Innovative Solutions for Concrete Applications

Abstract

Concrete, having evolved over the last 2000 years, is integral to modern infrastructure, with continuous innovations aiming to address sustainability challenges. From Roman concrete mixes to the invention of Portland cement (PC), concrete has evolved to meet growing infrastructure demands. As urbanization and energy consumption increase, the construction industry is focusing on high-performance materials, recycling, and minimizing harmful substances. Research on sustainable concrete alternatives shows promising reductions in global warming potential and other environmental impacts compared to traditional PC. However, challenges such as higher material costs and performance limitations remain. Alternatives such as alkali-activated concrete (AAC), self-healing concrete, and bacterial concrete (BC) have emerged in response to environmental concerns, along with fiber-reinforced AAC, waste-based concrete composites, and the reuse of construction and demolition waste (CDW), further enhancing sustainability. Foamed concrete, with its lightweight and insulating properties, offers additional potential for reducing environmental impact due to its ability to incorporate recycled materials and reduce raw material consumption. Technologies like three-dimensional concrete printing (3DCP) are improving resource efficiency and reducing carbon footprints while also lowering labor and material waste. However, concerns regarding cost-effectiveness and social sustainability persist. Overall, continued innovation is the key to balancing performance, cost, and sustainability in the development of concrete and to meet the growing demands of global infrastructure.

Concrete, having evolved over the last 2000 years, is the second most used material on the Earth. The Romans first combined lime, pozzolan, and aggregate to create a durable material, and in 1756, John Smeaton used a similar mixture to rebuild the Eddystone Lighthouse. In 1824, Joseph Aspdin patented Portland cement (PC), which spread worldwide, with annual production now around 4.2 billion tons. Over time, alternatives like alkali-activated concrete (AAC) have emerged, particularly in response to cement shortages during the 20th century. As the demand for more resilient and sustainable infrastructure grows, prioritizing investments in both new projects and the maintenance of existing structures is crucial. Research programs have focused on key performance indicators (KPIs) such as RAMSSHE€P—Reliability, Availability, Maintainability, Safety, Security, Health, Environment, Economics, and Politics—to guide these efforts [1]. In this context, innovative concrete solutions are being explored to ensure that infrastructure meets these objectives of durability and sustainability. As the construction industry faces growing demand for infrastructure, research on sustainable alternatives to traditional PC has intensified, leading to materials like “green concrete”. Research highlights the potential for a more sustainable built environment through innovative concrete formulations, recycling efforts, and advanced technologies. Several promising solutions were showcased at the IOCI2022 conference [1], with some of them being additionally highlighted in this editorial.

Twenty-five years ago, innovations began revolutionizing concrete technology with breakthroughs like self-healing concrete, which was first pioneered by researchers in the Netherlands through bacterial concrete (BC)—a type of concrete that uses biological processes to repair cracks. Meanwhile, researchers around the globe have developed various other self-healing methods, including those using chemical reactions and encapsulated healing agents. For instance, Snoeck [2] examined the self-healing capabilities of cement-based samples after ten years of maturation under different conditions. The results showed that cracks can still heal, primarily through the formation of calcium carbonate crystals, contributing to a partial recovery in mechanical properties. Samples with superabsorbent polymers (SAPs) demonstrated better healing compared to reference samples, even after a decade, highlighting their potential as a sustainable solution for cement composites. In addition to self-healing concrete, other innovations such as AAC present opportunities to reduce environmental impact. The construction industry is also exploring the use of waste materials like crumb rubber (CR) and developing non-traditional supplementary cementitious materials (SCMs) to minimize the carbon footprint and optimize resource use in construction projects. Furthermore, three-dimensional concrete printing (3DCP) has also reshaped concrete construction by enabling designs that traditional methods cannot achieve. Urbanization and demographic growth have significantly increased global energy consumption, with the construction sector being a major contributor due to its energy-intensive processes and environmental impact. The extraction and processing of raw materials for construction lead to high energy use and carbon emissions. In response, architectural design and civil engineering are focusing on using high-performance materials while minimizing toxic or harmful ones. There is also a strong emphasis on reducing energy consumption throughout a building’s life cycle. One promising solution is reusing construction and demolition waste (CDW), which, for example, accounts for over 30% of waste in the EU.

