Construction and Building Materials 58 (2014) 225–233

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Construction and Building Materials

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Combined effect of fine fly ash and packing density on the propertiesof high performance concrete: An experimental approach

http://dx.doi.org/10.1016/j.conbuildmat.2014.02.0240950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Institute of Civil Engineering, TU Berlin, Germany.Tel.: +49 30 314 72 109.

E-mail address: mohattia76@gmail.com (M. Abd Elrahman).

Mohamed Abd Elrahman a,b,⇑, Bernd Hillemeier a

a Institute of Civil Engineering, TU Berlin, Germanyb Structural Engineering Department, Mansoura University, Egypt

h i g h l i g h t s

� HPC with superior properties has been optimized using 312 kg/m3 binder.� Fine FA is more effective than normal FA on improving concrete properties.� Total porosity is reduced to 3% by combination of fine FA and SF.� The optimized HPC has a chloride diffusion coefficient of 1.4 � 10�13 m2/s.

a r t i c l e i n f o

Article history:Received 16 October 2013Received in revised form 5 February 2014Accepted 8 February 2014Available online 13 March 2014

Keywords:Packing densityIdeal Fuller curveFine fly ashPorosityDurability

a b s t r a c t

High performance concrete often contains large amount of cement which makes ecological, economicaland technical problems. This study provides a new approach to optimize high performance concrete withlow cementitious materials content. The ideal grading curve according to Fuller has been used in concretemix design to ensure high packing density of concrete mixtures and to reduce the required binder con-tent. Several systems comprising various pozzolanic materials (silica fume, fly ash and fine fly ash) havebeen prepared and tested. The role of fine fly ash on concrete performance has been estimated by mea-suring the concrete mechanical properties, porosity and durability. The mechanical properties wereassessed from compressive strength and modulus of elasticity, whilst the durability characteristics wereinvestigated in terms of water permeability, water absorption and chloride diffusion. The results showedthat fine fly ash performed better than normal fly ash for the strength development and durabilityaspects. The ternary system containing slag cement, fine fly ash and silica fume with low w/b ratio per-formed the best amongst all the systems regarding concrete mechanical properties and durability. Com-bination of fine fly ash and silica fume with OPC or with slag cement resulted in a significant reduction inconcrete porosity. All mixes containing fine fly ash exhibited high performance concrete with excellentdurability properties.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Nowadays, progress in science and technology in the field ofconstruction industries and usage of new materials have resultedin the use of reinforced concrete in special structures such assewage systems, nuclear power containments, cooling towers ofpower plants, high way bridges and tunnels. In such aggressiveenvironments, high durability, stability and resistance to chemicalattack are of more concern. High performance concrete (HPC)provides an attractive option for such conditions. According to

Mehta and Monteiro [1], three main characteristics make theconcrete with high performance: high workability, high strengthand high durability. Because of the various requirements includingworkability, durability, strength and dimensional stability, the mixdesign of HPC is a challenging task. Unlike normal concrete, it is nolonger sufficient to base the mix composition on the principle ofcompressive strength and w/c ratio relationship. HPC is producedusing carefully selected ingredients and low water/binder (w/b) ra-tio. In addition, it requires high amount of cementitious materials(400–550 kg/m3) and also high dosage of superplasticizer [2].However, the use of high amount of cement can lead to environ-mental, economic and technical problems. For example, using highcement content increases the hydration heat and shrinkage whichare critical issues for concrete [3]. To cope with these problems,

226 M. Abd Elrahman, B. Hillemeier / Construction and Building Materials 58 (2014) 225–233

more optimization is needed to minimize the cement content inorder to reduce the ecological and economic impact of HPC. Onepossibility of optimizing the HPC mixture is the selection of con-crete constituents in such a way that the packing density of thewhole granulometric assemblage is maximized. Basically, increas-ing the packing density of aggregate would decrease the volume ofpaste needed to fill up the voids and increase the amount of addi-tional paste that could be utilized to improve the workability.Additionally, the concrete will have less durability problems suchas permeability, shrinkage, and thermal degradation.

2. Particle packing optimization

The packing density can be defined as the ratio of volume frac-tion occupied by the solids to the volume of the surrounding con-tainer. It is a matter of interest in many fields of material sciencesuch as packed beds, ceramics, asphalts and concrete [4]. The opti-mum packing density of the system could be attained only ifspheres with smaller sizes are added to the assemblage. The smallsize spheres can fill the voids between the large spheres, and there-by the packing density is significantly increased. In 1961, McGearyreported that it is possible to achieve a packing density of 95.1using four sizes of spheres with diameter ratios of 1, 7, 38 and316 with fraction volumes of 6.1%, 10.2%, 23% and 60.7% respec-tively. However, the maximum density of infinite differences insizes can be attained is 97.5% [5]. For concrete, the situation ismore complex since the system is composed of various particlesizes with different shapes and sizes. Effective packing can be at-tained by selecting proper proportions and sizes of small particlesto fill in the voids between the bigger particles. The important ef-fects of aggregate grading on the properties of concrete have beenemphasized in very early reports [6]. In 1892, Feret concluded thatthe maximum strength can be attained when the voids in the mix-ture is minimum [7]. Fuller reported that the best grading curve ofaggregate to get the maximum density is a parabolic shape [8].Both Feret and Fuller confirmed that concrete properties can besignificantly improved by using continuous grading [7]. In 1923,Talbot developed the well-known equation [9]:

P ¼ ðd=DÞq ð1Þ

where P is the total percent passing through a sieve, d is the diam-eter of the current sieve, D is the maximum aggregate size and q isthe gradation ratio. The maximum packing density can be achievedwhen q = 0.5, which is close to the Fuller curve [10,11], but theresulting concrete is harsh and unsuitable. In 1930, Andreasen triedto improve the Fuller curve. He suggested using the exponent q inthe range of 0.33–0.5, because fine particles are not able to packsimilar to bigger particles [12].

3. The work of Fuller and Thompson

Fuller and Thompson studied the grading analysis for a widevariety of aggregate types and mixtures to achieve the maximumpacking density. They found that the best density of aggregatecan be attained when the particle size distribution of aggregate iscontinuously graded and the grading curve takes a parabolic shape(Fig. 1). This curve is known as Fuller parabola and can be appliedfor calculating the optimum grading of aggregate only (not for amixture of aggregate and binder) as Fuller mentioned later [8]. Thisis because the mixture of aggregate which gives the maximumdensity in the dry state does not necessarily achieve the greatestdensity when combined with cement and water. The very low voidcontent between the aggregate prevent the cement and water to fitin perfectly [8]. In addition, Fuller parabola leads to low powder

content, whereas, more fines are needed to maintain good cohe-sion and to prevent segregation.

In 1903, Fuller and Thompson began an intensive work toachieve the greatest packing density for mixtures of aggregateand fine materials with maximum size of 2.25 inch. The ideal grad-ing curve has been obtained by trial mixes without referring tomathematical basis. To get this curve, at least 7% of the solid mate-rials should be finer than the No. 200 sieve. It composed basicallyof an ellipse at the lower part and merging into a straight line tan-gent to the elliptical part [11]. The ellipse begins from 0.0029 inch(sieve No. 200) and runs to a value of x equals to one-tenth themaximum grain size (Fig. 1). At this point, the straight line beginsand continues to y = 100% and x = D (where D is the maximumgrain size). After finding the ideal curve, equation was fitted to thiscurve. The equation covering this ideal grading curve is dividedinto two parts:

For the elliptical part:

ðy� 7Þ2

b2 þ ðx� aÞ2

a2 ¼ 1 ð2Þ

For the straight line part:

y ¼ 100� y1

D� x1ðx0 � x1Þ þ y1 ð3Þ

where a and b are the axis of the ellipse and their values dependmainly on the shape of the particles and the maximum aggregatesize [11], x0 = D/10 to D, y1 = y of the ellipse at D/10 and x1 = D/10[13].

In 1992, Puntke reviewed Fuller work and redraw the idealcurve in a semi-logarithmic scale for the sake of simplicity(Fig. 2) [13]. This curve has been used for designing concrete mixesfor several applications, particularly those, which need high den-sity and high resistance to acid attack. For example, in 2000, thehighest cooling tower in the world (200 m, Niederaußem, Ger-many) has been constructed of acid resistant concrete. The usedconcrete has been designed on the basis of the ideal Fuller curve.By applying this concrete in the cooling tower, the tank did notneed any internal protective layer (as normal) because the usedconcrete has high density as well as high resistance to acid attack[14,15].

4. Mix design

In this investigation, the concrete mixture proportioning isbased on the granular optimization of all concrete constituentsaccording to the ideal Fuller curve. The maximum grain size ofcoarse aggregate was 16 mm. For this size and according to theideal Fuller curve (Fig. 2), the required aggregate volume(d > 125 lm) is 85.13%, while the binder volume (d < 125 lm) is14.87%. According to this calculation, the required amount ofcementitious materials and aggregates are 312 and 1984 kg/m3

respectively. On the other hand, to obtain a good size distribution,the skeleton of aggregate size fractions should be viewed as awhole rather than two separate entities; coarse and fine aggregate.Aggregate as they come from the quarry do not normally have sizedistributions that fit the dense packing curve as can be seen inFig. 2. There are some deviations between the non-optimized mix-ture and the ideal Fuller curve. The mixture has more materials inthe range of 0.5–2 mm. Therefore, sieve analysis of aggregate isnecessary to be done and the required amount of each size is taken.On the contrary, the mixture has low fine materials in the sizerange 0.063–0.25 mm. Thus, quartz sand and quartz powder areadded to fill this gap and to densify the matrix.

