procena ČvrstoĆe betona pri pritisku, koriŠĆenjem … · slika 1. uzorci za kontrolu čvrstoće...

GRAĐEVINSKI MATERIJALI I KONSTRUKCIJE 61 (2018) 3 (55-65)BUILDING MATERIALS AND STRUCTURES 61 (2018) 3 (55-65)

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PROCENA ČVRSTOĆE BETONA PRI PRITISKU, KORIŠĆENJEM RAZLIČITIHFUNKCIJA ZRELOSTI BETONA: PRIMER IZ PRAKSE

ASSESMENT OF CONCRETE COMPRESSIVE STRENGTH USING DIFFERENTMATURITY FUNCTIONS: CASE STUDY

Dragan BOJOVIĆNevena BAŠIĆKsenija JANKOVIĆAleksandar SENIĆ

STRUČNI RADPROFESSIONAL PAPER

UDK: 620.178:691.32doi: 10.5937/GRMK1803055B

1 UVOD

Čvrstoća pri pritisku jeste svojstvo betona, najvažnijeprilikom projektovanja i izvočenja konstrukcija. Često seostala svojstva – poput đvrstoće pri zatezanju, modulaelastiđnosti i trajnosti – povezuju s đvrstoćom pri pritiskubetona. U nauđnoj i struđnoj zajednici, najviše sepoklanjala pažnja modelovanju razvoja đvrstoće pripritisku betona tokom vremena. Većina modela ukljuđujeu razmatranje više parametara od kojih su najznađajnijioni koji se odnose na svež beton, kao što su vrsta ikoliđina pojedinih komponentnih materijala. Ipak, narazvoj đvrstoće betona veliki uticaj ima i temperatura,koja se uglavnom ne razmatra, jer je većina modelazasnovana na ispitivanjima u laboratoriji pri nekojkonstantnoj temperaturi.

Kada je beton u pitanju, neophodno je razdvojitiuticaj temperature kao uslova sredine odnosno negebetona od temperature koja je posledica hidratacijecementa. Posledice delovanja povišene temperatureumnogome se razlikuju u zavisnosti od mesta nastanka.Ovo je posebno bitno u sluđaju masivnih betona, gde sepovećana temperatura usled hidratacije cementa nemože zanemariti.

Dragan Bojović, istraživađ saradnik, dr, Institut IMS,Beograd, Srbija; [email protected] Bašić, dipl. grač. inž., ASTM d.o.o., Beograd,Srbija; [email protected] Janković, nauđni savetnik, dr, Institut IMS, Beograd,Srbija; [email protected] Senić, asistent, dipl. grač. inž., Gračevinskifakultet, Beograd, Srbija; [email protected]

1 NTRODUCTION

Compressive strength is a property of concrete of thehighest importance throughout design and constructionstage. Other properties, such as tensile strength,modulus of elasticity and durability are often associatedwith concrete compressive strength. Consequently,scientific and practitioners’ communities pay mostattention to the development of models for this particularconcrete property. Most of these developed models takeinto account two or more parameters among which theparameters related to fresh concrete such as type andquantity of component materials are the most significant.In addition, temperature has great influence on thedevelopment of concrete strength. However, thisparameter is almost never taken into consideration sincemost models are based on laboratory tests performed ata constant temperature.

When it comes to concrete, it is necessary toseparate the impact of temperature as an environmentalcondition, i.e., concrete curing condition, and tempera-ture generated by cement’s hydration. Consequences ofelevated temperature differ substantially depending onthe place of origin. This is especially important in thecase of massive concrete elements, in which tempe-rature increases since the hydration of cement cannot beignored.

Dragan Bojović, Research associate, PhD, IMS Institute,Belgrade, Serbia [email protected] Bašić, MScCE., ASTM d.o.o., Belgrade, Serbia,[email protected] Janković, Scientific advisor, PhD, IMS Institute,Belgrade, Serbia [email protected] Senić, Assistant, MScCE, Faculty of CivilEngineering, Belgrade, Serbia, [email protected]


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Kontrola čvrstoće betona u konstrukciji, premaameričkim propisima, podeljena je prema vrstama iveličini konstrukcije. Propisane su metode koje seprimenjuju za pločaste elemente do 300 mm debljine,kao i više metoda za sve ostale tipove i veličinekonstrukcija. Najčešće primenjivane metode jesuotpornost na utiskivanje, pullout i metode na bazi zrelostibetona. Za svaki od navedenih pristupa, standardom supropisane metode ispitivanja i izračunavanja čvrstoćebetona u konstrukciji.

Kontrola saglasnosti sa uslovima projektakonstrukcije pomaže u tome da se obezbedi sigurnostda proizvod – beton – ispunjava očekivanja odnosnopropisane kriterijume [1]. Prema zvaničnim zakonskimpropisima Republike Srbije u oblasti betona, za dokazčvrstoće betona mogu da se koriste uzorci kao što sukocke, prizme i cilindri različitih dimenzija. Za konačnidokaz, svi rezultati preračunavaju se na kocke ivice 20cm. Uvođenjem evropskih propisa, u Republici Srbiji –na gradilištima i u laboratorijama – osnovni uzorci će bitikocke ivice 15 cm. U skladu s važećim standardima, zaocenu betona u konstrukciji propisana je odgovarajućanega koja je u potpunosti drugačija od nege elementa ukonstrukciji. Za neke faze građenja konstrukcije uzimajuse dodatni uzorci koji se čuvaju u istim uslovima kaoodgovarajući elementi u konstrukciji. Oni se koriste kakobi se odredilo optimalno vreme oslobađanja elementa odoplate, utezanja konstrukcije i slično. Ipak, u slučajumasivnih konstrukcija i elemenata konstrukcije saznačajnim dimenzijama, poređenje betona u kocki ikonstruktivnom elementu pod znakom je pitanja. Okakvom disparitetu je reč, prikazano je na slici 1, na kojojse jasno vidi kolika može biti razlika u veličini izmeđukontrolnog uzorka i konstrukcije. U slučaju, na primer,prednapregnutih konstrukcija, broj uzoraka na gradilištuznatno se povećava i neophodna je dobra koordinacijakontrolne laboratorije sa izvođačem radova, kako bi sena vreme dobio podatak o čvrstoći betona.

