The construction of highways and road networks in the Middle East region has experienced significant development in size and standards in the last 50 years. In Saudi Arabia, for instance, the total length of paved roadways in 1975 was 12,200 km, but, by 2000, the length had exceeded 40,000 km (Bubshait, 2001). Middle Eastern countries have invested hundreds of billions of dollars in the construction of new highways and paved roads to accommodate the rapid development rates that generated extremely large traffic volumes (Al- Abdul Wahhab, et al., 1999). However, the performance of these new highways against major distresses still needs a great deal of improvements and enhancement.
Only a few studies have been conducted in evaluating and improving the performance of the asphalt pavement mixtures and structures in Middle Eastern countries. Most of these regional studies considered the significant increase in the traffic loading, and the hot weather, which are the most important factors affecting the performance of the asphalt pavement designs (Al-Abdul Wahhab & Balghunaim (1994) and Al-Abdul Wahhab et al. (2001)). However, most of the studies focused on rutting as a major and prominent failure mode in the region; they did not give much attention to fatigue cracking, which is another major failure mode. In this
10 subsection, the main regional studies on performance evaluation and improvement are introduced and summarised. The main studied aspects and conclusions are highlighted, which will introduce the importance of the work of this thesis.
Bubshait (2001) discussed the main factors that contribute to the quality and the service life of asphalt concrete pavement structures in the Kingdom of Saudi Arabia. These factors are managerial-related factors (e.g. the contractor), design- and specifications-related factors, and construction-related factors. In order to identify the most affecting factors, a survey was prepared and forwarded to 61 highway contractors in Saudi Arabia. The results of the survey revealed that the asphalt pavement structures are not designed for regional conditions and the most important factors are the aggregate quality, aggregate characteristics and the mix design including bitumen characteristics. For example, unmodified 60-70 Pen bitumen is used widely in the Middle East; however, this bitumen is too soft given the high temperatures in the region. Therefore, it was suggested by Fatani, et al. (1992), Al-Abdul Wahhab and Balghunaim (1994) and Elseifi, et al. (2012) that either a harder bitumen or a polymer-modified bitumen (PMB) could be used to mitigate the poor performance that is associated with the use of unmodified 60-70 Pen bitumen. PMB has been found to enhance several properties of asphalt concrete (AC) mixtures such as temperature susceptibility, fatigue life, and resistance to permanent deformation (Al-Hadidy & Yi-qiu, 2010).
Al-Abdul Wahhab and Balghunaim (1994) stated that many parts of the highways in Saudi Arabia experienced severe levels of rutting just after being opened to traffic due to heavy traffic loading and high temperatures. The primary objective of this study was to demonstrate data on variations of temperatures of asphalt pavement structures in the arid coastal and inland Saudi environment. Asphalt concrete slabs of 150 mm, 200 mm and 300 mm thicknesses were constructed in Riyadh, the capital of Saudi Arabia, and a 250 mm asphalt-bound layer on top of a compacted aggregate sub-base was constructed in Dhahran, another city in Saudi Arabia.
11 The bitumen used was unmodified 60-70 Pen, which is locally produced and widely used in the region. Temperatures were measured for two years at the pavement surface, and at depths of 20, 40, 80 and 160 mm, as well as at the bottom of the pavement structure. In addition, the air temperature at a height of 1.5 m above the surface was measured as well. The data collected were statistically analysed and showed that the highest temperatures were at a depth of 20 mm and that, during the summer months, the temperature decreased as the pavement section depth increased. Moreover, it was concluded that rutting could be minimised and resisted by using modified bitumen that can withstand high temperatures.
Another regional study, by Al-Abdul Wahhab, et al. (1999), was conducted to specify possible factors for the premature rutting which occurred on many roads and highways in Saudi Arabia after a rapid increase in traffic volumes. Nineteen different test sections, each one kilometre in length, located on 12 major highways in Saudi Arabia were selected and subjected to field investigation and laboratory characterisations. Results depicted that rutting is affected directly by the percentage of air voids (AV), the percentage of voids in mineral aggregate (VMA), the percentage of voids filled with bitumen (VFB), resilient modulus (Mr) at 25 ºC,
and bitumen viscosity. Furthermore, the rutting in the cores obtained from rutted test sections was localised only in the top 100 mm of the asphalt concrete layers, underneath the wheel path of the truck slow lane. Moreover, the study revealed that the pavement temperature approaches a high value of around 70 ºC during the months of May to September, which is one of the major external factors for rutting.
