Alkali-activated binder (AAB) has been extensively researched in recent years due to its potential to replace portland cement (PC) and lower carbon footprint. However, major barriers to its commercialization are related to the inadequate characterization of mechanical properties and long-term durability. The mechanical and durability performance of AAB is highly influenced by its microstructure. There is minimal research on correlating the microstructural changes to the specimen-level performance of alkali-activated concrete (AAC). Among AAB's primary advantages as a building material is its superior performance at high temperatures and lower environmental impact. The performance of reinforced concrete to function as a composite at high temperatures is evaluated through its bond strength. However, there is no reported study on the bond strength of ambient cured (fly ash + slag)-based AAC. To validate the practical sustainability of AAC, life cycle assessment (LCA) can be used to evaluate the environmental impact.

Therefore, the present study evaluates the effect of varying precursor proportion (fly ash: slag varied as 100:0, 70:30, 60:40, and 50:50), activator modulus (Ms, varied as 1.0 and 1.4), and high-temperatures (538 °C, 760 °C, and 892 °C) on the mechanical properties and microstructure of AAC. The effect of varying precursor proportions and Ms on the mechanical performance of AAC is evaluated through compressive, bond, flexural, and split tensile strength testing. The performance of AAB at extremely high temperatures is assessed in terms of residual compressive and bond strength. LCA of AAC is conducted using the ReCiPe 2016 methodology. Furthermore, since the commercialization of any novel alternative material depends on cost-effectiveness, a simplified cost analysis is performed.

The results from microstructural experiments show the formation of new crystalline phases and decomposition of reaction products when exposed to high temperatures, and they correlate well with the observed mechanical performance. The 28-day compressive strength with slag content is enhanced by 151.8 - 339.7 %, depending on the mix. In ambient conditions, lower Ms improves mechanical performance. When exposed to high temperatures, specimens with a high slag content and a low Ms suffered significant deterioration. AAC with fly ash: slag ratio of 70:30 and Ms of 1.4 is proposed as optimal from the results obtained in the present study. The results reveal that the biggest impact on climate change comes from transport (45.5 – 48.2 %) and sodium silicate (26.7% – 35.6 %). The proposed optimal AAC mix has a global warming potential 42.6 % lower than PC concrete. ReCiPe midpoint approach is more efficient in analyzing all impact categories except freshwater ecotoxicity (FETP) and human toxicity potentials (HTP) for which USEtox is recommended. The proposed AAC mix has a higher cost than PC concrete in the present scenario. In contrast, if a carbon tax is enacted, the cost of the proposed AAC mix will rise by only 18.4 %, whereas PC concrete prices will rise by 81.7 %. This proposed AAC mix is an environmentally sustainable replacement for PC concrete specifically intended for applications requiring the superior high-temperature performance of reinforced concrete.

The industry of building materials and construction, despite of their obviously conservative character, quite often has to face so-called “industrial revolution of the XXI century”. New trends, new methods of experiments and research are becoming perspective foundation for creation of high-tech products and processes characterized by guaranteed reliability index, developing principles of manufacturing up-to-date “supermaterials” and are marking the start of the sixth technological wave.

An impressive breakthrough in the construction technologies in the 21st century was achieved due to the properties of modern concrete which have recently seemed unattainable. These include extremely low values of water/cement ratio and air content of the concrete mixture with long-lasting flowability, cohesion and uniformity; the fresh concrete ability to easily and completely fill in the formwork to concrete of any configuration with dense reinforcement without the use of energy with horizontal or vertical mix pouring; the concrete ability to achieve a given strength with an adjustable strength development subject to climatic factors; dense concrete structure at the nano-, micro- and macrolevel as a factor ensuring high strength, resistance and durability.

The presence of nanomaterials and nanotechnologies in the construction segment is becoming more prominent. The detailed analysis and long-term forecast for the development of research and application of nanomaterials and nanotechnologies in construction shows that the cement and concrete cover over 40 % of the nanotechnology products in construction materials (market value is about $ 5.6 billion) with a predicted annual growth more than 10 %.