Zakonska regulativa Republike Srbije, u oblastiarmiranog betona, oslanja se u svemu na još uvekvažeći pravilnik BAB’87 [2]. Kada je u pitanjutemperatura u betonu, važeći pravilnik BAB’87 definišemaksimalnu temperaturu u masivnim betonskimelementima na nivou od 60°C. Evropske norme su neštoblaže, pa maksimalnu temperaturu u betonuograničavaju na 70°C [3]. Američki propis za beton ACI318 [4] ograničava maksimalnu temeperaturu u betonuna svega 55°C. Izgradnja masivnih delova konstrucijaposebno je kritična u zemljama hladnih klimatskih prilika.Tako, propisima u Švedskoj, definisana je maksimalnadozvoljena temperatura u betonu svega 55°C, aograničen je i maksimalni temperaturni gradijent uelementu konstrukcije na 24°C [4–7]. Pored ograničenjatemperature i temperaturnog gradijenta, u Švedskoj supropisana tri nivoa proračuna uticaja temperature nakonstrukciju pre početka izgradnje, u zavisnosti od vrstei veličine konstrukcije koja se gradi [7].

According to US regulations the control of in-placeconcrete strength depends on the types and size ofstructures. There are special methods which areapplicable to plate-like elements, whose thickness goesup to 300mm thick, as well as several different ones forall other types and sizes of structures. The most oftenapplied methods are the pull-through resistance testmethod, the pull-out method and methods based onmaturity of concrete. For each of these approaches,there are standard prescribing methods for testing andcalculating in-place concrete strength.

The control of compliance with the structure designrequirements help to ensure that the product – concretemeets the requirements, i.e., prescribed criteria. [1]According to the legislation of the Republic of Serbiarelevant to concrete, samples in the form of cubes,prisms and cylinders of various dimensions are used forthe purpose of proving required concrete strength. Thefinal proof is presented as the strength of concreteobtained in 20cm cubes. With the introduction ofEuropean regulations, the construction sites across theRepublic of Serbia will take a 15cm cube as the basictest cube. For the purpose of evaluation of in-placeconcrete strength, appropriate curing has beenprescribed, according to the applicable standards. Thecuring prescribed by applicable standards is entirelydifferent from the curing of elements in-situ. For somephases in construction stage additional samples shouldbe taken and kept under the same conditions as thecorresponding in-place elements. Such samples areused to determine the optimum time for strippingformwork, for tightening the structure, etc. However, inthe case of massive structures and structural elementsof significant dimensions, comparing concrete cured intest cubes to in-place concrete is questionable. The kindof required disparity is shown in Figure 1 which clearlyshows how much the difference can be in size betweenthe control sample and the construction. For instance, ifprestressed structures are used at a construction site,the number of samples is significantly increased and it isnecessary to have a good coordination between theactivities of the test lab and the contractor of the worksfor obtaining reliable and timely data on concretestrength.

Serbian Republic regulations related to reinforcedconcrete rely on the still applicable Rulebook BAB’87 [2].Regarding temperature in the concrete, the RulebookBAB’87 allows the maximum temperature in massiveconcrete elements of 60°C. European norms are a bitmore relaxed, limiting the maximum temperature inconcrete to 70°C [3]. American concrete institutestandard, ACI 318 [4] limits the maximum concretetemperature to only 55°C. The execution of massiveparts of structures is critical in the cold climate countries.Thus, Swedish regulations set the maximum concretetemperature at only 55°C, at the same time also limitingthe maximum temperature gradient in a structuralelements to 24°C [4,7]. Beside maximum temperatureand temperature gradient, the Swedish regulationsbefore construction stage also prescribe three levels ofcalculations of the impact of temperature on thestructure, depending on the type and size of thestructure. [7].


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Slika 1. Uzorci za kontrolu čvrstoće betona, negovani u uslovima kao elementi konstrukcijeFigure 1. Samples for concrete strength testing cured under the same conditions as in-place members

Ocena saglasnosti s propisanim uslovima kvalitetabetona na mestu ugrađivanja – putem uzoraka kocki ibez uzimanja u obzir uticaja temperature – jestediskutabilna. Rađena su brojna istraživanja i razvijene sumetode koje povezuju čvrstoću pri pritisku i temperaturubetona u starostima do 28 dana [8,9]. U istraživanjimasu prikazani različiti rezultati i pouzdanost usvojenihmetoda, jer su ulazni parametri različiti, a samim tim izaključci se razlikuju. U prikazanim istraživanjima,različito je i dobijanje ulaznih parametara, kao i njihovousvajanje, što je najznačajniji uzrok razlika u analizama izaključcima.

Cilj rada jeste da se na primeru iz prakse građenjamasivnih konstrukcija uporedi više predloženih pristupakontrole i procene čvrstoće betona. Prvo, potrebno jeuraditi laboratorijske probe pri konstantnimtemperaturama nege, a nakon toga sprovesti merenjatemperature u masivnim delovima konstrukcije mosta,uraditi proračun čvrstoće betona i nakon toga uporeditidobijene rezultate. Pored teorijskih pristupa za procenučvrstoće, koristiće se uređaj ConReg 706, pomoću kogmože da se procenjuje čvrstoća betona.

2 TEORIJSKE POSTAVKE

Kvalitet ugrađenog betona, po pravilu, u praksi sekontroliše na uzorcima koji se uzorkuju prilikombetoniranja konstrukcije. Uzorci se neguju u svemuprema standardu. Kada su u pitanju uzorci betona zakritične faze tokom građenja, oni se uglavnom čuvaju uuslovima istim kao i elementi u konstrukciji. Nakondostizanja zahtevane starosti, uzorci se transportuju ulaboratorije radi ispitivanja; nakon dobijanja rezultata,donosi se odluka o izvođenju planiranih faza tokomgradnje. Uzorci – koji se čuvaju u uslovima kao ielementi u konstrukciji – koriste se kako bi se dokazalačvrstoća betona, neophodna za skidanje oplate, fazeutezanja i/ili skidanje potpornih elemenata u slučajuploča i sličnih elemenata.