0

10

20

30

40

50

60

70

80

90

100

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pass

ing

% (V

olum

e)

Size (d/D)

Fuller Parabola

Ideal Fuller curveD/10

b

a7

Fig. 1. Comparison between Fuller parabola for best grading of aggregate and the ideal Fuller curve for grading of aggregate and fine materials.

Fig. 2. The grading of non-optimized mixture, Fuller parabola and the ideal Fuller curve in semi logarithmic scale.

M. Abd Elrahman, B. Hillemeier / Construction and Building Materials 58 (2014) 225–233 227

5. Experimental details

5.1. Materials

Natural quartzite aggregates with maximum size of 16 mm has been used inthis investigation. CEM I 32.5 R and CEM III B 32.5 N-LH/HS/NA with slag contentof about 68% according to DIN EN 197 have been implemented. Normal fly ash(FA), fine fly ash (M20) according to DIN EN 450-1, and silica fume (SF) comply-ing with DIN EN 13263-1 have been also used as cement supplementary materi-als. Quartz sand (QS) and quartz powder (QP) were used as correcting aggregate

Fig. 3. Particle size distribution of fine mater

to adjust the mixture with ideal grading curve. Fig. 3 shows the particle size dis-tribution (PSD) of all fines measured with laser diffraction spectrometry. Physicalproperties and chemical composition of the used materials are shown in Table 1.On the other hand, it is theoretically known that, capillary porosity begin to format water/binder ratio higher than 0.42. Therefore, two values are chosen to re-duce the probability of capillary pores formation: 0.42 and 0.27. The effectivenessfactor (k factor) is assumed to be 0.4 and 1 for fly ash and silica fume respectively(K factor represents the efficiency of supplementary materials compared to OPC.The quantity of the SCM in the mixture can be multiplied by the k value to esti-mate the equivalent cement content, which can be added to the existing cement

ials measured with laser granulometry.

Table 1Physical and chemical properties of the used fine materials.

Material CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O SO3 Cl Specific density Surface area (cm2/g)

CEM I 63.25 20.8 4.61 2.59 4.17 0.16 0.50 2.70 3.17 3450CEM III B 48 29.7 6.5 1.2 8 0.31 0.65 2.49 0.02 2.96 4156Fly ash 3.1 49.2 27.6 7.6 2.1 0.9 5 5.0 2.29 2877M20 5 52 25 6 1 1.5 1.9 1.1 2.45 6000Silica fume 0.2 98.4 0.2 0.01 0.10 0.15 0.20 0.10 0.01 2.2 20,000Quartz powder 98.5 0.7 0.06 2.69 2683Quartz sand 0.013 99.6 0.11 0.012 0.004 0.006 0.024 2.67 760

228 M. Abd Elrahman, B. Hillemeier / Construction and Building Materials 58 (2014) 225–233

content for the determination of the water-to-cement ratio [16,17]). In addition,a polycarboxylate-based superplasticizer complies with DIN EN 934-2 has beenused to ensure a desirable consistency (class F3–F4 according to DIN EN 206-1).

5.2. Mix composition and mixing

In order to optimize a high performance concrete mixture, several supplemen-tary materials have been implemented. In this investigation, in order to achievedense and homogeneous cement matrix, it is suggested to use CEM III/B. One con-trol mix incorporating OPC only and eight other mixes containing supplementarymaterials were prepared. To estimate the influence of fine fly ash on concrete prop-erties, normal fly ash with the same replacement level (30%) has been investigated.In addition, silica fume with 10% replacement level has been used. Likewise, to en-hance the packing density of fine materials which can be achieved by using a widerange of particle sizes, combination of 25% fine fly ash with 10% silica fume withOPC as well as with slag cement have been implemented. To evaluate the perfor-mance of fine fly ash at low w/b ratio (0.27), two mixes were prepared; the firstwith 30% of fine fly ash and the second with combination of 25% fine fly ash with8% silica fume. The details of concrete mixtures proportioning are given in Table 2.The concrete was mixed in 60 liters mixer. After mixing, fresh concrete propertieswere measured, and the moulds were cast and compacted according to EN12390-2. After 24 h of adding the water, the test specimens were demoulded. Then,the curing process took place in water basin at room temperature (20 ± 1 �C).

5.3. Determination of concrete properties

5.3.1. Mechanical propertiesThe compressive strength has been determined using 100 mm cubes at ages of

28 and 91 days according to EN 12390-3. In addition, the modulus of elasticity ofconcrete is determined on cylinders (150 mm � 300 mm) according to Europeanstandard EN 1048-5 at age of 91 days.

5.3.2. Concrete porosityThe concrete porosity and pore size distribution have been measured using the

mercury intrusion porosimetry (MIP) at age of 91 days. The principle of this tech-nique is based on penetrating a non-reacting liquid such as mercury, into the poresof a dried and evacuated porous medium. The Washburn equation is used to calcu-late the porosity. The relationship between the pore size and the exerted pressure isexpressed as:

r ¼ ð2c cos hÞpm

ð4Þ

where r is the radius of the intruded pore (nm), c is surface tension of mercury (N/m), pm is the applied pressure (bar), and h is contact angle between mercury and thepore walls. Details of measurements procedures can be found in [18].

