Modified Cement-based Mortars Crack Initiation And

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  See discussions, stats, and author profiles for this publication at: Modified cement-based mortars: Crack initiation and volume changes  Article   in  Materiali in Tehnologije · August 2015 DOI: 10.17222/mit.2014.179 CITATIONS 3 READS 63 6 authors , including: Some of the authors of this publication are also working on these related projects: Quasi-brittle fracture   View projectExperimental analysis of the early-age volume changes in cement-based composites   View projectIvana HavlikovaBrno University of Technology 19   PUBLICATIONS   27   CITATIONS   SEE PROFILE Vlastimil BílekBrno University of Technology 33   PUBLICATIONS   45   CITATIONS   SEE PROFILE Libor TopolarBrno University of Technology 84   PUBLICATIONS   127   CITATIONS   SEE PROFILE All content following this page was uploaded by Libor Topolar on 06 April 2016. The user has requested enhancement of the downloaded file.  I. HAVLIKOVA et al.: MODIFIED CEMENT-BASED MORTARS: CRACK INITIATION AND VOLUME CHANGES MODIFIED CEMENT-BASED MORTARS: CRACK INITIATIONAND VOLUME CHANGES MODIFICIRANE MALTE NA OSNOVI CEMENTA: INICIACIJARAZPOK IN VOLUMENSKE SPREMEMBE Ivana Havlikova 1 , Vlastimil Bilek Jr. 2 , Libor Topolar 1 , Hana Simonova 1 ,Pavel Schmid 1 , Zbynek Kersner 1 1 Brno University of Technology, Faculty of Civil Engineering, Veveri 331/95, 602 00 Brno, Czech Republic 2 Ditto, Faculty of Chemistry, Purkynova 464/118, 612 00 Brno, Czech Republichavlikova.i @ Prejem rokopisa – received: 2014-08-05; sprejem za objavo – accepted for publication: 2014-09-18 doi:10.17222/mit.2014.179The aim of this paper was to quantify the crack initiation and volume changes of three types of fine-grain cement-based compo-sites: the reference one including Portland cement only and the other two with the mass fraction 20 % of Portland cementreplaced by granulated blast-furnace slag or pulverized-coal fly ash. Half of the specimens from each mixture were curedaccording to ASTM C1260-94 and ASTM C1567-07, other specimens were cured in water. The length changes were measuredduring the curing (a test for the risk of the alkali-silica reactivity). Furthermore, a three-point-bending test was performed onthese specimens to obtain the fracture parameters, which were determined using a double- K   fracture model. It was found thatthe type of specimen curing has a significant effect on the volume changes and a moderate influence on the elasticity modulus,the crack initiation and fracture-toughness values.Keywords: cement-based composite, slag, fly ash, alkali-silica reaction, crack initiation, double- K   fracture modelNamen tega ~lanka je oceniti iniciacijo razpoke in spremembe volumna treh vrst drobnozrnatih kompozitov na osnovi cementa:osnovna samo s cementom Portland, pri drugih dveh je bil masni dele` 20 % cementa Portland nadome{~en z granuliranoplav`no `lindro ali s prahom elektrofiltrskega pepela premoga. Polovica vzorcev iz vsake me{anice je bila strjena skladno zASTM C1260-94 in ASTM C1567-07, drugi vzorci so bili strjeni v vodi. Med strjevanjem je bilo merjeno spreminjanje dol`ine(preizkus za rizik reaktivnosti alkalija-silicijev dioksid). Poleg tega je bil izvr{en pri teh vzorcih trito~kovni upogibni preizkus,da bi dobili parameter preloma, kar je bilo dolo~eno z uporabo lomnega modela z dvojnim  K  . Ugotovljeno je bilo, da ima na~instrjevanja vzorcev mo~an vpliv na spremembe volumna in zmeren vpliv na modul elasti~nosti, na iniciacijo razpoke, kot tudi navrednosti lomne `ilavosti.Klju~ne besede: kompozit na osnovi cementa, `lindra, lete~i pepel, reakcija med alkalijami in silicijevim dioksidom, iniciacijarazpoke, lomni model z dvojnim  K  1 INTRODUCTION The utilization of supplementary cementing materials(SCM) in the Portland-cement-based composite manu-facturing is a current topic in materials engineering.Besides the ecological and financial aspects, it alsoallows us to improve the mechanical properties and dura-bility of a composite. One of the main durability prob-lems is the risk of an alkali-silica reaction (ASR). ASRcan be very harmful to a concrete construction because itis accompanied by a significant volume expansion. As aconsequence, deleterious cracking of the aggregate andthe binder phase can develop, leading even to a failure of the concrete structure. ASR can be suppressed throughseveral ways, but some