nondestructive assessment of the actual compressive strength of high-strength concrete
TRANSCRIPT
Nondestructive Assessment of the Actual CompressiveStrength of High-Strength Concrete
Giovanni Pascale1; Antonio Di Leo2; and Virna Bonora3
volving50 MPa.ods. Theeping theanged forystems areensitivity,
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Abstract: This paper deals with nondestructive testing of high strength concrete. An experimental program was carried out, inboth destructive and nondestructive methods applied to different concrete mixtures, with cube strength varying from 30 up to 1Relationships were derived for pulse velocity, rebound hammer, pull-out, probe penetration, microcoring, and combined methresults show good behavior for some methods, like pulse velocity, rebound hammer, and combined SonReb methods. By keanalytical form of the relationship usually adopted for normal-strength concrete unchanged, only the constants have been chthese methods. Some problems arose with the pull-out and probe penetration tests, for which the available commercial testing snot adequate at very high strength levels. The relationships for the various methods were compared in terms of dimensionless sfor different strength levels. The results showed decreasing sensitivity, with increasing strength.
DOI: 10.1061/~ASCE!0899-1561~2003!15:5~452!
CE Database subject headings: High strength concretes; Nondestructive tests; Compressive strength.
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Introduction
The evaluation by nondestructive methods of the actual comprsive strength of concrete in existing structures is based on empcal relations between strength and nondestructive parameters~DiLeo et al. 1983, 1984; Carino 1993!. Manufacturers of devicesusually give empirical relationships for their own testing systemSuch relationships are not suitable for every kind of concreTherefore they need to be calibrated for different mixtures.
The most commonly used testing methods are rebound nuber, pulse velocity, pull-out, probe penetration, small-diamecores, and combined methods. The validity of the above metioned relations is actually limited to normal strength concrete,to 50 MPa, due to the absence of tests on higher strength ccrete.
High-strength concrete~HSC! has been employed in recenyears, with compressive strength in excess of 150 MPa. Thelationship functions currently used to evaluate the compressstrength of normal strength concrete by nondestructive tests mnot be valid for HSC. Also the ACI 228.1R-95~1995! gives somesuggestions, dealing in particular with some methods and with
1Associate Professor, DISTART—Univ. of BolognaViale Risorgimento, 2-40136 Bologna, Italy. E-mail: giovanni.pascalemail.ing.unibo.it
2Associate Professor, DISTART—Univ. of BolognaViale Risorgimento, 2-40136 Bologna, Italy. E-mail: [email protected]
3Research Assistant, DICASM—Univ. of BolognaViale Risorgimento, 2-40136 Bologna, Italy. E-mail: [email protected]
Note. Associate Editor: Roberto Lopez-Anido. Discussion open unMarch 1, 2004. Separate discussions must be submitted for individpapers. To extend the closing date by one month, a written request mbe filed with the ASCE Managing Editor. The manuscript for this papwas submitted for review and possible publication on May 23, 200approved on December 12, 2002. This paper is part of theJournal ofMaterials in Civil Engineering, Vol. 15, No. 5, October 1, 2003.©ASCE, ISSN 0899-1561/2003/5-452–459/$18.00.
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limitation of some statistical parameters, in order to determineacceptability of the methods.
Specifically, in 5.1.2 and in Table 5.1, the ACI 228~1987!gives some strength limitations, but it points out that these ranare approximate and may be extended if the user can shoreliable strength relationship at higher strengths.
The bond at the matrix-aggregate interface is very goodsuch concrete~Iravani 1996; Pascale et al. 1999!, and this couldmodify the response and the sensitivity of nondestructive pareters to strength variations~Di Leo et al. 1984; Pascale and DLeo 1984b; Pascale et al. 2000!.
The following observations can be made for the differmethods.
Pulse Velocity
The method is based on measuring the velocity of compresstress waves~P-waves!. The pulse velocity is related to Youngmodulus of elasticity by the well known law~Di Leo 1981!
VP5AEd
rf ~n! (1)
whereVP5velocity of compressional stress waves;Ed5dynamicYoung’s modulus of elasticity;r5mass density;n5Poisson’sratio; andf (n)5function dependent on the shape and dimensiof the solid~Di Leo 1981!.
The possibility of correlating the pulse velocityVP to the com-pressive strengthRc is a consequence of the above physical lain conjunction with the empirical relation between moduluselasticity and compressive strength of the concrete~Pascale andDi Leo 1984a!. In a concrete without macroscopic defects, puvelocity can be affected slightly by defects. So we can statein HSC, defects have a high influence on the compresstrength, but low influence on pulse velocity.
