Micromechanical Structure-Property Relationshipsfor the Damage Analysis of Impact-Loaded

Sustainable ConcreteSavaş Erdem1; Andrew Robert Dawson2; and Nicholas Howard Thom3

Abstract: In this study, quantitative microstructure-property relationships are mainly used to characterize the damage due to high-strain-rateimpact loading and the mechanical behavior of concretes prepared by substituting natural aggregate (gravel) with recycled aggregates havingdifferent rigidities (blue brick and rubber). Based on the results obtained, a possible mechanism for microstructural damage in concrete isproposed. It is concluded that the aggregate causes a change in the initial interfacial transition zone (ITZ) condition, and it is this altered ITZcondition that has a major effect on overall mix behavior. The analysis also indicates that there is almost a linear correlation between theroughness values (Ra) of the region near the paste–aggregate interface and the dissipated surface fracture energy values of the specimens.Moreover, three-dimensional topographic images of the specimens constructed using a vertical nanotech scanning interferometer show thatthe paste region of the gravel specimen has the smoothest profile due to the relatively strong hydrated paste.DOI: 10.1061/(ASCE)MT.1943-5533.0000616. © 2013 American Society of Civil Engineers.

CE Database subject headings: Sustainable development; Impact loads; Damage; Micromechanics.

Author keywords: Sustainable concrete; Micro and pore structure; Impact loading; Interfacial zone; Damage mechanism.

Introduction

The response of concrete to dynamic loading is of interest in bothcivilian and military applications. Understanding the behavior ofconcrete subjected to impact or explosive loading is crucial forthe effective protection of defense structures (Grote et al. 2001).Apart from deliberate impact loads, concrete structures in practicecan frequently be subjected to a range of accidental impacts such ascolumns in underground car parks, overpass bridges, and medium-to low-rise buildings located close to major roads and intersections(Thilakarathna et al. 2010) or rock falls on roadways in mountain-ous areas (Mouging et al. 2005).

In general, when a concrete material is subjected to high ratesof loading, three major damage regions (Fig. 1) are formed: a craterregion, a crushed aggregate region, and an extensive crackingregion due to the propagation and reflection of the impact-inducedstress waves from specimen boundaries (Zhang et al. 2007) wherescabbing may occur. However, more specifically, the entiredamage process for a brittle material could consist of initial elasticdeformation, microcracking, fragmentation, rubblization, andpostrubblization flow, but the primary damage stems from distrib-uted microcracking and plastic flow followed by localization,which causes global failure of brittle materials (Park et al. 2001).

In addition, highly dynamic loading results in a high strain rate(up to 106 s−1) and generates additional nonlinear effects in the brit-tle material such as the failure of micropores (Larcher 2009).Because loading rates are so high, the effect of strain relief aroundone growing crack has no influence on the material response inother zones of the material during an impact event; the stress–strainwaves resulting from strain relief take longer to reach those zonesthan the period for which they experience the direct effects of themain load application. Hence, because stress and strain cannot beredistributed as the load is applied, the strain energy remains atthe crack tip and the extension of microcracks would be very rapid.In turn, this could force the cracks to develop along a shorterfracture path of higher resistance (Zhang 2008). Clearly, themicrostructure of the impacted brittle material undergoes dramaticchanges through the process.

Concrete has been traditionally evaluated through its mechani-cal, physical, and functional properties. However, developmentof advanced and effective inspection techniques during the last de-cade has demonstrated that the atomic-level properties of concretehave a profound effect on its macro-level properties, and thesestructure-property relationships lie at the heart of modern concretetechnology. For example, Zhou and Hao (2008) concluded, basedon their own mesoscale model, that the failure time and thedynamic tensile strength of concrete under high strain rates aresignificantly affected by the thickness of the interfacial transitionzone (ITZ) between the aggregate and the matrix. Such a conclu-sion leads to the questions of what happens at the microstructurallevel during an impact event and how changes at the micro scaleaffect failure behavior at a macro level.