Recent advances in nano-chemistry and the development of new methods for synthesis of nanoparticles are now expected to offer a new range of possibilities for improvement of concrete performance. Incorporation of nanoparticles into conventional construction materials can provide the materials with advanced or smart properties that are of specific interest for high-rise, long-span, or intelligent infrastructure systems.

Self-regulating concrete (SRС) is one the most in-demand subjects of the modern concrete science. The choice of components and the design of SRC compositions are based on a prognostic assessment of the direction of spontaneous processes to ensure high functionality at any technological and operational stage. The concept of “self-regulation” should be interpreted as the technologically predicted course of spontaneous processes in order to achieve the maximum possible functionality of the interacting components and concrete mixes, which meets the concept of defined performance concrete (DPC).

Examples of successful applications of SiO₂, TiO₂, Fe₂O₃, Al₂O₃, CaCO₃ nanoparticles, nanosized spinel MgAl₂O₃, nanoferrit ZnFe₂O₄, and nanoclays in concrete are given. The most promising contemporary developments include the synthesis and application of new forms of carbon, viz fullerene (C₆₀, C₇₀, C₅₄₀), graphene oxide (GO) and new types of carbon nanotubes.

In conditions of the planet population growth and inevitable emergence of raw material and power shortage in construction quite rapid displacement of traditional materials and technologies by energy-saving and material-efficient solutions must be a determining factor. Nano-binders and nano-engineered cement-based materials with nano-sized cementitious component or other nano-sized particles may be the next ground-breaking development.

Due to the interest to increase the durability and sustainability of concrete structures and construction techniques, a wide range of novel cementitious materials are being designed and investigated. One such recent material is a cementitious material containing superabsorbent polymers (SAPs) studied only from 1999 onwards, mainly for its internal curing purposes with mitigation of autogenous shrinkage and sealing characteristics. Other positive influences are the change in rheology, the increase in freeze-thaw resistance, amongst others. From 2010 onwards, a combination of addition of synthetic microfibers and SAPs was studied, for their improved influence on autogenous healing in cementitious materials. It was found that optimal self-healing features were possible, as the crack widths were limited and water was available during dry periods. Some of those first samples now have an age of over 10 years.

As the autogenous healing capacity is dependent on the age of the material, so will be the possible influence of added materials to promote this healing. The effects beyond one year are not omnipresent in literature. The effect of the age cannot be investigated as long as the actual specimens do not reach the required maturity. In a previous study, the age was studied up to 8 years’ time. In this study, specimens from the same batch were studied after a decade of maturing in different storage conditions.

Typical strengths and crack widths were obtained. Due to the stress initiator property of SAPs, the number of cracks increases. Due to the macro-pore formation, the strength is lowered. However, the healing ratios are always higher for SAP compared to REF samples. This is due to the water action by the SAPs during dry periods and the ability of SAPs to extract moisture from the ambient environment. This leads to better conditions for healing products to form as water is available. The main visual appearance of the healing products was the whitish calcium carbonate crystallization.

The small crack widths after 10 years are still able to be partially healed. The main visual healing product is calcium carbonate. Further hydration was less likely as most binder already hardened during storage conditions. Generally, the samples containing SAPs show more prominent healing and they are still able to swell almost completely after a decade storage in an alkaline cementitious environment. This makes them a sustainable option for the future as less maintenance and repair will be required.

