PSI - Issue 2_B

Lee Leon et al. / Procedia Structural Integrity 2 (2016) 2913–2920 Author name / Structural Integrity Procedia 00 (2016) 000–000

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because the specimen first reacts to the load as a purely elastic homogenous solid. However, the plastic properties of the specimen arise when at some point the granular aspects of the specimen bring about friction as the grains make contact, therefore, a shear plane will begin to develop and the specimen begin to behave as a frictional solid. Stress increases until the peak friction coefficient is reached and will flatten for a short period (2). This horizontal segment of the curve represents the period at which the peak friction coefficient, µ equals the friction angle, Φ and this relationship suggests pure shear. The graph is purely horizontal because the sample does not increase in stress because of the purely frictional force and there is a constantly increasing strain. After this short period the friction angle will change causing the coefficient of friction to drop and so the graph descends linearly until it reaches a residual friction coefficient (3). At this point the graph will again become purely horizontal and continue to increase in strain only (4). This idealized graph, is seen in figure 2 where the experimental results are also superimposed on the graph. This can also be seen in figure 4 which shows real results from a 150 mm sample tested under uniaxial unconfined compression.

Fig. 2. Idealized stress-strain curve showing elastoplastic behaviour.

Applying pressure to AC through compression causes unbalanced forces in the system from which results in deformation, cracks and fatigue failure. These applications are useful in determining the plastic and elastic limits of AC. Plastic deformation and fatigue failure occur when the yield point of the material has been exceeded and this is a problem seen in many AC pavements today, hence, a connection between the stress and strain in AC needs to be explored especially for the purpose of designing. The research aims to investigate the failure mode of AC cylindrical samples when subjected to unconfined uniaxial compression. It will conclude that the general attributes of the stress strain curve and AC failure mode as affected by the physical and mix properties of an AC layer. 2. Materials and testing 2.1. Materials Commonly used construction materials such as Crushed Blue Limestone and Sharp Sand were the types of aggregates used in preparing the different mixes. The aggregate blending produced respective gradations as shown in Table 1. The specimens were dense graded (HMA 2, HMA 3) and stone matrix (SMA 3) mixes, which classification were governed by the gradation specifications and NMAS (nominal maximum aggregate size). The Trinidad Lake Asphalt (TLA) with a penetration of (60/75) as specified by ASTM (2015) was the study binder.

Table 1. The properties of the mix gradations Mix type

Coarse to fine ratio

NMAS

HMA 2 HMA 3 SMA 3

50% - 50% 49% - 51% 71% - 29%

12.5 mm 9.5 mm 12.5 mm