Journal of Materials in Civil Engineering

Although the concept of geopolymers was proposed over 40 years ago, there still remains a lack of clarity regarding their atomic-level structure and formation. In this study, the ${\mathrm{Dmol}}^3$ quantum chemistry calculation program, based on density functional theory (DFT), was used to determine a range of electronic structural properties associated with the initial, intermediate (IM), transition (TS), and final states of ${\mathrm{Al}}_2{\mathrm{O}}_3/[\mathrm{Al}(\mathrm{OH}{)}_4{]}^{-}$ conversion reactions in the alkali-activator. The properties analyzed included total energy, Gibbs free energy, electrostatic potential (ESP), Fukui functions, and frontier orbitals, comprising the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO). The simulation results indicated that electrons were transferred from the HOMO of O in ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{OH}}^{-}$ to the LUMO of Al in $({\mathrm{AlO}}_2{)}^{-}$ or $(\mathrm{AlO}{)}^{+}$, leading to an increase in the LUMO energy level of Al and a reduction in the chemical reactivity of the newly formed Al monomers. The transformation processes from ${\mathrm{Al}}_2{\mathrm{O}}_3$ to $[\mathrm{Al}(\mathrm{OH}{)}_4{]}^{-}$ involved varying numbers of steps, energy release, and energy barriers. Notably, during the transition state conversion process, the breaking and reformation of O─ H bonds often occurred as necessary conditions for the formation of transition states. These findings have significant implications for the advancement of new technologies based on geopolymer conversion processes.

This paper aims to study the impact splitting tensile properties of fly ash–recycled aggregate concrete (FARAC). The splitting tensile tests of FARAC with different replacement rates of recycled aggregate (0%, 30%, 50%, 100%) and different fly ash contents (0%, 10%, 20%, 30%) were carried out by using split Hopkinson pressure bar (SHPB) under different impact pressures (0.30 MPa, 0.35 MPa, 0.40 MPa, 0.45 MPa). The results show that the dynamic tensile strength of concrete with 30% recycled aggregate replacement rate and 10% fly ash content can be higher than that of natural concrete. It is also found that the increase of impact pressure value will enhance the dynamic tensile strength of FARAC. Eventually, the dynamic constitutive model based on ZWT constitutive model was established by considering the material damage, which can excellently forecast the dynamic stress-strain curve of FARAC and the dynamic mechanical properties.

This paper presented an experimental study on the dynamic biaxial mechanical behavior of steel fiber-reinforced recycled-aggregate concrete (SFRRAC) samples. Five control variables, including lateral pressure (${\sigma }_2$), strain rates ($\dot{\epsilon }$), volume content of steel fibers ($V_{sf}$), volume content of polyvinyl-alcohol fibers ($V_{pf}$), and substitution ratio of recycled coarse aggregate (SRCA) were designed. Under different values of ${\sigma }_2$, two distinct failure modes of samples were observed: columnar failure and oblique shear failure. However, the failure modes showed no substantial differences at various values of $\dot{\epsilon }$, $V_{sf}$, $V_{pf}$, and SRCA. The results indicated that the biaxial strength exhibited a general trend of initially increasing and then decreasing with increasing ${\sigma }_2$. Specially, the biaxial strength exhibited a maximum increase of approximately 67% when ${\sigma }_2$ reached 40 MPa. The lateral pressure was more significant than the strain rate in terms of material strengthening. The dynamic biaxial strength exhibited a linear correlation with the logarithm of the strain rate. The incorporation of steel fibers and polyvinyl-alcohol (PVA) fibers resulted in a slight 9.3% improvement in biaxial strength but significantly improved its deformation capacity. To achieve maximum strength, the optimal values of $V_{sf}$ and $V_{pf}$ were found to be 1.2% and 0.1%, respectively. With SRCA increasing from 0% to 100%, the biaxial strength decreased by approximately 11%. Moreover, this paper proposed two dynamic failure criteria, one in the principal and the other in the octahedral stress spaces. These criteria exhibited good agreement with the experimental data.

Abstract

Despite being economically and environmentally friendly, the use of cold bitumen emulsion mix (CBEM) is still limited to minor repair work and construction of low volume roads. One of the reasons behind this underutilization is its poor cracking resistance. Fillers have significant effects on the cracking resistance of asphalt mixes. So, the present study aims at examining the effect of different fillers, i.e., stone dust, cement, lime, fly ash (FA), and ground granulated blast furnace slag (GGBFS) on the cracking resistance of CBEM. The cracking resistance of CBEM was examined by indirect tensile strength (ITS) and semicircular bending (SCB) tests. Results showed that the inclusion of active as well as waste fillers significantly improved the cracking resistance of all modified CBEM. However, the cracking resistance developed by FA and GGBFS modified CBEM were not comparable to cement modified CBEM. So for further modification, lime was added to FA and GGBFS modified CBEM, which further enhanced the cracking resistance of CBEM, making it comparable to that of cement modified CBEM. This experimental study was further extended to examine the effect of these fillers on the properties of cold bitumen emulsion mastic (CBEMa) by conducting scanning electron microscopy (SEM), X-ray diffraction (XRD), and linear amplitude sweep (LAS) tests. The SEM and XRD results confirms the presence of hydration products in the mastic phase of CBEM. In addition, a Pearson correlation analysis was carried out between the LAS parameters of CBEMa and the cracking resistance parameters of CBEM, confirming a linear association between both characteristics.