When recycling reclaimed asphalt (RA) in new hot-mix asphalt (HMA), higher temperatures of the virgin aggregate allow the mobilization of a higher amount of the binder in the RA. However, this implies a more severe short-term aging of the virgin bitumen and poorer properties of the aged-virgin bitumen blend, due to the lower virgin bitumen/RA bitumen ratio. On the contrary, the adoption of lower temperatures has the opposite effect (lower mobilization of the RA binder but higher performance of the bituminous blend). In addition, the reduction of material heating results in a lower bitumen viscosity, which may determine lower compactability and lower adhesiveness. Previous studies showed that a reduction of 30 °C in the mixing temperature of AC containing RA does not imply a significant increase of the air voids content but allows improving the material performance against cracking, fatigue, and rutting. Moreover, the lower mixing temperature also preserves the effectiveness of the rejuvenating agent. To have a deeper understanding of this phenomenon, the objectives of the research were: (i) evaluating how the binder adhesive properties changes when varying the content of aged bitumen; (ii) assessing if the adhesion is higher on virgin aggregate or on RA particles, coated with aged bitumen; (iii) understanding how the blending temperature influence the binder-aggregate adhesion.

To this aim, binder bond strength (BBS) tests were carried out at 25 °C. The experimental program provided 2 types of substrates, simulating virgin limestone aggregate and RA, 3 RA/virgin binder proportions (20/80, 35/65 and 50/50), 2 types of rejuvenator in the binder (coded with the letters A and B), 2 bitumen application temperatures (140 °C and 170 °C), and 5 repetitions.

The results showed that the adhesive properties of the binder decreased when increasing the aged bitumen content from 20% to 50%. The t-test α values obtained when comparing the pull-off tensile strength (POTS) of the blends with 20% and 35% of RA bitumen and the blends with 35% and 50% of RA bitumen were respectively 1.2×10^-6 and 6.1×10^-7, confirming the decreasing trend of POTS with aged bitumen content. Moreover, for high RA bitumen contents (35% and 50%), the adhesion on the RA substrate was higher than on the limestone substrate (α = 0.004). Between the two rejuvenators, the type B allowed obtaining higher POTS values for high RA bitumen contents (35% and 50%), as confirmed by α = 0.008. Differently, the bitumen application temperature (140 °C or 170 °C) did not significantly influence the POTS (α = 0.50). This indicates that the increase of adhesiveness that can be obtained at higher temperature was approximately balanced by the more severe aging underwent by the binder. However, as in site the lower mixing temperature implies the lower mobilization of the RA binder, thus a lower RA/virgin bitumen proportion, from the experimental results it can be stated that the reduction of the mix temperature is beneficial for the adhesion between the binder and both the virgin and the pre-coated RA aggregates.

Introduction

The use of alternative materials in asphalt pavements has become a critical matter in pavement engineering due to sustainability issues. These issues include the utilisation of finite resources, the need to re-use of wastes and reduce the generation of Green House Gases emissions. To cope with these issues, two approaches can be considered. Firstly, the use of Reclaimed Asphalt (RA) in new asphalt mixtures has become a common practice in the last decades in the asphalt industry particularly in small amounts (<20%). However, there are still some concerns on the use of higher amounts (>20%), due to uncertainties in its performance. Secondly, the use of non-petroleum-based binders as alternatives to conventional bitumen is starting to gain force in this field. Recently, the combination of both approaches has been shown to be feasible and could lead to more sustainable solutions in pavement engineering. Nevertheless, more research is needed to give confidence to these innovative asphalt mixtures towards their final implementation. On this regard, the aim of this investigation is to optimise the combination of a RA source and alternative binders, made from vegetal by-products of other industries (biobinder) and targeting the maximum content of both materials in new asphalt mixtures.

Methodology

For this purpose, two sources of reclaimed asphalt (RAs) and two type of biomaterials were characterised, namely a biobinder and a bioemulsion. The cohesion and stiffness properties of the RAs were studied by means of ITS and ITSM testing 100% RA specimens manufactured at different temperatures. With this, the degree of activation of the RA binders was estimated. On the other hand, the biobinder, bioemulsion and the extracted binder from the RA were conventionally and rheologically characterised at the whole range of service temperatures of pavements.

