Scholarly article on topic 'Mix proportions and mechanical properties of concrete containing very high-volume of Class F fly ash'

Mix proportions and mechanical properties of concrete containing very high-volume of Class F fly ash Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — Chung-Ho Huang, Shu-Ken Lin, Chao-Shun Chang, How-Ji Chen

Abstract Two types of Class F fly ash with 4.6% and 7.8% loss on ignition were used for an experimental investigation dealing with concrete incorporating very high volumes of Class F fly ash (HVFA). A rational mix design method was developed for concrete with 20–80% fly ash replacement for cement. Tests were performed for fresh and hardened concrete properties. Test results indicated that the setting times and the air content of fly-ash concrete increased as the fly ash replacement level increased. The compressive and flexural strength of the HVFA concrete mixtures demonstrated continuous and significant improvement at late ages of 91 and 365days. Relation was formulated for flexural and compressive strength for all grades of HVFA concrete. The concrete mixture containing low-LOI fly ash exhibited superior mechanical properties than those of the corresponding mixture containing high-LOI fly ash. These results confirm the feasibility that up to 80% of Class F fly ash can be suitably used as cement replacement in concrete by using a rational mixture proportions.

Academic research paper on topic "Mix proportions and mechanical properties of concrete containing very high-volume of Class F fly ash"

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Construction and Building Materials

journal homepage: www.elsevier.com/locate/conbuildmat

Mix proportions and mechanical properties of concrete containing very high-volume of Class F fly ash q

Chung-Ho Huanga* Shu-Ken Linb, Chao-Shun Changc, How-Ji Chenb

a Department of Civil Engineering and Environmental Resources Management, Dahan Institute of Technology, Hualien, Taiwan, ROC b Department of Civil Engineering, National Chung-Hsing of University, Taichung, Taiwan, ROC

c Department of Construction Engineering, Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan, ROC

HIGHLIGHTS

• We investigate the mix proportions and properties of concrete containing very high-volume of Class F fly ash (HVFA).

• A rational mix design method was proposed for the very HVFA concrete.

• 62 MPa 80% fly ash concrete may be obtained using 136 kg Portland cement.

• Relation was formulated for flexural and compressive strength for all grades of HVFA concrete.

• The very HVFA concrete was found to be an adequate material for both structural and pavement application.

ARTICLE INFO ABSTRACT

Two types of Class F fly ash with 4.6% and 7.8% loss on ignition were used for an experimental investigation dealing with concrete incorporating very high volumes of Class F fly ash (HVFA). A rational mix design method was developed for concrete with 20-80% fly ash replacement for cement. Tests were performed for fresh and hardened concrete properties. Test results indicated that the setting times and the air content of fly-ash concrete increased as the fly ash replacement level increased. The compressive and flexural strength of the HVFA concrete mixtures demonstrated continuous and significant improvement at late ages of 91 and 365 days. Relation was formulated for flexural and compressive strength for all grades of HVFA concrete. The concrete mixture containing low-LOI fly ash exhibited superior mechanical properties than those of the corresponding mixture containing high-LOI fly ash. These results confirm the feasibility that up to 80% of Class F fly ash can be suitably used as cement replacement in concrete by using a rational mixture proportions.

© 2013 Elsevier Ltd. All rights reserved.

Article history: Received 10 March 2013 Received in revised form 9 April 2013 Accepted 12 April 2013 Available online 17 May 2013

Keywords:

Mixture proportioning Fly ash

Mechanical properties Shrinkage

1. Introduction

The usage rate of fly ash in Taiwan is currently quite low. The disposal of fly ash has become a considerable environmental problem, because the dumping of fly ash as a waste material may cause substantial environmental hazards. To increase the usage rate, large quantities of fly ash are proposed to be incorporated in structural and paving concrete mixes. Numerous studies have been focused on the development of structural and high-strength concretes containing large amounts of fly ash [1-7].

q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author. Address: Department of Civil Engineering and Environmental Resources Management, Dahan Institute of Technology 1, Shuren St., Dahan, Shincheng, Hualien, Taiwan, ROC. Tel./fax: +886 3 8210880. E-mail address: cdewsx.hch@gmail.com (C.-H. Huang).