Rebound Hammer
The rebound indexI r is the ratio between the returned amountenergyWr and the incident energyW0 . The dissipated energ
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Fig. 1. Compressive strength of cores versus diameter
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WP , which is equal to the difference betweenW0 and Wr , isconsumed in damaging the material around the rod in contwith the concrete.
We can distinguish diffused damage~ductile! from concen-trated damage~brittle!. The first is predominant for normastrength concrete, the second one for high strength concrete~Pas-cale et al. 1999; Dong and Keru 2001!. As a consequence, therebound index for a HSC could be expected to be only slighgreater than for a normal strength concrete.
Probe Penetration Method
The empirical relation between the exposed probe length andcompressive strength is expected to be different from the relatused for normal strength concrete. In general, the greater parthe energy consumed during the penetration of the probe is dipated, in a considerable volume of material near to the probecrush the aggregate, the cement paste, and to cause the failuthe interface.
In HSC, the mechanical properties of the cement paste andcharacteristics of the interface are greater than in normal strenconcrete, and the defectiveness is much lower. As a result,material is stronger, but more brittle and more sensitive to defe
For this reason, it is important to test the method, in orderevaluate the possibility to extend its use to HSC.
Pull-Out Method
In case of use of postinstalled undercut inserts, the operatineeded for drilling and milling the hole and expanding the insemay cause geometric discontinuity and stress concentrations.to the brittleness of the high-strength concrete, cracking moriginate and propagate in a different way, compared with normconcrete. For this reason the ACI 228.1R-95 takes into accoonly the pull-out method with metal inserts with enlarged heapreviously embedded in the concrete.
Small Diameter Cores
The method is based on the size-effect empirical curve shownFig. 1, where Bs5diameter of a normal diameter coreBm5diameter of a small diameter core.
For normal strength concrete, experimental research~Yip andTam 1995! showed that a diameterBm528 mm is most suitableto give the same compressive strength of a normal diameter cThis value could be inadequate for HSC, because the brittlehavior could give rise to a different crack pattern.
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In this paper, an experimental program is presented, aimeverifying the possibility of applying the known nondestructivtesting ~NDT! methods to HSC, to determine the limits of thtesting equipment available, and to extend the relations availin literature, or to determine new ones between nondestrucparameters and compressive strength of HSC.
A concrete with fixed constituents has been evaluated. Difent mixtures have been designed, resulting in a compresstrength range as wide as possible. This has been achievevarying the water/cementitious materials~w/cm! ratio and the cur-ing time. Thus each nondestructive testing method used is inenced mainly by strength variations and curing time.
Sensitivity Analysis of Nondestructive Testing
The sensitivity of a nondestructive method is the capability ofmethod to estimate strength variations.
Different analytical formsRc,s (x), which are continuous functions with continuous derivative, are usually assumed to estaba relationship between the estimated compressive strengthRc,s
and a nondestructive parameterx.If the experimental values are included within the followin
intervals: DRc5interval of values of the cube compressivstrength; andDx5interval of values of the nondestructive parameter, the sensitivityd of a nondestructive parameterx to strengthvariations can be expressed in a dimensionless form~Di Leo et al.1984; Pascale and Di Leo 1984b!
1
d5
dRc,s
dx•
Dx
DRc(2)
According to the considerations previously shown for the variomethods, the values ofd are expected to be different for HScompared with those of normal concrete.
Experimental Program
Materials
The materials used in this investigation and their characterisare summarized here.• Cement: High early-strength portland cement CEM I 52.
conforming to the Italian standard UNI EN 197/1,• Supplementary cementitious material: Silica fume, in the fo
of powder with the characteristics shown on Table 1,• Fine aggregate: Commercial, locally available sand was u
Based on petrographic analysis, 94% of the fine aggregatecomposed of limestone. The fineness modulus, density~on asaturated and surface-dried basis! and water absorption wer2.67, 2.70 g/cm3 and 0.23%, respectively,
• Coarse aggregate: Commercial, locally available crushed saggregate with a nominal maximum aggregate size of 1mm was used~Walker and Bloem 1960!. Based on petro-graphic analysis, 95% of the coarse aggregate was compof limestone. The fineness modulus, density~on a saturatedand surface-dried basis!, and water absorption were 6.78, 2.7g/cm3 and 0.71%, respectively, and
• Admixture: A new high-range water-reducer based on cboxylic ether polymer with long side chains was used.