The optimisation of the design of the sustainable asphalt mixtures was performed using the rheology and performance-related properties of the RA, the extracted binder from RA and the biomaterials, and the different results and hypothesis on the degree of blending between both binders obtained from the RAs characterisation.

Results

The results of the RAs characterisation show the potential of the testing programme used to determine the degree of binder activation of RA as an important intrinsic property to consider in the design of recycled asphalt mixtures. The characterisation of the biomaterials reveals their ability to fully replace asphalt binders in hot and cold recycled asphalt mixtures reducing global warming potential. Finally, the optimisation of the asphalt mixture design using the rheological and performance-related properties of the individual components, plus the estimation obtained of degree of blending between binders show the key role of these parameters to perform the adequate design of sustainable asphalt mixtures including alternative materials.

It is essential for the use of Reclaimed Asphalt (RA) to be established as a standard practice since it has been proven that it can serve an end-of-waste product that complies with the principles of circular economy within both an open and closed loop approach and can provide environmental and economic benefits. The European average of RA reused in pavement construction is 60%, however, some nations greatly outperform this average, to understand why these nations outperform Italy, which reuses only 25% of available RA, it is necessary to understand the regulatory framework in each nation, promoting the use of RAs in pavement design. In the context of making recommendations to Italian regulatory bodies. Spain has a similar economic capacity and climatic conditions to Italy, but reuses 72.2% of RA. What causes the chasm between Spanish and Italian figures? In the authors’ opinion, Spain sees the use of RA in pavement management as a sustainable and cost-effective solution. Moreover, Spain was an early adopter of hot and warm mix asphalt recycling, since the 1980s. These facts combined with experience gained over the past three decades, have changed the Spanish lawmakers’ perspective, who understand that RA pavements are equal to conventional pavements. Additionally, research supports RA use and advocates higher RA% in pavements. Combining the use of rejuvenators, asphalt mixtures with a RA content of 40% allows the amount of virgin aggregates and the quantity of virgin binder to be reduced. Assuming that Spain is as receptive to this new research as it has been in the past, it is likely that their rate of reuse of RA will continue to increase. In Italy, there seems to be little push to increase the use of RA in road construction; the Italian association of pavement design and bitumen – SITEB, cites several obstacles; complex bureaucracy and the slow rate of change to regulations, non-uniform regulations which vary on a municipal level, and lastly a prejudice among engineers, road authorities, and governmental bodies, against the use of RA. Moreover, the Italian regulatory context allows for only 30%, 25%, and 20% of RA usage in base, binder, and surface courses. A fact that significantly limits and hinders the exploitation of RA as an End-of-Waste product. On the contrary, in Spain, although mixtures composed of 60-70% recycled materials can be produced, the most common practice is the production of asphalt mixtures with an RA content below 50%. Thus, it becomes evident that the increase of the allowed RA% in the recycling process of asphalt mixtures can significantly impact the recycling and sustainability implications of a country. It is disheartening to see that the improvement in Italy’s use of RA has been slow, therefore recommendations are needed for Italian regulatory bodies, in the hope that they could learn, not only from their most adept and advanced in terms of RA recycling European partners but also from those in a similar economic condition who understand that the use of RA is not only environmentally sustainable but also economically sustainable.

