Clearly, all these ingredients, especially water, superplasticizer, and aggregate, affect the rheological properties differently. G., “A Viscometric Study of Cement Pastes Containing Superplasticizers with a Note on Experimental Techniques,” Magazine of Concr. The results of this study indicate that early addition of superplasticizer enhances dispersion of cement and increased the flowability of the RSSCC mixture, especially for mixtures mixed in mortar mixer.
Similarly, the robustness of the flow is proportioned to the agg:b volume and SP:b wt%. It was observed that for the same mixing sequence and mixing time, the mixtures produced using the pan mixer had higher viscosity as compared to the mixtures mixed in the mortar mixer. "Influence of Mixing Efficiency on the Mixture Proportion of General Purpose Self-Compacting Concrete " International Symposium on High-Performance and Reactive Powder Cements Sherbrooke, Canada, 19-39. While such results might not be representative, interpretation of the results suggests that it is not the mixing time, but rather the sheer energy and sheer rate that count.
In addition, Ye, et al.[xiv] showed that the fluidity can easily be manipulated by changing the c:f ratio, and the slopes of curves are about the same for the powder-type and VMA-type SCC (Fig 2). Effect of coarse-fine aggregate ratio on slump flow (Ye, et al.) Provided the aggregate is spherical, the beneficial role of fine aggregate is related to its ball-bearing effect. showed that the use of coarse aggregate and sand combinations that enable the increase in packing density can reduce the superplasticizer demand and plastic viscosity of SCC. Mixtures mixed in the mortar mixer required lower dosages of polycarboxylate based superplasticizer to produce rapid-set SCC (RSSCC) with the same slump flow as compared to the dosages required for the mixtures mixed in the pan mixer. "Final Report of Task 8.1." Proposal No0BE96-3801, 1-65. "State of the Art Report on Manufacturing of Self-Compacting Concrete." Proceedings of the International Workshop on Self-Compacting Concrete, Kochi, Japan, 360-367. "Effect of Mixing Energy on Fresh Properties of SCC." Proceedings of the Fourth International Rilem Symposium on Self-Compacting Concrete and Second North American Conference on the Design and Use of Self-Consolidating Concrete, Chicago, USA. It is recognized that with a given mixture, completely different flow curves are obtained by varying these two parameters. P., “The Effect of ingredients and shear history on the thixotropic rate of rebuilding of SCC,” pp.
the shear energy and shear rate, and the specific application. Thereby, in most cases, addition of fly ash and silica fume does not affect the matrix density. The effects of coarse aggregate, VMA, and superplasticizer on segregation (Ye, et al.) To reiterate, the resistance to segregation should not be based on visual inspection of the slump flow. showed that high superplasticized SCC mixtures that did not show segregation during slump flow test were prone to high segregation, and addition of VMA was instrumental for controlling it (Fig. Nevertheless, even at relatively high dose of VMA of 0.08%, sedimentation was not completely eliminated. The MWC is determined for concrete-equivalent mortar as the slope of the increase in flow diameter determined using a mini slump flow cone vs. Concrete-equivalent mortar exhibiting greater MWC can result in lower degree of increase in flow after a given increase in water content, hence more robust. These effects were reduced when a viscosity agent was added to these mixtures. H., Mesbah, H., “Variation of formwork pressure with thixotropy of Self-Consolidating Concrete,” ACI Materials Journal, Vol. Assaad, J., “Formwork Pressure of Self-Consolidating Concrete – Influence of Thixotropy,” Doctoral Thesis, Université de Sherbrooke, 2004, 450 p.
SCC might be more susceptible to changes than ordinary concrete because of a combination of detailed requirements, more complex mix design, and inherent low yield stress and viscosity. This observation is in agreement with similar results reported by Khayat and Guizani.[ii] [i] Bonen, D. P., ”The effects of formulation on the properties of self-consolidating concrete,” pp. Mixtures made with naphthalene-based superplasticizer are shown to have greater robustness (greater RWD) than similar mixtures with polycarboxylate-based superplasticizer. Similar observations of slump flow variations were made by Ushijima, et al.[iii] They varied the amount of water added to the mixture in such a way so as to simulate a change of aggregate moisture content between -1% to 1.5%.
and Oberg, P., “Effects of constituents on the workability and rheology of self-compacting concrete,” pp. Consequently, robustness increases by incorporation of density modifiers, and with regards to fines, the best density modifiers is slag, followed in decreasing order by ground dolomite, ground limestone, and ground quartz. The incorporation of VMA controls bleeding and segregation and increases the robustness of the SCC, while the low water content provides mostly the required level of viscosity. Mori et al.[i] examined mixes with 74 different types of aggregate and varying water absorption values.
