Crumb rubber modified binders are environmental-friendly alternatives to polymer modified bitumens in asphalt mixtures. This paper compares the performance of both types of binders with different modifier contents. Six binders were characterised by conventional tests and analysed using the UCL method. This method evaluates different properties of binders regarding their role in asphalt mixtures (cohesion, water and thermal sensitivity and resistance to ageing). Results showed that i) crumb rubber concentration has to be higher than that of SBS-polymers in order to obtain a similar performance to that of SBS-polymer modified bitumen; ii) crumb rubber modified binders are more stable than SBS-polymer modified binders in terms of modifier concentration; iii) crumb rubber modified binders exhibited less water sensitivity and similar thermal and ageing susceptibility to SBS-polymer modified binders; iv) linear relationships have been found between modifier concentration and the properties studied for both kind of binders.
The use of polymers as binder modifiers in asphalt mixtures is currently a common practice in pavement engineering, patented since 1843. Over the last few decades, the addition of these materials, chains of repeated small molecules, to bitumen has shown great improvements in its performance. Pavements with polymer modification exhibit greater resistance to rutting and thermal cracking, and decreased fatigue damage, stripping, plastic deformations and temperature susceptibility (
However, because of the increase of environmental requirements, other alternative modifiers have been developed to obtain high performance, environmentally sustainable binders (
Although the influence of the concentration of modifiers has been studied in those different aspects, the research described in this paper provides a new approach to the subject, focusing on the analysis of the most important engineering properties of binders for asphalt mixtures: cohesion, water, thermal and ageing sensitivity. These properties have been studied by means of the UCL (Universal de Caracterización de Ligantes) method. This method was developed by the Road Research Laboratory in the Transportation Department at the Technical University of Catalonia in 1994 to characterize binders (
In order to analyse the influence of the concentration of modifiers in crumb rubber-modified binder (CRMB) and SBS polymer-modified binder (PMB), three CRMB and three PMB with different modifier concentrations were studied. In order to have representative results to compare the behaviour of both modifiers, both types of binder had the same base bitumen (50/70 penetration grade). Then, starting with an initial concentration of modifiers (PMB-1 and CRMB-1), each concentration was reduced in the same percentage in both binders, with the reduction being 30% (PMB-2 and CRMB-2) and 75% (PMB-3 and CRMB-3).
Concentration of modifier on each binder by weight of bitumen
Bitumen | Modifier concentration (%) |
---|---|
PMB-1 | 3.75 |
PMB-2 | 2.63 |
PMB-3 | 1.31 |
CRMB-1 | 5.0 |
CRMB-2 | 3.5 |
CRMB-3 | 1.75 |
In the UCL method, binders are mixed with a particular mineral skeleton. This mineral skeleton consists of a gap-graded gradation with a maximum aggregate size of 5 mm to highlight the effect of the binder (
UCL aggregate grain-size curve.
UCL samples properties
Bitumen | Bitumen Content, (%) | Bulk Density, Mg/m3( |
Voids in mix,% ( |
Voids in aggregates (%) ( |
---|---|---|---|---|
PMB-1 | 4.3 | 1.987 | 25.1 | 33.4 |
PMB-2 | 4.3 | 1.988 | 25.1 | 33.4 |
PMB-3 | 4.3 | 1.975 | 25.9 | 34.1 |
CRMB-1 | 4.3 | 2.013 | 24.1 | 32.5 |
CRMB-2 | 4.3 | 1.996 | 24.8 | 33.1 |
CRMB-3 | 4.3 | 1.976 | 25.6 | 33.8 |
The effect of modifier content on the PMBs and CRMBs was studied and compared through the conventional characterisation of the binders and the UCL method.
Penetration test at 25 °C (
The UCL method studies the behaviour of asphalt binders in terms of cohesion of the binder, water, thermal and ageing sensitivity (
The Cantabro test consists of testing a Marshall specimen (
Cohesion was assessed by keeping the specimens at 25 °C for 24 hours and submitting them to the Cantabro test (dry set). After that, water sensitivity was evaluated by measuring the increase of the Cantabro losses of the dry set when it is kept immersed in water at 60 °C for 24 hours (wet set). Thermal sensitivity was assessed by studying the variations of the Cantabro losses of the standard mix with temperature. For this purpose, specimens were conditioned at 25 °C, 10 °C and −10 °C for 12 hours before being tested. Finally, to evaluate the ageing resistance, the mix was exposed to different ageing conditions and compared with the standard one. Specimens were kept in a convection oven at 163 °C for 2 hours, 4 hours and 8 hours respectively before being tested. The Cantabro test was always performed at 25 °C.
During the ageing process, specimens could suffer deformations and losses of material as well as runoff of the binder to the lower part of the sample due to the high temperature. In order to avoid the former, specimens were laterally confined by a wire mesh with a mesh size big enough to allow the hot air circulation but small enough to avoid material losses. However, to minimize the runoff of the bitumen, specimens were located in a special device which consists of two plates full of holes and joined by screws (
Device for ageing process.
Results have been analysed in two different ways: (
Binder properties vs. Modifier concentration: a) Needle penetration; b) Softening point; c) Fraass breaking point; d) Viscosity at 140 °C; e) Elastic recovery and f) Force-ductility.
In order to study the performance of both modified binders in terms of cohesion, water sensitivity, thermal and ageing susceptibility, Cantabro losses are compared before and after immersion of the samples and versus temperature and ageing time respectively. These results are shown in
Water sensitivity test.
Thermal susceptibility results.
Ageing susceptibility results.
In
Cantabro losses increase after water immersion (%)
Bitumen | Cantabro losses increase after water immersion (%) |
---|---|
PMB-1 | 161.3 |
PMB-2 | 134.9 |
PMB-3 | 75.0 |
CRMB-1 | 146.2 |
CRMB-2 | 42.6 |
CRMB-3 | 67.4 |
Cantabro losses before and after immersion in water are displayed in
Cantabro losses of dry and wet sets vs. modifier concentration.
Thermal sensitivity vs. modifier concentration.
Ageing resistance vs. modifier concentration.
Lastly, the linear relationships for all the properties studied exhibited high goodness of fit values, which suggests that the change in modified binder properties is lineally proportional to the change in modifier concentration within the range of concentrations tested in this investigation.
This paper analyses and compares the performance and the influence of additive concentration on three crumb rubber and three SBS-polymer modified binders in terms of their most important properties for pavement engineering. Based on the results of the tests carried out, the following conclusions can be drawn: Crumb rubber concentration in the binder has to be higher than that of SBS-polymers in order to obtain a similar performance. Crumb rubber modifier binders are more stable than SBS-polymer modified binders in terms of modifier concentration for all the properties studied. This fact can be seen as an advantage in pavement engineering due to the fact that if concentration of the modifier has to be decreased (implying budget savings), the properties of the binder would change less in the case of CRMB and therefore the expected performance would be better than that of PMB if the concentration of modifiers has to be decreased. UCL allowed the analysis of important properties of asphalt binders for pavement engineering. In this sense, crumb rubber binders exhibited less water susceptibility and similar thermal and ageing sensitivity to SBS-polymer modified binders. Thereby, CRMBs are a real alternative to PMBs in terms of performance and with environmental-friendly added value. Linear relationships have been found between modifier concentration and the properties studied, which suggest that the change in modified binder properties is lineally proportional to the change in modifier concentration within the range of concentrations tested in this investigation.
Authors would like to acknowledge the Agencia Andaluza de la Obra Pública de la Junta de Andalucía, Spain, for the financial support received. These results come from a project that has been financed with resources from the European Regional Development Fund (ERDF).