Experimental study of the e ect of water to cement ratio...

Scientia Iranica A (2019) 26(5), 2712{2722

Sharif University of TechnologyScientia Iranica

Transactions A: Civil Engineeringhttp://scientiairanica.sharif.edu

Experimental study of the e�ect of water to cementratio on mechanical properties and durability ofnano-silica concretes with polypropylene �bers

K. Rahmania, M. Ghaemianb;�, and S.A. Hosseinia

a. Department of Civil Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.b. Department of Civil Engineering, Sharif University of Technology, Tehran, Iran.

Received 26 August 2017; received in revised form 20 November 2017; accepted 24 December 2017

KEYWORDSConcrete;Nano-silica;Polypropylene �ber;Mechanical propertiesof concrete;ASTM standard.

Abstract. This study investigates the e�ect of nano-silica on mechanical propertiesand durability of concrete containing polypropylene �bers. Herein, the length and thelength-to-diameter ratio of used polypropylene �bers were considered �xed and equal to18 mm and 600, respectively, and the cement content was 479 kg/m3. The e�ects of�bers and nano-silica in four di�erent percentages of 0.1, 0.2, 0.3, and 0.4% volumefor �bers and 3% for nano-silica in concrete with water to cement ratios of 0.33, 0.36,0.4, 0.44, and 0.5% were compared and evaluated. In total, more than 425 cubic andcylindrical specimens were made according to ASTM standards. Finally, samples ofpolypropylene �ber containing nano-silica were tested under compressive loads, exuralstrength, indirect tensile strength (Brazilian test), abrasion resistance, permeability, andporosity; in addition, their mechanical properties were evaluated. The test results showed asigni�cant improvement of mechanical properties and durability of concrete. Compressivestrength, tensile strength, exural strength, and abrasion resistance (of concrete) increasedby 55%, 25%, 49%, and 45%, respectively. In addition, a considerable reduction in hydraulicconductivity coe�cient by 50% indicates the high durability of these types of concrete.

© 2019 Sharif University of Technology. All rights reserved.

1. Introduction

The use of various additives in concrete and cementproducts has attracted considerable attention in recentyears. So far, many research studies have been carriedout on �ber concrete; however, very few studies havebeen devoted to the nano-silica concrete containingpolypropylene �bers. Particularly, the e�ect of nano-

*. Corresponding author.E-mail addresses: [email protected] (K.Rahmani); [email protected] (M. Ghaemian);abbas [email protected] (S.A. Hosseini)

doi: 10.24200/sci.2017.5077.1079

silica on �ber concrete, especially polypropylene �bers,has been scarcely investigated.

To this end, the use of �ber concrete, especiallypolypropylene �bers with nano-silica, was considered inthis study. High-Strength Fiber Reinforced Concrete(HSFRC) has a wide range of applications in whichdi�erent types of �bers are used to increase the tough-ness, exibility, and tensile and exural strength of theconcrete in order to increase the strength of structuralmembers under static and dynamic loads and to reducecracking and crushing [1].

Polypropylene �ber was introduced in the mixtureto minimize the brittleness of the matrix, thereby re-ducing the susceptibility to cracking of concrete [2]. Itwas also reported that polypropylene �ber was e�ective

K. Rahmani et al./Scientia Iranica, Transactions A: Civil Engineering 26 (2019) 2712{2722 2713

in resisting the development of cracks caused by dryingshrinkage [3]. The inclusion of polypropylene �ber inconcrete showed excellent durability on exposure tofreeze-thaw cycling [4].

Studies have indicated that the explosive spallingof regular High-Strength Concrete (HSC) could benoticeably alleviated by embedding an appropriatevolume of strong, exile polypropylene �bers [5,6].This improvement is supposedly attributed to thedi�erence of the thermal expansion capability betweenpolypropylene �bers and cement pastes, which canintroduce micro-fractures to release the vapor pres-sure[7,8].

