Construction and Building Materials 51 (2014) 207–214

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

An analysis of shear fracture toughness KIIc and microstructurein concretes containing fly-ash

0950-0618/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.10.044

⇑ Corresponding author. Tel.: +48 81 538 43 86; fax: +48 81 5384173.E-mail address: t.sadowski@pollub.pl (T. Sadowski).

G.L. Golewski a, T. Sadowski b,⇑a Faculty of Civil Engineering and Architecture, Department of Civil Engineering Structures, Lublin University of Technology, Nadbystrzycka 40 str., 20-618 Lublin, Polandb Faculty of Civil Engineering and Architecture, Department of Solid Mechanics, Lublin University of Technology, Nadbystrzycka 40 str., 20-618 Lublin, Poland

h i g h l i g h t s

� Estimation of the shear fracture toughness KIIc in concretes containing 0%, 10%, 20% and 30% volume content of FA additive.� Determination of two regions of the fracture toughness depending on the amount of FA additive and maximum Kmax

IIc .� The microstructural observations of type of phases in the interfacial transition zone between paste and coarse aggregates.� The SEM analysis of microcracks in the interfacial transition zone between paste and coarse aggregates.

a r t i c l e i n f o

Article history:Received 31 May 2013Received in revised form 28 October 2013Accepted 29 October 2013

Keywords:Fly ashConcreteFracture mechanicsMode II fractureFictitious crackCompact Shear SpecimenMicrostructureSEMITZ

a b s t r a c t

The present study was focused on estimation of the fracture toughness in Mode II (shear) and internalmicrostructure of concretes containing 0%, 10%, 20% and 30% volume content additive of the Fly ash(FA). The addition of the FA changes the shear fracture toughness KIIc. The regular value of the shear frac-ture toughness is included within the additive range from 0% to 23%. The volume content of the FA, cor-responding to i.e. maximum increase of KIIc is equal to 17%. If the FA content in higher than 23%, thecomposite becomes weaker as for response against to shear cracking.

The most compact microstructure with well developed fibrous C-S-H phase were characterized by aconcrete containing 20% FA additive. The biggest micro-cracks (from 2.5 to 3.0 lm) in interfacial transi-tion zone (ITZ) between paste and coarse aggregates were observed in concretes containing 30% FA, whilethe smallest one (to 700 nm) in concretes with 20% FA additive.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

1.1. Siliceous fly-ash – origin and applications

Modern structural concretes are composites which compositionis often modified by addition of mineral additives and chemicaladmixtures. Such mineral additions and chemical admixtures areintroduced to concrete mainly in order to satisfy more and moredemanding requirements concerning strength parameters, durabil-ity and resistance of concrete to various types of aggressive influ-ences, e.g. [1].

Siliceous FA is a by-product obtained in the process of hardcoal combustion performed in electric power stations and in

thermal-electric power stations [2]. It is removed by mechanicalcollectors or electrostatic precipitators as a fine particulate resi-due from the combustion gases before they are discharged tothe atmosphere. Nowadays the FA is probably the most fre-quently used pozzolanic waste materials in concrete productionworldwide, mainly because it is available in large amounts forlow price. Therefore the FA is a subject of intensive research.The first paper, which described the properties of concretecontaining the FA was published already in 1937 [3], whereasthe first critical review of using FA in concrete was published in1980 [4]. Nowadays good quality siliceous FA is used for produc-tion of Portland-composite cement [5] and plain concretes as wellas high performance concretes [6], self-compacting concretes [6],roller-compacted concretes [7] or steel–fiber reinforced concretes[8]. Moreover with the right quality (low loss of ignition, highfineness) and quantity (up to 15%) of the FA selection is also

208 G.L. Golewski, T. Sadowski / Construction and Building Materials 51 (2014) 207–214

possible to use them for the production of polymer concrete [9]and Ultra High Performance Concrete.

The implementation of FA into these composites influences theproperties not only of the concrete mixture but also of hardenedconcrete [10,11]. Advantageous impact of the FA addition in con-cretes is related to the morphology of its particles, pozzolanicactivity and microfiller effect [12,13].

The FA is also used in various applications e.g.: transformingsiliceous FA into a zeolitic material [2,14], as fire resisting cement,to produce glass ceramics or as a admixture of the cement pastefor electromagnetic interface shielding. The FA is a valuable andnecessary raw material for the building materials industry. Theuse of the FA in the building materials industry brings aboutreduction of concrete production costs (due to decreased contentof cement in the concrete mixture) as well as improvement ofnumerous parameters of hardened composite. The influence ofthe FA additive on basic parameters of concretes was describede.g. in [10–13].

1.2. Mode II fracture toughness of concretes. Mixed mode of cracking

The problem of the FA influence on the concrete behaviour wasanalysed in [15], but in the context of assessment of a mode I frac-ture toughness, i.e. for tension. Up till now the effect of FA influ-ence on the shear fracture toughness of the concrete was notinvestigated systematically.

