NEW ADMIXTURE-SYSTEM FOR LOW-FINES SELF-COMPACTING CONCRETE

Transcription

1 NEW ADMIXTURE-SYSTEM FOR LOW-FINES SELF-COMPACTING CONCRETE Joana Roncero (1), Mario Corradi (2) and Rabinder S. Khurana (2) (1) BASF Construction Chemicals España, S.L., Barcelona, Spain (2) BASF Construction Chemicals Italia, Spa, Treviso, Italy Abstract The use of self-compacting concrete (SCC) as an every-day s concrete is, nowadays, difficult especially in the case of the Ready-Mixed Concrete Industry. This is mainly due to the higher cost of this material, which, very often, is supplied with excess in the required mechanical performances with the aim of fulfilling the fresh requirements. On the other hand, the use of extra fines implies, sometimes, new silos and more quality control increasing, consequently, the production costs. Along these lines, the development of new and more efficient Viscosity Modifying Agents (VMA) permits designing SCC with high stability and compressive strengths in the range of 25 to 35 MPa without incorporating extra fines to improve the cohesion of the concrete. These new molecules help to produce SCC having total fines content (cement + fines) lower than kg/m 3. The use of this new family of VMAs, along with new superplasticizers specially designed for SCC, leads to an improvement in the cost-effectiveness of the concrete, which can contribute to expand the use of SCC in the Ready-Mixed Concrete Industry. Along these lines, the present work shows the results obtained using a new VMA compared to traditional ones. Additionally, the robustness of mixes has been evaluated. 1. INTRODUCTION Generally, Self Compacting Concrete (SCC) is characterized by a significantly high volume of cement paste (between 35 to 40%) along with low volume of coarse aggregates. The high volume of paste is obtained by using high content of fines including both cement and filler, which is usually within the range of kg/m 3. This high volume of fine particles is needed in order to obtain the required cohesiveness into the mix in order to avoid bleeding and segregation. Nowadays, the use of SCC with high amount of fines represents a technology extensively implemented specially in the precast industry. These concretes are considered as a type of 875

2 High Performance Concretes that provide high mechanical performance, as well as, the required fresh properties without bleeding and segregation. On the other hand, the characteristics of the locally-available materials (cement, aggregates and fillers) for SCC production are not always the most appropriate ones and lead to a significant amount of laboratory work to adjust the mix design in order to obtain a stable concrete. Note that, unfortunately, all the methods for mixture proportioning described in the literature [1-6] are empiric-based. Moreover, variations in the moisture content of the aggregates and of the filler influence significantly the stability of the SCC implying several adjustments in the mix design and further control of the concrete in the plant. Consequently, the use of SCC as an every-day s concrete is, nowadays, difficult especially in the case of the ready mixed concrete industry. Note that, the required mechanical performances in most of the applications are significantly lower than those provided by standard SCC. Consequently, SCC is supplied, very often, with excess of the required mechanical performances with the aim of fulfilling the requirements of the fresh state. This leads, of course, to a higher cost of the concrete. On the other hand, the use of extra fines implies, sometimes, new silos and more quality control leading, therefore, to an increase in production costs. Along these lines, the development of new and more efficient Viscosity Modifying Agents (VMA) allow to design SCC without incorporating extra fines and, consequently, having a total content of fines (cement + filler) in the range of 350 to 370 kg/m 3, providing the required stability on the concrete. The use of this new family of VMAs, along with superplasticizers especially designed for SCC, leads to an improvement in the cost-effectiveness of the concrete that can contribute to expand the use of SCC in the ready mixed concrete industry. The present work summarizes the results of an experimental study performed in BASF Construction Chemicals with the aim of developing Low-Fines Self-Compacting Concrete (lfscc) that could contribute to extend the use of SCC in the ready mixed concrete industry. Along these lines, a new VMA has been developed and compared to traditional ones. 2. EXPERIMENTAL DETAILS Different cements of type CEM I 42.5R, CEM II/A-V 42.5R and CEM II/A-L 42.5R were used in the study. Locally-available crushed limestone sands with the grain size ranges of 0-2 mm and 0-4 mm and crushed limestone gravels with the size ranges of 5-12 mm and mm were used in the concretes. State of the art polycarboxylate-based superplasticizer was also used. The new VMA consists on a synthetic polymer specially developed to optimize the cohesion of the cement paste by influencing significantly the plastic viscosity without affecting the yield point. The fresh properties of the lfscc were evaluated through slump-flow test, V-funnel test and the J-Ring Test [7]. 876

