Effect of pre-setting pressure applied to mechanical behaviours of reactive powder concrete during setting phase

Effect of pre-setting pressure applied to mechanical behaviours of reactive powder concrete during setting phase

Construction and Building Materials 25 (2011) 61–68 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: ...

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Construction and Building Materials 25 (2011) 61–68

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Effect of pre-setting pressure applied to mechanical behaviours of reactive powder concrete during setting phase Metin Ipek a,*, Kemalettin Yilmaz b, Mansur Sümer b, Mehmet Saribiyik a a b

Sakarya University, Technical Education Faculty, Construction Education Department, 54187 Sakarya, Turkey Sakarya University, Engineering Faculty, Civil Engineering Department, 54187 Sakarya, Turkey

a r t i c l e

i n f o

Article history: Received 18 February 2010 Received in revised form 18 June 2010 Accepted 19 June 2010 Available online 13 July 2010 Keywords: Reactive powder concrete Pre-setting pressure Fiber Curing Compressive strength

a b s t r a c t In this study, mixing rates, fiber content and pre-setting pressure of reactive powder concrete (RPC) were examined. Mixing ratios were determined by different mixing techniques in literature using material characteristics via computer software. The samples were prepared according to these mixing ratios. The fresh (plastic) concrete samples were filled in a specified mould. Six different (0, 5, 10, 15, 20, 25 MPa) pre-setting pressure were applied. It is observed that application of pre-setting pressure increased compressive strength of the samples about twice. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Researchers at the laboratory of Bouygues’s company in Paris developed the first RPC in the early 1990s [1]. RPC firstly used in the Sherbrooke Pedestrain Bikeway Bridge Quebec, Canada and its application areas have gradually increased in the following years [2]. RPC is a rather new cement-based material. RPC is composed of cement and very fine powders such as crushed quartz, silica fume. RPC also has an ultra-dense microstructure as ultra high strength concrete. RPC based on the densest packing theory with heat curing was investigated and it was observed that it exhibits compressive strength of more than 200 MPa with great ductility [3–7]. RPC is a cement-based ultra high performance concrete which has superior mechanical and physical properties, exhibiting excellent ductility and durability characteristics. RPC has compressive strength of 150–800 MPa, while its, tensile strength changes between 25 and 150 MPa. Moreover, specific gravity of RPC is between 2.5–3 t/m3 and its fracture energy changes between 1200–40,000 J/m2. Finally its ultimate tensile strain is at the order of 1%. The durability properties of RPC are better than current high performance concrete in orders of magnitudes [1–8]. The basic principles for the development of RPC was explained by Richard and Cheyrezy [1,9]. These principles can be listed as be-

* Corresponding author. E-mail address: [email protected] (M. Ipek). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.06.056

low: not using coarse aggregate in order to increase the homogeneity of the material:  increased compactness through optimization of the grading,  improvement of the microstructure by pressure-casting with heat treatment curing,  further addition of steel fibers to improve ductility. Fine powders such as crushed quartz (100–600 lm) are used instead of coarse aggregate in order to increase the homogeneity of RPC. The water/cement ratio of RPC is reduced to less than 0.20 by using superplasticizers. Addition of silica fume to RPC reduced the total pore volume of the cement paste and the average diameter of the pores. Furthermore researchers reported that application of different heat cure processes improve mechanical properties of RPC substantially after the application of 50 MPa pre-setting pressure to fresh RPC [1,3,10]. Application of pressure to fresh RPC during setting phase for 6–12 h can eliminate some amount of pores caused by autogenous shrinkage. Pressure applied during setting stage caused micro cracks in the sample. Micro cracks which are in fresh RPC are improved in consequence of expansion of aggregate after discharging the applied pressure [1]. Dugat et al. [3], applied 60 MPa pre-setting pressure to samples of fresh (plastic) RPC800. In addition to that, they applied heat treatment cure to these samples at 90 °C and 250 °C, respectively. Consequently, they reached compressive strength of about 500 MPa, static young’s modulus of 36,000–74,000 MPa for RPC800.

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Bonneau et al. [10], studied confinement behaviour of the RPC in a steel tube. Confinement of the RPC was obtained with a compressive strength of 285 MPa. They reported that compressive strength up to 200 MPa could be achieved in both examined cases: which are the case involving after curing in hot water at 90 °C and the case involving in low-pressure steam chambers at the precast plant. Teichman and Schmidt [11] studied structural properties of RPC. Effects of these structural properties were investigated on strength and durability. In addition, they compared them with other concrete types. Steam curing were applied to ordinary and high strength concrete samples for 2 days at 90 °C and RPC samples were exposed to heat curing for 7 days at 250 °C after 2 days of demoulding. However, cement content of this mixture was as high as 1900 kg/m3. Furthermore, plastic RPC samples were exposed to pre-setting pressure of 50 MPa. The highest compressive strength value reached was 487 MPa with RPC samples. Pulverized fly ash (FA), pulverized granulated blast furnace slag (PS) and silica fume (SF) were quantitatively studied with the incorporation of Portland cement (PC) by Yazici [12]. PC was replaced with FA or PS at percentages of 0–20–40–60–80%, respectively. Three different curing methods (standard, autoclave and steam curing) were applied to the specimens. Consequently, compressive strength of control mixture was 170 MPa, while compressive strengths of PS20, PS40 and PS60 mixtures were 178.7, 185 and 168.9 MPa, respectively. In this case, UHSC could also be achieved with high volume PS60 binder phase. Cement content of this mixture was only 340 kg/m3. Topçu and Karakurt [13] applied pre-setting pressure of 2.5 MPa to RPC mixture. This mixture was exposed to steam curing for 7 days at 250 °C and then kept in water for 7 days at 90 °C. For this mixture highest compressive strength of 253.2 MPa and flexural strength of 63.67 MPa were obtained. In this study, effect of pre-setting pressure, which was applied during setting phase to reactive powder concrete in order to improve its mechanical behaviours was investigated. Mechanical and chemical properties of materials, constituting the mixture up, were determined. In order to determine the mixture ratios, computer software and basic principles from literature were used. At specified ages, compressive strength tests are applied to the samples prepared according to this mixing ratio and the sample with the highest compressive strength value was selected. Micro size fibers were replaced at different six ratios percentage of 0, 2, 4, 6, 8, 10 of all volume. Samples had a length of 200 mm and a diameter of 100 mm. The compressive strength, young modulus and poisson ratio tests were applied to the samples and optimum fiber content was determined as 4% by volume. Test samples were prepared according to this optimum fiber content. These samples were exposed to cure program given in Experimental Procedures and then samples were kept in the moulds for 24 h at room temperature of 20 °C. These fresh (plastic) concrete samples prepared with the optimum fiber content were filled to the mould which was specially designed for this study and six different pre-setting pressure (0, 25, 50, 75, 100, 125 MPa) were applied by concrete test press.

Table 1 Properties of cement and silica fume. Component

Chemical composition (%) Cement

Silica fume

64.47 20.09 – 5.01 2.73 1.72 0.66 0.01 0.21 – 3.03 – 2.11 – 60.7 11.8 8.6 8.3 2.6 1.8 2.3 0.58

0.50 96 1.50 0.70 0.25 0.60 0.85 0.10 0.25 0.10 0.50 0.80 1.50 5.0–8.0 – – – – – – – –

Blaine specify surface Unit volume weight Specific gravity Initial setting time Final setting time Volume expansion Genlesßmesi

Cement 5162 cm2/g – 3.14 154 min 191 min 0.8 mm

Silica fume 200,000 cm2/g 0.650 g/cm3 2.26 – – –

Compressive strength of cement (MPa) 2 days 7 days 28 days

39.8 MPa 54.2 MPa 61.8 MPa

– – –

CaO SiO2 C Al2O3 Fe2O3 MgO K2O Cl Na2O P2O5 SO3 H2O Fever loss pH value C3S C2S C3A C4AF Silicate modulus Alumina modulus Hydraulic modulus Total alkaline Physical properties

zolanic material that will fill voids of micro particulates in binder paste and will contribute strength by producing secondary hydrates by puzzolanic reaction with the lime resulting from primary hydration. This necessity is met by silica fume (SF) the best [1,9,14–16]. In this study undensified SF provided from Elkem Company in Norway was used. The physical, chemical and mechanical properties of Portland cement (PC 52.5 CEM I) and silica fume presented in Table 1. Two different quartz sands and powder used as aggregate with maximum particle size of 0.6 mm, 0.3 mm and 0.100 mm, respectively. Figs. 1 and 2 show the granule

2. Experimental 2.1. Material RPC is a concrete whose main constituent is cement and its quantity is (about 1000 kg/m3) higher than that of a custom concrete. Earlier studies have shown that high performance cement was used in RPC [1,2,9,12]. Therefore PC 52.5 CEM I R type high performance cement was preferred for this study. RPC needs a puz-

Fig. 1. Granule structure of the quartz powder.

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mixer should be specified according to a particular system in order to produce the desired mixture. Otherwise with same, either high or low strength concrete can be obtained. Therefore following mixing process was used:  All the granule materials without fiber were filled to mixer and they were mixed for five minutes at low speed of 400 rpm.  The speed of mixer was adjusted to 1400 rpm as soon as water and SP were sprayed into the mixer. The mixing process continued for about five minutes at this speed.  The mixture was additionally mixed for two minutes after the addition of fiber. Samples which pre-setting pressure was not applied were produced using standard vibration method in the sismic table. These samples were used to determine the mixture ratios, fiber ratio and cure type.

Fig. 2. Particle size analysis of granule materials.

Table 2 Cure programme and sample codes. Sample code

Cure type

W 3HW 3S 3S2A 3S3A 3W3HW 5W3HW 7W3HW

28 days 20 °C water cure 3 days 90 °C hot water cure 3 days 90 °C steam cure 3 days 90 °C steam cure later12 h 200 °C dry air cure 3 days 90 °C steam cure later 12 h 300 °C dry air cure 3 days 20 °C in water later 3 days 90 °C hot water cure 5 days 20 °C in water later 3 days 90 °C hot water cure 7 days 20 °C in water later 3 days 90 °C hot water cure

structure of the quartz powder and particle size analysis of granule materials, respectively. A polycarboxylate based superplasticizer was used to fluidify the mixture. Effect of this superplasticizer is to maintain fluidity within time of fresh concrete and to attain high strength in a short time. The short brass coated steel fibers without hooked ends with length of 6 mm and diameter of 0.16 mm were used. The steel fiber had tensile strength of 2250 MPa, specific gravity of 7.181 g/cm3 and aspect ratio of 37.5. 2.2. Experimental procedures Different mixing theories were used such as; Mooney suspension model, Fuller (the laws of proportioning concrete), mathematical relation for beds of broken solids of maximum density, packing calculations and concrete mix design, software programs based on models given here and also we used mixture ratios which were chosen in previous works within literature [1,10,11,16–22]. By using these theories, 33 different mixtures including no fiber were prepared. Mixture having the highest compressive strength was selected and used in following experiments. Mixing process was adopted from literature [1,23]. The order of the ingredients to be added into the mixture and the speed of the

Cure type which was taken from literature and applied to the samples is shown in Table 2 [1,3,11,12,16,24,25]. The remaining specimens were cured in standard conditions (water curing at 20 °C) for 28 days. Maximum and minimum temperatures were reached with increment or decrement of 10 °C per hour and that concludes several experiments. In curing experiments, dry heat air curing was directly applied to the samples without steam curing whose effect was negative in spite of low heat rise rate. These samples were cracked or broken into pieces without reaching the temperature of 100 °C (Fig. 3). Compressive strength testing of cured samples was determined for 10, 28 and 56 days. Placement and compression factors affect all properties of concrete. Concrete must be placed to minimum voids. It is well known that reducing voids positively affected mechanical and durability properties of hardened concrete [26]. The increment of air volume clearly understood with examination of samples due to air-entrainer property of SP during application of vibration to RPC. Flowability can be brought to desired level due to high ratio of SP, although RPC has low water cement ratio. However, water and compressed air of closed pockets found in the RPC. It was seen that swelling overflow from mould or forming of bubbles at surface due to air bubbles of concrete. The voids were formed between fiber and concrete interface. It was also seen that these voids produced defects in interface when samples were analyzed by microscopy (Fig. 4). In this condition adherence affected negatively between fiber and concrete interface and consequently the strength reduced. It was decided to compress the fresh RPC with applying pressure (pre-set-

Fig. 4. Non pre-setting pressurized specimens.

Fig. 3. Damaged samples due to thermal stresses.

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Fig. 5. Pre-setting pressure mould.

Fig. 6. Experimental set-up of compressive strength and young’s modulus.

Table 3 Compressive strength values and compositions of different mixtures (kg). Material

Cement Silica fume Q. Powder Q. Sand (100–300) Q. Sand (300–600) Water SP Air (%) Steel fiber 7 days Compr. Strength (MPa) 28 days Compr. Strength (MPa)

Fiber ratio (%) Ctrl.

2

4

6

8

10

900 270 360 258 258 225 27 2 0 113.2 125.63

882 265 353 253 253 221 27 1.8 144 129.8 154.83

864 259 346 248 248 216 26 1.8 287 145.8 170.29

846 254 338 243 243 212 25 1.9 431 151.86 181.52

828 248 331 237 237 207 25 2 575 156.67 206.74

810 243 324 232 232 203 24 2,1 718 177.89 230.28

Fig. 7. Compressive strength of 4% fiber content RPC in comparison to deformation.

Fig. 8. Compressive strength and young’s modulus in comparison to fiber content.

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Fig. 9. SEM images of different cure condition, (a) 20 °C water cure, (b) 3 days 90 °C hot water and steam cure, (c) 3 days 90 °C steam cure later 12 h 200 °C dry air cure and (d) 3 days 90 °C steam cure later 12 h 300 °C dry air cure.

ting pressure) to minimize the negative effects. Due to of high dosage of superplasticizer with vibration effect, swelling overflow of RPC from mould or forming of bubbles on entire RPC was clearly seen. Application of pre-setting pressure is applied for minimizing adverse effects of autogenous shrinkage and for removing water and air in the RPC. RPC is needed to stay under pressure for minimizing adverse effects of autogenous shrinkage during setting phase [1]. Moreover, the sample was expanded due to pushing of the pressed grains to each other. When the pressure was removed consequently, density of the sample was decreased. The desired compression level was not obtained. During the setting phase of RPC samples must be stayed at press for desirable compression level. This process was very difficult in terms of applicability. For this reason, some changes in mould design were made to prevent pressure discharge and setting of RPC under pressure was provided and then plastic RPC in mould was taken away from the press. A mould was specially designed for application of pre-setting pressure process. As can be seen in Fig. 5, the mould has internal diameter of 50 mm, height of 140 mm and it is made of 1040 hardened steel by heat treatment. The prepared plastic RPC having optimum fiber ratio was filled to the mould. The pressure (control-without pressure, 25, 50, 75, 100, 125 MPa) was applied to plastic RPC by this mould mechanism. Specific gravities of samples were measured to see the effects of compression. It is well known that specific gravity values of samples will increase with increasing pre-setting pressure. However, specific gravity values may be inconsistent due to eccentricity of piston, friction to the side wall of the mould or unestimated negative situations. Therefore, specific gravity values are extremely important in terms of the experimental control. Pre-setting pressurized samples were cured during three days at 90 °C water steam then for 12 h at 300 °C dry air was applied and in remaining days it was hold in water at 20 °C after demoulding. Samples were cut to height by 100 mm before the experiment. Compressive strength and young’s modulus tests were made on these samples. Young’s modulus and poisson ratio tests were made by computer-aided system was able to measure load, axial and lateral deformation at the same time (Fig. 6). For determination of young’s modulus

of pre-setting pressurized samples, specific elasticity module frame were designed.

3. Results and discussion Compressive strength results and material quantities of the samples which are based on fiber ratios are presented in Table 3. The compressive strength of RPC increased with increasing of the fiber ratio. Compared to control (zero fiber as percentage) RPC, samples which include fiber ratios of 2, 4, 6, 8, 10 as percentage have increment ratios of 23%, 36%, 44.5%, 63.8% and 83.3%, respectively depending on 28 days compressive strength test results (Fig. 7). The compressive strength was increased by micro fibers that were scattered as a rigid aggregate in RPC. As fiber ratio was increased especially when it exceeded 6% several serious difficulties were experienced in the mixing and placement processes. In addition, increase in fiber ratio has negative effects on cost of RPC. Based on previous criteria, such as; difficulties, lowest increment strength ratio (between 4% and 6%) and increase in cost due to high fiber ratio, 4% fiber ratio was determined as optimum during performing our experiments.

Fig. 10. Compressive strength in comparison to cure type.

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Fig. 11. Failure forms of pre-setting pressurized samples.

Fig. 13. Pre-setting pressure to the compressive strength and young’s modulus effect.

Fig. 12. Images of the between fiber and paste interface (a) pre-setting pressurized sample and (b) non pre-setting pressurized sample.

Fig. 14. Pre-setting pressure of 125 MPa applied sample.

Young’s modulus of the RPC was between 50,000 and 70,000 MPa. Young’s modulus of the samples using different fiber ratios are shown in Figs. 7 and 8.

It is known that for the custom concretes the higher compressive strength value, the more Young’s modulus increment. Increment of fiber ratio at RPC increased ductility as well as

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compressive strength. This can be explained by fibers acting as aggregate in concrete due to shortness of fiber length. It was seen that Young’s modulus increased with the high increment rates of compressive strength. Poisson ratio of RPC was between 0.20 and 0.22 as custom concrete. Optimum fiber ratio was determined as 4% when all results were taken into account. Treatment of hydration increased or continued for a long time due to heat cure process of RPC. Puzzolanic activate was increased by high ratio SF in the samples and it directly affected compressive strength. Compressive strength value of heat cured specimens was higher than that of water cured specimens. The hydration of the cement compounds proceeded more rapidly but the hydration reactions were basically similar. C–S–H remained as an amorphous material, although its compositions changed with temperatures. The reactions of pozzolans were also accelerated by the higher curing temperatures. As seen in Fig. 9, pores of the microstructures of hot water and steam cure at 90 °C are less than the standard cure. The pores were reduced with increasing temperatures. These microstructures explained the compressive strength changes. 3S3A of the samples had the highest compressive strength following that of 3S2A of the samples (Fig. 10). According to application time, compressive strength increased a small amount in the applied samples hot steam curing after 3, 5 and 7 days. Pre-setting pressure caused water and air exiting from the samples and drew near of grains each other. As a result, specific gravity of pre-setting pressurized samples increased. Specific gravity of pre-setting pressurized samples at 25 MPa which had the highest ratio reached 2688.33 kg/m3 while specific gravity of without pre-setting pressure samples were 2498.70 kg/m3. Failure forms of pre-setting pressurized samples are showed in Fig. 11. Compressive strength highly increased by increasing presetting pressure until 100 MPa. In addition, it was seen from SEM images that pore structure was improved and formed a perfect structure which was between fiber and concrete interface (Fig. 12). These effects of pre-setting pressure directly increased the compressive strength. When the samples were pre-setting pressurized at 25 MPa their compressive strength reached to 420.31 MPa (about twice) while samples which were not pre-setting pressurized had the compressive strength of 206.36 MPa. Other pre-setting pressures had low increment rate as seen in Fig. 13. The samples which were pre-setting pressurized at 100 MPa had the compressive strength of 475 MPa. That was maximum compressive strength for 125 MPa which was decreased. As it can be seen in Fig. 14, microcracks were created due to expansion of the aggregate when pressure was released. In addition, water needed for hydration could be gone out under pressure of 125 MPa or it could be explained that materials which constitute to RPC could be deformed under pressure of 125 MPa. It was seen that Young’s modulus was increased by increasing of pre-setting pressure (Fig. 13). In addition, when the samples were pre-setting pressurized at 125 MPa their young’s modulus reached to 84,562 MPa which was maximum young’s modulus while samples which were not pre-setting pressurized had the young’s modulus of 58,469 MPa. The young’s modulus increment rate was lower than that of the compressive strength. It could be explained that pre-setting pressure increased adherence between fiber and concrete interface hence ductility was increased. It can be seen in Fig. 13 that fracture energy highly increased by increasing of pre-setting pressure in spite of short fiber length.

4. Conclusions Mixing ratio with the highest compressive strength value was selected. Test samples were produced by adding micro size fiber which was used in RPC at six different ratios by volume. The com-

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pressive strength, young modulus and poisson ratio tests were applied to the samples and optimum fiber content was determined as 4% by volume. When pre-setting pressure of 100 MPa was applied to the samples, their compressive strength became 475 MPa. It was seen that pre-setting pressure of 25 MPa was sufficient for going out of large air spaces and free water in the samples. Permeability of pre-setting pressurized samples could be decreased owing to increment of specific gravity values. Hence, properties of durability could be improved. This is very important for the elements which are exposed to external factors. In terms of applicability, pre-setting pressure of 25 MPa was more suitable. Although, the highest compressive strength was reached to 475 MPa by choosing presetting pressure at 100 MPa. Pre-setting pressure method was used in production of prefabrication although it was applied in lower pressures. It is estimated that when this method is used in production prefabrication elements with RPC, strength based cost would be decreased. RPB element can be used not only in concrete materials but also in other materials as alternative in industry. Acknowledgement The authors would like to thank Sakarya University and TCMA due to its financial support for this study. References [1] Richard P, Cheyrezy MH. Composition of reactive powder concrete. Cem Concr Res 1995;25:1501–11. [2] Aitcin PC. Concrete the most widely used construction materials. ACI SP-154; 1995. p. 257–66. [3] Dugat J, Roux N, Bernier G. Mechanical properties of reactive powder concretes. Mater Struct 1996;29:233–40. [4] Morin V, Cohen-Tenoudji F, Feylessoufi A, Richard P. Evolution of the capillary network in a reactive powder concrete during hydration process. Cem Concr Res 2002;32:1907–14. [5] Matte V, Moranville M. Durability of reactive powder composites: influence of silica fume on the leaching properties of very low water/binder pastes. Cem Concr Compos 1999;21:1–9. [6] Chan Y, Chu S. Effect of silica fume on steel fiber bond characteristics in reactive powder concrete. Cem Concr Res 2004;34:1167–72. [7] Aitcin PC. Cement of yesterday and today concrete of tomorrow. Cem Concr Res 2000;30:1349–59. [8] Yazıcı H, Yardımcı MY, Aydın S, Karabulut AS. Mechanical properties of reactive powder concrete containing mineral admixtures under different curing regimes. Constr Build Mater 2009;23:1223–31. [9] Richard P, Cheyrezy MH. Reactive powder concretes with high ductility and 200–800 MPa compressive strength. Concrete technology: past, present, and future. In: Proceedings of the V. Mohan Malhotra symposium, ACI SP-144; 1994. p. 507–18. [10] Bonneau O, Lachemi M, Dallaire E, Dugat J, Aitcin PC. Mechanical properties and durability of two industrial reactive powder concretes. ACI Mater J 1997;94:286–90. [11] Teichman T, Schmidt M. Influence of the packing density of fine particles on structure, strength and durability of UHPC. Ultra high performance concrete (UHPC). In: International symposium on ultra high performance concrete, September 13–15; 2004. p. 312–23. [12] Yazıcı H. The effect of curing conditions on compressive strength of ultra high strength concrete with high volume mineral admixtures. Build Environ 2007;42:2083–9. [13] Topçu IB, Karakurt C. Reaktif pudra betonları. Turk Eng News J 2005;437:25–30. [14] Aitcin PC, Sarkar SL, Ranc R, Levy C. A high-silica-modulus cement for high performance concrete. Adv Cem Mater Ceram Trans Gaithersburg 1991;16:103–21. [15] Goldman A, Bentur A. The influence of microfillers on enhancement of concrete strength. Cem Concr Res 1993;23:962–72. [16] Talebinejad I, Bassam SA, Iranmanesh A, Shekarchizadeh M. Optimizing mix proportions of normal weight reactive powder concrete with strengths of 200– 350 Mpa. Ultra high performance concrete (UHPC). In: International symposium on ultra high performance concrete, September 13–15; 2004. p. 133–41. [17] Larrard F, Sedran T. Optimization of ultra-high-performance concrete by the use of a packing model. Cem Concr Res 1994;24:997–1009. [18] Mooney M. The viscosity of concentrated suspension of spherical particles. J Colloid 1951;6:162–70. [19] Furnas CC. Grading aggregates – I. Mathematical relations for beds of broken solids of maximum density. Ind Eng Chem (ACE) 1931;23:1052–8.

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