Experimental research and analytical study byOlivito and Zuccarello [9] provided reinforced concretewith steel �ber with mixing patterns and di�erentlengths of di�erent �bers. They evaluated compressiveand tensile strengths, fracture, and ductility. The addi-tion of �bers increased the ductility, which is associatedwith energy absorption during fracture. Samples oflonger �bers showed higher strain as compared to thesamples with shorter �bers.

Zhang and Li [10] performed an experimentalstudy to investigate the combined e�ect of silica fumeand polypropylene �ber on the workability and dryingshrinkage of concrete composite containing y ash.They concluded that the workability of the concretecomposite improved, and the drying shrinkage straindecreased gradually with the increase of y ash content.However, polypropylene �ber has an inconsiderableadverse e�ect on the workability of concrete composite.

Authors [11-13] showed that the behavior ofconcrete at high temperatures could be improved byadding polypropylene �bers (with a ratio varying from0.5 kg/m3-3 kg/m3). The melting (160�C-170�C) andvaporization (350�C) of polypropylene �bers generatenot only new pores, but also create microcracks at thetip of �bers connecting the already existing pores [14].Ozawa and Morimoto [15] carried out permeabilitytests on high strength concretes (72 MPa) including0.15% of volume of PPF. Results showed that theresidual permeability increased 12 times after heatingthe PPF concrete to 500�C, compared to the referencedconcrete. Thus, the improvement of permeabilityreduces the likelihood of explosive spalling [16].

Consoli et al. [17] showed that the reinforcinge�ect of polypropylene �bers became less e�ectivein high cement contents for a stabilized sandy soil,i.e., the peak strength only increased until 4% ofcement content and, after that, the e�ect of the �berreinforcement had the opposite e�ect. In fact, when thecement content is high, the sti�ness of the stabilized soilis fundamentally governed by the high sti�ness of thecement, and the polypropylene �bers are, therefore, un-able to mobilize the tensile strength before peak failure,since the deformations obtained are very low. Thus,

increasing the amount of �ber for high cement contentsdoes not have repercussions on the increase of thestrength [17]. Thus, polypropylene �ber, carbon �ber,plastic-glass-based �ber, and steel �bers started to beused in concrete. It is known that steel, nylon, andmixed �bers insigni�cantly a�ect mechanical properties(e.g., compressive strength and elasticity module) andhighly increase the mechanical properties (bending-tensile strength, ductility, and toughness) [18,19].

It is well understood that silica fume, due to highpozzolanic activity, is a necessary material when pro-ducing high-strength concrete. Silica fume e�ectivelyimproves the structure of the transition zone, elimi-nates the weakness of the interfacial zone, reduces thenumber and size of cracks, and enhances the ability ofsteel �ber to resist cracking and restrain damage. Bothnormal-strength concrete and high-strength concreteare brittle, with a degree of brittleness increasing withincreasing strength [20].

Nili and Afroughsabet [21] showed that the silicafume improved the compressive strength of the con-cretes with higher ages by improving the conjunctionof aggregates with cement paste. In addition, thesimultaneous use of silica soot and �ber in concretemixtures results in an e�ective increase in the exuralstrength of concrete.

Pore structure parameters, such as porosity, poresize distribution, etc., are progressively employed toevaluate permeability, frost resistance, carbonationresistance, and physical strength of concrete [22-24].

Tijani et al. [25] added arti�cial carbon �ber andmicrosilica to some concrete mixtures to improve theirmechanical properties. Maslennikov et al. [26] analyzedthe e�ect of di�erent modi�ers on the properties ofconcrete mixtures using microsilicon additive.

Subramanian et al. [27] showed that nano-silicaand micro-silica could be successfully used in thepreparation of Self-Compacting Concrete (SCC).

Khanzadi et al. [28] measured compressivestrength, tensile strength, and water absorption per-centage and chloride penetration depth in concretecontaining nano-silica. They showed that the measuredmechanical properties and the durability of the con-crete mixed with the nano particles were better thanthose of a plain concrete were.