In recent years, continuous attempts have been made by the pavement industry to explore the opportunities that assist in bringing down the environmental footprint of roadway infrastructure as well as mitigate the harmful impacts of climate change on the quality-of-life. The construction of pervious interlocking concrete pavement (PICP) in parking areas is gaining widespread acceptance attributed to their: (a) ease of installation, (b) high durability and skid resistance, (c) low repair and maintenance requirements, (d) ability to mitigate floods, and (e) potential to purify the stormwater. However, very little research has been conducted to investigate the environmental impacts associated with the installation of such pavement systems. Therefore, the objective of this cradle-to-gate research study was to quantify the environmental footprint of PICP for a 75 m × 16.5 m parking lot that was constructed in the premises of the Indian Institute of Technology Tirupati, India. Further, the quantified impacts were compared to that of traditional asphalt concrete (AC) and cement concrete (CC) parking lots. The scope of the effort encompassed: (a) design of three pavement systems based on site specific requirements as per relevant design codebooks, and (b) quantification of the environmental impacts using systematic lifecycle assessment (LCA) approaches that are in accordance with the international standards. The results indicated that construction of AC parking lot had lower environmental footprint compared to CC pavement and PICP systems. Further, the environmental impacts associated with the construction of CC pavements were the highest. Based on the results, it was understood that though PICP system has intermediate environmental footprint, it provides additional benefits such as infiltration of stormwater into the ground. Further, the PICP blocks have higher design life compared to CC and AC pavements. However, additional research must be conducted in future to ascertain the environmental impacts of the three pavement systems from cradle-to-grave perspective. Such an approach will assist in the integration of LCA toolkits with existing pavement design methods, and further contribute to the development of resilient and sustainable pavement infrastructure.

1. Introduction

The demolition of old flexible pavements layers generates large quantities of reclaimed asphalt pavement (RAP). This material can be milled and reused for new pavement sections as aggregates. Alkali-activated materials (AAM) are alternative cementitious binders with high strength and chemical resistance [1]. The main problem associated with RAP in cementitious materials is the strength loss due to the porous interface [2–5]. The use of metakaolin (MK) can improve slag based AAM's properties [6,7]. The objective of this study is to investigate if the properties of RAP-AAM produced with low alkali concentration (4% Na₂O and Ms = 0 and 1) can be improved with 5% MK replacement. This investigation compared isothermal calorimetry, compressive and flexural strength results for RAP-AAM produced with and without MK replacement.

2. Materials

The RAP-AAM was produced mixing a powdered precursor with an alkali solution. The precursors used were ground granulated blast furnace slag (GGBFS) supplied by Ecocem and MK (Caltra). The alkali solution was prepared using sodium hydroxide sodium silicate solution form VWR. Fine RAP aggregate was obtained by removing the fine fraction (< 4mm) of a locally milled flexible pavement supplied by Willemen Infra Recycling. Table 1 shows the compositions studied.

Mortar mixing details and experimental procedures can be found elsewhere [5].

Table 1. Compositions (Ms = silica modulus, w = water, p = precursor, a = fine RAP aggregate)

3. Results and discussion

Figure 1 presents the calorimetry results for the different RAP-AAM samples studied. The sharp peak at the start of the experiment was only partially captured. The second and main peak is related to the precipitation of the reaction products [8,9]. Samples with sodium silicate (R4-1 and 5MK4-1) have a delayed second peak due to the workability retention and the reduced availability of OH- [10,11]. Replacing 5% of GGBFS with MK decreased the intensity and delayed the second peak. It also reduced the cumulative heat of the samples. The retarding effect of MK in the formation of reaction products was also observed in another research [12].

Figure 1. Calorimetry results of RAP-AAM

The compressive and flexural strength of the samples is presented in Figure 2. The use of MK reduced the early strength of the samples (both compressive and flexural). At later ages, however, the use of MK caused some slight improvements in strength. This result differs from other studies [7,13] that reported a reduction in compressive strength and gains in flexural strength for sodium hydroxide alkali-activated pastes and mortars.

Figure 2. Compressive and flexural strength results for RAP-AAM.

4.Conclusion

The use of MK delayed the formation of main reaction products, which significantly impacted the early strength of the studied mixes. The filler effect of MK may have helped anchor RAP particles to the matrix and caused slight improvements in compressive strength observed at 28 days. This study did not see significant improvement in flexural strength as observed elsewhere [7], most likely due to the low alkali concentration used. Further studies of the benefit of MK for RAP-AAM at higher concentration of alkalis is needed.

