FractureToughnessOfHVFAMortar_500566

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SP-242—25

Fracture Toughness of Cement MortarContaining High Volume Fly Ash

by S.F.U. Ahmed and M. Maalej

Synopsis: In this paper experimental evaluation on the effect of high volume yash as partial replacement of cement on fracture toughness of cement mortar arepresented. The y ash replacement level was 50%, 60% and 70% by weight ofcement. Three-point bend notch beams were used to measure the fracture toughnessof mortar. Results show that the use of 50% y ash as partial replacement of cementreduces the fracture toughness values between 38% and 58% compared to thatwithout y ash. Reduction of compressive strength and Young’s modulus in mortarcontaining 50% y ash as partial replacement of cement compared to that withouty ash is also observed in this study. The use of 60% and 70% y ash as partialreplacement of cement is found to have negligible effect on the reduction of fracturetoughness of cement mortar. Long term effects of high volume y ash (50% cementreplacement) on fracture toughness, compressive strength and Young’s modulusof cement mortar are also evaluated in this study. Tests were conducted at 28, 56and 91 days and at 5, 7, 10 and 12 months. Results show that the rate of increase offracture toughness of cement mortar containing 50% y ash with time is very slow.Compressive strength and Young’s modulus also increase with time.

Keywords: fracture toughness; high volume y ash; long termevaluation; mortar



322 Ahmed and MaalejShaikh Faiz Uddin Ahmed graduated from Bangladesh Institute of Technology, Khulna(B.Sc. Engg. (civil) 1994), the Asian Institute of Technology, Thailand (M.Eng. 1998),and the National University of Singapore (PhD 2004). His research interests include useof supplementary cementitious materials in concrete and fiber reinforced cementitious

composites, corrosion durability of reinforced concrete structures, high performancesfiber reinforced cementitious composites, numerical modeling and structural healthmonitoring using fiber optic sensor. He is currently in Tohoku University, Japan as JSPS

post-doctoral fellow.

ACI member Mohamed Maalej is Associate Professor in Civil engineering departmentof the National University of Singapore. He graduated from Colorado State University(BS 1989), the Massachusetts Institute of Technology (MS 1991), and the University oMichigan (PhD 1994). His research interests include Fracture mechanics,

Micromechanics of fibrous composites, Engineered cementitious composites (ECC),Repair and strengthening of RC structures, Corrosion durability of reinforced concrete,Externally-bonded FRP reinforcement, Structural health monitoring and Fiber opticsensing technology.

INTRODUCTION

Supplementary cementitious materials (SCM) like fly ash, slag and silica fumeare industrial by-products, which are being used in concrete for better mechanical anddurability properties (1,2). In order to increase the utilization of SCM that are otherwisebeing wasted, it will be useful to use large amounts of SCM in concrete as partialreplacement of ordinary portland cement (3,4). Their use will recycle waste products andalso reduce the production of cement, which releases large amount of CO 2 into theatmosphere (5).

Fly ash, a principal by-product of the coal-fired power plants, is well accepted asa SCM that may be used either as a component of blended portland cements or as mineraladmixtures in concrete. It is widely being used as a cement replacement to produce high-

performance concrete and high-volume fly ash concrete. High volume fly ash concretes,with 50% or more cement replacement by fly ash, show high workability, high ultimatestrength, and high durability. High volume fly ash concretes also show excellent longterm mechanical and durability properties due to its slow pozzolanic reaction with thehydration products of cement (6-10). Fly ash is also used in the development of fiberreinforced concrete (FRC) and high performances fiber reinforced cementitiouscomposites (HPFRCC). HPFRCC shows strain hardening and multiple crackingbehaviors in tension and bending (11-17). They also exhibit excellent toughness andenergy absorption capacities compared to concrete and regular FRC.

