THE EFFECT OF FIBER CONTENT AND AGGREGATE TYPE ON THE PERFORMANCE OF UHPC

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1 THE EFFECT OF FIBER CONTENT AND AGGREGATE TYPE ON THE PERFORMANCE OF UHPC G. Agranati and A. Katz National Building Research Center, Technion, Israel Abstract Mixtures of Ultra High Performance Concrete (UHPC) are generally composed of cement, micro silica, quartz powder, sand, short steel fibers, high range water reducing admixtures, and usually do not include coarse aggregates. Research work carried out at the National Building Research Center of the Technion, Israel, aimed to optimize the cost-effectiveness of these mixtures by incorporating coarse aggregated and improve the tensile properties of the composite with steel fibers, while maintaining reasonable workability of the fresh mixture. In the study, the compressive strength and workability of mixtures with various types of aggregates (quartz sand, flint, basalt, and dolomite limestone) were evaluated. In order to improve the tensile properties, mixtures with different fiber content were evaluated as well. 1. INTRODUCTION Ultra high performance concrete (UHPC) usually refers to cementitious composites with a compressive strength of at least 150. The constituents of UHPC mixtures include cement, silica fume, quartz powder, high range water reducing agents and short steel fibers. The high compressive strength is obtained due to a low water/binder ratio, which is usually less than 0.2 and the silica fume content is coonly around 25% of the cement content [1]. Reaching such a low water/binder ratio while maintain adequate workability of the mixture is achieved due to the densely packed powder system and the use of high range water reducing agents. One of the objectives of this study was to develop a more cost effective UHPC mixture with special emphasis on incorporating coarse aggregates in order to reduce the cement and micro silica contents. Also, in order to produce UHPC on larger scale, local materials that are more readily available were used. Therefore, in the first part of the study three types of strong aggregates i.e., basalt, flint, and Korodur 0/4, which is a coercially distributed aggregate that is used in highly resistant industrial floors, were evaluated. It is coonly accepted that strong aggregates contribute to a higher compressive strength of the concrete [2]. Various combinations of aggregates at different contents were considered and their effect on the strength and workability was evaluated. 95

2 Since crushed dolomite is a coon aggregate used in the local concrete market, it was decided to evaluate the possibility of incorporating this type of aggregate too. Two size fractions were used, 9.5 and 14 maximum nominal diameter. The aggregate content constituted ~60% of the volume of the mixture. In the second part of the study, the effect of different fibers on the tensile behaviour of UHPC was evaluated. UHPC is highly brittle material, and as a result short steel fibers are coonly added to reduce brittleness and improve the fracture properties. Various publications indicate that the fiber content should be 2-2.5% by volume [3]. The most coonly used steel fibers in UHPC are the short straight fibers, which have a length of approximately 10 and a diameter of 0.2. However, these fibers do not have sufficient anchorage length to reach the yield strength before slippage of the fiber occurs. In this work, mixtures with short straight steel fibers at different contents were compared. The compressive strength, tensile (splitting) and flexural strength, and the load deformation behaviour were evaluated. In order to improve the post cracking behaviour of the concrete, the effect of combining short fibers with long fibers was studied. 2. EXPERIMENTAL PROGRAM The experimental program of this study included four groups of mixtures. The first series evaluated the effect of strong aggregates (basalt, flint, and Korodur) on the compressive strength. The second group of mixtures evaluated the use of dolomite coarse aggregates and the effect of reducing the silica fume content. In the third group of mixtures, short fiber content was evaluated. The fourth set of mixtures included mixtures that included short and long fibers. 2.1 Materials, specimen preparation and test methods All the mixtures included cement type V with low aluminate content, densified micro silica, quartz sand, and high range water reducing agent. The coarse aggregates and steel fibers varied between the different groups of mixtures. The characteristics of the aggregates are listed in Table 1. The characteristics of the fibers are listed in Table 2. The compressive strength specimens were cubes. These were demolded after 24 hours and stored in a water bath until tested. The specimens for the flexural strength test and splitting test were beams. The specimens were demolded after 24 hours and stored in a water bath until tested. Table 1: Characteristics of the aggregates Quartz sand Flint sand Basalt aggregates Dolomite aggregate Small Dolomite Aggregate Medium nominal max size, D Fineness modulus