In practical engineering the shear failure is a fundamentalproblem, e.g. in such structures as: reinforced concrete beams,wall–beams, slabs [16] or concrete dams. As it is demonstratedby the experimental research, e.g. [17,18] and analytical works,e.g. [19,20], the micro-defects develop under loading process instructural materials are mainly according to the mixed crackingmodel, i.e. the longitudinal shearing towards the crack edges(Mode II) must be necessarily taken into account. It was demon-strated in [19–23] that the importance of the shear fracturetoughness is essential in analytical description of the concretedegradation under compression, where so called a wing crackmodel should be taken into account. Fig. 1 presents an exampleof the wing crack propagation during compression test. The wingcrack consists of:

� A central straight crack (1).� 2 wings (2).

Therefore in this paper we focused on the estimation of theinfluence of the FA additive on the shear fracture toughness KIIc,important characteristic for designing and modelling of the matureconcrete response to mechanical loading.

Fig. 1. An example of the wing crack; 1 – the central straight crack, 2 – wings,3 – cavities in the place of separation grains from the matrix.

2. Experimental details

2.1. Purpose and scope of the tests

The aim of tests was estimation of the impact of the FA additive included in con-struction concretes on the fracture toughness at Mode II, i.e. KIIc. In addition, effortswere made to determine the relationship between the phase composition and thesize of microcracks occurring in the composite structure with the FA additive.

The fracture toughness tests and the compressive strength tests were con-ducted with four types of concretes of different amount of the FA additive: withoutthe FA additive (FA-00), with 10% FA additive (FA-10), with 20% FA additive (FA-20)and with 30% FA additive (FA-30). In other papers composites with the sameamount of the FA additive were used in the testing of:

� Concrete properties, in which the coarse aggregate was replaced by waste fer-rochromium aggregate [24].� The lightweight concrete behaviour in high temperatures [25].� The assessment of radiation safety of concretes [26].� Assessment of permeability of chlorides in mortars [27].

2.2. Materials

The following materials were used for making the mixtures: Ordinary PortlandCement (OPC) CEM I 32.5 R from Chełm cement plant, with the compressivestrength equal to 23.3 MPa in the age of 2 days and 50 MPa after 28 days of curing,natural gravel aggregates of maximum grain size up to 8 mm (from Las Suwalskideposit), a pit sand from Markuszów deposit, Class F FA and a plasticizer (0.6% ofbinding material weight). The siliceous FA is a result of energetic combustion ofhard coal in the Puławy thermal-electric power station.

2.3. Mix proportion, preparation and curing

The methodology used for determining the composition of the mixtures relieson replacing a suitable amount of the cement volume (the weight volume content)in the mixture with a corresponding amount of the FA to ensure that the totalamount of binding material is always constant. All mixtures had the same water-binding material indicator w/B = 0.4. The mix proportion of the concrete composi-tions are given in Table 1. The cast specimens were covered with polyurethanesheet and damped cloth and placed in 20 ± 2 �C chamber. All specimens weredemoulded at the age of 2 days. Then they were kept for 14 days in a chamber witha moisture-saturated atmosphere and for another 14 days under laboratory condi-tions (20 ± 2 �C). After 28 days of curing the compressive strength tests of and otherbasic tests were carried out.

2.4. Compressive strength analysis

The uniaxial compression strengths were tested using a compression machine(Walter + Bai ag) with a maximum load of 3000 kN. The compressive strength(fcm15) were tested with application of cubic specimens (150 mm) according tothe standards of series EN 12390 at the age of 28 days. The reported results arethe averages of six specimens.

2.5. X-ray diffraction (XRD) analysis

The crystalline phases of the FA were identified by XRD patterns. Radomly ori-ented powder specimen (about 1 g weight) for XRD analysis were prepared. XRDgraph were obtained at room temperature by a PANalytical X’Pert PRO MPD diffrac-tometer (with the PW 3050 goniometer), Cu lamp, and graphite monochromator.The diffractogram from 5� to 65� in the 2h and scanning rate was at 0.05� intervals.PANalytical X’Pert HighScore and the ClayLab ver. 1.0 software were used to processdifraction data. The identification of mineral phases was based on the PCPPDFWINver. 2.1 database formalized by the JCPDS-ICDD.

Table 1Mix proportion of the concrete compositions.

Concrete OPC(kg/m3)

FA(kg/m3)

Water(kg/m3)

Sand(kg/m3)

Coarseaggregate(kg/m3)

Plasticizer(kg/m3)

FA-00 352 0 141 676 1205 2FA-10 317 35 141 676 1205 2FA-20 282 70 141 676 1205 2FA-30 246 106 141 676 1205 2

Fig. 3. Specimen with: (a) loading conditions, (b) dimensions (in mm), (c)exemplary crack paths in the FA-30, (d) blunted notch.

G.L. Golewski, T. Sadowski / Construction and Building Materials 51 (2014) 207–214 209

2.6. Pozzolanic activity analysis

EN 450–1 defines as a standard pozzolanic activity index of the FA the StrengthActivity Index which is the ratio (in%) of the compressive strength of mortar con-taining 75 wt% of cement CEM I 42.5R, 25 wt% of the FA and cement mortar withoutaddition. According to EN 450–1 standard the pozzolanic activity index of the FA isdetermined after 28 and 90 days of hydration. The compressive strength of cementwas measured according to procedure described in EN 196-1 standard, using40 � 40 � 160 mm prisms of the mortar. The water to solid ratio in mortars was 0.5.

