EFFECT OF MAXIMUM AGGREGATE SIZE IN AIR-ENTRAINED ECO-SCC

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1 EFFECT OF MAXIMUM AGGREGATE SIZE IN AIR-ENTRAINED ECO-SCC Florian V. Mueller, Olafur H. Wallevik ICI Rheocenter, Reykjavik University, Innovation Center Iceland Abstract This paper evaluates an extreme type of SCC, which has very little powder content compared to other SCC, and therefore high water-binder ratio. Such a ratio leads to very low plastic viscosity, which acts the opposite of stability in fresh concrete. Investigations cover the effect of different maximum aggregate sizes on rheology and some hardened concrete properties. Measurements are taken on fresh concrete properties with a co-axial cylinders viscometer, and compressive strength and drying shrinkage are tested. For comparison, measurements are also taken on air-entrained mixes. The resulting mixes revealed self-compacting properties for the smaller aggregate size, and air entrainment was beneficial in revealing self-compacting properties up to the midsize aggregates. The differing aggregate sizes had here no effect on compressive strengths or shrinkage strain. 1. INTRODUCTION Self-compacting concrete (SCC) often contains a smaller aggregate size than conventional concrete, primarily to reduce the risk of bigger particles settling [1,2,3,4]. The historical implementation of SCC as high-performance (HPC) or even ultra-high-performance concrete (UHPC) indicates the main application of this early SCC was as high-strength concrete in highly reinforced structures. Here, the limited maximum particle diameter together with increased paste volume reduces risk of blocking and ensures sufficient form filling ability. The more prevalent concretes in building are lower and medium strength concrete, where the common degree of reinforcement (ρ) is lower and therefore the requirements allow a larger diameter of aggregate. In these cases, the primary concern is only the maximum resistance against particle precipitation The concept of Eco-SCC [7], calls for a reduced paste volume to lubricate the aggregate particles relative to most SCC [3, 4]. A binder content of 1 percent by volume results in relatively high water content, which is required for a sufficient lubrication layer. Both of these factors result in a water/binder-ratio of about.59. Using an air-entraining agent (AEA) reduces water surface tension, incorporates air, and thereby increases the density difference between the matrix (paste) and aggregates can therefore increase the risk of segregation. The resulting effect to the w/b-ratio is a drop to approximately.54. The addition of stabilizing 664

2 admixtures (ST) or supplementary materials such as silica fume increases stability. Furthermore, the relatively high w/b-ratio causes relatively low plastic viscosity, meaning the inner resistance of the concrete against movement is low. It follows then, that large particles would be susceptible to settling with such a paste, even more so with a high paste volume, which provides space for settlement. Here, aggregate grading helps prevent settlement due to the particle lattice effect of the continuous particle grading [7]. The particle lattice effect is, in short, the stabilising effect that smaller particles have on the next-larger group of aggregates. Along with the stabilising effects, the structure and volume of the particles causes a relatively high yield stress, corresponding to lower fluidity in Eco-SCC compared with standard SCC consisting of high powder volume. 2. MATERIALS AND METHODS 2.1. Materials The cement used is a Danish rapid-hardening Portland cement, classified according to EN 196 as CEM I 52.5 N. As dispersing admixture (SP), a polycarboxylate ether-based polymer from ResconMapei is used. The air-entraining admixture (AEA) used is a surface-active polymer from ResconMapei. The stabilizing admixture (ST) used is a polysaccharide based, semi-natural gum from CP-Kelco. The aggregate grading curves consist of two different fractions: one fraction sand to 8 mm and a coarse fraction. Here, the aggregate consists of one fraction 8 to11.2 mm, one fraction 8 to 16 mm or one fraction 8 to 22.4 mm. They have only slightly different physical properties (table 1). Table 1: Images and physical properties of investigated aggregates (a) Sand -8 mm (S8) Density: 265 kg/m³. Water of absorption (SSD):.6% (b) Coarse mm (St11.2) Density: 279 kg/m³. Water of absorption (SSD):.4% 665

3 (c) Coarse 8-16 mm (St16) Density: 267 kg/m³. Water of absorption (SSD):.4% (d) Coarse mm (St22.4) Density: 275 kg/m³. Water of absorption (SSD):.5% 2.2. Methods The most important property of SCC that differs from conventional concrete is the ability of the fresh concrete to consolidate without external energy. This is possible because of the significantly reduced yield stress of SCC compared with conventional concrete. The other property involved is the inner resistance against flow and deformation of the fresh concrete. Assuming the materials behaviour is related according to the Bingham-model ( τ = τ + & µ γ ), having the material parameters plastic viscosity (µ), which is in linear relation to the shear rate (γ& ) and shear stresses and the yield value ( τ ). Coaxial cylinder viscometer, such as the ConTec Viscometer 5 (Figure 2) a more advanced version of the co-axial cylinders BML-Viscometer [6, 8], can determine both these parameters for a given shear history (figure 1). The spread of slump flow out of Abram s cone mainly reflects the yield value, but the homogeneity is in away an indicator of plastic viscosity. The rule of thumb is (without the particle lattice effect) the lower the plastic viscosity, the higher the yield value has to be to avoid segregation. Such mixes are also naturally less fluid. 666

