Effects of Polypropylene Fibers on the Properties of High-Strength Concretes

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1 Effects of Polypropylene Fibers on the Properties of High-Strength Concretes T. Budi Aulia SUMMARY Beside its salient characteristics the high-strength concrete is well known having some poor performances particularly in the case of ductility and fire resistance. Recently the use of polypropylene fibers is often chosen to overcome these matters, mainly due to their excellent characteristics and low price. The use of a certain amount of fibers in the concrete mixture, e.g.,2 vol.-%, did not influence detrimentally the main mechanical properties of high-strength concretes, both in fresh and hardened concrete, but led assuredly to ductile fracture of such brittle concrete. 1 INTRODUCTION High-strength concrete offers many advantages in the utilization, due to its improved mechanical characteristics and low permeability, as well as due to higher resistance against chemically or mechanically penetrating attacks into the structure of concrete. By such outstanding characteristics one uses this material particularly for the constructions which are extremely influenced by the environmental conditions, e.g. offshore constructions and large bridges, or essentially to increase the structural load-carrying capacity, while a sufficient durability is ensured to the structures. Although high-strength concrete offers many advantages regarding mechanical characteristics of the concrete and economic aspects of the construction, the brittle behavior of the material remains a larger handicap for the seismic applications. Since their strength and ductility are proportionally opposite, high-strength concretes are importantly more brittle than conventional normalstrength concretes. The linear elastic part at the pre-peak phase of the stress-strain curve of a non-reinforced high-strength concrete is enormously increased: almost to approx. 9% of the peak load. After achieving the peak load the stress-strain curve drops rapidly, which is typical for a brittle material. The absorbed energy during the elastic phase is not dissipated in the even proportion in the crack initiation and propagation at the fracture process, so that a stable crack growth up to failure of the concrete is not achieved. This entails a sudden, explosive failure of the concrete, whereby the surfaces of fracture are fewer rough and interlocking of the crack surfaces is substantially reduced. Moreover, the very low permeability of high-strength concrete causes some further difficulties. One of them is the fire resistance. At the fire the temperature of concrete increases rapidly. Thus, because of the very low proportion of capillary pores, the water, which is not hydrated yet, can entrap itself in the interior of concrete. Consequently, the developing water vapor pressure cannot be relaxed over the capillary pores, which leads partly to internal tensile stresses. The result is the flakings of the surface and cracking in the concrete. In this case, the chemically bounded water by the hydration process can have also evaporated. To overcome such problems, the polypropylene fibers are often used at present due to their advantageous characteristics and beneficial price. Polypropylene fibers cause mainly on the one hand for increasing the ductility behavior and, on the other hand for improving the fire resistance of high-strength concrete. Since the fibers melt at approximately 16 C, they produce the expansion channels in the case of fire, whereby liquid and steam transport are enabled for the relaxation of the internal pressures (fig. 1). This can prevent the harmful flakings of the concrete surface, because there is an additional porosity in the concrete structure, in which its volume adjusts to the fiber content in the concrete mixture. If the gas movements entrap in the interior of concrete structure, the fire with 3 C can lead to internal tensile stresses of approx. 8 N/mm 2, which can accrete themselves more than two times during a further heating on 35 C. Hence, these stresses lie far over the tensile strength of the concrete, which amounts to a C 9/15 concrete about 5 N/mm 2 [2]. 1

2 Spalling of homogenous structure of the highstrength concrete due to insufficient capillary pores Developed explosion channels because of melting of polypropylene fibres Fig. 1: Flowing out of steam pressure through the melted polypropylene fibers in the case of fire [1] Polypropylene belongs to the polyolefin family of the chemicals. The polypropylene fibers are hydrophobic, which do not absorb the water, and not corrosive. Moreover, the polypropylene fibers have the excellent resistances against alkalis, chemicals and chloride, and have the low heat conductivity. By these characteristics polypropylene fibers have therefore no significant effect on the water demand of the fresh concrete. They do not intervene the hydration of cement and do not influence unfavorably the effects of all constituents in the concrete mixtures. Fig. 2: Fibrillated and bundled polypropylene fibers of 19 mm length Fig. 3: Fiber morphology of polypropylene in SEM The fibers are manufactured either in the pulling wire procedure with circular cross-section or by extruding the plastic film with rectangular cross-section. They appear either as fibrillated bundles or mono filaments. The fibrillated polypropylene fibers are formed by expansion of a plastic film, which is separated into strips and then slit. Thereby the meshed fiber bundles are formed, which are rectangular in the cross-section. These fiber bundles are cut into specified lengths and fibrillated. The available fibrillated fibers in the trade range from approx. 6,5 up to,5 mm in the length. Table 1: Characteristics of the polypropylene fibers 2