0950-0618/$ - see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.04.016

Pioneering work for production of structural concrete incorporating large quantities of fly ash was done by Malhotra et al. [811]. Nailk and Ramme [12,13] investigated mix proportions for high-volume fly ash (HVFA) concrete. Both air-entrained and non-entrained concrete mixes were proportioned to contain fly ash to replace cement up to 55% by weight at three strength levels of 21, 28, and 35 MPa. The results showed that the initial and final setting times were not significantly affected by the inclusion of fly ash for up to 55% cement replacement. In addition, the concrete containing 40-60% cement replacement exhibited lower compressive strength at early age, but had higher strength compared to the companioned concrete containing no fly ash at 28 days and beyond.

Malhotra [14] reported that concrete containing high volumes of Class F fly ash exhibited adequate mechanical properties, excellent durability, and low permeability to chloride ions. Superplasti-cized high-volume fly-ash concrete had inferior abrasion resistance to concrete without fly ash [15]. For concrete blocks containing high volumes approximately 55% of Class F fly ash, the ratio of the 42-days core compressive strength to the 28-days laboratory

measured strength ranged from 78% for the control concrete to 120% for the HVFA concrete. At 365 days, these ratios were 78% and 92%, and at 730 days, the ratios were 88% and 98% for the control concrete and the HVFA concrete, respectively [16].

Naik et al. [17] studied the mechanical properties and durability of concrete made with blended fly ash. The test results showed that blending of Class C fly ash with Class F fly ash demonstrated comparable or superior results than the control mixture without fly ash and the unblended Class C fly ash. Poon [18] investigated high-strength concrete with 45% low-calcium fly ash replacement and reported that 28 day cube compressive strength of 83 MPa can be obtained with a water-to-binder ratio of 0.24. Jiang and Malhotra [19] studied on no-air-entrained concrete containing 55% fly ash as cement replacement by mass. The water-cementitious material ratios ranged from 0.34 to 0.39. The cement content was 400 kg/ m3. There were reported ranges of compressive strength of concrete 18.0-42.2 MPa at 7 days, 30.7-55.8 MPa at 28 days and 43.9-65.2 MPa at 3 months. Siddique [20] experimentally investigated concrete incorporating high volumes of Class F fly ash with replacement percentages of 40%, 45%, and 50%. Tests were performed for the fresh concrete and hardened concrete properties. The test results indicated that Class F fly ash can be suitably used up to 50% level of cement replacement in concrete for reinforced concrete construction.

Berry, Malhotra, Lane and Lee [21-24] reported that fly ash with high LOI value may affect the concrete properties. High LOI fly ash absorbed more water and chemical admixtures, such as AE agent and SP, resulted in increasing the slump loss, decreasing the air-entraining effect and bleeding, and decreasing the strength of concrete.

Numerous studies investigated the characteristics of concrete containing high volumes of fly ash. However, only a few studies [2,25] focused on the mixture design and strengths of concrete containing high volumes of fly ash (i.e., 80% replacement) with very low and optimal water-cementitious material ratios (i.e., 0.24-0.27). Therefore, this study provides further data on mix proportions and the strengths of very high volume fly-ash concrete. In addition, this study verified that a high-performance concrete with moderate and high strength can be produced using very high volume of fly ash as cement replacement. This was achieved by designing superplasticized workable concrete containing HVFA for structural purposes. In addition, this study also assessed the influence of the loss on ignition (LOI) of fly ash on the fresh concrete properties and the mechanical properties of concrete.

This study was conducted to assess the feasibility that a fly-ash concrete with moderate and high strength can be produced using very high volume of up to 80% of Class F fly ash as cement replacement. Mix proportions and further data for the strength of very high volume fly-ash concrete are provided for reference.

2. Experimental procedure

2.1. Testing program

A laboratory study was performed to determine the effects of fly ash on the mixture proportioning procedures required for concrete to satisfy workability and strength. Two groups of fly-ash concrete mixtures were made with low-calcium Class F fly ash. The first group contained fly ash with a low-LOI of 4.6%, and the other group contained a high-LOI of 7.8%. The primary design variables included compressive strengths of 24 MPa and 35 MPa and 20-80% fly ash replacement for Portland cement. The 80% replacement mixture was designed according to a preliminary experiment to select a minimal cement content of 112 kg/m3 for the L24 series and 136kg/m3 for the L35 and H35 series. Table 1 shows the test factors and parameters.