Mode-I fracture toughness, K IC , is an important material property that affects thefirst crack strength and energy necessary for crack initiation of ordinary concrete andfiber reinforced cementitious composites. Low K IC is in favor of low first crack strength.By lowering the first crack strength and increasing the gap between the first crackstrength and ultimate strength in fiber reinforced cementitious composites the strain



Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete 323hardening behaviour can be ensured. There are many factors that affect the Mode Ifracture toughness of concrete or mortar e.g. amount of fine and coarse aggregates (18),water content (18, 19) and other additives (such as air content (19), Mica flake, etc.).While the high volume fly ash is found to improve both mechanical and durability

properties of concrete and fiber reinforced cementitious composites only few studies haveevaluated its effect on the fracture toughness of matrix (14, 16). Therefore, this study isdesigned to evaluate the effect of high volume fly ash as partial replacement of cement onthe fracture toughness of cement mortar. The fly ash contents used were 50%, 60% and70%. Moreover, the slow pozzolanic reaction of fly ash with hydration products ofcement will affect the long term value of K IC of concrete and fiber reinforcedcementitious composites containing high volume fly ash. Long term effects of highvolume fly ash (50%) on fracture toughness, compressive strength and Young’s modulusof cement mortar are also evaluated in this study. Tests were conducted at 28, 56 and 91days and at 5, 7, 10 and 12 months.

MATERIALS AND MIX PROPORTIONS

The cement used in this study was ordinary portland cement which correspondsto ASTM type I. The fly ash used in this study was class F fly ash. The physical

properties and chemical analysis of cement, fly ash and silica fume are given in Table 1.The mix proportions are given in Table 2. Series 1 is for cement paste. Series 2 and 3 arefor cement mortar with water/binder ratio of 0.3 and 0.45, respectively. In series 1-3 nofly ash was added. However, in series 1 only 10% cement was replaced by silica fume. Inseries 2-7 5% silica fume was added. Series 4 and 5 are for mortar containing 50% fly ash(FA) as partial replacement of cement with water/binder ratio of 0.3 and 0.45,respectively, while series 6 and 7 are for mortar containing 60% and 70% fly ash (FA) as

partial replacement of cement, respectively. Series 5 (Matrix type 50FA45) was used tomeasure and monitor the development of fracture toughness, Young’s modulus andcompressive strength of mortar containing 50% fly ash over a period of one year.

For each series, six cylinders of 100mm diameter and 150mm height and threenotch beams of 650x150x60 mm in dimensions were cast. Out of six cylinders, three

were used to measure the compressive strength while the rest were used to measure theYoung’s modulus. Cylinders and notch beams for both short term and long term studieswere kept in the fog room with a temperature of about 20 0 C until the dates of testing. Allnotch beams were tested in a closed-loop servo-controlled Instron testing machine. Aschematic of the bending test setup is shown in Fig. 1. Detail of the experimental

program is given in Table 3.

MEASUREMENT OF FRACTURE TOUGHNESS

The mode I fracture toughness, K IC of various mixes was determined accordingto the Two-Parameter Fracture Model proposed by Jenq and Shah (18), where thenonlinear slow crack growth was taken into account and the values of K IC determinedwere presumed independent of specimen dimensions. The crack mouth openingdisplacement (CMOD) was measured by clip gauge. Cyclic beam tests were performed



324 Ahmed and Maalej

in order to determine the value of the model parameter α , which is the ratio between totalCMOD (CMOD T ) and inelastic CMOD (CMOD * ) at peak load (Fig. 2). The calculationof K IC involved an iterative numerical scheme, where the effective crack length l e at peakload was evaluated. For a given peak load, initial notch length and measured total

CMOD, an effective crack length l e is first assumed and verified only when thecalculated value of elastic CMOD according to equation 1 (18) agreed with the measuredvalue. Once an effective crack length l e is obtained the K IC is then calculated according toequation 2 (18).

⎟ ⎠ ⎞

⎜⎝ ⎛ =

W

aV

BE W

Sa P CMOD

12

max6

(1)

where,

⎥⎥

⎥

⎥

⎥

⎦

⎤

⎢⎢

⎢

⎢

⎢

⎣

⎡

⎟ ⎠ ⎞

⎜⎝ ⎛ −

+⎟ ⎠ ⎞

⎜⎝ ⎛ −⎟

⎠ ⎞

⎜⎝ ⎛ +−=⎟

⎠ ⎞

⎜⎝ ⎛

2

32

1

1

66.004.287.328.276.0

W

aW

a

W

a

W

a

W

aV

a=a 0 +l e

a0 =Initial notch lengthl

e =Effective crack lengthW=Depth of beamB=Width of beamS=Loaded spanPmax = Peak loadE=Young’s modulus