of them are impractical or expen-sive, such as the choice of non-alkali-reactive aggregates,restriction of the access of moisture to the concrete massor the use of ASR inhibitors 1 . From this point of view, avery suitable possibility of ASR mitigation can be theuse of SCM, which is also the topic of this paper.In this paper, the authors examine the effect of re-placing 20 % of the mass of Portland cement (PC) in afine-grained composite by selected admixtures on me-chanical-fracture-parameter values and volume changes.The used admixtures are granulated blast-furnace slagand pulverized coal fly ash. Specimens of three differentmixtures were prepared and, for comparison, two typesof curing were used – the first one according to ASTMC1260-94 2 and ASTM C1567-07 3 , while the second onewas performed in water. Length changes were measuredto assess the effect of the admixtures on the suppressionof ASR during this process. The result of the length-change measurement is the relative expansion at the ageof 16 d.The values of the mechanical-fracture parameters of quasi-brittle materials are usually determined via an eva-luation of the records of the experiments on specimenswith stress concentrators. In this paper, three-point-bend-ing fracture tests were conducted on the above-men-tioned specimens, which were evaluated using adouble- K   fracture model 4 . The advantage of this model isthat it describes different levels of crack propagation: theinitiation which corresponds with the beginning of astable crack growth (at the level where the stress inten- Materiali in tehnologije / Materials and technology 49 (2015) 4, 557–561  557 UDK 691.5:620.1 ISSN 1580-2949Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 49(4)557(2015)  sity factor  K  Icini is reached) and the part featuring anunstable crack propagation (after the unstable-fracturetoughness  K  Icun has been reached). The results obtainedwith the double- K   model are the modulus of elasticity,the fracture toughness and the relative resistance tostable crack propagation. 2 EXPERIMENTAL PART  2.1 Materials and specimen preparation Specimens with the nominal dimensions of 25 mm ×25 mm × 285 mm were prepared from three types of mixture. The first mixture was the reference (R), thuscontaining one part of PC CEM I 52.5 R to 2.25 parts of natural sand (0–4 mm) by mass. In the other two mix-tures 20 % of PC was replaced with granulated blast-fur-nace slag (S) or pulverized-coal fly ash (F). All theseadmixtures srcinated from Czech plants. The water-to-binder ratio was 0.47 for all the mixtures. A total of 18beams were prepared, thus, six specimens from eachtype of mixture.  2.2 Curing of specimens and the measuring of length changes The specimens were cured and their length changesmeasured according to ASTM C1260-94 and ASTMC1567-07. The specimens were demolded 24 h after themixing, fitted with gages and stored in tap water of 80°C for another 24 h. Then a zero reading of the specimenlength compared to the reference rod was taken. Afterthis, a half of the total number of the specimens, i.e.,three specimens from each mixture were cured for 14 dat 80 °C in a 1-N sodium hydroxide solution (H). Forcomparison, other specimens were cured for the sametime in water (W) of the same temperature. To illustrate, Figure 1  shows the classification and designation of thespecimens by mixture and their curing conditions.The relative length changes (in %) of the specimenswere recorded during both types of curing and subtractedeach workday. The relative expansion of the specimensfor the total age of 16 d   16d , was calculated using thefollowing relation:  160 100 d  = −⋅∆ ∆ l ll k  (%) (1)where   l  is the specimen length compared to the refe-rence rod for the age of 16 d in mm,   l 0  is the specimenlength compared to the reference rod at the zero readingin mm, and  l k   is the effective gage length in mm (gene-rally  l k   = 250 mm).  