Osnovno pitanje koje se postavlja jeste da li takviuzorci (kocke ili cilindri koji se neguju na gradilištu) na

The evaluation of conformity with prescribedconcrete quality requirements for in-place concrete viatest cubes is questionable without taking into accountthe impact of temperature. Numerous researches havebeen done and methods have been developed toconnect compressive strength and temperature inconcrete aged up to 28 days [8,9]. The research showsthe different results and reliability of the adoptedmethods, since the input parameters are different, andthus, the conclusions differ. In the presented research,the acquisition and adoption of input parameters isdifferent, which is the most significant cause ofdifferences in analyzes and conclusions.

The aim of this paper is to compare the proposedapproaches to the control and assessment of theconcrete strength in the case study of the construction ofmass concrete constructions. First, it is necessary to dolaboratory tests at constant temperature of curing, andthen perform temperature measurements in massiveparts of the bridge construction, perform a calculation ofconcrete strength and then, compare the resultsobtained. In addition to theoretical approaches forstrength assessment, the ConReg 706 will be used toassess the strength of the concrete.

2 THEORETICAL POSTULATES

As a rule, the quality of in-place concrete in practiceis controlled using samples taken during the buildingworks. For the first 24 hours, the samples are curedunder conditions applicable to in-place elements, i.e.,under conditions as similar as possible to thoseprescribed by the applicable standard. Thereafter, thesamples are cured fully in accordance with prescribedstandards. Samples of concrete taken in critical phasesof building works are kept, as a rule, under the sameconditions as in-place elements. When reaching therequired age, such samples are transported tolaboratories for further testing and, based on theobtained results, decisions are made on proceeding withplanned stages in building works. The samples curedunder the same conditions as in-built elements are usedto prove the strength of the concrete required for strip-


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pravi način reprezentuju stanje u betonskoj konstrukciji.Ukoliko se razmatra betoniranje u uslovima niskihtemperatura, uzorci koji se neguju na gradilištu imajuznatno niže temperature. U takvim uslovima, uzorcimogu imati drastično manje čvrstoće betona od betona umasivnim delovima konstrukcije. Ako se, pak, razmatrabeton u predelima s visokim temperaturama, može doćido znatno viših temperatura u betonskim telima zakontrolu, nego u samoj konstrukciji. U tim slučajevima,mogu se očekivati uočljivo veće čvrstoće betona nauzorcima za kontrolu kvaliteta od betona u konstrukciji.Opisane situacije ukazuju na to da je neophodnokontrolisati temperaturu betona, kako bi se stvorila jasnaslika o dostignutim čvrstoćama betona i uporedilo stanjeu uzorcima s konstrukcijom.

Istraživači i brojni proizvođači opreme razvili su nizmetoda za kontrolu postignute čvrstoće betona ukonstrukciji, u zavisnosti od temperature koja se javlja uelementu. Sve razvijene metode zasnivaju se naprincipu zrelosti betona. Smatra se da hidratacijacementa prestaje na temperaturama oko -10°C, a zrelostbetona zavisi od starosti i temperature kojoj je beton bioizložen. Osnova za primenu zrelosti betona uodređivanju čvrstoće betona jesu laboratorijskaispitivanja pomoću kojih se određuje zavisnost čvrstoće izrelosti, nakon čega se dobijena funkcija možeprimenjivati prilikom izgradnje na terenu, odnosnogradilištu. Funkcije zrelosti betona jesu matematičkiizrazi za računanje kombinovanog efekta vremena itemperature na razvoj čvrstoće betonskih mešavinaodnosno cementnih kompozita. Dva u svetu najvišeprihvaćena pristupa jesu: da je nivo razvoja čvrstoćelinearno zavisan od temperature odnosno da se razvojčvrstoće odvija po eksponencijalnoj Arrhenius jednačini.Oba pristupa imaju svoje prednosti, kao i nedostatke.Prvi pristup je veoma jednostavan za korišćenje, ali jemanje pouzdan, dok je drugi pristup nešto složeniji, jerzahteva određene dodatne parametre na osnovu većegbroja laboratorijskih ispitivanja, ali je precizniji od prvogpristupa.

Prvi pristup određivanja faktora temperatura–vremezasnovan je na funkciji zrelosti (1), koju su pedesetihgodina prošlog veka predložili Nurse & Saul [8],usvajajući to da je T0 u rasponu od -8 do -10°C, kaotemperatura na kojoj prestaje hidratacija cementa.

ping the formwork, for re-tightening and/or removingsupporting structures in the case of slabs and similarmembers.

The basic question is: whether a cube or a cylindersampled at the construction site properly reflects thestate within the concrete structure. This issue is ofparticular importance from the aspect of in-placeconcrete strength, and achieving designed in-placeconcrete strength. If laying concrete is considered underlow environment temperatures, the samples taken in situwould be also considerably cooler. In such cases, thestrength of the sampled concrete can be significantlylower than that of the concrete built in massive parts ofbuilding structures. On the other hand, in areascharacterized by high daily temperatures, concrete intest cubes can be exposed to much higher temperaturesthan in-place concrete. Hence, test cube concrete canreach much higher strengths than in-built concrete. Thisproves that in order to obtain a clear and realistic pictureof achieved in-place concrete strength and to be able tocompare in-place and test concrete samples it isnecessary to control the temperature in concrete.