Table 2Composition of concrete mixes.

Mix Cement FA M20 SF SP Aggregate QSType (%) (%) (%) (%) (kg/m3) (kg/m3) (kg

1 CEM I 100 2.8 1854 462 CEM I 65 25 10 3 1854 463 CEM III 100 2.2 1854 464 CEM III 70 30 3.3 1854 465 CEM III 70 30 3 1854 466 CEM III 90 10 2.5 1854 467 CEM III 65 25 10 3 1854 468 CEM III 67 25 8 8.8 1947 489 CEM III 70 30 6.6 1947 48

5.3.3. DurabilityIn this investigation in order to assess the durability of concrete, permeability,

absorption and diffusion tests have been made. The water penetration depth underpressure according to EN 12390-8 was used to measure the permeability. The waterabsorption coefficient has been measured according to EN ISO 15148. In addition,the chloride diffusion coefficient was measured according to BAW procedure [19].The following equation was used to determine the diffusion coefficient:

Dcl ¼RThzFU� xd � ad

ffiffiffiffiffixdp

tð5Þ

With

ad ¼ 2�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiRThzFU� erf�1 1� 2cd

co

� �sð6Þ

where Dcl is the migration coefficient (m2/s), Z is valency, for chloride ions Z = 1, F isFaraday constant, F = 9.649 � 104 J (V* mol)�1, U is absolute potential difference (V),R is gas constant, R = 8.315 J (K* mol)�1, T is absolute mean temperature of the solu-tions during the test (K), h is height of the test specimen (m), xd is mean penetrationdepth of the chloride ions in each half of the test specimen (m), t is duration of thetest (s), erf�1 is inverse error function, cd is chloride concentration at which the col-our changes, cd = 0.07 (mol/L ).

6. Results

6.1. Mechanical properties

6.1.1. Compressive strengthTable 3 shows the results of compressive strength of concrete

mixes at age of 28 and 91 days. It is clear that, the main factor,which controls the compressive strength, is the w/b ratio. Mixes8 and 9 with w/b ratio of 0.27 had the highest compressivestrength at both 28 days and 91 days. It was 82 MPa for mix 8 at28 days and increased to 99 MPa after 91 days. In addition, the ce-ment type has also an important role on the development ofstrength especially at early ages. At 28 days, the compressivestrength of mix 1 with OPC was 62 MPa, however for mix 3 withslag cement it was 52 MPa. Nevertheless, at 91 days both mixesexhibited similar strengths (70 ± 2 MPa). On the other hand, theaddition of pozzolanic materials has an important influence onconcrete compressive strength; depending on its type, contentand properties. Combination of pozzolanic materials with Portlandcement enhances the strength of concrete more than with slagcement as can observed from the results of mixes 2 and 7 respec-tively. In addition, the strength improvement is more pronounced

QP w/b Flow diameter Air content Fresh density/m3) (kg/m3) ratio (cm) (%) (t/m3)

84 0.42 52 1.5 2.4584 0.42 46 1 2.4684 0.42 53 1.5 2.4384 0.42 52 1.3 2.4584 0.42 51 1.3 2.4484 0.42 48 2.3 2.4284 0.42 50 2 2.4488 0.27 44 1.5 2.5088 0.27 48 1.4 2.49

Table 3Compressive strength and modulus of elasticity of concrete mixes.

Mix Compressive strength (MPa) Modulus of elasticity (MPa)

28 days 91 days

Mix 1 62 72 43,100Mix 2 78 82 44,500Mix 3 52 68 43,500Mix 4 56 64 40,200Mix 5 68 72 42,900Mix 6 66 78 45,200Mix 7 66 71 40,800Mix 8 82 99 50,200Mix 9 80 91 46,900

M. Abd Elrahman, B. Hillemeier / Construction and Building Materials 58 (2014) 225–233 229

in mixes with silica fume (mix 2 and 6). However, the strengthdevelopment of mix 4 with normal fly ash is slow compared toother mixes, particularly at early ages.

6.1.2. Modulus of elasticityThe experimental results of elastic modulus of concrete are

found in Table 3. All mixes have modulus of elasticity more than40,000 MPa. Mix 8 with low w/b ratio has the highest elasticityof about 51,000 MPa. Mix 1 and mix 3 showed roughly the sameelasticity, that mean slag cement and OPC have the same effecton elasticity [20]. On the other hand, the elastic modulus of mixeswith silica fume is higher than mixes without silica fume. Mix 6with silica fume showed the highest elasticity at w/b of 0.42, itwas 45,200 MPa. However, mix 4 with normal fly ash showed thelowest elastic modulus, it was 40,200 MPa.