Mixture Proportions
Ten concrete batches were prepared with five different w/cmtios. The amount of silica fume was 11% of the mass of cem
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Table 1. Silica Fume Characteristics
Chemical composition Physical characteristics
SiO2 Minimum 85% Fineness Grading 100%,100 mm
CaO ;1% BET Spec. surface area 22 m2/gMgO Maximum 1% Density 2.2 kg/dm3
Al2O3 Maximum 1% Apparent density 0.2 kg/dm3
Fe2O3 ;2%
Na2O 0.50%
Carbon ;2%Loss of ignition Maximum 5%Moisture content Maximum 1%
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Two batches were made for each ratio, at different times. Thiswas aimed at taking into account the natural variability of theproduction process. The concrete mixture proportions are summa-rized in Table 2.
Mixture 1 was chosen as the reference mixture and the otherswere obtained by varying the w/cm ratio and, as a consequence,the nominal strength.
The water content was kept constant and the amount of ce-mentitious material was changed to affect the w/cm ratio.
The actual mix proportions were then determined. The actualvalues of the w/cm ratio are shown in Table 3.
The large differences for Mixtures 4 and 5 were caused by thedifferent moisture of the aggregate.
Test Specimens and Schedule
For each mixture, different specimens were prepared, as follows:1. Thirteen 150 mm cubes for density, pulse velocity, rebound
hammer, and standard compressive tests at different ages;2. One 60032003200 mm prism and one 40032003200 mm
prism, which were used for pull out, probe penetration, andpulse velocity tests at different ages; and
3. One 200 mm cube used to drill small core samples of 28 and50 mm diameter; the cores were drilled perpendicular to thecasting direction; the height of the cores after cutting wasequal to the diameter. The actual dimensions of specimenswere taken into consideration in calculating the compressivestrength.
The following tests were conducted at each age and for eachbatch:• Compression tests on two cubes,• Rebound index and pulse velocity on cubes,• Compressive tests on 12 small core specimens of 28 mm di-
ameter and five specimens of 50 mm diameter,• Three penetration tests on prisms, and• One pull-out test on prisms.
Only one pull-out test was done for each location, becauseaim of the experimental program was to obtain a preliminaverification of the applicability of this testing apparatus to HSC
Batching, Casting, and Curing
A horizontal rotary drum mixer with 50 dm3 capacity was used.The interior of the mixer was wetted before mixing. First, coarsaggregate, fine aggregate, cement, and silica fume weremixed and then water and admixture were added. Mixing wcontinued for about 5 min. Slump tests were done accordingItalian standard UNI 9418~UNI EN 12350-2, ASTM C 143!. Thecubes and the prismatic blocks were cast in steel molds. All spemens were compacted on an standard vibrating table for 2 mThe molds were wrapped in polyethylene sheets. After 24 h, tspecimens were removed from the molds and transferred tocontrolled environment room for curing. A curing regimen of 262°C and 7265% of relative humidity was used. The tests wercarried out at 1, 3, 7, 28, and 90 days. Two 150 mm cubes wecured in water for standard compressive testing at 28 days.
Nondestructive Tests and Equipment
The following testing equipment was used for nondestructive teing.
Pulse Velocity TestA microseismic analyzer was utilized, with the following characteristics:• Piezoelectric transducers: Either emitting or receiving, with 3
mm diameter flat active surfaces,• Operator-adjustable pulse repetition rate: Avoiding reverber
tion and resonance which would lead to disturbed measuments or to the impossibility of carrying them out,
• Pulse frequency: This was set at 70 kHz, being the resonfrequency of the emitting and receiving transducers,
2
Table 2. Nominal Concrete Mixture Proportions
Mix 1/1–1/2 2/1–2/2 3/1–3/2 4/1–4/2 5/1–5/
Water/~cement1silica fume! 0.21 0.26 0.30 0.35 0.40Water ~kg/m3! 126 126 126 126 126Cement~kg/m3! 540 436 378 324 284Silica fume~kg/m3! ~powder! 60 48 42 36 32Fine aggregate~kg/m3! 467 673 775 904 940Coarse aggregate~kg/m3! 1,367 1,273 1,232 1,190 1,166High-range water-reducer~L/m3! 17.9 14.5 12.5 10.7 9.4
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Table 3. Actual Values of Water/~Cement1Silica Fume! Ratio
Mix 1/1–1/2 2/1–2/2 3/1–3/2 4/1–4/2 5/1–5/2
Water/~cement1silica fume! 0.21–0.21 0.26–0.27 0.47–0.31 0.50–0.35 0.44–0.