Practical Applications

Cold mix asphalt or cold bitumen emulsion mix is a cost-effective, emerging pavement construction practice. Unlike traditional hot mix asphalt, cold bitumen emulsion mix is prepared at ambient temperature, requiring minimal energy consumption during production and transportation, which makes it suitable to be used in colder weather conditions. However, due to its poor early strength and poor performance, its application areas are limited to constructing low-volume roads and pavement maintenance works. The presence of water in the compacted mix is the primary cause of poor early strength in cold bitumen emulsion mix. Nowadays, researchers are investigating various methods to improve its performance and expanding its applications. This study promotes the use of fillers such as cement, lime, fly ash (FA), and ground granulated blast furnace slag, as well as their combinations, in order to mitigate one of the recognized pavement distresses, i.e., cracking failure. These fillers provide hydraulic behavior in the presence of water in the compacted mix, facilitating a secondary binding network. This additional binding network strengthens the mixes and reduces crack propagation. This in-depth study will inspire politicians, policymakers, and transport officials to consider the benefits of this technology and implement it in the field.

To promote green and low-carbon development in the rehabilitation of damaged asphalt pavements, a cold patching material combining magnesium phosphate cement (MPC) with emulsified asphalt (EA) was developed. This study systematically examined the impact of each preparation parameter on the performance of the composite system, including the EA to MPC ratio (A/C), the mass ratio of dead-burned magnesium oxide (MgO) and phosphate (M/P), and the dosing ratio of borax, a retarder (B/M). It also investigated their effects on workability, hydration process, mechanical behavior, and microstructure of MPC-EA. Furthermore, it elucidated the mechanism behind the loss of strength in MPC-EA. The results demonstrate that each component of MPC-EA has interdependent influences on workability. When $\mathrm{A}/\mathrm{C}<0.2$ and M/P increased to 4, there was a 5.4% to 31.5% reduction in setting time and an increase in compressive strength as well. However, when $\mathrm{A}/\mathrm{C}>0.2$, M/P gradually lost its regulating effect. EA significantly prolonged the setting time of the MPC-EA system; for instance, when $\mathrm{A}/\mathrm{C}=0.5$ (compared with MPC), setting time more than doubled but fluidity was severely impaired due to EA incorporation into the system. Additionally, borax in the MPC-EA system exhibited significant delaying effects on setting time, while positively affecting fluidity as well. Moreover, borax modulated heat release rate and cumulative heat release in MPC-EA, which influenced mechanical properties and improved microstructure within this system too. Furthermore, when A/C is not greater than 0.2, the pore structure becomes refined; however, when A/C is greater than 0.2, the porosity increases along with an increase in harmful micropores proportion within MPC-EA. The scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) test clarified microscopic mechanisms behind strength loss in MPC-EA: inhibition of hydration product development, poor crystalline morphology, and encapsulation by asphaltene film leading to decreased mechanical strength due to incomplete reaction between hydration products and reactants.

Two emerging challenges that could impede infrastructure development in India are achieving 100% utilization of fly ash generated by Indian thermal power plants and meeting the demand for aggregate in the construction sector. The paper discusses the engineering properties and performance of a novel angular-shaped fly ash aggregate (AFA) as a complete replacement for natural stone aggregate in wet-mix macadam (WMM) layer of pavement through laboratory investigation. After curing fly ash blocks in a hot water bath or autoclave, the high-strength blocks were crushed to produce AFA of the required sizes. The study used 98% Class C fly ash with 2% lime mix and 88% Class F fly ash with 12% lime mix for manufacturing two types of AFA in the laboratory. The properties of AFA, such as specific gravity, angularity number, water absorption, impact value, crushing value, abrasion value, and soundness, were compared with the required specifications given in the relevant Indian standards. Compaction characteristics, particle breakage, slake durability and leachability of AFA, were also investigated. The performance of AFA under cyclic and shear loading was investigated using cyclic triaxial tests and large box direct shear tests, respectively. AFA was found to be well-graded before and after the compaction. The results of the slake durability tests showed that AFA performs well even when subjected to severe wet and dry conditions. AFA exhibited resilient modulus ($M_r$) value of 129.1 to 149.7 MPa and internal friction angle of 42.73° to 50.75°. Based on the cyclic triaxial and shear test results, it was found that replacing natural aggregate with AFA in the WMM layer has satisfactory performance under traffic and shear loading. The results of leachate test showed that the AFA is safe for the environment. Depending on the type of fly ash used, the approximate production cost of AFA was estimated to be 16% to 65% lower than the cost of natural aggregate.