aW

a F BW

S P K IC π

⎟ ⎠ ⎞

⎜⎝ ⎛

= 12

max5.1

(2)

where,

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎪⎭

⎪⎬

⎫

⎪⎩

⎪⎨

⎧

⎟ ⎠ ⎞

⎜⎝ ⎛ +⎟

⎠ ⎞

⎜⎝ ⎛ −⎟

⎠ ⎞

⎜⎝ ⎛ −−

⎟ ⎠ ⎞

⎜⎝ ⎛ −⎟

⎠ ⎞

⎜⎝ ⎛ +

=⎟ ⎠ ⎞

⎜⎝ ⎛ 2

231

7.293.315.2199.1

12

1

1

W

a

W

a

W

a

W

a

W

a

W

aW

a F

π





326 Ahmed and Maalejbehaviour is also reported by several researchers (10,22). This is due to slow pozzolanicreaction of fly ash with hydration products of cement. The development of fracturetoughness of cement mortar containing 50% fly ash over a period of one year is shown inTable 4. The values of K IC increase with time with some scattering of K IC values for 5 th

and 7th

month. However, the rate of increase of K IC values is relatively slow as comparedto compressive strength and Young’s modulus. The increase of K IC value with time, onceagain, is believed to be due to slow pozzolanic reaction of mortar containing fly ash.With continued hydration the pores in mortar containing fly ash are expected to reducewith time which might be responsible for the increase of K IC values with time observed inthis study. The reduction of pores in mortar containing 50% fly ash at 6 months (22) andat 2 years (24) has already been reported by the researchers.

The reduction of matrix fracture toughness achieved in this study due to the useof high volume fly ash will have great impact in the development of high performancefiber reinforced cementitious composites with strain hardening and multiple crackingbehavior. The reduction of the matrix fracture toughness will ensure low first crackstrength in fiber reinforced cementitious composites and by increasing the gap betweenthe first crack strength and ultimate bridging strength in fiber reinforced cementitiouscomposites the strain hardening and multiple cracking behavior can be ensured, which isthe most desirable property for fiber reinforced cementitious composites.

CONCLUSIONS

The use of 50% fly ash as partial replacement of cement reduces the fracturetoughness values between 38% and 58% compared to that without fly ash. Reduction ocompressive strength and Young’s modulus in mortar containing 50% fly ash as partialreplacement of cement compared to that without fly ash is also observed in this study.The use of 60% and 70% fly ash as partial replacement of cement is found to havenegligible effect on the reduction of fracture toughness of cement mortar containing 50%fly ash. Results also show that the rate of increase of fracture toughness of cement mortarcontaining 50% fly ash is relatively slow. The increase in compressive strength andYoung’s modulus of high volume fly ash mortar with time over a period of one year is

also observed in this study.REFERENCES

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2. Langley, W.S., “Practical use of high volume fly ash concrete utilizing a low calciumfly ash”, Concrete technology for sustainable development in the twenty first century ,ed. P.K. Mehta, 1999, pp.65-96.

3. Swamy, R.N., `Design for durability and strength through the use of fly ash and slagin concrete’, ed. V.M Malhotra, ACI special publication, SP-171, 1-71.



Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete 3274. Mehta, P.K., `Role of pozzolanic and cementitious materials in sustainable

development of the concrete industry’ Sixth CANMET/ACI conference , ed. V.MMalhotra, ACI special publication, SP-178, 1998, pp.1-20.

5. Malhotra, V.M., (1987) “Supplementary cementing materials in concrete”, CANMET .

6. Malhotra, V.M., Carette, G.G. and Bilodeau, A. “Mechanical properties and durabilityof polypropylene fiber reinforced high volume fly ash concrete for shortcreteapplications” ACI materials Journal , Vol. 91, No. 5, 1994, pp. 478-486.

7. Gebler, S. H., and Klieger, P. “Effect of fly ash on some of the physical properties oconcrete.” Publication SP—American Concrete Institute, v 1, Fly Ash, Silica Fume,Slag, and Natural Pozzolans in Concrete, Proc., 2nd Int. Conf. , Madrid, Spain, 1986,

pp.1–50.

