APPLICATION OF HYBRID FIBER REINFORCEMENT AND HIGH VOLUME COARSE FLY ASH IN SELF COMPACTING CONCRETE

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1 CD APPLICATION OF HYBRID FIBER REINFORCEMENT AND HIGH VOLUME COARSE FLY ASH IN SELF COMPACTING CONCRETE A.A. Mounesan 1, K. Siamardi 2, M.M. Khodavirdi Zanjani 2 1 Civil Engineer, Sharif University of Technology, Head Manager of Atisaz Company 2 Civil Engineer, Concrete Research Center, Atisaz Company ABSTRACT Self compacting concrete is termed as a concrete with high flow ability and cohesiveness which can fill its mold without the need of any extra vibration effort. Fiber inclusion to concrete enhances the mechanical properties, while making the concrete less workable. This article presents a study on the fresh and mechanical properties of a fiber reinforced self compacting concrete incorporating highvolume fly ash that does not meet the fineness requirements of ASTM C 618. A poly carboxylic based superplasticizer was used in combination with a viscosity modifying admixture. In mixes containing fly ash, 50% of cement by weight was replaced with fly ash. Two different types of steel fibers were used in combination, keeping the total fiber content constant at 60 kg/m3. Slump flow time and diameter, V funnel, and air content were performed to assess the fresh properties of the concrete. Compressive strength, split tensile strength, and ultrasonic pulse velocity of the concrete were determined for the hardened properties. It can be concluded that high-volume coarse fly ash could successfully be used in producing fiber reinforced SCC. Even though there is some reduction in the concrete strength, because of the use of high-volume coarse fly ash, it is possible to achieve self compaction with considerable fiber inclusion. Keywords: self compacting concrete, fiber reinforcement, high volume coarse fly ash, fresh properties, ultrasonic test 1. INTRODUCTION Use of self compacting concrete (SCC) in the construction industry has been increasing [1] because of its technical advantages such as flowing through the reinforcement and filling every corner of its mold without any need for vibration and compaction during its placement. Generally, SCC is achieved using new generation superplasticizers to reduce the water binder ratio. In addition, supplementary cementitious or inert materials such as limestone powder, natural pozzolans, and fly ash is also used to increase the viscosity and reduce the cost of SCC. Among these materials, fly ash, a by-product of thermal power plants, has been reported to improve the mechanical properties and durability of concrete when used as a cement replacement material [2]. Concretes having large amounts

2 1062 / Application of Hybrid Fiber Reinforcement. of fly ash are termed as high-volume fly ash (HVFA) concrete. HVFA concrete was initially developed for mass concrete applications to reduce the heat of hydration, but with its sufficient mechanical and excellent durability properties it has been used in structural and pavement applications [3]. Fly ash is usually separated at the power plants and high quality (fine) fly ash meeting the fineness requirement of ASTM C 618 can be used in producing blended cements or added as a separate ingredient at the ready mixed concrete batching plants. In addition to this fine fly ash, there are vast amounts of substandard (coarse) fly ash that can be utilized in the concrete industry. A successful application of the coarse fly ash in producing blended Portland cements was published by the researchers at CANMET [4]. Fly ash has also been increasingly used in the Turkish concrete industry. Recently, to increase the use of fly ash, investigations on HVFA in producing SCC are being performed [5]. In this article another application of this type of coarse fly ash will be presented on SCCs incorporating hybrid fiber reinforcement. The term fiber reinforced concrete (FRC) is defined by ACI 116R, Cement and Concrete Terminology, as a concrete containing dispersed randomly oriented fibers. Inherently, concrete is brittle under tensile loading and mechanical properties of concrete may be improved by randomly oriented short discrete fibers which prevent or control initiation, propagation, or coalescence of cracks [6]. The character and performance of FRC changes depend on the properties of concrete and the fibers. The properties of fibers that are usually of interest are fiber concentration, fiber geometry, fiber orientation, and fiber distribution. Using a single type of fiber may improve the properties of FRC to a limited level. However, the concept of hybridization, adding two or more types of fiber into concrete, can offer more attractive engineering properties as the presence of one fiber enables more efficient utilization of the potential properties of other fibers [7-8]. Previous investigations showed that the use of steel fibers in SCC is feasible [9-10]. In these mixes, steel fibers can decrease workability of SCC as the fiber amount and slenderness ratio (length/diameter) increase. However, in case of well-proportioned SCC the workability is not influenced by the steel fibers [10]. The incorporation of fibers in concrete improves mechanical properties of concrete such as ductility, toughness, tensile strength, impact resistance and fatigue. The objective of this study is to assess the effects of HVFA replacement on the fresh and hardened properties of SCCs incorporating different types of steel fibers. Moreover, the fly ash used in this study was a coarse fly ash that does not meet the fineness requirements of ASTM C 618. Even though, the suitability of using such a substandard fly ash needs much detailed investigations, this study covers the fresh and some hardened properties of such mixes. In addition to the fly ash, two different sizes of steel fibers were used at different proportions in making the concrete. Total mass of cementitious materials is 500 kg/m3, in which 50% of cement is replaced by the coarse grained fly ash. For comparison, a control SCC mix without any fly ash was also produced. The commercially available chemical admixtures used in this study included a viscosity modifying admixture (VMA) and a polycarboxylic based superplasticizer (SP).