3 Color white Morphology fibrillated or mono filament Specific weight [g/cm 3 ],95 Diameter [ m] Modulus of elasticity [GPa] 5-1 Tensile strength [MPa] 5-75 Ultimate strain [%] 5 15 Elongation at fracture [%] Approx. Melting point [ C] 16 Bonding with cement good Stability in cement good 2 MIX DESIGN AND TESTING 2.1 Mix Design To study the effects of polypropylene fibers on the properties of high-strength concretes, many tests regarding the compressive strength and modulus of elasticity were carried out on the concrete specimens. Various types of aggregate, i.e. basalt, diabase, gabbro, granulite, limestone, quartzite, serpentine and steatite (high-performance ceramics) were embedded in two different concrete mixtures: 1. High-strength normal concretes (mixture A) 2. High-strength concretes by using silica fume in suspension slurry 1:1 (mixture B). The aggregates were obtained from different regions in Germany: Bavaria, Saxony, Lower Saxony, Thuringia and Hesse. Cement I 52,5 R from Schwenk GmbH and the sulphonated, naphthalene formaldehyde condensate based superplasticizer FM 26 from Woermann, Darmstadt were used for all mixtures. The coarse aggregates were crushing stone with the nominal maximum size 16 mm and the fine aggregate was local natural sand from Kleinpösner (Saxony). 3

4 Table 2: Concrete mix design for mixture A No. of mixture Type of aggregate Basalt Diabase Gabbro Granulite Limestone Quartzite Serpentine Steatite Specific gravity [g/cm 3 ] 3,13 2,85 2,91 2,8 2, 2, 2, 2,65 CEM I 52,5 R 3 ] Water 3 ] Sand /2 3 ] /5 3 ] ) 5/8 3 ] ) 8/11 3 ] ) 11/16 3 ] ) 3 ] 126 5) 3 ] 237 6) Superplasticizer Woermann FM 26 3 ] , Retarder Woermann VZ 32 3 ] 2,34 Polypropylene fibres,2 vol.-% 3 ] w /c -,26,27,25,27,25,27,25,27 w */c -,32,33,31,33,31,33,31,33 1). Granulates 2,5-3,5 2). Pellet 6 3). Pellet 8 4). Pellet 1 5). Pellet 12 6). Pellet 16 Table 3: Concrete mix design for mixture B No. No. of of mixture mixture Type Type of of aggregate aggregate Basalt Basalt Diabase Diabase Gabbro Gabbro Granulite Granulite Limestone Limestone Quartzite Quartzite Serpentine Serpentine Steatite Steatite Specific Specific gravity gravity [g/cm [g/cm ] 3,13 3,13 2,85 2,85 2,91 2,91 2,8 2,8 2, 2, 2, 2, 2, 2, 2,65 2,65 CEM CEM I 52,5 52,5 R ] Water Water ] Sand Sand /2 /2 ] /5 2/5 ] ) 5/8 5/8 ] ) 8/11 8/11 ] ) 11/16 11/16 ] ) ] ) 3 3 ] 228 6) 228 6) Silica Silica fume fume (slurry (slurry 1:1) 3 1:1) 3 ] Superplasticizer Superplasticizer Woermann Woermann FM FM ] Retarder Retarder Woermann Woermann VZ VZ ] 2,34 2,34 Polypropylene Polypropylene fibres fibres,2,2 vol.-% vol.-% ] w /c /c -,,,22,22,,,21,21,22,22,21,21,,,, w */c */c -,25,25,26,26,25,25,26,26,27,27,25,25,25,25,24,24 w/c w/c = (Water (Water + SF/2) SF/2) / (Cement (Cement + SF/2); SF/2); w*/c w*/c = (Water (Water + SF/2 SF/2 + FM FM VZ VZ 32) 32) / (Cement (Cement + SF/2) SF/2) The grain-size distribution was according to grading curves between curves A and B to DIN 145. The polypropylene fibers were mixed at,2% by volume of the concrete, which was equivalent to 2 kg/m 3. The mix design C /85 was selected for this research programme, in order to obtain the typical strength of high-strength concrete. The water-cement ratios ranged from,25 to,27 for high-strength normal concretes (mixture A), and from, to,22 for high-strength concretes (mixture B) as shown in tables 2 and 3. The mix designs contained either with,2 vol.-% polypropylene fibers or no fibers. The mixtures with polypropylene fibers were defined with P. The water-cement ratios were strongly dependent on the aggregate characteristics, particularly from the density, water absorption, porosity and cleanliness. The same volume ratio between sand and aggregate of,47 was used in all mix designs, so that the influence of the different density of aggregates on the water-cement ratio could be minimized. In order to keep this volume ratio constant, the larger density of aggregate was proportionally distributed to the amount of sand. The larger the density of aggregate, the higher the amount of sand in the concrete. 2.2 Testing Cube specimens of 1 mm were tested for the uniaxial compressive strength and cylinders with a diameter of 1 mm and a height of mm were tested for the modulus of elasticity of the concrete. To obtain the entire stress-strain curve of concrete specimens under the centric uniaxial compressive stresses, the cylinders with a ratio of diameter : height of 1 : 3 were chosen in order to avoid the influence of the hindrance of the lateral stress expansion at the upper and bottom edge of specimens. Hence, the dimensions were kept constant with a diameter of 1 mm and a height of 3 mm. These cylindrical specimens were tested then by using a servo hydraulic testing machine made by Schenck. The testing machine had a stiffness of 1x1 1 N/m and the 4

