Today, virgin polymer modified asphalt mixes are comparatively more expensive for road pavement. One way to reduce the expense of such construction and to make it more convenient is the application of inexpensive polymer, such as waste polymer. The primary aim of this study was to investigate the effect of adding waste tyre rubber (crumb rubber modifier (CRM)) on the stiffness and fatigue properties of stone mastic asphalt (SMA) mixtures. Various percentages of waste CRM with size of 0.60 mm were added to SMA mixtures. Indirect tensile stiffness modulus test was conducted at temperatures of 5, 25, and 40°C. Indirect tensile fatigue test was conducted at three different stress levels (2000, 2500, and 3000 N). The results show that the stiffness modulus of reinforced SMA samples containing various contents of CRM is significantly high in comparison with that of nonreinforced samples, and the stiffness modulus of reinforced samples is in fact less severely affected by the increased temperature compared to the nonreinforced samples. Further, the results show that CRM reinforced SMA mixtures exhibit significantly higher fatigue lives compared to the nonreinforced mixtures help in and promotion of sustainable technology by recycling of waste materials in much economical and environmental-friendly manner.
Roadways are considered as one of the most important elements of infrastructure. They play an essential role in our daily lives for the present and in the future. Thus, highway construction engineers must consider the primary user’s requirements of safety as well as the economy. To achieve this goal, highway construction designers should take into account three fundamental requirements which include environmental factors, traffic flow, and asphalt concrete mixtures materials.
In asphalt concrete (AC), bitumen as a binder serves two major functions in road pavement: first, to hold the aggregates firmly and, second, to act as a sealant against water. However, due to some distresses like fatigue failure, the performance and durability of bitumen are highly affected by changes with time in terms of its characteristics which can lead to the cracking of pavement [
The design of bituminous mixture involves the selection and proportioning of materials to obtain the desired properties in the finished product. Asphalt concrete (AC) is designed to resist rutting, fatigue, low temperatures, cracking and other distresses. The serious distresses associated with flexible pavements are cracking, which occurs at intermediate and low temperatures, and permanent deformation, which occurs at high temperatures. These distresses reduce the services of life of the pavement and elevate the maintenance costs [
The dynamic properties and durability of conventional asphalt, however, are deficient in resisting pavement distresses. Hence, the task of current asphalt researchers and engineers is to look for different kinds of polymer modified bitumen with rheological properties that would directly affect the asphalt pavement performance [
Virgin polymer offers the possibility of producing mixtures that can resist both rutting and cracking; however, using virgin polymer is costly. Thus, using recycled polymer such as crumb rubber is a good alternative and is inexpensive. Also, it is considered as sustainable technology, that is, “
Crumb rubber or waste tyre rubber is a blend of synthetic rubber, natural rubber, carbon black, antioxidants, fillers, and extender type of oils which are soluble in hot paving grade. Rubberised asphalt is obtained by the incorporation of crumb rubber from ground tyres in asphalt binder at certain conditions of time and temperature using either dry process (method that adds granulated or crumb rubber modifier (CRM) from scrap tires as a substitute for a percentage of the aggregate in the asphalt concrete mixture, not as part of the asphalt binder) or wet processes (method of modifying the asphalt binder with CRM from scrap tires before the binder is added to form the asphalt concrete mixture). There are two rather different methods in the use of tyre rubber in bitumen binders: first is by dissolving crumb rubber in the bitumen as binder modifier and second is by substituting a portion of fine aggregates with ground rubber that does not completely react with bitumen [
The main characteristic of rubber is its property of high elasticity which allows it to undergo large deformations from which almost complete, instantaneous recovery is achieved when the load is removed [
The properties of an elastomer rubber are as follows. The molecules are very long and are able to rotate freely about the bonds joining neighbouring molecular units. The molecules are joined, either chemically or mechanically, at a number of sites to form a three-dimensional network. These joints are termed cross-linked. Apart from being cross-linked, the molecules are able to move freely past one another; that is, the Van der Waal’s forces are small.
Similar to bitumen, rubber is a thermoplastic, visco-elastic material, whose deformation response under load is related to both temperature and rate of strain. However, the deformation of rubber is relatively incentive to temperature change where at both low rates of strain and at temperature well above the ambient, the material remains elastic. The wider range of elastic behaviour of rubber compared to that of bitumen largely results from the cross-linking of the long rubber molecules. Rubber is also much more ductile than bitumen at low temperatures and high loading rates.