The authors were unable to discover a systematic study of themicrostructure-property relationship for the impact response ofconcrete. The paucity of data leads to the objective of this study,which is to link quantitatively and qualitatively the micro-scaleproperties, in particular the effects of the ITZ, to the macro-scaleimpact properties and physical properties such as strength and

1Lecturer, School of Civil Engineering, Univ. of Istanbul, AvcilarCampus, 34320, Turkey (corresponding author). E-mail: savas.erdem@istanbul.edu.tr

2Professor, School of Civil Engineering, Univ. of Nottingham, Univer-sity Park, NG7 2RD, UK. E-mail: andrew.dawson@nottingham.ac.uk

3Lecturer, School of Civil Engineering, Univ. of Nottingham, Univer-sity Park, NG7 2RD, UK. E-mail: nicholas.thom@nottingham.ac.uk

Note. This manuscript was submitted on November 20, 2011; approvedon June 12, 2012; published online on August 27, 2012. Discussion periodopen until October 1, 2013; separate discussions must be submitted for in-dividual papers. This paper is part of the Journal of Materials in CivilEngineering, Vol. 25, No. 5, May 1, 2013. © ASCE, ISSN 0899-1561/2013/5-597-609/$25.00.

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stiffness of concrete produced with different elasticity conventional/unconventional aggregates.

Experimental Methodology

Materials Used and Concrete Mixtures

The cement used in the study reported here was general-purposePortland-fly ash cement CEM II/B-V 32.5 R. Natural gravel witha size of 4=14 mm and local river sand with a specific gravity of2.66 constituted the reference mix. In addition, two types of uncon-ventional coarse aggregate were used in this study. In the firstcase, new solid engineering blue bricks of 215 × 102.5 × 65 mmworking size were crushed down into a coarse aggregate of size4/14 mm and used to produce concrete. The second type wasrecycled shredded tire obtained by mechanical shredding into a4=14 mm particle size. Table 1 shows the physicomechanical prop-erties of the aggregates used.

The mix proportions are given in Table 2. In all mixes, theamounts of cement, sand, and free water were the same. The massof coarse aggregate was adjusted for each mix to keep the samevolume fraction (0.420). Thus, the only difference was the typeof coarse aggregate used in the mixtures. Concrete with gravelaggregate was used as the control concrete. The coarse aggregateof the control mix was totally replaced by rubber and blue brickparticles. The designation was on a total volume basis. The samesize percentages were selected for the gravel aggregate and uncon-ventional aggregates to eliminate the effect of grading differenceon concrete performance. Before the mixing, the aggregates werefirst immersed in water for 24 h until all particles were fully satu-rated before the mixing and subsequently air dried in the laboratoryto obtain, approximately, a saturated surface dry condition. In thiscondition, the aggregates cannot absorb any more water and soare not expected to pull water away from cement hydration sites.

All concrete mixtures were batched using a mechanical pan mixer,placed in oiled steel molds in two layers. The slump test was notperformed on the concrete samples, but visual observation indi-cated that the consistencies of the different types of concrete werequite stiff, between 40 and 50 mm. ASTM C192/C192M (ASTM1998) recommends the use of vibrating and rodding for compactionfor such types of concrete. In the case of this project, each layerwas compacted using a vibration table before being covered withplastic sheets. The specimens were left in their molds for 1 daybefore demolding and cured at 20� 2°C in a water tank until theday of testing.

Tests and Analysis Performed

Engineering PropertiesFour cubes (100 × 100 × 100 mm) were used to measure thecompressive strengths of the mixtures at the age of 28 days inaccordance with the relevant British European standard. Threecylinders, 150 mm in diameter and 300 mm long, were also pre-pared for each mix in order to determine the modulus of elasticity.The specimens were end-capped using sulfur mortar prior to testingto ensure parallel loading faces and thus to prevent stress con-centrations. Each specimen was fixed with four potentiometersat different quadrants to measure the deformations. The staticmodulus of elasticity in compression was determined from theslope of stress–strain curves.

Drop Weight Impact TestingImpact tests were carried out using a Rosand Type 5 instrumentedfalling weight impact tester, shown schematically in Fig. 2. Theimpact machine is capable of dropping a 120 kg mass from heightsof up to 3 m onto the target specimen. Disk specimens 150 mm insize were placed vertically on a cylindrical steel base (diameter ¼150 mm) located at the center of the impact machine (Fig. 2).The hammer was dropped from a height of 500 mm to provide astriking velocity of 3 m=s. An accelerometer attached to theimpact hammer was used to measure acceleration during the impactprocess. From this information displacement was calculated.

