Laboratory fatigue evaluation of modified and unmodified asphalt binders in Stone Mastic Asphalt mixtures using a newly developed crack meander technique

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  • Accepted Manuscript

    Laboratory Fatigue Evaluation of Modified and Unmodified Asphalt binders in

    Stone Mastic Asphalt Mixtures using a newly Developed Crack Meander Tech

    nique

    Ratnasamy Muniandy, Nor Azurah Binti Che Md Akhir, Salihudin Hassim,

    Danial Moazami

    PII: S0142-1123(13)00236-3

    DOI: http://dx.doi.org/10.1016/j.ijfatigue.2013.08.021

    Reference: JIJF 3198

    To appear in: International Journal of Fatigue

    Received Date: 7 December 2012

    Revised Date: 16 August 2013

    Accepted Date: 20 August 2013

    Please cite this article as: Muniandy, R., Akhir, N.A.B., Hassim, S., Moazami, D., Laboratory Fatigue Evaluation

    of Modified and Unmodified Asphalt binders in Stone Mastic Asphalt Mixtures using a newly Developed Crack

    Meander Technique, International Journal of Fatigue (2013), doi: http://dx.doi.org/10.1016/j.ijfatigue.2013.08.021

    This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

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    Laboratory Fatigue Evaluation of Modified and Unmodified Asphalt binders in Stone Mastic Asphalt Mixtures using a newly Developed Crack Meander Technique

    Ratnasamy Muniandy , Nor Azurah Binti Che Md Akhir, Salihudin Hassim, Danial Moazami

    Department of Civil Engineering, University Putra Malaysia, ratnas@eng.upm.edu.my Department of Civil Engineering, University Putra Malaysia, azurahcma@yahoo.com Department of Civil Engineering, University Putra Malaysia, hsalih@eng.upm.edu.my

    Department of Civil Engineering, University Putra Malaysia, d_moazami@mshdiau.ac.ir

    Corresponding Author E-mail: ratnas@eng.upm.edu.my +60-3-89466373/7847 (+60)123396917

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    ABSTRACT

    This paper looks into the fatigue evaluation of modified and unmodified asphalt binders in Stone

    Mastic Asphalt (SMA) mixtures using a Crack Meander (CM) technique. Specimens images were

    taken during the repeated load indirect tensile fatigue test (ITFT) and crack initiation, propagation and

    failure were analyzed using a developed "Measurement and Mapping of Crack Meander" (MMCM)

    Software. The results of crack analysis on every SMA specimens were compared with tensile strain

    plots obtained from the ITFT test. It was concluded that, in addition to strain or dynamic modulus

    plots, fatigue behavior can be determined using crack appearance as an alternative method.

    Keywords: Stone Mastic Asphalt; Fatigue Strength; Crack Formation; Crack Meander; Repeated Load Indirect

    Tensile Fatigue Test.

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    1 INTRODUCTION Fatigue cracks are one of the major distresses on the roads worldwide. In fatigue studies, there are

    many approaches to define and evaluate the fatigue strength of asphalt mixtures such as the traditional

    method, by using stress or strain against number of cycles (S-N plot), the dissipated energy approach,

    and visco-elastic continuum damage method. However, there is not a clear or specific standard that

    states which one is the best method to compare the performance between various bituminous mixtures.

    The failure point in the traditional fatigue models considered at 50 percent reduction in the stiffness

    modulus for controlled strain testing [1]. However, Lundstrom et al. [2] reported the traditional failure

    criterion unsuitable, since at that point there is often no sign of real failure leading to inconsistent

    fatigue results. Dissipated energy is another approach used instead of stress or strain while this method

    does not consider progressive damage of material and crack development. Continuum mechanics also

    does not accurately identify the fatigue crack development in the secondary and tertiary stages [3].

    Therefore, in order to portray the nature of cracks, studying the fatigue crack network and its pattern

    seems necessary. Braz et al. [4] used computed tomography technique to detect crack evolution in

    asphaltic mixtures submitted to fatigue test. Birgisson et al. [5] used a Digital Image Correlation (DIC)

    system to obtain displacement/strain fields and to detect crack patterns. In this study a different

    approach is presented to fully quantify the fatigue strength of asphalt mixtures until failure. Some

    preliminary works were undertaken at Universiti Putra Malaysia (UPM) in 2004 and 2010 to establish

    a protocol for Crack Meander technique (CM) to determine the fatigue strength. Some unique features

    of this method include investigation of all aspects of fatigue distress (crack length, area and density),

    simplicity of the test and the high precision of the image processing technique. In this study the fatigue

    strength of modified and unmodified asphalt binders in Stone Mastic Asphalt mixtures (SMA) were

    evaluated by using the developed crack meander method. The obtained crack data was validated as

    compared to real strain data from the repeated load indirect tensile fatigue test (ITFT).