Researchers and manufactures of concrete-relatedequipment have developed a number of methods fortesting in-place concrete strength, depending ontemperatures inside the element. All these methods arebased on the principle of concrete maturity. The idea ofconcrete maturity is quite old and based on simpleprinciples. The basic principle is that cement hydrationstops at around -10°C, and that maturity of concretedepends on the age, as well as on the temperature thatconcrete has been exposed to. Laboratory tests, whichmake correlation between concrete strength andmaturity, serve as the basis for using concrete maturityto define concrete strength and later applied in situ, i.e.,at the construction site. The functions of concretematurity are mathematical expressions used forcalculating the time-temperature effects on the strengthdevelopment of concrete mixtures, i.e., cementcomposites. In general, there are two widely-usedapproaches; one assumes that the rate of the strengthdevelopment is a linear function of temperature, and theother assumes that the rate of the strength developmentfollows the exponential Arrhenius equation. Bothapproaches have advantages and weaknesses. The firstis very simple to use but is less reliable while the secondis more complex requiring certain additional parametersbased on more extensive lab tests but its accuracy ismuch higher compared to the first approach.

The first approach for defining the time-temperaturefactor is based on the maturity function (1) proposed in1950’s, by Nurse & Saul [8], which is adopting T0, in therange between -8 and -10°C, as temperature belowwhich hydration stops.

(1)

gde su: M(t) – faktor temperatura – vreme koji seizražava u stepen–dan ili stepen–sat, Ta – srednjatemperatura u usvojenim vremenskim intervalima, T0 –početna temperatura odnosno temperatura nultehidratacije u betonu, Δt – usvojeni vremenski intervalmerenja temperature.

Drugi pristup, dosta kasnije predstavljen, zasnovanje na određivanju ekvivalentne starosti pomoćuArrhenius-ove [8] funkcije (2).

where: M(t) = temperature-time factor expressed asdegree-days or degree-hours, Ta = average temperaturein adopted time intervals, T0 = starting temperature, i.e.,temperature for staring concrete hydration, Δt – adoptedtime interval for temperature measurement.

The second approach, presented much later, isbased on determination of equivalent age usingArrhenius [8] function (2).


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(2)

gde su: te – ekvivalentna starost pri specificiranojtemperaturi Ts izražena u danima ili satima, Q –aktivaciona energija, podeljena s gasnom konstantom, ugranicama je od 4800 do 5000°K, u zavisnosti od vrsteprimenjenog cementa u betonu, Ta – srednjatemperatura betona u intervalu Δt (°K), Ts – usvojena ilispecificirana temperatura izražena u °K i usvaja se nanivou od 20±2°C, Δt – interval merenja, a izražava se udanima ili satima.

Oba pristupa zahtevaju prethodna ispitivanja ulaboratorijskim uslovima i na recepturama za beton kojeće biti korišćene prilikom betoniranja elemenatakonstrukcije.

3 EKSPERIMENTALNI RAD

Kao što je već prethodno rečeno, kontrola postignutečvrstoće betona u konstrukciji jeste obavezna zapojedine faze izgradnje. Naročito je značajno kontrolisatipostignutu čvrstoću betona u masivnim elementima,odnosno elementima čija je najmanja dimenzija veća od1 m. Kada su u pitanju ovakvi elementi, posebno suznačajni vremenski uslovi u kojima se radovi izvode,odnosno da li su visoke ili niske ambijentalnetemperature. Prilikom izgradnje mostova, svi delovikonstrukcije – osim ploča i glavnih nosača – svrstavajuse u masivne konstrukcije, odnosno primenjuju sepravila za masivni beton.

Prilikom izgradnje mosta preko reke Save kodOstružnice, za naglavne grede, stubove i ležišne gredenajmanja dimenzija elemenata bila je veća od 1 m.Samim tim, ovi elementi morali su se razmatrati kaomasivni betonski elementi. Zbog ubrzanja dinamikeizvođenja radova i boljeg iskorišćenja oplata, neophodnoje bilo da se ovi elementi što pre oslobađaju od oplate.

Urađene su analize receptura za beton klase C30/37(MB35 i MB40), kao i laboratorijska ispitivanja zadobijanje odnosa čvrstoće pri pritisku i faktora vreme–temperatura ili ekvivalentne starosti.

Za izradu betona, korišćeni su: cement CEM II A/L42,5R, agregat iz reke Morave, separisan u četiri frakcijes maksimalnim zrnom agregata 32 mm, voda izvodovoda i superplastifikator na bazi polikarboksilata.Količina cementa bila je 370 kg/m3 i 400 kg/m3 za betonMB35 i MB40 respektivno. Količina superplastifikatorabila je konstantna 0.6% u odnosu na masu cementa.Voda je usvojena tako da se dobije beton konzistencije19 cm ± 2 cm. Spravljanje betona u laboratoriji rađeno jeu mešalici kapaciteta 60 l, dok je na gradilištu bilaangažovana fabrika betona s kapacitetom mešanja 2 m3

u mešalici.Laboratorijska ispitivanja obuvatila su izradu

receptura pri spoljašnjoj temperaturi od 20°C iodređivanje čvrstoće pri pritisku u starostima od 12h,18h, 24h (jedan dan), tri dana, sedam dana, 14 dana i28 dana. Komponentalni materijali za beton čuvani su ulaboratorijskim uslovima na temperaturi od 20°C, pa jespravljeni beton, pri mešanju, imao temperaturu od 22°C± 2°C. Nakon 24 sata nege na vazduhu pri temperaturiod 20°C, dalja nega sprovođena je u vodi konstantnetemperature 20 ± 1°C. Temperatura u uzorcima jekontrolisana; već nakon 12 sati od izrade, bila je do

where: te = equivalent age at specified temperature, Ts =the same expressed in days or hours, Q = activationenergy divided by the gas constant, (°K), Ta = averageconcrete temperature in time interval Δt, (°K), Ts =adopted and specified temperature, (°K), Δt = timeinterval of measurement, days or hours.

Both of the above approaches require prior testing inlaboratory on the concrete mixtures that will be usedduring the casting.