3 3. RESULTS AND DISCUSSION Several mix designs with cement contents below 350 kg and without extra filler were prepared in order to evaluate the robustness of the new VMA towards changes of the mix design. Table 1 summarizes some of the mix-compositions tested. Table 1: Mix-composition of preliminary lfscc Composition (kg/m 3 ) lfscc-1 lfscc-2 lfscc mm sand mm sand mm gravel mm gravel Cement I 42.5R total water added total w/c 0,6 0,6 0,6 effective w/c (saturated aggregates with dried surface) 0,55 0,55 0,55 Superplasticizer dosage, in % by cement weight 2,90% 2,90% 2,90% VMA dosage, in % of actives by cement weight 0,15% 0,12% 0,09% Slump Flow, in cm Note that all the concretes show good stability without bleeding and segregation. As can be seen in the table, lfscc-1 and lfscc-2 concretes were prepared using a continuous grading curve, whereas, lfscc-3 presents a gap-grading curve with a significant decrease in 5-12 mm gravel. Results obtained show good robustness of the new VMA in front of changes in mixdesign of the lfscc. Further testing was performed in order to optimize even more the mixture composition of the concrete by decreasing the cement content up to 320 kg/m 3. Table 2 show the mixture composition of the concrete prepared. The concrete shows good appearance without bleeding or segregation as can be seen in detail in Figure 1. In order to evaluate the robustness to water content of the lfscc incorporating the new VMA, the same concrete was prepared again but incorporating 10 liters/m 3 more of water. This leads to an increase in Slump-Flow up to 66 cm and to a decrease in V-funnel time up to 7 seconds. The appearance of this concrete is quite good without showing bleeding and segregation. It is important to note that, using the same mix-design, lfscc prepared with cement II/A-L 42.5 R and II/A-P 42.5 R show, also, good results. This seems to indicate that the new VMA also provides good robustness towards changes in cement types. Further testing is still being performed in order to confirm the results. 877

4 Table 2: Mix composition of lfscc with 320 kg/m 3 of cement Composition (kg/m 3 ) lfscc mm sand mm sand mm gravel mm gravel 657 Cement I 42.5R 320 total water added 185 total w/c 0,58 effective w/c (saturated aggregates with dried surface) 0,53 Superplasticizer dosage, in % by cement weight 2,90% VMA dosage, in % of actives by cement weight 0,045% Fresh Properties Slump Flow, in cm 64 J-Ring, in cm 62,5 V-funnel, in s 12 Figure 1: Detail of lfscc-4 described in Table 2. On the other hand, the performance of the new VMA was compared to those of several commercial VMA belonging to different chemical families (natural gums, starches, synthetic polymers, others). In order to evaluate the effect of the different VMA on the rheology of the concrete, the mortar from concretes prepared with several VMA were sieved at 2 mm and 878

5 tested with a Viskomat NT rheometer. Note that the dosage of each VMA was adjusted in order to obtain a slump flow of about 65 cm in each concrete. Results are shown in Figure 2. Torque (Nmm) Starch A Synthetic Polymer B Starch B Natural Gum D NEW VMA for LF-SCC Synthetic Polymer A Natural Gum C REFERENCE Natural Gum B Natural Gum A Speed (rpm) Figure 2: Influence of several VMA on the rheology of mortar compared to a reference mortar without VMA and to the new VMA developed for lfscc. Note that the mortars tested in the rheometer were obtained by sieving the concretes. Note that, all the concretes with standard VMA show bleeding, which was extremely high in some cases. The concrete prepared with the new VMA developed for lfscc shows good stability without bleeding or segregation. Along these lines, as can be seen in Figure 2, the new VMA developed for lfscc is able to increase significantly the plastic viscosity of the mortar without influencing the yield value and, consequently is able to provide stable concrete without affecting significantly the slumpflow. Some standard VMA lead to a significant increase in the yield value but without providing significant improvement of the plastic viscosity. However, in other cases, the VMA does not affect the yield value but without providing a significant increase in the plastic viscosity. Note that Natural Gum A and B do not affect either the yield value or the plastic viscosity when compared to the reference mortar. 879

6 4. CONCLUSIONS The following conclusions can be drawn in the range of materials and variables examined: The new VMA developed for lfscc permits obtaining SCC with fines content (cement + filler) lower than kg/m 3. This can contribute to expand the use of SCC in the ready mixed concrete industry as an every-day s concrete The new VMA for lfscc shows good robustness in front to changes in mix design, cement type and water content. The new VMA for lfscc provides enough cohesion and stability in concretes with low amount of fine particles by means of increasing significantly the plastic viscosity of the mortar without affecting the yield value. REFERENCES [1] Okamura, H., Self-Compacting High-Performance Concrete, Concr. Intnl. 19 (7) (1997) [2] Ouchi, M.; Hibino, M. and Okamura, H., Effect of Superplasticizer on Self-Compactability of Fresh Concrete, Transportation Research Record, 1574 (1997) [3] Gomes, P.C.C.; Gettu, R.; Agulló, L. And Bernard, C., Experimental Optimization of High Strength Self-compacting Concrete, Proceedings of the Second Intnl. Symp. On Self-Compacting Concrete, Tokio, 2001 (Eds. K. Ozawa and M. Ouchi, COMS Engng. Corp.) [4] Sedran, T.; De Larrard, F.; Hourst, F. and Contamines, C, Mix Design of Self-Compacting Concrete, Production Methods and Workability of Concrete, Eds. P.J.M. Barros, D.L. Marrs and D.J. Cleand (E&FN Spon, London, 1996), [5] Petersson, Ö.; Billberg, P. and Van, B.K., A Model for Self-Compacting Concrete, Production Methods and Workability of Concrete, Eds. P.J.M. Barros, D.L. Marrs and D.J. Cleand (E&FN Spon, London, 1996), [6] Skarendahl, Å and Petersson, Ö., Self-Compacting Concrete, State-of-the-Art Report of RILEM TC 174-SCC, Report 23 (RILEM Publications S.A.R.L., Cachan, France, 2000) 154p. [7] EFNARC, Specification and Guidelines for Self-Compacting Concrete, (2002) 32p. 880