incorporating structural steel - ubi...as reference. fornormal strength ·events·concrete...
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Developments fromCSIROT
C-Y'T'TT '. SSerdang Hospital in Kuala LumpurInterview with MSSA PresidentProfile on SSSS President
Radar as nondestructive testing tool for concreteNon-destructiveTests On ConcreteCubelnfoControls equipment for Mapei France lab
f"'f"' T{ E TECHNOLOGYConstructiVisionConcreteKioskTermacolRecycled aggregates for precast concrete
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Cone rcrc cornpressivc "trength ofviandnrd samples curing undervtnndard conditions are of urmovrimportance for quality control ofconcrete as It IS produced, Ilowever, those values can only be atveptcd as mdrcarmg the 'porcnnal"rength' of concrete and rnav <lIt"kr from the acrual 1S1 siru concretecornprcwrve vrrcngrh.
I he knowledge, a, close a,1)()'~Jblc.of the ,"-~I(Uconcretecornpressrvc vtrcngth i,. In somexuuanonv, necessary in order [0
evrrmure, for instance, t he timewhen precast concrete rucmbervcan be moved and transported, rhelime of prestressing, removal ofmould, or propping. etc,
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by Miguel C. S. Nepomuceno, Lecturer,University o[Beirn Imerior, Portugal andsergio M. R. Lopes, AssiSIlIIJl Professor,Utliversityo{Coimbra, Porwgal
concretetests on
Non-destructive
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This fact is a direct consequence of the high numberof practical factors that can affect the rebound of theincident mass, due to the difficulty of taking into account the correct influence of such factors or to reproduce similar condirions as those using on calibrarion test.
Some professionals have pointed out that even insituations where these conditions are weUreproducedin-situ and specifically developed correlations areused, the 95 percent confidence limits on the estimation of compressive strength deviate 25 percent fromthe mean value [4,6]. Under ideal laboratory condiLions it is probable that this difference would be reduced to 15percent [4.6J.
Figure 2 - LoadingSchmidt ReboundHammer.
Figure 1- Schmidt ReboundHammertype N.
Surface Hardness TestThis type of resri ng has being applied [0concrete sineethe 30'S and from the early attempts different methods and techniques have been proposed. The methodproposed by the Swiss engineering Ernst Schmidt inthe 40'S is based on the rebound of an incident masshas been recognised as the most reliable and it is stillused nowadays (3,6).
The Schmidt hammer has been used with successto evaluate the surface hardness, with special evidencefor such apparatus type N, with an impact energy of2..207 N.m <Figure I and Figure 2). Its use is coveredbyvarious national standards such as the Recommendations from British Standards BS:1881:Part202 {4)
The surface hardness res is easy to operate, onlyrequires a free surface, is cheap, and causes Lessdamage than the majority of the other non destructivetests. However its use to estimate directly the in-situcompressive strength using general established correlations is not recommended by many authors [6}.
Non-destructive tests (NUT) have been increasingly used in the last few decades for the assessmentof in-situ quality anti integrity of concrete elements,especially for estimation of in-situ concretecompressive strength. National standards are available in a number of countries, notably in the UK andUSA, detailing procedures for the most firmly established testing methods. It is important to mentionthat procedures from different national standardspresent small differences.
Non-destructive tests present some significantadvantages such as speediness of operation, immediate availabili ty of results. less costs and, above all, lessdamage for concrete elements under test. The NDTmethods arc specially designed for estirnarion of thein-situ concrete compressive strength and do not alIowa direct quanrirarivc measurement of such property. Therefore, it would be necessary to develop anduse empirical correlations. Such correlations introduceuncertainties on the results, which represent the maiodisadvantage of these tests when compared to thosemethods based on drilled cores. 10spite of that, whenbased on fum correlations, the accuracy of such estimations improves significantly and it could overcomesuch drawbacks [3].
Tbe NDT techniques actually available vary significantly in terms of accuracy; degree of destructioncaused to concrete elements, speed of test, immediacyof results and costs involved. Those methods havebeen traditionally used on normal strength concrete.Although, since high strength concrete is being increasingly used all over the world. many researchworks have been developed during rhe lase ten yearsto rest the applicability of such methods to highstrength concrete.
Tbe use of NDT methods on normal and highstrength concrete clements will be qualitatively discussed on this paper, taking as reference five differem test methods, which imply the measurement ofother five different properties. Such properties indude the measurement of the surface hardness, ultra-sonic pulse velocity, penetration resistance, pullout force and direct tensile strength (pull-off test).
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Figure 4 -Windsor Probe TestSystem.