Abstract

As three-dimensional printing (3DP) technology continues to develop, it has successfully prepared rock mass samples repetitively and reproducibly. Previous studies primarily focused on comparing the mechanical properties and the pore structure between 3DP rocklike samples and natural rocks based on traditional uniaxial compression and CT scanning tests at single-stress conditions. In this study, 3DP gypsum samples were prepared for uniaxial compression tests equipped with the high-resolution in situ X-ray micro-computed tomography (Micro-CT) apparatus to obtain two-dimensional (2D) Micro-CT scanning images of 3DP gypsum samples at different stages of deformation. Based on the Micro-CT images, the cracking and pore structure evolution of 3DP gypsum samples were quantitatively analyzed. Results indicate that as the load increases, the void ratio of 3DP gypsum samples first decreases gradually and then increases slowly, which is later followed by a rapid rise along with failure. The cracks could be categorized into three types based on their initiation and growth mechanism: cracks that initiate and grow along the interface (Crack I), pores (Crack II), and residual binders (Crack III). Compared with pores in the compression region, pores were more prone to connect with adjacent pores in the extension region, forming larger pores. The qualitative and quantitative results are essential to characterizing and understanding the failure mechanism and microstructural evolution of 3DP gypsum samples for modeling natural rock behavior.

Practical Applications

3DP technology, with its highly customizable and precisely controllable features, presents an innovative approach for the fabrication of rock analogs. Currently, a topic of considerable debate revolves around the efficacy of utilizing 3DP as a viable replacement for natural rocks. To address this issue, we conducted an investigation into the cracking mechanisms and pore evolution of 3DP rocks under in situ compression experiments. First, we investigated the particle size distribution and composition of ZP150 powder. Subsequently, 3DP gypsum samples were manufactured using the ZPrinter 450 using ZP150 powder and Zb63 binder as the printing materials in this study. By complying with an in situ Micro-CT scanner, we conducted a microscopic investigation of crack behavior and pore evolution in the 3DP gypsum samples during uniaxial compression experiments. Through comprehensive analysis of visualized cracks and pores, a novel failure mode of 3DP gypsum samples under loading was proposed, considering the impacts of pores, bedding planes, and residual binder. The results presented in this study are important for characterizing and understanding the failure mechanism and microstructural evolution of 3DP gypsum samples when modeling natural rock behavior or conducting physical model experiments.

To enhance the early strength development of lithium slag composite binder, synergistic action of calcium silicate hydrates (C─ S─ Hs)–polycarboxylate (PCE) and ${\mathrm{Al}}_2({\mathrm{SO}}_4{)}_3$ on the hydration properties of lithium slag–cement binder was studied. Results showed that C─ S─ H─ PCE and ${\mathrm{Al}}_2({\mathrm{SO}}_4{)}_3$ synergistically facilitated the solidification and hardening of composite binder. The combination of 2% C─ S─ H─ PCE and 6% ${\mathrm{Al}}_2({\mathrm{SO}}_4{)}_3$ enhanced the compressive strength of the composite cement mortar at 1 day, 3 days, and 28 days by 37.6%, 33.1%, and 9.9%, respectively. By providing nucleation sites, C─ S─ H─ PCE facilitated the early hydration of silicates and the pozzolanic reaction of lithium slag, leading to shortened induction period, advanced heat release peak, increased total hydration heat release, and increased production of portlandite (CH) crystals. Aluminum sulfate facilitated dissolution of lithium slag (LS) and expedited the production of ettringite (AFt). The synergistic adoption of C─ S─ H─ PCE and ${\mathrm{Al}}_2({\mathrm{SO}}_4{)}_3$ facilitated the hydration of both clinker and LS, and the hydration products were more abundant, contributing to a more compact matrix.

With India importing more than 35% of its bitumen demand, there is a significant variation in the cost of bitumen in the international market. There is a need to identify suitable parameters that can help in selecting the right bitumen for a given viscosity grade (VG). In this study, the physical, rheological, and chemical characteristics of bitumen(s) acquired from different sources and falling under the same viscosity grade were explored under unaged, short-term aged, and long-term aged conditions. Two grades of bitumen (VG30 and VG40) obtained from three sources (P1,J1,I1 and P2,J2,I2) were evaluated. Though the binders obtained from different sources were found to satisfy the criterion as per Indian specification, they showed dissimilar short- and long-term characteristics. The bitumen(s) had unique temperature susceptibility and aging indices, which revealed the effect of the source on the performance of the binders. The difference in short- and long-term aging indices obtained from rheological data was in agreement with the FTIR results and asphaltene–maltene ratio(s). This study recommends using the short-term aging index (STAI) and long-term aging index (LTAI) obtained from frequency sweep data for selecting the appropriate source of bitumen. Aging indices at lower loading frequency and high in-service temperature should be selected for comparing the binders. Future study should focus on establishing limits of aging indices for characterization and selection of bitumen for practical conditions.