6.2. Porosity measured with MIP

Table 4 presents the experimental results of concrete totalporosity at age of 91 days. Compared to literature, all mixes havelower porosity than traditional concrete which lies between 15%and 20% [21]. Mix 1 with OPC has the highest porosity (10%), whilethe use of slag cement reduces the porosity to about 8.9% (mix 3).However, replacing OPC by 25% fine fly ash and 10% silica fume asin mix 2 dramatically reduces the porosity to 6.2%. Moreover,mixes 8 and 9 with low w/b ratio exhibited porosity in the rangeof UHPC (about 4–6%) [21]. The total porosities of mix 8 and mix9 were 3% and 4.9% respectively.

On the other hand, regarding durability and deterioration ofconcrete, capillary porosity is of most concern. Most of the trans-port mechanisms take place via the capillary pore framework.Therefore, capillary porosity of concrete mixes has been calculatedfrom the data sheet of MIP measurements. It is assumed to be inthe range of 30 nm to 10 lm [21]. Table 4 shows also the capillaryporosity of different concrete mixes. Similar to total porosity re-sults, mix 1 with w/b of 0.42 has the highest capillary porosity(5.6%). At w/b ratio of 0.42, the capillary porosity of mix 2 was4.5%. Mixes 8 and 9 with w/b ratio of 0.27 have very low capillaryporosity of 1.9% and 2.96% respectively. Fig. 4 presents the poresize distribution of concrete mixes. Mix 1 has the largest volumeof pores in the size range of 0.01–0.1 lm. It is clear from the figurealso that combined addition of fine fly ash and silica fume along

Table 4Total and capillary porosity of concrete mixes measured at the age of 91 days using MIP.

Mix Mix 1 Mix 2 Mix 3 Mix 4

Total porosity (%) 10.30 6.20 8.90 7.80Capillary porositya (%) 5.60 4.50 4.50 3.89

a The capillary porosity is assumed to be in the range of 30 nm to 10 lm.

with the use of low w/b ratio (mix 8) resulted in large reductionin pore sizes, particularly in the capillarity range.

6.3. Durability

6.3.1. Water penetration depth (permeability)Table 5 illustrates the results of water penetration depth tests.

All mixes exhibited low permeability (depth < 20 mm). The resultsrevealed also that the main factor governs the permeability is thew/b ratio. The penetration depth of mix 8 with w/b ratio of 0.27was about 3 mm which is the lowest penetration depth, whilefor mix 9 it was 5 mm. In addition, cementitious materials havecrucial influences on pore volume and connectivity which controlthe water permeation. Mix 5 with fine fly ash has lower penetra-tion than mix 4 with normal fly ash; it was 8 and 18 mm respec-tively. At the same time, mix 5 has lower depth than mixes 6and 7 with silica fume. Mix 3 with slag cement showed higherresistance to penetration than mix 1 with OPC (15 and 18 mmrespectively).

6.3.2. Absorption (capillary suction)Table 5 shows the results of the absorption coefficient of differ-

ent concrete mixes. Mix 8 made with slag cement, fine fly ash andwith w/b ratio of 0.27 has the lowest absorption coefficient ofabout 0.08 kg/m2 h0.5. However, for mix 1 with w/b ratio of 0.42and with OPC, the water absorption coefficient was 0.32 kg/m2 h0.5, which is about 4 times higher than that of mix 8. All con-crete mixes with slag cement have lower absorption than mix 1.For all these mixes, the absorption coefficient ranges between 0.1to 0.2 kg/m2 h0.5. Compared to mix 1 with OPC, replacing a partof OPC with 25% of fine fly ash and 10% silica fume (mix 2) reducedthe absorption to about 0.1 kg/m2 h0.5.

6.3.3. Chloride diffusionTable 5 shows the experimental results of chloride diffusion

coefficient of concrete. Mixes with slag cement exhibited higherresistance to chloride diffusion than mix 1 with OPC alone. How-ever, mix 2 with partial replacement of OPC with silica fume andfine fly ash showed significant reduction in the diffusion coeffi-cient. It reduced from 28 � 10�13 m2/s for mix 1 to4.5 � 10�13 m2/s for mix 2. At w/b ratio of 0.42, mix 5 with finefly ash exhibited higher resistance to chloride diffusion than mix6 with silica fume alone and higher than mix 7 with both silicafume and fine fly ash. Furthermore, due to incorporation of finefly ash (mix 5), the chloride diffusion is reduced to around the halfof the value of mix 3 with slag cement only. As expected, reducingthe w/b ratio resulted in high resistance to chloride diffusion. Withw/b ratio of 0.27, combination of silica fume and fine fly ash (mix8) resulted in a very high reduction in chloride diffusion to1.39 � 10�13 m2/s compared to mix 1 with w/b of 0.42 and OPC,which is about 20 times less. On the other hand, contrary to the re-sults of mixes with w/b ratio of 0.42, combination of silica fumewith fine fly ash is more efficient in reducing the chloride diffusionthan fine fly ash alone at w/b ratio of 0.27. Proof for that are the re-sults of mixes 5, 7, 8 and 9. Similar conclusion has been empha-sized by Bentz, who reported that silica fume is more efficientfor reducing diffusivity in low w/c ratio concretes [22].