Ultra-sonic Pulse VelocityTestThe ultra-sonic pulse velocity testis well studied and its use is generalised worldwide. This methoddocs not cause any damages onconcrete surface and it is believedto be one of the methods that canbe used to evaluarc the interiormassof concrete elements (6}.Oneof the most generalised equipmentused on concrete is the PortableUltra-sonic Non-destructive Digrral Indicating Tester (pUNDTnWithelectro-acoustical transducersof natural frequency of 54 kHz(Figure 3) [3). Recommendationsfor Its usc can be found, for example, on British Standards Institution BS:1881:Part203 [2]
The factors which affect thetest results have been studied formanyyears and, in some cases, correcrion factors were developed totake into account, for example, thepresence of reinforcement bars.The ultra-sonic pulse velocity depends on the modulus of elasticityof the material which is related [0
The experience has sbown thatthe surface hardness test can bevery useful in situations in whichcorrelations arc not needed, forexample, to monitor large surfacesfor comparative analysis and localisation of crrtical zones, whereother more reliable tests can beapplied, which unfortunatelycauses more surface damage andarc more expensive.
Penetration Resistance TestPenetration resistance tests havebeen used for many years mainlysupported by the Wmdsor ProbeTest System (WPTS) and its uschas been more generalised inNorth America [6]. USA Standards ASTM C803-90 [I] orBS:1881:Parr207[5) have been usedas reference. For normal strength
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concrete compressive strength.Despite all [he recognised practical advantages. the CPV test hassome of the problems referred torbe penetration resistance test, i.e., this rcsr doe.. not give a directreading of the concretecompressive strength, and the correlation .. are affected by a largenumber of factors. There existsome correction factors, but pracrice has demonstrated that it introduces largeuncertainties on theresult, with consequences on theaccuracy of rhc estimation value.
It has been pointed out that,even using correlauons specificallydeveloped for a given concrete under good in-situ conditions, it isunlikely that the 95 percent confidence limits for the estimation ofthe compressive strength are better than 20 percent of the meanvalue [2,6]. Using general correlation curves. thi' difference couldarise to 50 percent [2,6].Using specific correlation curves developedin the laborarory such 95 percentconfidence hmirs could dcviarc 10percent from the estimated meanvalue (2.6).
Correlation betweencompressive strength and pulsevelocity has shown exponentialbehaviour [2,6,12], which leads tothe loss of sensibility of the lest forhigh strength concrete, I. c., a little increase on pulse velocity corresponds to a great increase oncompressive strength. For this reason the applicability LO high
Figure 3 - Ultra-sonic PulseVelocity Test Equipment.
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strength concrete needs to be further investigated. A first attemptwas made by the authors [12] andthe obtained correlation curveshave shown a considerable variation on estimation of concretecompressive strength when dealingwith concrete elements ofcompressive strength of more than70 MFa. Probably, it is a naturalconsequence of exponential correlation which increases considerably at this level of strength.
As for surface hardness test,unless a specific calibration curvecan be obtained, it is virtually impossible to predict the absolutestrength of a mass of in-situ concrete by pulse velocity measurements [2,6,u]. However, this testis useful for comparative and uniformiry assessments.
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Figure 6 • Removing the drilled core InIts full 65 mm length.
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Figure 5 - Drilling a 18 mm centre hole with adrill unit.
Pull-Out TestPull-out Test is recognised. amongthe main non-destructive tests. asthat which offers more accuracy onthe estimntion of the in-situ concrete compressive strength [),6].Recommendations for its use areprovided by British Standards BS188[:Part 207 (5l
Onc of the techniques used forthis purpose is the CAPO.:rESTSystem developed by 'Germann IJ1-
struments AlS' (Figures 5 to rr), TheLoadis applied to the capo-insert bymeans of an hydraulic jack and areaction ring of 55mm inner diameter. The capo-insert (2.5mm diamerer ring) is placed at a depth of 15rom.
In the last few decades, independent analytical and experimental studies have been developed tounderstand how failure mechanismworks during the pull-out tests. Aconsensus has been achieved regarding the existence of a triaxialstate of stress highly non uniformon the concrete involved the capoinsert during extraction [6,7]. Inspite of some divergence as far as
tribute to a more extensive application of this method.
test can produce on the surface andthe high cost of ammunitions andprobes are the reasons why thismethod has not been extensivelyused [9,10,11). For this reason it isunlikely that penetration resistance replaces the surface hardnessor the ulna-sonic pulse velocitytests for comparative analysis ofconcrete.
The traditional WPT systemhas shown to be limited to concrete compressive strength of lessthan 40 MPa (on cylinders)[5,9,IO,lT]. However, the authorshave confirmed that the measurement of penetration resistance canbe extended to concretecompressive strength of at least 85MPa (on cubes of 150 mm side)using a new technique supportedby a new equipment named 'Alternative Firing Apparatus' [9.10,11].This new technique has also shownto be adequate for normal strengthconcrete offering the advantagesof reducing the damages in comparison to the rraditional WPTsystem [o.ro.n]. It is also important to note that the applicationcosts of this new equipment islower compared to the traditional\VPT system. This (act may con-
concrete the WPT system is usedwith silver coloured probes ofhardened steel alloy With 6.35 mmdiameter. 79.; mm length. bluntconical end with a plastic guide andthe standard power (Figure 4).
The WPTSystem causes somedamages on the concrete surfaceand. in some circumstances itsreparation can be necessary. Onthe other hand its application islimited by restrictions of minimumedge distances (to prevent splitting) and minimum distance between rwo probes (to avoid overlapping of zones of influence).Thismay limit the use of the techniqueIn some situations. Usually thismethod is recommended for clements of large surfaces like slabsor clements oflarge cross sections[3]·
The penetration resistancetests can be used in the situationsmentioned above for comparativeanalysis of similar concrete elements, showing as an advantage,comparatively to the surface hardness and ultra-sonic pulse velocitytests. more accuracy on estimationof the concrete compressivestrength (3].
However,the damages that this
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The Lok-tesr and Capo-testwere developed in Denmark andhave the same rest geometry andare a reference test inEurope.
Both tests have an apex angleof 61" and measure the force withwhich a 15 mm diameter discplaced at 15mm depth is extractedfrom concrete by means of an hydraulic jack reacting over bearingring of 55 mm interior diameter.placed on concrete surface.
The pull-out rest probes can beintroduced in fresh concrete applying the lok-tcst system, which requires the previous test planning.The capo-test system can be alternatively used in hardened concreteat any time, through the openingand wideness of an inside hole,where an expansive ring wirh mesame diameter at the same depthas the lok-rest system is placed
The capo-test system producessome surface damages on concreteelements and, in some circumstances, its reparation may beneeded. The lok-rest system maybe optionally applied with or wrrhout full extraction of the insert .The second option does nor ereate visible surface damages. The
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methods. Examples of which arethe excellent correlation curves forthe compressive concrete strengthversus the pull-out force; the facrthat the number of factors thataffect the results arc considerablyreduced compared to other nondestructive tests; the possibility ofthe correlation curves to be generalised to natural aggregateclasses; the cost and the procedureof this methods, which is cheapand relatively quick to operate; andfinally,(he fact that the results arcimmediately available. Its applicability has been mainly recommended in situations where a highaccuracy of results is needed [5,6,7).
Using specifically developedcorrelations (0 the studied concrete, the 95 percent confidenceLirnirs of in-situ compressivestrength estimation in a locationcould be anticipated in the [0 percent deviation interval from themean value, obtained through the4 valid readings of pull-our test[5,6). Using general correlations,such as those suggested by Loktest and Capo-test manufacturers,that interval would probably bewidened to 20 percent [5,6}.
Figure 8 - The expansion unit is inserted In the hole and the capoInsert is fully expanded to 25 mm diameter.
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the basic failure mechanism is concerned, a consensus exists regarding the fact that the last pull-ourload is influenced by the sameproperties that influence the concrete compressive strength [6,7].
.Manyfactors have contributedto the dissemination and to thegeneralised use of pull-out test
Figure 9 - Coupling and the 55 mm inner diametercounterpressure.
Figure 7 - Using the diamond recess router to open anInside hole at 25 mm depth.
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Figure 11 - Failure type Is observed for validation of resulL
minimum distances usually required for locations are limiting for small elements as for penetration resistance tests.
Figure 10 - Loading the instrument by turning theloading handle.
Pull-Off TestThe pull-off tests enable the dcrerminarion of direct tensile strength in-situ (Figures 11 and 13).The possibility ofusing this method with partial coring enables the measurement of such property ar different depths. For this restmethod, recommendations arc available on British Standards BS 1881: Parr 207 [5]. Figures I;and 16 show a technique developed by' Germann Instruments AIS' to support rhrs method and called BOND- TEST System. Thissystem Includes a portable hand operated hydraulic jack,a circular steel ruskwith 75mm diameter and 30mm thickness and the equipment for preparation of surfaces andpartial coring of concrete.
The possibihry of esrabhshing, for a given test equipment and agiven concrete. a correlation between the pulloff test and the compressive strength has shown to beuseful for estimation of the In-)I(Uconcrete compressivestrength [5,6). In spite of other practical factors that canaffect rhe results, one fact that should nor be forgorren isthar pull- off test results on the surface differ (rom thoseusing partial conng and that the depth of partial coringshould be also taken into account (5.6].