In 1950, the use of scrap tyre in asphalt pavement was reported [
Stone matrix asphalt (SMA) is a gap-graded hot mix that has gained popularity world-wide. SMA was first developed in Germany during the mid 1960s [
The dynamic stiffness or “resilient modulus” is a measure of the load-spreading ability of the bituminous layers; it controls the levels of the traffic-induced tensile strains at the underside of the lowest bituminous bound layer which are responsible for fatigue cracking, as well as the stresses and strains induced in the subgrade that can lead to plastic deformations [
Asphalt concrete (AC) should have high stiffness to be able to resist permanent deformation. On the other hand, the mixtures should have enough tensile stress at the bottom of the asphalt layer to resist fatigue cracking after many load applications. Figure
Stress beneath a rolling wheel load after [
Different testes and approaches have been used to evaluate asphalt concrete mixtures properties. Several material properties can be obtained from fundamental, mechanistic tests that can be used as input parameters for asphalt concrete performance models. Hadley et al. [
Fatigue is one of the most important distresses in asphalt pavement structure due to repeated load of heavy traffic services which occur at intermediate and low temperatures. There are different test methods used throughout the world to measure fatigue resistance for asphalt concrete mixtures.
A study by Aflaki and Memarzadeh [
Cracking is normally considered to be low temperature phenomena while permanent deformation is considered to be the predominant mode of failure at elevated temperatures. Cracking is mainly categorised into thermal cracking and load-associated fatigue cracking. Large temperature changes that occur in pavement usually result thermal cracking. This type of failure occurs when the thermally induced tensile stress, together with stresses caused by traffic, exceeds the tensile strength of the materials. It is often characterised by transverse cracking along the highway at certain intervals. Load-associated fatigue cracking is the phenomenon of fracture as a result of repeated or fluctuated stresses brought about by traffic loading. Traffic loads can cause a pavement structure to flex, and the maximum tensile strain will occur at the base of the bituminous layer. If this structure is inadequate for the imposed loading conditions, the tensile strength of the materials will be exceeded, and cracks are likely to initiate, which will be manifested as cracks on the surface of the pavement [
Raad and Saboundjian [
These models are created based on the relationship existing between stress, or strain, and fatigue life as are presented in the equations below:
For the purposes of this research investigations bituminous binder of 80/100 penetration grade was used. Table
Properties of base binder grade 80/100 penetration.
Test properties | Test result |
---|---|
Viscosity @ 135°C (pas) | 0.65 |
|
1.35 |
Ductility @ 25°C | 100 |
Softening point @ 25°C | 47 |
Penetration @ 25°C | 88 |
SMA 20 aggregate gradation.
SMA 20 | |||||
---|---|---|---|---|---|
B.S Sieve | % Passing | % Retained | Weight (G) | ||
Min. | Max. | Mid. | |||
19 | 100 | 100 | 100 | 0 | 0 |
12.5 | 85 | 95 | 90 | 10 | 110 |
9.5 | 65 | 75 | 70 | 20 | 220 |
4.75 | 20 | 28 | 24 | 46 | 506 |
2.36 | 16 | 24 | 20 | 4 | 44 |
0.6 | 12 | 16 | 14 | 6 | 66 |
0.3 | 12 | 15 | 13.5 | 0.5 | 5.5 |
0.075 | 8 | 10 | 9 | 4.5 | 49.5 |
pan | 0 | 0 | 0 | 9 | 99 |
|
|
Physical properties of the crushed aggregate.
Properties | Value | Standard Test Method | |
---|---|---|---|
L.A. abrasion (%) | 18.5% | <30% | ASTM C-131 |
Flakiness index (%) | 5.0% | <20% | BS 182: part 3 |
Elongation index (%) | 11.7% | <20% | BS 182: part 3 |
Soundness (%) | 4.7% | <12% | BS 12: part 3 |
Impact value (%) | 12.5% | <15% | BS 12: part 3 |
Polished stone value (%) | 17.7% | <30% | BS 12: part 3 |
The specimens were prepared at optimum asphalt content (OAC) using Marshall method. Five various amounts of OACs have been obtained for five various CRM contents, 6.70%, 6.5%, 6.40%, 6.20%, and 6.31% of OAC each for 0%, 6%, 12%, 16%, and 20% (all by weight of aggregate particles) of CRM content, respectively.