2.2. Materials

Ordinary Portland cement Type I (OPC), which is similar to ASTM C150 [26], was used for test. Locally available crushed sandstone with a maximal nominal size of 19 mm and specific gravity of 2.61 was used as the coarse aggregate. The fine aggre-

gate was locally available natural sand with a specific gravity of 2.69 and fineness modulus of 2.62. The fly ash was obtained from the Taichung Power Station in central Taiwan. It was classified as low-calcium Class F fly ash. Two types of fly ash with the low-LOI and high-LOI of 4.6% and 7.8%, respectively, were used for the test. According to ASTM C618 [27] and CNS 3036 (Chinese National Standard) [28] the qualified LOI value of fly ash is limited to 6.0%. They exhibited a total SAF (SiO2 + -Al2O3 + Fe2O3) of 85.4% and 80.9%, which complies with the ASTM criterion for a total SAF of at least 70%. A polycarboxylate based superplasticizer (SP) confirming to ASTM C494 [29] Type G was used in the concrete mixtures. Tables 2 and 3 list the properties of the constituent materials.

2.3. Mixture proportions

Concrete is basically a three main constituent material matrix that includes binder (cement and mineral), aggregate (fine and coarse), and water. The binder is in a finely powdered form to facilitate chemical reaction with water, thereby creating the necessary dense concrete matrix. The chemical admixtures, such as air-entraining agents, plasticizing admixture, and set-controlling admixture, are also occasionally added to concrete to improve the physical and mechanical properties of fresh and hardened concrete.

Based on the proposed method of mix design by Malhotra and Mehta [30], fly-ash concrete consists of five constituent materials (cement, mineral admixture, fine aggregate, coarse aggregate, and chemical admixture), among which the cement and mineral admixture are in a finely ground state and form the powder constituent of fly-ash concrete. The fly ash facilitates superior flow characteristics of concrete in the fresh state. The adequate workability and segregation resistance can be obtained with a relatively high cementitious paste volume, low aggregate and water content, and the use of suitable dosage of SP.

The concrete mixtures were mixed to have a slump in the range of around 200 mm. Because the water demand for a slump of concrete containing fly ash differs from that of Portland cement concrete, the required adjustments to maintain a constant yield are made using the fine aggregate content. Thus, although the water to cementitious material ratios (W/cm, and cm = C + FA) varied between mixes, the workability of each was nearly identical.

The mix design procedure was developed by considering the following aspects:

1. Consistent with good workability and segregation resistance, the cementitious material (cement and fly ash) content was fixed at an adequate range of 280 kg/m3 and 340 kg/m3 (except the L24F80, L35F80, and H35F80 mixtures) to achieve concrete strength levels of 24 MPa and 35 MPa, respectively. The cementitious material contents for the exceptions (L24F80, L35F80, and H35F80) were varied at 560 kg/m3, 680 kg/m3, and 680 kg/ m3 according to the previously investigated and determined minimal cement contents of 112 kg/m3,136 kg/m3 and 136 kg/m3, respectively.

2. The strength controlling factor was the W/cm ratio. Several trials were conducted to determine the optimal W/cm ratio for the specified targeted design strength. A prediction model was developed for the optimal W/cm ratio using local materials; the basic properties of the materials are shown in Tables 2 and 3. The prediction model is as follows.

fc;28 = a[1/W/cm + b(FA/cm)+y] (1)

where fc2S is the compressive strength (MPa) at 28 days, FA/cm the proportion of fly ash in the total binder, b the strength effective factor, and a and y are the constants.

3. A practical model was developed for the determination of W/cm in the mix design:

fc;28 = 240[1 /W/cm + 3.2 (FA/cm)+0.476] (2)

However, it is suggested that similar such prediction models can be developed to determine the optimal W/cm ratio depending on the characteristics of the aggregate and fly ash.

4. Using the particle packing theory, the amount of well-graded fine aggregate and coarse aggregate with a maximal size of 19 mm were experimentally determined as approximately 750-850 kg/m3 and 1020-1100 kg/m3, respectively.

This study used two mix proportions for various grades of fly-ash concrete according to the method proposed by Malhotra and Mehta [30]. We determined the mix proportions (L-series and H-series) for two strength levels (24 and 35 MPa). The L-series containing two strength levels was obtained using fly ash with the low-LOI content as the mineral admixture and H-series using fly ash with the high-LOI content, as shown in Table 4. L24F20 stands for mix L-series with strength level of 24 MPa and fly ash content of 20% of the total cementitious material, and H35F80 stands for mix H-series with strength of 35 MPa and fly ash content of 80%. The percentage of cement content varies from 100% to 20% of the total binder; the fly ash content varies from 0% to 80% of the total binder for the target strength levels (24 and 35 MPa) of both L-series and H-series.

