Restrained Shrinkage of Fly Ash Based Geopolymer Concrete ...

Journal of the Arkansas Academy of Science

Volume 71 Article 31

2017

Restrained Shrinkage of Fly Ash BasedGeopolymer Concrete and Analysis of Long TermShrinkage Prediction ModelsMd R. IslamSouthern Arkansas University, [email protected]

Mahbub K. Ahmedsouthern arkansas university, [email protected]

Hosney Ara BegumBangladesh Council of Science and Industrial Research, [email protected]

Erez N. AlloucheLouisiana Tech University, [email protected]

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Recommended CitationIslam, Md R.; Ahmed, Mahbub K.; Begum, Hosney Ara; and Allouche, Erez N. (2017) "Restrained Shrinkage of Fly Ash BasedGeopolymer Concrete and Analysis of Long Term Shrinkage Prediction Models," Journal of the Arkansas Academy of Science: Vol. 71 ,Article 31.Available at: http://scholarworks.uark.edu/jaas/vol71/iss1/31

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Restrained Shrinkage of Fly Ash Based Geopolymer Concrete andAnalysis of Long Term Shrinkage Prediction Models

Cover Page FootnoteThe author would like to thank Trenchless Technology Center of Louisiana Tech University for the use of itsfacilities. Special thanks to Dr. Carlos Montes for his suggestions and help to access the materialcharacterization lab of the micro-manufacturing institute of Louisiana Tech University.

This article is available in Journal of the Arkansas Academy of Science: http://scholarworks.uark.edu/jaas/vol71/iss1/31


Journal of the Arkansas Academy of Science, Vol. 71, 2017180

Restrained Shrinkage of Fly Ash Based Geopolymer Concrete and Analysis of LongTerm Shrinkage Prediction Models

M.R. Islam*, M.K. Ahmed, H.A. Begum, and E.N. Allouche

Department of Engineering and Physics, Southern Arkansas University, Magnolia, AR 71753

*Correspondence: [email protected]

Running Title: Restrained Shrinkage of Geopolymer Concrete and Shrinkage Prediction Model for GPC

Abstract

The research presented in this manuscript describesthe procedure to quantify the restrained shrinkage ofgeopolymer concrete (GPC) using ring specimen.Massive concrete structures are susceptible to shrinkageand thermal cracking. This cracking can increase theconcrete permeability and decrease the strength anddesign life. This test is comprised of evaluatinggeopolymer concrete of six different mix designsincluding different activator solution to fly ash ratiosubjected to both restrained and free shrinkage. Testresults obtained from this experimental setup wereplotted along with the available empirical equation toobserve the shrinkage strain of GPC and a model wassuggested to predict the shrinkage strain of GPC. It wasfound from this study that along with activator solutionto fly ash ratio the final compressive strength of GPCplays an important role on shrinkage strain.

Introduction

In high strength concrete structure and concreterepair, overlay, long span slab, differential dryingthrough the thickness of the large mass cause internalrestraint and buildup tensile stress within the material(Palomo et al. 1999). Tensile stress in the structure alsodepends upon the external restraint of the structuralelement. Time to crack depends not only on the tensilestrength of the concrete but also on the tensile creepcharacteristics of the material (Duxon et al. 2007). Oneof the popular tests to determine the early-age-behaviorof concrete under restrained shrinkage is the ring test(Moon and Weiss 2006). When the concrete ringdeforms due to shrinkage the steel ring restrains theconcrete which causes tensile stress in the specimen. Inthe ASTM C 1581 the ring provides a high degree ofrestraint while still allowing sufficient strain in the steelas the concrete shrinks (Ryan et al. 2010). Cracking inthe ring specimen are assessed from the readingobtained from the strain gages attached to the inner

surface of the steel ring. This method provides the straindata which can be converted with suitable mathematicalequations to the stress developed in the concrete ring(See et al. 2003). An instrumented ring similar to theASTM C1581 ring was evaluated in this study and usedto obtain the restrained shrinkage behavior of sixgeopolymer concrete mixtures. Dimension of the ringspecimens and thickness of the steel and concrete ringwas selected according to the standard to follow theempirical equations already developed for stresscalculation for restrained ring specimen (Jun et al.2011). Testing and analysis procedure presented in thisstudy illustrates how instrumented ring specimen canprovide data on restrained stress and strain ofgeopolymer concrete. These results provide a basis forcomparing the performance of different GPC mixturesunder restrained shrinkage in the same environmentalcondition (Swayze 1942). This study deals with theresult from shrinkage tests on the geopolymer concretemix on both restrained and free shrinkage condition.Test was conducted to see the age of cracking and freeshrinkage strain of geopolymer concrete. Data analysiswas performed to evaluate the effect of various factorson the shrinkage behavior. Statistical analysis wasconducted to establish the relationship betweencompressive strength at the age when shrinkage test wasstarted and ultimate shrinkage strain. A theoreticalmodel was emphasized and compared with existingempirical models to see the effectiveness of the bestprediction equation for GPC.