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• Pulse energy: The instrumentation was designed to power temitting transducers at various energy levels from 0.25 to 5mJ, according to the absorbing characteristics of the specimand trajectory lengths,
• Amplification of received signal: Amplification varying be-tween216 and180 dB, in 1-dB steps, was used for the electric signal of the receiving probes. The amplification value waboth digitally displayed to the operator and made availableelectric output for recording equipment during measuremenand
• Transit time and intensity variations: The values of transit timand intensity were displayed by the unit, and the cathode rtube of an oscilloscope was used for continual display of timfunction vibrations, as received by the probe and suitably amplified.
The tests were done according to the Italian standard UNI 95~1989!.
Rebound TestA standard Schmidt rebound hammer Type N was used, andtests were done according to the Italian standard UNI 918~1988!.
Pull-Out TestThe commercial apparatus Fischer TCP was used, with poinserted, forced expansion inserts, according to the Italian stadard UNI 10157~1992!. This testing method deals with pullingout a steel expansion rod after concrete drilling and milling.
Probe Penetration TestThe Windsor Probe System was used, according to ASTM803M-97.
MicrocoringA Hilti DCM-1 drilling machine was used to obtain the coresfrom the 200 mm cubes. An Italian Remet MT70/LS3 apparatuwas used to cut and grind the ends of the cores. A standard laratory testing machine was used to do the compressive strentests@UNI 10766 ~1999!#.
Fig. 2. Cube compressive strength versus time
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Experimental Results and Comments
The development of the cube strength for the mixtures testeplotted in Fig. 2. The hyperbolic function presented in Brans~1977! for the strength development curve appears to be aadequate for HSC.
In order to obtain relationships between nondestructiveparameters and cube strength, all results, for all mixturesages, were treated together. This was possible, because onltest age and mixture proportions were varied. For each relatship, the independent variables refer to nondestructive pareters, and the dependent variables to the estimated cube stre
The regression curves were represented by the followpower function:
Rc,s5axb (3)
whereRc,s5estimated cube strength~MPa! andx5nondestructiveparameter typical of the method used.
The coefficienta and the exponentb were determined by tak-ing the natural logarithms of both the independent and the depdent variables and using the ordinary least-squares method totimate the slope and the intercept. The 90% confidence limwere determined for each method.
The values of the rebound index and the pulse velocity wcorrelated to the compressive strength separately, as well acombination, according to the ‘‘SonReb’’ method. In the secocase, the following form was adopted for the relationship:
Rc,s5aI rbVP
g (4)
All the experimental results are plotted in Figs. 3–12 togethwith the plots of the best-fit curves and the 90% confidence limThe following analytical forms of the relationships were obtaine
Rebound hammer
Rc,s52,39231024I r3,299 (5)
whereI r5rebound number.Pulse velocity
Fig. 3. Cube compressive strength versus pulse velocity
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Fig. 5. Relationship between measured and estimated cube stre
Fig. 4. Cube compressive strength versus rebound index
Fig. 6. 3D plot of cube compressive strength estimated by SonRmethod
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Rc,s51.942310227VP7,776 (6)
whereVP5P-wave pulse velocity~m/s!.Pull-out method
Rc,s52.85931022P1,537 (7)
whereP5pull-out pressure~bar!.Probe penetration method
Rc,s52.45L4,484 (8)
whereL5exposed probe length~in.!.Small diameter cores
Rc,s529.3310.614Rc,28 (9)
Rc,s519.8910.763Rc,50 (10)
whereRc,28, Rc,505compressive strength ofB 28 mm andB 50mm cores, respectively~MPa!.
Pulse velocity-rebound hammer combined method
Rc,s51.691310219I r1,704VP
4,839 (11)
We can observe that the exponents in the above relationshiare generally higher, compared with the values reported in literature for normal concrete~Malhotra 1984; Pascale and DiLeo
th
Fig. 8. Concrete cracking after pull-out testb
Fig. 7. Cube compressive strength versus pull-out pressure
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1984b!. This results in lower sensitivity of nondestructive tesparameters to strength variations, when the strength is very hig
In Figs. 3 and 4 the relationship of pulse velocity and rebounnumber versus compressive strength are plotted, for the whrange of the experimental results. A lot of results are locatedthe zone of the highest strength values.
