Concrete, which was hitherto considered as a brittle material, has shown much better softening behavior after the post peak load than anticipated. This behavior of concrete did put the researchers in a quandary, whether to categorize concrete under brittle materials or not. Consequently concrete has been called a quasi-brittle material. Fracture mechanics concepts like Linear elastic fracture mechanics (LEFM) and Plastic limit analysis applicable to both brittle and ductile materials have been applied to concrete to characterize the fracture behavior. Because of quasi-brittle nature of concrete, which lies between ductile and brittle response and due to the presence of process zone ahead of crack/notch tip instead of a plastic zone, it is found that non-linear fracture mechanics (NLFM) principles are more suitable than linear elastic fracture mechanics (LEFM) principles to characterize fracture behavior. Fracture energy, fracture process zone (FPZ) size and the behavior of concrete during fracture process are the fracture characteristics, which are at the forefront of research on concrete fracture. Another important output from the research on concrete fracture has been the size effect. Numerous investigations, through mathematical modeling and experiments, have been carried out and reported in literature on the effect of size on the strength of concrete and fracture energy. Identification of the sources of size effect is of prime importance to arrive at a clear analytical model, which gives a comprehensive insight into the size effect. With the support of an unambiguous theory, it is possible to incorporate the size effects into codes of practices of concrete design. However, the theories put forth to describe the size effect do not seem to follow acceptable regression. After introduction in Chapter-1 and literature survey in Chapter-2, Chapter-3 details the study on size effect through three point bend (TPB) tests on 3D geometrically similar specimens. Fracture behavior of beams with smaller process zone size in relation to ligament dimension approaches LEFM. The fracture energy obtained from such beams is said to be size independent. In the current work Size effect law (Bazant et al. 1987) is used on beams geometrically similar in three dimensions with the depth of the largest beam being equal to 750mm, and size independent fracture energy G Bf is obtained. In literature very few results are available on the results obtained from testing geometrically similar beams in three dimensions and with such large depth. In the current thesis the results from size effect tests yielded average fracture energy of 232 N/m. Generally the fracture energies obtained from 2D-geometrically similar specimens are in the range of 60-70 N/m as could be seen in literature. From 3D-geometrically similar specimens, the fracture energies are higher. The reason is increased peak load, could be due to increased width. The RILEM fracture energy Gf , determined from TPB tests, is said to be size dependent. The assumption made in the work of fracture is that the total strain energy is utilized for the fracture of the specimen. The fracture energy is proportional to the size of the FPZ, it also implies that FPZ size increases with increase in (W−a) of beam. This also means that FPZ is proportional to the depth W for a given notch to depth ratio, because for a given notch/depth, (W−a) which is also W(1 − a ) is proportional to W`because (1 − a ) is a constant. WWThis corroborates the fact that fracture energy increases with size. Interestingly, the same conclusion has been drawn by Abdalla & Karihaloo (2006). They have plotted a curve relating fracture process zone length and overall depth the beam. In the present study a new method namely Fracture energy release rate method is suggested. In the new method the plot of Gf / (W−a) versus (W−a) is obtained from a set of experimental results. The plot is found to follow power law and showed almost constant value of Gf / (W−a) at larger ligament lengths. This means that fracture energy reaches a constant value at large ligament lengths reaffirming that the fracture energy from very large specimen is size independent. The new method is verified for the data from literature and is found to give consistent results. In a quasi-brittle material such as concrete, a fracture process zone forms ahead of a pre-existing crack (notch) tip before the crack propagates from the tip. The process zone contains a scatter of micro-cracks, which coalesce into one or more macro-cracks, which eventually lead to fracture. These micro-cracks and macro-cracks release stresses in the form of acoustic waves having different amplitudes. Each micro or macro crack formation is called an acoustic emission (AE) event. Through AE technique it is possible to locate the positions of AE events. The zone containing these AE events is termed the fracture process zone (FPZ). In Chapter-4, a study on the evolution of fracture process zone is made using AE technique. In the AE study, the fracture process zone is seen as a region with a lot of acoustic emission event locations. Instead of the amplitudes of the events, the absolute AE energy is used to quantify the size of the process zone at various loading stages. It has been shown that the continuous activities during the evolution of fracture process zone correspond to the formation of FPZ, the size of which is quantified based on the density of AE events and AE energy. The total AE energy released in the zone is found to be about 78% of the total AE energy released and this is viewed as possible FPZ. The result reasonably supports the conclusion, from Otsuka and Date (2000) who tested compact tension specimens, that zone over which AE energy is released is about 95% can be regarded as the fracture process