Construction and maintenance of the built environment consume a large quantity of resources and energy and contribute to the emission of a significant amount of greenhouse gases. Hence, improving resource efficiency and resource cycles are crucial to reducing environmental, economic and social impacts. The construction of roads mainly consumes mineral aggregates and binders, has the advantage of high recycling rates and is a consumer of cascading materials. However, infinite recycling is impossible and recycled road construction material is often cascaded due to quality, quantity and economic aspects. In addition, the extending and ageing road network faces increased traffic and climate changes, which might increase the probability of failure, inducing an increased maintenance effort. Maintenance has a minor contribution to resource consumption in developing countries, but it can have a major contribution in developed countries in the range of 50% to 75%. The increasing production of asphalt for surface wearing courses in Austria indicates the increase in materials used in maintenance. The production of surface course asphalt was about 25 to 35% before 2016, roughly reflecting the 3 cm asphalt surface layer of the total asphalt layer thickness of 15 cm to 20 cm used in municipalities’ roads. An increase to 55% to 60% from 2017 to 2019 indicates the increasing maintenance work done on the Austrian asphalt network. Reconstructions of roads, maintaining the road network, accounted for about 65% in an Austrian municipality, reflecting the efforts of the municipality’s administration to improve traffic concepts (increasing roadway width, adding cycle lanes and paths, reducing traffic speed and improving townscape), as well as addressing structural problems and long-term solutions of degrading road surfaces. Since the reconstruction process is similar to initial construction, it consumes an identical amount of resources for asphalt layers. Hence, local factors, such as traffic development, economic viability and road lifespan, are important to determine long-term resource efficiency. The amount of reclaimed asphalt of about 25% of the Austrian asphalt production in 2018 and 2020 corresponds to the increased surface course asphalt production. It shows that system improvements are required to record waste generation, treatment and utilisation. Processed reclaimed asphalt is officially used to 70% to produce new asphalt. However, a cascading material flow of reclaimed asphalt pavement (used in unbound layers, gravel roads, road shoulders and backfilling) depends on local factors like short transportation distances of primary materials, low binder prices and administrative recycling commitments. A deeper understanding of the material flows related to asphalt roads, including primary and secondary material resources and resource consumers, and economic interaction between industries related to these flows are necessary to establish sustainable asphalt roads without causing unwanted shifts of material flows, sustaining resource depletion.

Environmental issues are a main concern for the society. The construction field largely affects the consumption of energy and the environment. In this concern, the infrastructure, in particular streets, bridges and tunnels are a common needs for today life. They link people speeding up the meeting capability. Tunnels continuously gained a relevant importance to shorten the travel distances and to save the landscape surface. In this work, the demolished rock materials coming from the construction of part of a 50 km long tunnel through the alps, was characterized and used to produce shotcrete to secure the tunnel walls. Several samples of demolished aggregates were investigated with respect to the granulometric curves. They needed to match with reference curves in the content and amount of stone aggregates. This was particularly difficult in some cases because of the different mineralogy encountered. The type and form of the aggregates were evaluated as well. These latter parameters have an influence on the workability and on the mechanical properties. Especially angular and subangular aggregates needed a special attention. Then, the material was mixed by adding silica fume. This enabled a more dense microstructure by closing the porosity at a later stage. The steel fibres were also added to the mixtures in different amounts to produce the shotcrete. The fresh concrete properties were measured directly on site. Furthermore, the hardened state was controlled on site and in laboratory. The compression strength exhibited variable values, which could be related to the mixing proportion on the ingredients. The punch tests indicated a similar fracture behavior but were very important for the safety of the worker inside the tunnel, in particular where material enrichment was present on the roofing parts. The steel fibre content generally increased the ductility of the specimens. The porosity and the water permeability were controlled as well as the freeze / thaw resistance. The mixtures were continuously optimized by taking the water / cement ratio and the superplasticizer dosage under control. All these adaptations allowed to reuse the big amount of tunnel demolition material. The concrete was produced in a special mixing plant on site. This fact reduced the transportation and increased the environmental sustainability of such a long infrastructure.