7. Laboratory Performance Evaluation of SBS Polymer Modified Bituminous Concrete Mixes
Date of Start

August 2005
Indian Institute of Technology Madras, Chennai (R, C)


Scope and Objectives

• Carry out various tests on conventional bitumen and polymer modified binder

• Compare the rutting parameter of unaged samples and thin film oven residue of both binders

• Determine optimum bitumen content and polymer modified binder content for bituminous mixes

• Carryout indirect tensile tests on bituminous mixes

• Carryout fatigue tests on cylindrical specimens with and without modifier

• Study the effect of variation in load and temperature on the fatigue behaviour of bituminous mixes

• Develop fatigue prediction equations for bituminous concrete mixes with conventional bitumen and modified binder

The laboratory tests have been carried out on 60/70 grade bitumen and SBS based polymer modified bitumen (PMB70) in the present study to achieve the objectives mentioned hereinbefore. The Marshall mix design has been carried out for Bituminous concrete (BC, Grading 1) to determine the optimum bitumen content for both binders. The complex shear modulus and phase angle for the two binders i.e. 60/70 grade bitumen and PMB – 70 on unaged and TFO aged samples have been determined by dynamic shear rheometer. This has been done with a view to evaluate the rutting parameter (G* / Sin ). The fatigue tests have been carried out on the “Repeated Indirect Load Test Equipment”. The cylindrical specimens with both binders have been prepared separately at optimum binder content obtained by Marshall mix design and tested to study the effect of variation of load and temperature. The regression analysis has been carried out using SPSS software to develop equations to predict the fatigue life taking initial tensile strain as the independent variable.
Findings and Conclusions
(i) SBS polymer modified binder is found to have a high elastic recovery of 79%.

(ii) Modified binders have better age resistance properties. The loss in weight on heating in thin film oven is 6 times higher in case of 60/70 grade bitumen.

(iii) Modified binders show increased viscosity at a given temperature. The viscosity of 60/70 grade bitumen at 1350C is 3.93 poise which is approximately equal to the value of 3.97 poise for PMB-70 at 1500C. This shows that the mixing and laying temperatures for polymer modified binder should be higher by approximately 150C.

(iv) SBS polymer modified binder shows 150 to 370% increase in rutting resistance of the binder.

(v) Marshall stability of the mix increases by 27% when polymer modified binder was used.

(vi) The indirect tensile strength of the bituminous concrete mix prepared with SBS polymer modified binder is 20% higher at 300C.

(vii) Resilient modulus of the mix prepared with SBS polymer modified binder is 19 to 30% higher at the test temperatures of 30 to 400C.

(viii) The tensile strain increases by 83% with increase in temperature from 30 to 400C and the increase in tensile strain reduces the fatigue life of the mix.

(ix) At constant tensile stress and pavement temperature, the fatigue life of the SBS polymer modified bituminous concrete mix increases by 95% to 360%. The magnitude of increase in fatigue life, however, decreases with increase in tensile stress.

(x) Overloading of vehicles upto 20% causes 30 to 40% reduction in life of the pavement.