Permanent Deformation Characterisation

2.2.1 Definition and Mechanism

Permanent deformation is a common mode of failure that degrades flexible pavement and is a concern for the asphalt community. It manifests itself as rutting in the wheel path caused by accumulated strains (deformation) due to the repeated traffic loading (Figure 2.1) (Lundy et al., 2004). The mechanism by which rutting occurs in pavement can be explained by two concepts:

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densification (decrease in volume) and shear displacement (movement of the material without volume changing).

Figure 2.1 Accumulation of permanent strain due to repeated loading

Densification tends to appear in the early life of the pavement when the binder is still soft and fresh (un-aged), while shear displacement is a continuous process during the lifetime of the pavement (Khanzada, 2000).

While densification can be minimised by proper compaction during construction, several laboratory studies support the primary reason for rutting is shear displacement (Hofstra et al., 1972, Collop et al., 1995).

However, these are coupled mechanisms for a nonlinear behaving material such as asphalt. Yet, it is the material property that enables rutting to occur.

When the pavement is being passed by a wheel loading, there will be some irrecoverable strains generated in the material. It is the viscous property of the binder that is responsible for these permanent strains, and it dominates more at high temperatures and/or long loading times (Khanzada, 2000).

Consequently, factors that affect permanent deformation can be classified into material properties and working condition.

Initial depth

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2.2.2 Determination Factors 2.2.2.1 Material Properties

The structural strength of the asphalt mixtures relies primarily on the mechanical properties of bitumen and interlock between aggregate particles (Khanzada, 2000). Different proportions of the components produce various types of asphalt mixtures with different performances. In order to optimise the composition for maximum permanent deformation resistance, understanding the effect of each component and their influential properties is inevitable.

Bitumen is the main element in the asphalt mixture governing the behaviour.

The amount of bitumen added to the aggregates is critical. An overfilled mixture has low air voids content and results in a thicker bitumen film between aggregate particles that raises the permanent deformation susceptibility. On the other hand, too little binder leads to aggregates separated by a very tight network of air voids. Therefore, optimum bitumen content in the mixture is required with no less than about 3% air voids content as recommended by (Khanzada, 2000, Suo et al., 2009). In addition to the quantity, the grade of the bitumen plays a vital role in determining the deformation resistance.

Bitumen with higher stiffness considerably reduces the susceptibility to permanent deformation (Hofstra et al., 1972, Lundy et al., 2004). This is partially limited by the aggregate gradation type of the mixture. The effect is more significant in gap graded mixtures than when the resistance is

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primarily dependent on aggregate interlocking as typically observed in continuously graded mixtures (Taherkhani, 2006).

Aggregates which occupy a large volume of the mixture, with their physical properties (type, shape, size, texture) have a great impact on the asphalt mixture performance. Aggregates resistance to sliding over each other and aggregates adhesion with the binder are the characteristics of aggregates to resist deformation. Rough surface aggregates allow more bond with bitumen and greater friction between particles than smoothed surface particles (Garba, 2002). In terms of physical properties, angular shape are more preferable due to the increased contact points between particles (Kim et al., 1992). Mixtures with large aggregates have less rutting potential than mixture with small size aggregates because of better load transfer (Lundy and Sandoval-Gil, 2004).

Gradation or particle size distribution is a measurement of aggregate sizes and their proportions in the mixture, determined by sieve analysis.

Superpave introduced the ‘restricted zone’ that lies between 2.36 and 0.3 mm in which aggregate gradation was recommended not to pass. In their study, Kandhal et al. (2001) reported significant difference in performance between asphalt mixtures that share similar mineralogical composition and had aggregate gradation below, above, and through the restricted zone, Figure 2.2. Test results in the Asphalt Pavement Analyser (APA) exhibited highest and intermediate rutting in mixes of gradation below and above the restriction zone respectively, whilst gradation through the intermediate zone generally had the lowest rutting.

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Figure 2.2 Aggregate gradation and restricted zone (Kandhal et al., 2001)

Cross et al. (1992) argued that influence of the aggregates properties and gradation on rutting is limited when the air void content is low (<2.5%).

Field samples were collected and tested in laboratory. They found that even for above 2.5% air void content, the fine aggregate angularity had more influence than aggregate gradation on rutting resistance.

2.2.2.2 Working Condition

Working conditions include mainly the environment (temperature, humidity, moisture content, etc.) (Qiao et al., 2013) as well as wheel loading in terms of frequency, type and weight. Bitumen and bituminous materials behave in a more viscous manner at high temperature and low frequency loading. The more viscous the material is, the more severe is permanent deformation.

Increasing the temperature from 20 to 60⁰C in the wheel tracking test, Collop et al. (2001) in their study revealed that the resistance to permanent deformation reduced by a factor of 1/250 to 1/350. Several researchers pointed out that heavy loading has a huge negative influence on deformation resistance resulting in higher rutting (Muraya, 2007). Paving process of

Restricted zone

Sieve size (mm)

%passing

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roads construction also has an influence as previously noted in which compaction is a crucial part for limiting early rutting.

Gaskins et al. (1960) stated that, during the paving operation, hardening of bitumen takes place substantially when the heated aggregate is mixed with the hot binder. During this time (short term ageing) the thin bitumen film is exposed to high temperatures which increase the viscosity. The hardening process continues till laying and compacting, and afterwards by 2 to 3 years and under the traffic loading the hardening (long term ageing) carries on at a lower rate. Yet, the structural linkages of the unaged bitumen molecules are less susceptible to strained rupture than an aged one (Cheung, 1995).