That depends on: maximum aggregate size: asphalt characteristics; temperature and mix design There is no simple answer.

you mean the mixture stiffness as van der poil charts, can the marshal stiffness, ultrasonic waves and indirect tensile explain the meaning ?

Dear Zaid.. this term originally developed by SHELL..its analogues modulus in other science branches...but they use stiffness term to focus the light that the modulus of asphalt depends on time of loading and temperature..regards 

thanks Dr. Amjad, i thought that the stiffness means apply the static load on sample til destroyed in term exceeding the elasticity point. my target is, can i adapt the indirect tensile test and marshall stiffness instead of resilent modulus test ASTM D 4123 and stiffness by van der poel nomographs ?? with my best regards

Dear Zaid,,,,,ASTM D4123 refer to repeated load indirect tensile test..you can not go to temperature more than 40 degree Celsius in this test type because  the loading type (strips) and the specimen geometry..if you we want high temperature you can try axial testing method with specimen dimension (4inch x 8inch)... regarding marshall...the result of dividing the stability by flow does not considered stiffness due to the difference in measurement unit (kNL/mm) . van der poel predict the stiffness from simple parameters of asphalt cement...you can try witczak equation since it existed in AASHTO standard and i check it it give reliable estimate for resilient modulus. 

thank you Dr. Amjad this is good advice i will depend it. can you show me this relationship?

thanks a lots