The study by Ramagiri et al. [3] evaluated the environmental impact and cost of three sustainable concrete mixes: AAC with natural aggregates, AAC with recycled coarse aggregates (RCA), and BC—compared to Portland cement concrete (PCC). Using a life cycle assessment (LCA), this study revealed that AAC mixes were most affected by transportation and sodium silicate, while BC mixes had the highest impacts compared to PC, nutrient broth and coarse aggregates. PCC had a 1.4–2 times higher global warming potential (GWP) than the other mixes. BC, without nutrients, had the lowest environmental impact overall, except for GWP. AAC mixes were 98.8–159.1% more expensive, while BC mixes were 21.8–54.3% costlier than PCC. AAC, with a 0.7 activator modulus, showed the lowest environmental impact, while AAC, with RCA, had higher impacts. This LCA and life cycle cost analysis (LCCA) provided insights into sustainable concrete alternatives in the industry.

Another study by Ramagiri et al. [4] explored AAC made from fly ash and slag, focusing on its high-temperature performance, microstructural changes, and environmental impact. The research examined how different fly ash–slag ratios, activator moduli (Ms), and temperature exposures affect AAC’s microstructure and mechanical properties. High temperatures caused the formation of crystalline phases like gehlenite, akermanite, and nepheline, which were linked to improved compressive and bond strengths. Ground granulated blast-furnace slag (GGBFS)-based AAC resulted in a 151.8–339.7% increase in 28-day compressive strength. The optimal mix was found to be a fly ash–slag ratio of 70:30 and Ms of 1.4. However, exposure to temperatures above 760 °C led to a significant drop in compressive strength for mixes with higher slag content due to pore pressure build-up. LCA revealed that transportation and sodium silicate production had the highest environmental impact, accounting for 45.5–48.2% and 26.7–35.6% of the total emissions, respectively. Compared to PC, AAC demonstrated lower global warming potential, though its manufacturing process still contributed to higher impacts in ecotoxicity, eutrophication, and ozone depletion. This study also found that the cost of AAC was competitive, ranging from INR 16,532 to INR 17,265 (EUR 200–210) per m³, and that implementing a carbon tax would raise costs by 18.4% for AAC compared to an 81.7% increase for PC, underscoring AAC’s potential as a sustainable alternative.

The study by Ganapathi Chottemada et al. [5] evaluated the environmental impacts of AAC with different precursors using a cradle-to-grave approach. It compared the impacts of AAC mixes with 28-day compressive strengths ranging from 35 to 55 MPa, selecting the most sustainable option for further assessment with fiber reinforcement. The results showed that PCC has an 86% and 34% higher environmental impact on ecosystem quality and human health, respectively, compared to AAC. Sodium silicate in AAC production accounted for 30–50% of its total environmental impact. Among fiber-reinforced AAC (FRAAC) mixes, glass fibers increased the environmental impact more than steel or polypropylene fibers. The FS50 AAC mix, containing 50% fly ash and 50% GGBFS, had the lowest environmental impact. LCCA showed that AAC is 132% more expensive than PCC, primarily due to sodium silicate. However, polypropylene fibers result in the lowest production cost among FRAAC mixes. This comprehensive LCA highlighted the environmental and economic implications of using FRAAC, offering valuable insights for policymakers and the construction industry to adopt more sustainable alternatives, particularly in regions with similar economic and climatic conditions to the Indian subcontinent.

