Special Issue on Materials Innovations for Sustainable Infrastructure

Recycling of solid waste materials into paving materials reduces not only environmental issues associated with waste disposal but also the demand for virgin asphalt cement or Portland cement binder, and coarse and fine aggregates, leading to potential cost savings. This is especially important when faced with the ever-growing world population and increasingly dwindling natural resources. Recycled asphalt pavement (RAP), recycled asphalt shingles (RAS), crumb rubber, and steel slag have been widely accepted by state highway agencies and recycled into asphalt pavements (Schroeder 1994; Shelburne and DeGroot 1998; Huang et al. 2002, 2005a, 2007, 2010a, b, 2011; Xiao et al. 2007, 2009; Shu et al. 2008, 2012; Zhao et al. 2012, 2013). In addition to the four types of supplementary cementitious materials (SCMs)—fly ash, slag, silica fume, and natural pozzolans—many other materials such as recycled concrete aggregate (RCA), cement kiln dust (CKD), foundry sand, waste glass, bottom ash, crumb rubber, plastics, ceramics, and even RAP have been recycled into Portland cement concrete (PCC) materials (Ansari et al. 2000; Naik 2002; Huang et al. 2004, 2005b, 2006b, 2009b, 2013; Li et al. 2004a, b). Like SCMs contributing to the properties of hardened PCC through hydraulic and/or pozzolanic reactions, RAP and RAS can also improve the binding properties of asphalt paving materials through the aged asphalt binder in them, which makes them more technically and economically beneficial.

This special issue presents six technical papers regarding the use of recycled materials such as RAP, RCA, waste phosphorous slag, sludge, low-quality fly ash, and waste ceramic aggregate. Lin et al. investigated the effects of rejuvenator sealer material on aged asphalt binder through laboratory testing. Their test results show that rejuvenator can serve as a softening agent, reducing viscosity and complex modulus of aged asphalt binder. Chemical analysis indicates that addition of rejuvenator can slightly change the fractions of aged asphalt binder and thus balance its asphaltene-maltene ratio. Because rejuvenator is not very common in the recycling of RAP in asphalt mixtures, this study may shed new light on combining rejuvenator with RAP and serve as a booster to incorporate more RAP in asphalt paving materials.

Self-healing is an intrinsic property of asphalt binder and may potentially be beneficial in improving the properties of recycled asphalt mixtures. Qiu et al. developed three new test methods to simulate the self-healing process of asphalt binder, mastic, and mixture in the laboratory and investigate the self-heading mechanism. In their study, the self-healing process was to be hypothesized to the reverse process of cracking, consisting of two phases—crack closure and strength gain. They also found that the self-healing capability of asphalt materials is significantly affected by two factors—temperature and healing time.

Qian et al. and Feng et al. both used waste industrial by-product in asphalt mixtures. Qian et al. incorporated phosphorus slag as mineral filler in hot-mix asphalt (HMA) and used multiple characterization techniques to assess the feasibility of recycling this industrial waste into HMA mixtures. Their test results indicate that phosphorus slag filler significantly increases the resistance of HMAs to rutting and moisture damage. Feng et al. attempted to recycle ceramic waste aggregate (CWA) into wearing layer of asphalt pavement. Their concern is on the thermal effect and the mixture performance attributed to adding CWA. Their finite element method (FEM) simulation results show that CWA can reduce the thermal conductivity of asphalt mixtures and subsequently reduce the temperature gradient of pavement. Basing on the performance test results, they recommended that less than 40% CWA be added into asphalt mixtures to replace natural coarse aggregates.

Qiao and Chen introduced an innovative chemical admixture called mesoporous inorganic polymer (MIP) to improve the mechanical properties and early-age shrinkage resistance of recycled aggregate concrete (RAC). The properties of RAC are usually compromised because of the addition of RCA, such as increased shrinkage and creep, reduced strength and modulus, and poor durability. To overcome these inferior properties, a promising chemical admixture, an atomic polymer technology (APT) in the form of MIP, was added and successfully enhanced the mechanical and early-age shrinkage performance of RAC. Combined with this technology, more RAC can potentially be made to promote sustainability in the PCC industry.

Yang et al. used river sludge, low-quality fly ash, and glass powder to produce high-strength ceramsite. They looked into different recipes and different fabrication conditions and successfully produced high-strength ceramsite that meets China standards. Their method provides an outlet for the sludge from river or lake dredging and low-quality fly ash.

In this category, four technical papers were included, involving electrically conductive materials, pervious concrete, and ${\mathrm{TiO}}_2$ materials.

Conventional asphalt or PCC mixtures generally behave as an electrical insulator. The addition of electrically conductive fillers or fibers will make them electrically conductive, leading to many practical applications, such as electrically thermal deicing for airport runways or highways, cathodic protection of concrete bridge decks, infrastructure health monitoring, and traffic monitoring (Chung 2002, 2004; Wu et al. 2005; Huang et al. 2006a, 2009a; Liu and Wu 2011). In this special issue, Wu et al. presented a study on the characteristics of electrically conductive asphalt concrete (ECAC). They investigated the effects of different factors such as filler type, filler content, and mixed fillers on the electrical conductivity of asphalt concrete. They found that a combination of mixed fillers has an appreciable advantage over just a single type of powder. Through X-ray computed tomography, they found that the conductivity of asphalt concrete is largely dependent on the conductive network formed by the filler particles dispersed in the asphalt mortar.