It is commonly accepted that the relationship obtained with thtwo methods used separately becomes more reliable whenmethods are combined, which is known in Europe as the SonRmethod. The results obtained are plotted in Figs. 5 and 6.
For the other test methods, it was not possible to use all texperimental results, for different reasons. In particular• The pull-out method~see Fig. 7! showed good performance
for the entire strength range, but it was necessary to discasome results, because of anomalous cracking around the ting area~Fig. 8!, and
• The probe penetration method gave unreliable results fstrength values greater than about 100 MPa, as can be seeFig. 9. The probes did not penetrate correctly and in mocases were bent or the prism was cracked~Fig. 10!.
With regard to small diameter cores, drilling, cutting, and engrinding are very important and they can greatly affect the resulAt first, a conventional apparatus, suitable for normal streng
Fig. 10. Cube compressive strength versus exposed probe leng
Fig. 9. Concrete cracking after probe penetration test
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concrete, was adopted for drilling, cutting, and end grinding, buit did not work correctly because of the brittleness of HSC. Inparticular• Edge breaking occurred during cutting,• The damage during drilling was very significant, and• The faces were not sufficiently flat.Thus a more suitable apparatus, designed for mechanical testsrocks, was used.
For this reason, the results plotted in Figs. 11 and 12 refer onto the mixtures for which this new apparatus was used. A linearelationship appeared to be suitable for this method.
In Fig. 13 the plots of the dimensionless sensitivity are shownas a function of concrete strength. The curves have been obtainaccording to Eq.~2!, merging the results of previous research~Pascale and Di Leo 1984b! with those of the present one. Thevalues ofDRc and Dx in Eq. ~2! refer to the whole range ofstrength and of ND parameters. It is seen that the sensitivity dcreases with increasing compressive strength, and that the curbecome closer together for HSC.
Conclusions
Relationships have been established, in order to verify the posbility to extend the use of the most commonly used nondestrutive testing methods to high-strength concrete. It is concluded th
Fig. 11. Cube compressive strength versus small diameter core copressive strength~B 28 mm!
Fig. 12. Cube compressive strength versus small diameter core copressive strength~B 50 mm!
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• The pulse velocity method, as well as the rebound hammerthe combined SonReb method, are suitable for HSC,
• The working range of the probe penetration method, foremployed configuration of the commercial test system us~silver probes and standard power!, can be extended up tostrength values of about 100 MPa,
• The pull-out method, with the instrumental apparatus usedthis research, can be used for HSC; it is noted that the distaof the testing area from the edges of the element under tesmust be sufficiently high, to avoid spalling or cracking,
• The effectiveness of small-diameter cores depends onpreparation of the specimens; the relationships are linearthe correlation is good, in particular for the 50 mm diamecores, and
• For all NDT methods, the sensitivity decreases with increasstrength levels; this can affect the variation coefficient of testimated strength, for HSC.
Acknowledgments
The technical staff of the Experimental Laboratory for thStrength of Construction Materials at the University of Bolognnamely, Mr. Roberto Carli, Mr. Davide Betti, and Mr. GregoriBartolotta, is gratefully acknowledged. Graduate students MaBallestrazzi and Michele Colangelo are acknowledged for thcollaboration and active participation in the experimental woThe assistance provided by Livabeton S.p.A.~Bologna, Italy! andMAC S.p.A. ~Treviso, Italy!, which supplied the materials, isgratefully acknowledged. This research was supported financiby the Italian Ministry of University and Scientific Researc@MURST Contract No. 9708165746I006#.
Notation
The following symbols are used in this paper:Ed 5 dynamic Young’s modulus of elasticity~MPa!;
f (n) 5 function depending on shape of solid and on stressstate;
I r 5 rebound index;L 5 exposed probe length~in.!;P 5 pull-out pressure~bar!;
Rc 5 concrete cube compressive strength~MPa!;Rc,s 5 estimated compressive strength~MPa!;
Fig. 13. Relative sensitivity of nondestructive methods versus cucompressive strength
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Rc,28 5 compressive strength of small-diameter coresB28 mm ~MPa!;
Rc,50 5 compressive strength of small-diameter coresB50 mm ~MPa!;
VP 5 velocity of longitudinal pressure waves~m/s!;x 5 nondestructive parameter;d 5 sensitivity of a nondestructive parameter;n 5 Poisson’s ratio;r 5 mass density~kg/m3!;
Bs 5 diameter of a standard core~mm!; andBm 5 diameter of a small diameter core~mm!.
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