Microstructural Features for Impact DamageThe change in pore structure before and after impact testingwas one of the available evaluation indices for the impact damage.The pore structure of the mixes was measured by the mercuryintrusion porosimetry (MIP) technique with a pressure appliedup to 410 MPa using a Quantachrome PoreMaster mercury poros-imeter. For the porosity characteristics of the specimens beforeimpact load, cores were first taken from the middle of thecubes. Then, small concrete pieces were cut from each con-crete core sample. To prepare the samples for the pore structureanalysis of the impacted specimens, the fragments were selectedfrom the center of the failed disks. Pores with a radius less than

Fig. 1. Fracture regions in concrete subjected to an impact loading[data from Zhang et al. (2007)]

Table 1. Physicomechanical Properties of Aggregates Used

Characteristics Gravel Rubber Blue brick

Specific gravity 2.65 1.15 2.85Surface roughness: Ra (μm) 5.6 1.4 13.2Angularity 1.12 1.16 1.14Impact value (%) 12 — 6.1Fractal dimension 1.08 1.02 1.02Modulus of elasticity (GPa) 56.9 10.7 65.8

Table 2. Concrete Mixtures

Material (kg=m3) Gravel RubberBluebrick

Volumefraction (%)

Cement 330 330 330 11.1Water 198 198 198 19.8Sand 678 678 678 25.1Gravel (coarse aggregate) 1,115 0 0 42.0Rubber (coarse aggregate) 0 485 0 42.0Blue brick (coarse aggregate) 0 0 1,200 42.0

Note: Replacement was based on volumetric measurement with theproportions of all the other ingredients remaining same.

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100 nanometer (nm) are considered micropores, whereas poreswith a radius greater than 100 nm are classified as macropores.The percentage of content of the micro- and macropores was cal-culated after impact.

The loss of microhardness in the ITZ due to impact loading waschosen as another index of impact damage evaluation. A Vickersmicrohardness test was carried out to determine the microhardnessin the transition zone between the aggregates and the hardenedcement paste. The analysis was obtained between 5 and 50 μmfrom the aggregate interface. Another important feature of theITZ is its thickness. The ITZ thickness has been regarded as thedistance from the aggregate surface to the point where the localporosity value approaches the observed porosity of the bulk paste.The quantification of the porosity in the ITZ was performed bymeans of the technique developed by Scrivener and Pratt (1986).An example of this type of analysis is given in Fig. 3. Then theinfluence of the ITZ thickness values on the failure propertiesdue to impact (the travel time of stress waves and the correspondingload) was analyzed.

Finally, three-dimensional micrographs of the fracture surfaceswere constructed using a vertical scanning interferometer (Nano-tech Photomap 3D), making it possible to focus solely on thepaste regions and, more specifically, on the interfacial zone inthe vicinity of the aggregate particles. Microsurface roughness(Ra) values, which were defined as the mean value of surfacerelative to the center plane, were calculated for each mix. A meanvalue of three measurements was used as a response value for eachexperiment. These values were then used to help explain the micromechanism of fracture. In addition, the quantitative analysis ofsurface macrocracks was also performed using image software(Image Pro Plus).

X-Ray Computed Tomography InvestigationTo investigate the void characteristics of the mixtures, sequences oftwo-dimensional images were captured throughout the height ofthe 100 mm cube specimens with a slice thickness of 1 mm using

the nondestructive X-ray computed tomography (CT) system.The system had a 350 kV X-ray source and a line detector. Thecaptured images from the X-ray CT system were then analyzedusing an image analysis software package (Image J). Finally, theconverted images were used to identify and quantify air voidswithin the specimens.

Thermogravimetric AnalysisThermogravimetric analysis was conducted on the hardened mortarsamples taken from the crushed concrete cubes, after 28 days ofcuring, to determine the quantity of Ca ðOHÞ2 according to theprocedure described by Peschard et al. (2004). Dried pieces fromthe fractured cubes were ground to a fine powder, and samples of10 mg were then placed in preweighed aluminum containers. ATGAQ500 thermogravimetric analyzer with ceramic pans wasused in the execution of this test. The analyzer was programmedto heat the sample up to 600°C at a rate of 3°C per minute upto 220°C and 10°C per minute thereafter. The tests were conductedin a nitrogen atmosphere. The weight loss–temperature (TGA)combined with derivative weight loss–temperature (DTG) profileswere recorded.