The study by Tahwia et al. [6] aimed to develop a more sustainable and eco-friendly engineered geopolymer composite (EGC) by using common, low-cost pozzolanic waste materials—rice husk ash (RHA), granite waste powder (GWP), and volcanic pumice powder (VPP)—as partial replacements (10–50%) of GGBFS. These materials were chosen due to their high content of aluminum and silicon, which are key for geopolymer formation. The research focused on evaluating how these waste materials affected the workability, mechanical properties, durability, and microstructure of EGC. The results showed that using RHA and GWP as 50% replacements for GGBFS reduced workability by up to 23% and 31%, respectively, whereas VPP increased workability by up to 38.5%. Among the different mixes, the optimal performance was found with 30% RHA, 20% GWP, and 10% VPP, which achieved the highest compressive, tensile, and flexural strengths and the best residual compressive strength when exposed to elevated temperatures. In contrast, water absorption and porosity increased significantly with higher amounts of RHA and GWP, while VPP showed the opposite effect, decreasing both water absorption and porosity. When exposed to high temperatures, the optimal mixes (RHA-30, GWP-20, VPP-10) retained strength better than other mixes at 200 °C. However, at 400 °C and 600 °C, all mixes experienced substantial strength loss. This suggests that while these alternative materials improve some properties, they may not perform as well in high-temperature applications compared to conventional EGC. Scanning electron microscopy analysis indicated that the VPP mix had the densest matrix, while RHA and GWP mixes showed more porosity. The study suggested further research on the durability of EGC and further investigation of long-term strength development for RHA-based EGC.

The review by Kara De Maeijer et al. [7] highlighted 30 years of research on CR in concrete, identifying up-to-date key barriers in the construction industry, such as high recycling costs, reduced mechanical properties, limited research on environmental risks, and recyclability concerns. Improving the effectiveness of CR particles through surface treatments and optimized concrete mix designs could provide significant benefits as a replacement for natural aggregates. However, the application of CR in concrete is often region-dependent and may be limited to environmental concerns. A few promising key points were highlighted. Workability can be improved with the addition of admixtures like superplasticizers. CR also lowers concrete density, making it ideal for lightweight applications. Pre-treating CR enhances the bond at the interfacial transition zone (ITZ), mitigating strength loss. In general, the cementitious materials surrounding CR effectively confine the trace metals or volatile organics that exist in rubber particles. The optimal CR replacement is 10–15% for fine aggregates and 5% for coarse aggregates. Crumb rubber concrete (CRC) shows improved resistance to freeze–thaw cycles, chloride penetration, acid resistance, and abrasion, though it is more susceptible to sulfate attacks. Additionally, CR improves vibration and moisture absorption, making it useful for dynamic structures like railway sleepers, bridges, and seismic-prone structures. However, further cost-effective studies are needed to support its broader use in the construction industry.

A major challenge in the construction industry is the absence of scientifically supported, versatile building materials that perform effectively in aggressive environments, such as those exposed to chloride attack. The study by Shcherban’ et al. [8] compared the durability of conventional and variotropic concrete mixes modified with microsilica under cyclic chloride exposure using three different production methods: vibrating, centrifuging, and vibro-centrifuging. The results showed that vibro-centrifuged concrete exhibited the highest resistance to chloride attack, with a significantly lower decrease in compressive strength compared to vibrated (87%) and centrifuged concrete (24%). Adding 2–6% microsilica improved the concrete’s resistance, with the best results seen at 4%, reducing strength loss by 45–55% after 90 wet–dry cycles. The combination of vibro-centrifuging and microsilica led to an 188% decrease in strength loss as a result of cyclic chloride attack, significantly improving concrete durability. The authors concluded that variotropic concrete, especially when combined with microsilica, offers superior resistance to chloride attack. These findings are valuable for designing more durable infrastructure, and it was indicated that further research will focus on full-scale tests of reinforced concrete structures exposed to chloride attack, with potential adjustments based on the technological capabilities of production plants.

The study by Jahami et al. [9] investigated the effects of replacing cement with steel dust in reinforced concrete beams, focusing on workability, mechanical properties, and durability. Steel dust was added at 0%, 10%, 20%, and 30% replacement levels, with a constant water–cement ratio of 0.55. The results showed that increasing steel dust content reduced workability and density and significantly affected the elasticity modulus. At 10% replacement, compressive, tensile splitting, and flexural strengths improved but declined with higher steel dust levels. The best results in terms of ductility and maximum load were seen at the 10% replacement level, where ductility increased by 13% and load capacity improved by 5%. However, at 30% replacement, ductility and load capacity dropped significantly. The SD₁₀ beam (with 10% of steel dust) exhibited fewer microcracks and better deformation resistance, sustaining a 20 kN load without yielding, while other beams with higher steel dust cement replacement levels resulted in premature failure. This suggests that steel dust can enhance concrete properties at moderate levels, but caution is needed for higher replacements, as they can weaken the material. However, further research is necessary to assess the long-term durability and corrosion potential of steel dust-based concrete.

