2 rekayasa ndt akmaluddin

7/31/2019 2 Rekayasa NDT Akmaluddin

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Volume 9 No 2, Desember 2008

93

UTILISING NDT APARATUS FOR STRENGTH ASSESMENT OF CONCRETESTRUCTURAL ELEMENT

Penggunaan Peralatan NDT Untuk Menilai Kekuatan Elemen Struktur Beton

Akmaluddin *

ABSTRACT

Non-destructive test (NDT) method was preferred due to it has advantaged to solve the problem when the structural elements constructed are questionable by the client. PUNDIT was one of NDT equipment applied to measure concrete modulus of elasticity non-destructively whilst compression machine was used to obtain the modulus of elasticity of concrete destructively. For more convenient with the NDT result, it needs to validate the result with the standard test using destructive method.

Twenty seven cylinder specimens together with nine beams of 150x250x2500 mm were used. Prior to assess the beam specimens, both non-destructive and destructive test apply to the cylinder specimens for verifying the NDT equipment.

Results show that density of material affects the value of modulus of elasticity significantly.A new relationship between static modulus of elasticity, E c , and dynamic modulus of elasticity, E d ,was proposed. Applying the model proposed to assess the beam specimens produce strength varies from 0.72 to 0.90 toward strength obtain using cylinder test for normal weight concrete.However, for lightweight concrete the proposed model produced strength prediction varies from 1.13 to 1.22 toward cylinder strength.

Keywords: concrete, modulus of elasticity, compression strength, PUNDITplus, NDT

ABSTRAK

Metode pengujian dengan cara tidak merusak benda uji (NDT) lebih disukai oleh karena manfaatnya yang besar dalam mengevaluasi atau menilai kekuatan elemen struktur bangunan yang diragukan kualitasnya karena kesalahan pengerjaan. PUNDIT merupakan salah satu peralatan NDT yang digunakan untuk mengetahui nilai modulus elastis dengan cara tidak merusak benda uji sedangkan mesin tekan digunakan untuk mengukur modulus elastis dengan cara merusak benda uji. Untuk memberikan hasil yang lebih memuaskan dan meyakinkan dari peralatan NDT diperlukan verifikasi dan validasi hasil NDT terhadap hasil uji metode standar.

Silinder sebanyak 27 buah dan balok berukuran 150x250x2500 digunakan dalam pengujian ini. Sebelum pengujian dilakukan untuk mengestimatsi kekuatan benda uji balok terlebih dahulu dilakukan validasi alat tersebut pada benda uji silinder dengan menggunakan kedua metode pengujian.

Hasil menunjukkan bahwa modulus elastis sangat tergantung dari nilai berat jenis atau

kepadatan bahan. Hubungan antara modulus elastis statis, E c, dan modulus elastis dinamis, E d,diperkenalkan. Dengan menggunakan model tersebut dalam menilai kekuatan benda uji balok diperoleh bahwa hasil prediksi kekuatan balok underestimate atau bervariasi dari 0,72 sampai 0.90 terhadap hasil uji silinder untuk balok beton normal. Namun untuk balok beton ringan hasil prediksi overestimate kekuatan silinder yaitu bervariasi dari 1.13 sampai 1.22.

Kata kunci : beton, modulus elastisitas,kuat tekan,Pundit,NDT

* Akmaluddin, ST., MSc(Eng), Ph.D., Pengajar pada Jurusan Teknik Sipil Fakultas Teknik Universitas MataramSanggahan dan diskusi tentang tulisan ini harus sudah diserahkan ke redaksi sebelum 30 Maret 2009


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INTRODUCTION

Non-destructive testing (NDT) is aneffective method for quickly testing andevaluating the properties of materials, whichdoes not destroy the physical, mechanical,even chemical properties of materials and hasno influence on future performance. Thismethod of NDT is preferred because of itsdistinct advantage over the physical propertiestest. Portable Ultrasonic Non-destructiveDigital Indicating Tester (PUNDIT) is one ofthe NDT equipment specially designed fornon-destructive assessment of massivematerial. The exploitation and application ofthis technology have been quickly developedin concrete fields for its evident advantages.