zone. As pointed out earlier, among the fracture characteristics, the determination of fracture energy, which is size independent, is the main concern of research fraternity. Kai Duan et al. (2003) have assumed a bi-linear variation of local fracture energy in the boundary effect model (BEM) to showcase the size effect due to proximity of FPZ to the specimen back boundary. In fact the local fracture energy is shown to be constant away from boundary and reducing while approaching the specimen back boundary. The constant local fracture energy is quantified as size independent fracture energy. A relationship between Gf , size independent fracture energy GF , un-cracked ligament length and transition ligament length was developed in the form of equations. In the proposed method the transition ligament length al is taken from the plot of histograms of energy of AE events plotted over the un-cracked ligament. The value of GF is calculated by solving these over-determined equations using the RILEM fracture energies obtained from TPB tests. In chapter-5 a new method involving BEM and AE techniques is presented. The histogram of energy of AE events along the un-cracked ligament, which incidentally matches in pattern with the local fracture energy distribution, assumed by Kai Duan et al. (2003), along the un-cracked ligament, is used to obtain the value of GF , of course using the same equations from BEM developed by Kai Duan et al. (2003). A critical observation of the histogram of energy of AE events, described in the previous chapter, showed a declining trend of AE event pattern towards the notch tip also in addition to the one towards the specimen back boundary. The pattern of AE energy distribution suggests a tri-linear rather than bi-linear local fracture energy distribution over un-cracked ligament as given in BEM. Accordingly in Chapter-6, GF is obtained from a tri-linear model, which is an improved bi-linear hybrid model, after developing expressions relating Gf , GF , (W−a) with two transition ligament lengths al and blon both sides. The values of Gf , and GF from both bi-linear hybrid method and tri-linear method are tabulated and compared. In addition to GF , the length of FPZ is estimated from the tri-linear model and compared with the values obtained from softening beam model (SBM) by Ananthan et al. (1990). There seems to be a good agreement between the results. A comparative study of size independent fracture energies obtained from the methods described in the previous chapters is made. The fracture process in concrete is another interesting topic for research. Due to heterogeneity, the fracture process is a blend of complex activities. AE technique serves as an effective tool to qualitatively describe the fracture process through a damage parameter called b-value. In the Gutenberg-Richter empirical relationship log 10N=a−bM, the constant ‘b’ is called the b-value and is the log linear slope of frequency-magnitude distribution. Fault rupture inside earth’s crust and failure process in concrete are analogous. The b-value, is calculated conventionally till now, based on amplitude of AE data from concrete specimens, and is used to describe the damage process. Further, sampling size of event group is found to influence the calculated b-value from the conventional method, as pointed out by Colombo et al. (2003). Hence standardization of event group size, used in the statistical analysis while calculating b-value, should be based on some logical assumption, to bring consistency into analytical study on b-value. In Chapter-7, a methodology has been suggested to determine the b-value from AE energy and its utilization to quantify fracture process zone length. The event group is chosen based on clusters of energy or quanta as named in the thesis. Quanta conform to the damage stages and justify well their use in the determination of the b-value, apparently a damage parameter and also FPZ length. The results obtained on the basis of quanta agree well with the earlier results.

This research focuses on the characterization of damage accumulation in concrete specimens. Specifically, a nonlinear vibration technique is used to characterize the damage introduced by ongoing alkali-silica reactions (ASR). The nonlinear resonance testing consists of an analysis of the frequency response of concrete specimens subjected to impact loading. ASR introduces a third gel like phase, which can be expansive in the presence of moisture. The result of ASR is the formation of microcracks and debonding between aggregate and cement phases. Collectively, these changes act to increase the specimens' nonlinearity. As a result, it is found that the concrete samples exhibit nonlinear behavior; mainly a decrease in resonance frequency with an increasing level of excitation strain. The relationship between the amplitude of the response and the amount of frequency shift is used as a parameter to describe the nonlinearity of the specimen. The specimens used in this research are of varying reactivity with respect to ASR, which is induced in accordance with ASTM C 1293. The level of nonlinearity is used as a measure of damage caused by the progress of ASR throughout the one year test duration. These nonlinear resonance results are compared to the traditional measures of expansion described in the standard. The robustness and repeatability of the proposed technique is also investigated by repeated testing of samples assumed to be at a specific damage state. Finally, a petrographic staining technique is used to complement nonlinearity measurements and to further gain understanding of ASR. The results of this study show that the proposed nonlinear resonance methods are very sensitive to microstructural changes and have great potential for quantitative damage assessment in concrete.