2.3 Fracture tests A quantification of the mechanical fracture parame-ters was performed using tests on the specimens withstress concentrators. In this paper, three-point-bendingtests were performed on the beams with a central-edgenotch. The geometry of a specimen used in this test isshown in  Figure 2 , where  D  is the specimen depth,  B  isthe width,  L  is the length,  S   is the span,  a 0  is the initialnotch length,  H  0  is the thickness of the edge of the holderclip on the extensometer (for all the specimens  H  0  = 2.5mm) and  CMOD  is the crack-mouth-opening displace-ment at load  P . In this case, the specimens were cut from I. HAVLIKOVA et al.: MODIFIED CEMENT-BASED MORTARS: CRACK INITIATION AND VOLUME CHANGES 558  Materiali in tehnologije / Materials and technology 49 (2015) 4, 557–561 Figure 3:  P – CMOD  diagrams recorded during testing – the referenceset Slika 3:  Diagrami  P – CMOD , posneti med preizku{anjem – refe-ren~na vrsta Figure 1:  Classification of the specimens by mixture and curing con-dition Slika 1:  Delitev vzorcev po me{anicah in razmerah pri strjevanju Figure 2:  Three-point-bending-fracture test geometry Slika 2:  Geometrija trito~kovnega upogibnega preloma  the original beams, the nominal dimensions of thespecimens were 25 mm × 25 mm × 200 mm and the spanlength was 150 mm. The initial notch was made with adiamond-blade saw. Note that the depth of the notcheswas about 1/3 of the specimen depth. The specimen agewas 28 d. Fracture tests were carried out using a HeckertFP 10/1 testing machine within a range of 0–400 N.Load-versus-crack-mouth-opening-displacement ( P – CMOD ) diagrams were recorded during the tests ( Figure3  for R,  Figure 4  for the specimens of the mixturemodified with S and  Figure 5  for the specimens with F).From these diagrams we deducted the input data for thedouble- K   fracture model ( Figure 6 ), namely, the maxi-mum load  P max  and its corresponding critical crack-mouth-opening displacement  CMOD c , and load  P i deducted from the linear part of a diagram and its corres-ponding crack-mouth-opening displacement  CMOD i .  2.4 Double-K fracture model  In principle, a double- K   fracture model 4 combines theconcept of cohesive forces acting on the faces of cracksduring a fictitious (effective) crack increment with thecriterion based on the stress-intensity factor.As previously mentioned, the  P – CMOD  diagramswere used to determine the fracture parameters of thedouble- K   model. In this case, the unstable fracturetoughness  K  Icun was the first to be numerically deter-mined, followed by a determination of the cohesivefracture toughness  K  Icc . When both of these values wereknown, the following formula was used to calculate theinitiation fracture toughness  K  Icini : K  Icini =  K  Icun –  K  Icc (2)Details regarding the calculation of both the unstableand cohesive fracture toughness can be found in nume-rous sources 5,6 .To determine the cohesive part of the fracture tough-ness  K  Icc it is necessary to accept the assumption of thedistribution of the cohesive stress     along the fictitiouscrack. Generally, in a cohesive crack model, the relationbetween the cohesive stress     and the effective crack-opening displacement  w  is referred to as the cohesivestress function    ( w ). The cohesive stress    ( CTOD c ) at thetip of the initial notch length  a 0  at the critical state can beobtained from the softening curve. In this paper, theexponential softening curve suggested by Reinhardt etal. 7 was used. The value of     ( CTOD c ) was determinedusing the following formula:    ( ) exp CTOD f c CTODwc CTODw c tcccc = +      −   1  132     − + −  cc CTODw c c ( )exp( )1 132 (3)where  f  t  is the tensile strength in MPa,  CTOD c  is the cri-tical crack-tip-opening displacement in mm (the detailsregarding the calculation can be found in 4 ),  w c  is themaximum crack-tip-opening displacement in mm (inthis case  w c  = 0.06 mm for all the specimens), and  c 1 ,  c 2 are the material constants (in this case constants  c 1  = 3and  c 2  = 6.93 were considered as recommended in 4 ). I. HAVLIKOVA et al.: MODIFIED CEMENT-BASED MORTARS: CRACK INITIATION AND VOLUME CHANGESMateriali in tehnologije / Materials and technology 49 (2015) 4, 557–561  559 Figure 5:  P – CMOD  diagrams recorded during testing – the set of specimens with pulverized-coal fly ash (F) Slika 5:  Diagrami  P – CMOD,  posneti med preizku{anjem – vrstavzorcev z elektrofiltrskim pepelom premoga (F) Figure 6:  Deduction of the input data for the double- K   model fromthe  P – CMOD  diagram Slika 6:  Vhodni podatki za model z dvojnim  K   iz diagrama  P – CMOD Figure 4:  P – CMOD  diagrams recorded during testing – the set of specimens with blast-furnace slag (S) Slika 4:  Diagrami  P – CMOD , posneti med preizku{anjem – vrstavzorcev s plav`no `lindro (S)

Tataaig Pearl

Sep 11, 2019
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