Properties of self consolidating concrete for prestressed members
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.
It was shown that for any content of superplasticizer-to-binder (SP:b) ratio, the slump flow increases as the aggregate-to-binder (agg:b) ratio decreases (Fig 1). G., “A viscometric study of cement pastes containing superplasticizers with a note on experimental techniques,” Magazine of Concr. By contrast, SCC produced in pan mixer was found to be prone to changes, which compromised the robustness of the mixtures. Mixtures with the same w:p volume ratio were mixed in a mortar mixer and a conventional laboratory pan mixer. "Effect Of Mixing Equipment And Mixing Sequence On Rapid -Setting Self-Consolidating Concrete." pp. However, in the case of forced pan mixers, in order to obtain the same degree of deformability, mixtures that mixed for 5 minutes required higher dosages of SP than the corresponding mixtures with the same Vw/Vp that mixed for 3.5 minutes.
Robustness can be regarded as the ability of the SCC mixture to maintain both the fresh properties and composition pre- and post-casting of one batch or successive batches, due to the composition of the mixture and due to some small changes in the contents of the ingredients of the mixture. 143-153, in Self-Compacting Concrete, Proceedings, 3rd Intern. By contrast, neither silica fume nor most types of fly ash can be considered as density modifiers. Sakata et al.[i] reported that SCC made with low w:p ratio of 0.33 (powder containing limestone filler), the incorporation of a small concentration of welan gum of 50 g/m3 can reduce the variability in slump flow of SCC due to changes in cement Blaine (318 to 342 m2/kg), fineness modulus of sand (2.08 to 3.06), and temperature of fresh concrete (10 to 30°C). The authors concluded that the slump flow value tends to prominently decrease with an increase in natural moisture content of fine aggregate for mixtures with 0.35 w:c ratio as opposed to 0.5 w:c ratio.
Robustness depends on a number of different attributes including the specific composition of the mixture, the mixing history, i.e. The density of fly ash varies over a large range, commonly from about 2 to 2.5 g/cm³ and that of silica fume is about 2.24 g/cm³. Hwang and Khayat[ii] suggested using the minimum water content (MWC) index to determine the robustness of SCC. A strong influence on slump flow was observed by Sakai et al.[ii] when the amount of water was changed by 5 kg/m2.
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.
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.
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.