Mix 5 Mix 6 Mix 7 Mix 8 Mix 9

6.98 7.20 6.50 3 4.903.50 3.90 3.80 1.90 2.96

Fig. 4. Pore size distribution of concrete mixes measured at age of 91 days using MIP.

Table 5Durability properties of concrete measured at age of 91 days.

Mix Water penetration depth (mm) Water absorption coefficient (kg/m2 h0.5) Chloride diffusion coefficient �10�13 (m2/s)

Mix 1 18 0.32 28Mix 2 14 0.095 4.5Mix 3 15 0.178 9Mix 4 18 0.132 8Mix 5 8 0.105 3.81Mix 6 11 0.148 6Mix 7 13 0.15 5Mix 8 3 0.074 1.39Mix 9 5 0.088 4.33

230 M. Abd Elrahman, B. Hillemeier / Construction and Building Materials 58 (2014) 225–233

M. Abd Elrahman, B. Hillemeier / Construction and Building Materials 58 (2014) 225–233 231

7. Discussion

7.1. Mechanical properties

By having a close look to the experimental results, it can be ob-served that the compressive strength and modulus of elasticity ofall mixes are higher than 60 MPa and 40 GPa respectively at91 days with total cementitious materials of about 312 kg/m3.However, for traditional concrete without optimization, the com-pressive strength and modulus of elasticity are in the range ofabout 40 MPa [21] and 25 GPA respectively. This significantenhancement in mechanical properties of concrete can be directlyattributed to the high packing density of solid particle system dueto applying ideal Fuller curve. On the other hand, the addition ofsupplementary materials has important influences on concretemechanical properties. The pozzolanic reaction turns CH into anew pozzolanic C–S–H gel which tightens and strengthens theaggregate-paste interface, therefore modulus of elasticity and latestrength are markedly improved. In addition, supplementary mate-rials enhance the packing density which results in better compact-ness, thus the strength increases [23]. These effects are morenoticeable with using fine fly ash and silica fume.

In fact, it should be pointed out here that, at different w/b ratiosthe behaviour of fine materials is somewhat different. At w/b ratioof 0.42, the comparison of mix 5 and 7 showed that the compres-sive strength and modulus of elasticity of mix 5 (72 and42,900 MPa) with fine fly ash was a little bit higher than mix 7(71 and 40,800 MPa) with fine fly ash and silica fume. While, atlow w/b ratio (0.27), the situation is totally reversed. The additionof both silica fume and fine fly ash (mix 8) enhanced the mechan-ical properties (99 and 50,200 MPa) more than mix 9 with fine flyash (91 and 46,900 MPa). On the other hand, although the cementcontent is low (can be said lean mixes), the mechanical propertiesof mixes 8 and 9 are in the range of high strength concrete whichneeds more cement, more silica fume and more superplasticizerper cubic meter. This can be reasoned by sequential filling of thevoids in the system. The voids between each particle class are filledwith particles from smaller class. Especially at the micro scale, thegap between the cement particles and silica fume with averagediameter of about 30 lm and 0.5 lm respectively are filled withfine fly ash with diameter of 1–10 lm. As a consequence, the bondbetween aggregate and cement paste is increased which improvesthe mechanical properties as the experimental results revealed.

7.2. Porosity

In order to evaluate the experimental results, a small compari-son with total and capillary porosity of concrete as reported in lit-erature will be presented. It is stated that the total porosity ofnormal concrete, HPC, and UHPC are 15%, 8.3% and 6% respectively,while the capillary porosity (0.03–10 lm) are 8.3%, 5.2% and 1.5%by volume respectively [21]. The experimental results indicatedthat the maximum total porosity was 10% for mix 1. In addition,the capillary porosity was 5.6% which is lower than porosities ofconventional concrete as reported in the literature. These resultssignificantly manifest the influence of enhancing the packing den-sity on concrete porosity. The porosity of concrete is resulted fromthe porosity of cement paste only, however the voids inside theaggregate do not participate in the porosity of concrete. By maxi-mizing the packing density of aggregate, the volume of cementpaste (cementitious materials and water) required to fill the gapsbetween the aggregate is reduced to about 23%. In this investiga-tion, the cement content is about 312 kg/m3 and the water contentis about 131 liters (w/c = 0.42). So, the total volume of cement andwater (cement paste) equals about 230 liters for one cubic meter of

concrete (23%). However, normal high performance concrete needsat least 400 kg cement and about 168 liters of water (w/c = 0.42).The total volume of cement paste (cement and water) is about300 liters for one cubic meter of concrete (about 30%). So, if the ce-ment pastes of both concrete have the same porosity, then the totalporosity of the densely packed concrete will be lower by 23.33%because it has less cement paste.

