Special Issue on Innovation on Paving Materials

Increasing awareness in sustainability has prompted many innovations in transportation infrastructure industry in the past several decades (Huang and Liu 2011; Shu et al. 2013). This special issue contains eight technical papers in this area, which cover the topics of alternative paving materials, recycled asphalt pavement (RAP) and recycled asphalt shingles (RAS), new and innovative methods for evaluating the distress of asphalt and concrete materials, as well as the latest advances in pavement base construction and railroad ballast research.

With the increasing price of petroleum-based asphalt in recent years, people have started to seek alternative binders to petroleum asphalt that can be used in pavement construction (Aziz et al. 2015; Huang et al. 2012). Some examples of asphalt alternatives include bioasphalt derived from waste cooking oil (Wen et al. 2013), waste engine oil (Jia et al. 2014, 2015), and biochar derived from bio-oil used as a biomodifier for asphalt cement (Zhao et al. 2014a, b). In this special issue, Ji, Yao, Suo, You, Li, Xu, and Sun evaluated the efficacy of waste ooking vegetable oil as a rejuvenator in restoring the desired properties of aged asphalt binder from RAP. They developed two types of rejuvenators by replacing heavy oils with waste cooking vegetable oils (corn oil and soybean oil) and compared the rejuvenating effects of vegetable oils with those of a commercial asphalt rejuvenator at different contents. They found that vegetable oil can successfully restore the properties of aged asphalt binder by reducing its viscosity and stiffness. At 6% content, vegetable oil–rejuvenated RAP binder showed rutting and fatigue cracking properties similar to those of asphalt binder PG 64-22. At a higher content ($>6\%$), the rejuvenated binder can meet the low-temperature cracking criteria according to Superpave binder standards.

Coal tar pitch is a by-product of the coking industry and has similar chemical compositions and physical properties to asphalt binder (Brown et al. 2009). When used as a paving material, coal tar pitch exhibits the following advantage: excellent wetting and adhesion to stone and mineral aggregates, resistance to oil corrosion, and a large road surface friction coefficient (Andreikov et al. 2010; Chambrion et al. 1995; Li et al. 1999; Yan 1986). However, it also suffers many disadvantages, two of which are a high temperature susceptibility and a small temperature range of viscoelasticity. To overcome these disadvantages, Ji, Yao, Suo, Zhang, Cao, You, and Li adopted a modification technique to improve the high temperature property of coal tar pitch and to reduce its temperature susceptibility. They used different combinations of four polymeric additives, including polyethylene glycol (PEG), paraformaldehyde (PFA), polystyrene (PS), and polyphosphoric acid (PPA), to modify coal tar pitch and investigated the effects of these modifiers on the rheological properties of coal tar pitch. They found that polymer modification can increase the elasticity and thus the high-temperature property of coal tar pitch. They also observed a better dispersibility and compatibility between modifiers and coal tar pitch through microscopic images, which resulted in a better performance of the modified coal tar pitch.

Use of RAP and RAS has gained widespread acceptance in the asphalt paving industry (Huang et al. 2011; Shu et al. 2008, 2012; Zhao et al. 2012, 2013, 2015d, 2016). However, when RAP or RAS is added into asphalt mixture, one critical concern arises: How much of the aged binder in RAP or RAS can be blended into virgin asphalt binder and thus be available to coat aggregate particles (Huang et al. 2005)? If RAP or RAS binder cannot be 100% used for coating aggregates, the performance of recycled asphalt mixtures may be compromised. This question has attracted many researchers looking into the blending efficiency of RAP and RAS (Bowers et al. 2014a, b; Zhao et al. 2015c). Use of recycling agent or rejuvenator is one of the viable ways to improve blending efficiency of RAP or RAS mixtures and thus enhance the performance of recycled asphalt mixtures (Cooper et al. 2015; Tran et al. 2012; Zhao et al. 2015a). In this special issue, Cooper et al. evaluated the effects of recycling agents on the blending between aged and virgin asphalt binders and further their effects on the laboratory performance of asphalt mixtures containing RAP and RAS. They used a vegetable oil derived from the pyrolysis of pine tree and a PG 52-28 soft binder as recycling agents in their study and compared the high-, intermediate-, and low-temperature properties of recycled asphalt mixtures with and without recycling agents. They found that use of recycling agent can improve the blending between aged and virgin binders. However, the increased blending adversely affected the intermediate- and low-temperature properties of the mixtures and improved the rutting performance of the mixtures.