Construction activities are generating substantial amounts of construction waste due to the rapid increase in economic growth and infrastructure developments. A series of laboratory model tests were conducted to investigate the feasibility of using recycled aggregate as backfills to construct the rammed stone columns. Recycled aggregates and gravel aggregates with the same particle size distributions were used to construct the rammed stone columns. The direct shear test results showed that the recycled aggregate exhibits similar behavior of initial compression but more significant expansion compared to those of the gravel aggregate. The critical bearing capacity of the rammed stone column using recycled aggregate as backfills was 6.2%–6.5% lower than that using the gravel aggregate as backfill, but was 36.1% higher than that of the traditional stone column. The rammed stone column was failed in bulging failure mode with the maximum bulging position of approximately $2d$ from the column head. The critical bearing capacity of the rammed stone columns could be estimated according to that of the traditional stone column by considering the effects of the tamping energy.

The stability of soil is an essential requirement for various geotechnical engineering projects. The application of composite materials made from cemented soil has become prevalent in road subgrade engineering and foundation treatment due to their affordability, quick construction, and ability to withstand high compression forces. However, the mechanism about the incorporating fibers into cemented soil to enhance strength characteristics, mitigate the formation of microcracks in the soil matrix, and increase frost resistance is still unclear. In this study, a composite improvement method of adding basalt fiber (BF) to cemented soil is proposed, which is to select a single subgrade filling material with most significant freeze–thaw (FT) durability on the basis of traditional cement improvement methods. A series of static/dynamic triaxial compression tests were performed with cemented soil samples reinforced by three BF contents (0, 0.25%, 0.50%, and 0.75%) after FT cycles. The physical properties of these samples were studied, such as the optimal ratio of fiber content, the stress–strain relationship, failure strength, shear strength, and shear modulus, among others. The results revealed that both the shear modulus and failure strength of cemented subgrade soil reinforced with BF showed a significant increase. Compared with cemented soil, fiber-cemented soil exhibited a lower reduction rate in its mechanical properties after 15 FT cycles. The cohesion of the reinforced soil exhibited a gradual decrease as the number of FT cycles increased. Conversely, the friction angle initially decreased but later exhibited an increase. Compared with the reinforcement effects of BF at 0.25% and 0.75%, fiber-reinforced cemented soil with BF content of 0.5% demonstrated the highest strength and performed well in minimizing the effect of FT cycles. It is therefore recommended that ratio of 6% cement and 0.5% BF should be used to enhance the integrity of subgrade filling materials on silty clay.

Characterizing the fatigue behavior of cement-stabilized aggregates (CSAs) is essential to semirigid base asphalt pavement design. However, the relevant research is relatively limited and retains significant challenges. Therefore, fatigue tests subjected to multilevel loads were designed, and a mechanical damage evolution rule was proposed to describe the CSA’s damage behavior better. Four-point bending fatigue tests were conducted following the designed loading steps composing different cyclic stress levels and frequency combinations. The damage evolution patterns in sequential and disorder loading cases, together with the plastic strain accumulation trends, were analyzed to uncover the factors influencing CSA’s damage evolution. It was found that the general damage evolution of CSA exhibited a three-stage pattern, and it was affected by the cyclic stress level, loading sequence, and history of plastic and damage evolution. Remarkably, loading frequency appeared to have a negligible impact. Plastic strain and damage evolution demonstrated congruent evolution trends in most cases; however, they differed in disorderly loading. To address these, a new damage evolution rule was proposed based on the continuum damage mechanics and driven by the equivalent plastic strain rate. The damage dissipation rate was introduced to characterize the impact of effective stress level, and the damage variable and plastic strain path were also included in the proposed rule to reflect the effect of loading or damage history. The comparison results between the fitted and measured damage evolution curves validated the effectiveness in characterizing the fatigue damage behaviors of CSA when subjected to multilevel loads. Furthermore, the proposed damage evolution rule was also employed to model the damage curves of CSA’s uniaxial and indirect tensile fatigue tests with single-level loads. The commendable agreement between the fitting and experimental results confirmed the validity of the proposed rule and showed its broad applicability under different fatigue loading forms.

Rapid freeze-thaw (F-T) tests were conducted to study the frost resistance of desert sand concrete (DSC) at different stress levels (SL), desert sand replacement rate (DSRR) and the number of F-T cycles. The impact of the SL, DSRR, and number of F-T cycles on the mass loss rate, ultrasonic wave velocity, and stress-strain curve of DSC was investigated through uniaxial compression tests. Scanning electron microscope (SEM) was used to examine the microstructure of DSC. The constitutive relationship was established considering the influence of the SL and number of F-T cycles. The results indicated that the frost resistance and uniaxial compressive mechanical properties of DSC could be effectively enhanced when desert sand was added at 40%. The peak strain initially decreased and then increased as the DSRR increased. In contrast, the peak stress first increased and reached a maximum value as the DSRR increasing to 40%, followed by a gradual decrease. The F-T cycles gradually deteriorated the macroscopic properties of DSC. The proposed constitutive model of DSC was established by combining the two classical models as the ascending and descending sections, respectively. The model prediction results matched well with the experimental results, which can provide a theoretical basis for the engineering application of DSC under F-T cycles and loading environments.