4 Angular velocity [rad/s] Data points Figure 1: Shear history applied during test. The data points are detected within 2 seconds at each velocity. Figure 2: ConTec Viscometer 5 The compressive strength is determined for all mixes, and beam drying shrinkage is calculated using measurements similar to ASTM C157. Shrinkage specimens undergo air curing in 23 C and 5%-RH straight after demoulding after 24 hours Mix design The mix designs (table 2) include reference mixes for each particle size distribution (PSD) without AEA and one related mix, aiming for air-entrainment of about 7 percent. Table 11: Mix-designs (recalculated) [kg/m³] St11.2 St 11.2+air St 16 St 16+air St 22.4 St 22.4+air CEM 52.5 N water total Sand Stone Stone Stone SP(PC)* ST stabilizer AEA # * Solution of polymers with dry-content of 28% # Solution of polymers, originally dry-content of 6.3%, then diluted further in water in ratio 1:9 667

5 Aggregates moisture and water from admixtures included in total water content, aggregates in SSD. The composition of the grading curve with different coarse particle fractions yields three grading curves, predominantly differing in content of particles beyond 8 mm (figure 3). 1 Cumulative PSD [%] S 8 + St 22.4 S 8 + St 11.2 S 8 + St 16,63,125,25,5 1, 2, 4, 8, 11,2 16, 22,4 32, Particle size [mm] Figure 3: Grading curve of aggregates 3. RESULTS AND DISCUSSION 3.1. Fresh concrete properties The increased D max (maximum aggregates size) does not change the surface of solids significantly compared with increasing fines content. Only little reduced surface with increased D max can be calculated. It usually results in higher paste volume available as lubricant layer and therefore increased fluidity. The results show opposite effect (figure 4), which leads to the conclusion that for this paste volume the aggregates bulk rheology becomes significant. Addition of water or SP cannot increase the fluidity further, as the slump flow (figure 6) already shows segregation signs (figure 5). Here, the doubled data points indicate burst spread with an inner diameter including all coarse aggregates and an outer diameter including the mortar or paste separated. 668

6 12 Yield value [Pa] St11.2 St16 St22.4 St11.2+air St16+air St22.4+air Plastic viscosity [Pa s] Figure 4: Yield value and plastic viscosity of mixes with different D max, with and without AEA Note: The dotted line expresses approximated recommendations [9] for rheological parameters to obtain self-compacting properties in fresh concrete: The higher the plastic viscosity, the lower the required yield stress. Air-entrainment, by use of AEA, increases the paste volume but lowers the plastic viscosity (figure 4). The surface of the air bubbles also requires paste and therefore reduces the volume available for bleeding or segregation (figure 7). This can be beneficial for mixes in the initial state of segregation. More severe segregation can be reduced slightly by air-entrainment but really needs other mix-design variations for best effect. Hence, the rheological properties of the air-entrained mixes, for the small and midsize aggregates, exceed values that correspond with very good self-compacting abilities (figure 4). This ability could work in practise for the largest maximum aggregates size of 22.4 mm. Fluidity, expressed by the spread of slump flow (figure 5), increased remarkably for the air-entrained mixes compared to the corresponding mixes without AEA. Here it resulted in homogenous dispersed spread without signs of segregation (figure 7). 669

7 7 Slump flow [mm] 6 St11.2 St16 St22.4 St11.2+air St16+air St22.4+air Time [min] Figure 5: Spread of Slump flow at different testing times Note: The doubled points reflect burst slump flow. Here, the smaller diameter includes coarse aggregate and mortar, whereas the larger diameter includes mortar/paste-rich circle. It is a sign for occurring segregation. Compared with other possible segregation corona, the ones documented here are very decent; see also figures 6 and 7. Figure 6: Slump flow of St mm + paste 2x5 mm (burst) Figure 7: Slump flow of St 11.2+air 65 mm 3.2. Hardened concrete properties Varied D max between 11.2 mm and 22.4 mm did not affect compressive strength (figure 8). The increase of voids by about 5 percent, i.e. from 2 percent in the reference to about 7 percent with AEA, yield in average reduction of about 1 MPa, which is about 2 MPa per one volume percent air. The concrete fulfils a strength criterion according to EN 26-1 for class C25/3 in the case of air-entrainment, and for class C35/45 without air-entrainment (figure 8). The density is coherently reduced by air-entrainment in fresh concrete (figure 9). 67

