Self-Consolidating High Performance Concrete – SCC – Self-Compacting & Highly Flowable Concrete

A lack of robustness can be manifested in several ways that affects workability and the other assigned properties of SCC, i.e., flowability, passing ability, and stability. Following is a review of the effects of ingredients on the rheological properties that affect robustness.

Tattersall and Banfill[i] and Banfill[ii] showed that the yield stress and plastic viscosity values are exponential functions of the water and superplasticizer contents, and the flow characteristics of the cementitious materials are related to structural buildup during rest and structural breakdown due to remixing. Roy and Asaga[iii] concluded that a change from the least severe to the most severe mixing procedure caused both the yield stress and plastic viscosity to decrease by about 60%. 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.

Cyr, et al.[vii] have shown that different superplasticizers and mineral admixture affect differently the rheological properties including shear thickening. Accordingly, the shear thickening is increased in the presence of metakaolin, ground quartz and fly ash have no effects on it, whereas silica fume reduces it. Banfill[viii] however reported that substitution of up to 60% of the cement by fly ash reduces the yield stress, but has little effects on the plastic viscosity.

Carlsward, et al.[ix] have studied the effects of entrapped air, silica fume, limestone, and moisture on the rheological properties. It has been shown that the air content increases the slump flow, reduces the plastic viscosity, but has little effect on the shear stress. Silica fume thickens the mixture, the shear stress is substantially increased, the plastic viscosity is moderately increased, and the slump flow is strongly decreased. By contrast, limestone has little effects on the plastic viscosity and the slump, but increases the shear stress. Similarly, Assaad and Khayat,[x],[xi] showed that incorporation of pozzolanic materials such as the silica fume, fly ash, and blast furnace slag can increase internal friction of the cement paste and the shear stress.

In addition, the mean interparticle distance play a significant role on the flow characteristics of SCC as it affects the rheological properties and the capacity to flow through obstacles. 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.

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. 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). Similarly, the robustness of the flow is proportioned to the agg:b volume and SP:b wt%. 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). Provided the aggregate is spherical, the beneficial role of fine aggregate is related to its ball-bearing effect. Khayat et al. 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. This was especially the case for concrete with low water-to-binder (w:b) ratio of 0.33. The increase in paste volume is also shown to reduce the plastic viscosity of SCC.

Similar to the fluidity, the viscosity is readily changed by changing the superplasticizer and aggregate contents (Fig. 3). The effects of the latter on viscosity cannot be overlooked as the effects are as important as the role of superplasticizer. The figure also shows that the effect of aggregate on viscosity is exponential.

Clearly, all these ingredients, especially water, superplasticizer, and aggregate, affect the rheological properties differently. Thus, in order to minimize variations that stem from small changes in quantities of these ingredients in successive batches, it is advantageous to add VMAs (viscosity-modifying admixtures) to the mixtures as even small additions of VMA considerably increase the plastic viscosity and cohesion of the mixture. VMAs are often water-soluble polymers or inorganic substances with very high surface area that bind water upon mixing. A review on the effects of the various VMAs is given elsewhere.[xv] Addition of VMA counters the strong adverse effects of small variations in the water and superplasticizer contents, and the contents of the other ingredients. Indeed, Shi, et al.[xvi] has shown that the flow loss of VMA-free mixtures is higher than in VMA mixtures.

The high flowability and deformability of SCC derives from the characteristically low values of yield strengths and plastic viscosity. 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.

---------------------------------------------------------------------------------------------------------------------------

[i] Tattersall, G.H. and Banfill, P.F.G., The Rheology of Fresh Concrete, Pitman Advanced Publishing Program, Boston, London, Melbourne, 1983, p. 356.

[ii] Banfill, P.F.G., “A Viscometric Study of Cement Pastes Containing Superplasticizers with a Note on Experimental Techniques,” Magazine of Concr. Res. 33(114), 37-47, 1981.

[iii] Roy, D.M. and Asaga, K.,” Rheological properties of cement mixes: III The effects of mixing procedures on viscometric properties of mixes containing superplasticizers,” Cem. Concr. Res. 9, 731-739, 1979.

[iv] Banfill, P.F.G., “A viscometric study of cement pastes containing superplasticizers with a note on experimental techniques,” Magazine of Concr. Res. 33 (114), 37-47, 1981.

[v] Jones, T.E.R. and Taylor, S.A.,” Mathematical model for the flow curve of cement paste,” Magazine Concr. Res. 29 (101) 207-212, 1977.

[vi] Douglas, R., Gregori, A., Sun, Z., Bonen, D. and Shah, S.P., “The Effect of ingredients and shear history on the thixotropic rate of rebuilding of SCC,” pp. 591-596 in 2^nd North American Confer. & 4^th Intern. RILEM Confer. On Self-Consolidating Concrete, Chicago, 2005.

[vii] Cyr, M., Legrand, C. and Mouret, M.,” Study of the shear thickening effect of superplasticizers on the rheological behaviour of cement pastes containing or not mineral additives.,” Cem. Concr. Res. 30, 1477-1483, 2000.

[viii] Banfill. P.F.G.,” An experimental study of the effect of pfa on the rheology of fresh concrete and cement paste,” pp 161-171 in Proceedings, Intern. Symp. on the Use of pfa in Concrete, Leeds, eds. J.G. Cabrera and A.R. Cusens, University of Leeds, April 1982.

[ix] Carlsward, J., Emborg, M., Utsi, S. and Oberg, P., “Effects of constituents on the workability and rheology of self-compacting concrete,” pp. 143-153, in Self-Compacting Concrete, Proceedings, 3^rd Intern. RILEM Symp. Eds. O. Wallevik and I Nielsson, RILEM Publications S.A.R.L. August 17-20, 2003.

[x] Assaad, J., Khayat, K.H., Mesbah, H., “Variation of formwork pressure with thixotropy of Self-Consolidating Concrete,” ACI Materials Journal, Vol. 100(1) 29-37, 2003.

[xi] Assaad, J., “Formwork Pressure of Self-Consolidating Concrete – Influence of Thixotropy,” Doctoral Thesis, Université de Sherbrooke, 2004, 450 p.

[xii] Geiker, M.R., Brandl, M., Thrane, L.N. and Neilsen, L.F. “On the Effect of Coarse Aggregate Fraction and Shape on the Rheological Properties of Self-Compacting Concrete,” Cem. Concr. Agg. 24[1] 3-6, 2002.

[xiii] Assaad, J., and Khayat, K., “Effect of coarse aggregate characteristics on lateral pressure exerted by self-consolidating concrete,” ACI Materials Journal, Vol. 102(3) 145-153, 2005.

[xiv] Ye, Y., Bonen, D. and Shah, S.P., “Fresh Properties and Segregation Resistance of Self-Compacting Concrete,” pp. 621-627 in 2^ndt North American Confer. & 4^th Intern. RILEM Confer. On Self-Consolidating Concrete, Chicago, 2005

[xv] Khayat, K.H., “Viscosity-enhancing admixtures for cement-based materials-an overview,” Cem. Concr. Composites, 20, 171-178, 1988.

[xvi] Shi, C., Wu, Y., Shao Y. and Riefler, M.,” Comparison of two design approaches for self-consolidation concrete,” pp. 313-317 in 1^st North American Confer. On the Design and Use of Self-Consolidating Concrete, Eds. S.P. Shah, J.A.Daczko and J.N. Lingscheit, November12-13, 2002.

[xvii] Wallevik, O.H. and Nielsson, I.,“Self-compacting concrete- a rheological approach,” Proceedings of the International Workshop on Self-Compacting Concrete, Japan, 21 p., 1998.