5 maximum load of 25 kn. This equipment was able to record the effects of the deformation behavior of high-strength concretes by measuring the longitudinal strain and circumferential strain and adjusted these both signals in combination for controlling the pressure. Consequently, an explosive failure of specimens could be avoided resulting in a stable descending branch of the stressstrain curve. 3 RESULTS 3.1 Effects of Polypropylene Fibers on the Properties of Fresh Concrete The workability of fresh high-strength concrete is occasionally impaired by its higher content of binder and its lower watercement ratio. This can be minimized by using the suitable type and correct amount of the superplasticizer. The main parameter, which is often used to determine the workability of fresh concrete, is the slump test. The slump value depends mainly on the water absorption and porosity of the aggregates, water content in the mixture, amount of the aggregate and fine material in the mixture, shape of the aggregates and surface characteristics of the constituents in the mixture. The slump values decreased significantly with the addition of polypropylene fibers as shown in tables 4 and 5. The concrete mixture became rather clingy resulting in increasing of the adhesion and cohesiveness of fresh concrete. During mixing the movement of aggregates sheared the fibrillated fibers apart, so that they opened into a network of linked fiber filaments and individual fibers. These fibers anchored mechanically to the cement paste because of their large specific surface area. Without compacting, the clumps of fibers with the cement paste might thus occur in the mixture. As the consequence, the segregation occurred due to non-uniform concentration of the constituents in the mixture. It could also lead to bleeding at the same time. All of these resulted in the slump loss. If bleeding occurred, a water layer on the surface of fresh concrete and the localized water canals could be developed, by which the water seepage could transport the important cement paste to the surface. Then a layer containing the diluted cement paste was formed in the still plastic fresh concrete, which led later to formation of the early microcracking. However, the concrete mixture with polypropylene fibers resulted in the fewer rate of bleeding and segregation, and they occurred more slowly than in plain concrete. The cause is, as mentioned above, the fibers held the concrete together and thus slowed the settlement of aggregates. With the use of sufficient compaction, the fresh concrete would flow satisfactorily again and the polypropylene fibers would be uniformly dispersed in the mixture. The fibers should not float to the surface nor sink to the bottom of the fresh concrete. Due to their high tensile strength and pull-out strength, the polypropylene fibers even could reduce the early plastic shrinkage cracking by enhancing the tensile capacity of fresh concrete to resist the tensile stresses caused by the typical volume changes. The fibers could also distribute these tensile stresses 5

6 No. of mixture Type of aggregate Basalt Diabase Gabbro Granulite Limestone Quartzite Serpentine Steatite Slump value accord- ing to DIN 148 [cm] Slump value accord- ing to DIN 148 with [cm] PP fibres strength W 1 (7d) [MPa] 61,7,5 71,9 71,4 65,9 73,6 73, 67,6 strength W 1 (7d) [MPa] 69, 73,7 71,6 73,6 66,2 77,5 71,,7 strength W 1 (28d) [MPa],4 78,6 88,7 86,8 75,4 91,4 84, 83,3 strength W 1 (28d) [MPa] 77,3 81,2 84, 87,6 8,3 86, 82,8 77,9 Mod. of elasticity [GPa] 48,5 41,3 42,2 33,6 37,1 37,1 45,7 56,6 Mod. of elasticity [GPa] 45, 45,1 39,9 32, 37,9 37,4 45,4 55,8 Inelastic deformati- on ( v ) [ ],84,93 1,25 1,7 1,26 1,44 1,28,74 Inelastic deformati- on ( v ) with PP fibres [ ] 1,5 1,15 1,44 1,22 1,31 1,52 1,71,91 Fracture energy [MN/m],1,11,191,138,223,268,213,9 Fracture energy [MN/m],154,178,239,167,23,275,283,118 Table 4: The main concrete properties for mixture A No. of mixture Type of aggregate Basalt Diabase Gabbro Granulite Limestone Quartzite Serpentine Steatite Slump value accord- ing to DIN 148 [cm] Slump value accord- ing to DIN 148 with [cm] PP fibres strength W 1 (7d) [MPa] 88,1 88,6 91, , ,4 93 strength W 1 (7d) [MPa] 95, 82,6 83,8 62,7 85,8 99,1 85,3 9,1 strength W 1 (28d) [MPa] 117, 113,4 16,8 15,4 15, 114,5 112,2 96,8 strength W 1 (28d) [MPa] 122,1 18,2 12,7 95, 17,3 129,4 13,8 113,5 Mod. of elasticity [GPa] 59,2 45,9 42,9 38,3 4,7 39,6 51,2 61, Mod. of elasticity [GPa] 54, 48,9 46,8 35,5 39,6 45,4 47,5 62, Inelastic deformati- on ( v ) [ ] 1,14 1,23 1,44 1, 1,1 1,35 1,53,86 Inelastic deformati- on ( v ) with PP fibres [ ] 1,43 1,29 1,51 1,6 1,17 1, 1,64,95 Fracture energy [MN/m],255,296,341,3,222,341,3,161 Fracture energy [MN/m],354,38,36,344,241,467,387,175 Table 5: The main concrete properties for mixture B more evenly throughout the concrete. It is well known that the tensile strength of fresh concrete at the early age is very low. According to [3], the tensile strength of normal concrete during the first 4 hours is as low as,2 MPa. At the same time the,2 vol.-% random fibers can supply the fiber stresses of 1 MPa, by assuming the fiber tensile strength is equal to 5 MPa (see table 1). Hence, it is obvious that all cracking stresses (,2 MPa) are sustained by the fibers. As the plastic shrinkage cracking decreased, the number of cracks in the high-strength concretes under loading could also be reduced, due to decreasing of the propagating cracks from the existing shrinkage cracks. If shrinkage cracks are still formed, the fibers bridged these cracks, reduced at the same time their length and width. Furthermore, with the constant water-cement ratio, the slump values of the concrete mixtures containing polypropylene fibers were not significantly affected by the aggregate types. They fluctuated just in a small range as shown in tables 4 and 5. As aforementioned, this was primarily caused by the good adhesion in the fresh concrete, which was created by the polypropylene fibers. On the contrary, the aggregate types affected significantly the slump values of the plain concrete mixtures. The water absorption and the porosity of aggregate, as well as its shape contributed to the loss of slump value. In addition, the slump values of high-strength concrete mixtures with and without polypropylene fibers decreased, compared to those of high-strength normal concrete mixtures. The cause were the increase of fine materials, due to the use of silica fume, and the decrease of the watercement ratio in comparison with the mixture without silica fume. Moreover, as the rate of bleeding was decreased, the use of polypropylene fibers may accelerate the time to initial and final set of the concrete, because this led to a slower rate of drying in the concrete. 6

7 3.2 Effects of Polypropylene Fibers on the Properties of Hardened Concrete The observed properties of the hardened concrete in this research programme are the uniaxial compressive strength and the modulus of elasticity. The results can be seen in tables 4 and 5, and in figures 4, 5 and 6. It can be noted that the production of high-strength concretes with the strength of C /85 needs the addition of silica fume absolutely. In concrete mixtures B, the strength class of C /85 was achieved at all types of aggregate, while this was not attainable for all types of aggregate embedded in the concrete mixtures A. 1 Strength 7 d [MPa] Basalt Diabase Gabbro Granulite Limestone Quarzite Serpentine Steatite Concrete-A Fig. 4: strength of concretes (7 d) Concrete-B Concrete-AP Concrete-BP 7

8 Strength 28 d [MPa] Basalt Diabase Gabbro Granulite Limestone Quarzite Serpentine Steatite Concrete-A Fig. 5: strength of concretes (28 d) Concrete-AP Concrete-BP Concrete-B Mod. Of Elasticity [GPa] Basalt Diabase Gabbro Granulite Limestone Quarzite Serpentine Steatite Fig. 6: Modulus of elasticity of concretes Concrete-AP Concrete-A Concrete-BP Concrete-B With the use of many types of aggregate, the addition of polypropylene fibers in the concrete did not significantly affect the compressive strength and the modulus of elasticity of high-strength concretes. The average increase for the 7-day compressive strength of concrete with various aggregate types was 4,62% and on the other hand, the average decrease was 5,86%. For the 28- day compressive strength the average increase was as much as 6,79% and the average decrease was 5,61%. Furthermore, the average increase of the modulus of elasticity of concrete with different aggregates containing polypropylene fibers was 6,2% and the average decrease amounted to 5,%. It means that the use of,2 vol.-% polypropylene fibers alone resulted in the low influence on both the compressive strength and modulus of elasticity of concrete rather than the influences contributed by the other constituents of concrete. The positive effects of polypropylene fibers, which may contribute to the increase of the mechanical properties of high-strength concrete, are concluded as follows: The fibers function as micro-reinforcement in the concrete. This results in better mechanical anchoring to the concrete. The fibers act as crack arresters. This is due to their high tensile strength and pull-out strength. The fibers stop the propagating of cracks by holding the cement matrix together or bridge the cracks. Hence, the cracks cannot grow longer and wider, and propagate gradually. 8

9 The high ultimate strain of fibers enables the concrete having an ability to restrain the large deformations without crushing. The fibers may reduce the early plastic shrinkage cracking, so that the number of cracks under loading due to propagating of these existing cracks is decreased. The fibers may avoid the formation of the single shear band, which is typical for the fracture of plain high-strength concrete, by ramifying the microcraking that spread over the entire concrete mesh. On the contrary, the negative effects of polypropylene fibers, which may lead to the decrease of the properties of high-strength concrete, are summarized as below: The fibers function as initiator of the microcracking because of their low modulus of elasticity compared to the cement matrix. The formed mechanical bond with the cement matrix is thus low. On the other hand, the early formation of microcracking represented in the ascending branch of the stress-strain curve may dissipate the high elastic potential energy and then absorb them for creating the new crack surface. The result, in stead of increasing of maximum strength, is highly improved cracking and deformation behavior. The fibers cause the enhancement of the pores volume of concrete by creating more micro-defects in the cement matrix. 3.3 Effects of Polypropylene Fibers on the Fracture Properties of High-Strength Concrete Compared to the effects of polypropylene fibers on the properties of fresh concrete and hardened concrete, the fracture properties of high-strength concretes were significantly influenced by the polypropylene fibers. The failure behavior of high-strength concretes was effectively improved by the use of fibers. The typical shear band rupture due to strain localization could be avoided (fig. 7). Instead of this, a large number of the longitudinal cracking, which was predominantly oriented in the direction parallel or sub-parallel to the external compressive stresses, was formed at the entire concrete specimens as shown in fig Fig. 7: Fracture shape of plain high-strength concrete Fig. 8: Fracture shape of high-strength concrete containing polypropylene fibers Regarding to cracking development, the polypropylene fibers improved the deformation behavior of high-strength concretes both in the pre-peak and post-peak region. The higher inelastic deformation at the ascending branch of the stress-strain curve ( v ), as presented in tables 4 and 5, showed the good ability of the fibers to create the microcracking at the fiber-cement matrix bond short before reaching the peak load. Determination of the v value is described in fig. 9. At the post-peak region, the concretes containing polypropylene fibers showed the larger strain softening and residual strength than the plain concretes. This can be thoroughly observed from the descending branch of the stress-strain curve (fig. 1). The larger strain softening was ascribed to the macrocraking propagation process induced by the fibers. On the other hand the higher residual strength exhibited that the stresscarrying capacity of the concrete fell off gradually due to crack bridging and material interlocking process created by the fibers, as well as due to pull-out process of the fibers themselves. Thus, the fibers still could transfer the stresses over the macrocracks with decreasing of load. These led to stabile fracture process and hence, to higher fracture energy. The stress-strain curve of the high-strength concretes containing polypropylene fibers displayed the better ductility behavior than 9

10 the plain concretes owing to increasing of the deformation under compressive forces both in the pre-peak and post-peak load (fig. 1). This was assuredly achieved, although the maximum compressive strengths were slightly higher or lower than their plain concretes. As the consequence, the fracture energy of the high-strength concretes containing polypropylene fibers was highly improved as presented in tables 4 and 5. The fibers helped dissipate the huge absorbed elastic energy by initiating the formation of microcracking at the pre-peak region, and by creating the new cracks and also by bridging the cracks at the post-peak region. The fracture energy of the concrete specimens subjected to uniaxial compression was calculated by means of applying the Damage Zone (CDZ) model from Markeset (1993), as already discussed in [4]. max v Fig. 9: Determination of inelastic deformation ( v ) at the ascending branch of the curve 1

11 Basalt P Diabase P Gabbro P Granulite P Limestone P Quartzite P Serpentine P Steatite P

12 Fig. 1: Stress-strain curves of all high-strength concretes 4 CONCLUSION Regarding their advantageous properties and beneficial price, the use of polypropylene fibers is often recommended recently to enhance some performances of high-strength concrete, particularly its poor ductility behavior and fire resistance. Besides that, some mechanical properties of high-strength concrete can also be improved by the use of,2 vol.-% polypropylene fibers. Due to their high tensile strength and pull-out strength, the fibers could reduce the early plastic shrinkage cracking by enhancing the tensile capacity of the early age concrete to resist the typical volume changes. The fibers could also act as crack arresters by stopping the crack propagating or by bridging the cracks. When the polypropylene fibers were added into the concrete mixture, they precipitated the slump loss of the fresh concrete. The fibers anchored mechanically to the cement paste due to their large specific area and then held the concrete together. With the use of various aggregate types, the effects of fibers on the compressive strength and modulus of elasticity of the high-strength concrete were less significant. The compressive strength and modulus of elasticity of concretes containing fibers were slightly higher or lower than their plain concretes (under 1%). It means that the use of,2 vol.-% polypropylene fibers alone contributed to the low influences on such concrete properties rather than the influences raised by the other concrete constituents. On the contrary, the fracture properties of high-strength concrete were significantly affected by the polypropylene fibers. The fibers improved the cracking behavior of concrete both in the pre-peak and post-peak region of the stress-strain diagram. The higher inelastic deformation ( v ) at the pre-peak phase, as well as the larger strain softening and the higher residual strength at the post-peak phase were assuredly gained from the stress-strain curve of the concretes containing fibers. As the consequence, the fracture energy of concrete was highly improved leading finally to the improved ductility behavior of the high-strength concretes. 5 REFERENCES [1] Walraven, J.: The Evolution of Concrete. Structural Concrete, P1, No. 1, March 1991, pp [2] Kützing, L.: Tragfähigkeitsermittlung stahlfaserverstärkter Betone. Institut für Massivbau und Baustofftechnologie, Universität Leipzig, [3] Ma, Y. et. al.: Effect of Different Geometric Polypropylene Fibres on Plastic Shrinkage Cracking of Cement Mortars. Materials and Structures, Vol. 35, April 2, pp [4] Aulia, T. B.: Strain Localization and Fracture Energy of High-Strength Concrete under Uniaxial Compression. LACER No. 5,, pp [5] Malisch, W. R.: Polypropylene fibers in concrete. Concrete Construction, April 1986 [6] Laning, A.: Synthetic Fibres. Concrete Construction, July 1992 [7] Aulia, T. B., Deutschmann, K.: Effect of Mechanical Properties of Aggregate on the Ductility of High Performance Concrete. LACER No. 4, 1999, pp [8] König, G. et. al.: Entwicklung zäher Hochleistungswerkstoffe. Abschlußbericht der Forschungsgruppe Hochleistungsbeton des Institutes für Massivbau & 12

13 Baustofftechnologie, Universität Leipzig, [9] Deutschmann, K., Sicker, A.: Time Dependent Change of the Ductility of High Strength Concrete. LACER No. 3, 1998, pp