effects of fly ash, silica fume, and ground- granulated ... of fly ash, silica fume, and...

Effects of Fly Ash, Silica Fume, and Ground-Granulated Blast Slag on Properties of Self-

Compacting High Strength Lightweight Concrete Sherif Yehia, Sharef Farrag, Kareem Helal, and Shahinaz El-Kalie

Abstract—In this paper, the effect of utilizing Fly ash (FA), Silica fume (SF), Ground granulated blast slag (GGBS), and various combinations of them is assessed. Their effect on the fresh stage and mechanical properties of Self-compacting Lightweight Concrete (SCCLWC) is investigated and compared to a control mix without Supplementary Cementitious Materials (SCMs). Flowability, compressive strength, and flexural strength were the main criteria considered in the evaluation. Moreover, the applicability of the ACI 318 reduction factor (λ) for flexural strength was

assessed for all mixes to capture the effect of various SCMs based on the lower and upper limits of the proposed ACI 318 equation. Results from the evaluation show that SF greatly improved the compressive strength and GGBS increased flexural strength of SCCLWC. However, SF reduces the flowability of SCCLWC. Equally important, FA achieved the lowest increase in compressive strength compared to the control mix. Furthermore, the λ value of 0.85 proposed by

ACI 318 for sand-lightweight provides a good estimate of LWC properties even when different SCMs are utilized. However, fly ash can affect the λ value at early age.

Keywords-component; Lightweight aggregate, Lightweight concrete, Supplementary cementitious materials, Mechanical properties, SCCLWC

I. INTRODUCTION

A vast and ever-increasing utilization of Lightweight Aggregate (LWA) in concrete production is taking place nowadays [1]. This is due to the LWA’s comparative

advantage over Normal Weight Aggregate (NWA) since it can be used in producing concrete with enhanced durability and comparable mechanical properties, which lead to lighter, smaller, and more durable structural elements [2]. Common examples of LWA include pumice, volcanic ash, and sintered pulverized fuel-ash. Nevertheless, Supplementary Cementitious Materials (SCMs) are usually required to achieve the desired

properties [1, 3]. Typically, SCMs are industrial by-products, and utilizing/reusing them not only leads to an overall improvement of concrete properties, but also promotes concrete as a sustainable material since this reduces CO2 emitted by the cement production process [4-7]. Moreover, SCMs are typically included in a concrete matrix as replacement of cement, either by volume or by weight. Silica Fume (SF), Fly Ash (FA), and Ground-granulated Blast Slag (GGBS) are among the most widely SCMs in concrete applications [6-10], especially Normal Weight Concrete (NWC).

Generally, SCMs, with the exception of SF, increase the workability/flowability of fresh concrete, reduce heat of hydration, and mitigate the alkali-aggregate reactivity depending on their type and dose [11, 12]. Such features are crucial especially in harsh environments. Nevertheless, the mix proportioning and materials selection has to be carefully conducted in order to ensure an adequate performance, especially for lightweight aggregate concrete (LWC). LWC is greatly influenced of the physical and mechanical properties of the LWA used, including the specific gravity factor, shape, and absorption in addition to mixing procedures [2]. Therefore, it is essential to evaluate the effects of utilizing SCMs in LWC mixture, in order to achieve a good synergy. This is the case since the influence of SCMs on LWC is expected to be different than that on NWC due to the variability of the aggregate properties. In this paper, the influence of cement replacement by SF, FA, and GGBS on the properties of LWC is investigated.

II. BACKGROUND

LWA properties play a crucial role in determining several properties of LWC, including mechanical performance, durability, and microstructure [2]. In

DOI: 10.5176/2251-3701_3.3.138

GSTF Journal of Engineering Technology (JET) Vol.3 No.3, October 2015

©The Author(s) 2015. This article is published with open access by the GSTF

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Received 10 Aug 2015 Accepted 15 Sep 2015

DOI 10.7603/s40707-014-0021-3

addition, aggregate handling and mix procedure are also important for ensuring that the desired features are achieved. Specifically, pre-soaking the LWA, which are porous by nature, can govern several properties of LWA [13-15]. On the other hand, the inclusion of SCMs and other natural pozzolans in concrete enhances its properties by improving the hydration of process of the binder paste (cement and SCMs) [16]. SCMs provide more siliceous materials to enable Ca(OH)2, a weak crystalline formation produced by cement hydration, to form more C-S-H from C2S and C3S, the main skeleton of the hardened binder. This is owed to the fineness of SCMs, which leads to a higher grain surface area. Various SCMs possess different physical properties and chemical compositions. Thus, each

SCM can be useful for improving various concrete features, such as fresh stage properties, hardened stage properties, and several durability aspects. Furthermore, different combinations of inclusion, whether as addition of replacement for cement, can lead to a wide range of properties that need to be investigated. Table 1 provides an overall expected influence of various SCMs on the properties of LWC as found from the literature. It shows that the effect of SF on mechanical properties is more prominent. However, FA and GGBS, when used individually, can be also as important as SF in enhancing mechanical and durability properties of LWC. In addition, various combinations of these SCMs can also lead to improvement of several concrete properties.