8 Compressive strength [MPa] St11.2 St16 St22.4 St11.2+air St16+air St22.4+air Density (fresh) [kg/m³] Density [kg/m³] Figure 8: Compressive strength on cylinders at 28 days, in relation to density of hardened concrete Air content [%] Figure 9: Density of fresh concrete in relation to air-content Drying shrinkage (figure 1) of beams in 23 C and 5%-RH revealed no significant influence when increasing maximum aggregate size from 11.2 to 22.4 mm while keeping the paste volume constant. The air-entrained mixes tend to reveal little reduced shrinkage but it remains unclear whether the air-entrainment or other, methodical, influences affect the results, due to the sensitivity of the method (figure 11). A reduction could be confirmed by findings of others, for example Kronlöf et al [5] who reported reduction of shrinkage strain in air-entrained mortars, probably due to lowering the capillary pressure in the presence of AEA. Hence, in concrete it reveals an insignificant reduction. 1 5 Shrinkage strain [mm/m],8,6,4, St 11.2 St 16 St 22.4 St11.2+air St16+air St22.4+air Evaporation [%] Time [d] Time [d] Figure 1: Drying shrinkage strain of beams in 23 C and 5%-RH Figure 11: Evaporation of water on shrinkage beams in 23 C and 5%-RH 671

9 4. FINAL REMARKS The use of different coarse aggregate fractions with maximum diameter of 11.2, 16, and 22.4 mm revealed rheological properties that most likely required some form of compaction energy for full consolidation. Incorporating about 7 percent air by AEA enhanced the rheological properties to probably self-compacting properties. For the grading curve containing the largest D max of 22.4 mm, this ability might have to be proven for the practical application and placement conditions. Due to air-entrainment, the compressive strength was reduced by some 2 MPa per 1 litres of additional air, while the air-entrained SCC fulfils the strength criterion for Class C25/3 according to EN Shrinkage measurements on beams revealed no significant effects in respect of the different aggregate sizes. The air-entrained mixes tend to exceed little reduced shrinkage strain (-.5 mm/m) compared to the references without AEA. After a drying time of about 18 months, this difference is not very significant, especially when compared to effects caused by small variations in ambient environment. From the aggregates investigated, the recommended maximum aggregate size would be the grading curve with 16 mm. Air-entrainment of about 7 percent is recommended and beneficial to reveal self-compacting properties in Eco-SCC. It results in a fresh concrete that reveals self-compacting properties and fulfils strength requirements of C25/3 that is an unusual low value for SCC, but the strength class very often used in practical application. ACKNOWLEDGEMENT The authors gratefully acknowledges the support of Aalborg Portland (Denmark), CEMEX Research Group AG (Swiss), CP-Kelco (USA), Icelandic Research Foundation Rannis, MEST Steypustadin (Iceland), Norstone (Norway), Norcem (Norway), ResconMapei (Norway) for the project Eco-SCC. REFERENCES [1] Alexander, M. and S. Mindess: Aggregates in Concrete, Modern Concrete Technology 13, Taylor & Francis Group, 28 [2] Saak, A.W., H. M. Jennings, and S.P. Shah: New Methodology for Designing Self-Compacting Concrete, ACI Materials Journal, V. 98, No.6, , 21 [3] European Project Group: The European Guidelines for Self-Compacting Concrete - Specification - Production and Use, 25 [4] Domone, P.L.: Self-Compacting concrete: An analysis of 11 years of case studies, Cement and Concrete Composites, Vol. 28, , 26 [5] Kronlöf, A., M. Leivo, and P. Sipari: Experimental Study on the Basic Phenomena of Shrinkage and Cracking of Fresh Mortar, Cement and Concrete Research, Vol. 25, No. 8, , 1995 [6] Wallevik, O.H.: The Rheology of Fresh Concrete and its Application on Concrete with and without Silica Fume, PhD-thesis, The Norwegian University of Science and Technology, Trondheim, Norway, 199 (in Norwegian) [7] Wallevik, O.H., F.V. Mueller, and B. Hjartarson: Eco-SCC, an environmental and economical 672

10 alternative, Proceedings of the ICCX Oceania 29, Sydney, Australia, 16-2, 29 [8] Wallevik, O.H. and O.E. Gjørv: Development of a Coaxial Cylinder Viscometer for Fresh Concrete, Properties of Fresh Concrete Proceedings of the RILEM Colloquium, Chapman & Hall Hanover, , 199 [9] Wallevik, O.H., Rheology A Scientific Approach to Develop Self-Compacting Concrete, 3rd Int. Symposium on SCC, Rilem, Reykjavik, August, 23, pp