3 EXPERIMENTAL WORK

As already mentioned, the control of achieved in-place concrete strength is mandatory in some buildingstages. It is very important to test concrete strength inmassive elements, i.e., in elements whose minimumdimension is 1m or more. Weather conditions at the timeof building works have particular importance for suchelements, i.e., whether ambient temperatures are high orlow. In the bridge construction industry, apart deck slaband main girders, all other elements are considered tobe massive parts, i.e., massive concrete elements.

During the construction of the bridge over the riverSava near Ostružnica, the smallest dimensions of builtelements, pile cup, piers, and bearing beams, were morethan 1m. Therefore, these parts had to be viewed asmassive concrete elements. However, for the purpose ofacceleration of the works, and better utilization offormwork, it was necessary to strip formwork as soon aspossible.

The mix design analyses were carried out and usedto prepare concrete class C30/37 (Serbian: MB40 andMB 35), as well as laboratory tests in order to establishthe relation between compressive strength and time-temperature or equivalent age factors.

For the production of concrete we used: CEM II A/L42,5R cement, aggregate from river Velika Moravaseparated into four fractions with a maximum aggregategrain 32mm, water from water supply and apolycarboxylate based superplasticizer. The amount ofcement was 370 kg/m3 and 400 kg/m3 for concreteMB35 and MB40 respectively. The amount ofsuperplasticizer was constant at 0.6% relative to theweight of the cement. The water was adopted so as toobtain a concrete consistency of 19cm ± 2cm. Concretepreparation in the laboratory was done in a 60 l mixer,while a concrete plant with a mixing capacity of 2 m3 inthe mixer was engaged on the construction site.

Laboratory tests included production of concreteusing the recipes at 20°C and determining compressivestrength for concretes aged 12h, 18h, 24h (1 day), 3days, 7 days, 14 days and 28 days. The constitutivematerials for concrete were stored under laboratoryconditions at a temperature of 20°C, and the producedconcrete was at a temperature of 22°C+2°C at most.After 24 hours of air cure at a temperature of 20°C, thefurther curing was carried out in a water with constanttemperature of 20°C±1°C. The temperature in thesamples was controlled and after 12 hours of productionit was up to a maximum of 22°C. A constant temperatureof 20°C in the case of laboratory tests is taken for thecalculation. Three 150mm-test cubes were examined for


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najviše 22°C. Za proračun je uzeta konstantnatemperatura od 20°C u slučaju laboratorijskih proba. Usvakoj od predviđenih starosti betona, ispitane su trikocke ivice 150 mm. Kao rezultat ispitivanja, uzeta jesrednja vrednost od tri uzorka, s tim što je rasipanjerezultata ograničeno na 15% od srednje vrednosti. Naosnovu dobijenih rezultata, dobijene su funkcije čvrstoćepri pritisku i faktora vreme–temperatura. Takođe, naosnovu istih rezultata dobijena je i funkcija čvrstoće pripritisku i ekvivalentne starosti betona. Laboratorijskidobijene funkcije prikazane su na slici 2 za odnosčvrstoće i faktora vreme–temperatura i na slici 3 odnosčvrstoće i starosti betona, a eksperimentalne vrednostidate su u tabeli 1. Za izračunavanje faktora vreme–temperatura, kao referentna temperatura korišćeno je -3°C prema uputstvima brojnih autora [8].

Kompletan eksperimentalni rad urađen je na dverecepture betona – MB35 i MB40. Recepture za betondate su u tabeli 1, kao i postignute čvrstoće betona upredviđenim starostima.

each of the above listed concrete ages. The averagevalue of the results obtained from all three cubes wasused as the final result, with scattering of results limitedto 15%. The functions of compressive strength and time-temperature factors were calculated based on theseobtained results. Also, these results were used togenerate the function of concrete compressive strengthand equivalent age. The functions obtained by labtesting are shown in Figure 2 for the strength and time-temperature factor relation and in Figure 3 for theconcrete strength and age relation, and experimentalvalues are shown in Table 1. Reference temperature of-3°C was used for calculating the time-temperaturefactor following recommendations of numerous authors[8].

The entire experimental work was done for twoconcrete recipes, MB35 and MB40. The concreterecipes are given in table 1 as well as achieved concretestrengths at specified age.

Tabela 1. Rezultati laboratorijskih ispitivanja za MB35 i MB40Table 1. Results of laboratory tests for MB35 and MB40

Markabetona

Concrete

CementCementkg/m3

VodaWaterkg/m3

AgregatAggregate

kg/m3

Plastifik.Plast.kg/m3

Aerant"Aerant"additivekg/m3

12hMPa

18hMPa

24hMPa

3dMPa

7dMPa

28dMPa

MB35 371 162 1795 2.6 0.03 5.1 12.1 17.1 34.1 42.0 48.1MB40 400 170 1742 2.8 0.032 9.3 18.1 18.7 35.9 43.2 52.5

Slika 2. Odnos čvrstoće i vreme–temperaturaFigure 2 Strength and time-temperature relation

Slika 3. Odnos čvrstoće i starosti betonaFigure 3 Strength and concrete age relation

Kada je završena faza laboratorijskih ispitivanjabetona za dve marke betona, pristupilo se merenjima nagradilištu. Merenje je sprovedeno pomoću aparataConReg 706 firme ASTM iz Švedske. Uređaj imamogućnost da meri temperature na maksimalno šestmesta u konstrukciji i – na osnovu sopstvene funkcije zaodređivanje zrelosti betona i čvrstoća određenih ulaboratorijskim uslovima – daje procenu čvrstoće usvakom momentu. Funkcija zrelosti betona zasnovana jena principu određivanja ekvivalentne starosti koju surazvili proizvođači opreme, a koja predstavljamodifikovanu Arrhenius funkciju (3).

In situ measurements started once the laboratory testingfor both above mentioned concrete mixtures wasfinished. Measurements were taken using a deviceConReg 706 provided by ASTM, a Swedish company.The device has a possibility to take temperaturemeasurements at up to 6 points on the structure and toproduce its own evaluation of concrete strength at alltimes based on its ability to determine concrete maturityand taking into account strengths determined under labconditions. The concrete maturity function is based onthe principle of establishing equivalent age developed bythe equipment manufacturer and it represents a modifiedArrhenius function (3).