Materials and mix design

Concrete mixtures were selected with differentactivator solution to fly ash ratio and for different targetstrength of the hardened concrete. Variables wereselected to see the effect of activator solution to fly ashratio on the shrinkage strain of geopolymer concrete.Compressive strength of the concrete varied in rangesbetween 25 MPa to 55 MPa. Samples were preparedwithout using any shrinkage reducing admixtures

180

Journal of the Arkansas Academy of Science, Vol. 71 [2017], Art. 31

Published by Arkansas Academy of Science, 2017

Restrained Shrinkage of Geopolymer Concrete and Shrinkage Prediction Model for GPC


(Guneyisi et al. 2010). Total amount of coarse and fineaggregate was constant for different mix design to seethe effect of geopolymerization on the short and longterm properties of GPC. All the mixes showed morethan an 8 inch slump and the air content was below 4%.Concrete rings were kept on a vibrating table for 30seconds to remove any entrapped air bubble inside it. Atotal of 6 different GPC and 1 ordinary portland cement(OPC) concrete mixture were evaluated. OPC mix wasused as a control sample to monitor the shrinkageproperty from the testing and adjustment of the testsetup.

Mix Proportion of ConcreteConcrete mixtures were selected from the specific

strength range using the particular mix design of theactivator solution and fly ash type. Strength of theconcrete was controlled by the variation in activatorsolution to fly ash ratio (AS/FA). Class F fly ash wasused for the design of concrete cylinders. Four differentmix designs were produced by varying the AS/FA. Mixdesign was selected from the preliminary test. Thedetailed mix proportion for this group of specimens ispresented in Table 1.

The second set of mix design was prepared toobserve the effect of the extent of geopolymerization. Inthis test program, the aggregate to fly ash ratio was keptconstant. Minimum compressive strength was attainedusing N silicate and 10M sodium hydroxide solution,and high strength was achieved using D silicate and14M sodium hydroxide solution (Table 2). Activatorsolution to fly-ash ratio was 0.35 for both mix design.

A control mix of OPC to compare the results withthe GPC was designed following the ACI guideline.OPC mix design was prepared to see the propagation ofcracks and to use as a reference. Mix proportion wasselected to get a hardened concrete with nominalstrength of 55 MPa. Water cement ratio for this mix was0.3.

Table 1. Mix design of GPC with the variation in AS/FAratio.

Raw

Material

Mix Design for different activator solution tofly ash ratio (kg/m3)

0.35 0.45 0.55 0.65

NaOH (12M) 78.3 100.9 122.8 145.4

Silicate (N) 117.5 151.3 184.6 218.4

Fly Ash 559.6 559.6 559.6 559.6

River Sand 719.8 719.8 719.8 719.8

Pea Gravel 868.8 868.8 868.8 868.8

Table 2. Mix design of GPC with the variation incompressive strength.

Mix design for 25 MPa GPC(kg/m3)

Mix design for 50 MPa GPC(kg/m3)

NaOH (10M) 78.3 NaOH (14M) 78.3

Silicate (N) 117.5 Silicate (D) 117.5

Fly Ash 597.5 Fly Ash 597.5

River Sand 612.4 River Sand 612.4

Pea Gravel 881.5 Pea Gravel 881.5

Table 3. Mix design of high strength OPC.Working mix design in (kg/m3)

Cement (type-I) 692.5

Water 207.5

River sand 630.2

Pea gravel 868.5

The particular mix design in Table 3 was used tomake a set of samples to find the strength gain over timeand other mechanical properties. OPC samples wereprepared and stored according to ASTM C31.

