Concrete workability cannot necessarily be sacrificed for improved hardened properties, such as durability or strength. Workability is typically quantified in the field by the result of the slump cone test. Nevertheless, a survey [1], conducted by the National Ready-Mixed Concrete Association (NRMCA) and the National Institute of Standards and Technology (NIST), determined that for high performance concrete (HPC) the slump cone value is not representative of the ease of handling HPC in the field. It was reported that concrete mixtures with the same slump might not behave in the same way during placement. This implies that the slump value does not give enough data to fully characterize concrete flow.
In the construction field, terms like workability, flowability and cohesion are used, sometimes interchangeably, to describe the behavior of concrete under flow. The definitions of these terms are very subjective. Therefore, there is a need for a more fundamental and quantitative description of concrete flow. Rheological measurements of concentrated suspensions can be used to describe the flow of concrete. Numerous researchers [2, 3, 4] have successfully used the Bingham equation. Two parameters define the flow: the yield stress and the plastic viscosity [5]. The yield stress is related to slump [6, 7], but the plastic viscosity is usually ignored because only a few type of instruments exist to measure it [8]. However, the viscosity may be related to properties such as stickiness, placeability, pumpability, and finishability. Also, segregation could be defined as the ability of the aggregate to migrate (or sink) in the cement paste. This phenomenon is linked to the viscosity of the cement paste and the concrete mixture design. Therefore, methods to predict concrete workability need to take into account more than just the yield stress.
Admixtures mainly affect the flow behavior of the cement paste without altering the composition or behavior of the aggregates. Therefore, it seems reasonable to try to select admixtures, chemical and mineral, by only testing the cement paste. Ideally, the results will then be related to the concrete workability. Unfortunately, the relationship between cement paste rheology and concrete rheology has never been completely established. The main reason for this is that cement paste rheology is typically measured under conditions that are never experienced by the cement paste in concrete. Therefore, the measured cement paste rheological parameters may differ from those estimated from concrete parameters. The values usually reported in the literature for cement paste do not take into account the contribution of the aggregates [9]. The aggregates act as heat sinks and shear the cement paste during the mixing process. A computerized model to simulate the concrete being sheared is under development at NIST [10]. This model will predict concrete rheology from constituent properties that will include rheological measurements on cement paste.
However, the cement paste flow properties, if measured "properly," can be used to screen mineral admixtures. The details of the methodology are presented elsewhere [11] but the following principles should be reiterated. The cement paste needs to be mixed and tested under the conditions similar to those it will experience in concrete, primarily shear and temperature history. Therefore, a high-shear, temperature controlled mixer should be used. To measure the rheological parameters of cement paste flow, a parallel plate rheometer must be used because it is the only rheometer with variable geometry. It has been established [12] that the rheological properties of cement paste change if the material is squeezed between two surfaces or aggregates as in concrete. The distance between the surfaces is called the "gap." In a parallel plate fluid rheometer, the gap or distance between the plates can be varied easily to more closely represent the shearing action imposed on paste in concrete. In this paper, cement paste rheological parameters were measured using the above method as a function of type and dosage of mineral admixture.
It is usually reported that, if the volume concentration of a solid is held constant, the addition of mineral admixtures improves concrete performance but reduces workability. The most common reason for poor workability is that the addition of a fine powder will increase the water demand due to the increase in surface area. This belief is supported by test results that show that the addition of fine silica fume particles increases the water demand to attain specific workability levels. However, in certain cases it is reported in the literature that the use of fine mineral admixtures can reduce the water demand or increase the slump. Lange et al. [13] measured the water demand of mortars with increasing additions of a very fine blast furnace slag. He found that, for a specific flow an optimum amount of blast furnace slag reduced the water demand of the mortar. A popular hypothesis put forward to explain the workability enhancement due to the use of certain fine mineral admixtures, especially fly ash or silica fume, is that the spherical particles easily roll over one another, reducing interparticle friction [14]. The spherical shape also minimizes the particle's surface to volume ratio, resulting in low fluid demands. Out of all 3-d shapes, a sphere gives the minimum surface area for a given volume [15]. Sakai et al. [16] reported that a higher packing density was obtained with spherical particles as compared to crushed particles in a wet state. This resulted in lower water retention in the spherical case and subsequently lower water demand for a specific workability. A strong dependence of fluidity (defined as the inverse value of the viscosity) on the average particle size was reported with a pessimum value [16]. It was explained that, at an optimal particle size, the packing density was maximum, which helped to achieve maximum fluidity. Recently, Collins et al. [17] reported that in concrete containing alkali-activated ground granulated slag as the binder, the workability was improved by replacing part of the binder with ultra-fine materials. This material had 90 % by mass of the particles smaller than 13.7 µm. It was also reported that some similar materials were not effective in improving the workability. It can be concluded from this survey of the literature that the selection of a fine mineral admixture for improved concrete workability is not a trivial problem. At present, this selection cannot be predicted from the physical or chemical characteristics of the admixture, and can only be determined using a properly designed test.