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(3)

gde se koeficijenti k3 i θ izračunavaju iz laboratorijskihispitivanja cementa i betona.

Merenja su urađena na više vrsta elemenata - nakolovoznoj ploči, stubu, naglavnoj gredi i ležišnoj gredi.Od izabranih elemenata, samo kolovozna ploča ne pri-pada masivnim delovima konstrukcije, jer je njena naj-manja dimenzija svega 22 cm, što je daleko manje oduslova da najmanja dimenzija elementa bude veća od 1m.

Kao proba, prvo su urađena merenja temperaturebetona na kolovoznoj ploči za koju je korišćen betonMB40. Rezultati ispitivanja na kockama čuvanim uuslovima elementa i prognozirane čvrstoće pomoćuaparata ConReg 706 firme ASTM dati su u tabeli 2 zaprva 24 sata starosti betona.

where coefficients k3 and θ are derived based on theresults of laboratory tests of cement and concrete.

Measurements were taken on several types ofmembers: deck slab, pile cup and bearing beam. Amongall these elements, only the deck slab does not belong tomassive parts of a structure since its shortest edge isonly 22cm long, that is, far below the 1m requirement forconsidering a structure to be a massive one.

Temperature measurements were taken on the deckslab first. The results of tests performed on cubes keptunder the same conditions as elements and strengthsforecasted by ConReg 706 devices provided by ASTMcompany are shown in table 2 for the fists 24 h ofconcrete aging process.

Tabela 2. Uporedna ispitivanja na kolovoznoj pločiTable 2 Comparative tests for deck slab

Marka betonaConcrete Class

VremeTime

Čvrstoća na kockamaStrength on cube

Procena na ConRegEvalution on ConReg

12h 13.75 N/mm2 14.61 N/mm2

18h 24.16 N/mm2 23.13 N/mm2MB4024h 30.14 N/mm2 30.05 N/mm2

Nakon ispitivanja urađenih na kolovoznoj ploči,pristupilo se pripremi i ispitivanjima na naglavnoj iležišnoj gredi koje su betonirane betonom MB35. Tokompripremnih radova, urađena je analiza oba elementa iodređena su potencijalna mesta na kojima će biti rađenomerenje i paralelno ispitivani kontrolni uzorci. Naglavna iležišna greda su izabrane jer je reč o dva potpunorazličita elementa iz aspekta temperaturnih uticaja.Naglavna greda je ukopan element, odnosno radi se uširokom iskopu, ali je donja strana kompletno na zemlji,odnosno na mršavom betonu, te s donje strane nemaoplate. Bočne strane su na 1–1.5 m od iskopa i ovakavelement, osim s gornje strane, zaštićen je od direktnoguticaja sunca. Gornja površina se na svim elementimaposle 6 do 8 sati od betoniranja zaštićuje geotekstilom ipočinje se s negom betona već posle 12 sati. Drugiizabrani element – ležišna greda – jeste deo konstrukcijeiznad stuba i za takav element, osim na relativno malomdelu stuba na koji se oslanja, oplata je neophodna sasvih strana. Kod ležišne grede vazduh je sa svih strana.Kod ovakvog elementa konstrukcije spoljašnji uticaji suvrlo važni, naročito leti kada direktni sunčevi zraci mogupovisiti temperaturu oplate, a samim tim i betona.

Prvo je urađeno ispitivanje na naglavnoj gredidimenzija 9.5x2.4x2.5 m, a potom i na ležišnoj gredidimenzija 12.6x2x (~2 m) – širina grede promenljiva jeod 2 do 1.5 m. Temperatura je merena na četiri mesta ukonstruktivnom elementu i u okolini elementa koji jebetoniran. Šematski prikaz rasporeda senzora ukonstrukciji tokom merenja temperature u oba elementadat je na slici 4.

After the tests on the deck slab were performed,there have been some preparations and testing on thepile cup and bearing beam. The preparation worksincluded an analysis of both elements and choosingpotential spots in the elements suitable formeasurements and parallel control of test samples.These components had been selected because theywere completely different from the aspect of temperatureimpact. The first component was the pile cup that is anembedded structure. More precisely, there was a widerexcavation and the bottom side of the component waslaid completely on the ground, i.e., lean concrete surfacewhich means no formwork was placed there. The sidesof the components were located 1-1.5 m far from theexcavation and this member, save for its upper side iscompletely protected from direct impact of the sun. Theupper surface of all these elements should be protectedfrom the sun 6 to 8 hours after concreting with geotextilefabric material and the curing process should start 12hours after casting. The second structural componentselected for the test – the bearing beam is a part of thestructure above the pillar and as such it requiresformwork to be placed from all sides except for arelatively small part where the component rests of thepillar. The bearing beam is exposed to air from all sides.In the case of such element, external impacts aresignificant, especially in summer when direct sunexposure can increase the temperature of formwork andthus, of the concrete as well.

Measurements were taken first on the pile cup dim.9.5x2.4x2.5m, and then on the beam dim. 12.6x2x(~2m)(the width of the beam varies between 2 and 1.5m).Temperature was taken at four points in the element aswell as the temperature of the external air close to thecasting place. Schematic diagram of measurementpoints in both elements is shown in Figure 4.


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Slika 4. Raspored senzora u ispitivanim elementimaFigure 4. Disposition of sensors in tested elements

Pored prikazanih senzora, korišćen je i peti senzorza merenje temperature vazduha u okolini elementa. Tajsenzor postavljen je u zaštitnu drvenu kutiju koja jesprečavala direktni uticaj sunca i na taj načinomogućeno je pravilno merenje temperature vazduha uokolini elementa.

Sva merenja rađena su tokom sedam dana. Uređajfirme ASTM ConReg 706 podešen je da beleži rezultateu intervalu od 30 minuta, pa je dobijena baza od oko 300merenja po razmatranom elementu. Uređaj beležitemepraturu i daje procenu čvrstoće prilikom svakogmerenja.