Test Method and Sample PreparationThe shrinkage test apparatus was prepared

following ASTM C1581. The mold was prepared with ametal pipe section as the inner ring and a PVC two-partouter ring. Strain gages were attached to the innersurface of the metal ring to calculate the shrinkage straincaused by the drying of concrete. The data acquisitionsystem was used to calculate the deformation occurredin the strain gage and the stress in concrete was alsoanalyzed from this data. Ring specimens are morecommonly used because of the benefits that those caneasily be cast and the end effects are removed providingan axi-symmetric geometry (Kovler 1994). If thethickness of the steel is too large, deformation cannot bedetected from the experiment. Such test setups providequalitative evaluations, but do not establish a simpleprocedure to routinely quantify the restrainedcharacteristics of the material (Grzbowski and Shah1989). Strain at the inner surface of the steel ring ismeasured by the foil strain gage, which provides anaccurate assessment of the time to crack. Cracking of thetest specimens are indicated by a sudden decrease incompressive strain in the steel ring. The measured strainprovides the basis for quantifying the restrainedshrinkage behavior of the concrete specimen. Straingages were placed at mid-height of the steel annulus,where the average strain is measured. Thickness of the

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M.R. Islam, M.K. Ahmed, H.A. Begum, and E.N. Allouche


concrete wall was maintained at 1.5 inch for allspecimens (Kwesi et al. 2014). The steel ring for theinner part of the mold was prepared from the steel pipesection of the standard size. The dimension of the steelring was selected following ASTM C1581. Thickness ofthe steel ring was 0.5 inch and the inside diameter of thering was 12 inch (30.48 cm). Steel pipe was cutaccording to the specified height 6 inch (150 cm) givenin the standard. The edge of the ring was ground withfine sand paper. The inner and outer surfaces of the steelring were cleaned using the sand blasting apparatus toremove any oil and grease. The rings made from the pipesection were further prepared to install the foil straingage at the inner surface. Two strain gages wereattached to the surface 180o apart. Data collection fromthe acquisition system was stopped when the crackformed at the outer surface of the concrete propagatedto the inner ring, and there was no change in the readingobtained from the strain gages. The rate of shrinkage canchange due to temperature and relative humidity. It isvery important to keep concrete specimen inside acontrolled environment to measure the shrinkageaccurately. For this test an environment chamber with adimension of 30ft x 15ft x 8ft was made with thickinsulated aluminum wall.

The environmental control chamber kept thespecimens at controlled temperature and humiditywithout too much stress on the mechanical devices.There was an arrangement to read the actual temperatureand humidity by the digital panel from outside thechamber. For this experiment the environment chamberwas kept at a constant temperature of 73±3o F and arelative humidity of 50±4% (Qiao et al. 2012).

Figure 1. Test setup for strain measurement

Strain Data CalculationEach sample was equipped with two strain gages. In

order to record the strain for three months of testing, thedata acquisition system is essential. Data logger usedwas manufactured by Hewlett Packard with 96 channelsfor strain readings. This arrangement allowed multipledata to be recorded at the same time. Every three daysdata was collected from the data logger to a computer.Data obtained from the data acquisition system wasprocessed through the data logger software. Regularobservations were made to see whether there is a trendof cracking in any of the ring specimen. Cracking straincapacity on the other hand was also determined by theelastic modulus test and splitting tensile strength test(Temuujin et al. 2009). Drying shrinkage strain wascalculated considering the elastic and tensile creep strainin the concrete and balanced with the elastic contractionstrain in the steel (Shah and Weiss 2006).

)()()()( tttt stcpesh (1)

Where εsh(t) is the shrinkage strain, εe(t) is elasticconcrete strain, εcp(t) is tensile creep strain and εst(t) isthe elastic steel strain at time t. Tensile stress in theconcrete σt(t) at time t is obtained from the followingequation

twr

wrEt st

cis

sticstt (2)

Here Est is the modulus of elasticity of steel. wst and wc

are the wall thickness of the steel and concrete and ric

and ris are the internal radius of the concrete and steelrespectively.

TheoryIn 1982, the American Concrete Institute (ACI)

recommended the procedure for the prediction of creepand shrinkage in its ACI-209R-82 code provisions (ACI1982). The main inputs for shrinkage prediction arerelative humidity, specimen size, curing period and ageof loading. This model predicts the shrinkage strain.Correction factors are applied if the conditions aredifferent from the ideal condition stated in the standard(Hardjito et al. 2004). This model can be applied todifferent kinds of concrete and is very simple to apply.The ACI-209R-82 code recommends the followingexpressions for shrinkage:

shu

cc

ccsh

ttT

tttt

)(),(

(3)

According to CEB-FIP code proposed in 1990 and

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is restricted to ordinary structural concrete. This modelis based on the work of Muller and Hillsdorf (Hossainet al. 2003). The main input factors for the prediction ofshrinkage are ultimate compressive strength, volume tosurface ratio, age of curing, age of loading, and relativehumidity. Unless special provisions are given, the modelis valid for ordinary structural concrete having acompressive strength of 3000 psi (20 MPa) to 15000 psi(100 MPa), mean relative humidity 40-100% and meantemperature 5°C-30°C. Shrinkage strain was calculatedfrom