    2 FATIGUE CRACK MECHANISM

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    In general, fatigue life is defined as the number of load cycles to failure for a bituminous mixture and

    fatigue resistance indicates its ability to resist repeated cyclic loading that cause fracture although

    other stress inducing factors are not mentioned here. Technically, because of continuous cyclic

    loading, the bottom of the pavement layer experiences tensile strains thus forms cracks that continue

    to propagate upward until failure [6, 7]. Fatigue behavior of asphalt mixtures is determined either by

    controlled stress (load) or controlled strain (deflection) mode in the laboratory [8]. Because of the high

    similarity with site conditions, controlled stress mode is widely used [7]. In controlled stress mode, a

    constant amplitude of repeated stress or load causes the increasing strain while in controlled strain

    mode, the amplitude of constant strain is applied in form of repeated deflection which results in stress

    decrease [9].

    Dynamic reactions are responsible in evaluation of fatigue resistance in bituminous mixtures [10].

    Dynamic complex modulus is defined as the ratio of sinusoidal amplitude of stress to strain at angular

    frequency for any given time. The dynamic complex modulus (E*) plot is normally used to represent

    the relationship between stress and strain [11, 12]. During a fatigue test, modulus value decreases [13]

    according to Figure 1 [14]. The first phase shows a fall in stiffness modulus due to repetitive load

    excitation. Phase II, shows a quasi-linear decrease in stiffness, after which the sample starts to fracture

    rapidly at the early of phase III due to non-uniformity in the strain field.

    < Insert Figure 1 about here >

    3 STONE MASTIC ASPHALT AND THE MODIFIERS

    SMA is a dense and gap-graded bituminous mixture contains coarse and fine aggregates, filler, and

    bitumen. The binder is typically modified with suitable binder carrier such as fiber or polymer [15,

    16]. Earlier, SMA was known by its great potential to resist rutting and to decrease wear due to the

    studded tires [15, 17]. Cubical, hard, crushed and durable aggregates are adhered with optimum

    quantity of moisture-resistant mortar, and produce stone-on-stone contact. SMA contains about 93 to

    94 percent of aggregates by weight of total mix, less than 1 percent fiber and about 6 percent binder

    [16]. Although SMA is rut resistance, due to high proportions of coarse aggregates, it shows poor

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    performance in fatigue resistance due to the reduced amount of fine aggregates [16]. In total, because

    of its good potential in pavement performance, detailed consideration should be taken in the selection

    of materials to produce suitable mixtures.

    In this study two different common modifiers were selected for use in the SMA mixture in order to

    improve its performance in fatigue strength. Cellulose Oil Palm Fiber (COPF) is widely available in

    Malaysia and therefore it was selected as one of the stabilizers. In addition to COPF, Ethylene Vinyl

    Acetate (EVA) was selected as a traditional asphalt modifier. The cellulose fiber and EVA materials

    are shown in Figures 2 and 3 below.

    < Insert Figures 2 and 3 about here (in one line) >

    COPF is a non-hazardous biodegradable material that is produced from the empty fruit bunch of oil

    palm tree through various pulping methods. It was proven that COPF greatly minimizes drain down of

    asphalt mixtures and tends to improve the fatigue resistance [16].

    EVA is a type of polymer in plastomer group. For over twenty years, it has been used in pavement

    construction to improve the performance of asphalt mixtures since it has great potential to resist

    permanent deformation [18, 19], thermal cracking [20] as well as fatigue of asphalt mixtures [21]. By

    blending EVA with the original bitumen, the physical properties of binder such as penetration,

    softening point, loss on aging and viscosity improve which indicates the stiffening effect of EVA

    blended binders [22].

    4 CRACK MEANDER CONCEPT AND APPROACH

    Indirect tensile fatigue test is widely carried out to estimate the resistance of a bituminous mixture

    sample to fatigue failure in accordance with BS EN 12697-24 [23] by using the Universal Testing

    Machine (UTM). Laboratory investigation of fatigue has shown that visual cracks that appear on the

    trimmed test samples seem to have a unique relationship with fatigue resistance. This observation

    leads to the idea of "crack meander" study to be developed.