Orientador: José Luiz Antunes de Oliveira e Sousa / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Civil, Arquitetura e Urbanismo / Made available in DSpace on 2018-08-26T02:16:10Z (GMT). No. of bitstreams: 1 GeadosSantos_FabioLuis_M.pdf: 8564383 bytes, checksum: 72e9522142ccde293049d4563ab69368 (MD5) Previous issue date: 2014 / Resumo: Nos últimos anos, o aumento no número de construções mais altas e complexas fez com que o estudo das propriedades mecânicas do concreto na ruptura em tração fosse foco de muitas pesquisas. O concreto desenvolve, durante o carregamento, diferentes processos de coalescência e crescimento de micro-fissuras e, devido a sua natureza heterogênea, um enorme esforço ainda é requerido para o desenvolvimento de um modelo constitutivo capaz de descrever aspectos relevantes do seu comportamento. A essa premissa soma-se a discussão da validade dos procedimentos de ensaio em descrever o comportamento real do material, devido à influência estrutural (forma da geometria, condições de contorno e tamanho) do espécime de teste. O comportamento quase-frágil do concreto, caracterizado pelo desenvolvimento de uma zona de processos inelásticos e/ou zona de processos de fratura (ZPF) ao redor do fronte da trinca, quando na ruptura em tração, requer uma lei constitutiva ¿ chamada lei coesiva, curva tensão-abertura de fissura (? ¿ w), curva de amolecimento ou função coesiva (?=f(w)) ¿ que relaciona a tensão desenvolvida no fronte da fratura, isto é na ZPF, e a sua abertura como ingrediente para modelagem da nucleação e da propagação da fratura, seja esta representada de maneira implícita e/ou explícita. Desenvolveu-se um software para a determinação automática da lei constitutiva utilizando-se curvas da relação carga versus abertura da boca fratura (load versus crack mouth opening displecement ¿ P-CMOD) ou da relação carga versus deslocamento no ponto de aplicação (P-?) obtidas em ensaios com vigas prismáticas ranhuradas em flexão em três pontos (Three-Point-Bend Test), com taxas de carregamento variada. O objetivo desse trabalho é avaliar a lei coesiva-viscosa proposta por Rosa et al. (2012) utilizando o software desenvolvido para o ajuste de curvas P-? numéricas com curvas de vigas ranhuradas de concreto de alta resistência, ensaiadas com taxas de carregamento variando da ordem de ? ?=?10?^(-5) mm/s a ? ?=?10?^(+1) mm/s / Abstract: In recent years, the increase of high and more complex constructions, many researches focus on the study of the mechanical properties of concrete at failure. The concrete under loading develops different processes of coalescence and growth of micro-cracks, and due to its heterogeneous nature, a great effort is still required to develop a constitutive model capable of describing the relevant aspects of its behavior. In addition to this premise is the discussion of validity of tests that describe the real behavior of the material, due to the structural influence (shape of the geometry, boundary conditions and size) of the test specimen. The quasi-brittle behavior of concrete, characterized by the development of a zone of inelastic processes and/or fracture processes (ZPF) around the crack front at failure, requires a constitutive law ¿ called cohesive law curve stress-crack opening (? ¿ w), softening or cohesive function (?=f(w)) ¿ curve that relates the stress developed across the front of the fracture, i. e., the ZPF, and its opening displacement as an ingredient for modeling fracture nucleation and propagation, represented implicitly and/or explicitly. A software developed for automatic determination of a constitutive law using curves of load versus relative fracture mouth opening (P-CMOD) or the load versus displacement relationship at the application line (P-?) obtained in tests with notched prismatic beams in Three-point Bend Test, with varying loading rates. This study aims at the evaluation of the viscous-cohesive model proposed by Rosa et al. (2012), using a software developed for the curve fitting P-? numerical curve with notch beams of high strength concrete, tested with loading rate numerical in the order between ? ?=?10?^(-5) mm/s and ? ?=?10?^(+1) mm/s / Mestrado / Estruturas e Geotécnica / Mestre em Engenharia Civil

The thesis deals with nonlocal model simulations of the three-point-bening test series. The model is applied to set of beams of variable size and notch depth. The intention is to identify such parameters that would provide the response of the nonlocal model similar to experimental data from the comprehensive fracture tests performed at the Northwestern University. Size and shape of the process zone are estimated from the discrete model results and according to that the parameters of weight function and material for the nonlocal model are identified. Results obtained with the model are compared to the experimental data.