To incorporate rubber in bituminous mix, a wet process was conducted. In the wet process, the rubberised bitumen samples were prepared by blending the CRM with bitumen of 80/100 penetration grade (at the velocity speed of 200 rpm at 180°C and 30 minutes) before mixing the resultant rubberised bitumen with aggregate [
Later, for preparing SMA mixtures, 1100 gm of aggregate was placed in the oven and heated to 160°C for 2 hours. Rubberised bitumen required for the specimen was simultaneously heated up to temperature of 160°C for one hour. Once the aggregate and the bitumen reached the required temperature, the needed quantity of heated rubberised bitumen was added into the aggregates. Later, the crumb rubber modified bitumen binder and aggregates were mixed together (mixed by hands) at mixing temperature of 160°C until the aggregate was coated totally with bitumen. The mixture was transferred into a Marshall mould. The stainless steel thermometer was put in the centre of the mould, and the mixture was then ready for compaction at the temperature of 160 ± 5°C. All samples were subjected to 50 blows of compaction by Marshall hammer on each side of specimen. After finishing compaction, each sample was left to cool at room temperature before being extruded from the mould. Samples were removed from Marshall mould using hydraulic jack and stored at room temperature to be used later for further testing.
The dynamic stiffness or “resilient modulus” is a measure of the load-spreading ability of the bituminous layers; it controls the levels of the traffic-induced tensile strains at the underside of the lowest bituminous bound layer which are responsible for fatigue cracking, as well as the stresses and strains induced in the subgrade that can lead to plastic deformations [
This test covered the procedure for testing laboratory or filed recovered cores of bituminous mixtures to determine resilient modulus (MR) value using load indirect tensile test, under specified conditions of temperature, load, and load frequency. The test was conducted by applying compression loads with a prescribed sinusoidal waveform. The load was applied vertically in the vertical dimension plan of cylindrical specimen of bitumen sample. The resulting horizontal deformation of the specimen was measured with an assumed Poisson’s ratio to calculate the resilient modulus values.
In the current study, the testing parameters were as follows: temperature = 5, 25, and 40°C, Poisson ratio = 0.34, force = 20 × specimen depth, rise time = 70 ms, pulse period = 1 s.
Horizontal tensile stress and stiffness modulus of AC mixtures can be obtained by the following:
Fatigue test procedure is used to rank the bituminous mixture resistance to fatigue as well as being a guide to evaluate the relative performance of asphalt aggregate mixture and obtaining data and input for estimating the structural behaviour in the road.
During the fatigue test, modulus value decreased as indicated in Figure Phase I: initially there is a rapid diminution of the modulus value. Phase II: modulus variation is approximately linear. Phase III: rapid decrease of the modulus value.
The three phases of fatigue test [
Damage is defined as any loss of strength that takes place in a specimen during a test.
The fatigue life is defined as the number of load cycling application (cycles) resulting in either disintegration or a permanent vertical deformation. A stiffness reduction of 50% was used to present the sample failure due to fatigue deformation.
Horizontal tensile strain also can be obtained as the function of stress and stiffnes of mixture by using (
Universal materials testing apparatus (UMATTA) was used to determine the repeated load indirect tensile test as a method of assessing the fatigue resistance of bituminous materials. In this research study, three cyclic loading forces were used (2000, 2500, and 3000 N), respectively. Loading cycle width was 100 ms, load cycle repeated time was 500 ms, and temperature was 25°C with axial displacement of about 5-6 mm.
Resilient modulus is a primary variable in mechanistic design approaches for improved pavement structures, with regards to dynamic stresses and corresponding strains in pavement response [
Figure
Stiffness modulus versus temperature.
IDT results (stiffness modulus) indicate that the increase in CRM content produces an improvement in the elastic properties of the studied mixtures. Modified bitumen improves the resilient modulus of asphalt mixtures compared to the control mixtures, due to higher viscosity and thick bitumen films leading to better resilience properties. Thus, modified bitumen produces asphalt concrete mixtures with improved stiffness and subsequently higher load bearing capacity. Furthermore, crumb rubber modified binders indicated lower temperature susceptibility. Mixes with modified binders indicated increased flexibility at decreased temperatures. This is due to the lower resilient modulus and higher stiffness as well as tensile strength at higher temperatures [
The fatigue characteristics relating the accumulated strain with the number of cycles to failure for the SMA mixes with and without CRM reinforcement are illustrated in Table
Fatigue test results.
