STEEL FIBER REINFORCED SELF- COMPACTING CONCRETE INCORPORATING CLASS F FLY ASH

Transcription

1 STEEL FIBER REINFORCED SELF- COMPACTING CONCRETE INCORPORATING CLASS F FLY ASH B. Krishna Rao Department of Civil Engineering, JNT University Kakinada, Kakinada, Andhra Pradesh, India Abstract: Professor V. Ravindra Department of Civil Engineering, JNT University Kakinada, Kakinada, Andhra Pradesh, India Self-compacting concrete (SCC) offers several economic and technical benefits; the use of steel fibers extends its possibilities. Steel fibers acts as a bridge to retard their cracks propagation, and improve several characteristics and properties of the concrete. Fibers are known to significantly affect the workability of concrete. Therefore, an investigation was performed to compare the properties of plain normal compacting concrete (NCC) and SCC with steel fiber. Ten SCC mixtures and one NCC were investigated in this study. The content of the cementitious materials was maintained constant (600 kg/m 3 ), while the water/cementitious material ratio is kept constant The self-compacting mixtures had a cement replacement of 35% by weight of Class F fly ash. The variables in this study were aspect ratio (0, 15, 25 and 35) and percentage of volume fraction (0, 0.5, 1.0 and 1.5) of steel fibers. Slump flow time and diameter, J-Ring, V-funnel, and L-Box were performed to assess the fresh properties of the concrete. Compressive strength, splitting tensile strength and flexural strength of the concrete were determined for the hardened properties. A marginal improvement in the ultimate strength was observed. The addition of fiber enhanced the ductility significantly. The optimum volume fraction (V) and aspect ratio (A) of fiber for better performance in terms of strength was found to be 1.0 percent and 25. The results indicated that high-volume of fly ash can be used to produce Steel fiber reinforced selfcompacting concrete, even though there is some increase in the concrete strength because of the use of steel fiber and high-volume of fly ash. Keywords: Self-compacting concrete, fiber reinforcement, fly ash, workability, strength 1. Introduction Self-compacting concrete (SCC) is considered as a concrete which can be placed and compacted under its self weight with little or no vibration effort, and which is at the same time cohesive enough to be handled without segregation or bleeding. It is used to facilitate and ensure proper filling and good structural performance of restricted areas and heavily reinforced structural members. SCC was developed in Japan [1] in the late 1980s to be mainly used for highly congested reinforced structures in seismic regions. Recently, this concrete has gained wide use in many countries for different applications and structural configurations. SCC can also provide a better working environment by eliminating the vibration noise. There are many advantages of using SCC, especially when the material cost is minimized. These include: Reducing the construction time and labor cost; Eliminating the need for vibration; Reducing the noise pollution; Improving the filling capacity of highly congested structural members. Facilitating constructability and ensuring good structural performance. Such concrete requires a high slump that can easily be achieved by super plasticizer addition to a concrete mixture. However, for such concrete to remain cohesive during handling operations, special attention has to be paid to mix proportioning. To avoid segregation on super plasticizer addition, a simple approach consists of increasing the sand content at the cost of the coarse aggregate content by 4% to 5% by weight [2, 3]. But the reduction in aggregate content results in using a high volume of cement which, in turn, leads to a higher temperature rise and an increased cost. An alternative approach consists of incorporating a viscosity-modifying admixture to enhance stability [4]. ISSN:

2 Chemical admixtures are, however, expensive, and their use may increase the materials cost. Savings in labor cost might offset the increased cost, but the use of mineral admixtures such as fly ash, blast furnace slag, or limestone filler could increase the slump of the concrete mixture without increasing its cost. Previous investigations show that the use of fly ash and blast furnace slag in SCC reduces the dosage of super plasticizer needed to obtain similar slump flow compared to concrete made with Portland cement only [5]. Also, the use of fly ash improves rheological properties and reduces cracking of concrete due to the heat of hydration of the cement [6]. Kim et al. [7] studied the properties of super flowing concrete containing fly ash and reported that the replacement of cement by 30% by weight (40% for only one mixture) fly ash resulted in excellent workability and flowability. Other researchers [8] evaluated the influence of supplementary cementitious materials on workability and concluded that the replacement of cement 30% by weight of fly ash can significantly improve rheological properties. But to the best knowledge of the authors, the percentage replacement of cement by fly ash, in the various published studies, did not exceed 30% (except the one 40% mixture by Kim et al) by weight of the total cementitious materials. In the 1980s CANMET designed the so-called high volume fly ash (HVFA) concrete. In this concrete 55-60% of the Portland cement is replaced by Class F fly ash and this concrete demonstrated excellent mechanical and durability properties [9-12]. In order to extend to general concept of HVFA concrete and its applications to a wider range of infrastructure construction, this paper outlines the preliminary results of a research project aimed at producing and evaluating SCCs incorporating high volumes of fly ash. The objective of this study is to assess the effects of high volume fly ash replacement on the fresh and hardened properties of SCCs incorporating steel fibers. Even though, the suitability of using such a fly ash needs much detailed investigations, this study covers some fresh and hardened properties of mixtures In addition to the fly ash; steel fibers were used at different aspect ratio and volume fraction in making the concrete. Total mass of cementations materials is 600 kg/m 3, in which 35% of cement is replaced by the fly ash and the water powder ratio, was constant at For comparison, a control plain normal compacting concrete (NCC) of M40 grade mixture without any fly ash and a control plain self compacting concrete (A0V0) was also produced. The commercially available chemical admixtures used in this study included a viscosity modifying admixture (VMA) and a polycarboxylic based superplasticizer (SP). 2. Materials 2.1. Cement: The cement used in all mixtures was commercially available Ordinary Portland cement (OPC) of 53 grade manufactured by penna power company confirming to IS: was used in this study. The specific gravity of the cement was The initial and final setting times were found as 36.2minutes and 313 minutes respectively Fine aggregate Locally available Godavari river sand passed through 4.75mm IS sieve was used. The specific gravity 2.66 and fineness modulus of 2.72 were used as fine aggregate. The loose and compacted bulk density values of sand are 1600 and 1688 kg/m 3 respectively, the water absorption of 1.1% Coarse aggregate Crushed granite aggregate available from local sources has been used. The coarse aggregates with a maximum size of 16mm having the specific gravity value of 2.78 and fineness modulus of 7.36 were used as coarse aggregate. The loose and compacted bulk density values of coarse aggregates are 1437 and 1526 kg/m 3 respectively, the water absorption of 0.4% Fly ash Fly ash obtained from Vijayawada thermal power plant (VTPS), Vijayawada, Andhra Pradesh, India was used in this study. Its chemical admixture is given in Table: 1. The fly ash had a relatively low specific gravity and fineness modulus of and respectively. ISSN:

3 Table: 1 Chemical Analysis of Fly Ash S.No Property Formula % content 1. Silicon Dioxide Sio Aluminum Oxide Al 2 O Iron Oxide Fe 2 O Lime Cao Sulphur Trioxide SO Magnesium Oxide Mgo Alakalies NA 2 O Alakalies K 2 O Loss of ignition LOI Chemical admixtures A polycarboxylic type superplasticizer (SP) (Structuro 100) was used in all concrete mixtures. In addition to the SP, a viscosity modifying admixture (VMA) (Structuro 480) was also used. The properties of both admixtures, as provided by their manufacturers, are shown in Table: 2. Table: 2 properties of chemical admixture Chemical Specific ph color Dosage(l/m 3 ) Main component admixture gravity SP opaque 1 to 4 Polycarboxylic ether VMA Light yellow 0.5 to 2 Aqueous dispersion of microscopic silica 2.6. Fiber Steel fiber of diameter 0.92mm, fiber had a tensile strength and a specific gravity of 331Mpa and 7.850, respectively. The main variables used in the study are three different values of aspect ratios (15, 25 and 35) and the corresponding lengths are 13.8mm, 23 mm and 32.2mm respectively. Three different values of percentage of volume fraction of steel fibers (0.5%, 1.0% and 1.5%) and the corresponding weights are 39.25, 78.50, and kg/m 3 respectively. 3. Experimental procedures 3.1Mixture proportions Eleven concrete mixtures, with the proportions provided in Table 3, are prepared. The two control mixtures did not contain any steel fibers. Out of these two one is normal compacting concrete (NCC) of M 30 grade and the other one had fly ash replacing 35%, by weight of cement (A0V0). All of the remaining mixtures had the same amount of fly ash replacing 35% by weight of cement. These were named as A1V1, A1V2, A1V3, A2V1, A2V2, A2V3, A3V1, A3V2 anda3v3 indicating the three different values of aspect ratios (i.ea1=15, A2=25 anda3=35), and three different values of percentage of volume fraction (i.ev1=0.5%, V2=1% and V3=1.5%) of steel fiber incorporated in the mixture. For all the mixtures, the total amount of binder (cement + fly ash), the amount of cement, the amount fly ash and the amount of water were all kept constant. Therefore, the water/powder ratio (w/p) was kept constant Chemical admixture was added to the mixture until the SCC characteristics were obtained. ISSN:

4 Table: 3 Mixture Proportions S.No Mix A V %Fly Cement Fly ash Water w/b- F.A C.A SP VMA ID ash (kg/m 3 ) (kg/m 3 ) (l/m 3 ) Ratio (kg/m 3 ) (kg/m 3 ) (l/m 3 ) (l/m 3 ) 1. NCC A0V A1V A1V A1V A2V A2V A2V A3V A3V A3V Preparation and casting of test specimens The following mixing sequence was arrived after several trials for optimizing the workability. All the ingredients were first mixed in dry condition in the concrete mixer for one minute. Then 70% of calculated amount of water was added to the dry mix and mixed thoroughly for one minute. The remaining 30% of water was mixed with the super-plasticizer and viscosity-modifying agent and was poured into the mixer and mixed for five minutes. At this stage, 20% by weight of the water mixed with superplasticizer (struturo100) was poured into mixer and mixed for four minutes. Then, 10% by weight of water mixed with viscosity modifying agent (structuro 480) was poured into the mixer and mixed for one minute. Later required quantities of steel fiber were sprinkled over the concrete mix and mixed for one minute to get a uniform mix. Thus, the total mixing time was 7 minutes. After the mixing procedure was completed, tests were conducted on the fresh concrete to determine slump flow time and diameter, J-Ring test, L-Box ratio test, and V-funnel flow time. Segregation and bleeding were visually checked during the slump flow test and was not observed in any of the mixtures. From each concrete mixture, twelve 150-mm cubes, nine cylinders 150 mm in diameter and 300 mm height and nine beams of size 100x100x500 mm were cast. All specimens were cast in one layer without any compaction. The cubes were used for the compressive strength, the cylinders were used for the splitting tensile strength and beams were used for the flexural strength tests. After demoulding, all specimens were placed in a curing tank at room temperature Tests on fresh concrete Deformability and viscosity of fresh concrete is evaluated through the measurement of slump flow time and diameter, J-Ring test, L-Box ratio test and V-funnel flow time (Fig.1). 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 is determined as the slump flow diameter (D). According to Nagataki and Fujiwara [13], a slump flow diameter ranging from 500 to 700 mm is considered as the slump required for a concrete to be classified as SCC. According to Specification and Guidelines for SCC prepared by EFNARC [14] (European Federation of National Trade Associations), a slump flow diameter ranging from 650 to 800 mm can be accepted for SCC. In the slump flow test concrete s ability to flow and its segregation resistance can also be measured. To measure these properties, the time 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 [14] suggests a slump flow time (t 500mm ) of 2-5s for a satisfactory SCC. In addition to the slump flow test, J-Ring test, L-Box test and V-funnel test, is also performed to assess the flow ability, passing ability and stability of the SCC. The L-box ratio was in the range of , the J-ring test values were in the range of 0-10mm. The V-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 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 [15], a flow times were in the range of 6-12 sec. It is recommended for a concrete to qualify as a SCC. According to EFNARC [5], time ranging from 6 to 12 sec is considered adequate for a SCC. ISSN:

5 3.4. Tests on hardened concrete Fig: 1 workability test on the HVFA-SCC Tests performed on cured concrete specimens consist of the specimens compressive strength, the splitting tensile strength and flexural strength. For each mixture, cube specimens were tested at 7d, 28d, 56d and 90d and the cylinder, beam specimens were tested at 7d, 28d and 90d.The compressive, split tensile and flexural strength was computed from the average of three specimens. In addition to the above tests, the rebound hammer test was conducted on cube specimens at 7, 28, and 90 d. 4. Results and discussion 4.1. Fresh concrete properties The results of fresh concrete tests are shown in Table 3, which included the w/p of the mixture, slump flow diameter and time, V-funnel flow time, L-Box ratio test and J-Ring test. As seen in that table, the slump flow diameters of all mixtures were in the range of mm, The slump flow times were in the range of 3.3-5s, the L-box ratio were in the range of , the J-ring test values were in the range of 5-10mm,and the V- funnel flow times were in the range of s. Therefore, all concrete mixtures were considered as Self compacting concrete (SCC). In all of the SCC mixtures, there was no segregation of aggregate near the edges of the spread-out concrete as observed from the slump flow test. Also observed in Table 3, is the constant water-powder ratio for the same workability measure, i.e. the same slump flow diameter and time, V-funnel flow time, L-Box ratio, and J-Ring test. As seen in that table, the slump flow diameters of all mixtures D, The Control A0V0 mixture had the highest slump flow diameter, but as part of the cement were replaced by fly ash and without using steel fiber content. The w/p of for all mixtures the water-powder kept constant. This phenomenon was also observed by other researchers [6, 12]. In such studies, even though fly ash were used, which is expected to increase the water requirement of a concrete mixture, the smooth surface characteristics and spherical shape of the fly ash improved the workability characteristics of concrete mixtures, and similar workability properties was achieved by a smaller w/p. Therefore, using a fly ash with higher volumes naturally decreased the water demand of a SCC mixture for similar workability measures. The steel fibers also affected the fresh properties of the concrete mixtures. The addition of steel fibers did not affect the water requirement of the mixture for the same workability. However, addition of three different aspect ratios and three different volume fractions of steel fibers which had different length. This could be explained by the length of the fibers as well as the percentage of volume fraction of these fibers. The mixture (A1V1) of fibers had smaller lengths and volume fractions when compared to mixture (A3V3) of fibers, thus had less ISSN:

6 potential to prevent the movement of aggregates. Therefore, these fibers reduced the energy loss during the movement of concrete ingredients Hardened concrete properties Table: 3 Fresh Properties of Self Compacting Concrete S.No Mix ID Slump flow mm T 50cm Slump 2-5 s V-Funnel 6-12 s L-Box Ratio J-Ring 0-10mm 1. A0V A1V A1V A1V A2V A2V A2V A3V A3V A3V The results of hardened concrete tests are presented in Table 5, which included the 7d, 28d, 56d and 90d compressive strength and 7d,28d and 90d split tensile strength, flexural strength tests. To evaluate the effect of fibers of the hardened properties, the 90d properties are normalized with respect to the Control NCC, A0V0 mixture and the results are presented in Fig. 2, Fig.3 and Fig.4. As seen from that figure, the most significant changes were observed on the compressive strength, splitting tensile strength and later on flexural strength. Substitution of fly ash without a steel fibers resulted in lower compressive, split tensile and flexural strengths at 7d, 28d, and 56d even though the w/p of the mixture was kept constant. The reduction in compressive strength was 8.35% at 7d, 2.37%at 28d, and2.16% at 56d. The reduction in split tensile strength was 2.79% at 7d, and 5.79%at 28d. The reduction in flexural strength was 5.03% at 7d, and 2.99%at 28d. This reduction could be attributed to the low pozzolanic activity of the fly ash. But the increase in compressive, split tensile and flexural strength was 2.19%, 2.25% and 1.69% at 90d. Fiber inclusion did not significantly affect the measured mechanical properties. From Table 5, it may be noted that maximum strength, in general, improve as the fiber content increases from 0.5 to 1.0%, and from 1 to 1.5% decreases. However, only marginal increase is noticed in the f ultimate strength. On the other hand maximum compressive, split tensile and flexural strength is found to increase markedly and maximum increase is about 7.20%, 11.07% and 8.77% at 90d in the case of specimen with aspect ratio 25 and volume fraction 1%. This could be attributed to the effect of fiber bridging. As and when micro cracks develop in the matrix, the fiber in the vicinity of such micro cracks will try to arrest these cracks and prevent further propagation. Hence the cracks that are appearing inside the matrix have to take a meandering path, resulting in the demand for more energy for future propagation, which in turn increases the ultimate load. However, at higher values of fiber content, in fact a reduction in ultimate load has been found. This may be due to the insufficient matrix around the fibers for transfer of stress from concrete to fibers through bond. In general, for specimens with aspect ratio 15, 25 and 35, the volume fraction of 1% was found to be optimum. Fig.2 Compressive Strength (Mpa) ISSN:

7 Split tensile strength(mpa) NCC A0V0 4.4 A1V1 A1V2 4.0 A1V3 A2V1 3.6 A2V2 3.2 A2V3 A3V1 2.8 A3V2 A3V Age(days) Fig.3 Split Tensile Strength (Mpa) Fig.4 Flexural Strength (Mpa) 9.2 Flexural Strength(Mpa) Age(days) NCC A0V0 A1V1 A1V2 A1V3 A2V1 A2V2 A2V3 A3V1 A3V2 A3V3 Table: 5 Hardened Concrete Properties S.No Mix Compressive strength (Mpa) Split tensile strength (Mpa) Flexural strength (Mpa) ID 7d 28d 56d 90d 7d 28d 90d 7d 28d 90d 1. NCC A0V A1V A1V A1V A2V A2V A2V A3V A3V A3V Conclusion The present investigation has shown that it is possible to design a steel fiber reinforced self-compacting concrete incorporating high volumes of class F fly ash. The HVFA SCCs have a slump flow in the range of mm, a flow time ranging from 3.3 to 5 s, a J-Ring test value ranging from 5 to 10mm, a L-Box ratio ISSN:

8 ranging from 0.8 to 0.94, and V-funnel flow in the ranging from 7.1 to 12s. It was observed that it is possible to achieve self compaction with steel fiber (three different aspect ratio (15,25and 35) and three different volume fraction (0.5%,1%,and1.5%)) inclusion. Although results obtained from all of the mixes satisfy the lower and upper limits suggested by EFNARC, all mixes had good flowability and possessed self-compaction characteristics. The SCC developed compressive strengths ranging from to 37.89Mpa and from to 55.23Mpa, at 7 and 90 days, respectively. The SCC developed split tensile strengths ranging from3.83 to 4.28Mpa and from 5.41to 5.92Mpa, at 7 and 90days, respectively. The SCC developed flexural strengths ranging from 6.98 to 8.70Mpa and from 9.04 to 9.86Mpa, at 7 and 90 days, respectively. The strength and ductility of fiber reinforced SCC specimens were found to increase substantially in the case of specimens with volume fraction of 1% for aspect ratios of 15, 25 and 35. The optimum volume fraction (V) and aspect ratio (A) of fiber for better performance in terms of strength was found to be 1.0 percent and 25. Results indicate that pozzolanic reactions of high volume fly ash in the matrix composite were low in early ages, but by aging the specimens to more than 56 days considerable effect have been seen in strength. According to our study, addition of steel fiber reinforcement and mineral admixture like fly ash to the concrete, can improve the mechanical properties of specimens. SFRSCC mixes show higher compressive, split tensile and flexural strength rather than normal compacting concrete. In this paper, it was observed that, incorporation of HVFA reduced the water requirement of a SCC mixture. In other words, using high-volumes of a fly ash increased the workability characteristics of SCC mixtures. References [1] K. Ozawa, K.Mackawa, M.Kunishima, H. Okamura, Performance of concrete based on the durability design of concrete structures, Proc.of the Second East Asia-Pacific Conference on Structural Engineering and Construction, [2] P.K.Mehta, Concrete Structures, Properties and Materials, Prentice-Hall, 1986, pp [3] A.M.Neville, Properties of Concrete, 4 th ed., Longman Group, 1995, pp [4] K.H.Khayat, Z.Guizani, Use of viscosity-modifying admixture to enhance stability of fluid concrete, ACI Mater.J.94(4) (1997) [5] A. Yahia, M.Tanimura, A. Shimabukuro, Y. Shimoyama, Effect of rheological parameters on self compactability of concrete containing various mineral admixtures, in:a. Skarendahl, O.Petersson(Eds.), Proceedings of the First RILEM International Symposium on Self- Compacting Concrete, Stockholm, September,1999, pp [6] M.Kurita, T.Nomura, Highly-flowable steel fiber-reinforced concrete containing fly ash, in: V.M.Malhotra(Ed.), Am. Concr. Inst. SP 178 (1998) (June). [7] J.K.Kim, S.H.Han, Y.D.Park, J.H.Noh, C.L.Park, Y.H.Kwon, S.G.Lee, Experimental research on the material properties of super flowing concrete, in: P.J.M. Bartos, D.L.Marrs, D.J.Cleland (Eds), Production Methods and Workability of Concrete, E&FN Spon, 1996, pp [8] N.Miura, N.Takeda, R.Chikamatsu, S.Sogo, Application of super workable concrete to reinforced concrete structures with difficult construction conditions, Proc.- ACI SP (140) [9] V.Sivasundaram, Thermal crack control of mass concrete, MSL Division Report MSL (IR) Energy Mines and Resources Canada, Ottawa, 1986, 32pp. [10] V.M.Malhotra, Superplasticizer fly ash concrete for structural applications, Concr. Int. 8(12) (1986) [11] V.M.Malhotra, High performance high-volume fly ash concrete, in B.VRangan, A.K. Patnaik (Eds.). Proceedings of International Conference on High-Performance High-Strength Concrete, 1998, pp [12] A. Bilodeau, V.M. Malhotra, High-volume fly ash system: Concrete solution for sustainable development, ACI Mater. J. 97(1) (2000) [13] Nagataki S, Fujiwara H. In; Malhotra VM, editor. Self-compacting property of highly flowable concrete, Vol. Sp 154. American Concrete Institute: P [14] EFNARC. Specification and guidelines for self-compacting concrete, English edition. European Federation for Specialist Construction Chemicals and Concrete systems. Norfolk, UK February [15] Khayat KH, Guizani Z.Use of viscosity-modifying admixture to enhance stability of fluid concrete. ACI Mater J 1997; 94(4); ISSN: