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Revue Française de Génie CivilPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tece18
Is laboratory concrete representative of building siteconcrete?Jean-Luc Clément a & François Le Maou aa Laboratoire Central des Ponts et Chaussées Division Bétons et Composites CimentairesSection Comportement Mécanique et Modélisation , 58 boulevard Lefebvre, F-75732,Paris cedex 15 E-mail:Published online: 04 Oct 2011.
To cite this article: Jean-Luc Clément & François Le Maou (2002) Is laboratory concrete representative of building siteconcrete?, Revue Française de Génie Civil, 6:3, 373-382, DOI: 10.1080/12795119.2002.9692372
To link to this article: http://dx.doi.org/10.1080/12795119.2002.9692372
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RFGC – 6/2002. Fiabilité : conception, maintenance, pages 373 to 382
Is laboratory concrete representativeof building site concrete?
Consequence on the creep prediction
Jean-Luc Clément — François Le Maou
Laboratoire Central des Ponts et ChausséesDivision Bétons et Composites CimentairesSection Comportement Mécanique et Modélisation58 boulevard LefebvreF-75732 Paris cedex 15
{clement; [email protected]}
ABSTRACT. The use of probabilistic methods to analyse the reinforced or prestressed concretestructures long term behaviour experimental data on material properties. The aim of thispaper is to present the main experimental results obtained at the LCPC after two years ofconcrete creep and shrinkage measurement. We present first some results about thecompressive strength of concrete in a real case and in the laboratory, then the experimentalprocedure, the statistic treatment and the main result related to the long term complianceestimation for some concrete.RÉSUMÉ. L’application des méthodes probabilistes à l’analyse du comportement desstructures en béton armé ou précontraint nécessite, outre l’identification des variables duproblème étudié, l’introduction de données statistiques relatives au comportement physiquedes matériaux. C’est en particulier le cas des données relatives au fluage en compression etau retrait des bétons. L’objectif de cet article est de présenter les principaux résultats d’uneétude de répétabilité sur la résistance à la compression et la complaisance de fluage propre,effectuée récemment au LCPC. Il s’agit d’abord de comparer les variabilités en laboratoirepar rapport à celles de chantier à l’aide de mesures de résistances à la compression, puisd’estimer la complaisance à long terme, affectée d’une précision parfaitement définie demanière statistique.
KEYWORDS: Concrete, creep, shrinkage, experiment, repeatability, statistic treatment,compliance.
MOTS-CLÉS : Béton, fluage, retrait, essai, répétabilité, traitement statistique, complaisance.
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1. Introduction
The knowledge of the concrete long term behaviour is of fundamentalimportance for reinforced or prestressed concrete structures:
– for the works under construction, it is often necessary to be able to estimatewith sufficient accuracy the value of the short-term deflection. The concrete will becharged at the youth, on low levels of variable stresses;
– the analysis of the long term behaviour starting from observations carried outon existing structures generally requires a recalculation, which must make it possibleto qualify the state of the structure, and to define for example its repair orstrengthening.
One of the difficulties of these analyses results to the fact that the mechanicalproperties of concrete are changing with the time: its mechanical characteristicsdepend on its age, of the variations in internal and external temperature andhygrometry.
For the new structures, the clients of relevant authorities in general order longterm tests on concrete samples made with the real composition from the concrete.These tests require an important and expensive environment and equipment. Twocouples of creep/shrinkage tests are realised per load level, one in endogenouscondition (the samples are protected from drying) and the other in conditions ofdrying (exchange of moisture between the sample and outside), over a durationvarying between 6 months an 1 year. Design rules models as those of [AFR 96] arethen identified on these experimental curves and some extrapolations are thenpossible.
But up to now, we did not have any idea of the repeatability of the shrinkage andcreep tests. Thus, some calculations, like those recommended by the SETRA orbased on probabilistic methods [HEI 98] can be done only starting from dispersionestimations which are not fully justified.
Other difficulties still emerge for old structures: the variability of the concretecasting and consequently the concrete distribution in the structure are unknown.
The only data relate to the characteristic strength aimed and controlled ofconcrete at 28 days, but not the variability of this resistance in various parts of thestructure. As the long term behaviour of concrete is connected to its strength, it willbe difficult to really estimate the global behaviour of the structure. With that thevariations in temperature are added (face exposed to the sun or the shade), thevariable hygrometry conditions during cycles day/night and seasons.
Some studies are considered today starting from concrete samples taken on realstructures and tested in creep at various levels of load, this in parallel with creeptests on samples made from materials of origin. In this case, it is necessary to definethe minimal number of tests which can make it possible to answer the problem
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Concrete laboratory 375
arising: to be able to explain the value of real displacements observed on thestructure and to estimate, with an accuracy to be determined, the long termbehaviour of the structure.
If it is possible, for a controlled concrete formulation (in laboratory), to estimatethe repeatability of the creep tests in laboratory, it will be possible to give anindication of the variability of the long term characteristics in the structure. Ourapproach, which is not exhaustive, brings a first brief reply. It is based on therealisation of an important and expensive experimental campaign, using theexperimental means of the “bétons et composites cimentaires” research division ofthe LCPC.
2. Variability of the compressive strength at 28 days
To illustrate our matter it is first of all advisable to be able to qualify, from arepeatability point of view, the formulations of concrete carried out in laboratorystarting from the standard size of control which is the compressive strength at 28days, and to compare it with the real measurements made on real structure: we mustbe able to carry out various batches in laboratory which are representative of what isactually obtained on building site.
2.1. Real measurement data
Measurements of compressive strength of a fluid concrete C50 used indeed forthe construction of a real bridge pier, the viaduct of Verrières, are presented figure 1.
Each point represents the average of three compressive tests. The averagecorresponding resistance is MPaf
mc 5028 .
The points on the left are those relating to the tests of suitability, and the othersare strengths actually obtained 28 days after the casting of various parts of the pier(control tests).
Simple statistical treatment of these recording led to an average value equal to
MPaf 44,78 and a standard deviation equal to MPas 66,8 . Imposed
specifications are respected. But one also notices an increase in resistance with thedate of casting: concretes executed in winter are more resistant. In fact, this well-known phenomenon is magnified by a modification of the procedure of casting: theadditives delivered in diluted form were well mixed in their cans before theiremployment.
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376 RFGC – 6/2002. Fiabilité : conception, maintenance
Figure 1. Concrete compressive strength variability for a bridge pierVariabilité de la résistance à la compression d’un béton d’ouvrage d’art
Figure 2. Precast slab C40 concrete compressive strength variabilityVariabilité de la résistance à la compression sur chantier pour un B40 TP
This example illustrates the variability of the concrete properties in the structure.
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contrôle 2C40 concrete
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Concrete laboratory 377
The value of the coefficient of variation equal to 11% gives an indication on thevariability of strength; The same type of analysis is employed in the case of C40concrete used for the realisation of prefabricated slabs of composite steel andconcrete bridges (figure 2).
In this case, one does not find the problems of casting met for the concrete of thepiers, and the coefficient of variation is equal to 8,3%.
2.2. Laboratory measurement
The aim of the study is to allow an estimation of the repeatability of creep tests,in laboratory, starting from eight successive batches of concrete which isrepresentative of what is actually obtained on construction sites. For each of thebatches, nine 16*32 concrete samples and three 16*100 concrete samples are cast.16*32 specimen are tested in compression at 3, 28 and 90 days, based on thefollowing procedure LCPC [TOU 99][TOR 99].
The complete results obtained are available in [CLE 01].
Figure 3. Laboratory concrete compressive strength variabilityVariabilité de la résistance à la compression en laboratoire
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378 RFGC – 6/2002. Fiabilité : conception, maintenance
The characteristic resistance aimed for our concrete is MPafc 4028 . The
evolutions of resistance with the age of the concrete are plotted in figure 3. At 28days and for the whole set of results, we obtain an average resistance at 28 daysequal to MPaf mc 12,4528 , a standard deviation equal to 50,3 s and a
coefficient of variation of 7,8%. The criteria of conformity of this concrete,according to the current French standard, are checked (Leaflet 65A of FrenchCCTG).
The C.O.V. measured from laboratory tests is slightly lower but of the sameorder of magnitude, than that obtained on construction site: several probable causeswhich can explain the laboratory results were identified. During the pouring ofconcretes, the content of included air was measured. It is shown figure 4 for eachbatch. This representation highlights the existence of two fabrication classes: thefirst class relates to concretes of the series S1-S5, and the second to concretes of theseries S6-S8. The variations of included air are due to a sedimentation inherent inthe mode of storage of the fine aggregates in metal vats, during their handling: theaggregates top of the vat are less fine and the concrete then includes more air than ifit is manufactured with the bottom of the vat.
Figure 4. Concrete air included quantity measurementsMesure de l’air occlus par gâchée
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Concrete laboratory 379
2.3. Conclusion of the compressive strength study
From this analysis and this differentiation between two classes of casting, weobtain coefficient of variations of 3,6% for the S1-S5 class and of 2,3% for the S6-S8 class. The C.O.V. of the whole concretes (7,8%) may be compare to actualvalues obtained on building sites (11% in an extreme case and 8,3% for a commoncase): our laboratory concrete is representative of a cast in situ concrete, andconsequently the analysis of the dispersion of the creep tests can be considered asbeing representative. The statistical analysis of the tests of Young modulus carriedout leads to a C.O.V. between 1,3% and 3% following the age of the concrete at thetime of the test: like the initial basic creep compliance needs the instantaneousmodulus knowledge, one will be able to consider that these initial values areidentical.
3. Creep test repeatability
3.1. Creep/shrinkage test principle
The traditional creep test consists in measuring the longitudinal variations ofdisplacement of 16*100 loaded cylindrical concrete samples, between two sectionsapart 50 cm. In parallel with this test, the shrinkage measurement is measured on anunloaded 16*100 concrete sample. The assumptions of no coupling resulting fromlinear viscous elasticity make it possible to distinguish the effects of creep fromthose of shrinkage. During tests, the average relative displacement of the twosections is automatically recorded, by the means of an extensometer made up ofInvar stems. The relationship between this basic displacement and the length of 50cm gives us the deformation then. The measurement of deformations is carried outautomatically, as soon as a threshold of 2.10-6 is reached.
For the unloaded shrinkage sample, let us note � �tretH the point in time strain. It
is due to endogenous shrinkage. For the creep sample, subjected to a constant
compressive stress appV , let us note � �ttotalflH the recorded total strain. The
concrete of the creep sample is subjected to shrinkage. A common assumptionmakes it possible to dissociate the effects of shrinkage from the effects of creep.Moreover, assuming a linear viscoelastic behavior, the loading stress may be
ignored by defining the compliance function � �tJ by:
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380 RFGC – 6/2002. Fiabilité : conception, maintenance
� �� � � �
app
rettotalfl tt
tJV
HH �
This function is given starting from a test and which allows, knowing theprobable stresst applied in a structure, to calculate long term deformations.
3.2. Data processing statistical method
We tested eight 16*100 concrete samples (series S1 in S8) loaded at 3 days andduring 1 month, eight samples loaded at 28 days for 6 months and we recorded theendogenous shrinkage since the youth, during 6 months. The data processingstatistical method, developed more precisely in [CLE 00], and based on a method oflinear regression linear after change of variable and with an initial threshold, is notpointed out here.
3.3. Example of result
The results of the treatment samples loaded at 28 days are shown in table 1. with inparticular the limiting values obtained for each sample, the average value, thestandard deviation and the confidence interval of 95%. The average of the long termestimated compliance is equal to %8,523,45 r [µm/m/MPa].
Test N° Limit value
[µm/m/MPa]
Mean value
[µm/m/MPa]
Standarddeviation
Confidence
interval
S1-28j 45,047 P=95%
S2-28j 46,947
S3-28j 41,008 f,28 jmoyenJ js28
Maximal value
S4-28j 40,723 45,231 3,153 47,87
S5-28j 43,844
S6-28j 49,201 Minimal value
S7-28j 47,302 42,59
S8-28j 47,777
Tableau 1. Statistical treatment results of concrete samples loaded at 28 daysRésultats du traitement statistique des éprouvettes chargées à 28 jours
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Concrete laboratory 381
In the case of the statistical processing of only one test with 153,328 js we have a
Student coefficient equal to � � 365,27975,0 C . The confidence interval on the test
carried out is of � �
%5,16.7
,2828975,0
r|rfj
moyen
j
J
sC. This value relates to the laboratory
tests, and it is legitimate to think that the confidence interval on real work will besimilar or slightly superior.
4. Conclusion
We illustrated, through examples of control measurements of compressivestrength, the variability of the concrete in real structures. From laboratorymeasurements on a big number of 16*32 concrete samples, we also showed that thedispersion of compressive strength of laboratory concrete is similar to that obtainedon construction site. This result enabled us to validate some assumptions of thestatistical analysis of the creep tests repeatability, and to define a confidence intervalon the long term basic creep compliance.
The method of statistical treatment of the experimental compliance, makes itpossible to estimate a value of long-term compliance, using only one experimentalshrinkage/creep test, and to define a confidence interval which allows structuraldesigners to choose design values.
In addition, the shrinkage and creep test must have a minimal duration of 6months.
For a C 40 concrete, the C.O.V. of the compressive strength is about 8% (both inthe laboratory and in the real structure). The C.O.V. of the Young modulus is lessthan 3%, and the confidence interval on the long term basic creep compliance isof %5,16r .
This study will have now to be repeated for other concrete formulations.
Acknowledgement
We thank the A.I.O.A. which gave us the results of compressive strength relatingto the Verrières Bridge.
5. References
[AFR 96] Sous-groupe Règlements de l’AFREM, « Extension du domaine d’application desrèglements de calcul BAEL/BPEL aux bétons à 80 MPa », Bulletin des laboratoires desPonts et Chaussées, Numéro spécial XIX, mai 1996.
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[HEI 98] HEINFLING G., COURTOIS A., HORNET P., « Application des méthodes probabilistes àl’analyse du comportement des structures vieillisantes : une approche industrielle »,Proceedings of 2nd National Conference JN-FIAB’98 Actes de la 2ème conférencenationale JN-FIAB’98 Fiabilité des matériaux et des structures, Hermès, p. 232-245,1998.
[TOU 99] TOUTLEMONDE F., « Quelques réflexions sur la détermination du moduled’élasticité du béton, en vue de l’élaboration d’un mode opératoire LPC », Bulletin deslaboratoires des Ponts et Chaussées, Numéro 220, Note technique, p. 75-78, mars-avril1999.
[TOR 99] TORRENTI J.M., DANTEC P., BOULAY C., SEMBLAT J.F., « Projet de processusd’essai pour la détermination du module de déformation longitudinale du béton », Bulletindes laboratoires des Ponts et Chaussées, Numéro 220, Note technique, p. 79-81, mars-avril 1999.
[CLE 01] CLEMENT J.L., LE MAOU F., « Comportement incertain et amélioration de laprédiction du fluage en compression des bétons de structures », Proceedings of 3rd
National Conference JN Fiab3, 1-2/2/2001, 15 pages.
[CLE 00] CLEMENT J.L., LE MAOU F., « Etude de la répétabilité des essais de fluage suréprouvettes de béton », Bulletin des laboratoires des Ponts et Chaussées, 228, p. 59-69,septembre-octobre 2000.
[LER 96] LE ROY R., « Déformations instantanées et différées des bétons à hautesperformances », Collection Etudes et Recherches des laboratoires des Ponts etChaussées, série ouvrages d’art – OA22, ISBN 2-7208-2520-4, 376 pages, 1996.
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