2.7. Methodology of the mode II fracture tests for concretes containing the FA

2.7.1. Choice of the test specimen and its preparationIn order to understand crack propagation process under shear loading of any

concrete, it is necessary to perform Mode II fracture tests with application of suit-able specimens. Different types of Mode II specimens can be found in the literature,e.g. [17,18,28–32] and in the stress intensity factors handbook [33]. In case of con-crete materials the most popular is Compact Shear Specimen (CSS), e.g. [17,28]. Forthis type of test an analytical model of crack activation in CCS’s was proposed in[34,35], treating the composite concrete as pure elastic material. A more realisticand a new 3-D numerical model corresponding to the CSS test which includes bothstages, i.e. the crack activation and propagation was created with application of theABAQUS program [36]. In this model the numerical results are convergent qualita-tively and quantitatively with the experimental ones.

Based on the above theoretical analyses cubes with 150 mm long edge and twofictitious cracks – CSS were used for the experiments [17,30,36]. The target cracksizes were obtained by embedding in concrete the cubes being formed two 4 mmsteel sharpened flat plates. Preparation of the CSS’s was shown in Fig. 2. After form-ing of the samples 2 sharpened flat plates were immediately removed from thestand. Therefore after curing process we get blunted end of the cracks, seeFig. 3d. The fictitious cracks may also be prepared by cutting diamond saw. Thisway of the fictitious cracks preparation was shown in [37].

2.7.2. Testing procedureIn order to assess the influence of the FA volume content on the shear fracture

toughness in the considered concrete composites 6 CSS’s (for each type of concrete)were prepared for the basic tests [17,36]. The experiments were performed for theloading scheme presented in Fig. 3. Specimen dimensions in detail and exemplarycrack paths, which were observed during experiment were also shown in Fig. 3.

The tests were performed on the MTS 810 servo-hydraulic testing machine.During the experiment we applied a displacement controlled type of tests withthe MTS head velocity equal to 0.25 mm/min. We observed almost a quasi-static in-crease of the loading force F up to the final failure.

2.7.3. Procedure of fracture mechanics parameters calculationThe shear fracture toughness (Mode II) can be estimated by two fracture

mechanics parameters: the fracture toughness – KIIc [17,29,30] or the fracture en-ergy – GIIc [18,31,32,38]. The last parameter was used, e.g., in the research study[31,38] in description of damageability of concretes with different internal struc-ture created by crushed granite stone fine aggregate replacing sand [31] and inthe analysis fracture toughness in castable refractories [38]. Application of-theabove fracture mechanics parameters in the case of linear elastic response of thematerial due to mechanical loading is equivalent.

In this paper, basing on the experimental results, we calculated KIIc, which wasdetermined according to the formula proposed by Watkins [30]:

K IIc ¼5:11FQ

2Bb

ffiffiffiffiffiffi

pap

ð1Þ

Fig. 2. CSS for the Mode II fracture tests during preparation (dimensions in mm).

where: FQ is the value of critical force corresponding to beginning of the crackgrowth. This stage is identified on the failure diagrams as a small collapse or extre-mum of the curve. The following denotations were used in Eq. (1): b is the specimenheight above the fictitious crack, B is the thickness of specimen, a is the length of thefictitious crack.

2.7.4. Microstructural analysisThe analysed composite concrete, containing the FA, is a complex multi-phase

and non-homogenous material. Therefore during the loading process different de-fects can be initiated and further developed inside the material. Damage beginsin the concrete matrix or in the interfacial transition zone (ITZ), i.e. between cementpaste and aggregate [39]. The crucial points in the concrete microstructure are con-tact layers of coarse aggregate because they are stress concentration sites, whichcan lead to defects initiation or activation, e.g. [40]. Information regarding thephases arrangement inside material structure is essential because some of them,such as gel C-S-H phase (e.g. [41] – the most important component of the cementmatrix) strengthens the composite structure and improves its fracture toughness.In contradistinction to gel C-S-H phase, large portlandite crystals (CH) appearingin the ITZ can cause a significant weakening and greater susceptibility to brittlefractures [42]. However, the ITZ is commonly regarded as the weakest element ofthe concrete composites. This zone is characterized by considerably higher porosityresulting from so called ‘‘wall’’ effect of packing of cement grains against the rela-tively flat aggregate surface. Application of the Scanning Electron Microscope (SEM)to assess the morphology of the fractured concrete surfaces allows to estimate themanner of material destruction and provides an answer to the question what typeof microcracks initiate in the composites.

The paper [43] presents the microstructural analyses of concrete with the FAadditive of class C and F in the range between 1 and 365 days of curing, based onthe SEM images with the use of two electron detectors: secondary electrons (SE)and backscattered electrons (BSE). The tests were performed in terms of analysisof the concrete porosity changes and micro-damages. Particular attention was paidto damages occurring in the cement matrix around the FA grains treated as activemicro-aggregates. Based on the experimental studies [43] it was observed thatthe reaction in the cement matrix containing the FA starts after 7 days of the con-crete mixture preparation, and lasts even after 365 days. Typical reaction productsin this type of composites are mainly C-S-H phase, visible around the FA grain evenafter one day of curing. It can be concluded also that addition a large amount of theFA for concrete significantly change the microstructure, mechanical properties (e.g.modulus of elasticity) and the manner of crack propagation. In this type of compos-ite microcrack propagation generally deviates around the FA particles.

