Effect of Anisotropy on Tensile

Materials: Tensile testing technology and applications

Effect of Anisotropy on Tensile Properties of Asphalt Mixtures


Anisotropy is a term used to describe the characteristic of exhibiting different values of a property in different directions with respect to a fixed reference system in the material. An example is wood which is stronger along the grain than across it. (Gauthier, 1995). Tensile means that a physical material can be stretched. (Anon. 2008). This report contains results from tension tests that were performed at Arizona State University by Mamlouk et al in 2002.


Tensile testing is necessary in industrial situations such as asphalt production. Current research efforts under the National Cooperative Highway Research Program (NCHRP) Superpave Project 9-19 are directed towards the development of advanced fundamental characterization methods for asphalt concrete mixtures that can support necessary enhancements to the Superpave distress prediction models. Anisotropy testing constituted a part of the exploratory testing of the project (Mamlouk, 2002b).

Effect of Anisotropy on Tensile
Properties of Asphalt Mixtures


Highways Agencies across America are interested in and greatly benefit from tensile testing of properties in hot mix asphalt (HMA). The objective of the study undertaken by the NCHRP was to evaluate the anisotropy of HMA mixtures for selected test properties using specimens prepared in the Superpave gyratory compactor (Mamlouk et al. 2002c). The Superpave Gyratory Compactor produces asphalt mix specimens to densities achieved under actual pavement climate and loading conditions (Anon. 1996). Equivalences of properties from the samples cored in different directions were compared. The effect of anisotropy on tensile properties was evaluated. (Mamlouk et al. 2002d).


In order to achieve the best results vertical, horizontal, and diagonal cores were taken from the samples created in order to see the full effects of anisotropy on the material. Most existing testing procedures and constitutive models for HMA materials assume isotropic properties. A material which shows isotropic properties would mean that under tension or compression along any axis the resulting effect would be the same (Gere. 2002). In order to attach sprung loaded gauge instruments to the samples, LDVT brackets were glue to each core (Mamlouk et al. 2002e). All tests were performed using an IPC Universal Testing Machine 100. This versatile machine is based on a 100kN capacity hydraulically-driven load-frame and was designed to meet the needs of laboratories wishing to carry out a range of tests on either bound or unbound pavement construction materials (Anon. 2003). The load was measured through the load cell whereas the deformations were measured through the sprung loaded gauge instruments. A strain-controlled vertical load was measured using the load cell connected to the machine. Deformations through the vertical and horizontal were recorded throughout testing (Mamlouk et al. 2002f).

The following conditions were imposed by Mamlouk et al.

  • Core orientation: Vertical, horizontal, and diagonal
  • Test temperature: 25oC
  • Strain rate: 0.01 and 0.001 m/m/s

The strain rate is the rate by which the deformation occurs, i.e. deformation or strain per time unit (Stylen 2008) Three replicate specimens were tested for each factor combination. The results are shown in table 1.

Coefficient of Variation

This is the ratio of the standard deviation to the mean:

The coefficient of variation describes the magnitude sample values and the variation within them (Anon. 2006).

Table. Averages and coefficients of variation of tension test results

Core Orientation                                 Vertical                Horizontal                 Diagonal          
Strain Rate (m/m/s)                              0.01     0.001          0.01       0.001           0.01     0.001
Tensile strength               Average (kPa)     1925     1167           1509       1081            1782     659
                               Coeff. Of var.(%) 35       8              31         49              11       24
Initial modulus of elasticity  Average (MPa)     1011     284            707        323             913      274
                               Coeff. Of var.(%) 40       3              50         44              30       3
Axial strain at failure        Average (m/m)     0.0068   0.0080         0.0075     0.0075          0.0072   0.0055
                               Coeff. Of var.(%) 12       9              19         37              19       20
Lateral strain at failure      Average (m/m)     0.0047   0.0049         0.0055     0.0054          0.0040   0.0057
                               Coeff. Of var.(%) 29       21             30         54              18       54
Secant modulus at 90% of peak 
deviator stress                Average (MPa)     657      239            423        269             456      244
                               Coeff. Of var.(%) 45       2              32         57              9        6

(Source: Mamlouk et al. 2002g).


Tension tests were performed on the cores at strain rates of 0.01 and 0.001m/m/s. For the tension test the evaluated test parameters were tensile strength, initial modulus of elasticity, axial and lateral strains at failure, and secant modulus at 90% of the peak deviator stress. All test results were analysed statistically. Tension tests results showed no significant effect of core orientation on all tensile measurements at a significance level of 0.05. These results indicated that for the asphalt mix used anisotropic effects may be ignored for the test conditions and parameters used. The results may have been affected by large variation between some of the samples used, and a different method of specimen preparation may need to be used to gain more precise specimens. In conclusion anisotropy and isotropy of HMA materials greatly depends on specimen preparation and test parameters under consideration. (Mamlouk et al 2002h).

Effect of Anisotropy on Tensile
Properties of Asphalt Mixtures 08220000

References. Listed in alphabetical order (of the author's surname).

Anon. 2003. pavement materials testing [Online]. IPC Global. Available from http://www.ipcglobal.com.au/pavement.html# [Accessed 21 October 2008].

Anon. 2006. the superpave gyratory compactor [Online]. The University of Texas at Austin Superpave Asphalt Technology Program. Available from: http://www.utexas.edu/research/superpave/mix/gyrate.html [Accessed 13 October 2008].

Anon. 2006. coefficient of variation [Online]. Brighton Webs Ltd. Statistical and Data Services for the Industry. Available from http://www.brighton-webs.co.uk/Statistics/coeff_of_variation.asp [Accessed 21 October 2008].

Anon. 2008. cambridge advance learner's dictionary. [Online]. Cambridge Dictionaries Online. Available from http://dictionary.cambridge.org/define.asp?key=81949&dict=CALD [Accessed 20 October 2008].

Gauthier , M. 1995. Engineered materials handbook. Desk ed. Materials Park, OH: ASM International.

Gere, J. M. 2002. Mechanics of materials. 5th ed. Cheltenham: Nelson Thornes.

Mamlouk, M. S., Witczak, M. W., Kaloush, K. E., and Ho, Y.-S.2002. Effect of Anisotropy on Compressive and Tensile Properties of Asphalt Mixtures (Publishing details not given). Journal of Testing and Evaluation. Vol. 30, No. 5, Sept. 2002.

Stylen, A. 2008. strain rate imaging [Online]. Norwegian University of Science and Technology. Available from http://folk.ntnu.no/stoylen/strainrate/#strain_rate [Accessed 21 October 2008].

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