    The term meander is derived from the river meandering concept with a convoluted path which is

    known as Maiandros or meander by ancient Greeks. According to Oxford dictionary, meander is

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    defined as "to curve a lot rather than being in a straight line" or "to walk slowly and change direction

    often, especially without a particular aim". To summarize, meander in this context can be defined as

    initiation and propagation of cracks, meandering due to crack pinning through the cross section of the

    trimmed specimens.

    The new approach is divided into a few stages. In the first stage, initiation and propagation of cracks

    which appear on the sample surface during the diametral fatigue test is monitored and captured via a

    SLR camera. For this purpose, instead of using the existing frame for indirect tensile fatigue test in

    universal testing machine, the frame was specially fabricated; so that the surface image can be

    captured directly without any barrier as shown in Figure 4. The images were recorded at a

    predetermined interval of cycles depending on the speed of crack migration from start of the test until

    failure. The SLR camera brand Nikon D300 was used to capture images which can capture up to six

    frames per second with 12.3 megapixel resolution. This criterion is important to capture few images in

    one second and to provide more than one image of crack at certain cycle so that the best image can be

    selected for use in the new Measurement and Mapping of the Crack Meander (MMCM) software. The

    diametral surface of the specimen must face the UTM machine glass door. Furthermore a fixed

    distance, between the camera lens and the glass door, and a fixed height, between the camera and the

    ground, must be provided using a tripod. This is important because later the photos will be uploaded

    into the MMCM software to measure and compare the crack development at the same resolution. Each

    image is coded based on the number of load cycles.

    < Insert Figure 4 about here >

    In the second stage, crack analysis and measurement are performed using the MMCM software.

    4.1. Measurement and Mapping of the Crack Meander (MMCM) Software

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    Sample information and test control parameters in MMCM software were designed based on ITFT

    format. This software was developed at UPM [24] based on 2004 original concept [25].

    The images of samples taken during the fatigue test were inserted as the inputs into this software for

    crack measurement and analysis. Frame size of the picture was set to a standard dimension, by

    changing the pixels (usually 30mm equal to 10 pixels), before any analysis in order to remove the

    possible errors occurred due to slight variation in camera distance.

    In order to specify the initial crack, MMCM used the first image of each specimen before the ITFT

    test as a guide. Since the surface of specimen is painted with white color, MMCM converts each white

    surface to specific number of white pixels. By using image processing and comparing the other images

    with the first image, MMCM is able to recognize any black pixel which represents the initiation of

    crack in any image. As illustrated in Figure 5, crack measurement is based on measuring different

    groups of small cracks which can be highlighted in even different colors. Adding up all the groups of

    cracks leads to the total crack measurement which includes total area, average width and length of

    cracks. This information is presented at the bottom of each sample for comparison purpose. MMCM

    maps all the cracks in each specimen effectively using image processing technique.

    4.2. The results of each image analysis, include crack length, crack width, crack area and crack density as

    shown in Figure 6, are summarized in Microsoft Excel format as well. Furthermore, as illustrated in

    Figure 7, the software is able to compare the results based on the number of cycles. The image

    comparison among different kinds of samples is a useful tool in crack propagation and samples

    behavior analysis to evaluate the performance as well.

    < Insert Figure 6 about here >

    < Insert Figure 7 about here >

    5 MATERIALS AND METHODOLOGY

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    In this study granite aggregate from Kajang quarry, Malaysia was used. Non-hydrated calcium

    carbonate powder obtained from the limestone processing plant in Ipoh, Malaysia was the source of

    filler. Asphalt binder with 80/100 penetration grade was used. The aggregate and binder physical

    properties were evaluated which fulfilled the JKR (Malaysia Public Works Department) requirements.

    For the mix design seven different combinations including 1) Control sample;

    2) SMA mix with 0.3% COPF; 3) SMA mix with 0.6% COPF; 4) SMA mix with 0.9% COPF;

    5) SMA mix with 3% EVA; 6) SMA mix with 6% EVA and 7) SMA mix with 9% EVA were used

    which produced 105 Marshall samples (15 samples each). However, the results of mix design stage are

    not presented here since the details are beyond the scope of this paper. In the next stage the Optimum

    Asphalt Content (OAC) was determined and three different types of SMA mixtures including SMA

    Control Samples (CS), SMA mixtures with 0.6 percent of COPF (COPF P0.6) and SMA mixtures with

    6.0 percent of EVA...

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