Table 1

Test factors and parameters studied.

Test factor Level of parameter

1 2 3 4 5 6 7 8

Loss on ignition, LOI (%) 4.6% 7.8%

Fly ash replacement ratio 0 20 40 60 80

Comp. strength (MPa) 24 35

Age of testing Comp. strength 1 3 7 28 56 91 182 365

(days) Flexure strength 7 28 56 91 365

Length change 4 7 14 28 56 112 224

Young's modulus 28 56 91 365

Poisson's ratio 28 56 91 365

Table 2

Properties of the constituent materials.

Material used

Specific gravity

Fineness modulus

Bulk density (kg/cm3)

Fineness, retained on 325 sieves (%)

Cement

Fly ash (Class F)

Coarse aggregate River sand

LOI =4.6% LOI = 7.8%

3.15 2.31 2.28 2.61 2.69

6.79 2.62

13.24 26.60

Table 3

Composition and physical properties of Class F fly ash.

Components Low-LOI High-LOI

SiO2 (%) 50.0 50.0

Al2O3 (%) 28.4 23.0

Fe2O3 (%) 7.0 7.9

CaO (%) 6.0 6.0

MgO (%) 1.4 2.0

Alkalies as Na2O (%) 0.09 0.17

Sulfur as SO3 (%) 0.47 0.68

K2O (%) 0.13 0.28

Loss on ignition, LOI (%) 4.6 7.8

PAC (%)a 7 days 86.8 84.5

28 days 97.8 86.0

Specific gravity 2.31 2.30

Soundness (%) 0.056 0.053

a PAC: pozzolanic activity index.

2.4. Casting and testing of specimens

All concrete mixtures were mixed for 5 min in a laboratory pan mixer. The fresh concrete properties were recorded for each batch. The tested properties of the concrete included slump, air content, unit weight, and setting time according to ASTM C143, C231, C138, and C403 [31-34], respectively. The specimens were cast from each mixture to test the compressive strength (three 100 mm diameter and 200 mm height cylinders each for testing at 8 ages), modulus of elasticity, Poisson's ratio, drying shrinkage, and flexural strength. After removal from the molds, all specimens were moved to a standard moist-curing room until date for testing. The tests were performed following the relevant ASTM standards.

3. Experimental results and discussion

3.1. Properties of fresh concrete

Table 5 shows the properties of the fresh concrete including slump, air content, unit weight, and setting times. As shown in Table 5, the dosage of the superplasticizer in all concrete mixtures was adjusted to produce a slump of 160-230 mm, and ranged from 0.4 to 9.5 l/m3 of concrete. The dosage amount increased in conjunction with the fly ash content, whereas the fly ash with the high-LOI content requires more SP than that with the low-LOI content. These results indicate that concrete containing fly ash of up to 80% of the binder content can be proportioned to have adequate workability when suitable SP is used.

The air contents of L24, L35, and H35 series were in the ranges of 2.2-3.3%, 2.1-3.6%, and 2.1-4.3%, respectively. The specimens of L24 and L35 series had adequate air content of less than 3.2%, except the L24F80, L35F80, H35F60, and H35F80 specimens, which contained 60% or 80% fly ash of the total binder and had a measured air contents larger than 3.5%. The air content increased as the percentage of fly ash increased for the three series. In addition, fly ash with the high-LOI content for the H35 series exhibited increased air content.

The unit weight of each series of concrete decreased with the increase of the cement replacement percentage of fly ash. Moreover, the concrete mixtures of the H35 series with high-LOI fly ash content may produce less unit weight than those of the L35 series with low-LOI, because fly ash with the high-LOI has a lower specific gravity and the mixtures of the H35 series were mixed with a higher dosage of SP.

Table 5 also shows that the initial- and final-setting times of the concrete ranged from 4 h and 50 min to 13 h and from 5 h and 15 min to 15 h and 10 min, respectively. For each series, the fly-ash concrete mixtures, such as L24F20, L35F40, and H35F60, exhibited longer setting times than the corresponding control mixtures without fly ash (L24F00, L35F00, and H35F00). The setting time of the fly-ash concrete increased in conjunction with the fly ash content. The setting times were 1.25 h to 11 h longer than those for the control concrete. In addition, the concrete mixtures containing 80% fly ash of the total binder (L24F80, L35F80, and H35F80) exhibited a particularly longer final setting time of up to 15-18 h, which may attributed to the more fly ash content and the higher dosage of SP.