Results and Discussion

Engineering Properties

Fig. 4 represents the relationship between the average cube com-pressive strengths and the average hardened densities of the mix-tures. It can be seen that there is a very drastic drop in strength whenthe gravel aggregate is totally replaced by rubber particles. The de-crease in the compressive strength ranks the same as density. Theseresults are consistent with those of other studies (Eldin and Senouci1993; Khatib and Bayomy 1999; Taha et al. 2008). To understandwhy there is such a change in the static strength of the unconven-tional mixes compared to that with conventional aggregates, themicrostructure of the mixes was studied more closely.

It is well known that the compressive strength of plain concretecorrelates to the formation of the structure of the hardening cementmatrix as a result of a chemical reaction between cement and water—the degree of hydration (Schutter 2004). Data obtained from the ther-mogravimetric analyzer program are shown graphically in Fig. 5.

Fig. 2. Schematic diagram of impact test setup [data from Brown(2007)]

Fig. 3. SEM micrograph of paste surrounding aggregate

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The points of interest are labeled 1. These peaks show calciumhydroxide (CH) being decomposed in a temperature range of ap-proximately 425–475°C. The percentage loss of CH was 5.20,5.70, and 4.30% for the blue brick, gravel, and rubberized mortar,respectively, signifying that the most pronounced chemical activityin the development of calcium silicate hydrate (CSH) and CH crys-tals occurred for the gravel concrete mix, which was not the strongestmix. The blue brick mix gave the highest strength yet its lowest levelof CH, suggesting that less hydration has taken place and, hence, alower mechanical strength could be expected. This mismatch sug-gests that the strength was dependent on the aggregate strength ratherthan the degree of hydration. Weakness can also suggest that micro-structural development is less uniform. Thus, the large decrease instrength of the concrete with rubber particles relative to the other twomixes could suggest a partial absence of adequate CSH gels inthe ITZ.

Fig. 6 shows a typical image of the blue brick mix obtained bythe X-ray technique coupled with digital image analysis. The imageafter pseudo color transformation is also included for comparison.The difference in colors shows the different material phases in themixture. The images for the gravel and rubberized mixes are notpresented here due to space limitations, but the calculated voidcontents of the mixtures derived from multiple horizontal sliceassessments made through each specimen’s height are presentedin Fig. 7. The analysis clearly shows that the inherent void distri-bution is not homogeneous for the mixtures, and the blue brick mixpossesses the lowest air void content. Such a distribution in the bluebrick mix could be attributed to a lower interconnectivity of the ITZ(more homogeneous ITZ) and orientation of the brick particles,which could allow the further movement of the voids during com-paction. However, the void content in the rubberized specimen wasmuch higher than in the other two specimens, which would cause adramatic increase in the destructive transverse tensile strains duringloading, leading to premature failure.

Fig. 5. TGA–DTG curves of mixes Fig. 6. Typical X-ray CT image of blue brick mix

Fig. 4. Relationship between strength and densities of mixes

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Fig. 8 shows the typical stress–strain curves of the mixes incompression. Results for the modulus of elasticity determined fromthe stress–strain curves are also presented in the figure. As can beseen from Fig. 8, the rigidity of the mix with crushed brick ismuch higher than those of the other mixes. By contrast, the mix

with rubber particles exhibited a greater deformability and energy-absorbing capacity (ductile plastic failure), although its peak stressis very low. There was some similarity in the failure mechanism forthe blue brick and gravel samples. After testing, the blue brick andgravel concrete showed signs of failure emanating from crackingpredominantly through the ITZ. However, in the case of rubberizedconcrete, the crack surfaces were bridged by rubber particles, andeventually the particles debonded along both sides of the interfaces,leading to a more tortuous failure surface. In addition, the distinctcompressible characteristics of the rubber particles increasedtoughness due to microcrack path lengthening and energy dissipa-tion. Based on the visual observations carried out on the crackingfaces, the failure states of the mixtures containing rubber particlesdid not exhibit any separation as the rubber particles form a tightbridge across cracks in the matrix. The nature of the aggregate-matrix bond may be responsible for the decrease/increase in theelastic modulus of concrete. For instance, an interfacial bond con-taining severe defects would quickly degrade and lose its loadingcapacity, and thus the composite would not fully achieve thepotential modulus of elasticity (Chan 1998). However, basedon composite mechanics, the elastic modulus of concrete depends,to a great extent, on the modulus of aggregate. For the same mixdesign conditions, the stiffer the aggregate, the stiffer the concretebecomes.