Currently, the maintenance and repair of civil engineering infrastructures (especially bridges and highways) have become increasingly important, as these structures age and deteriorate. Interface between two different mixes or strengths of concrete also appear in large concrete structures involving mass concreting such as dams, nuclear containment vessels, cooling towers etc., since joints between successive lifts are inevitable. These joints and interfaces are potential sites for crack formation, leading to weakening of mechanical strength and subsequent failure. In case of a bi-material interface, the stress singularities are oscillatory in nature and the fracture behavior of a concrete-concrete bi-material interface is much more complicated. A comprehensive experimental work has been undertaken for characterization of the behavior of different concrete-concrete interfaces under static and fatigue loading. The effect of specimen size on the concrete-concrete interfaces is studied and the non-linear fracture parameters such as fracture energy, mode I fracture toughness, critical crack tip opening displacement, critical crack length, length of process zone, brittleness number, size of process zone, crack growth resistance curve and tension softening diagram. These parameters are required for modeling the concrete-concrete interfaces in non-linear finite element analysis. Presently, the advanced non-destructive techniques namely acoustic emission, digital image correlation and micro-indentation have great capabilities to characterize the fracture behavior. The damage in plain concrete and concrete interface specimens is characterized both qualitatively and quantitatively using acoustic emission technique by measuring the width of fracture process zone and width of damage zones. The DIC technique is used to obtain the fracture parameters such as mode I and mode II fracture toughness and critical energy release rate. The micro-mechanical properties are obtained by performing depth-sensing micro-indentation tests on the concrete-concrete interfaces. Civil engineering structures such as long-span bridges, offshore structures, airport pavements and gravity dams are frequently subjected to variable-amplitude cyclic loadings in actual conditions. Hence, in order to understand the fracture behaviour under fatigue loading, the fatigue crack growth in plain concrete and concrete-concrete interface is also studied using the acoustic emission technique. An attempt is made to apply the Paris’ law, which is applicable to mechanical behaviour of metals, for acoustic emission count data. All these studies show that, as the difference in the compressive strength of concrete on either side of the interface increases, the load carrying capacity decreases and the fracture parameters indicate the increase in the brittleness of the specimens. It is concluded that the repair concrete should be selected in such a way that its elastic properties are as those of the parent concrete.

Structural concrete is the most widely used material in the construction of bridges, pave-ments, runways, dams and other infrastructures which are subjected to uctuating loads during its service period. Concrete contains internal aws in the form of micro-cracks as an inherent property. When subjected to fatigue loading, distributed micro-cracks are formed at the sites of pre-existing aws, which subsequently, localize to form a major crack and propagates. The crack growth curve of a structural component when subjected to fatigue loading depicts a sigmoidal pattern. This curve is divided into three distinct regions namely sub-threshold crack propagation (short crack), stable crack propagation (long crack) and unstable crack propagation depending on the crack propagation rate. Most of the fatigue life is spent in the sub-critical stage (small crack) before the for-mation of long cracks. Hence, from the view of estimating the fatigue life, the crack initiation and early crack propagation (short crack stage) phase are the most important and correct concepts need to be developed. Hence, in this work, the behavior of propa-gation and non-propagationof short cracks in concrete when subjected to fatigue loading is addressed. Small non-propagating cracks are usually found at notch roots when the nominal stress range is below certain limits that depend on the notch sensitivity. Analysis is performed on geometrically similar three-point bend beams of three di erent sizes and subjected to fatigue loading in order to determine the important factors that a ect the notch sensitivity and to determine the minimum stress range required for the initiation and propagation of short cracks. A criterion for crack initiation and propagation is proposed based on linear elastic fracture mechanics. Using