3.2. Compressive strength

The compressive strengths of the concrete mixtures were determined at the ages of 1, 3, 7, 28, 56, 91, 182, and 365 days. Table 6 shows the results. The control mixtures for the L24F0, L35F0, and H35F0 series were proportioned to have compressive strengths of 25.0 MPa and 34.5 MPa, which are approximately equivalent to those of the fly-ash concrete at 28 days. This was achieved for most mixtures, except those containing high volumes of 60% and 80% fly ash of the total binder. In these cases, the strength equivalence was achieved between the ages of 56 and 91 days.

Table 4

Proportions of concrete mixtures.

Mixture no.a W/cm W/C Cement Fly ash Sand Coarse aggregate Water Superplasticizer Percent of

(kg/m3) (kg/m3) (kg/m3) (kg/m3) (kg/m3) (l/m3) binder (%)

L24F00 0.72 0.72 280 0 777 988 202 0.0 0.0

L24F20 0.66 0.83 224 56 788 1003 185 0.5 0.2

L24F40 0.56 0.93 168 112 802 1041 157 2.1 0.8

L24F60 0.44 1.11 112 168 801 1106 124 3.9 1.4

L24F80 0.27 1.34 112 448 418 1101 150 3.7 0.7

L35F00 0.60 0.60 340 0 737 977 203 0.4 0.1

L35F20 0.55 0.69 272 68 743 985 188 0.8 0.2

L35F40 0.48 0.80 204 136 752 1017 163 1.7 0.5

L35F60 0.36 0.91 136 204 756 1089 124 3.3 1.0

L35F80 0.24 1.20 136 544 295 1062 163 4.9 0.7

H35F00 0.60 0.60 340 0 737 977 203 0.4 0.1

H35F20 0.55 0.69 272 68 743 985 188 1.5 0.4

H35F40 0.48 0.80 204 136 752 1017 163 2.7 0.8

H35F60 0.36 0.91 136 204 756 1089 124 5.0 1.5

H35F80 0.26 1.32 136 544 279 1038 180 8.7 1.3

a L24F20: low-LOI content fly ash, compressive strength of 24 MPa, fly ash content of 20%. H35F60: high-LOI content fly ash, compressive strength of 35 MPa, fly ash content of 60%.

Table 5

Properties of fresh concrete mixtures.

Mixture W/ SP Slump Air Unit Setting time

no. cm (l/ (mm) content weight Initial Final

m3) (%) (kg/m3) (h:min) (h:min)

L24F00 0.72 0.0 160 2.2 2347 6:10 7:20

L24F20 0.66 0.5 170 2.5 2340 7:25 9:10

L24F40 0.56 2.1 180 2.6 2311 8:25 10:40

L24F60 0.44 3.9 210 2.8 2280 10:55 13:30

L24F80 0.27 3.7 220 3.5 2218 12:20 15:50

L35F00 0.60 0.4 220 2.1 2352 5:50 7:00

L35F20 0.55 0.8 210 2.7 2348 7:00 8:55

L35F40 0.48 1.7 160 2.9 2334 8:10 10:00

L35F60 0.36 3.3 230 3.2 2289 10:30 13:05

L35F80 0.24 4.9 230 3.6 2230 12:00 15:20

H35F00 0.60 0.4 220 2.1 2352 5:50 7:00

H35F20 0.55 1.5 210 2.8 2307 7:15 8:55

H35F40 0.48 2.7 230 3.2 2280 8:25 10:50

H35F60 0.36 5.0 220 3.5 2265 11:30 14:20

H35F80 0.26 8.7 230 4.3 2090 14:05 18:10

The compressive strength of concrete increased with age for all mixtures, as shown in Figs. 1-3. The strength developments of the three concrete series presented a similar trend. At 1-7 days, the compressive strengths of the fly-ash concretes were lower than

Fig. 1. Compressive strength development of concrete series L24.

that of the corresponding control concrete. Regardless of the decrease of W/cm, the strength of the fly-ash concretes decreased as the replacement percentage of fly ash increased at these ages. After 7 days, the strength gain of the control concrete was less than that of the fly-ash concretes. At 28 days, the compressive strength of the fly-ash concretes approached that of the control concrete,

Table 6

Compressive strength of concrete mixtures (MPa).