In civil engineering application, thisequipment has advantaged to solve theproblem when the structural elementsconstructed are questionable by the client.Basically the equipment give real timemodulus of elasticity (MOE) reading ofmaterial tested. However, for more convenientwith the result produced by the equipmentwhen utilising it in specific structural concretematerial, it should be validated. Figure 1shows example application of the equipmenton beam structural element of Mataram MallCar Park.

Figure 1. Application of Pundit on concrete beam elementBy the contractor request, the

equipment was applied for assessment of thecar park building element due to constructiondoubted as the material used to perform theelement did not compliance with specificationof the concrete strength determined. Beforeutilising the equipment, it has been donetesting on laboratory prior to test existing ofbeam specimens.

In this paper however, the primaryobjective of this study is to investigate thedynamic MOE of normal weight concrete(NWC) and lightweight concrete (LWC) beamobtained from Pundit aparatus in laboratoryonly.

In the present study, the difference andrelationship between dynamic MOE and staticMOE were analyzed and the accuratenessand reliability of MOE evaluated by the NDTtechniques were discussed. The findings ofthis study can provide scientific references forquickly testing concrete structure.

LITERATURE REVIEW

Physical properties of concrete can bedetected by, for example the speed of anultrasonic pulse propagation through theconcrete. The application of ultrasonic pulsevelocity (UPV) to the non-destructiveevaluation of concrete quality has been widelyinvestigated. However, their effects on theultrasound and the relationship betweencompressive strength and UPV have receivedlittle attention (Tanyildizi and Coskun, 2007).The pulse velocity can be determined from thefollowing equation (BS 1881-203, 1986)

= .....................................(1)

where V is pulse velocity in km/s, S is path

length and t represent transit time ( s).The MOE, one of primary indexes inevaluating mechanical properties of concrete,indicates the degree of concrete resistingdistortion. A higher value of MOE indicatesthat the material is not easy to be distortedand has a high rigidity. A prediction model ofMOE using NDT technique has beendeveloped (Neville and Brokes, 1987). TheMOE increases more rapidly than strength.The MOE of lightweight aggregate concrete isusually between 40 and 80 per cent of theMOE of normal weight concrete of the same

strength, and, in fact, is similar to that of thecement paste.The MOE obtained destructively using

standard test in laboratory namely static MOE, E c, whilst dynamic MOE, E d, obtained fromnon-destructive test. The PUNDITplusequipment is developed with consider to someparameters such as path length, density andpoissons ratio and dynamic MOD, Ed, isgiven by equation below (BS 1881-203, 1986;CNS Farnel Ltd, 2006).


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= 2 1 + 1 2 /(1 ) .......(2)

where, = density, v = velocity and =poissons ratio.

The relationship between static and dynamic

modulus of elasticity is given in the equationbelow (Nevile and Brokes, 1987).

= 1.25 19 ....................(3)

where E c and E d are expressed in GPa. Therelation does not apply to concrete containingmore than 500 kg of cement per cubic metreof concrete.When it is required to relate the dynamicmodulus to strength, the static modulus maybe estimated using equation (3) andsubstituted into either equation (4a) for normalweight concrete or equation (4b) forlightweight concrete where applicable.

= 4700 .........................(4a)or

= 0.75 4700 .................(4b)

where E c and f

c are expressed in MPa.Modulus of elasticity obtained from cylinderstandard test can be obtained from thefollowing equation.

= 2 1 2 0.00005 ...........(5)where S2 is stress about 40% of ultimate

stress (0.4 f c), S1 is stress at strain value of0.00005 and 2 is a strain value at the levelstress of S2.