)(100

2350

10

1.0910160),(

2

6

cc

cRH

cmsccsh

ttA

tt

ftt

(4)

B3 model as proposed by Bazant and Baweja (1995). Itwas developed at Northwestern University and is basedon the statistical analysis of shrinkage data in acomputerized data bank involving about 15,000 datapoints and about 100 test series. The latest B3 modelconsiders more parameters than other prediction models(Bazant and Baweza 1995). The following parametersare used: a) relative humidity, b) exposure of concretespecimen to temperature prior to drying, c) size, d)cement type, e) coarse and fine aggregate, f) concretedensity, g) concrete age, h) specimen ultimate strength.This model is predicted for w/c ratio of 0.30 to 0.85 andstrength 2500 psi (17 MPa) to 10000 psi (65 MPa), a/cratio 2.5-13.5 and cement content 160-720 kg/m3. Themean shrinkage strain in the cross section is expressedas:

)(

6007(

)270(091.0(

,28.01.2

21

shcc

c

cmshu

chshucsh

tE

tE

fw

ttStt

(5)

Sakata proposed this model for creep and shrinkageon concrete by a statistical method on the basis ofexperimental data. The equation can estimate theconcrete creep and shrinkage strain (Sakata 1993).These prediction equations of shrinkage were adoptedas the Japanese standard methods by the Japan Societyof Civil Engineers (JSCE) in the revised standardSpecification for Design of Construction and ConcreteStructure published in 1996.

4.31log16

)/(121177.0

0/

tf

cwcsht(6)

Gardner and Lockman (2001) proposed the GL2000 model following the factors: a) relative humidity,b) average compressive strength, c) concrete membersize, d) water to cement ratio, e) cement type, f) modulusof elasticity of concrete at the age of loading, g) concreteage at drying and h) concrete age at loading. This modelis calibrated for compressive strength in the range of2320 psi (16 MPa) to 11890 psi (80 MPa), with volumeto surface ratio larger than 0.76, and w/c ratio between0.40 to 0.60 (Gardner and Lockman 2001). The creepcoefficient in this model is dependent on volume tosurface ratio, age of drying, age of concrete at loading,and relative humidity. Following equations are used tocalculate the creep compliance.

2

4

/15.0

)18.11(),(

SVtt

tt

htt

c

c

shucsh

(7)

Mix designs that survive longer without crackingare considered to perform better than those which crackearlier. The cracking area is also an indication of theperformance of a mix. Some of the specimens whichmay crack early but have small cracks, and may notpropagate toward the steel ring. Usually, the ringspecimens start cracking from the outer surface neareither the top or the bottom, and then the crack continuesto move inward toward the ring over time (Bentz et al.1995). The speed at which the crack propagates towardthe steel ring depends on the mix design. It is possiblethat the crack does not propagate fully towards the ring.When the crack reaches the ring, it causes a release ofcompressive stress upon the steel ring. Shrinkage strainvalues for different mix designs were calculated usingthe empirical equations and test data obtained from thedata acquisition system were compared. In this studyACI, Bazant B3, CEB, GL2000, and Sakata modelswere evaluated on their effectiveness and accuracy inpredicting the shrinkage strain of the different GPCmixes. Tensile creep parameters and restrainedshrinkage strain calculation was performed using thefree shrinkage strain, steel ring stain, modulus ofelasticity and flexural tensile strength of GPC.Empirical equations given in ACI 209 were used tocalculate the predicted strain at time t. Given the elastic

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strain at cracking, an analysis based on free shrinkagestrain alone without considering the tensile creep willgive cracking of the concrete much earlier than theactual cracking days. Thus, the tensile creepsignificantly increased the time to cracking of allconcrete mixtures. As expected the lower activatorsolution to fly ash ratio for GPC mixtures yield longertimes to cracking (Jensen and Hansen 2001). Thisdifference is explained by considering the magnitude oftensile creep effect on the cracking resistance. Thelarger magnitude of tensile creep coefficient of highstrength low activator solution to fly ash ratio mixturesalso corresponds to the longer days to cracking. Thisresult is also linked to the higher geopolymeric reactionin high strength GPC. The tensile creep coefficientsunder restrained shrinkage are smaller than thecoefficient under free shrinkage and fixed stress(Zuanfeng et al. 2011). A lower tensile creep under astate of increasing stress occurs when the specimen isrestrained.