In recent years, using di�erent additives in con-crete and cement products have attracted considerableattention. Fiber and nano-silica concrete can beconsidered as the most commonly used additives. Inthis study, by reviewing the advantages of using nano-silica and polypropylene �bers in concrete, 450 sampleconcretes were prepared using 20 mixture designswith water to cement ratios of 0.33, 0.36, 0.4, 0.44,0.5, �ber volume ratios of 0.1, 0.2, 0.3, 0.4, and3% volume of nano-silica based on ACI and ASTMstandards. The purpose of the present study is three-

2714 K. Rahmani et al./Scientia Iranica, Transactions A: Civil Engineering 26 (2019) 2712{2722

fold: to determine the e�ect of polypropylene andnano-silica �bers on behavioral properties of concreteunder stress conditions due to exural loads, to studythe permeability and porosity of the concrete, and toobtain their optimal percentage. In this research, thelength of polypropylene �ber is considered 18 mm.Eventually, compressive strength, exural strength,tensile strength, abrasion resistance, permeability, andporosity tests conducted on the samples containingnano-silica and their mechanical properties are investi-gated.

2. Experiments performed on samples

In the present study, the concrete samples with 3%nano-silica and water to cement ratios of 0.33, 0.36,0.4, 0.44, 0.5 and �ber contents of 0, 0.1, 0.2, 0.3 ofconcrete volume are investigated experimentally:

- The cement used in these experiments is of ordinaryPortland cement (ASTM Type I) in accordance withTable 1;

- For all samples, the amount of �xed nano-silica and3% consumption cement are replaced, as shown inTable 2;

- The amount of polypropylene �ber varies and isequal to 0, 0.1, 0.2, and 0.3 volumetric percentalternatives of the cement used; The properties of theconsumed polypropylene �ber are shown in Table 3,and its mixture design with the nano-silica is shownin Table 4;

Table 1. Properties of ordinary Portland cement (ASTMType I) used in the present tests.

No. Property of cement Value

1 Normal consistency 31%

2 Initial setting time 120 min

3 Final setting time 6 hour

4 Speci�c gravity 3.11

5Compressive strength of

cement on the 28th day55.6 MPa

Table 2. Properties of nano-silica [29].

SI. no Dispersed in water

Active nano content (%) 40-41.5

Ph (20�C) 9-10

Speci�c gravity 1.4

Particle size 10-40 nm

Table 3. Properties of polypropylene �ber [29].

Properties Value

Density (kg/m3) 900Reaction with water HydrophobicTensile strength (MPa) 300-400Melting point (�C) 175Length (mm) 18

- Compressive strength of the 28-day specimen is40 MPa;

- The maximum diameter of aggregates is 15 mm. Ta-ble 5 summarizes the properties of required materialsfor a samples design. Actually, this table shows theconcrete mixture design proportions (kg/m3) used inthe present study;

- Super lubricant used in these experiments is superplasticizer 400 according to ASTM-C494 Type 4standard.

In the production of test specimens, excesswater was computed and added to the dry state(S.S.D state) to prevent the free water absorption ofthe concrete into the empty space of the aggregate;

- A exural strength test was carried out on concretebeams of 10�10�50 cm on the 28th day. All stagesof testing were performed according to ASTM-C1018standard. Eq. (1) can be used in order to calculatethe exural strength [30]:

Flexural strength = [(PL=bd2)� 1000]: (1)

In the above equation, P , L, b, and d are theapplied load at fracture moment, the length of thespecimen, the section's width, and the section'sheight, respectively;

- An indirect tensile strength test or tensile strengthof halving is performed on cylindrical specimens of300 � 150 mm on the 28th day. To do so, the loadneeds to be done continuously and without impact.The load speed of 80 kN/min was selected accordingto the ASTM C496 standard. Eq. (2) illustrates howto determine tensile strength [31]:

Tensile strength = [(2P=�dL)� 1000]; (2)

where P is the applied load at the fracture moment;L and d are the length and diameter of the cylinder,respectively;

- Abrasion strength test on cube samples 150� 150�150 mm on the 28th day was performed by the watersand blast method based on ASTM-C778 standard;

- The hydraulic conductivity of the concrete wastested by penetration method according to ASTM-C1920-5 [32]:

Hydraulic conductivity = Hp2V=(2TH); (3)


Table 4. Nano-silica �ber concrete mixture designs.

