Emerging Materials and Technologies for Next-Generation Sustainable and Resilient Polymer Concrete

The importance of infrastructure resilience cannot be overstated, as it ensures that critical services and systems can function during and after disruptive events such as natural disasters or cyberattacks. Resilient infrastructure can minimize downtime, protect public safety, reduce economic losses, and enhance community well-being [1‐4]. Infrastructure resilience is critical in ensuring essential services and systems continuity during and after disruptive events such as earthquakes or tornadoes [5, 6]. The significance of resilient infrastructure lies in its ability to minimize downtime, protect public safety, reduce economic losses, and enhance community well-being [5]. These benefits underline the importance of infrastructure resilience in maintaining community safety, security, and stability [1‐3]. Emerging materials and technologies (EM&Ts) can be defined as the materials and technologies whose development and/or field application is still in a state of growth. Rotolo et al. [7] characterized emerging technologies on the basis of five attributes—(i) radical novelty, (ii) relatively fast growth, (iii) coherence, (iv) prominent impact, and (v) uncertainty and ambiguity. EM&Ts have been identified as a potential solution to enhance the resilience of civil infrastructure, providing improved absorptive, adaptive, and restorative capabilities [6]. EM&Ts are increasingly becoming an integral part of the engineering community's work and will significantly impact the future of civil infrastructure.

Using EM&Ts in civil infrastructure is expected to improve infrastructure resilience capacities, enabling infrastructure to withstand or efficiently recover from disruptive events. Three disruptive technologies affecting infrastructure resilience have been identified in the literature [6]: innovative materials, advanced construction technology, and advanced sensing technology. These EM&Ts will enhance the four characteristic elements of infrastructure resilience: redundancy, robustness, rapidity, and resourcefulness. For instance, the significance of using advanced construction technology, such as 3D printing, and advanced materials, such as self-sensing polymer concrete, can improve infrastructure resilience in the case of an extreme event, as depicted in Fig. 1. A framework has been proposed to provide common ground for stakeholders such as ET companies, infrastructural professionals, and infrastructure owners to evaluate, position, and communicate ET's performative roles in establishing or enhancing civil system resilience [8, 9]. This framework emphasizes five properties: resourcefulness, robustness, redundancy, responsiveness, and rapidity.

Polymer concrete (PC) has the potential to enhance infrastructure resilience due to its ability to withstand harsh conditions and maintain structural integrity over time [10]. PC is highly durable and resistant to environmental degradation factors such as extreme temperatures, chemicals, and moisture. Its good impact resistance makes it an ideal choice for infrastructure exposed to high traffic or heavy loads [11‐13]. Moreover, PC has a faster setting time than traditional concrete [10], which means that infrastructure projects can be completed more quickly and with less disruption to the surrounding community. The main applications of PC in infrastructure include bridge decks, industrial overlays, waste-water pipes and containers, manholes, underground communication and transmission line boxes, building façade panels, machine foundations, and other elements subjected to dynamic and cyclic loads [12]. The reduced maintenance needed for PC due to its long-term durability and resistance to performance drops due to extreme events (e.g., earthquakes) would limit damage and help infrastructure resilience. Polymers have also been used as a partial substitute of the cementitious material for concrete applications that require meeting high structural and durability demands [14]. Details regarding characterization of polymer-cement concretes incorporating polymers as a partial replacement of cement can be found elsewhere [15, 16].

However, it is essential to note that polymers, like other industrially processed materials, produce greenhouse gases (GHG), contributing to climate change [17, 18]. Thus, reducing the carbon footprint of PC is necessary to limit the adverse effects of GHG on the environment and public health [17‐19]. Many countries have introduced regulations to reduce greenhouse gas emissions, including those related to the production and use of polymers [20]. Promoting sustainable PC can be achieved by decreasing our reliance on non-renewable polymers and examining alternative polymers from natural resources.

In this context, this paper provides an overview of the latest advancements in EM&Ts that promote resilient and sustainable PC. Through exploring EM&Ts, we propose solutions that can help mitigate the environmental impact of PC while enhancing infrastructure resilience.

In the past decades, nanotechnology has revolutionized a vast majority of industries. The use of nanotechnology in PC has the potential to enhance the material's properties at many levels. Using nanotechnology in PC has been shown to improve its properties, including fracture toughness, self-sensing capabilities, impact resistance, and electrical and thermal conductivity, thus improving infrastructure resilience.