In geotechnical engineering, the small-strain shear modulus and its attenuation characteristics are pivotal for analyzing and evaluating soil vibration responses to various engineering construction projects. This study conducts the resonant column test on undisturbed fissured clay samples, exploring the impacts of fissure inclination and confining pressure on the shear modulus in small-strain range. Results indicated that the shear modulus and its attenuation behavior in undisturbed fissured clay are substantially affected by both the fissure inclination angle and the confining pressure. With constant confining pressure, the shear modulus increases as the fissure inclination angle grows, reaching its maximum value at a fissure angle of 90°. In addition, as the confining pressure rises, there is a notable increase in the shear modulus and a corresponding reduction in the decay rate. Through the threshold strain, the elastic deformation of the specimen increases as the fissure inclination angle increases, and the confining pressure increases the ability of the fissured soil to deform at small strains elastically. Based on the acquired data, this research analyzes the relationship between the fitting parameters $A$ and $N$ and the fissure angle in the context of the Harding–Drnevich formula. Consequently, a mathematical model based on the fissure inclination angle and the effective confining stress was established to predict the maximum dynamic shear modulus ($G_{\max }$) and decay attributes of undisturbed fissured clay. Additionally, the study offers a comparative analysis of the maximum shear modulus and its attenuation features in clay with varied degrees of fissure development. The stiffness anisotropy is related to the orientation of particles and the normalized decay rate of the fissured clay has a certain relationship with the fissure density.

This study focuses on the development and evaluation of a novel cement-free gamma-ray-resistant ultrahigh-performance concrete (UHPC), known as UHPC-CAS. UHPC-CAS is composed of calcium oxide–activated slag, iron and barite powders, and steel fibers, making it a type of ultrahigh-performance geopolymer concrete (UHPGC). In this study, the effects of replacing silica sand with varying percentages of iron and barite powders on the mechanical and radiation properties of UHPC-CAS were investigated. The optimal replacement ratio of iron powder was found to be 50%, resulting in the highest compressive, tensile, and bending strengths among all mixtures without fibers. Furthermore, the UHPC-CAS samples reinforced with steel fibers and containing barite powder exhibited a more pronounced softening zone than those containing iron powder. With 100% iron powder and 3% steel fibers, UHPC-CAS achieved superior resistance to gamma rays, as evidenced by the highest attenuation coefficient of 0.234 cm and the lowest half-value layer of 2.96 cm. This research demonstrates the potential for developing sustainable cement-free UHPC for use in nuclear facilities and other applications requiring high radiation shielding.

Utilizing hazardous solid waste to produce autoclaved aerated concrete (AAC) has been a prominent area of research in the building materials industry. This paper proposes the preparation of AAC by substituting quartz sand with solid waste textile-dyeing sludge incineration ash, which met A5.0 (compressive strength $\ge 5\mathrm{MPa}$) and B07 (Bulk density $\le 750\mathrm{kg}/{\mathrm{m}}^3$) grade requirements. The hydration products in AAC by using textile-dyeing sludge incineration ash (TDSIA) were evaluated by X-ray diffraction, thermogravimetry, and field emission scanning electron microscopy. The results indicated that ${\mathrm{CaSO}}_4,{\mathrm{Fe}}_3{\mathrm{O}}_4$, and others in TDSIA were generating hydrogarnet, calcium aluminate hydrate, ettringite, and calcium iron aluminum silicate. Grinding was found to promote the autoclaved hydration of ${\mathrm{CaSO}}_4$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4$ in TDSIA. The AAC process effectively immobilized heavy metals from TDSIA and led to a lower concentration of heavy metals in the leachate. These findings offer a promising and environment-friendly method for the resource utilization of TDSIA.

Due to the large specific surface area and strong van der Waals force, bulk aggregation of nano-${\mathrm{TiO}}_2$ immediately occurs when it is mixed with fresh cement mixture, limiting its reinforcing efficiency to cement-based materials. This study proposes a novel strategy to improve the mechanical and photocatalytic properties of cement mortar using nano-${\mathrm{TiO}}_2$ hydrosol instead of conventional nano-${\mathrm{TiO}}_2$ powder. Compared with pure mortar, the 28-day compressive and flexural strength of mortar with 0.6% by weight nano-${\mathrm{TiO}}_2$ hydrosol were enhanced by 51% and 15%, respectively, the water absorption and chloride ion diffusion coefficient were decreased by 21% and 31%, respectively. The observed improvements were attributed to the denser microstructure and enhanced micromechanical properties in the bulk matrix and around interfacial transition zone (ITZ), resulting from the better dispersion of nano-${\mathrm{TiO}}_2$ hydrosol in alkaline environment. In addition, compared with the mortar with nano-${\mathrm{TiO}}_2$ powder, the degradation efficiency of Rhodamine B and NO by the mortar with nano-${\mathrm{TiO}}_2$ hydrosol (0.6% by weight) was enhanced by 18.3% and 17.1% at 28 days, respectively. The enhancement resulted from the higher intrinsic photocatalytic activity and larger exposed surface area of nano-${\mathrm{TiO}}_2$ hydrosol due to better dispersion. The research outcomes demonstrate the potential of using nano-${\mathrm{TiO}}_2$ hydrosol in cement mortar to significantly increase the mechanical strength, durability, and photocatalytic activity, which can lead to the development of cement-based constructions with the integrated design of functionality and structure.