TABLE I. PROPERTIES AND EFFECTS OF SF, FA, AND GGBS ON VARIOUS FEATURES OF CONCRETE

Moreover, the effects of SCM on Self-compacting Lightweight Concrete (SCCLWC) can vary from those mentioned on Table 1. Therefore, the current study is aimed towards assessing and verifying the contribution of SF, FA, GGBS, and various combinations of them to the improvement of the mechanical properties of SCCLWC when they are utilized as partial weight replacement of cement. In addition, the validity of the reduction factor (λ)

recommended by the ACI 318 to reflect the reduced mechanical properties of lightweight concrete relative to normal weight concrete of the same compressive strength was assessed for the various mixes, in order to capture the effect of utilizing various SCMs in SCCLWC.

III. EXPERIMENTAL PROGRAM

Various concrete mixes were considered in the current investigation as discussed in the following subsections. The mechanical properties evaluated in the current study were the compressive strength, flexural strength, and modulus of elasticity. In addition, the slump flow test was used to assess the flowability of concrete for the fresh stage evaluation. All mixes in the current study are considered sand-lightweight mixes according to ACI 318-14 [9]; fine lightweight aggregate was excluded from the current study in order to avoid problems associated with shrinkage and static stability of the mix.



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A. Mixes

Mixes prepared in the current study were based on [2], the volumetric ratios are shown in Table II. ACI-318 allows for up to 50% replacement of cement with SCMs by weight, depending on the exposure conditions [9]. In addition, the typical maximum replacement percentage of each of the SCM is shown Table I. SF and FA typically are utilized at a cap of 10% and 25% by weight respectively, whereas GGBS can be used up to 40% which means that GGBS can be accommodated more than other SCMs. Thus, in a mix with 50% replacement of cement, the authors chose the combination of 10%-SF, 15%-FA, and 25%-GGBS to be considered. From those ratios, mixes with individual SCMs and several combinations were prepared as shown in Table III. Since FA and GGBS have the same influence on the mechanical properties of concrete as shown in Table I, the GGBS+FA combination mix was excluded from the current study. All mixes were evaluated and compared against the control mix (without SCMs). The water-to-binder ratio (w/b) was kept the same for all mixes at 0.34. This ratio was selected to facilitate achieving high strength and acceptable flowability.

B. Materials

The coarse lightweight aggregate used in all mixes was sintered pulverized-fuel ash aggregates (LYTAG) with a specific gravity factor ~ 1.34 and a bulk density of 790 kg/m3 . The aggregate has a 30-minute absorption of 15.7% and 24-hour absorption of 30.1%. Aggregate with size of 4- 8 mm were used as-received. In addition, Portland cement Type-I (SG = 3.14), silica fume (SG = 2.22), fly ash (SG = 2.19), GGBS (SG = 2.9), normal weight dune sand (particle size 100% passing 0.6 mm, SG = 2.60), and coarse sand maximum particle size = 4.75 mm, SG = 2.60) were used. Fine lightweight aggregate was excluded from the current study. A commercially available admixture (GLENUIM SKY 504 from BASF) was used to adjust workability/flowablity of all mixes.

TABLE II. BASIC MIX DESIGN

Material Volume Ratio

Cementitious Material 0.17

Water 0.18

LYTAG 0.38

Crushed Sand 0.13

Dune Sand 0.14

TABLE III. MATERIALS AND MIX PROPORTIONS

TABLE IV. SUMMARY OF TESTS, TESTING SAMPLES AND TESTING EVENTS



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Fresh Stage Hardened stage

Figure 1. Samples during fresh and hardened stage evaluation

C. Sample Preparation and Testing

At least 15 minutes prior to mixing, the lightweight aggregate was presoaked with 10% of the lightweight aggregate as recommended by the supplier of the aggregate [35]. This is typically carried out to overcome the high absorption in order to prevent the alteration of the water-tobinder ratio due to absorption. Concrete specimens were cast and moist cured for 7 days to ensure proper hydration.