In our study the microscopic analysis of the composite concretes containing theFA was carried by microscopic observations of the CSS fragments obtained from thedestruction zones after fracture toughness testing. The specimens with dimensionsof 10 � 10 � 3 mm from cubes (Fig. 3) at the age of 28 days were prepared. 3 sam-ples of concrete were prepared from each series. Thirty pictures were taken for eachsample, from which the representative pictures were selected. A half of capturedpictures were images showing the phases occurring within the material, whilethe other half was with distinct damages in the ITZ area. The attention was mainly

210 G.L. Golewski, T. Sadowski / Construction and Building Materials 51 (2014) 207–214

drawn on the microstructure of C-S-H phases and to the size and type of cracksoccurring in the ITZ. The width measurements of the cracks occurring in the con-crete are of significant importance in assessing the mechanical parameters anddurability; e.g. the paper [44] discusses width of the microcracks in concretes withthe addition of silica flour, exposed to high temperatures.

The SEM observation of cracks in the tested samples allowed for assessment ofthe average width of the cracks (Wc) for each type of concrete. Average value of theWc was calculated after observations of 30 representative SEM images.

The microstructural testing was carried out using a QUANTA FEG 250 at magni-fication from 200 to 80,000 times equipped with an energy dispersive Spectroscopy(EDS EDAX). The tests were performed in both the low and high vacuum. Prior totesting in a high vacuum, the samples were dried for one hour at the temperatureof 70 �C and then they were sprayed with carbon or alloy of gold and palladium in ahigh vacuum sputter coater Q 150 E. The samples tested in the low vacuum did notrequire drying and spraying prior to testing.

Fig. 5. XRD pattern of the Fly ash.

3. Results and discussion

3.1. Characteristics of OPC and FA

The Blaine fineness of the OPC and the FA are 3300 and3600 cm2/g. The specific gravities of the OPC and the FA are 3.11and 2.14 respectively. Fig. 4 presents characteristic pictures of atypical grains of used FA at various magnifications with EDS anal-ysis. The pictures show very small finess of the FA (a few to 50 lm)and characteristic cenospheres. Physical properties of the OPC andthe FA, and Fig. 4 suggest that the FA is finest, followed by the OPC.

The chemical constituents of the OPC and the FA is shown inTable 2. The FA is a Class F with 85.09% of SiO2 + Al2O3 + Fe2O3,0.65% of SO3 and 3.2% of LOI meeting the requirement of ASTMC618.

3.2. Xrd

The X-ray diffractogram of the FA (Fig. 5) shows that there are,beside glass, four major crystalline components in the phase com-position of the FA: quartz (SiO2), mullite (Al6Si2O13), magnetite

Fig. 4. Microstructures and grains of used FA; (a) grains (Mag: 500�), (b) grain withcenospheres (Mag: 4000�), (c) grains (Mag: 20,000), (d) EDS analyze.

Table 2Chemical composition of the OPC and the FA.

Constituent (wt.%) SiO2 Al2O3 CaO MgO SO3

OPC 21.37 5.02 63.95 2.47 3.0FA 50.96 25.88 2.15 2.6 0.65

(Fe3O4) and hematite (Fe2O3). On the diffractogram, a characteristicincrease in intensity at the angle ranging from 21� to 28� 2h can beobserved as well as relatively low intensity of the peaks of quartzand mullite phases. This image of the phase composition of theFA indicates their fine particle size and a high strength activityindex.

3.3. Pozzolanic activity

Strength activity index tests of the FA carried out in accordancewith the EN 450–1 standard indicated that this activity is: after28 days – 92%, while after 90 days – 111%. The high activity ofthe used FA can also be indicated by its small particle size, whichwas observed in physical properties and SEM studies (Fig. 4).

3.4. Compressive strength

Table 3 shows the average results of the compressive strengthof concretes containing the FA with statistic parameters (standarddeviation – s and coefficient of variation – v). Based on the obtainedresults, it can be concluded that the 10% and 20% additive causesan increase fcm15 after 28 days of curing by respectively, 2.2% and3.1%, while the 30% FA additive causes a drop of this parameterby 5.1%. Similar positive results of compressive strength of mortarwith the 10% and 20% FA additive were observed e.g., in the paper[1].

3.5. Mode II fracture tests

3.5.1. Fracture toughness KIIc

The average values of the fracture toughness after 28 days ofcuring with statistic parameters s and m are gathered in Table 4.The obtained results lead to conclusion that the 10% and 20% FA

Fe2O3 Loss C3S C2S C3A C4AF

2.4 1.24 60.69 15.82 9.24 7.288.25 3.2 SiO2 + Al2O3 + Fe2O3 = 85.09

Table 3Compressive strengths of concretes.

Concrete Compressive strength (MPa) s (MPa) m (%)

FA-00 47.51 – (100% – comparative concrete) 2.55 4.58FA-10 48.55 – (102.2% in relation to FA-00) 2.93 6.54FA-20 48.96 – (103.1% in relation to FA-00) 3.02 7.53FA-30 45.10 – (94.9% in relation to FA-00) 3.25 7.87

Table 4Fracture toughness KIIc of concretes.