Microbial cement (MC) can generate biocalcium carbonate on the surface of mortar in the liquid environment, which shows potential advantages to enhance the surface property of prefabricated components. This paper is aimed to investigate the cementation solution (CS) concentrations (0.5, 1.0, 1.5 M) and treatment times (3, 6, 9, 12 time) on capillary water absorption coefficient (coefficient k) and compressive strength of MC treated mortar, and attempted to point out the macroproperties and microstructure evolution mechanism of mortars based on adsorption and deposition of MC. The results indicated that when CS concentration was 0.5 M, the reduction rate of coefficient k increased from 29% to 76% with increasing treatment times from 3 to 12, and the growth rate of compressive strength also increased from 7.4% to 27.5%. For a CS concentration of 1.5 M, shorter less treatment times resulted in a significant reduction in coefficient k, whereas it had little effect on compressive strength. The adsorption and deposition of MC treated mortar surface led to an increase in the pore volume fraction of less harmful and harmless pore ($<50\mathrm{nm}$) in the near-surface layer by filling the pores with calcite and altering the content of hydration products. This process decreased the content of portlandite and increased that of calcite. Increasing the content of surface deposition was beneficial for reducing coefficient k, whereas improving the inner pore structure was beneficial for increasing compressive strength.

This study aimed to evaluate the mechanical performance of asphalt mixtures with tire rubber powder (TRP) and graphene composite-modified asphalt binder. Asphalt mixtures with graphene/TRP-modified binders, as well as TRP-modified and conventional asphalt mixtures, were prepared. The performance of the mixtures was evaluated using dynamic creep, resilient modulus, semi-circular bending fracture, indirect tensile fatigue, and freeze-thaw splitting tests. Results showed that the graphene/TRP composite asphalt mixtures exhibited superior rutting resistance, fatigue performance, fracture resistance, and moisture stability over the TRP-modified and conventional asphalt mixtures. The findings demonstrated that incorporating graphene into TRP-modified binders may result in asphalt mixtures with more desirable mechanical performance. This study may shed more light on the application of TRP and graphene in asphalt to improve pavements’ overall performance and sustainability.

Dispersive soil is susceptible to water erosion and could cause damage in geotechnical engineering or hydraulic engineering projects. Recycled clay brick powder (RCBP) was used as a modifier to improve the dispersivity and water stability of dispersive soil in this study. Pinhole tests, crumb tests, disintegration tests, particle analysis tests, exchangeable sodium percentage (ESP) tests, pH tests, conductivity tests, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were conducted to explore the modification effects and corresponding mechanisms of RCBP on dispersive soil. The results revealed that the dispersivity of the soil significantly weakened as the RCBP content increased and curing time extended. Specifically, adding 4% RCBP to the soil and curing for 7 days effectively transformed dispersive soil into nondispersive soil. Furthermore, the final disintegration time of the soil sample with 10% RCBP cured for 28 days was 273% longer than that of the soil sample without curing. Moreover, the treatment led to decreased fines content, ESP value, and pH value in the soil samples. The decrease in ESP value indicated the replacement of sodium ions adsorbed on the soil particle surfaces with calcium ions, resulting in a reduction in the thickness of the diffuse electric double layer of soil particles, and subsequently reduced soil dispersivity. Additionally, the decrease in pH also contributed to the reduction of the diffuse electric double-layer thickness. XRD and SEM analyses confirmed the formation of cementing materials between soil particles due to the modification, which filled gaps and cemented particles to create a waterproof barrier between soil particles. In conclusion, the utilization of RCBP as a modifier for dispersive soil could be a win–win measure with promising outcomes. It is recommended that more than 4% RCBP should be added in engineering applications.

This study draws inspiration from the chelating abilities of dopamine and tannic acids with ${\mathrm{Ca}}^{2+}$ in cement-based materials and explores the potential of green tea water (GTW) with tea polyphenols (TP) as the major soluble substance as a natural source of polyphenols available in people’s daily lives. Hydration kinetics analysis reveals that GTW has both retarding and accelerating effects on cement hydration, with the latter being more prominent at lower concentrations. Properly adding GTW to cement can increase the compressive strengths of mortars by up to 16.7% at 3d, 12.2% at 7d, and 18.2% at 28d compared to the blank mortar. The microstructure of the optimal mortar shows grape-cluster-like nanoparticles that originate from the TP∼Ca intermediates that act as seeds for more hydration products. This effect is confirmed by using analytically pure TP to analyze its influence on the compressive strength of mortar.