Fig. 8. Stress–strain curves of mixes in compression

Fig. 7. Air void distributions of specimens

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Microhardness is a parameter dependent on various character-istics of the ITZ including the mean crystal size, crystal orientationindex of CH, and pore microstructure (Gao et al. 2005). Fig. 9shows microhardness test results at the ITZs along with the calcu-lated modulus of elasticity of the specimens. It is seen in Fig. 8that the trend for microhardness variation is quite similar to thatof the compressive strength and the modulus of elasticity. It is sup-posed that a weaker ITZ could yield lower microhardness valuesand, correspondingly, poor mechanical performance in rubberizedspecimens. It is established that, generally, normal gravel and rub-ber are inert and therefore will not trigger any chemical activity inconcrete under normal circumstances. Further, X-ray diffraction(XRD) results (Fig. 10) show that the blue brick possessed no com-pound that was potentially reactive (crystal structure) in concrete.Hence it can be concluded that changes in the ITZ were not theresult of any chemical reaction between the respective type of

aggregate and the cement paste. It is postulated that the utilizationof water in fresh concrete is partly influenced by the surface andgeometrical characteristics of the aggregate particles. It is thereforebelieved that these aggregate characteristics influenced the effectivewater/cement ratio and, hence, on the pore water in the ITZ. Forexample, less free water will be available for the transition zonein the blue brick concrete due to a much higher surface roughnessand large overall surface area of the brick particles. This will, inturn, result in a better packing of cement particles against aggregatesurfaces and less thick and strong ITZs.

Microstructure-Impact Response Relationship

Fig. 11 shows load-time histories for the concrete mixes duringthe first drop of impact. As can be seen, the magnitude of loadfor the blue brick concrete is much higher than those of both graveland rubberized specimens in the first cycle of wave propagation.The peak value is 21.20 kN compared to a peak load of 11.8and 4.20 kN for the gravel and rubberized concrete, respectively.Since the loading is constant (drop height ×mass × gravitationalconstant) in this study, different peak heights correspond to differ-ent times of loading, indicating different dynamic stiffness andplastic strains. Therefore, the blue brick mix (highest peak force)is likely to convey stress waves with less crack propagation whenit is subjected to impact loading. Based on composite mechanics,the elastic modulus of concrete is, to a great extent, positivelyrelated to the modulus of the aggregate. According to this theory,when a specimen is subjected to high rates of loading, a con-crete with rigid aggregates (such as blue brick) would mainly beable to dissipate energy by cracking rather than rebounding ordeforming. As cracks preferentially travel through the path ofleast resistance, the ITZ properties become of major importancefor the behavior of this concrete under loading. The strongerITZ in this concrete would efficiently participate in transferringstress through the composite, resulting in a greater vertical impactreaction force.

Fig. 10. X-ray diffraction pattern of blue brick particle

Fig. 9. Microhardness versus static strengths of specimens

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Fig. 12 shows the changes in microhardness values afterfirst impact and provides a strong support for the foregoing inter-pretation. It is obvious that the impacted specimens displayeddegradation in the mean hardness values, likely due to crackdevelopment in the ITZ, but this was less pronounced for theblue brick specimen. The strong ITZ is also expected to providemuch greater distances between cracks. For these reasons, thespecimen will be able to withstand more loading before failureis observed.

It is also worth highlighting that the time taken for the compres-sion stress waves to travel from the impact point to the bottom ofthe rubberized specimen was almost three and five times higher

than for the gravel and blue brick specimens, respectively. Thisis a further demonstration of the lower overall stiffness of therubberized mix and a weaker structural integrity at the micro scale.Quantification of the ITZ porosity results (Table 3) shows thatwhen the ITZ is thicker, it requires a longer time for the stresswave to travel through the specimen body. This informationcan be used in the qualitative analysis of fracture mechanicsbecause a thinner ITZ approximately corresponds to less fracturedamage.