A vast number of concrete structures are approaching the end of their expected service life, creating a growing need for repairs. Reinforcement corrosion is a major cause of damage, leading to cracking and concrete spalling, which require effective repair techniques. As Europe shifts to a circular economy, considering both environmental impact and life cycle costs in repair decisions is crucial. The review by Renne et al. [10] highlighted the gaps in the existing literature, comparing repair methods in terms of the differences in structures, damage causes, repair techniques, estimated and expected life span assumptions, etc. Although refurbishment is often seen as more environmentally beneficial than new construction, economic outcomes vary. Preventive maintenance is typically more cost-effective over the long-term, while curative repairs may be better for short-term life extension. In terms of specific repair methods, low-labor options like patch repair are favored for short-term fixes, though they may not always be the most economical for longer-term extensions. To determine the most sustainable concrete repair, LCA and LCCA should be performed, considering life cycle perspectives for optimal environmental and economic outcomes. While sustainability frameworks exist, no research compares repairs using LCA and LCCA with all five EN1504-9 [11] repair principles related to reinforcement corrosion. Further research is needed on the leaching behavior of concrete with sacrificial galvanic anodes. Service life prediction should be more integrated into LCA and LCCA for accurate service life assessments.

The interface between old and new concrete is critical in construction, but it often remains a weak point despite various bonding treatments. Traditional design methods overlook the complexity of this interface, leading to uncertainties in structural performance. The study by Zhang and Lu [12] introduced a novel framework using X-ray computed tomography and finite element-based numerical homogenization to quantify the anisotropic properties of the old–new concrete interface. The analysis showed that the interface has significantly lower stiffness and greater anisotropy compared to non-interface regions due to microcracks and voids. This study identified the “weakest vectors” for normal and shear stresses, revealing an orthogonal relationship between them with slight local deviations. Cosine similarity analysis demonstrated more consistent directional features at the interface, further highlighting its heterogeneous nature. These findings challenge traditional design assumptions and provide crucial insights for improving concrete structure rehabilitation and design.

The growing issue of construction waste, which is costly to dispose of, can be mitigated by reusing it in concrete production. This approach not only reduces waste disposal but also offers a sustainable solution for the depletion of natural concrete resources. Pervious concrete, which can contain up to 80% coarse aggregates, is a promising medium for recycling construction waste. The study by Sangthongtong et al. [13] explored the mechanical properties of pervious concrete made with both natural and recycled aggregates, with recycled aggregates enhanced by natural fibers from sackcloth. The research involved 45 samples, focusing on air void ratios and aggregate sizes. The results showed that increasing the air void ratio led to a 40–60% decrease in compressive strength, regardless of aggregate type or size. The permeability of pervious concrete remained unaffected by the type of aggregate, while the temperature increased as the air void ratio rose. Specifically, for small-size aggregates with 10% designed air voids, the permeability was 0.705 cm/s for both natural and recycled aggregates with sackcloth.

The review by Suarez-Riera et al. [14] explored various strategies for producing more sustainable cement and concrete, with an emphasis on leveraging CDW as a valuable resource. Sustainable improvements in cement-based materials can be pursued in two main ways: first, by using eco-friendly cements that have a lower environmental footprint than traditional materials, and second, by fully exploiting CDW as aggregates while also improving the properties of recycled aggregates to enhance their performance. These efforts are central to the work of architects and engineers striving to create high-performance, eco-friendly construction materials. The EU’s ongoing support of CDW recovery strategies further emphasizes the importance of managing construction waste in a way that benefits both the environment and the economy. A key advancement in sustainable cement is green cement, which lowers energy use and carbon emissions compared to traditional PC. The use of crystallizing agents in concrete also boosts durability and self-healing properties and reduces maintenance. These innovations improve both the environmental impact and lifespan of cement materials. By incorporating CDW, green cement, and crystallizing agents into construction, the industry can make strides toward sustainability. Ongoing research and collaboration are needed to refine these strategies and ensure broad adoption for a more sustainable built environment.