Concrete Fracture toughness K IIc MN=mð3=2Þ s (MN/m3/2) m (%)

FA-00 4.24 – (100% – comparative concrete) 0.65 9.43FA-10 4.32 – (101.9% in relation to FA-00) 0.74 10.15FA-20 4.39 – (103.5% in relation to FA-00) 0.81 11.63FA-30 3.65 – (86.1% in relation to FA-00) 0.78 12.11

Fig. 7. Force – displacement diagrams for FA-30 specimen.

G.L. Golewski, T. Sadowski / Construction and Building Materials 51 (2014) 207–214 211

addition caused small increase of KIIc by 1.9% and 3.5%, while thereplacement of a part of cement with a 30% additive of the FAbrings about 13.9% reduction of the fracture toughness in compar-ison to the concrete without the FA. The above results are partiallyconvergent with the results of the compression strength tests(Table 3). The increase of fcm15 in case of composites FA-10 andFA-20 and the decrease of this parameter in case of the compositeswith a 30% addition of this filling material were observed.

Detailed analysis of the influence of the FA addition on KIIc

(Fig. 6) leads to conclusions that it changes the shear fracturetoughness KIIc. The volume content of the FA, which correspondto the maximum of the KIIc, is equal to 17%. The average width ofthe cracks would be only 600 nm. However, if the contribution inthe concrete mixture of the FA in higher than 20%, the compositebecomes weaker because of destructive action of the shear force(Fig. 6). The results obtained in experimental studies were alsoconfirmed by numerical analysis with application of ABAQUS com-mercial code, Fig. 7. Convergence of results with regard to value ofthe maximum force FQ , the force (F) –displacement (D) diagramwas obtained. Fig. 7 shows results for the composite FA-30. Theappropriate solutions for FA-20 were presented in [36].

3.5.2. MicrostructureFig. 8 shows the characteristic images of the microstructure

obtained from the SEM/EDS testing for all types of analyzedcomposites, while the average values of the Wc are gathered inTable 5.

Fig. 6. Dependence of KIIc – FA and Wc – FA replacement: (a) dependence KIIc – FA,(b) dependence Wc – FA .

The widest cracks in the ITZ area occurred in the concrete with30% FA addition (from 2.5 to 3.0 m) (Fig. 8j and k). Much less crackswere observed in concretes without the addition of the FA (Fig. 8aand b) and their average width was equal to 1.1 m (Table 5). SmallWc (to 800 nm) were observed in concretes FA-10 (Fig. 8d), whilethe most compacted microstructures had the concretes FA-20(Fig. 8g) with the smallest cracks in the ITZ area (up to 700 nm).This composite contains additionally:

� Mainly fibrous C-S-H phases of type (I) (Fig. 8h).� C-S-H phase type (II) in the form of a honeycomb structure

(Fig. 8g).� C-S-H phase type (IV) in the form of spherical agglomerates

(Fig. 8h), but with a much less intensity than in the phase oftype (I).

In the case of the concrete of series FA-00, a cluster of Ettringite(E) needles occurred in the matrix pores and CH crystals aroundthe ITZ area between paste and coarse aggregate (Fig. 8a). The mainphase present in those composites was large hexagonal plates ofportlandite with overgrowing E needles (Fig. 8b), see also [46]. Incase of concretes from series FA-10, phases C-S-H of type (I), (II)and (III) could also be observed, however, in quite small quantities(Fig. 8d and e). In addition, CH plates packed in the structure of thephases C-S-H were visible, which may indicate that reactions ofportlandite transition into phase C-S-H. This SEM image may indi-cate that the 10% siliceous FA additive is too small and the pozzo-lanic reactions occurring in the concrete after 28 days did not allowfor the overreaction of CH phase. On the other hand the effect ofthose reactions is already partially visible. However, the 20% FAadditive causes a distinct replacement of weak CH phase for a com-pact and durable phase C-S-H (Fig. 8h), see also [45]. The results ofthe experiments presented in this paper carried out for cementpastes with the addition of 2 types of FA (original and classified)showed that the concretes with the 20% FA additive have loweramount of CH and smaller pore sizes in comparison to the con-cretes without the FA additive. This compact concrete structurewith pores of small diameter has a positive influence on thestrength and resistance to corrosion as well as, own testing indi-cated, on the fracture toughness at shear. The concretes of seriesFA-30 were clearly characterized by amorphous clusters of C-S-Hphase (III) (Fig. 8k).