3 3 rd International Conference on Concrete & Development / MATERIALS 2.1. Portland Cement The cement used in all mixes was a commercially available Portland cement (PC), which corresponds to ASTM Type I cement. It had a specific gravity of 3.09 and Blaine fineness of 3030 cm2/g. Chemical composition of the PC is given in Table Limestone Powder Limestone powder (LP) was used as a mineral viscosity enhancing admixture. LP was a by product of marble extraction with a CaCO3 content of 98% and a specific gravity of The chemical composition of the limestone powder is also presented in Table Fly Ash A fly ash (FA) from Çayırhan, Turkey was used in this study. Its chemical composition is given in Table 1. The FA had a relatively low specific gravity and Blaine fineness of 2.01 and 2420 cm2/g respectively. The percentage of fly ash retained when wet sieved on a 45-μm sieve was 46. Therefore, this FA failed to meet the fineness requirements of ASTM C 618. To confirm the fineness of the FA, the particle size distribution of the FA was also determined. Figure 1 shows the particle size distribution of the FA, as well as the LP, and PC used in this study. As can be seen from that plot, FA was much coarser compared to both PC and LP. Table 1: Chemical composition of the Portland cement and mineral admixtures Chemical Portland Limestone Fly ash analyses (%) cement powder CaO SiO Al2O Fe2O MgO SO K2O Na2O LOI Fiber Two cylindrical steel fiber types, one with hooked ends (SF1) and one straight (SF2) were used. Their specific gravities were 7.85 and 7.17 respectively. The length and aspect ratio of the SF1 was 30 mm and 55, respectively, compared to 6 mm and 37.5 of SF2. The SF2 fiber was made of high strength steel with a brass coating, which provides it a relatively smooth surface. The total fiber content was kept constant at 60 kg/m3 for all the mixes Aggregates As for the aggregates, crushed limestone and crushed sand from the same local

4 1064 / Application of Hybrid Fiber Reinforcement. source were used. As can be seen from the gradation of the aggregates presented in Table 2, the maximum aggregate size was 19 mm. Both the coarse and fine aggregate had a specific gravity of 2.70, and water absorptions of 0.5 % and 1.2 % respectively. Figure 1. Particle size distribution of PC, FA and LP Table 2: Aggregate grading % passing Sieve size Coarse Fine (mm) CHEMICAL ADMIXTURES A polycarboxylic type superplasticizer (SP) was used in all concrete mixes. In addition to the SP a viscosity modifying admixture (VMA) was also used. The properties of both admixtures, as provided by their manufacturers, are shown in Table 3.