The variation of load-displacement histories of the mixesduring the first drop of impact is displayed in Fig. 13. In all cases,the specimens exhibited a quite irregular load-deflection responseover a relatively large strain range followed by an elastic recovery.The recovered strain in the case of the blue brick mix lower inthe same way because it is much stiffer (Fig. 8). The figures alsoindicate that the rubber mix exhibits high plastic deformationwhereas the gravel and brick mixes have a similar plastic response,but these are much smaller than that of the rubber mix.

Taking these observations together, it appears that the rubberaggregate has deleteriously affected a wider zone of paste aroundeach particle, resulting in a thicker ITZ with less ability to carrystress waves effectively and a weaker overall performance. Poorer

Fig. 12. Changes in microhardness values after first impact

Table 3. Relationships between ITZ Thickness, Stress Wave TransmissionTime, and Impact Reaction Force

Concrete IDITZ thickness

(mm) Time (s) Force (N)

Gravel concrete 28 0.0053 11.8Rubberized concrete >50 0.0140 4.20Blue brick concrete 18 0.0032 21.20

Fig. 11. Load-time histories of mixes during first drop of impact

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aggregate characteristics alone could be responsible for a greaterITZ deterioration after loading because it might be expected to havehad to play a greater role in carrying the stress of the first loadpulse. But the initial condition demonstrates that, in fact, theaggregate causes a change in the initial ITZ condition, and it isthis altered ITZ condition that has a major effect on overall mixbehavior. The good performance of the blue brick mix also supportsthis interpretation because, although the individual pieces of brickare not as strong as those in the gravel, yet the mix is the strongest.Thus the benefit must be coming from the paste, and the resultsshow that it is in the form of an improved ITZ. It is also supposedthat the high rates of loading would tend to form a network ofinterconnected cracks between the micropores and preexisting

microcracks. Microcracks exist even before loading due to theincompatibility between the modulus of elasticity and shrinkageof the aggregate and the matrix (Acker et al. 1987; Van Mier1997), thereby leading to stress concentrations around the interfa-cial zone. These dissipate some of the impact energy and reducethe peak stress, which is very obvious in the rubberized specimen.A possible mechanism of the pore connectivity under impact isschematically illustrated in Fig. 14.

Fig. 15 shows cumulative MIP intrusion volume against porediameter curves before and after exposure to impact loading.The calculated percentage total pore space (micro- and macropores)is also tabulated in Table 4. In general, the analysis showed that theimpact loading resulted in a significant increase in the cumulative

Fig. 13. Load-deformation histories of mixes during first drop of impact

Fig. 14. Schematic representation of pore connectivity in matrix under loading: (a) before loading; (b) after loading

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pore volume and caused coarsening of the pore structure of theconcrete samples, which was found to be less in the rubberizedmix. The MIP test results clearly indicated that the percentageof the pore volume in the rubberized specimen occupied by macro-pores (pores larger than 100 nm) after impact was significantlylower than in both the gravel and blue brick mixes, and the leastdegeneration in the pore structure occurred in this mix. In addition,the analysis indicated that a refinement of pore structure, as is thecase in which the volume of micropores is greater than 65%, tookplace for the rubberized specimen. The cement matrix of themixtures used in this study show a higher fluidity due to the highwater/cement ratio. In this case, stabilization of air voids is moreimportant than generation of air voids (Lee et al. 2010). The non-polar nature of rubber particles may attract air voids and air voidsmay get trapped in their jagged surface textures (Fedroff et al.1996). In addition, the rubber particles absorb a relatively lowproportion of the energy that is required for the effective flowof the cementitious material due to their low density. For thesereasons, on the one hand, pores will inevitability expand in the tran-sition area and will be very hard to stabilize. On the other hand,microcracks will develop around the perimeter of the rubber par-ticles, which provides a more gentle crack propagation compared toother mixes.

Typical topographic images of the paste surfaces of the mixesare also displayed in Fig. 16. Zampini et al. (1995) showed that theroughness of the cracked surfaces near the paste–aggregate inter-face was higher than that of the cracked surface of the paste far fromthe aggregate, and there is a clear correlation between the fractureparameters (KIC and Δac critical stress intensity factors) and theaverage roughness of the paste. As can be seen from the figure,the rubberized paste region has a highly irregular and rougherfracture surface, which is beneficial under dynamic loading as itincreases the effective (or necessary) surface area on the fractureplane and consumes more surface energy before complete crackfailure. By contrast, the paste region of the gravel specimen hasa relatively smoother fracture profile. As concluded by Fickeret al. (2010), relatively stronger hydrated Portland cement pasteswill have smoother fracture surfaces because the finer productsof hydration reaction fill the gaps between the cement grains,resulting in a more uniform and less porous structure, hence deliv-ering greater strength.