METHODOLOGY

Test specimens Nine beams of 150x250x2500 mm

reinforced with three different reinforcementratio were prepared to be measured theirmodulus of elasticity. Three groups of cylinderspecimens of 150 x 300 mm length taken fromthe beam concrete mixture were used withthree different mix proportions. Each groupconsisted of nine specimens from each batchof the concrete mixture. The cylinders weretested at age 28 days after water curing.Table 1 presents detail mix proportion toproduce two normal weight concrete of 17 and30 MPa and a lightweight concrete of 17 MPaas refers to ACI 211.2-98.

Test procedure Prior to destructive testing using UTM

machine, specimen was scaled and testednon-destructively using Pundit equipment.Figure 2 shows the application of the Pundit

plus equipment to predict MOE of cylinderspecimen. The equipment display value ofMOE in GPa.

Table 1. Mix Proportions for 1 m 3 concretew/c

PC(kg)

Water(kg)

Sand(kg)

Gravel(kg)

Pumice(kg) ID

0,58 327 190 810 1073 -NWC

0,45 422 190 715 1073 -

0,40 507,5 203 467,23 - 382,3 LWC

Figure 2. Dynamic Modulus of elasticity, E d , measurementusing Pundit aparatus

Figure 3. Measurement of Static Modulus of elasticity, Ec.

In addition, compression test were done usingStandard Compression machine as shown inFigure 3 produced stress and strain


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relationship. From the relationship the MOEcan be generated by applying equation (5).Finally, to asses strength of beam, thePUNDIT was applied by direct transmissiontechnique to surface of the beam in threeplaces as shown in Figure 4 below.

Figure 4. Strength assessment of beam using PUNDITaparatus

RESULTS AND DISCUSSIONS

Strength test results obtained fromdestructive test on cylinder specimen ofnormal weight and lightweight concrete arepresented in Figure 4. From the figure it canbe seen that normal weight concrete producehigher strength than lightweight concrete. This

is caused by coarse aggregate used toperform normal weight concrete has specificgravity higher than pumice as lightweightcoarse aggregate. From the stress and strainrelationship as shown in Figure 4 it can becalculated modulus of elasticity (MOE) usingequation (4).

Figure 4. Typical relationship of Stress-Strain cylinderspecimen.

Cylinder specimens of normal weightand lightweight concrete were tested. Firstly,non-destructive test method was appliedproducing dynamic modulus of elasticity, E d followed by destructive test producing staticmodulus of elasticity E c. Both test results are

compared and presented in Figure 5 below.

Figure 5. Comparasion between Static and dynamic Ec

From the figure it can be seen thatthere is a linear relationship between E c and

E d. For more convenient the relationship ispresented as Equation (5). This equationproduced results with trend similar to resultsproduced by the British Standard as givenpreviously by Equation (3).

= 1.038 11,45 ....................(5)

For more comprehensive discussion the testresults obtained by both test method ispresented in Figure 6. Figure 6 shows that twotypes of concrete specimen of lightweightconcrete (LWC) and normal weight concrete(NWC) were tested using static and dynamictest method.

Figure 6. Concrete modulus of elasticity against density

0

5

10

15

20

25

30

0.0000 0.0010 0.0020 0.0030

S t r e s s

( M P a

)

Strain

NWC

LWC

5

10

15

20

25

30

35

10 20 30 40 50

E c

( G P a

)

Ed (GPa)

E c =1.038 E d -11,45

0

5000

10000

15000

2000025000

30000

35000

40000

45000

1000 1500 2000 2500 3000

E ( M P a

)

Density (kg/m 3)

Ed

Ec

Ec, Eq (3)

LWC

NWC

LeftEnd

Middle RightEnd

PunditsTransducer


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Table 1. Results of Beams assessment using PUNDITplus