5 3 rd International Conference on Concrete & Development / 1065 Chemical Admixture Table 3: Properties of chemical admixtures Specific Solid ph Main component gravity content (%) SP polycarboxylic VMA dispersed carbohydrate 4. EXPERIMENTAL PROCEDURES 4.1. Mix Proportions The mix proportions of the mixes are summarized in Table 4. As seen in that table, five concrete mixes are prepared. The two control mixes did not contain any steel fibers. As a binder, one of the control mixes included PC (Control_PC) and the other one had FA replacing 50 % by weight of PC (Control_FA). All of the remaining mixes had the same amount of FA as in Control_FA. These were named as FA_SF1, FA_SF1&SF2, and FA_SF2 indicating the type of steel fiber incorporated in the mix. For all the mixes, the total amount of binder (PC + FA), the amount of chemical admixtures, and the amount of LP were all kept constant. Water was added to the mix until the SCC characteristics were observed; therefore, the water/powder ratio was not kept constant and change was observed between 0.35 and Table 4: Mix proportions Mix Aggregate Steel fiber Water PC FA LP ID Fine Coarse SF1 SF2 SP VMA Control_PC Control_FA FA_SF FA_SF1&SF FA_SF Preparation and Casting of Test Specimens The mixes were prepared at about 5 min. with a 70-liter rotating planetary mixer. The sand, coarse aggregate and fibers were first dry-mixed followed by the addition of fine materials and 1/3 of water. Finally, water and chemical admixtures were pre-mixed and added to the mix. After the mixing procedure was completed, tests were conducted on the fresh concrete to determine slump flow time and diameter, V-funnel flow time, and air content. Segregation and bleeding were visually checked during the slump flow test and was not observed in any of the mixes. From each concrete mix, six 150-mm cubes and six _100*200-mm cylinders were cast. All specimens were cast in one layer without any compaction. The cubes were used for the compressive strength and ultrasonic pulse velocity tests and the cylinders were used for the splitting tensile strength tests. After demolding, all specimens were stored in a curing room at 21±2 0C, and 95±5% relative humidity until testing.

6 1066 / Application of Hybrid Fiber Reinforcement Tests on Fresh Concrete Deformability and viscosity of fresh concrete is evaluated through the measurement of slump flow time and diameter, and V-funnel flow time (Figure 2). The slump flow is used to assess the horizontal free flow (deformability) of SCC in the absence of obstructions. The procedure for the slump flow test and the commonly used slump test are almost identical. In the slump test, the change in height between the cone and the spread concrete is measured, whereas in the slump flow test the diameter of the spread concrete is determined as the slump flow diameter (D). According to Nagataki and Fujiwara, a slump flow diameter ranging from 500 to 700 mm is considered as the slump required for a concrete classified as SCC [11]. According to Specification and Guidelines for SCC prepared by EFNARC (European Federation of National Trade Associations), a slump flow diameter ranging from mm can be accepted for SCC [12]. In the slump flow test concrete s ability to flow and its segregation resistance can also be measured. To measure these properties, the time (t50) it takes for the concrete to reach a 500 mm spread circle and any segregation border between the aggregates and mortar around the edge of spread are recorded. EFNARC suggests t50 of 2 to 7 sec. for SCC. In addition to the slump flow test, V-funnel test is also performed to assess the flowability and stability of the SCC. The funnel is filled completely with concrete and the bottom outlet is opened, allowing the concrete to flow. The V-funnel flow time is the elapsed time (tv-f) in seconds between the opening of the bottom outlet and the time when the light becomes visible from the bottom, when observed from the top. Good flowable and stable concrete would consume short time to flow out. According to Khayat, a tv-f which is less than 6 sec. is recommended for a concrete to qualify as a SCC [13]. According to EFNARC, tv-f ranging from 6 to 12 sec. is considered adequate for a SCC [12]. Figure 2. Workability tests on the HVFA-SCC 4.4. Tests on Hardened Concrete Tests performed on cured concrete specimens consist of the specimen compressive strength, the splitting tensile strength, and the ultrasonic pulse velocity. For each mix, cubic specimens were loaded under compressive load to failure (ultimate