Mixture no. W/cm 1 day 3 days 7 days 28 days 56 days 91 days 182 days 365 days

L24F00 0.72 5.3 14.6 20.9 25.0 27.6 29.0 32.4 36.5

L24F20 0.66 5.0 12.9 18.9 25.4 28.5 32.1 35.9 43.2

L24F40 0.56 3.9 10.4 16.8 25.6 30.0 35.2 39.8 39.4

L24F60 0.44 2.6 9.7 14.5 23.5 25.6 30.6 37.3 41.2

L24F80 0.27 2.3 8.5 14.1 20.9 24.3 28.5 33.6 38.7

L35F00 0.60 7.6 20.4 27.2 34.5 37.2 40.3 41.8 44.6

L35F20 0.55 7.6 17.6 23.9 36.5 40.8 45.8 49.4 54.7

L35F40 0.48 5.8 17.8 24.7 40.3 42.5 51.3 56.1 62.4

L35F60 0.36 3.5 13.3 18.9 34.5 38.8 47.8 55.6 65.3

L35F80 0.24 3.3 9.8 16.5 30.0 34.4 40.4 48.6 61.6

H35F00 0.60 7.6 20.4 27.2 34.5 37.2 40.3 41.8 44.6

H35F20 0.55 6.9 16.8 22.4 34.9 36.1 39.6 43.2 46.2

H35F40 0.48 6.0 11.3 21.5 34.1 37.4 40.8 51.5 54.3

H35F60 0.36 2.5 12.1 17.8 30.5 33.9 39.0 47.3 55.8

H35F80 0.24 1.0 5.2 11.6 25.2 28.3 34.1 39.6 43.4

Fig. 2. Compressive strength development of concrete series L35.

U) c <u k_

] L35F00

t??^^ L35F20 j L35F40 I L35F60 L35F80

Age (days)

(a) series L35

Fig. 3. Compressive strength development of concrete series H35.

Fig. 4. Comparison of concrete strength versus age between L35 and H35 series.

except for the concrete mixtures containing 60% and 80% fly ash of the total binder. At 91 days, the strength of the fly-ash concretes for the L24 and L35 series consistently exceeded that of the control concretes, whereas the fly-ash concretes of the H35 series were extended to 182 days to exceed the strength of the control concrete. These results imply that the early strength gain of the control mixtures was superior to that of the fly ash mixtures. However, a significant strength gain was observed from 7 to 28 days and from 28

J H35F00 ; H35F20 | H35F40 3 H35F60 H35F80

Age (days)

(b) series H35

Fig. 5. Flexural strength of concrete versus age.

to 56, 91, 182, and 365 days for the mixtures containing high volumes of fly ash.

A comparison of the L35 and H35 concrete series in Table 6 shows that the compressive strength of the concrete mixtures for the H35 series are lower than those of the corresponding mixture of the L35 series; Fig. 4 shows a significant difference between the H35F80 and L35F80 mixtures. This may be attributed to the lower pozzolanic activity and greater particle size of the fly ash with higher LOI content for the H35 series, which increases the water demand and decreases the pozzolanic reaction.

3.3. Flexural strength

The flexural strength of concrete mixtures was determined at the ages of 7, 28, 56, 91, and 365 days. The results are shown in Table 8 and Fig. 5. Similar to the compressive strength, the flexural strength of the concrete mixtures increased with age. The flexural strength of fly-ash concrete decreased slightly as the percentage of fly ash increased. As expected, the early strength gain of the control concrete without fly ash was superior to that of the fly ash mixtures. However, most of the concrete mixtures, except the L35F80, H35F60, and H35F80 mixtures, met the minimum 7-day flexural strength requirement (4.1 MPa) of the Texas specifications

Table 8

Flexural strength of concrete mixtures (MPa).