Fatigue cracking is one of the three major types of distress of asphalt pavement (rutting, fatigue cracking, and low-temperature cracking) and has attracted a great deal of attention from asphalt researchers (Shu et al. 2008; Huang et al. 2011; Wu et al. 2014). Cong et al. studied the fatigue cracking of asphalt mixtures from a new perspective: surface energy. Surface energy, or surface free energy, is a term derived from surface physical chemistry and it is defined as the energy required to create one unit of surface area (Adamson and Gast 1997). Therefore, surface energy is directly related to cohesion of asphalt binder and adhesion between asphalt binder and aggregate and thus can be used to investigate the failure and healing of asphalt-aggregate system (Cheng 2002; Cheng et al. 2002a, b; Little et al. 2005). Cong et al. used the Wilhelmy plate method to measure the surface energy of asphalt at different temperatures and a contact angle meter to measure the surface energy of aggregate at room temperature, which were then incorporated into a micromechanics cracking model to predict the fatigue life of asphalt mixtures. Their results indicated that the surface energy of asphalt decreases with the increasing temperature. They found that the fatigue life of asphalt mixtures is proportional to the surface energy of adhesion between asphalt-aggregate and a greater adhesion leads to a long fatigue life.

Quality control (QC) and quality assurance (QA) are critical to ensure that asphalt mixtures are produced, placed, and compacted as designed so that asphalt pavements would perform and last as anticipated (Cominsky et al. 1998; Dobrowolski and Bressette 1998; Hughes 2005). Liu, Zhao, Li, and Saboundjian evaluated the variability in composition, and volumetric and mechanical properties between plant-produced and lab-designed Alaska asphalt mixtures. They found that composition of Alaska mixtures has the lowest variability, followed by volumetric and mechanical properties, but the variability in each tested property is still within the nationwide range. They recommended that composition and volumetric properties still be used for current QA purposes and meanwhile methods be sought to include mechanical properties in QC–QA because they correlate better to field performance of asphalt pavements.

In this special issue, one of the papers deals with a fundamental problem in durability of portland cement concrete—mechanisms of the salt–frost scaling of concrete. Concrete pavements in cold climates suffer freeze-thaw damage due to extreme temperature variations. Sufficient freeze-thaw resistance is a basic requirement of a durable concrete pavement. Powers put forth the hydraulic and osmotic pressure theories, which state that damage of concrete from freezing is caused by a buildup of hydraulic and osmotic pressure (Powers 1945; Powers and Helmuth 1953). However, Powers’ theories cannot explain the phenomenon of pessimum salt concentration, at which concrete suffers the maximum damage (Verbeck and Klieger 1957; Sellevold and Fastad 1991; Lindmark 1998). Yuan et al. investigated the mechanisms of salt–frost scaling of concrete. Their results indicated that with the increase in salt concentration, ice-formation pressure and volume expansion of solutions are significantly reduced, which are beneficial in reducing salt–frost scaling. However, the capillary-uptake degree of saturation and water-uptake rate in concrete increase with the increasing salt concentration, making salt–frost scaling worse. By calculation, they proved that 2–6% NaCl solution causes the maximum salt scaling damage to concrete.

Qian et al. attempted to treat laterite soil with cement for use in base layer of pavements. Laterite is a special type of soil rich in iron and aluminum oxides with a reddish color. Laterite is widespread in tropical and subtropical African countries and has a potential to be used as a pavement base material if properly treated (Autret 1983; Gidigasu 1976). Qian et al. found that the laterite from Mali is gap graded, lacking sand and silt size particles. After cement treatment, the laterite can meet the strength requirement of a pavement base material in Africa. However, its low content of fine particles makes it unable to effectively react with cement, leading to poor water stability.

Uniform and sufficient ballast support is critical for railroad track stability and safe operation of trains. Unfavorable ballast conditions can cause many types of track distress (Collingwood 1988; Selig et al. 1988, 1992). Studies have shown that ballast performance is dependent on individual ballast particle movement (Raymond 2002; Saussine et al. 2006; Tutumluer et al. 2006, 2007). To investigate the movement of a single ballast particle embedded in the track bed, Liu, Huang, Qiu, and Gao developed an innovative wireless device: SmartRock. SmartRock resembles real ballast particles not only in shape, but also in contact stiffness, specific gravity, and moment of inertia. Inside the SmartRock is a 9-degree-of-freedom motion/vibration sensor consisting of a triaxial gyroscope, a triaxial accelerometer, and a triaxial magnetometer, which records rotation, translation, and orientation, respectively. They used the SmartRock technique to monitor the movement of individual particles in a laboratory ballast box under cyclic loading. They then compared the results recorded by SmartRock with those from a discrete element modeling (DEM) simulation and found a good agreement between the two in terms of peak vertical, horizontal, and angular accelerations during ballast deformation. SmartRock is shown to be a promising tool for railroad ballast research and can also be used in monitoring the movement of individual aggregate particles in unbound pavement base layer.

We would like to thank Dr. Antonio Nanni, the editor-in-chief of the Journal of Materials in Civil Engineering, and the ASCE publishing office personnel for their help and support during the production of this special issue. We would also thank the reviewers of this special issue for their valuable comments and suggestions.