7 3 rd International Conference on Concrete & Development / 1067 load) at 28, and 56 days. The compressive strength was computed from the average of three specimens. The ultrasonic pulse velocities (UPV) of all six cubic specimens were measured on the two smooth sides of the specimen at 7, 14, 28, and 56 days. The UPV test was conducted with direct transducer arrangement using a pair of narrowband 54 khz transducers using a commercially available PUNDIT system. 5. DISCUSSION OF TEST RESULTS 5.1. Fresh Concrete Properties Table 5 lists the test results performed on fresh concrete. Included in that table are the w/p ratio of the mix, slump flow diameter (D) and time (t50), V-funnel flow time and air content. As seen in that table, the slump flow diameters of all mixes were in the range of 560 to 700 mm, slump flow times are less than 2.9 sec., and the V-funnel flow times (tv-f) were in the range of 2.4 to 4.3 sec. Therefore, all concrete mixes could be considered as SCC. In all of the SCC mixes, there was no segregation of aggregate near the edges of the spread-out concrete as observed from the slump flow test. Mix ID w/p* Table 5. Fresh properties Slump flow V-Funnel Fiber Air Content factor D flow time t50 (s) (%) (mm) tv-f (s) Control_PC Control_FA < FA_SF < FA_SF1&SF < FA_SF < Also observed in Table 5 is the change in w/p ratio for the same workability measure, i.e. the same D, t50, and tv-f. The Control_PC mix had the highest w/p ratio, but as part of the PC was replaced by FA the w/p ratio of all mixes decreased. This phenomenon is also observed by other researchers [4, 14]. In such studies, even though finer FAs were used, which is expected to increase the water requirement of a concrete mix, the smooth surface characteristics and spherical shape of the FA improved the workability characteristics of concrete mixes and the same workability was achieved by a smaller w/p ratio. Therefore, using a coarser FA with higher volumes is naturally going to decrease the water demand of a SCC mix for the same workability measure. The steel fibers also affected the fresh properties of the concrete mixes. The addition of SF1 type steel fibers did not affect the water requirement of the mix for the same workability. However, addition of SF2 type fibers which have smaller diameters and sizes reduced the amount of water. This could be explained by the geometry of the fibers as well as the surface characteristics of these fibers. SF2 fibers have smaller dimensions when

8 1068 / Application of Hybrid Fiber Reinforcement. compared with SF1 fibers, thus have less potential to prevent the movement of aggregates. In addition, SF2 fibers are coated with brass and have very smooth surfaces, which reduce the energy loss during the movement of particles Hardened Concrete Properties The results of hardened concrete tests are presented in Table 6. Included in that table are the 28 and 56 day compressive and splitting tensile strength tests and 7, 14, 28, and 56 day ultrasonic pulse velocity tests. Even though the w/p ratio of the mix was reduced, substitution of PC with a coarse FA resulted in lower strengths both at 28 and 56 days. This reduction was 43% at 28 days and 31% at 56 days. The low pozzolanic activity can be attributed to the coarseness of the FA used. Fiber inclusion did not significantly affect the measured mechanical properties; however, as seen in Figure 3 as the volume of the SF2 type fibers increased the compressive strength slightly increased. This is due to the relatively small dimensions of SF2 type fibers, which give these fibers the ability to delay the micro crack formation and to arrest and prevent their propagation afterwards up to a certain extent. Another explanation to the increase in the compressive strength could be the decrease in w/p ratio which decreased as the amount of SF2 type fibers increased. However, when the split tensile strengths are examined (Figure 4) it can be seen that there is a reduction in the split tensile strengths as the volume of SF2 type fibers are increased or the w/p decreased. The reduction in the split tensile strength is explained by the loss of the presence of longer SF1 type fibers which are responsible for the increase in tensile strengths. Mix ID Table 6: Hardened properties Compressive Split Tensile Ultrasonic Pulse Velocity (m/s) Strength(MPa) Strength (MPa) 28 d* 56 d* 28 d* 56 d* 7 d 14 d 28 d 56 d* Control_PC Control_FA FA_SF1 FA_SF1&S F2 FA_SF [0.5] 23.3 [1.0] 19.6 [0.1] 22.8 [0.5] 22.5 [2.9] 41.7 [0.4] 28.6 [1.2] 24.5 [0.6] 26.1 [2.0] 31.8 [0.8] 3.58 [0.3] 2.8 [0.4] 3.10 [0.2] 3.40 [0.0] 3.08 [0.1] 3.68 [0.1] 3.34 [0.5] 3.69 [0.6] 3.82 [0.3] 3.23 [0.2] * Tests are performed on 3 specimens Tests are performed on 6 specimens Numbers in parenthesis are the standard deviations 4565 [22] 4161 [31] 3963 [10] 3970 [42] 4142 [109] 4570 [45] 4260 [51] 4007 [44] 4100 [41] 4224 [109] 4578 [35] 4436 [74] 4157 [45] 4249 [40] 4359 [82] 4609 [7] 4564 [29] 4317 [28] 4383 [88] 4506 [114] Ultrasonic pulse velocity (UPV) is used to assess the hardening of the SCC mixes. As seen in Figure 5, as hydration continues the UPVs increased for all the SCC mixes. However, the slope of that curve is quite different for the PC and FA mixes. For the Control-PC mix the slope was much smaller as most of the hydration was

