KSCE Journal of Civil Engineering (2013) 17(1):117-121 DOI 10.1007/s12205-013-1698-6
Highway Engineering
www.springer.com/12205
Application of Response Surface Methodology to Evaluate Stone Matrix Asphalt Stripping Potential A. Khodaii*, H. F. Haghshenas**, H. Kazemi Tehrani***, and M. Khedmati**** Received September 13, 2011/Accepted May 8, 2012
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Abstract Stone matrix asphalt mixes are widely used in Western Europe due to its high strength properties in pavements, this type of mix is subjected to continuous wetting and drying cycles due to its porous structure leading to a higher potential for stripping. Lime has long been recognized and used in industry to reduce the stripping potential of SMA. Aggregate grading has also been identified to have an impact on the moisture damage potential. Response surface methodology was employed in this paper to evaluate the effect of lime content and grading on stripping potential of Stone Matrix Asphalt (SMA) using Tensile Strength Ratio (TSR) index. The statistical significances of linear, quadratic and interactive terms of the lime percent and grading were examined and second order polynomial regression equation was successfully fitted to the experimental results. The lime content of 1% and the finest aggregates resulted in a TSR optimum value of 91.8%±0.8%. It was further concluded that increasing the mastic asphalt would decrease the stripping potential of SMA and both of the two parameters evaluated, had approximately the same main effect on the TSR response. Keywords: lime, grading, indirect tensile strength, SMA (Stone Matrix Asphalt) mixture, response surface methodology ···································································································································································································································
1. Introduction The problem of moisture sensitivity, that constitutes itself as stripping, has been recognized since the advent of asphalt paving technology (Kim and Coree, 2005). Presence of moisture together with traffic loading, can lead to two events; the loss of adhesive bond between the asphalt and aggregate (a failure in asphalt bonding to aggregate) and/or softening of the cohesive bonds within the asphalt (a failure of the asphalt itself) (Kim and Coree, 2005). Stripping is among the most important failure modes of asphalt mixtures which can make the pavement layers prone to other types of failure such as rutting and cracking (Mehrara and Khodaii, 2011). It is believed that moisture-sensitive mixtures are more likely to exposure to stripping (Mehrara and Khodaii, 2011). Factors affecting moisture sensitivity have been identified as the type of mix, characteristics of the asphalt binder and the aggregate, environmental effects during and after construction, and the use of anti-stripping additives (Abo-Qudais, 2007; Gorkem and Sengoz, 2009; Hicks, 1991; Kiggundu and Roberts, 1988;
Sivilevicius et al., 2011; Stuart, 1990). Two of the pertinent factors, which influence the stripping characteristics, are the aggregate grading and the lime content of mixture (Sivilevicius and Vislavicius, 2008). The addition of hydrated lime - as an anti-stripping additive results in increasing stiffness, fracture toughness and oxidation protection as well as decreasing rutting, fatigue and cracking (Little and Epps, 2001; Khosla et al., 2000; Lee et al., 2010). The main role of adding hydrated lime is to increase bonding between bitumen and aggregates. Furthermore, it has been reported that hydrated lime, apart from the other mixture constituents, plays a major role in improving the stripping resistivity (Little and Epps, 2001). Stone Matrix Asphalt (SMA) is a gap-graded hot mix that gained popularity world-wide. SMA was first devised in Germany in the 1960s. Since the evolution of SMA technology in the early 1990s (Brown and Cooley, 1999), it has been widely used due to its great potential in resisting permanent deformation or rutting (Moghdas Nejad et al., 2010). However, little attention has been paid to the stripping potential of SMA.
*Associated Professor, Dept. of Civil and Environmental Engineering, Amirkabir University of Technology, Tehran 15914, Iran (Corresponding Author, E-mail:
[email protected]) **Research Assistant, Dept. of Civil and Environmental Engineering, Amirkabir University of Technology, Tehran 15914, Iran (E-mail:
[email protected]) ***Graduate Student, Highway Division, Dept. of Civil Engineering, Islamic Azad university South Tehran Branch, Tehran 4435/11365, Iran (E-mail: h.kazemi_
[email protected]) ****Graduate Student, Dept. of Civil and Environmental Engineering, Tarbiat Modares University, Tehran 14115-111, Iran, (E-mail: mahdieh.khedmati@ modares.ac.ir) − 117 −
A. Khodaii, H. F. Haghshenas, H. Kazemi Tehrani, and M. Khedmati
Some advantages of SMA mixtures are its high rut resistance (Richardson, 1997), high durability, improved resistance to reflective cracking and reduced noise pollution; however drainage of binder and higher primary costs are known as its disadvantages (Moghdas Nejad et al., 2010). Due to both the gap-graded nature of SMA mixes and relatively smaller amount of fine material there is a potential for drain down of asphalt binder from the mix and hence some form of stabilization is normally required This requirement is commonly achieved by adding fibers or polymer modifiers to the mix (Serfass and Samanous, 1996; Süreyya et al., 2007). A review on available literature shows that in previous studies one-factor-at-a-time methodology has been used to evaluate the effect of important parameters affecting on the stripping potential of SMA. This methodology is very inefficient and furthermore gives no information about possible interaction between parameters. One of the methodologies capable of providing an answer to this question is factorial Design of Experiments (DOE), which - through the use of techniques such as Response Surface Methodology (RSM) - is able to simultaneously consider several factors at different levels, and give a suitable model for the relationship between the various factors and the response. However, it seems that RSM has not yet been used to study the effect of lime and grading on moisture sensitivity. Amongst the two factorial designs, full and fractional, Fractional Factorial Designs (FFD) -such as Central Composite Design (CCD) or Box-Behnken- can give information regarding parameter interactions with the use of less experimentation; it should however be noted that, reliable information about first order interactions can only be obtained from the results of DOEs which are not highly fractionated (Montgomery, 2006). The aim of the present work was to examine the effect of grading and lime content on stripping potential of SMA, as well as interactions between them, using appropriate methodology, namely RSM. A half fractional factorial design was chosen as the design matrix since it allows reliable identification of first order interaction between factors and provides a second order polyno mial model which can be used to predict optimum level of these parameters.
Fig. 1. Size Distribution of the Three Aggregate Grading Selected Table 1. Mechanical Properties of the Siliceous Aggregate Tested Values Asphalt Institute MS-2 (%) Specifications (%) AASHTO T96 19 < 30 ASTM D5821 100 -
Test
Standard
LA Abrasion Loss Crushed in one Face Fractured Particles in ASTM D5821 two Face and More Coating of Aggregate AASHTO T182 Flakiness BS 812 Sand Equivalent AASHTO T176 Sodium Sulphate Soundness
AASHTO T104
93
90