Foamed concrete, also known as cellular concrete or aerated concrete, is an innovative solution that has gained significant attention in the construction industry due to its unique properties and wide range of applications. The study by Markin et al. [15] examined how different foamed concrete (FC) production methods—cavitation disintegration (CD) and turbulent mixing (TM)—affect pore size distribution, compressive strength, and water absorption. Six FC mixes with densities ranging from 820 to 1480 kg/m³ and compressive strengths up to 47 MPa were prepared and tested at 7, 28, and 180 days. Digital image correlation was used to analyze pore structure, revealing that the production method significantly influences pore formation, which in turn affects strength and water absorption. CD promoted a finer, more uniform pore structure, improving compressive strength. TM was effective in producing low-density FC with a lower amount of foaming agents. This study found that porosity directly impacted water absorption, though pore shape and distribution also played a role. At 28 days, compressive strength ranged from 9.4 to 47.4 MPa and continued to improve with curing time, reaching 53 MPa at 180 days. The findings highlighted the importance of choosing production methods that optimize pore structure for stronger, more durable FC. It was indicated that future research should refine these methods, expand the dataset, and use advanced imaging techniques, such as micro-computed tomography (micro-CT), to better understand how pore structure affects material performance.

With significant growth since 2014, 3DCP has gained attention for its potential to reduce the construction industry’s carbon footprint, though its impact on social sustainability and project success is often overlooked. The study by Shivendra et al. [16] examined how strategic decisions influence the balance between economic, environmental, and social sustainability in 3DCP adoption. Interviews with 20 Indian industry leaders revealed that companies invest in 3DCP primarily for automation and workforce development rather than for environmental benefits alone. A key barrier to wider adoption is the lack of government incentives for sustainable practices. This study identified five strategies companies use to promote sustainability through 3DCP and suggested government measures to accelerate its adoption. Additionally, 3DCP’s environmental impact, especially regarding raw materials, requires more assessment to explore alternative combinations that could further reduce its footprint. Design factors like modularity and structure thickness also need to be studied for their influence on circularity and overall performance. The social sustainability of 3DCP is complex. While it reduces reliance on seasonal labor, it could also reduce opportunities for low-skilled workers. The potential job displacement due to automation needs further exploration to understand its effects on the workforce. Finally, 3DCP’s cost-effectiveness remains unclear. Technoeconomic models that consider cost, quality, labor, and maintenance factors are needed, and studying cost trends in similar industries could help predict future expenses as 3DCP technology evolves.

Concrete is vital for infrastructure like bridges, tunnels, and power plants, which consume large amounts of the material. As infrastructure demand increases and sustainability concerns rise, alternative SCMs are needed. The industry is also turning to automated methods like 3DCP to address labor shortages. The study by Hanžič et al. [17] explored using oil shale ash (OSA) as an SCM in 3DCP and investigated collision milling as a pre-treatment. The research found that OSA from flue gases, particularly OSA-Ees(nid), showed the most promise due to its smooth, globular particles and high active β-calcium silicate content. Concrete with this ash achieved a 56-day compressive strength of 60 MPa, similar to conventional concrete. Collision milling, while effective for reducing particle size in bottom ash, did not significantly improve the performance of flue gas-derived ashes. Milling was only beneficial for bottom ash, which was too coarse for direct use as an SCM. Although milling increased the reactivity of the ash, it did not lead to significant improvements in concrete strength, except for bottom ash. Printability tests showed no major differences between untreated and milled OSA-Ees(nid) in terms of yield stress and buildability. This study concluded that OSA-Ees(nid) is the most suitable ash for 3D-printable concrete, and further research should focus on optimizing its use. The results highlighted the potential for non-traditional SCMs and digital fabrication methods in addressing sustainability, efficiency, and labor challenges in large-scale infrastructure projects.

This editorial highlighted key themes such as sustainability, performance, durability, cost, environmental impact, recycling, and construction waste management. It also explored emerging techniques and innovations, including AAC, BC, FRAAC, CRC, steel dust-based concrete, foamed concrete, and the use of CDW. In conclusion, sustainable concrete alternatives, innovative materials, and improved recycling and mixing techniques offer significant opportunities to reduce the environmental impact of concrete. However, challenges related to cost, long-term durability, and the application of LCA and LCCA still need to be addressed through further research. Furthermore, technologies such as 3DCP present advanced opportunities for innovation in the construction industry.