A comprehensive study by Al-Abdul Wahhab, et al. (2001) was carried out later based on his two previous studies, discussed earlier. This study was performed to discover trends of temperature variation and their consequences on the moduli of flexible pavement structures. A statistical model for the asphalt concrete resilient modulus (Mr), a second model to predict
12 for the resilient modulus-temperature correction factor correlation were all developed to be used in pavement analysis and design for Saudi Arabia. It was stated that no single study in Saudi Arabia had previously been conducted to develop mathematical models to simulate the temperatures to which local pavements are subjected and which are applicable to other parts of the Middle East region.
The advantages of utilising waste volcanic ash as a cheap local alternative to aggregate for pavement designs were investigated in a study conducted in Yemen by Naji and Asi (2008). This study contributed to waste management of this undesirable material. Four different mix designs of unmodified 60-70 Pen bitumen with replacement proportions for aggregate of 0, 10, 20, and 30% of the total weight of dry aggregate were used. The experimental results showed that the properties and characteristics of all mixes comprising volcanic ash aggregate, up to 20%, were within the specification limits of the Marshall mix design method. In addition, the resilient modulus (MR) was increased, and the creep resistance properties improved. Mixtures
with only 10% volcanic ash aggregate provided optimum results in terms of stripping resistance, creep resistance, fatigue and resilient modulus.
A study conducted in Jordan by Asi (2006) focused on a laboratory evaluation of the performance of stone matrix asphalt (SMA) mixtures compared to dense-graded mixtures in hot climates. SMA is a gap-graded mix with a high bitumen content, and is designed to provide better rutting resistance. The dense-graded mixture was designed according to the Marshall method and was used in this study with unmodified 60-70 Pen bitumen. Performance evaluation tests such as Marshall Stability, split tensile strength, resilient modulus, fatigue, and rutting testing were performed on both mixtures. Results concluded that the stability of Marshall mixtures is higher than SMA mixtures; however, the loss of stability of the Marshall mixtures is higher. In addition, SMA mixtures have better durability, lower fatigue life, and higher MR values than Marshall mixtures. However, the test results indicated that the Marshall
13 mixtures resisted rutting better than the SMA mixtures. This result is counter-intuitive and might be affected by the limited number of specimens tested. The field performance of the SMA mixtures proved their superiority over conventional asphalt mixtures in terms of durability and resistance to rutting.
Al-Hadidy and Yi-qiu (2010) investigated the effect of Styrene-butadiene-styrene (SBS) and Starch (ST) used to modify bitumen on the performance of asphalt pavements. Marshall Stability, Marshall Quotient, tensile strength, tensile strength ratio, rutting resistance, flexural strength and resilient modulus tests were carried out on specimens of stone mastic asphalt concrete (SMAC) mixtures containing SBS and ST. The results of the tests depicted that the performance of SBS-modified SMAC is slightly better than ST-modified SMAC. In addition, the temperature susceptibility could be reduced by the addition of SBS and ST to the mixture.
Rutting has been the main failure mode considered in regional studies. However, fatigue cracking – especially top-down cracking – is also a prominent distress in the region because of the high traffic loading, low bitumen content used and over-compaction at relatively low temperatures. In addition, unmodified 60-70 Pen bitumen is extensively used in the region, despite evidence that this bitumen does not have the stiffness required to resist deformation at high temperatures. Therefore, the research study presented in this thesis was conducted to investigate all factors affecting the performance of the pavement structures and mixtures in the region, in particular against rutting and top-down cracking.
As mentioned earlier, Middle Eastern countries and specifically the State of Qatar are currently investing hundreds of billions of dollars in constructing their road networks and highways. The objective is to build a sustainable road network that is designed according to state-of-the-art standards and methods. Several field studies have shown that the current and projected traffic loads (axle weights and design traffic) are unprecedented in the State of Qatar.
14 Consequently, there is a need to evaluate whether the current mix designs and materials are suitable for these projected loads.
The only study available in the open literature about performance of pavement materials in Qatar was conducted by Masad, et al. (2011). The study aimed to make recommendations for materials and test methods that can be used to construct long-life roads that are necessary to support economic growth. Asphalt concrete materials that are used in road construction in Qatar were collected from different sources and field cores were extracted from in-service asphalt pavement structures. Many laboratory tests were performed to assess the physical, chemical, and mechanical properties of these materials. The study concluded that the conventional imported aggregates, Gabbro (igneous rock), can resist skid and degradation better than the local Limestone aggregates (sedimentary rock). However, Gabbro has less resistance to moisture damage than Limestone aggregate. In addition, the results of dynamic mechanical analysis tests performed on the asphalt mixtures demonstrated the benefits of using modified bitumen instead of the unmodified bitumen currently used in the country. Moreover, the results of the laboratory tests conducted on extracted field cores showed good resistance to permanent deformation (rutting); however, the field cores could be susceptible to fatigue cracking, given the low bitumen content used in these mixtures.