Mixingschemenumber

Polypropylene�ber(%)

Polypropylene�ber content

(kg)

Waterto cement

ratio

Nano-silicacontent

(kg)

1 0.1 0.56

0.33 142 0.2 1.13

3 0.3 1.7

Table 5. Nano-silica �ber concrete mixture designs (kg/m3).

Water-to-cement ratio 0.33 0.36 0.40 0.44 0.5

Nano-silica weight (kg) 14 14 14 14 14

Cement weight (kg) 465 465 465 465 465

Water consumption (lit) 158+20 172+20 191+20 211+20 239+20

Weight of coarse gravel (kg) 470 470 470 470 470

Weight of tiny gravel (kg) 385 385 385 385 385

Sand weight (kg) 821 821 821 821 821

where:Hp Water penetration depth (m);T In uence time (sec);H Pressure height (m);V Concrete porosity.

The following formula can be used to calculate theporosity of the concrete [33]:

Concrete porosity = (w=c)� (100� 36:15�)

=(W + 100=g); (4)

where w=c is the water to cement ratio, � is thecement hydration degree, W is the pure water ofconcrete, kg/m3, and g is the cement speci�c weight,gr/cm3;

- Compressive strength tests on cube samples of 150�150� 150 mm on the 7th, 28th, and 91th days weredone with a pressure test device with a capacity of2000 kN (Tekno test-Italy) at a speed of 2.5 kN/S inaccordance to ACI-C330 standard. The compressivestrength value of samples can be evaluated usingEq. (5):

Compressive strength = [(P=A)� 1000]; (5)

where P is the applied load at fracture, and A is thecross-sectional area of cubic samples.

3. The mixture scheme of �ber concretecontaining nano-silica

In the mixture scheme of samples, the following pointsare considered:

- Slump test, according to the ACI, the 544 Commit-tee, is not suitable for �ber concrete, and one needsto use the inverted slump test. The inverted slumpfor samples is achieved in the period of 8-12 seconds;

- The aggregates are broken.

4. Analysis of the exural strength test results

According to Figure 1, it is shown that the exuralstrength increases with the addition of �bers. As thegraphs show, the addition of �bers up to 0.3% and3% of nano-silica has a signi�cant e�ect on exuralstrength. However, the exural strength decreases inhigher �ber ratios. It is anticipated that by increasingthe �ber content to more than 0.3%, the exuralstrength will be reduced. In this part, inappropriatedistribution of �bers can also be a�ected. Non-�brous

Figure 1. Flexural strength of the �ber-based samples fordi�erent water to cement ratios.


samples are fractured during failure, where this prob-lem is completely eliminated by the �ber. According toFigure 2, by decreasing the water to cement ratio from0.50 to 0.3 in non-�ber samples, the exural strengthhas improved by 70.17%. In the water to cement ratioof 0.33, by adding 0.1, 0.2, and 0.3% of the �bers to thesamples, according to Figure 3, the exural strengthhas improved by 31.81, 37.23, and 39.61%, respectively.