8. Haque, M. N., Kayyali, O. A., and Gopalan, M. K. “Fly ash reduces harmful chlorideions in concrete.” ACI Materials. Journal , Vol. 89, No.3, 1992, pp. 238–241.

9. Al-Amoudi, O.S.B., Rasheeduzzafar, Maslehuddin, M. and Al-Mana, A.I.,“Prediction of long term corrosion resistance of plain and blended cementconcretes”, ACI materials journal , Vol. 89, No.4, 1992, pp.337-344.

10. Gu, P., Beaudoin, J.J., Zhang, M.H. and Malhotra, V.M., “Performance of steelreinforcement in Portland cement and high volume fly ash concretes exposed tochloride solution”, ACI materials Journal , Vol. 96, No.5, 1999, pp.551-558.

11. Ramanalingam, N., Paramasivam, P., Mansur, M.A. and Maalej, M., Flexuralbehavior of hybrid fiber reinforced cement composites containing high volume flyash, Proceedings of the 7 th CANMET/ACI international Conference, 2001, ACI SP-199 .

12. Qian, C.X., and Stroeven, P., Development of hybrid polypropylene-steel fiber

reinforced concrete, Cement and Concrete Research , Vol. 30, 2000,pp. 63-69.

13. Zhang, Y., Sun, W., Shang, L., and Pan, G., The effect of high content of fly ash onthe properties of glass fiber reinforced cementitious composites, Cement anConcrete Research , Vol. 27, No. 12, 1997,pp. 1885-1891.

14. Li, V.C., Lepech, M., Wang, S., Weimann, M. and Keoleian, G., “Development ogreen engineered cementitious composites for sustainable infracture systems”, In the

proceedings of International workshop on sustainable development and concretetechnology , Beijing, 2004, pp.181-191.

15. Peled, A., Cyr, M.F., and Shah, S.P., High content of fly ash (class F) in extrudedcementitious composites, ACI materials journal , Vol. 97, No.5, 2000, pp.509-517.



328 Ahmed and Maalej16. Ahmed, S.F.U., Maalej, M., and Paramasivam, P., Strain-hardening Behavior o

Hybrid Fiber Reinforced Cement Composites, Journal of Ferrocement , Vol. 33, No.3, 2003, pp. 172-182.

17. Ahmed, S.F.U., Maalej, M., and Paramasivam, P., Flexural Responses of HybridSteel-Polyethylene Fiber Reinforced Cement Composites Containing High VolumeFly Ash, Journal of construction and building materials (in press)

18. Jenq, Y. and Shah, S.P., “Two parameter fracture model for concrete”, ASCE Journalof Engineering Mechanics , Vol. 111, No. 10, 1985, pp.1227-1241.

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Engineering Fracture Mechanics , Vol. 69, 2002, pp. 249-265.

20. Li, V.C. and Maalej, M., “Toughening in cement based composites, Part I: Cement,mortar and Concrete”, Cement and Concrete Composite , Vol 18, 1996, pp.223-237.

21. Sengul, O., Tasdemir, C. and Ali, M., “Mechanical properties and rapid chloride permeability of concrete with ground fly ash”, ACI materials journal , Vol. 102, No.6,2005, pp.414-421.

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23. Poon, C.S., Lam, L. and Wong, Y.L. Effect of fly ash and silica fume on interfacial porosity of concrete, Journal of materials in civil engineering , Vol. 11, No. 3, 1999, pp.197-205.

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Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete 329




Fig. 1—Three-point bend notch specimens for fracture toughness measurement.(all dimensions in mm)



Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete 331

Fig. 2—Composition of CMOD due to non-linear effect.

Fig. 3—Compressive strength of cement paste and mortars with different y ashcontents and water/binder ratios.




Fig. 4—Young’s modulus of cement paste and mortars with different y ash contentsand water/binder ratios.

Fig. 5—Fracture toughness of cement paste and mortars with different y ashcontents and water/binder ratios.

Fig. 6—Pore distribution of cement mortar with and without y ash.(Note: FA denotes y ash; A 0=Angstrom; 10,000 A 0=1 micrometer)

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