BEAMSPECIMEN

Ed (MPa) Ec(MPa)

f'c(MPa) 1

f'c(MPa) 2 RatioLeft End Middle Right End Average

(1) (2) (3) (4) (5) (6) (7) (8) (9)=(7)/(8)NWC17 2 34000 32000 34000 33300 23100 24,16 28,50 0,85

NWC17 3 32500 34900 35000 34100 23900 25,86 29,02 0,89NWC17 5 33000 34000 32000 33000 22800 23,53 29,25 0,80NWC30 2 34500 38300 35000 35900 25800 30,13 40,69 0,74NWC30 3 33800 35500 37400 35600 25500 29,44 41,05 0,72NWC30 5 34400 43400 33000 36900 26900 32,76 36,23 0,90LWC17 2 25000 26400 27400 26300 15800 20,09 17,83 1,13LWC17 3 26700 28000 26000 26900 16500 21,91 17,90 1,22LWC17 5 26800 25200 27200 26400 16000 20,60 18,11 1,14

Note: 1. PUNDIT assessment2. Cylinder test

In all cases dynamic test method producedhigher value of E than the static one.However, both methods have similar trendwhich is increasing as concrete densityincreased. This result suggested that densityof the concrete affect the values of modulus ofelasticity. Therefore it is reasonable to usePUNDIT plus for assessing concrete beam.

Three places on beam surface as shown inFigure 4 were scanned by the equipmentproducing results ( E d) as given in column (2),(3) and (4) for left end, middle and right end of

the beam respectively. The average value ofthe results was taken to represent dynamicMOE of the beam as given in column (5) ofTable 1. In addition, Equation (5) was used toobtain E c values and results presented incolumn (6) of Table 1. Furthermore, thestrength of concrete beam was obtained byapplying equation (4) and results shown incolumn (7). The strength values werecompared with the strength obtained fromcylinder test (column (8) Table 1) andrepresented in ratio between strength obtainusing PUNDIT and the test cylinder as given

in column (9) of Table 1.

From Table 1, it can be seen that the strengthprediction of the beam using PUNDITplus fornormal weight concrete, gave value lowerthan the strength value produce usingstandard test. However, for light weightconcrete produce over estimate predictionwhen compare to cylinder test results.

The different result showed in Table 1between column (7) and (8) is due to differentobject tested ie beams and cylinder

specimens respectively. Although the beamshave similar mix proportion to cylinderspecimens, however treatment given to thecylinder and the beam was different especiallyin compacting the specimens as a results thedensity could be different. Therefore, thevalue of MOE obtained from the beam testedgive more realistic value than the valueobtained from the cylinder test, because thevalue obtained has considered straightforwardthe density of the beam.

CONCLUSIONS AND RECOMMENDATIONS

The following conclusions are drawn from thestudy:1. The values of MOE rely on density of the

specimen tested. The more value of thedensity the more modulus of elasticityproduced.

2. Strength prediction of the beam studiedvaries from 0.72 to 0.90 toward cylinderstrength for NWC but varies 1.13 to 1.22for LWC.

3. Strength prediction using PUNDIT fornormal weight concrete underestimatethe strength given by the standard test.

4. Strength of lightweight concreteevaluated by PUNDIT overestimated thestrength obtained using standard test.

For more comprehensive evaluation it isneeded to study more specimens to improvethe model proposed.

REFERENCES

ACI Committee 211, Standard Practice for Selecting Proportions for Structural


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Lightweight Concrete (ACI 211.2- 98) , American Concrete Institute,Farmington Hills, MI, 20 pp.

BS 1881-203, 1986 , Testing concrete. Recommendations for measurement of velocity of ultrasonic pulses inconcrete, British Standards Institution.

Farnel, CNS, 2006, Manual instruction of PUNDITplus, CNS electronic ltd.

Neville A.M., Brooks, J.J., 1987, Concrete Technology , Longman

Tanyildizi, H. And Ahmet Coskun, 2007,Fuzzy logic model for prediction of compressive strength of lightweight concrete made with scoria aggregate

and fly ash , International earthquakesymposium Kocaeli.

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