9 3 rd International Conference on Concrete & Development / 1069 complete by 7 days. However, for the mixes with FA the hydration reactions continue after 7 days indicating a higher slope. Figure 3. Effect of steel fibers on the compressive strength Figure 4. Effect of steel fibers on the split tensile strength Figure 5. Strength gain of SCC mixes

10 1070 / Application of Hybrid Fiber Reinforcement. 6. CONCLUSIONS This paper discusses the part of the results of an experimental program carried out to investigate the effects of incorporation of HVFA, and steel fibers on the flow characteristics of SCC and mechanical properties in the hardened state. It can be concluded that it is possible to achieve self compaction with considerable fiber inclusion. Incorporation of HVFA may reduce the water requirement of a SCC mix. In other words, using high volumes of coarse FA may increase the workability characteristics of SCC mixes. Therefore the amount of SP and VMA to achieve self compaction could be reduced with proper adjustments to the FA amount. However, it is also seen that using coarse FA may cause significant strength losses to the SCC mixes, since they are used in high volumes. However, the strength reduction due to low pozzolanic activity of the FA was partially off-set by the use of smaller SF2 type steel fibers. It can also be concluded that the SF2 type steel fibers affect the properties of SCC mixes not only in the hardened state but also in the fresh state reducing the water requirement for the same workability measure. ACKNOWLEDGEMENTS The Atisaz Company has provided this research study for the Concrete Research Center. REFERENCES 1. H. Okamura, M. Ouchi, Self compacting concrete: development, present use and future. Proceedings of the First RILEM International Symposium on Self- Compacting Concrete (1999), pp A. Bilodeau, V. Sivasundaram, K.E. Painter, V.M. Malhotra, Durability of concrete incorporating high volumes of fly ash from sources in U.S. ACI Mater. J. 91 (1994), pp A. Bilodeau, V. M. Malhotra, High-volume fly ash system: concrete solution for sustainable development. ACI Mater. J. 97 (2000), pp N. Bouzoubaâ, M. H. Zhang, V. M. Malhotra, Mechanical properties and durability of concrete made with high-volume fly ash blended cements using a coarse fly ash.. Cem. Concr. Res. 31 (2001), pp N. Bouzoubaa, M. Laclemi, Self-Compacting concrete incorporating high volumes of class F fly ash preliminary results. Cem. Concr. Res (2001), pp D.J. Hannant, Fiber Cements and Fiber Concrete, Wiley, Chichester, A. Bentur, S. Mindess, Fiber Reinforced Cementitious Composites, Elsevier, London, B. Mobasher, C.Y. Li, Mechanical properties of hybrid cement-based composites, ACI Mater. J. 93 (3) (1996) K. H. Khayat, Y. Roussel, Testing and performance of fiber-reinforced, selfconsolidating concrete.proceedings of the First RILEM International Symposium on Self-Compacting Concrete (1999), pp P. Grouth, D. Nemegeer, The use of steel fibres in self-compacting concrete. Proceedings of the First RILEM International Symposium on Self-Compacting

11 3 rd International Conference on Concrete & Development / 1071 Concrete (1999), pp S. Nagataki and H. Fujiwara, Self-compacting property of highly-flowable concrete. in: V.M. Malhotra (Ed.), Am. Concr. Inst. SP 154 (1995), pp EFNARC, Specification & Guidelines for Self-Compacting Concrete (2002). 13. K. H. Khayat, Z. Guizani, Use of viscosity-modifying admixture to enhance stability of fluid concrete. ACI Mater. J (1997), pp A. Yahia, M. Tanimura, A. Shimabukuro, Y. Shimoyama, Effect of rheological parameters on self compactability of concrete containing various mineral admixtures Proceedings of the First RILEM International Symposium on Self- Compacting Concrete (1999), pp