Results and Discussion

The cracking behavior of a particular mix is verymuch dependent upon the liquid content of the mix. Theenvironmental factors such as humidity and temperaturethat changes the shrinkage behavior were kept constantfor all samples so the evaporation effect was neglected.In this study, one of the reason behind the early agecracking was found to be the liquid content of theconcrete mix. Non-structural causes were: plasticshrinkage, thermal deformation and autogenousshrinkage. Plastic shrinkage occurs because ofdifferential settlement and excessive evaporation ofwater from the concrete surface. Thermal shrinkage islargely due to considerable heat generated from thechemical reaction. Autogenous shrinkage is caused dueto reduction of volume and self-desiccation of internalpores. Free shrinkage test results were obtained from theprism specimen using the length comparator. The resultsobtained from the test are shown in Figure 2. Among themodels used to predict the free shrinkage of theconcrete, the Sakata model was very close to theexperimental data observed for various GPC samples.From the test results, it was observed that the watercontent has a significant effect on the drying shrinkageof the GPC. For a mix design with 0.35 AS/FA ratio, ittook 86 days and with 0.65 AS/FA ratio, the concretering was cracked in only 42 days. GPC with higherstrength took more time to form the surface crack. Thehigh strength of the GPC prevents the tensile crackformation. Test result obtained from 4000 psi and 8000

Figure 2. Free shrinkage strain.

psi GPC reflect the hypothesis that geopolymer withhigh strength has less drying shrinkage than that withlower compressive strength and lower polymerizationreaction. Test result obtained from this study puts fortha table with this coefficient for the shrinkagemeasurement of the GPC with a different activatorsolution to fly ash ratio. The sand to aggregate ratio hadlittle effect on the mechanical properties or the crackingpotential of the mixes.

From compressive strength test results, it was foundthat OPC had 28 days strength of 55 MPa. GPC sampledesigned for this similar strength acquired this after 24hours of heat curing. Figure 2 shows that the first twoweeks strain rate is steep for all specimens while thereis a change in strain rate at the end of two weeks. After120 days GPC sample with different AS/FA ratioreached a steady state. At the end of one-year strain inthe OPC sample is 200% more than the strain in the GPCsample having the same liquid content at the beginning(35%). The reason behind this can be associated with theformation of a dense polymer matrix that leaves littlespace for shrinkage in GPC. It can be observed from thegraph that the maximum strain from the GPC samplewas more than 500 micro strain (for 0.65 AS/FA) andthe minimum was around 200 micro strain (for 0.35AS/FA). This is important data to use in combinationwith the total strain to find the basic shrinkage for thecorresponding mix design of the GPC.

A characteristic comparison plot is shown in Figure3. All control data are available upon request. It can beobserved that ACI model overestimated the shrinkagestrain. Strain obtained from the SAKATA modelsuccessfully predicted and was very close to theexperimental data. Same phenomena were observed forthe other samples with different activator solution to fly-ash ratio. This is why the SAKATA model isrecommended to predict the shrinkage strain of GPC.

-6.5E-04

-4.5E-04

-2.5E-04

-5.0E-050 60 120 180 240 300 360

Shr

inka

geS

trai

n

Time (Days)

8000 GP C 0.35 GP C 0.45 GP C 4000 GP C

0.55 GP C 0.65 GP C 8000 O P C

184





Figure 3. Empirical and experimental data plot

Conclusion

In this paper free and restrained shrinkage ofgeopolymer concrete was measured at a constanttemperature and humidity. The effect of activatorsolution to fly ash ratio and final compressive strengthof GPC was observed on the shrinkage behavior overtime. It has been observed that the free shrinkage strainof GPC is less than the data predicted by the empiricalequation most of the cases. Each of the mixes had anelastic modulus in the range of about 5000 Ksi (34 GPa)and a tensile strength in the range of 650 Psi (4.5 MPa).Every mix in the AS/FA group cracked around 90 daysor stopped putting any compression on the inner ring.The free shrinkage at day 90 for each mix was in therange of 350 to 450 micro strain. Tensile stressgenerated by restrained shrinkage of the concrete aresignificant in the first week after casting and lead to afracture of the material. The role of the tensile creep inthe relaxing shrinkage stress is substantial and reducesthe stress. The SAKATA model had the closestagreement with the experimental data. The overallcomparison with the available models showed theproposed model in this study has the closest correlationwith the experimental data.

Acknowledgements

The author would like to thank TrenchlessTechnology Center of Louisiana Tech University for theuse of its facilities. Special thanks to Dr. Carlos Montesfor his suggestions and help to access the materialcharacterization lab of the micro-manufacturinginstitute of Louisiana Tech University.

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0.0E+00

2.0E+02

4.0E+02

6.0E+02

8.0E+02

1.0E+03

1.2E+03

0 60 120 180 240 300 360Shr

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icro

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in)

Time (Days)ACI B3 CEB

GL 2000 S akata 8000 GP C

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