Mixture no. 7 days 28 days 56 days 91 days 365 days

L35F00 4.9 5.1 5.4 5.9 6.5

L35F20 4.7 5.3 6.0 6.5 7.2

L35F40 4.5 5.2 6.0 6.6 7.1

L35F60 3.9 5.0 5.7 6.3 7.5

L35F80 3.3 3.7 4.7 5.8 6.7

H35F00 4.9 5.1 5.4 5.9 6.5

H35F20 4.5 5.0 5.6 6.1 6.6

H35F40 4.1 5.1 5.9 6.3 6.9

H35F60 3.0 4.5 5.3 5.9 6.8

H35F80 2.2 3.2 4.2 5.3 6.0

Fig. 6. The relation between flexural and compressive strength of fly-ash concretes. 3.5. Modulus of e^Stiaty and Poisson's mtio

[25], and exceeded the 28-day flexural strength limitation of the British Airport Authority [5]. Consequently, these concrete mixtures can be used in highway application and airport quality concrete.

In addition, Fig. 5 shows that all fly-ash concretes demonstrated larger increases in flexural strength from 28 days to 365 days, indicating that the fly-ash concrete may develop superior flexural strength at the age of 1 year. This is attributed to an interaction between fly ash and calcium hydroxide that results in the formation of re-crystallized calcium carbonate in the cementitious matrix, which causes a decrease in the porosity of the matrix and the transition zone.

3.4. Relation between compressive and flexural strength

The compressive strength and corresponding flexural strength of fly-ash concrete, regardless of fly ash content and curing, are patted in Fig. 6. A power relation was established for the current results, which is suitable for all grades of fly-ash concrete. The proposed equation is given by:

ft = 0.51/'c0'

(r2 = 0.880)

where ft is the flexural strength in MPa; fc' the compressive strength in MPa; and r is the correlation coefficient of the equation.

The relation obtained was compared with the relevant reports, such as ACI 1992 [35], ACI 1995 [36], and Ahmad and Shah [37], which are also represented in Fig. 6. It is observed that the equation of ACI 1995 highly underestimates the flexural strength from its compressive strength data. Ahmad and Shah and current relation give a better but conservative estimate of flexural strength from compressive strength. It is found in Fig. 6 that the current relation almost coincides with the equation given by ACI 1992.

Table 7 shows the results of modulus of elasticity (E value) and Poisson's ratio tests. The E values at 28 days for the concretes containing 20-60% fly ash of the total binder and the control concrete were 26.5 GPa in average and 26.9 GPa, respectively; the corresponding values at 365 days were 40.3 GPa in average and 33.0 GPa, respectively. The result indicates that the fly-ash concretes exhibited a substantial increase in the modulus of elasticity from the ages of 28-365 days. However, this was not the case for the control concretes. Consequently, the fly-ash concretes exhibited superior E values at the age of 1 year; the E values increased considerably between 28 and 365 days to reach values ranging from 38.6 to 41.4 GPa. Previous published data [9,15,38] also showed that the E values for the HVFA concrete are generally higher than those of the control concrete without fly ash with similar compressive strength at 28 days.

In addition, Table 7 shows that the concrete mixtures of the L35F80 and H35F80 series exhibited a significantly lower E value than the other fly-ash concrete mixtures with fly ash content of less than 60%. These results indicate that, similar to the compres-sive strength gain, a high volume fly ash content of up to 80% may inversely decrease the modulus elasticity of fly-ash concrete.

Table 7 shows that the Poisson's ratio of all fly-ash concrete mixtures randomly fall in the range between 0.122 and 0.181, which are comparable to the value of normal concrete; however, we observed inconsistent relationships between Poisson's ratio and concrete characteristics, such as water-cementitious material ratio, compressive strength, and fly ash content.

3.6. Drying shrinkage

The ASTM C157 [39] standard test method was used to assess the drying shrinkage characteristics for the concrete mixtures. All specimens were cured in lime-saturated water for 28 days prior to exposure to moderate conditions of 23 ± 2 °C and 50 ± 4% RH.

Table 7

Modulus of elasticity and Poisson's ratio test results.