this criterion, the maximum length of non-propagating crack that can be formed from fatigue loading alone and the minimum stress range required to propagate a crack without arrest are computed. It is observed that the notch sensitivity increases with increase in beam size, decrease in notch-tip aspect ratio and increase in the fatigue limit of the material. Since the probability of formation of a non-propagating crack at a notch tip decreases with increase in notch sensitivity, and since it is desirable not to have a non-propagating crack in experimental investigations, it is essential to design a specimen with higher notch sensitivity. A crack spends a considerable amount of time in the short crack regime. The short cracks are found to propagate at higher rates than the long cracks at the same nominal stress intensity factor which is known as the short crack anomaly. It is important to consider this anomaly in the prediction of the residual life of damaged concrete structures. Hence, in the present work, an analytical model is developed using the principles of dimensional analysis and self-similarity in order to estimate the rate of short crack growth in concrete. The important parameters such as load range, threshold value of stress intensity factor range, modulus of elasticity, tensile strength, fracture energy, stress ratio, crack size and the maximum aggregate size are considered in the development of the short crack growth model. The model is calibrated and validated using the experimental results that are available in the literature. A probabilistic analysis is carried out to determine the sensitivity of each of the di erent parameters that has been considered on the crack growth rate using the coe cient of variation method. It is found that the crack length is the most sensitive parameter to short crack growth rate followed by the load range. A term called `characteristic fatigue life of short crack' is de ned as the number of fatigue cycles that can be applied such that not more than ve percent of the short cracks is expected to proceed to the long crack regime. Furthermore, the fatigue life of a crack spent in the short crack regime is determined through a reliability based study using the Monte Carlo technique. It is found that the smaller sized specimens have larger fatigue life in the short crack regime than the larger specimens.

Evolution of concrete is continuously taking place to meet the ever-growing demands of the construction industry. Self compacting concrete (SCC) has emerged as a result of this demand to overcome the scarcity of labour. SCC is widely replacing normal vibrated concrete (NVC) these days owing to its advantages such as homogeneity of the mix, filling ability even in heavily congested reinforcement, smooth finish, reduction in construction time etc. The ingredients used for SCC is the same as that of the NVC. But the proportioning of ingredients to achieve self compactability alters the microstructure of SCC which in turn affects the mechanical and fracture properties. Moreover, the mineral admixtures such as fly ash and silica fume when used for improving the workability of SCC help in the development of the microstructural skeleton. In this study, three SCC mixes SCC1- made with only cement, SCC2 - with fly ash in addition to cement and SCC3 - with fly ash and silica fume in addition to cement for achieving normal, medium and high strength SCC respectively are cast. The microstructural changes in SCC with and without mineral admixtures over a period of time are studied using different techniques such as scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The modification of mechanical properties at microstructural level brings difference in the behavior at macro level. Hence in this study, the mechanical properties at microstructural are obtained by using microindentation test and are scaled up to the macro level to predict the influence of micromechanical properties on macro response. The fracture properties of SCC is considered to be the interest of this study and is carried out with the help of advanced techniques such as acoustic emission (AE) and digital image correlation (DIC). From the various studies carried out, it is inferred that the mixes with mineral admixtures behave in a more brittle manner when compared to mix having no mineral admixture. It is also observed that class ‘F’ fly ash hydrates at a slow pace and the strength gain is observed after 28 days and even beyond 90 days. Hence, it is concluded that it is appropriate to consider the strength at 90 days instead of 28 days for a SCC mix with class ‘F’ fly ash. Silica fume on the other hand is observed to result in a more rapid gain in strength and this can partially offset the delay in strength gain due to fly ash.