The EDS analysis, shown in Fig. 8, was performed at sites of theC-S-H phases. Analysis were performed at the points marked withx in the SEM images. The EDS analysis confirms clearly differencesin amount of the C-S-H. These differences can be attributed to thedifferent amounts of the FA in each composites. The EDS analysisconfirms that the CaO/SiO2 molar ratio of the C-S-H formed in poz-zolanic reaction in concretes containing FA additives is signifi-cantly lower than that molar ratio of C-S-H in FA-00 (Fig. 8c, f, i

Fig. 8. SEM micrographs of samples of analyzed concretes: (a) SEM of FA-00 showing microcracks in ITZ, CH crystal and ettringite (Mag: 2,40,000), (b) SEM of FA-00 showingCH plates, C-S-H phase and ettringite (Mag: 16,000), (c) EDS, C-S-H showed in point x in micrograph b, (d) SEM of FA-10 showing microcracks in ITZ and C-S-H phase (Mag:24,000), (e) SEM of FA-10 showing CH plates and C-S-H phase (Mag: 2400), (f) EDS, C-S-H showed in point x in micrograph e, (g) SEM of FA-20 showing microcracks in ITZ andC-S-H phase (Mag: 10,000), (h) SEM of FA-20 showing C-S-H phases (Mag: 10,000), (i) EDS, C-S-H showed in point x in micrograph h, (j) SEM of FA-30 showing microcracks inITZ (Mag: 4000), (k) SEM of FA-30 showing microcracks in ITZ and C-S-H phase (Mag: 16,000), (l) EDS, C-S-H showed in point x in micrograph k.

Table 5Width of cracks Wc.

Concrete Width of cracks Wc (lm) s (MN/m3/2) m (%)

FA-00 1.1 – (100% – comparative concrete) 0.075 4.63FA-10 0.8 – (72.7% in relation to FA-00) 0.058 5.21FA-20 0.7 – (63.6% in relation to FA-00) 0.061 6.12FA-30 2.7 – (245.4% in relation to FA-00) 0.13 6.89

212 G.L. Golewski, T. Sadowski / Construction and Building Materials 51 (2014) 207–214

and l). The change in the CaO/SiO2 ratio of C-S-H is clearly visible inthe comparison to the plain concrete and the concrete with 20%addition of the FA. In the case of FA-00 CaO/SiO2 ratio is equal to7, while in the concrete FA-20, only about 1.2 (Fig. 8c and i). Inthe concretes FA-10 and FA-30 CaO/SiO2 values are also lower thanin the FA-00, but slightly higher than in the FA-20 (Fig. 8f and l). Inthe concrete FA-10 CaO/SiO2 ratio is about 3 (Fig. 8f), while in theFA-30, about 2 (Fig. 8l). Differences in the CaO/SiO2 ratios in partic-ular concretes containing FA are probably related to the different

G.L. Golewski, T. Sadowski / Construction and Building Materials 51 (2014) 207–214 213

intensity in the pozzolanic reaction of these materials. This con-firms the SEM images and the fracture toughness tests. Lower va-lue of CaO/SiO2 molar ratio indicates that major part of the FAadditive reacted with CH, see also [47]. This causes significant in-crease in the C-S-H phases, improved sealing of the cement matrixand consequently the higher: concrete strength and shear fracturetoughness.

Basing on the variation of parameters KIIc and Wc in relation tothe amount of the FA additive, one can distinguish 2 regions of thefracture toughness (Fig. 6). When the FA addition is present in theamount ranging from 0% to 23% the level of the shear fracturetoughness varies within the range K IIc P 4:24MN=mð3=2Þ. The localmaximum corresponds to weight content of the FA = 17% and theminimum value of the mean cracks width Wc ¼ 0:6lm. This meansthat the internal structure of this concrete has higher cracks resis-tance in comparison to concrete without FA additive or with the FAcontent equal to 23%, i.e. when K IIc P 4:24MN=mð3=2Þ. One can ob-serve also by SEM a denser concrete structure in the ITZ area (dropof the Wc by 36.4% in comparison to the comparative concreteFA-00, for the maximum value K IIc ¼ 4:41MN=mð3=2Þ). When partof the cement is replaced by 30% FA, there is a sharp decrease ofthe KIIc by 13.9% and a significant increase in the Wc in concreteby 145.5%. With this amount of additive the concrete is more softand is characterized by the lower shear cracks resistanceðK IIc ¼ 3:65MN=mð3=2ÞÞ.

Modification of the concrete structure with active mineral addi-tives, such as the FA, is justified by ecological as well as economicreasons. The own tests showed that this active filling material hasbeneficial influence on delaying the destructive processes in thematerial and on the increase of the material’s fracture toughness.The replacement of 10% and 20% of cement with the additive ofactive pozzolana-siliceous FA caused the increase of the KIIc param-eter of the concrete after 28 days of curing. Moreover, it was ob-served that the concrete from the batch FA-10 and FA-20 hadhigher compression strength than the concrete without additives.The conducted tests revealed that the increase of the fracturetoughness KIIc occurs when the FA additive in the concrete rangesfrom 10% to 20%. Sealing of the composite structure can be thenobserved due to occurrence of smaller damages in the ITZ area(Table 4, Fig. 8) and this is related to limiting the amount of CH[46] due to its reaction with active silica included in the FA. As aresult of the formation of larger amounts of hydrated calcium sili-cates and due to formation of tobermorite [48] the concrete withthe 10% or 20% FA additive can reach higher strength and is char-acterized by the higher fracture toughness. When the FA additiveexceeds 20% there is a reduction in the formation of tobermorite[48] as well as decrease in the strength of the composite and muchlower fracture toughness. The FA addition of more than 20% mainlyplays a role of active micro-aggregate in the concrete [13,43]. After28 days of curing, the composites containing 30% addition of the FAwere characterized by lower strength parameters (Table 3) as wellas reduced fracture toughness KIIc (Table 4). The increase of thefcm15 and the KIIc in the concretes of series FA-10 and FA-20 isdue to changes of the internal structure of these composites bythe FA additive and smaller cracks sizes in the ITZ area in compar-ison to the comparative concrete FA-00 and the concrete FA-30.After 28 days of curing, 20% FA addition caused the very significantincrease of C-S-H phase mainly of type (I), which resulted instrengthening of the matrix and reducing its susceptibility to dam-age. The sharp increase in C-S-H phases in the composites FA-20was also visible in the EDS analysis. 20% addition of the FA resultedin a significant decrease of the CaO/SiO2 ratio from the value of 7for FA-00 to the value of 1.2 for FA-20 (Fig. 8). In concretes withoutaddition of the FA, the E needles (which occurred in the porousarea of the matrix) and CH crystals in the ITZ area were clearlyobserved. The composites with the 30% FA addition were

