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

In the last two sections, the effects of the major ingredients on the stability of SCC are discussed. To reiterate, because of the inherent low values of yield stress and viscosity, SCC is especially prone to segregation under static conditions (Figs. 4 and 5). In view of the central role of segregation (that is manifested by sedimentation of aggregate as well as migration of paste and air voids to the top of the element and bleeding, several test methods have been proposed for evaluating the stability of the mixture.

One popular method is based on visual stability index (VSI) of the slump flow of SCC and rating it visually from 0 to 3 in increments of 0.5, where a 0 rating represents no segregation and a rating of 3 represents severe segregation.[i] However, in accordance with Section 2.2, a visual inspection of slump flow is applicable to dynamic stability, but is an inadequate measure to evaluate the static stability of the mixture.

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. 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. Additional methods make a correlation between the measured rate of sedimentation of aggregate and the rate of a penetration device. Bui and his coworkers practiced with penetration apparatus that was placed on the leg of L-box two minutes after pouring the concrete into the L-box and measuring the depth of penetration after 45 s. It was claimed that a satisfactory segregation resistance is achieved if the penetration depth of the cylinder head of the apparatus is less than 8 mm.[iv] Another version of the above apparatus was based on the penetration depth of a hollow metal cylinder.[v]

More recently, Shen et al.[vi] developed a new penetration probe made of a 130 mm diameter ring connected with a 150 mm high rod marked with scale. A smaller 100 mm diameter ring has also been evaluated. The probe is made of 1.6 mm diameter steel wire and its total weight is about 18 g. Concrete is cast into a 150 × 300 mm cylinder, and after 2 min of undisturbed rest, the probe is placed on the concrete surface for 1 min. The stability rating is evaluated according to Table 1.[vii]

The robustness curves of the three base mixes made with graded aggregate, mineral filler, and VMA are compared in Figure 9. The mix design of the mixtures is given in Table 2.

Two parameters need to be examined when comparing the robustness curves: (a) the slope of the w/cm vs. penetration depth curve, and (b) the margin between target w/cm and the w/cm with maximum penetration depth. A flatter slope and larger margin indicate higher robustness. According to the slope and margin, the robustness of the three base mixes is rated in the order VMA > graded aggregate > mineral filler. The higher robustness of the VMA mix is attributed to the increase in viscosity. Graded aggregate also help to enhance robustness, probably because gradation of fine and coarse aggregates can achieve a lattice effect where small aggregates can resist the settlement of middle-sized ones, which in turn resists the settlement of large aggregates.[viii]

Table 2: Mix Proportions of SCC for Robustness Test

			Material kg/m³						Admixture ml/m³
Mix ID	Mix Modification	w/c	Cement (Type I)	Fly Ash (C)	CA1	CA2	FA	H20	SP (Grace Adva cast 530)	VMA (Master Builders)
	base	0.38	392	93	218	638	833	185	1377
Graded Aggregate	GA +5%	0.38	448	106	201	589	769	211	1236
	GA -5%	0.38	336	80	234	687	897	158	1413
	base	0.33	357	193	810	0	793	179	1413
Mineral Filler	MF +5%	0.33	404	218	741	0	734	202	1389
	MF -5%	0.33	309	167	869	0	862	155	1483
	base	0.41	407	0	966	0	824	165	1789	848
VMA	VMA +5%	0.41	474	0	896	0	766	192	1413	777
	VMA -5%	0.41	339	0	1034	0	884	138	1884	824

Figure 9. Effects of mix composition on robustness. The robustness increases

in the order: mineral filler (fly ash), graded aggregate, and VMA.

Figure 10 shows the effects of a modest variation of the paste content by ±5% on robustness. For all the three types of SCC, the increase in the paste content increases robustness, whilst a decrease of the paste content decreases the robustness. It should be noted, however, that the VMA mixture with 5% less paste could not achieve the same slump flow. Higher paste content improves robustness because it increases the viscosity, density, and yield stress of the matrix.

Figure 10. Effects of modest change in the paste content on the robustness of (a) graded aggregate, (b) mineral filler, and (c) VMA.

As discussed in Section 2.2, Fig. 11 shows the mixture with slag is more robust than with fly ash.

Figure 11. Effects of slag and fly ash mixtures on robustness

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[i] Interim Guidelines for the Use of Self-Consolidating Concrete in Precast/Prestressed Concrete Institue Member Plants, PCI, TR-6-03, 2003.

[ii] Brameshuber, W. and Uebachs, S., “ The application of self-compacting concrete in Germany under special consideration of rheological aspects,” pp. 211-126, in 1^st North American Confer. On the Design and Use of Self-Consolidating Concrete, November12-13, 2002.

[iii] Japan Society of Civil Engineers, Recommendation for Self-Consolidating Concrete, T. Omoto and K. Ozawa, eds., JSCE Concrete Engineering Series 31, 1999, pp.77

[iv] Bui, V.K., Montgomery D., Hinczak I., Turner K., “Rapid testing method for segregation resistance of self-compacting concrete”, Cem. Concr. Re.Vol 32, 1489-1496, 2002.

[v] Bui, V.K. and Shah, S.P., “Rapid methods for testing quality of fresh self-consolidating concrete,” pp. 281-285 in 1^st North American Confer. On the Design and Use of Self-Consolidating Concrete, November12-13, 2002.

[vi] Shen, L., Struble, L. J., Lange, D., “Testing static segregation of SCC”, pp. 729-735 in 2^nd North Amer. Conf. & 4^th Int’l RILEM Conf. on Self-Consolidating Concrete, Chicago, 2005.

[vii] Brinks, A. J., Lange, D. A., D’Ambrosia, M. D., and Grasley, Z. C., “A layered finite element model for the analysis of segregated concrete”, In draft.

[viii] Jolicoeur, C., Khayat, K.H., Pavate, T.V., and Page, M., “Evaluation of Effect of Chemical Admixture and Supplementary Materials on Stability of Concrete-Based Materials Using In-Situ Conductivity Method,” pp. 461-483 in Superlasticizers and other Chemical Admixtures on Concrete, Proc. 6th CANMET/ACI Intern. Confer. SP-195, V. M. Malhotra, ed., American Concrete Institute, Farmington Hills, Mich., 2000.