3 Table 2: Properties of the fibers # Fiber type Length, Diameter Aspect ratio, l/d yield strength, # fibers in kg 2 13/ ,000 Short 3 12/ ,000 Long 7 and hooked 60/ The workability of the mixtures detailed in section 3.1 was measured using a standard flow test for mortars following EN ; however, after the cone was raised the diameter of the mortar was measured without dropping the table (Fig. 1). The workability of the concretes in section 3.2 was measured using a standard concrete slump test EN or the slump flow test using EN RESULTS AND DISCUSSION 3.1 Effect of aggregated type on compressive strength and flowabilty of the mixture In order to evaluate the effect of the different aggregates, a reference mixture with a 28 day compressive strength of 151 and flowability of 172 was used. The steel fibers used were short 12/.4 fibers (#3 in Table 2). The characteristics of the reference mixture are listed in table 3. Table 3: Characteristics of the reference mixture w/c w/binder microsilica /cement sand/ cement steel fibers Flow, f c, 28days %v The different mixtures evaluated in the series were similar to the reference mixture; however, each mixture contained a different aggregate type or a combination of two different types. In two of the mixtures the aggregate to cement ratio was also changed. Table 4 includes the results for the evaluated mixtures. Table 4: The effect of different aggregates on the compressive strength (f c ) and flowabilty type of aggregates aggregate/ cement flow, f c, 28day 1 quartz sand (ref) quartz sand % flint sand % basalt aggregate, 50% quartz sand % basalt aggregate, 50% quartz sand % Korodur 0/

4 From these results it can be observed that increasing the quartz sand content did not change the compressive strength, however, the flowabilty of the mixture was substantially lower. The use of flint sand and Korodur aggregate at the same content as the quartz sand in the reference mixture increased the compressive strength by ~10%; however, the flowabilty of these mixtures has slightly reduced. Combining basalt aggregate and quartz sand did not improve the strength but resulted in a substantial increase in flowabilty. Using basalt and quartz sand, but at a higher content resulted in ~10% increase in the compressive strength. As can be seen from these results, the use of strong aggregates did not contribute substantially to an increase in the compressive strength of these mixtures. The effect of the different aggregates is more pronounced on the workability. Since the objective of this study was to develop mixtures that can be used on a larger scale production, the use of these aggregates is not justified when considering the increase in cost and operational complications. Figure 1: Flowabilty test of the mixtures 3.2 The use of coarse aggregates The second part of the optimization process of the mixtures included the use of coonly available coarse aggregates, in order to reduce the paste volume, without adversely affecting the compressive strength of the mixtures. Based on previous work done, we also reduced the micro silica content. At this stage of the study the tensile characteristics of the mixtures were also evaluated. All the mixtures had a w/c ratio of 0.2. Mixture #1 is the reference mixture used in section 3.1. The characteristics of the mixtures and the results for the 28 days compressive strength (f c ), 28 days flexural and splitting strength are included in Table 5. From these results it can be observed that coarse aggregates can be incorporated in the mix design without decreasing the compressive strength. Mixture #4 included 42%v coarse aggregate and reached a compressive strength of 166, which is 10% above compressive strength of the reference mixture. From the results of mixture #3 it can be seen that it is also possible to reduce the silica fume content to 12% without reducing the compressive strength. The flexural and splitting strengths of the mixtures were quite similar with no significant effect of the studied parameters. Higher variation of the results was noticed compared with the variability of the compressive strength tests. However, when comparing the results of mixture #5 which did not include steel fibers, with the other mixtures that included 2% short fibers, it is clear that including 2% steel fibers did not improve substantially the tensile strength of the mixtures whereas the compressive strength has significantly changed. 98

5 Table 5: Characteristics and results for the mixtures with coarse aggregates Coarse Coarse Agg. Agg. f SF/ Sand, c, 28 Flexural # w/c w/b Dmax Dmax days Strength cement %vol 14, 9.5, %vol %vol 1 Ref % 0% 0% w/coarse agg. Dmax 9.5 Splitting strength % 0 18% Less silica fume % 0 30% w/coarse agg. Dmax 14 w/coarse agg. Dmax 14 no steel fibers % 22% 20% % 22% 20% This can also be observed in Figure 2, that presents the mid span deflection vs. the flexural stress diagram for mixture #5 without the steel fibers and mixture #4 with 2% short steel fibers. The use of the short fibers did prevent a brittle fracture, but they did not increase substantially the toughness of the concrete. The maximum flexural strength was the same for both mixtures with a value of approximately 14. For mixture #4 with the 2% fiber, after the maximum strength has reached, strain hardening did not develop, and the strength decreased rapidly with an increase in the deflection. This probably occurred due to insufficient anchorage of the steel fibers, and they do not reach their yielding capacity. As a result, the fibers are pulled out before contributing to the load capacity of the beam. Figure 2: Mid span deflections vs flexural strength for mixtures #5 and #4 3.3 Effect of fiber contents on the workability and strength In order to evaluate the effect of the fiber content on the concrete, a series of 4 mixtures with increasing fiber content were evaluated. The fiber contents evaluated were 2%, 4%, 6%, and 8% by volume. The steel fibers tested were the 12/.4 fibers. It is important to note that the other types of short fibers could not be used, because it was impossible to adequately mix more than 2%v fibers. Above this content, the fibres ball together. The reference mixture is 99

