Specific Examples of Applications

3.4.1 Introduction

In this section some specific examples of applications using the various characterization techniques are discussed to illustrate the potential applications and benefits in pavement engineering.

3.4.2 Soil Characterization

The chemical and mineralogical composition of soils has long been characterized on the nano level using techniques such as Electron Probe Micro-Analysis (EMA), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analyzers (EDXRA) [Bullock 1981; Esawaran and Shoba 1981; Bisdom 1981; FitzPatrick 1984]. The application of these techniques has led to improved models and understanding of the macro-behavior of soils, based on an interpretation of their nano-and micro-structures. This knowledge is applied in pavement engineering through improved decisions regarding stabilization of specific materials and potential for compactability of materials.

3.4.3 Scanning Electron Microscopy (SEM) Applications

Mgangira [2007b] used the SEM to evaluate the micro-texture of two similar sands that behaved differently when being stabilized using various non-standard soil stabilizers. None of the typical engineering tests provided insight into the reason why the stabilization did not work similarly for the two types of sands evaluated. The SEM investigation showed the difference in terms of fines that were not identified in the standard grading analysis of the sands, which allowed the stabilizers to bond better with the sand grains. In Fig. 9 SEM images of the two types of sands are shown, with the fines visible on the right image.

Fig. 9 SEM images of two sands showing traces of small fractions on the right [Mgangira 2007b].

Mgangira [2010] used a SEM to evaluate the interaction between two different soil samples (black clay and reddish brown residual chert) and enzyme-based liquid stabilizer solutions. The results showed that the addition of the enzyme-based products modified the microstructure of the material matrix and that the type of microstructure changes that take place after treatment will be dependent on the product which in turn is a function of the compositional make up of the product itself. Whether this leads to significant improvement in engineering properties is another question. The observed connectivity of the microstructure and the presence of a surface network binding the particles, following treatment with product A differs from that seen on samples treated by product B and is considered to be the underlying explanation in the difference in performance of the two products. This is considered to be the result of the compositional characteristics of the products. The SEM images of the untreated and treated chert is shown in Fig. 10 to indicate the differences observed in the structure, and the presence of a tentacle-like network of structures on the surface of the sample.

Fig. 10 Microstructure image of untreated chert sample (magnification 1000x) (Left) and chert sample treated with enzyme-based liquid stabilizer (magnification 2000x (Right).

Muniandy [2002] used SEM to evaluate the differences between Malaysian cellulose oil palm fiber and traditional commercially available cellulose fibers used in Stone Mastic Asphalt (SMA) mixes. It was found that the Malaysian fiber sizes appeared to be less than 500 (im compared to the traditional fibers which had thicknesses of around 50 (im. Use of the SEM analysis assisted in the process of defining the differences between the two types of fiber.

Stulirova and Pospisil [2008] discussed procedures to enable SEM to be conducted on bitumen samples. The oil fraction (maltenes) in the samples needs to be eliminated prior to characterization as the resolution of the images is too low if the samples are non-conductive. This process leaves the asphaltene fraction to be studied. It is important to understand that only changes in the asphaltene structure can thus be studied using the SEM technique.

Peethamparan et al [2008] evaluated the physicochemical behavior of cement kiln dust treated kaolinite clay using, inter alia, SEM and EDX techniques. Analysis of the samples using SEM indicated that the most important morphological modification of the kaolinite due to the cement kiln dust treatment was the change from clay flake stacks observed in untreated kaolinite clay to a disaggregated and randomly oriented form. This detail assisted in the development of a model for the stabilization mechanism of cement kiln dust treated kaolinite clay.

Steyn and Jones [2006] evaluated the performance of an in-service pavement with a cemented base layer that was recycled and stabilized with bitumen emulsion using Accelerated Pavement Testing (APT). Upon analysis of the material properties (that behaved exceptionally well after 25 years of trafficking) SEM was used to evaluate the microstructure of the material. By comparing carbonated and un-carbonated material from the stabilized base layer using a SEM, it was possible to conclude that the matrix of the base appeared to have residual cement from the original recycled cement stabilized layer that was able to hydrate and form cemen-titious bonds (although no cement was added during the recycling). Although it is difficult to detect bitumen using the SEM, small quantities were detected in the base materials by the presence of high carbon concentrations identified using the EDX facility. An indication of the SEM images showing the cementitious bonds and the bitumen is shown in Fig. 11.

Fig. 11 SEM images showing location of bitumen in matrix and new cement reaction crystals.

3.4.4 Atomic Force Microscope (AFM) Applications

Pauli et al [2001] used AFM in characterizing bituminous binders and their respective properties. They correlated the surface morphology with the constituents in the bitumen and concluded that the AFM data may be used to improve the understanding of the precipitation of the asphaltenes in the bitumen and that this information may lead to an improved understanding of the interaction between the surface of the aggregate and the bitumen in asphalt.

The AFM was used to investigate the structural morphology of crumb rubber bitumen at the interfacial regions, especially during aging [Huang et al 2006]. Masson et al [2006] used phase-detection AFM to evaluate bitumen morphology and found that bitumen can be classified into three groups, based on the different domains or phases visible.

Steyn [2009a] evaluated the correlation between the aging of bituminous binders on the road (a property that severely affects the deterioration of the pavement surface) and the elastic stiffness of the binders as measured using the AFM. Initial data focused on the surface morphology and a clear difference could be observed between the surface morphology of a bituminous binder that was aged at different temperatures. In Fig. 12 the overall difference in the surface morphology for

Fig. 12 The surface morphologies of four bituminous binders as aged at three different temperatures and observed using the AFM [Steyn, 2009b].

four binder types aged at three temperatures can be observed. Research in this area is continuing, with the focus on improving the understanding and potential correlation between surface morphology and performance of bitumen.

Masson et al [2007] described a novel application of the AFM where they evaluated the structure of various bitumens at low temperatures using cryo-microscopy and related the results to material stiffness. AFM were conducted at imaging temperatures of 22°C, -10°C, -27°C, -55°C and -72°C. These temperatures were selected based on glass transition temperatures of the bitumens. At these low temperatures the bitumens did not pollute the tips with sticky residues. The effect of temperature on volume was specifically noteworthy. The AFM observations of new domains that became visible upon cooling of the bitumen samples were consistent with segregation. The microscopy work at -10°C to -30°C showed that the bitumen contracted, although not all bitumen phases contracted equally. Further, topographic features of around 85 nm in height were visible at the low temperatures and not at room temperature.

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