The use of supplementary materials largely reduces the porosityof concrete depending on their type and content at w/b ratio of0.42. The use of slag cement, with higher fineness than OPC, re-duces the total and capillary porosity to about 9% and 4.5% respec-tively (mix 3). More reduction of porosities was attained by theaddition of normal fly ash (mix 4). The total and capillary porositieswere reduced to about 7.8% and 3.9% respectively. Mix 5 with finefly ash showed lower porosity; about 7% and 3.5% for total and cap-illary porosity respectively, which are smaller than the aforemen-tioned porosities of HPC according to Teichmann [21]. This canbe attributed to the efficient packing of fine fly ash which has smallsize (<10 lm), spherical shape and smooth texture. On the otherhand, the addition of silica fume to the system of slag cementand fine fly ash showed a small effect at w/b ratio of 0.42. At thesame direction, mix 2 made with OPC, fine fly ash and silica fumeexhibited about 6.3% and 4.5% for total and capillary porosityrespectively. At low w/b ratio, the porosity is significantly reduced.For mix 9 with w/b ratio of 0.27 and with fine fly ash, the total andcapillary porosities were 4.9% and 2.9% respectively. More reduc-tion was gained by the addition of the silica fume to the system.The total and capillary porosity of mix 8 was about 1.9% 3% respec-tively which are much lower than normal HPC [24]. These valuesare comparable to those of UHPC which needs more fines, cement,fillers and superplasticizer and special mixing procedures.

7.3. Durability

The incorporation of fine fly ash or both fine fly ash and silicafume with low w/b ratio result in an effective reduction in thetransport of contaminants into concrete. The results of sorptionand chloride diffusion confirm this influence. The water absorptioncoefficient of mix 9 was about 0.09 kg/m2 h0.5. However, combina-tion of fine fly ash and silica fume leaded to more reduction inwater absorption coefficient to 0.07 kg/m2 h0.5. Similar resultswere obtained for the chloride diffusion which is considered themost important factor that can be used to evaluate the concretedurability. The chloride diffusion coefficient of mix 1 withoutpozzolanic materials was about 28 � 10�13 m2/s. However, theuse of slag cement (mix 3) reduced the diffusion to around9.3 � 10�13 m2/s. The use of fine fly ash at the same w/b ratio of0.42 (mix 5) reduced the diffusion to about 3.8 � 10�13 m2/s. How-ever, mix 8 with w/b ratio of 0.27 showed the lowest chloride dif-fusion (about 1.4 � 10�13 m2/s) which is in the range of UHPC [21].The role of high packing density in reducing the chloride diffusioncoefficient can be elucidated by comparing the result of mix 5 withliterature [25]. In spite of both concretes made with fine fly ash, thechloride diffusion of mix 5 is 3.8 � 10�13 m2/s which is 5 timeslower than that of Brandburger which was 1.5 � 10�12 m2/s. Fur-ther, by the use of silica fume it is reported that the diffusion coef-ficient reduced to 1.5 � 10�12 which is half the resistance of mix 7.

The transport of liquids and gases takes place through the con-tinuous pore system inside concrete. So, enhancing the packingdensity reduces the penetration of contaminates into concrete.The increased volume of aggregate closes the transport passesand makes it longer. The experimental results of water penetrationtest clarify the important role of the increased packing density intransport mechanisms. All concrete mixes exhibited water pene-tration depth lower than 20 mm which can be classified as imper-meable concrete under aggressive conditions (<30 mm) according

232 M. Abd Elrahman, B. Hillemeier / Construction and Building Materials 58 (2014) 225–233

to Neville [26]. Because of its higher fineness, the use of slag ce-ment results in lower permeability compared to OPC concrete[20]. This is attributed to the dense microstructure and the lowCH concentration in the transition zone in slag cement concrete.

The addition of both fine fly ash and silica fume notably reducesthe permeability and diffusivity [27]. The main effect of fine mate-rials is the enhancement of transition zone properties and makingit denser. This zone is known as the locus of micro-cracks whichinfluences not only the mechanical properties but also the perme-ability and durability. Due to its high pozzolanity, fine fly ash andsilica fume increase the homogeneity of the microstructure byreplacing CH crystals with additional C–S–H gel. Therefore, theprobability of micro-cracking is reduced and the transition zonebecomes thinner. Moreover, the size and content of capillary pores,as well as the CH crystals concentration are reduced with progressof the pozzolanic reaction. Furthermore, the produced C–S–H gelblocks the pores, reduces its size and interrupts its connectivity.In addition, it reduces the wall effect around the aggregate sur-faces, thus allowing better packing of cement particles at the inter-face between cement paste and aggregate.