Na osnovu merenja temperature, urađen je proračunčvrstoće betona pri pritisku na osnovu dva opisanapristupa. Pored ova dva pristupa, uređaj ConReg 706dao je procenu čvrstoće betona. Na osnovu dobijenihrezultata, data su tri para krivih za procenu čvrstoćebetona: prva korišćenjem Nurse & Saul funkcije, drugakorišćenjem Arrhenius funkcije i treća je dobijena izuređaja ConReg 706. Za sve navedene pristupe,dobijene su po dve funkcije: za mesta s najnižom inajvišom temperaturom u elementu. Svi dobijeni rezultatiprikazani su u obliku dijagrama na slikama 5 i 6. Naslikama su prikazane krive dobijene korišćenjem Nurse& Saul pristupa (Str-1 i Str-3), krive na osnovuArrhenius-ove formule (Str-1 Arrh i Str-3 Arrh) i krive

In addition to the above displayed sensors, a fifthsensor was also used for measuring environmental airtemperature. This sensor was encased in a protectivewooden box preventing direct exposure to the sun, thusenabling proper measurement of air temperature closeto the tested component.

The entire measurement lasted for 7 days. TheASTM supplied device, ConReg 706 was adjusted torecord results at 30-minute intervals, thus creating adatabase containing 300 measurements per testedelement. The device is designed to record temperatureand produce its own evaluation in terms of strength aftereach measurement.

Based on temperatures taken on the elements thecompressive strength of concrete was calculated usingtwo traditional approaches. In addition to theseapproaches, the ConReg 706 device also produced itsown concrete strength evaluation. On the basis of theobtained results, three pairs of curves for the concretestrength assessment were given: first using the Nurse &Saul function, the second one using the Arrheniusfunction, and the third was obtained from the ConReg706. For all the foregoing approaches, two functionshave been obtained: for the places with the highest andfor the places with the lowest temperature in themember. All of the results thus obtained are shown in


63

dobijene iz uređaja ConReg 706 (ConR1 i ConR2). Naslikama su paralelno prikazana i merenja spoljašnjetemperature (AirTemp) u okolini predmetnog elementa.

4 ANALIZA REZULTATA I ZAKLJUČCI

Prikazani rezultati ispitivanja navode na višezaključaka u razmatranju uticaja temperature na razvojčvrstoće betona.

Izabrane metode ispitivanja veoma su osetljive naizbor mesta merenja temperature betona. Iz prikazanihrezultata vidi se da u okviru svakog pristupa postojeznatna odstupanja, u zavisnosti od mesta merenja. Nadijagramima sa slike 5 i slike 6, vide se razlike za svakiod korišćenih pristupa. Ako se pojedinačno posmatrajuprimenjene analitičke metode za procenu čvrstoće, zamesta s najnižom temperaturom dobijaju se i preko 15%manji rezultati čvrstoće pri pritisku nego za mesta snajvišom izmerenom temperaturom u elementu.Čvrstoća betona, ako se procenjuje na bazi Arrhenius-ove formule, osetljivija je na promene temperature. Zaiste rezultate merenja temperature ovaj pristup dajemnogo veći raspon rezultata nego Nurse & Saul pristup,što se jasno vidi sa dijagrama na slikama 5 i 6.

the form of diagrams in Figures 5 and 6. Figures showthe curves obtained using the Nurse & Saul approach(Str-1 and Str-3), curves based on the Arrhenius formula(Str-1 Arrh and Str-3 Arrh) and the curves obtained fromConReg 706 (ConR1 and ConR2 ). The figures alsoshow the external temperature measurements (AirTemp)in the surroundings of the subject element.

4 THE ANALYSIS OF THE RESULTS ANDCONCLUSIONS

Based on the found results, several conclusions canbe drawn when considering the influence of temperatureon concrete strength.

The selected test methods are very sensitive to thechoice of spots for measuring concrete temperatures.The above results clearly show that each approach hassignificant deviations depending on the measurementplace. Figures 5 and 6 clearly show the differences foreach of the applied approaches. If the applied analyticalmethods for assessing the strength for places with thelowest temperature are observed individually, more than15% less results of compressive strength are obtained inrelation to the sites with the highest emissiontemperature in the element. The strength of theconcrete, if assessed on the basis of the Arrheniusformula, is more sensitive to temperature changes. Forthe same temperature measurement results, thisapproach gives a much greater range of the results thanthe Nurse & Saul approach, which is clearly seen in thediagrams in Figures 5 and 6.

Slika 5. Merenja temperature i procena čvrstoće betona u naglavnoj grediFigure 5. Temperature measurements and evaluation of concrete strength in pile cup


64

Slika 6. Merenja temperature i procena čvrstoće betona u ležišnoj grediFigure 6. Temperature measurements and evaluation of concrete strength in the bearing beam

Analizirajući rezultate dobijene primenom tri pristupa,uočavaju se znatne razlike u proceni čvrstoće pripritisku. Generalno uzevši, od svih proračunskih modela,najmanje rezultate daje Nurse & Saul pristup, dok sunajveći rezultati dobijeni pristupom koji je koristioArrhenius-ovu formulu. Proračunske metode veoma suosetljive na temperaturne razlike koje se svakako javljajuu masivnim betonskim elementima. Razlike izmeđuklasičnih metoda i proračunskog modela koji koristiuređaj ConReg 706 jesu značajne, što svakako trebaimati u vidu prilikom odabira metode koja će se koristiti.

Uređaji, kao što je i korišćeni ConReg 706 Švedskefirme ASTM, paralelno mere temparaturu i daju procenučvrstoće betona, pa samim tim mogu umnogomeolakšati procenu postignute čvrstoće betona ukonstrukciji.