Unsaturated transport of water and chloride into concrete was investigated using capillary absorption tests and molecular dynamics (MD) modeling. Results showed that a linear relation between capillary absorption rate and saturation degree (SD) of concrete was significantly influenced by water to cement ratio ($W/C$) due to different pore-size distributions. Chloride penetration depth was reduced at high SD and low $W/C$. The relationship model of chloride diffusion coefficient with $W/C$ and SD is proposed. MD modeling was conducted to simulate water molecules and ion transport in the nanopores of calcium silicate hydrate gel. The MD study indicated that the rapid capillary absorption of water and ions in the dry state followed the Lucas–Washburn equation, and the slow diffusion of solution species in the saturated state matched well with Fick’s second law. MD also attributed the slow transport rate of water and ions in cementitious materials with fine microstructures to the pronounced filter effect of nanometer confinement and the longer resident time on a hydrate’s surface as a result of the electrostatic trapping of surface silicate and calcium atoms.

The aim of this study is to develop an eco-friendly lightweight concrete that contains recycled expanded glass aggregate (EGA) and pine resin (PR) and to contribute to sustainable solutions for energy consumption, energy efficiency, and waste management in buildings. Waste EGA (2–4 and 4–8 mm) was sorted based on grain sizes and separately mixed with cement at 15%, 35%, 55%, and 75% ratios of the total volume. Pine resin was added into each mixture at 0.5% and 1% ratios of the total mixture (EGA + cement), and artificial micropores were created in order to enhance total porosity and produce specimens with superior thermal performance. Twenty-four combinations were produced from the specimens, and after they were dried at room temperature for 28 days, their thermal, physical, mechanical, and microstructure properties were analyzed. Results of the study indicated that some properties of the specimens significantly improved. As EGA grain size, ratio, and resin amount increased, density, thermal conductivity, compressive strength, modulus of elasticity, ultrasonic pulse velocity, and water absorption decreased, but porosity and abrasion loss increased. Notably, the water absorption ratios of the specimens remained below 30%, which is the critical value. Consequently, this study suggests that the developed lightweight concrete specimens have the potential to outperform traditional materials in various applications such as mortar, plaster, wall panels, brick or briquette works, and roof and floor concretes due to their superior insulation properties.

In order to determine the damaging effect of different fire temperatures on a segment, experimental research was carried out on the structural performance of segments subjected to different fire temperatures. A temperature plateau occurred when the temperature reached 100°C. There were two temperature layers across the 300 mm thickness of the segment due to heating on one side only, and the temperature gradient within 0–50 mm was much more significant than that within 50–300 mm. The concrete spalling and the cracking initiated on the fired surface of the segment were severe; the maximum depth of concrete spalling on the fired surface of the segment at different fire temperatures was 600°C: 15 mm; 800°C: 45 mm; and 1,000°C: 67 mm; further, the main reinforcement was exposed or even softened. Cracking was also initiated on the unfired surface, and even penetrating cracks developed on the side surface. The damage at both ends of the segment was more severe than that in the center. The fire temperatures were 600°C, 800°C, and 1,000°C, the maximum midspan deflections were 5.27, 7.83, and 11.79 mm, the maximum midspan axial forces were 65.58, 208.41, and 333.63 kN, and the maximum reinforcement stresses were 22.49, 35.08, and 52.63 MPa, respectively. The mechanical parameters of the segment increased during the firing process and recovered during the cooling process, which is equivalent to a loading and unloading cycle for the segment. This study advances the understanding of the structural performance of shield tunnel segments subjected to different fire temperatures.

Effects of polycarboxylate ethers (PCEs) with different charge densities on the time-dependent rheological property (yield stress) were systematically investigated by correlating the charge density of PCE with its adsorption behavior, dispersion capability (including the dispersion rate and dispersion extent), and packing density, fluidity, and hydration kinetics of the cement slurry. Results show that with the increase of charge density, the adsorption rate of PCE increases, which is attributed to the increase of affinity between PCE and cement particles. Based on this, the dispersion rate of PCE is effectively enhanced by increasing its density charge, which results in a faster packing rate, consequently lowering the decrease of the time-dependent yield stress of cement slurry and increasing the fluidity loss over time. Noticeably, the stronger affinity raised from higher charge density leaves fewer PCEs in the solution of cement slurry, which is remarkable at high dosage, and thus leading to a faster fluidity loss or time-dependent yield stress increase. This will also lead to slower hydration kinetics with the same PCE content. These findings can provide guidance for the design of PCE structures in regulating the workability of concrete.