5. Analysis of the tensile strength test results

Figure 4 illustrates that indirect tensile strength in-creases with the addition of polypropylene �bers. Asshown in the diagram, by adding 0.1% �ber, there isa signi�cant e�ect on tensile strengths. In the �berratio higher than 0.3%, the tensile strength decreases.In addition, according to Figure 5, one can see thatby reducing the water to cement ratio from 0.5 to0.33 in concrete samples without �ber, the tensilestrength has improved by 82.4%. By adding 0.1, 0.2,and 0.3% �bers for the water to cement ratio of 0.33,

Figure 2. The percentage of improvement in exuralstrength without �ber for water to cement ratio of 0.50.

Figure 3. The percentage of improvement in the exuralstrength of the �bers for di�erent ratios of water tocement.

tensile strength has improved by 11.96%, 16.13%, and17.68%, respectively, compared to non-�brous samples(Figure 6).

Figure 4. Tensile strength of the �ber-based samples fordi�erent water to cement ratios.

Figure 5. The percentage of improvement in tensilestrength without �ber for water to cement ratio of 0.50.

Figure 6. The percentage of improvement in the tensilestrength of the �bers for di�erent ratios of water.


6. Analysis of the abrasion strength testresults

Figures 7 and 8 show the abrasion resistance of re-inforced concrete. One can see the changes in theabrasion resistance of the samples by the �ber addition.Increasing the �ber content up to 0.2% can increase theabrasion resistance. However, by increasing the �bercontent to more than 0.2%, the abrasion resistance ofthe non-�brous samples decreases, since the �ber willmake the samples more porous. Figure 9 illustratesthat by reducing the water to cement ratio from 0.5to 0.33, the abrasion resistance of non-�brous concretesamples has improved by 46%. By adding 0.1, 0.2, and0.3% of the �ber to the concrete samples for the waterto cement ratio of 0.33%, the abrasion resistance im-proves by 14, 25.45, and 9.68%, respectively, comparedto the non-�brous samples (Figure 10). As the waterto cement ratio increases from 0.33 to 0.50, the slopeof the depth of the abrasion curve decreases gradually.This is related to the two-phase nature of concrete inabrasion (phase of mortar and phase of aggregates).

Figure 7. Depth of abrasion variation of �ber-basedsamples for di�erent water to cement ratios.

Figure 8. Abrasion strength variation of non-�broussamples for di�erent water to cement ratios.

Figure 9. The percentage of improvement in abrasionstrength of non-�brous samples for the water to cementratio of 0.50.

Figure 10. The percentage of improvement in theabrasion strength of the �ber-based samples for di�erentratios of water to cement.

As the ratio of water to cement increases, the abrasionstrength of the mortar phase decreases; however, theabrasion strength of the concrete will tend towards theabrasion strength of the aggregates.

7. Analysis of the hydraulic conductivitycoe�cient and porosity of concrete results

The analysis of Figures 11 to 13 shows the hydraulicconductivity coe�cient, which is changed by the ad-dition of �bers. Using �bers up to 0.2% reduces thedepth of water penetration and hydraulic conductivity.Nevertheless, these values increase by increasing �bersto more than 0.2%. The reason for this behavior ofmost porous samples is due to the addition of more�ber content. According to Figure 11, it is obviousthat by decreasing the ratio of water to cement from0.50 to 0.33, the hydraulic conductivity of non-�brousconcrete samples decreases from 43:03 � 10�15 m/sto 6:3 � 10�15 m/s. In other words, the hydraulicconductivity coe�cient of samples has improved byabout 85.36%. Figure 13 displays that in the water


to cement ratio of 0.33, the hydraulic conductivitycoe�cient of 0.1, 0.2, and 0.3% of the �bers improvedby 25.08, 41.43, and 17.9%, respectively. In addition,

Figure 11. Hydraulic conductivity coe�cient variation of�ber-based samples for di�erent water to cement ratios.

Figure 12. The percentage of improvement in hydraulicconductivity coe�cient of non-�brous samples for thewater to cement ratio of 0.50.

Figure 13. The percentage of improvement in hydraulicconductivity coe�cient of the �ber-based samples fordi�erent ratios of water to cement.

by reducing the ratio of water to cement from 0.50 to0.33, the concrete porosity decreases from 14.4 to 13.5%(Figure 14).