Najveće dobijene temperature u elementima dostiglesu 70°C, što je daleko više od 60°C, koliko je propisanou Pravilniku BAB’87. Ipak, nije premašena temperaturapropisana evropskim normama, te stoga nisuprimenjivane mere za smanjenje temperature betona uposmatranim elementima. Ukoliko se kao referentnenorme usvoje Švedske norme, premašena jemaksimalna temperatura u elementima od 55°C, kao itemperaturni gradijent u elementu od 24°C. Sve toukazuje na potrebu da se važeća regulativa RepublikeSrbije mora revidirati u pogledu temperaturnih uticaja ubetonskim elementima i približiti savremenim pogledimai istraživanjima u ovoj oblasti.

Analyzing the results obtained using the threeapproaches, significant differences in pressure strengthassessment are observed. Generally speaking, fromcalculating models, the least results are provided by theNurse & Saul approach, while the highest results arederived from the approach used by the Arrheniusformula. The classic calculating methods are verysensitive to temperature differences that certainly occurin massive concrete elements. Differences betweenconventional methods and calculation model which usesa device ConReg 706 are significant which shouldcertainly be kept in mind when selecting the method.

Instruments, as well as the used ASTM ConReg 706from Sweden, are simultaneously measuring the tem-perature and providing an estimate of the strength of theconcrete, and therefore can significantly facilitate theestimate of the strength of the concrete in the construction.

The highest measured temperature in the elementsreached 70°C, which is far more then 60°C maximumallowed in Serbian regulations, Rule book BAB’87.However, maximum temperature defined by Europeanstandards was not exceeded, thus no measures weretaken to reduce the concrete temperature in testedelements. If the standard adoption of the Swedishstandard, the maximum temperature in the elements of55°C is exceeded as well as the temperature gradient inthe element of 24°C is exceeded. All this points to theneed for the valid regulations of the Republic of Serbia tobe revised in terms of temperature influences in concreteelements and bring them closer to contemporary viewsand research in this area.


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5 LITERATURAREFERENCES

[1] Čerepnalkovska S., Vladičeska Lj., „Kako smanjitirizik u procesu dokazivanja kvaliteta građevinskihproizvoda”, Građevinski materijali i konstrukcije2017, 60(1), 65–75.

[2] Pravilnik za beton i armirani beton, PBAB87, 1987.[3] SRPS EN 1992-1-1, European Standard, 2004.[4] User guide for ConReg 706 with ConRegSoft – PC

Software.[5] SRPS EN 12390-2, European Standard, 2013.

[6] ACI 318, American Concrete Institute, 2008.[7] AMA, Swedish Norm for Roads[8] Carino, N. J., and Tank, R. C., ”Maturity Functions

for Concrete Made with Various Cements andAdmixtures”, ACI Materials Journal, Vol 89, No. 2,March–April 1992, pp. 188–196.

[9] Malhotra, V.M., and Carino, N.J., ”Handbook onNondestructive Testing of Concrete”, CRC PressInc., Boca Raton, FL 1991, 343 pp.

REZIME

PROCENA ČVRSTOĆE BETONA PRI PRITISKU,KORIŠĆENJEM RAZLIČITIH FUNKCIJA ZRELOSTIBETONA: PRIMER IZ PRAKSE

Dragan BOJOVIĆNevena BAŠIĆKsenija JANKOVIĆAleksandar SENIĆ

Čvrstoća pri pritisku jeste svojstvo koje je veomaznačajno u građevinarstvu, te postoji izražena potrebada se razviju metode za praćenje prirasta čvrstoće pripritisku betona u konstrukciji. Prema zakonskimpropisima Republike Srbije, procena čvrstoće pri pritiskubetona u konstrukciji radi se na osnovu rezultatadobijenih tokom laboratorijskih ispitivanja u konstantnimuslovima.

U radu se koriste rezultati dobijeni u laboratoriji iporede se rezultati dobijeni pomoću metode za merenjezrelosti betona, koje se baziraju na korelaciji izmeđučvrstoće betona, starosti betona i temperaturnih uslova irezultata dobijenih pomoću ConReg 706 uređaja, kojitakođe uzima u obzir temperaturu van betona.Primenjena su dva pristupa za određivanje zrelostibetona – Nurse & Saul i Arrhenius.

Za potrebe ovog istaživanja, pripremljene su dverazličite betonske mešavine – MB 35 i MB 40, obe klaseC30/37. Odabrani konstrukcijski elementi betonirani sutokom leta, na mostu preko reke Save, kod Ostružnice, ublizini Beograda. Testirani elementi mosta jesukolovozna ploča, naglavna greda i ležišna greda.

Ispitivanje je pokazalo da temperatura koja se razvijau masivnim delovima konstrukcije znatno utiče načvrstoću betona, pored već dobro poznatih faktora, kaošto su vodo-cementni odnos, tip i kvalitet komponentnihmaterijala i drugi.

Ključne reči: zrelost betona, masivni beton,temperatura

SUMMАRY

ASSESMENT OF CONCRETE COMPRESSIVESTRENGTH USING DIFFERENT MATURITYFUNCTIONS: CASE STUDY

Dragan BOJOVICNevena BASIC Ksenija JANKOVICAleksandar SENIC

Compressive strength is a property of significantimportance for civil engineering; consequently, therehave been strong need for developing method, which willestimate rise of concrete compressive strength inconstruction. According to Serbian legislation,assessment of compressive strength in constructionrelies on laboratory results obtained under constantconditions.

This paper presents and compares results obtainedin laboratory, results obtained by maturity method, whichis based on correlation between concrete compressivestrength, concrete age and ambient temperature, andthe results obtained by ConReg 706 instrument, whichalso takes in consideration external environment andconcrete temperature. Two approaches of maturitymethod, Nurse & Saul and Arrhenius have been applied.

For this research, concrete class C30/37 was used,prepared as two different mix designs, MB35 and MB 40.Casting was performed in summer time in Ostruznicabridge near Belgrade. Elements that were casted aredeck slab, pile cup and bearing beam.

Key words: concrete maturity, mass concrete,temperature

procena ČvrstoĆe betona pri pritisku, koriŠĆenjem … · slika 1. uzorci za kontrolu čvrstoće...

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