4 also indicates that the sedimentation rate can be reduced by increasing the content of the fines with high specific density. In such mixtures, the VMA is used to reduce the variability of the SCC that can arise from changes in material properties and placement conditions. If the natural moisture content of the aggregate is lower than SSD, then the amount of mixing water is increased. “On the Effect of Coarse Aggregate Fraction and Shape on the Rheological Properties of Self-Compacting Concrete,” Cem.
Indeed, Shi, et al.[xvi] has shown that the flow loss of VMA-free mixtures is higher than in VMA mixtures. Effects of superplasticizer and aggregate:cement ratio on viscosity The high flowability and deformability of SCC derives from the characteristically low values of yield strengths and plastic viscosity. The parameters varied in the study included the quantity of powder, use of limestone filler, and various types and contents of silica fume and SP. "Properties of Mortar for Self-Compacting Concrete " Proceedings of the First International Rilem Symposium on Self-Compacting Conrete Stockholm, Sweden, 109-120. Other common methods are based on column tests in which the mixture is cast into a few cylindrical sections that are mounted one on the top of the other, and at a predetermine time before hardening, the sections are removed and the content of aggregate in each of the sections is determined by wet sieving,[ii] After hardening, cylinders can be vertically sawed and the distribution of the aggregate along the vertical axis can be determined by visual inspection, point counting, or image analysis.
As an example, a typical yield stress of SCC is about one order of magnitude smaller than the corresponding yield strength of regular concrete.[xvii] These low yield stresses and plastic viscosity values inherently compromises the segregation resistance and countermeasures must be taken as discussed below. The authors concluded that although the SCC mixtures generally require longer mixing times than conventional mixtures, their mixing time can be reduced by increasing the fine particle content, (with a constant w:c ratio), increasing the total water amount, and replacing part of the cement by silica fume. Another approach is to measure the electrical conductivity along a vertical section as a function of time.[iii] This method is sensitive to bleeding, rather than settling of aggregate.
Indeed, some properties, such as loss of fluidity and compatibility are frequently encountered with regular concretes. P., “Fresh and hardened properties of self-consolidating concrete,” Progress in Structural Engin. Similar results were reported by others.[iv],[v] More recently, Douglas, et al.[vi] showed that the structural buildup and thixotropy are also related to the superplasticizer content, rest time, and mixing energy. It can be seen that when tested immediately after mixing, the V-funnel flow time for mixes with dry aggregate increases from 9s to 19s when tested at 0 and 20 minutes after mixing, respectively.
However, SCC might be more susceptible than ordinary concrete because: (a) the mix design is more complex as it contains more ingredients that each of them might affect the rheological properties differently, (b) SCC it is likely to be more thixotropic than ordinary concrete, and (c) the requirements from SCC are by far more demanding, thus what might be acceptable for ordinary concrete, might not meet SCC requirements. Cyr, et al.[vii] have shown that different superplasticizers and mineral admixture affect differently the rheological properties including shear thickening. For the same time intervals, the corresponding increase in the V-funnel flow time is only 2-second for mixtures with aggregates in the SSD condition.
Higher aggregate content increases the yield stress and viscosity, so does aggregate with high aspect ratio.[xii] Similarly Assaad and Khayat[xiii] showed that an increase in the coarse-to-fine aggregate ratio and an increase of size of aggregate bring about a significant increase in the rate of stiffening. The effects of the superplasticizer:binder ratio (by wt.) on slump flow of concrete at w:b ratio of 0.39 (Bonen and Shah) Bonen and Shah reported on the effects of the superplasticizer content, coarse aggregate-to-cement ratio, and fine aggregate-to-coarse aggregate (c:f) ratio on the flow properties of concrete. Forced pan mixers have higher mixing efficiency than drum or mortar mixers (Deshpande and Olek,[iv] and Takada et al.). performed laboratory investigation of the effect of mixer type on fresh concrete properties of SCC and concluded that for the same composition and mixing sequence, tilting drum mixer increases the V-funnel flow times of SCC as compared to SCC mixed in pan mixer, and to achieve the same slump flow (650±30 mm), smaller amounts of superplasticizer (SP) were needed in the tilting drum mixer. It was reported that for the same water-to-powder ratio by volume (Vw/Vp), longer mixing times for 7.5 and 3.5 minutes in gravity mixtures required lower SP dosages and resulted in higher slump flow values and low V-funnel flow times as compared to shorter mixing times of 5.5 and 2.5 minutes.