Předkládaná práce se zabývá matematickým modelováním chování betonu. K numerické analýze je použit diskrétní částicový model. Tento model zjednodušuje meso-strukturu materiálu na systém propojených diskrétních částic -- konvexních mnohostěnů. Tyto částice jsou uvažovány ideálně tuhé a jejich vzájemná interakce je předepsána na kontaktech sousedních částic. Při hledání nespojitého pole posunů a rotací jsou zjednodušeně předpokládány malé deformace. Práce popisuje dvě rozšíření implementovaná do stávající verze modelu, konkrétně (i) přidání reprezentace krátké rozptýlené výztuže a (ii) implicitní dynamický řešič. První z hlavních částí práce se zabývá modelováním kompozitů s krátkou rozptýlenou výztuží. Krátká vlákna rozptýlená v materiálu přispívají k zlepšení jinak nepříznivé tahové pevnosti betonu. Vlákna jsou v částicovém modelu reprezentována nepřímo, zohledněním tření mezi vláknem a cementovou matricí silami, které působí proti otevírání trhlin. S pomocí tohoto rozšíření model dokáže předpovídat chování vláknobetonu zahrnující tahové zpevnění i navýšení počtu trhlin. Druhá z hlavních částí se zabývá odezvou materiálu na dynamické zatížení. Beton vykazuje rozdílné chování pro různé rychlosti zatěžování. V případě pomalého, kvazi-statického, zatěžování se počáteční mikro-trhliny lokalizují v makro-trhlinu. Dochází-li k rychlému zatěžování, energie nahromaděná v tělese není spotřebována pouze jednou trhlinou, ale dochází k jejímu větvení. V případě rychlých procesů je hlavním faktorem setrvačnost hmoty, která je zatížením urychlována. Struktura materiálu a setrvačnost částic je v mezo-úrovňovém diskrétním modelu zahrnuta. Přesto ale další jevy probíhající pod rozlišovací úrovní modelu ovlivňují výsledné chování materiálu. Proto je do modelu přidána fenomenologická závislost konstitutivního zákona na rychlosti přetváření. Numerické simulace těles rozličných tvarů zatěžovaných různou rychlostí deformace jsou porovnány s experimenty z literatury.

Master´s thesis deals with some problems associated with utilisation of lightweight concrete from the porous aggregates in the load – carrying structures. The thesis focuses on the possibilities of the increase of the cement composites toughness using dispersed reinforcement. Lightweight concretes were reinforced with a combination of different lengths of polypropylene fibers Forta Ferro. There were used polypropylene fibers of three lengths 19, 38, 54 mm. The thesis is divided into theoretical, experimental and static part.

A major distress mode in asphalt pavements is low temperature cracking, which results from the contraction and expansion of the asphalt pavement under extreme temperature changes. The potential for low temperature cracking is an interplay between the environment, the road structure and importantly the properties of the asphalt mixture. The thermal cracking performance of asphalt concrete mixtures can be evaluated by conducting thermal stress restrained specimen tests (TSRST) which is known to be correlated well with the fracture temperatures observed in the field. Although TSRST provides a good estimation of the field performance, it may be unrealistic to implement the obtained results in a design framework. On the other hand, recent studies showed Superpave indirect tension tests can be used to evaluate fracture performance (fatigue, moisture damage, low temperature cracking, etc.) of the asphalt concrete  mixtures. In addition, the obtained elastic and viscoelastic parameters from the Superpave IDT tests can be used as an input parameter to establish a design framework. The study presented in this thesis has a main objective to develop a framework using Superpave IDT test results as input parameters in order to evaluate the low temperature cracking performance of asphalt concrete mixtures. Moreover, the study aims to investigate micro-mechanically the low temperature cracking behavior of bitumen using atomic force microscopy (AFM) as a tool. The numerical model has been developed by integrating fracture energy threshold into an asphalt concrete thermal fracture model, considering non-linear thermal contraction coefficients. Based on the asphalt concrete mixture viscoelastic properties, this integrated model can predict thermally induced stresses and fracture temperatures. The elastic, viscoelastic and fracture energy input parameters of the model were measured by conducting indirect tension tests and the thermal contraction coefficients were measured experimentally. The proposed model has been validated by comparing the predicted fracture temperatures with the results obtained from TSRST tests. It was found that, while there is a quantitative discrepancy, the predicted ranking was correct. In the measurement of the thermal contraction coefficients it was observed that the thermal contraction coefficient in asphalt concrete is non-linear in the temperature range of interest for low temperature cracking. The implications of having non-linear thermal contraction coefficient were investigated numerically. In an effort to understand the effect of bitumen properties on low temperature fatigue cracking, AFM was used to characterize the morphology of bitumen. The AFM topographic and phase contrast image confirmed the existence of bee-shaped microstructure and different phases. The bitumen samples were subjected to both environmental and mechanical loading and after loading, micro-cracks appeared in the interfaces of the bitumen surface, confirming bitumen itself may also crack. It was also found that the presence of wax and wax crystallization plays a vital role in low temperature cracking performance of bitumen. / <p>QC 20120828</p>