6 mixture #5 from Table 5, which included 42% coarse aggregates and reduced silica fume content. The results are included in Table 6. Table 6: Results for the mixtures with varying fiber content # Workability f c, 28day Flexural Strength Splitting strength 1 0 % fibers 830 flow % fibers 460 flow % fibers 155 slump % fibers 0 slump % fibers 0 slump From Table 6 and Figure 3 it can be seen that there is a uniform increase in the compressive strength with increasing the fiber content in the mixture. Using linear regression analysis, it appears that every 1% increase in the fiber content contributes to an increase in the compressive strength of approximately 5.5. Regarding the tensile strength, at 2% and 4% fibers, the effect on the tensile strength is limited, and a clear increase in the tensile properties was seen only at 6% and 8% of fiber addition. The mixtures with fiber contents of 2% and 4% had a reasonable workability, while the mixtures with 6% and 8% fibers, had zero slump. As can be seen in Figure 4 compared with Figure 2, the increase in fiber content did not contribute to an increase in the toughness behaviour of the concrete; however, the elastic limit has increased due to the composite behaviour of the concrete matrix and steel fibers. Figure 3: Experimental results for mixtures with different steel fiber content 100

7 Figure 4: Mid span deflection vs flexural strength for the mixture with 8% short steel fibers 3.4 Effect of fiber length and shape on the workability and strength In order to improve the post cracking behaviour of the concrete, a combination of long and short fibers was utilized using mix #4 from Table 5. The 2% steel fibers were composed of 1/3 long (60 ) fibers with double hooked ends (#3 in Table 2) and 2/3 short fibers (#1 in Table 2). As can be seen from Figure 5, the use of short and long steel fibers increased both the maximum strength of the matrix and substantially improves the toughness behaviour of the concrete. After the elastic limit has reached, the strength continued to increase until the maximum strength has reached (strain hardening). Figure 5: Mid span deflection vs. flexural strength for the mixture with 2% short and long steel fibers 4. CONCLUSIONS This study evaluated the use of strong aggregates, conventional dolomite coarse aggregate, and increasing the steel fiber content. The use of strong aggregates such as basalt and flint did not contribute substantially to an increase in the compressive strength of the mixture. It is possible to improve the cost effectiveness of the mixture by incorporate up to 45%v of coarse aggregate while maintaining the targeted compressive strength of

8 The use of short steel fibers contributes to the compressive strength of the concrete by ~5.5 for every 1%v of fiber. The use of short steel fibers did not improve much the toughness characteristics of the concrete, probably due to insufficient anchorage length of the fibers. The use of combination of short and long steel fibers increased the maximum flexural strength of the composite but more substantially improve the toughness behaviour of the concrete. REFERENCES [1] FHWA-HRT , Ultra-High Performance Concrete: A State-of-the-Art Report for the Bridge Counity, June [2] Ezeldin, A.S., Aitcin, P.C, Effect of Coarse Aggregates on the behaviour of Normal and High strength Concretes Cement, Concrete, and Aggregates, V.13, No.2, 1991, pp [3] Wille, K., Naaman, A.E., and El-Tawil, S., Optimizing Ultra-High-Performance Fiber- Reinforced Concrete, Concrete International, Vol. 33, No. 9, September 2011, pp [4] Kwon, S.,Nishiwaki,T., Kikuta, T., Mihashi, H., Development of Ultra-High-Performance Hybrid Fiber-Reinforced Cement-Based Composites, ACI Materials Journal, V.111, No.3, May-June 2014, pp [5] Murthy, A.R., Lyer, N.R., Prasad, B.K.R, Evaluation of mechanical properties for high strength and ultra-high strength concretes, Advances in Concrete Constructions, Vol. 1, No. 4, 2013, pp [6] Richard, P., Reactive Powder Concrete: A New-Ultra High Strength Cementitious Material, Proceedings of the Fourth International Symposium on the Utilization of High-Strength/High- Performance Concrete, May 1996, Paris, France, Ed., de Larrard, F.and Lacroix, R., Vol. 3, pp. 1,343 1,