8. Analysis of the compressive strength testresults

Figures 15 to 17 show the results of compressivestrength test and the percentage of improved com-pressive strength of �brous samples (0.1, 0.2 and 0.3),compared with non-�brous specimens. Investigationsof the compressive strength of the �ber-reinforcedconcrete show that the adhesive strength changes withthe addition of the �bers. Using 0.1% �ber with aconstant water to cement ratio of 0.33 causes a relativeincrease in compressive strength on the 7th, 28th, and90th days. By increasing �bers to more than 0.1%, thecompressive strength decreases. The reason for thisbehavior is that when the amount of �ber in concreteincreases by a certain amount, compressive strengthvalues are reduced due to poor �ber dispersion.

According to Figure 16, by reducing the ratio ofwater to cement from 0.50 to 0.33, the compressive

Figure 14. Percentage of concrete porosity variation fordi�erent water to cement ratios.

Figure 15. 7-, 28-, and 90-day compressive strengthvariations of �ber-based samples for di�erent water tocement ratios and �ber percentages.


strength of non-�brous concrete samples at the age of7, 28, and 90 days of concrete has improved by 27.92,33.04, and 31.88%, respectively. Figure 17 illustratesthat by adding �ber at 0.1, 0.2, and 0.3%, comparedto non-�brous samples, the compressive strength of thesamples on the 7th, 28th, and 90th days has improvedby (21.42, 15.29, and 13.44%), (13.52, 8.47, and 35.8%),and (8.42, 5.9, and 5.71), respectively.

9. Microstructures

Studying the microstructure of cement paste and con-crete by Scanning Electron Microscope (SEM) imageshas revealed a new horizon in the concrete technology

Figure 16. The percentage of improvement incompressive strength of non-�brous samples for the waterto cement ratio of 0.50.

in recent years. Concrete characteristics includingspeci�c gravity and all kinds of chemical reactions andhydration created in the initial and �nal setting pro-cesses of concrete have a direct and close relationshipwith the concrete microscopic structure. Therefore,this study investigates the microstructure of concretein the process of cement hydration and the placementstate of concrete besides various admixtures: materialsare added to concrete, including various types of �bersand pozzolans using Scanning Electron Microscope(SEM), which are very important.

In this investigation, polypropylene �bers andnano-silica were added to concrete, and a detailedinvestigation into their arrangement in the concretestructure was carried out.

Scanning Electron Microscope (SEM) was usedfor studying three types of cement pastes includinga control mix design, a mix design containing 2%(polypropylene) �bers, 3% nano-silica, a mix designcontaining 2% (polypropylene) �bers, and 4% nano-silica, whose hydration reaction stopped using acetoneat the age of 7 days. The taken images (by scanningelectron microscope) are divided into two groups: thefracture surface and the modeled transition zone (re-gion) between the paste and the �bers.

The results of tests and microscopic photos andtheir comparison with the voucher sample (Figure 18)show that the addition of nano-silica to concrete (3%by weight of cement) reduces the volume of cavitiesand creates a smooth surface with less porosity. Inaddition, adding polypropylene �bers (2% by volume)improved the concrete integrity (Figure 19). The

Figure 17. 7-, 28-, and 90-day compressive strength improvement percentage of �ber-based samples for di�erent water tocement ratios and �ber percentages.


Figure 18. The microscopic photo of referenced concretespecimen.

Figure 19. The microscopic photo of concrete specimencontaining (including) the optimum value (amount) ofpolypropylene �bers.

Figure 20. The microscopic photo of conglobationphenomenon of concrete specimen due to unsuitabledistribution of �bers and using more than the optimumvalue (amount) of polypropylene �bers.

di�erence between the density of control sample and�ber-containing sample is clear.