Slump flow test was used to evaluate workability of all mixes during the fresh stage. In addition, compressive strength, flexural strength, and modulus of elasticity were monitored during the hardened stage evaluation. Samples’

type, size, testing events and specifications are summarized in Table IV. Fig. 1 shows some concrete samples during preparation.

IV. RESULTS

A. Fresh Stage Evaluation

The slump flow test was performed to assess the flowability of each mix. Fig. 2 shows a sample of the flow level achieved by the mixes. All mixes achieved >500 mm spread in less than 15 seconds, satisfying the self-compaction target set in the current study. Mixes containing FA and GGBS achieved higher spread

diameters in the range of 620 – 660 mm, whereas mixes with SF showed a lower flowability, with a spread in the range of 540 – 580 mm. These results are reported at a fixed w/b ratio and superplasticizer dosage 0.34 and 3.53 L/m3.

Figure 2. Slump flow test spread sample

B. Hardened Stage Evaluation

All mixes achieved an equilibrium unit weight in the range of 1850 - 1900 kg/m3 , which complies with the ACI- 211 requirements for structural lightweight concrete [39]. The compressive and flexural strengths were evaluated for all mixes according to Table III. The



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reference mix used for comparison has a compressive strength of 50 MPa and flexural strength of 3.1 MPa, while the modulus of elasticity was 17 GPa. Fig. 3 and Fig. 4 illustrate the compressive and flexural strengths development for the mixes in the current study, respectively. All mixes achieved the target compressive and flexural strengths set from the control mix as shown throughout the strength development. As presented in Fig. 3, the SF mix achieved the highest 56-day compressive strength (67.3 MPa) with a 35% increase relative to the control mix, whereas the FA achieved the lowest increase of only 17% at 58.7 MPa. It can be observed that SCMs

variation led to a difference in the strength gain rates among the mixes. Moreover, the fluctuations of the flexural strength in the period between 7 and 14 days can be attributed to the incomplete hydration of the mixes in addition to the release/absorption of moisture by the LWA. Furthermore, all mixes achieved a modulus of elasticity in the range of 17-21 GPa at 56 days. Fig. 5 shows a concrete sample prior to being tested for modulus of elasticity. The failure modes of the tested samples are displayed in Fig. 6. It was observed that the failure plane passes through the aggregate, which due to the low aggregate strength compared to the binder matrix.

Figure 3. Compressive strength development

Figure 4. Flexural strength development



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Figure 5. Modulus of elasticity test sample

Compressive strength testing Flexural strength testing

Figure 6. Failure modes for beam and cube samples

V. DISCUSSION OF THE RESULTS

A. SCCLWC Properties

All mixes in the current study met the unit weight (<2000 kg/m3) and strength (>40 MPa) requirements for structural high strength concrete according to ACI 213 and ACI 211 [39, 40]. There was an overall improvement in properties of LWC when different SCMs were utilized, with a varying effect of each. FA and GGBS generally did not affect the workability of SCCLWC as much as SF did, which can be noted from the spread flow results in the current study. As for mechanical properties, FA greatly affected the early strength and strength gain of concrete compared to the control mix, although it helped achieve a higher late strength. Such outcome is in agreement with the literature findings reported in Table I. This behavior is

displayed in Fig. 3, in which the FA mix had a slow gain the in the period between 7 and 14 days compared to all the other mixes, with a sudden increase at 28 days. Ultimately, the FA mix achieved a 56-day compressive strength of 58.7 MPa due to the continuous hydration occurring through internal curing process in the presence of porous LWA [41]. Nevertheless, the FA mix achieved the lowest flexural strength (3.9 MPa), with an increase of 26%.

When SF was used in combination with FA, SF helped boost the strength development, nullifying the FA’s

negative effect while maintaining the same flowability level when comparing the SF+FA and FA mix. Therefore, SF is the SCM that would most enhance the compressive strength and gain rate of SCCLWC, since all mixes SF, SF+FA, and SF+FA+GGBS achieved 67.3 MPa, 60.9 MPa, and 65.3 MPa respectively with a relative increase of 22-35% compared to the control mix. This is due to the highly reactive nature of SF. Moreover, the flexural strength was also enhanced when SF was used in the mixes. However, the GGBS mix achieved the highest flexural strength (5.4 MPa) with a significant increase of 74%, as opposed to the 4.9 MPa of both SF and SF+FA mixes which achieved a 58% increase. Similar to FA, GGBS led to a low early strength, yet it attained a higher strength gain and ultimate strength compared to the FA mix. Combining the SCMs in the SF+FA+GGBS with the proposed ratios not only led to a significant increase in compressive and flexural strength, but it also achieved very good flowability and reduced cement by weight up to 50%. It is also expected that it would possess adequate durability features as addressed in Table I. Although FA and GGBS did not provide the same improvement in compressive strength compared to SF, they are usually utilized to improve durability features which are not addressed in the current study. FA is typically used for increasing the setting time and reducing the heat of hydration. As for the modulus of elasticity, the SCMs exhibited negligible influence since all mixes achieved a modulus of elasticity in the range of 17-21 GPa as opposed to that achieved by the control mix (17 GPa). As a result, it can be deduced that the main factor affecting the modulus of elasticity is the properties of the aggregate used, especially density [16].