characterized by smallest amount of clearly developed C-S-Hphases, which resulted in the significant decrease of their strengthparameters and the lower fracture toughness.

4. Conclusions

From this research the following conclusions can beformulated:

1. The FA additivies change the shear fracture toughness KIIc. Theconcretes with the FA additive ranging from 10% to 23% arecharacterized by the regular fracture toughness with the localmaximum corresponding to 17%. Addition of more than 23%of the FA has negative consequence – it creates the concretecomposites with the lower shear cracks resistance – up to13.9% for the FA additive content equal to 30%

2. After 28 days of curing the most compact microstructure withwell developed C-S-H phase were characterized by the concretecontaining 20% FA additives (Fig. 8).

3. The biggest cracks in ITZ area between paste and coarse aggre-gates were observed in the concretes containing 30% FA, whilethe smallest one in the concretes with 10% and 20% FA additives(Fig. 8).

Acknowledgements

The research leading to these results has received funding fromthe European Union Seventh Framework Programme (FP7/2007 –2013), FP7 - REGPOT – 2009 – 1, under Grant agreement No:245479; CEMCAST. The support by Polish Ministry of Science andHigher Education – Grant No 1471-1/7.PR UE/2010/7 – is alsoacknowledged.

This work was financially supported by Ministry of Science andHigher Education within the statutory research number S/20/2013.

References

[1] Chindaprasirt P, Rukzon S. Strength, porosity and corrosion resistance ofternary blend Portland cement, rice husk ash and fly ash mortar. Constr BuildMater 2008;22:1601–6.

[2] Ahmaruzzaman M. A review on the utilization of fly ash. Prog Energy CombustSci 2010;36:327–63.

[3] Davis RE, Carlson RW, Kelly JW, Davis HE. Properties of cements and concretescontaining fly ash. ACI J 1937;33:577–612.

[4] Berry EE, Malhotra VM. Fly ash for use in concrete – a critical review. ACI J1980;77:59–73.

[5] Tkaczewska E, Mróz R, Łój G. Coal-biomass fly ashes for cement production ofCEM II/A-V 42. 5 R. Constr Build Mater 2012;28:633–9.

[6] Raghu Prasad BK, Eskandari H, Venkatarama Reddy BV. Prediction ofcompressive strength of SCC and HPC with high volume fly ash using ANN.Constr Build Mater 2009;23:117–28.

[7] Mardani-Aghabaglou A, Ramyar K. Mechanical properties of high-volume flyash roller-compacted concrete designed by maximum density method. ConstrBuild Mater 2013;38:356–64.

[8] Duran Atis C, Karahan O. Properties of steel fiber reinforced fly-ash concrete.Constr Build Mater 2009;23:392–9.

[9] Garbacz A, Sokołowska JJ. Concrete-like polymer composites with fly ashes –comparative study. Constr Build Mater 2013;38:689–99.

[10] Sear Lindon KA. Properties and use of coal fly ash, a valuable industrial by-product. London: Thomas Telford Ltd; 2001.

[11] Wang A, Zhang Ch, Sun W. Fly ash effects: I. The morphological effect of fly ashCem. Concr Res 2003;33:2023–9.

[12] Wang A, Zhang Ch, Sun W. Cem. Fly ash effects: II. The morphological effect offly ash. Concr Res 2004;34:2057–60.

[13] Wang A, Zhang Ch, Sun W. Fly ash effects: III. The microaggregate effect of flyash. Cem Concr Res 2004;34:2061–6.

[14] Franus W. Characterization of X-type zeolite prepared from coal fly-ash. Pol JEnviron Stud 2012;21:337–43.

[15] Tang WC, Lo TY, Chan WK. Fracture properties of normal and lightweight high-strength concrete. Mag Concrete Res 2008;60:237–44.

[16] Naga Satish Kumar Ch, Gunneswara Rao TD. Punching shear resistance ofconcrete slabs using mode-II fracture energy. Eng Fract Mech 2012;83:75–8.

214 G.L. Golewski, T. Sadowski / Construction and Building Materials 51 (2014) 207–214

[17] Golewski G, Sadowski T. Fracture toughness at shear (mode II) of concretesmade of natural and broken aggregates. In: The Eight InternationalSymposium on Brittle Matrix Composites; 2006. p. 537-546.