According to Figure 20, using more than theoptimum value (0.4% by volume) of polypropylene�bers results in the conglobation phenomenon due to

Figure 21. The microscopic photo of agglomerationphenomenon of concrete specimen due to unsuitabledistribution and the use of more than the optimumamount of nano-silica.

unsuitable distribution of �bers. This phenomenonpractically causes the �bers to be ine�ective in improv-ing the mechanical properties of concrete.

In addition, according to Figure 21, when theamount of nano-silica addition is more than the op-timum value (4% by weight of cement), the particles ofnano-silica stick to each other due to a physical reac-tion, and they become unstable lumps, thus weakeningthe concrete structure.

10. Conclusion

In this paper, with an overview of the history and ad-vantages of using nano-silica concrete with polypropy-lene �bers, the mechanical properties of concrete sam-ples (e.g., compressive, exural, tensile, and abrasionstrengths, permeability, and porosity) were evaluated.The main and most important results are drawn, aspresented below:

- In the ratio of 0.1% �ber and 3% nano-silica, anoptimal compressive strength state occurs and con-tinues until the end of the experiments. The reasonfor this is the inappropriate di�usion of �bers andthe reduction of concrete slump, because the super-lubricant is constant during the test; thus, it is betterto increase the nano-silica in order to achieve higherresistance by increasing the �ber;

- In the ratio of 0.2% �ber and 3% nano-silica, anoptimal abrasion strength condition takes place. Byincreasing the �ber content to more than 0.2%, theabrasion strength is reduced due to the porosity ofthe samples;

- Fibers of 0.3% produced a signi�cant increase inthe tensile strength, which is more obvious by theincrease of �ber content. The presence of �bersin concrete, which is a brittle body, improves the


samples' ductility and improves tensile strength ofconcrete;

- In the ratio of 0.2% �ber and 3% nano-silica, anoptimal state occurs at a depth of water penetrationand hydraulic conductivity;

- The reason for reducing the permeability in thepresence of �bers is due to the placement of �bersbetween the porous bonding pathways and theirblocking, resulting in the removal of the capillaryproperty and the permeability decreases;

- In the ratio of 0.3% �ber and 3% nano-silica, anoptimum state of exural strength occurs. By adding�bers more than 0.3%, exural strength decreases,compared to non-�brous samples. In the presenceof samples of more than 0.3% �ber, the exuralstrength decreases due to the lack of suitable �bers;the balling phenomenon is evident here;

- It is clear based on the SEM images of the fracturesurfaces of hardened paste that the presence ofnano-silica and polypropylene �bers at optimumlevels has improved the density of the cement pastestructure, and the paste structure has clearly higherdensity and uniformity in the presence of these twomaterials.

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Biographies

Kamal Rahmani received his BS degree in CivilEngineering from Islamic Azad University and his MSdegree in Civil Engineering from Islamic Azad Univer-sity of Mahabad and Science and Research Branchesin 1998 and 2002, respectively. He is currently aPhD degree student at Science and Research Branch ofTehran-Islamic Azad University. His research interestsinclude hydraulic structures, concrete technology, andstructural engineering. He has presented and publishedseveral papers in various national and internationalconferences and scienti�c journals.

Mohsen Ghaemian is a Professor at the Civil En-gineering Department of Sharif University of Technol-ogy. His current activity concerns concrete technology,dynamic responses of gravity and arch dams. Damreservoir interaction e�ects, seismic response of damsdue to non-uniform excitations, and nonlinear behaviorof concrete dams have been his research interests inthese recent years.

Seyed Abbas Hosseini obtained his BS in CivilEngineering, MS in Hydraulic Structure, and PhDdegree in Water Engineering from Sharif Universityof Technology. He is currently an Assistant Professorof Technical & Engineering Department at IslamicAzad University, Science and Research Branch, Tehran,Iran. His research interests include experimental andnumerical modeling of incompressible ow, sedimenttransport, and scouring in hydraulic structures.

Experimental study of the e ect of water to cement ratio...

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