B. LWC Mechanical Properties Reduction Factor (λ)

ACI 318 introduces a factor for reduced mechanical properties of LWC relative to NWC. Such reduction affects several structural behavior including various shear and bond behavior as well as development length.



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Typically, a value of 0.85 is considered for sand-lightweight concrete, which is concrete that utilizes normal weight fine aggregate and lightweight aggregate as coarse aggregate only. The ACI formula can be rearranged as shown in (1).

𝜆 =𝑓𝑟

0.62√𝑓′𝑐 (1)

where fr is the flexural strength of concrete (in MPa) and f’c is the cylinder compressive strength of concrete (in

MPa). From the equation presented, it can be seen that fr and f’c are the only parameters governing its applicability. Despite that f’c and fr may be intrinsically affected by the

material constituents, the current form of the equation entails the assumption that λ would always be the same for

similar mechanical properties, which may not be true. The volumetric fractions used in the current study were the only fractions considered. Other factors such as aggregate

type can also play a role in such variation, but they are out of the scope of the current study.

Although the cylinder strength was not tested in the current investigation, all cube compressive strengths obtained from the results were reduced by a factor of 0.8. Fig. 7 shows the calculated λ at various age of concrete

using the test results from the current investigation. From the evaluation, 3 out of 24 data points yielded a λ value <0.85, corresponding to the SF, FA, and SF+FA mixes occurring only at 7 and 14 days as shown in Fig. 7. This may be important in the application of removing formwork from cast-in-place concrete, in which cracks due to self-weight may occur. However, it can be deduced that utilizing various SCMs, individually or in combinations, does not lead to a pronounced effect on the λ value for failure limit state design purposes.

Figure 7. Modification factor λ estimation- Lower Limit

VI. CONCLUDING REMARKS

In this paper, the influence of SF, FA, GGBS, and combinations of them on the mechanical properties and flowability of SCCHSLWC was examined. All mixes considered in the investigation was compared to mix without any use of SCMs. SF, being highly reactive, helped achieve the highest compressive strength among all mixes, while it noticeably affects flowability. On the other hand, FA and GGBS did not affect flowability as SF did, but they led to lower compressive strength than that of SF, lower strength gain rate, and lower early strength. Nevertheless, GGBS helped increase the flexural strength of the mixes. Equally important, the applicability of the ACI 318 mechanical properties reduction factor of LWC relative to NWC (λ) when different SCMs are utilized was assessed in a comparable range of compressive and

flexural strength. In general, the results show that the utilization of various SCMs, individually or in combination, does not lead to an alteration of the λ value.

However, the modification factor λ needs to be further investigated especially some of the supplementary materials have a retarder effect leading to slow gain of strength at early ages.

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AUTHORS’ PROFILE

Prof. Sherif Yehia earned a PhD in Civil Engineering from University of Nebraska, Lincoln (1999). He is the Co-developer of the newly conductive concrete application for deicing operations. His research interests include behavior of reinforced and prestressed concrete, composite structures, special concrete, infrastructure management systems and engineering database management and information technology.

Sharef Farrag earned his civil engineering Bachelor’s (2011) and Master’s (2013) degrees from the American University of Sharjah, UAE, with a focus on development of sustainable materials and non-destructive testing. Since 2009, Sharef has been involved in research with his advisor on various topics in field of construction materials and methods.

Kareem Helal is an Egyptian Civil Engineer from the American University of Sharjah. He is currently pursuing a Master’s degree in civil engineering and is working as a Project Engineer at Structural Technologies; a Structural Group Company.

Shahinaz El Kalie was raised in the United Arab Emirates (UAE), educated in private schools and graduated from high school in 2008. She earned her Bachelor’s degree in civil engineering from the American University of Sharjah, Sharjah, UAE, in 2012. Ms. El Kalie continued her academic path through finishing her Master’s degree in civil engineering, focusing on the behavior of construction materials, from the American University of Sharjah, Sharjah, UAE, in 2015.



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