The assessment of the influence of styrene-butadiene-styrene copolymer on the selected rheological properties characterising durability of modified bitumens used in road pavements

BULLETIN OF THE POLISH ACADEMY OF SCIENCES TECHNICAL SCIENCES, Vol. 68, No. 6, 2020 DOI: 10.24425/bpasts.2020.135391 CIVIL ENGINEERING The assessment of the influence of styrene-butadiene-styrene copolymer on the selected rheological properties characterising durability of modified bitumens used in road pavements M. MIELCZAREK 1*, S. DZIADOSZ 2 , M. SŁOWIK 1 , and M. BILSKI 1 1 2 Institute of Civil Engineering, Poznan University of Technology, ul. Piotrowo 5, 60-965 Poznan, Poland Doctoral Studies, Institute of Civil Engineering, Poznan University of Technology, Plac M. Skłodowskiej-Curie 5, 60-965 Poznan, Poland Abstract. The subject matter of the research pertains to the improvement of rheological properties of petroleum bitumens by their modification with SBS (styrene-butadiene-styrene) copolymer. The authors have determined selected rheological properties characterising the durability of modified bitumens used in road pavements. The bitumens were modified in laboratory conditions with modified bitumen concentrate of a known SBS copolymer content of 9%. The result was a binder containing the known percentage of the SBS copolymer of 3%, 4.5% and 6%. Rheological properties of the tested bitumens were determined by the use of a DSR dynamic shear rheometer (in a wide temperature range from 40°C to 100°C) and a ductilometer at 5°C. DSR was used for performing MSCR test to determine the resistance of the asphalt mixture with the SBS-modified binder to permanent deformations in the high temperature range (from 40°C to 82°C). The comparison of the values of the dynamic shear modulus jG*j of all the bitumens tested shows that with a growing content of the SBS copolymer in the tested binder the value of jG*j increases, which may indicate greater resistance to permanent deformation of the asphalt pavement. The MSCR test has shown that the increased use of the SBS copolymer addition in the bitumen translates to decreasing values of the non-recoverable creep compliance Jnr. The SBS copolymer accelerates stress relaxation in the bitumen sample, thus increasing pavement resistance to low-temperature cracks. Furthermore, modification reduces the negative impact of ageing on the properties of the binder, manifested by its stiffening and slowdown of relaxation. Key words: dynamic shear rheometer (DSR), modified bitumen, dynamic shear modulus, relaxation, Multiple Stress Creep Recovery (MSCR). 1. Introduction Consistently growing traffic loads and the influence of changing climatic conditions prevailing in Poland shorten the service life of asphalt pavements [1, 2]. Increase in heavy traffic is predominantly caused by high demands posed by passenger and freight transport, which contribute to rapid degradation of road pavements. Newly developed pavements are subject to ever higher requirements related to resistance to permanent deformations at high temperature, resistance to cracking at low temperature, resistance to fatigue under recursively repeated loads and resistance to water and frost [1]. Pavement’s service life is largely affected by the asphalt mixture’s composition and rheological properties of the applied bitumen binder, i.e., cohesion, viscosity, elasticity, stiffness and adhesion to mineral aggregates [3]. One way of improving binder properties is to modify their structure by introducing appropriate modifiers [4, 5]. At present, polymers are most commonly used for this purpose, especially elastomers, which improve the elastic properties of *e-mail: Manuscript submitted 2020-06-01, revised 2020-09-05, initially accepted for publication 2020-10-29, published in December 2020 Bull. Pol. Ac.: Tech. 68(6) 2020 bitumen, limiting the intensive formation of permanent deformations in asphalt pavements and increasing pavement durability [1, 6, 7]. According to SHRP (Strategic Highway Research Program), the rheological properties of modified bitumen are tested using, among others, BBR (Bending Beam Rheometer), ductilometer featuring the option to measure tensile force and DSR (Dynamic Shear Rheometer) [1, 3, 8]. Modifications with styrene-butadiene-styrene block copolymer have a remarkable influence on rheological and physicochemical properties of the asphalt binder [9‒11]. This influence depends also on the binder’s copolymer content [12, 13]. Increased content of SBS copolymer (in literature the threshold is set at about 6% [13]) is accompanied by a change in the structure and proportions of the binder components. The modifier in the binder becomes a dominant component and forms a polymer network, which significantly changes the binder’s properties [12, 14, 15]. Polymer modification is of key importance in the context of the binder’s resistance to degradation due to ageing. During the ageing process not only does the binder undergo oxidation, but it is also subject to gradual decomposition and disappearance of the polymer [14, 16, 17]. Zhang and others [18] proved in their research that regardless of the method of ageing, e.g., RTFOT (Rolling Thin Film Oven Test), PAV (Pressure Aging Vessel) or UV (ultraviolet), an increased content of 1471 M. Mielczarek, S. Dziadosz, M. Słowik, and M. Bilski 2. The features of the tested asphalt binders The tests were carried out on 50/70 penetration grade road bitumen, obtained as a result of distillation of Russian crude oil. The bitumen was modif ied in laboratory conditions by introducing into its structure a bitumen concentrate modified with SBS copolymer. The concentrate with SBS copolymer content of 9%, obtained from 160/220 penetration grade road 1472 bitumen, was combined with 50/70 penetration grade bitumen to arrive at the known SBS copolymer contents in the obtained bitumen, i.e., 3%, 4.5% and 6% of styrene-butadiene-styrene copolymer (in relation to the mass of modified bitumen obtained). Bitumen binders were heated to 140°C (190°C in case of SBS copolymer concentrate). Then they were mixed for 5 min using a laboratory mixer at an angular speed of 400 RPM. In order to obtain the determined contents, the mass proportions between the concentrate and 50/70 road bitumen were maintained as 1:2, 1:1 and 2:1, respectively. All tested binders were conditioned in the same manner. During the test, the heating process was controlled, and an attempt was made to prepare a set of studied samples during one heating cycle of the material. Bitumen was submitted to analysis both in its initial state and after a short-term (technological) ageing process simulated by the RTFOT method according to EN 12607-1:2014 “Bitumen and bituminous binders. Determination of the resistance to hardening under the inf luence of heat and air – Part 1: RTFOT Method”. Table 1 summarises the results for basic properties of the tested bitumens (penetration and softening point) carried out according to the standards EN 1426:2015-08 and EN 1427:2015-08, respectively. Binders Penetration after ageing by RTFOT method [mm/10] TR&B before ageing [°C] TR&B afte (...truncated)