Mixture no. W/cm Modulus of elasticity (GPa) Poisson's ratio

28 days 56 days 91 days 365 days 28 days 56 days 91 days 365 days

L35F00 0.60 26.9 28.5 28.3 33.0 0.153 0.173 0.157 0.165

L35F20 0.55 26.4 29.8 30.2 38.6 0.160 0.169 0.154 0.164

L35F40 0.48 26.8 32.4 32.6 40.8 0.164 0.173 0.167 0.176

L35F60 0.36 26.2 31.8 32.8 41.4 0.169 0.163 0.177 0.181

L35F80 0.24 18.7 23.5 23.9 29.8 0.167 0.181 0.168 0.175

H35F00 0.60 26.9 28.5 28.3 33.0 0.153 0.173 0.157 0.165

H35F20 0.55 27.1 31.0 31.2 36.0 0.160 0.167 0.166 0.172

H35F40 0.48 25.7 28.6 30.7 36.1 0.150 0.169 0.172 0.160

H35F60 0.36 23.3 29.3 30.9 37.3 0.149 0.175 0.171 0.178

H35F80 0.24 15.9 21.1 23.2 27.5 0.122 0.147 0.124 0.165

0 50 100 150 200 250

Age (days)

Fig. 7. Drying shrinkage development of concrete mixtures.

Fig. 7 shows the test results of the shrinkage versus age relationships. The drying shrinkages increased with age for all mixtures. The incorporation of fly ash in concrete leads to an increase of drying shrinkage at various ages. The shrinkage of fly-ash concrete mixtures increased in conjunction with the percentage of fly ash for all cementitious contents. Therefore, the fly-ash concrete mixtures exhibited higher shrinkage after a total drying period of 224 days; for example, the shrinkage values were 650 x 10~6 for the L35F20 series, 673 x 10~6 for the L35F60 series, 668 x 10~6 for the H35F40 series, and 683 x 10~6 for the H35F80 series. However, the increments of shrinkage with respect to increased replacement percentage of fly ash (Fig. 7) reduced after the age of 56 days.

4. Conclusion

Two types of ASTM Class F fly ash with low-LOI value of 4.6% and high-LOI value of 7.8% were successfully used to produce high volume fly-ash concrete, which contains fly ash of up to 80% of the total cementations material. A rational mix design method was proposed for HVFA concrete. Based on the test data on the properties of fresh concrete (air content, unit weight, and setting time), compressive strength, elastic modulus, flexural strength, and drying shrinkage, fly-ash concrete exhibits the following characteristics as compared to the normal concrete without fly ash:

1. Concrete containing fly ash of up to 80% of cementitious material content can be proportioned to have adequate workability when a more suitable SP is used.

2. The air content of fly-ash concrete increases with the increase of cement replacement percentage of fly ash. However, the unit weight decreases with the increase of fly ash content. Moreover, the fly ash with the high-LOI may produce larger air content and less unit weight of concrete than those with the low-LOI content.

3. The setting time of fly-ash concrete increased in conjunction with the fly ash content. The concrete mixtures containing 80% fly ash of the total binder exhibited a particularly longer final setting time of up to 15-18 h.

4. The early compressive strength gain of the control concrete without fly ash was superior to that of the fly-ash concretes, whereas we observed a significant strength gain from 7 to 28 days and from 28 to 56, 91, 182, and 365 days for the concrete containing high volumes of fly ash. In addition, the compressive strengths of the H35 series (containing high-LOI fly ash) were lower than those of the

corresponding mixtures of the L35 series (containing low-LOI fly ash); a larger difference was observed between the H35F80 and L35F80 mixtures.

5. The modulus of elasticity of fly-ash concrete has a similar development trend to that of the compressive strength. It increases considerably between 28 and 365 days and exhibits superior E values ranging from 40.8 to 41.4 GPa for L35 series. In addition, the concrete mixtures of L35F80 and H35F80 exhibit relatively lower E values than the other fly-ash concretes with fly ash contents of less than 60%. In addition, the Poisson's ratio of all fly-ash concrete mixtures randomly falls in the range of 0.122-0.181, which is comparable to the value of normal concrete.

6. Although the fly-ash concretes gained flexural strength slowly in the early age, most mixtures (except the L35F80; H35F60 and H35F80 mixtures) met the minimum 7-days flexural strength requirement (4.1 MPa) of the Texas specification, and exceeded the 28-day flexural strength limitation of the British Airport Authority. All fly-ash concretes demonstrated a larger increase in flexural strength from 28 days to 365 days, thereby developing superior strength at the age of 1 year.

7. Suitable relation is developed between flexural and com-pressive strength of all grades of fly-ash concrete, which is given by ft = 0.51fc0 653.

8. The addition of fly ash in concrete leads to an increase of drying shrinkage at various ages. The shrinkage of fly-ash concrete increases in conjunction with the replacement percentage of fly ash and exhibits a higher shrinkage strain level of approximately 750-795 x 10~6 after a drying period of 224 days.

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