demonstrate rheological properties of concrete on the field by using slump flow, T500 and V-funnel measurements. The development of an easy, comprehensive, and portable tool that can correlate the science of rheology (rheometer) with the concrete life (operators’ feeling) is a must: the Placing Simulator is able to bridge these two realities. The quantification of the stickiness reduction due to the low viscosity admixture technology can be easily measured and implemented in HPC. The rheology improvement achieved by MasterEase polymer allows one to enhance the concrete performances in terms of durability improvement (lower water content) and cost optimization (lower cement content).
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