Carbon nanotubes (CNTs) have been widely used as nanofillers to improve the properties of polymer composites. Daghash et al. proved that the impact resistance of PC was improved by adding CNTs. The results showed that the addition of CNTs significantly enhanced the toughness and impact resistance of the PC [21]. This is supported by other studies in which the addition of CNTs significantly improved the fracture toughness of PC [22]. Moreover, Soliman et al. demonstrated the importance of CNTs in enhancing the mechanical properties of polymer-modified concrete incorporating styrene butadiene rubber [23]. Alumina nanoparticles have also been investigated for their potential to improve PC properties. In the study by Emiroglu et al., a new PC with superior ductility and fracture toughness was developed using alumina nanoparticles. The study showed that adding alumina nanoparticles led to an increase in the fracture toughness of the PC [24]. Furthermore, Douba et al. developed a very ductile PC using CNTs, as depicted in Fig. 2. This PC also exhibited improved compressive and tensile strength [25]. This was further supported by the study of Douba et al. [22], where PC's fracture toughness was improved by adding a combination of CNTs and alumina nanoparticles. Reda Taha et al. developed electrically and thermally conductive PCs using CNTs. The study demonstrated the potential use of CNTs to improve PC's electrical and thermal conductivity, making them suitable for self-sensing smart infrastructure [26]. In smart infrastructure with self-sensing capability, damage can be detected by observing the change in the electrical conductivity of the PC. This is demonstrated in Fig. 3, where the change in the electrical resistance of an electrically conductive PC can be used for damage detection and quantification [27].

It is evident that implementing nanotechnology in PC has enhanced its properties, including fracture toughness, impact resistance, and electrical and thermal conductivity. CNTs and alumina nanoparticles have been identified as promising nano modifications for PC, potentially improving its resilience in infrastructure applications. These findings suggest that further research is needed to fully explore the potential of nanotechnology in PC and develop new and innovative applications for this material. The use of nanotechnology has also been applied to other aspects of resilience infrastructure such as fiber-reinforced polymer composites for reinforcement bars as a way to improve the mechanical performance of concrete structures [28].

The need to reduce GHG emissions has prompted researchers to investigate innovative solutions to investigate materials with limited carbon footprint [19]. One potential solution is to use sustainable bio-based polymers in infrastructure applications [17, 18]. Bio-based polymers, derived from renewable resources, have attracted increasing interest due to their limited carbon footprint of construction materials. These polymers can be used in various applications, such as adhesives, coatings, and composites, and can also be used as binders in concrete production. For instance, researchers explored using lignin-based polymers for CO2 sequestration [29‐31]. Lignin, a paper industry byproduct, is a complex polymer that can substitute synthetic polymers in various applications [32]. Using sustainable bio-based polymers for CO2 sequestration can potentially reduce the total carbon footprint of the construction industry. Recently, researchers have investigated the use of bio-based polyurethane as a binder in the production of PC [18]. The new PC, known as bio-PC (BPC), is produced by replacing cement with bio-based polyurethane. The main challenges in using Bio-based polyurethane include its very fast setting and foaming due to the reaction of polyurethane with moisture, as shown in Fig. 4.

It was suggested that the concrete set and foaming could be controlled by nano-modifying the bio-based polyurethane using carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs) dispersed in benzoic acid before mixing it with the aggregate [18]. Figure 5a shows PC samples produced using bio-based polyurethane with 30 min setting time and no foaming. Figure 5b shows the compressive strength of sustainable BPC incorporating bio-based polyurethane with and without heat curing. A PC with 20–30 MPa compressive strength is attainable using bio-based polyurethane. Researchers also reported that this BPC has a low carbon footprint, with a 50% reduction compared with ordinary Portland cement concrete, as depicted in Fig. 6. The BPC also displayed excellent durability characteristics, making it a promising alternative for infrastructure projects [18].

Concrete 3D printing commonly relies on the deposition of layered filaments of the material. Cement-based materials have been used for this application due to their ability to stay as pseudo-solid before hardening when their flow properties are properly designed1. Numerous PC applications are directed toward precast elements, making PC amenable to 3D printing technology. Researchers [10] reported 3D printing of PC incorporating Novolac epoxy resin mixed with 44% Benzyl alcohol hardener fumed silica and medium-graded silica sand with a nominal maximum size of 2.36 mm. The maximum aggregate size was selected to ensure proper printing resolution for the 40 mm nozzle diameter. Class-F fly ash and silica fume were also used as fillers to increase the packing fraction and, consequently, the mechanical performance of a 3D printed PC. The fly ash to silica fume weight ratio was kept constant at 2:1 since it has been proven to yield a good compressive strength for PC [33]. Different proportions of rheology modifiers (fumed silica), fillers (silica fume and fly ash), and aggregate (sand) were studied to investigate their effect on the rheological properties of PC. The PC mixes were changed to keep the polymer (resin and hardener) volume fraction of all the mixes around 40%. Rheology measurements were performed using a Brookfield RST soft solid tester rotational rheometer. For the PC, a shear vane spindle with a diameter of 20 mm, a length of 40 mm, and a shear stress range of 5.2 to 3400 Pa was used. The test was performed using the hysteresis technique widely used for thixotropic particulate suspensions. The shear rate was ramped up from 0 s⁻¹ to 100 s⁻¹ in 60 s and then down from 100 s⁻¹ to 0 s⁻¹ in 60 s, as shown in Fig. 7. The rheological behavior of the polymer (mixed resin and hardener) with no fillers were studied following the same procedure but using a coaxial spindle (40 mm diameter). The rheological testing allowed quantifying PC thixotropy and thus reaching a printable PC mix.