7.4. The role of dense packing and fine fly ash

To explain the role of the packing density and fine fly ash onconcrete properties, it is worth to repeat the fact that all of thestudied mixtures prepared roughly with the same volume fractionsof aggregate and cementitious materials. Applying ideal Fullercurve in the concrete mix design minimized voids content. Thesevoids should be also filled with dense cement paste. However, ce-ment particles with sizes of about 30 lm cannot achieve this tar-get alone. Therefore, another material with smaller size whichcan fit in the voids between cement particles should be imple-mented. Fine fly ash is the ideal in these conditions because ithas smaller size, spherical shape, smooth texture and pozzolanicreactivity. Going deeper, the voids between fine fly ash should befilled with smaller size particles. Silica fume with particle size ofabout 0.1–0.5 lm could be the best material filling the voids be-tween fine fly ash. By this mechanism, cementitious materialsare needed only to fill the voids between the aggregates, whichare minimized, and also to cover the aggregate surface to keepthe workability at a satisfied level. As a result, dense microstruc-ture are generated which enhances both mechanical propertiesand durability of concrete using low binder content.

The role of fine fly ash in reducing the porosity and enhancingthe concrete durability can be interpreted basically from severalaspects. Firstly, it enhances the packing density of the system byits ball bearing effect which reduces the particle interlocking[28]. Secondly, it enhances the packing of particles at the aggregatesurfaces. Thirdly, it reacts with CH, therefore, the transition zonebecome denser and the pores become finer [29]. Fourthly, the poz-zolanic reaction takes place within the capillary pores, and thehydration products block and reduce the size of capillary poresand prevent their connectivity. Fifthly, the pozzolanic reactiontakes some water of the free water in the system, which indirectlyreduces the porosity. Sixthly, it enhances the cohesion of fresh con-crete and reduces the amount of bleeding water beneath the aggre-gate, and as a result the transition zone is densified. Seventhly,because of its spherical shape and smooth texture, it reduces thewater demand and makes the microstructure more homogenous.Finally, because of its low hydration rate compared to cement,the hydration heat is low which result in smaller thermal stressesat early ages. Both normal fly ash and fine fly ash has similar influ-ences on concrete from chemical point of view because they havesimilar chemical composition as can be seen from Table 1. The onlydifference is the mean particle size (fineness): for normal fly ash itis in the range of cement (about 30 lm), while fine fly ash it is

lower than 10 lm as can be seen in Fig. 3. The benefits of fine flyash instead of normal fly ash can be drawn as follow:

1. Fine fly ash is more reactive than normal fly ash, therefore,mechanical properties are better enhanced particularly at earlyages [29–31].

2. Normal fly ash has relatively large particle size; therefore, itcannot fill the voids between cement particles which resultedin poor packing density on the cement matrix.

3. Fine fly ash enhances the packing density on the aggregate sur-face which cannot be achieved by normal fly ash.

On the other hand, the use of silica fume with cement (binarysystem) result in low packing density because of the high inter-particle forces and the possibility of agglomeration. Silica fumeshould be uniformly and homogeneously dispersed in order toachieve its pozzolanic and filler effects [32]. Therefore, the use ofadequate cementitious materials with optimum composition, withthe use of appropriate superplasticizer that is sufficient to effi-ciently disperse the fine particles will lead to concrete with densemicrostructure and superior properties.

8. Conclusion

Based on the above results and discussions, the following con-clusions can be drawn:

1. By applying ideal grading curve, it is possible to produce HPCwith superior properties using only 312 kg/m3 of cementitiousmaterials.

2. In the densely packed system, replacing OPC with 25% fine flyash and 10% silica fume (mix 2) exhibited compressive strengthhigher than 80 MPa with w/b ratio of 0.42, while the elasticmodulus reached around 45 GPa. However, the compressivestrength reached 99 MPa and the elastic modulus was morethan 50 GPa for mix 8 with combination of slag cement, finefly ash and silica fume along with w/b ratio of 0.27.

3. The maximum porosity of concrete mixes was 10% for mix withOPC only, (significantly lower than traditional concrete), whichrevealed the importance role of packing density. However, atotal porosity of 3% has been achieved by using supplementarymaterials with w/b ratio of 0.27.

4. A very low capillary porosity was obtained for all mixes com-pared to traditional concrete and HPC. In particular, mix 8 withw/b ratio of 0.27 has a capillary porosity of lower than 2%.

5. The optimized concrete showed very low water penetrationdepth. At w/b ratio of 0.42, it ranges between 8 and 14 mmand it decreased to 3–8 mm with reducing the w/b ratio to 0.27.

6. The chloride diffusion coefficient was reduced from28 � 10�13 m2/s for OPC concrete to 4.5 � 10�13 m2/s by using25% fine fly ash and 10 silica fume. More reduction of chloridediffusion is reached by using both silica fume and fine fly ashwith slag cement at w/b ratio of 0.27, it is reduced to 1.39� 10�13 m2/s (about 20 times lower than OPC concrete).

7. Silica fume is more effective in improving the durability andstrength of concrete in systems with low w/b ratio than systemswith high w/b ratio.

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