Pervious or porous concrete pavements have been increasingly used in the United States and other countries during the past three decades because of its various environmental benefits, such as controlling storm water runoff, restoring groundwater supplies, and reducing water and soil pollution, urban heat island effect, and traffic noise (Huang et al. 2010b; Wu et al. 2011; Shu et al. 2011).

This special issue presents two technical papers on either porous asphalt concrete or pervious PCC. Liu et al. added steel wool to impart electrical conductivity to asphalt mixture and evaluated the healing capacity of porous asphalt concrete beam through induction heating. They found that after induction heating, the beams can regain up to approximately 80% of their bending strength. They also showed that 85°C is the optimal temperature to heal and that the healing capacity of the beams is not compromised by multiple induction reheating. Dong et al. compared three laboratory test methods for characterizing the abrasion and raveling resistance of pervious PCC with eight pervious mixtures. Their results showed that the loaded wheel abrasion test with studded wheels and increased wheel load exhibited the best sensitivity and sufficient repeatability for abrasion testing.

Self-cleaning and air-purifying concrete or asphalt pavements are a relatively new but rapidly emerging technology. These pavements can be built by applying a thin coating of an air-cleaning agent, titanium dioxide (${\mathrm{TiO}}_2$) (Dylla et al. 2010; Hassan et al. 2013; Shen et al. 2012). ${\mathrm{TiO}}_2$, when activated by ultraviolet (UV) light (sunlight), can decompose air pollutants, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs), thus serving as an air-cleaning agent (Fujishima et al. 2000). In this special issue, Hanson and Tikalsky investigated the variables affecting NOx removal efficiency. They found that specific anatase ${\mathrm{TiO}}_2$ phases manufactured and sized to have high levels of photocatalytic activity could reduce NOx air pollutants at high efficiencies. Their results also show that UV irradiance is a dominating variable in determining NOx removal efficiency. A sunny day will result in a much higher efficiency than a cloudy one.

Transportation professionals always strive to extend the service life of pavements (Newcomb et al. 2001; Hall et al. 2007). Longer service life means less repair, rehabilitation, and reconstruction, leading to significant savings in pavement materials and money (Hall et al. 2007).

In this special issue, two technical papers were included in this category. Zheng proposed a new design guide for semirigid pavements of China that is based on the asphalt mixture critical states. He developed a method to convert the allowable surface deflection at the end of pavement service life to the design deflection at the initial stage of pavements, which was chosen as a design parameter for the China design guide for semirigid pavements. Li et al. attempted to address the more-severe rutting issue at intersections caused by slow-moving or standing vehicles. Through field testing, observations, and FEM simulation, they concluded that the voids in the mineral aggregate (VMA) at the intersections were lower than the required minimum value, making the mixtures unstable and easy to deform. They recommend that adequate VMA and a minimum air-voids content be maintained and limits be put on traffic load and standing time of vehicles.

The production of asphalt or PCC paving materials is energy intensive. With the concerns about global warming and energy consumption, both the asphalt and the PCC paving industries have been constantly looking for ways to reduce energy consumption and lower carbon footprint. In the asphalt industry, a good example is the widely adopted warm-mix asphalt (WMA) technologies (D’Angelo et al. 2008; Xiao and Amirkhanian 2010; Xiao et al. 2011; Liu et al. 2011; Liu and Li 2012; Shu et al. 2012; Zhao et al. 2012, 2013). On the PCC side, various alternative materials or binders have been proposed to replace part or all of the cement, which consumes significant amount of energy during its production (Schneider et al. 2011).

In this special issue, three technical papers are concerned with WMA technologies, and one paper deals with replacing partial cement content in engineered cementitious composites (ECC) with iron ore tailings (IOTs). Xiao et al. looked into the effects of various variables on the moisture susceptibility of WMA mixtures. The variables included five different WMA additives (Asphamin, Cecabase, Evotherm, Rediset, and Sasobit), two aggregate sources (granite and marble schist), one liquid antistripping agent (ASA), two moisture contents (0 and $\sim 0.5\%$), and two lime contents (1 and 2%). They found that long-term aging improves the moisture resistance of WMA mixtures, whereas aggregate type plays a key role in determining the moisture susceptibility of WMA. The effects of other variables on moisture resistance of WMA vary from case to case. Similarly, Yu et al. investigated the impacts of types and content of WMA additives on various properties of crumb rubber modified asphalt (CRMA) mixtures. Their test results indicate that types and content of WMA additives have significant impacts on CRMA performance. They recommend that optimal type and content of WMA additives be determined on the basis of specific conditions. Huang et al. utilized an innovative testing method—small-angle neutron scattering (SANS)—to detect water that may be trapped in foamed asphalt. Although they did not find any water entity of less than 0.1 μm present in foamed asphalt, they did not rule out the possibility that there might be some bigger water droplets. This new testing technique may also shed new light on the microstructure of asphalt materials.

Huang et al. reported on a greener ECC with IOTs as cement replacement. The powder IOTs were added into ECC to replace 40–80% of cement. With a cement content of only $117.2–350.2\mathrm{kg}/{\mathrm{m}}^3$, the developed ECC still showed good tensile ductility and good compressive and tensile strengths. The particle replacement of cement with IOTs results in 10–32% reduction in energy consumption and 29–63% reduction in carbon dioxide release.

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. Special thanks are extended to all the reviewers of this special issue for their valuable comments and suggestions, which significantly helped improve the quality of the technical papers.