CRM |
|
|
|
---|---|---|---|
0% |
2000 | 1185 | 15,476 |
2500 | 3354 | 3011 | |
3000 | 9893 | 345 | |
| |||
6% |
2000 | 677 | 19,999 |
2500 | 2735 | 2354 | |
3000 | 6656 | 490 | |
| |||
12% |
2000 | 568 | 22566 |
2500 | 2354 | 4657 | |
3000 | 4189 | 678 | |
| |||
16% |
2000 | 889 | 18767 |
2500 | 2890 | 2890 | |
3000 | 6788 | 543 | |
| |||
20% |
2000 | 989 | 16566 |
2500 | 3567 | 3567 | |
3000 | 7898 | 488 |
Fatigue life versus strain at 2000 N stress.
Fatigue life versus strain at 2500 N stress.
Fatigue life versus strain at 3000 N stress.
In order to obtain representation of the fatigue life, the regression equation for each mixture along with the regression parameters for various CRM and stress values are illustrated in Tables
Regression equations for fatigue life due to the variation of stress along with regression parameters.
Stress Values | Equation for |
|
|
|
---|---|---|---|---|
2000 N |
|
|
0.476 | 0.95 |
2500 N |
|
|
1.090 | 0.92 |
3000 N |
|
|
0.866 | 0.90 |
Regression equations for fatigue life due to the variation of CRM content at OBC along with regression parameters.
CRM % | Fatigue equations |
|
|
|
---|---|---|---|---|
0 |
|
|
1.345 | 0.96 |
6 |
|
|
1.898 | 0.92 |
12 |
|
|
1.212 | 0.90 |
16 |
|
|
1.676 | 0.92 |
20 |
|
|
1.888 | 0.94 |
For a long time, the crack potential of pavements under various loading conditions and temperature variations has been an issue for the purposes of application of asphalt pavements. The maintenance and rehabilitation pricing is drastically increased when crack generation in asphalt pavement appears [
Based on the study conducted, the following conclusions can be derived. The stiffness modulus of reinforced SMA samples containing various contents of CRM is significantly high in comparison with that of nonreinforced samples. This increased stiffness modulus however is not related to increased brittleness of reinforced asphalt samples. The stiffness modulus of reinforced samples is in fact less severely affected by the increased temperature compared to the nonreinforced samples. With the presence of crumb rubber, the fatigue life of CRM reinforced samples is significantly improved. The resistance of waste tyre rubber to generated horizontal tensile stresses decreases the formation of vertical cracks and prevents these cracks from propagating along the diameters of asphalt samples. This in turn improves the fatigue life of reinforced samples. The relationships obtained are rational; the higher the stress level is, the lower the fatigue life is and the higher the accumulative strain is. Due to the addition of CRM, there has been a marked improvement in fatigue life thus being more considerable at higher stress level than at lower stress level. In particular, when heavy traffic load is applied, the enhancement of crumb rubber reinforced bituminous mix as a fatigue barrier is more remarkable. Regression models (fatigue equation) of fatigue life and accumulated strain due to different CRM content were developed for all samples. It was evident that high Apart from decreasing the pile up of waste materials, utilising these waste materials has improved the characteristics of engineering structure and materials in asphalt production and similar industries. Consequently, it has also reduced construction rehabilitation and maintenance costs. Since different conclusions have been drawn from this research project, a list of recommendations is summarised as follows for further investigations in the future: use of different type of aggregate, aggregate gradation, different mixing methods, and different compaction methods, selection of different bitumen sources with various penetration grades and also the use of other kind of recycled polymer such as waste plastic bottle, a comparative assessment of the cost incurred for pavement constructions utilising various modified asphalt with those constructed using conventional binder, use scanning electron microscope images to evaluate binder-aggregate adhesion, conduct more studies for fatigue damage, including more mix variables and different rubber sizes to evaluate the effect of the particle size and texture of rubber.
This paper has no conflict of interest.