A research work carried our by the authors has shownthat the applicability of pull-out rem, using the Bondrest system, is lirmrcd to concrete clements of compressivestrength of no more then 45 MPa (on cubes of ')0 mmside) [to). It was also found OUt that rhe usc of partialcoring causes considerable surface damages and needs aconsiderable time to operate. Ilowever, partial coring ena-bles the evaluation of concrete strength at deeper layers Figure 12 - Loading the Instrument
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[I] ASTM C80)-90 - Standard Test Method for Penetration Resistance of Hardened Concrete,Philadelphia, American Society for Testing and Materials, 1990.
[21British Standard BS 1881:Part 203: 1986- Testing Concrete: Recommendations for Measurementof Velocity of Ultrasonic Pulses in Concrete, London, British Standard Institution.
(3) British Standard BS 1881:Part 201: 1986 - Tesnng Concrete: Guide to the Use of Non-dcscructiveMethods of Test for I Iardened Concrete. London, British Standards Institution.
[41British Standard BS 1881:Part 202: 1986-Testing Concrete: Rccommendations for Surface HardnessTesting by Rebound Hammer, London, Brirish Standards Institution.
(5) British Standard BS J881:Part 207: 1992- Testing Concrete. Recommendations for the Assessmentof Concrete Strength by Ncar-to-surface Tests. London, British Standards institution.
[61BUNGEY,J. H.i MILLARD, S. G. - Testing of Concrete in Structures. 3.' ed., London, Chapman& l lall, 1996.
[7] CARINO,). Nicholas - Pullout Test, to «Handbook on Non- destructive Testing of Concrete»,Florida (EUA). CRC Press Inc., I991, p. 39 - 82.
[8]KRENCHEL. H.; PETERSEN. C.G.-ln-situ Pullout Testingwith Lok-rest, Ten Years'Experience .• International Conference on In sirul Non-destructive Testing of Concrete», Ottawa, 2-5OC<.I984·
[9) LOPES, sergio; NEPOMUCENO, Miguel - A Comparative Study of Penetration ResistanceApparatus on Concrete, in «Proceedings of rCCE/4'). Hawaii, David Hui Edition, Jut 1997,p.6J5-616.
[LO]LOPES, Sergio, NEPOMUCENO, Miguel - Evaluation of In-place Concrete Strength byNearto-surface Tests. «rath European Ready Mixed Concrete Congress», Lisbon, APEB.June 1998,p. 338- 347·
en] LOPES, Sergio; NEPOMUCENO, Miguel- High Strength Concrete: Penetration ResistanceTests on High Strength Concrete, in "First International Conference on High StrengthConcrete.july 13-18,1997",ASCE, USA, 1999, ISBN 0-7844-0419- 4, p. 42,-433.
[12J LOPES, Sergio; NEPOMUCENO, Miguel- Non-Destructive Tests on Normal and I ligh StrengthConcrete, in "Proceedings of 26th Conference on Our World in Concrete & Structures, August27- 28, 2001, volume XX (20Ot)~, Singapore, ISBN 981-04-25'3-9. p. 53-67'
and probably, as a consequence ofthat, the obtained correlationcurves for normal strength concrete could show agood agreementbetween pull-off force and concrete compressive strength.
For tests performed under
Figure13- Failuretype is observed for validation ofresult.
About the authorsMiguel Costa Santos Nepomucenois a Lecturer at the University ofBeira Interior, Portugal .He has aFirst Degree inCivil Engineeringfrom the University of Beira Interior. 1994;MSc. Concrete Materials, University of Beira Interior,1999;Ph. D Srudenr; IIe is a Member of the Portuguese Associationof Engineers since T992. J lis areaof specialisation is in ConcreteMaterials.
Sergto Manuel RodriguesLopes is an Assistant Professor,University of Coimbra, Portugal.I Ie has a first Degree in Civil En-
laboratory conditions and using 6valid readings in each location, theBritish Standards BS1881:Pan 107[51 points our that the 95 percentconfidence limits for the estimation of compressive strength thevalues could vary (15 percent fromthe mean value).
gineering from the University ofCoirnbra, IC)84;MSe In StructuralEngineering, Technical Universityof Lisbon, 1988; Ph.D from theUniversity of Leeds, England, U.K.,1991.He is Senior Member of thePortuguese Association of Engineers, 1992, was Nominated forFerry Borges Prize as one of bestpapers in International Journals,1999. His area of specialisation isin Structural Concrete. He is aMember of FIP Commission 10Management, maintenance, andstrengthening of concrete structures; Mem bel' 0 f Porruguese Mirror groups for the norrnalizarion ofboth European Structural Concrete Code, Eurocodc 2, and of Eu- .ropcan Concrete Standard,ENV206; and Individual memberof AC I, Portuguese StructuralConcrete Group (PortugueseCharter of fib) and PortugueseGroup of Structural Engineering<portuguese Charter of lABSE).
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