[18] Reinhardt HW, Ozbolt J, Xu S, Dinku A. Shear of structural concrete membersand pure mode II testing. Adv Cem Bas Mater 1997;5:75–85.

[19] Sadowski T, Golewski G. Effect of aggregate kind and graining on modeling ofplain concrete under compression. Comput Mater Sci 2008;43:119–26.

[20] Van Mier JGM. Fracture processes of concrete. Boca Raton, NY, London, Tokyo,Florida: CRC Press; 2000.

[21] Nemat – Nasser S, Obata M. A microcrack model of dilatancy in brittlematerial. J Appl Mech 1988;55:24–35.

[22] Sadowski T. Modelling of semi-brittle MgO ceramics behavior undercompression. Mech Mater 1994;18:1–16.

[23] Sadowski T, Samborski S. Modelling of porous ceramic response tocompressive loading. J Am Ceram Soc 2003;86:2218–21.

[24] Gencel O, Koksal F, Ozel C, Brostow W. Combined effects of fly ash and wasteferrochromium on properties of concrete. Constr Build Mater 2012;29:633–40.

[25] Tanyildizi H, Coskun A. The effect of high temperature on compressivestrength and splitting tensile strength of structural lightweight concretecontaining fly ash. Constr Build Mater 2008;22:2269–75.

[26] Turhan S, Arikan IH, Yucel B, Varinlioglu A, Kose A. Evaluation of theradiological safety aspects of utilization of Turkish coal combustion fly ashin concrete production. Fuel 2010;89:2528–35.

[27] Alhozaimy A, Soroushian P, Mirza F. Effect of curing conditions and age onchloride permeability of fly ash mortar. ACI Mater J 1996;93:87–95.

[28] Reinhardt HW. A new Compact Shear Specimen. Int J Fract 1981;17:R105–7.[29] Reinhardt HW, Xu S. Experimental determination of KIIc of normal strength

concrete. Mater Struct 1998;31:296–302.[30] Watkins J. Fracture toughness test for soil–cement samples in mode II. Int J

Fract 1983;23:R135–8.[31] Rao KB, Desai VB, Mohan DJ. Probabilistic analysis of mode II fracture of

concrete with crushed granite stone fine aggregate replacing sand. ConstrBuild Mater 2012;27:319–30.

[32] Bazant ZP, Pfeiffer PA. Shear fracture tests of concrete. Mater Struct1986;19:111–21.

[33] Murakami Y. Stress intensity factors handbook. Oxford: Pergamon Press; 1987.[34] Petrova V, Sadowski T. Theoretical analysis of mode II cracks in a Compact

Shear Specimen. Comput Mater Sci 2012;64:248–52.

[35] Petrova V, Marsavina L, Sadowski T. Reviset of compact mode II crackspecimen: analysis and fracture interpretation. Theor Appl Fract Mech2012;59:41–8.

[36] Golewski GL, Golewski P, Sadowski T. Numerical modeling crack propagationunder mode II fracture in plain concretes containing siliceous fly ash additiveusing XFEM method. Comput Mater. Sci 2012;62:75–8.

[37] Rao GA, Rao AS. Toughness indices of steel fiber reinforced concrete undermode II loading. Mater Struct 2009;42:1173–84.

[38] Ribeiro S, Rodrigues JA. The influence of microstructure on the maximum loadand fracture energy of refractory castables. Ceram Int 2010;36:263–74.

[39] Prokopski G, Halbiniak J. Interfacial transition zone in cementitious materials.Cem Concr Res 2000;30:579–83.

[40] Xia C, Jianjun Y, Huaguan Y. Influence of aggregates on cracking sensitivity ofconcrete. Appl Mech Mater 2012;204–208:3299–302.

[41] S. Diamond, Cement paste microstructure: An overview at several levels. In:Hydraulic Cement Pastes. In: Their Structure and Properties, Proc SympCement and Concrete Association; 1977. p. 2-31.

[42] Beaudoin JJ, Gu P, Myers RE. The fracture of C-S-H and C-S-H/CH mixtures. CemConcr Res 1998;28:341–7.

[43] Zhang MH. Microstructure, crack propagation and mechanical properties ofcement pastes containing high volumes of fly ashes. Cem Concr Res1995;25:1165–78.

[44] Morsy MS, Alsayed SH, Aqel M. Effect of elevated temperature on mechanicalproperties and microstructure of silica flour concrete. Int J Civ Environ Eng2010;10:1–6.

[45] Chindaprasirt P, Jaturapitakkul C, Sinsiri T. Effect of fly ash fineness onmicrostructure of blended cement paste. Constr Build Mater2007;21:1534–41.

[46] Walsh D, Otooni MA, Taylor Jr ME, Marcinkowski MJ. Study of Portland cementfracture surfaces by scanning electron microscopy techniques. J Mater Sci1974;9:423–9.

[47] Tkaczewska E, Małolepszy J. Hydration of coal-biomass fly ash cement. ConstrBuild Mater 2009;23:2694–700.

[48] Jing Z, Jin F, Hashida T, Yamasaki N, Ishida EH. Influence of addition of coal flyash and quartz on hydrothermal solidification of blast furnace slag. Cem ConcrRes 2008;38:976–82.