An optimal PC mix was 3D printed using a gantry robot 3D printer with 3 degrees of freedom and a printable area of 2.0 x 2.0 x 2.0 m. Cube specimens of 45 x 45 x 45 mm were also printed by using a 40 mm diameter nozzle, 10 mm layer height, and 50 mm/s linear printing speed. The extrusion rate represents the rotational speed of the helicoidal extruder that pushes the material through the nozzle. The extrusion rate was kept constant at 19.2 L/min. A build-up rate (H) of 0.5 mm/s was used for the geometry printed and the printing settings reported herein. The 3D printing of PC is shown in Fig. 8. PC was air-cured for 7 days at controlled ambient conditions of 23 ± 2 ºC. The compressive strength of the 3D printed concrete cubes was tested in the vertical (Z) direction. The 3D printed PC showed a compressive strength of 30 MPa and a thixotropy growing from 10,000 to 60,000 Pa/s over 45 min. Both measurements proved the optimal PC mix able to produce good 3D printable material.

Textile Reinforced Concrete (TRC) is a composite material that was introduced to civil infrastructure as a new emerging technology to repair and strengthen deteriorated structures. TRC benefits included design flexibility, lightweight, and high durability[34‐37]. TRC typically consists of a matrix of cement binder and small-size aggregates reinforced with high-performance textile reinforcements, either in 2D or 3D [38, 39]. Several types of textile reinforcement materials have been used in TRC, such as glass, polypropylene, carbon, and basalt [40]. However, the cementitious matrix's improper impregnation of the fiber fabrics has led to premature debonding between the textile yarns and the cement matrix, potentially affecting TRC's load-carrying capacity and ductility [39, 41, 42]. Researchers have proposed using organic coats to mitigate this problem to enhance the bond behavior between the fibers and mortar.

A new type of TRC has been proposed to overcome the above performance limitations by replacing the cement binder with a polymer resin to produce textile-reinforced polymer concrete (TRPC) [42]. The new TRPC is created using a fine-graded aggregate, a polymer resin, and a fiber textile fabric. Researchers showed that a methyl methacrylate polymer and basalt fibers could be used to produce a good TRPC. Four different specimen configurations were manufactured by embedding 0, 1, 2, and 3 textile layers in concrete. Through 3-point bending tests, flexural performance was analyzed and compared with reference TRC specimens with similar compressive strength and reinforcement configurations. TRPC showed significantly improved flexural capacity, superior ductility, and substantial plasticity compared with TRC. TRPC represents an excellent new material for civil infrastructure applications. Figure 9 compares the performance of TRPC incorporating different layers of textile reinforcement with cementitious TRC. It is evident that TRPC overperforms TRC and can achieve a much higher structural capacity and ductility than conventional TRC. Figure 10a shows a light-microscopy picture at a sagittal cut (through the textile-matrix interface) for TRC (top) and TRPC (bottom) and demonstrates the significant ability of the polymer to impregnate the textile fibers compared with the cement binder and thus leading to the improved performance reported above. Finally, Fig. 10b shows the potential production of TRPC using 3D printing technology.

Infrastructure resilience ensures critical services and systems function during and after disruptive events such as natural disasters and cyberattacks. PC is a durable material that can withstand harsh conditions and maintain structural integrity over time. However, the production of polymers contributes to climate change, and reducing the carbon footprint of polymers is necessary to mitigate its impact on the environment and public health. We suggest that EMTs can help alter PC contributions and enable producing a sustainable and resilient PC.

Nanotechnology has been shown to enhance PC properties, including superior ductility and improved electrical conductivity. Such developments can lead to smart PC with self-sensing capabilities limiting damage propagation and improving PC ability to respond to loading in extreme events, thus improving infrastructure resilience. We also demonstrated that sustainable bio-based polymers derived from renewable oil could be used to produce a PC with appreciable strength and excellent durability. We showed the possible production of 3D printed polymer concrete, which can enable fast recovery in extreme events. We also showed that such polymer concrete could have superior load capacity and ductility by incorporating textile reinforcement during the 3D printing process. EMTs have the potential to facilitate a resilient and sustainable PC for future infrastructure applications.