Fig. 17 shows digitized contour crack maps of a typical failuresurface. These crack maps were used to calculate the fractal dimen-sions and fracture energies of the specimens based on surfacemacrocrack measurement using the formula suggested by Guo et al.(2007). Analysis using this approach indicates that concrete with

Table 4. Percentage of Pore Volumes and Critical Diameters before and after Impact

Specimen ID

Micropores (%) Macropores (%) Critical diameter (μm)

Before After Before After Before After

Gravel mortar 79.10 62.40 21.90 37.60 0.007 0.025Rubberized mortar 91.10 89.80 8.90 10.20 0.006 0.007Blue brick mortar 72.70 56.60 28.30 43.40 0.008 0.035

Fig. 15. Cumulative intrusion volume against pore diameter curves before and after impact: (a) gravel concrete; (b) rubberized concrete; (c) blue brickconcrete

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Fig. 16. Three-dimensional topographic images of paste fracture surfaces: (a) rubberized specimen; (b) gravel specimen; (c) blue brick specimen

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Fig. 17. Digitized surface crack maps of specimens

Fig. 16. (Continued.)

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rubber particles dissipates much greater fractal fracture energyduring impact, followed by the blue brick mix and the gravelmix. It is supposed that this was basically due to the fact that veryelastic rubber particles may produce local microinstabilities withinthe mix and, consequently, localized stress concentrations and,hence, greater crack initiation and propagation points, leading tolarger fractal dimensions. The results are also consistent withthe findings presented in the previous paragraphs and give somesupport to theory that higher microroughness usually results inhigher fracture energy consumption during fracture or vice versa(Wu et al. 2001). A quantitative analysis of the data presentedin Fig. 18 shows that a strong linear correlation exists betweenthe roughness number of the paste and dissipated surface fractureenergy of the specimens. Qualitatively, this signifies that therougher near-ITZ fraction of the bulk paste is more resistant tocracking at the macro level. The major findings of this study usinga grading system are summarized in Table 5.

Conclusion

In the light of the findings obtained from this experimental study,the following conclusions can be drawn:1. The analysis strongly suggests that the microscopic damage

mechanism under impact loading is highly complex, andthe aggregate causes a change in the initial ITZ condition;it is this altered ITZ condition that has a major effect on overallmix behavior.

2. The effect of the aggregate on the surface roughness of the ITZwas established for the first time in the concrete literature. Theroughness number of the area near the ITZ was found topositively correlate with dissipated surface fracture energy.An increase in the roughness number is associated with anincrease in the dissipated fracture energy.

3. The analysis reveals that when a specimen has both a thick andweak ITZ, it requires a longer time for the impact-inducedstress waves to travel through the specimen body. This reflectsthe fact that a thin but less strong interface would have arelatively lower ability to redistribute stress which makesthe material brittle.

4. The different types of aggregate produce unique changes thatresult from disturbances in the packing of the coarse and finehydration products and in the unhydrated cement particles.This is evident in the peaks and troughs of strength acrossthe length of the aggregate–paste boundary.

5. Chemical and porosity heterogeneities within the ITZ cancause fluctuations/disordering in the cracking path, resultingin an increase in the tortuosity and corresponding fractureenergy dissipation. In addition, a weak and porous ITZ trans-fers less stress from matrix to aggregate particles. This leads toa lower compressive strength but increased toughness due tomicrocrack path lengthening and energy dissipation.

6. Aggregates with low surface roughness increase the regionalporosity, which results in a reduction in the pore connectiv-ity, and aggregate with high surface roughness producesan opposite effect. Further, these changes help to controlthe moduli of elasticity at the micro level, in that thickerITZ results in a reduction of the modulus of elasticity andvice versa.

Acknowledgments

The authors would like to gratefully acknowledge Dr. M. T.Bassuoni and Dr. Kevin A Brown (University of Nottingham)for helpful discussions and advice. Special thanks must go todoctoral research student Ms. Lindy Heath (School of Chemistry-University of Nottingham) for her great help and valuable com-ments with regard to the TGA analysis.

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