The article presents the results of studies of composite magnesium oxychloride iron-containing binders. The research is aimed at solving the problem of increasing operational durability and expanding the scope of application of magnesium materials. The purpose of the work is to study hydrate formation, hardening, and properties of composite magnesium binders with varying contents of iron compounds. Compositions of caustic magnesite (magnesium oxide) with iron oxides, with ferruginous minerals (andradite and pyrite), and with waste from the enrichment of magnetite ores have been studied. The scientific novelty of the research results lies in the identification of patterns of hydration of magnesium oxychloride binder with a high content (at least 50%) of iron compounds. The mutual influence of magnesium and ferruginous components during hydration and formation of the binder structure has been established. Schemes for the formation of hydrates involving andradite and pyrite have been proposed. The qualitative composition of the main hydrates of composite binders was determined by diffractometric and differential thermal analysis methods. The microstructure of the materials was studied by electron microscopy. The structure of magnesium paste made from a composite binder is formed by crystals of magnesium pentahydroxychloride and amorphous gel-like iron-containing hydrates. In composite binders containing iron oxide or andradite, magnesium hydroxychlorocarbonate is formed. The dependence of the stability of magnesium hydroxychlorocarbonate on the content of magnesium oxide and chloride was revealed. The patterns of the influence of iron compounds on the hydration and hardening of magnesium oxide form the basis for the development of a composite binder containing 50%–70% magnetite ore enrichment waste. A composite binder containing a ferrous man-made component is not inferior in strength properties to caustic magnesite and is characterized by high structural resistance to long-term operational impacts.

Abstract

Expanded polystyrene (EPS) is a widely used material in multiple industries, especially in the construction and packaging of appliances and equipment, owing to its properties of lightweight, sound insulation, thermal insulation, and strength. However, in the final stage of the EPS life cycle, multiple environmental issues arise related to the management of this material’s waste due to the large volume occupied and greenhouse gas emissions. This issue has generated growing interest among researchers worldwide, seeking sustainable alternatives to mitigate these negative impacts. In this study, an alternative for utilizing EPS waste was explored, using it in a diluted form with gasoline to manufacture mortar, where EPS acts as a binder. The properties of the material were evaluated, including compressive strength (CS), density (D), and ultrasonic pulse velocity (UPV), using nonparametric statistical tests and multiple linear regression. Variables such as the weight ratio of EPS, the curing method and temperature, and the pressing load were considered in manufacturing 175 cylindrical samples. The results revealed notable strengths of up to 25 MPa, and correlations were established with nondestructive test data, such as UPV. This research opens new perspectives in the search for sustainable solutions for using EPS waste.

Practical Applications

Optimizing the proportion of diluted EPS using gasoline as a solvent and implementing thermal curing resulted in the material’s outstanding performance. The most imperative results were achieved with a 9% EPS content, followed by 24 h of air drying and an additional 24 h at 110°C. It is relevant to highlight that applying a pressing load of 35 kN led to an additional improvement in compressive strength. The research presents an environmentally friendly curing method characterized by its energy efficiency compared to conventional materials, such as clay blocks. In addition, up to 46% of the solvent evaporates during manufacturing, offering opportunities for solvent recovery techniques. The development of materials, such as dilute EPS mortars, is positioned as a promising and sustainable alternative in construction, with the potential to generate significant savings and environmental benefits. In the construction field, these innovations could find applications in manufacturing masonry blocks, both structural and nonstructural, paving stones, pavements, and filling in structural expansion joints for buildings, among other architectural and structural elements. These applications illustrate the broad spectrum of possibilities of these advanced mortars in the construction landscape.

Use of pozzolanic materials in concrete has become increasingly popular due to their ability to improve long-term strength and durability. To better understand their reactivity and reaction mechanism, proper characterization of their chemical properties is required. X-ray diffraction (XRD) is a widely used nondestructive testing method that can identify the different constituent phases of pozzolanic materials. However, the conventional qualitative approach for XRD analysis can be challenging due to the significant peak overlap and an amorphous hump in the baseline of XRD patterns. To address this issue, this study proposes a deep learning–based automated method for accurately identifying the minerals in pozzolanic materials using XRD patterns. The proposed method involves the deep learning–based peak picking algorithm, establishment of mineral candidates, and automated mineral identification using an optimization process. The performance of the model was validated using eight pozzolanic materials belonging to four classes (slag, fly ash, ground bottom ash, and silica fume).

With the extensive application of coal gasification technology, the production and storage of coal gasification slag continue to increase. Expanding the application scope of coal gasification slag is of positive significance for mitigating environmental pollution. Coal gasification slag has potential in the field of microwave-heated asphalt mixtures; however, there is limited research in this area. This study attempts to investigate the impact of utilizing coal gasification slags as fillers on the self-healing properties of asphalt mixtures. For this purpose, two different types of coal gasification slag in five ratios (within the range of $<0.075\mathrm{mm}$, 10%, 20%, 30%, 40%, and 50%) were used to replace limestone filler (LF) for preparing the Marshall test samples. Firstly, the micromorphology, chemical composition, and microwave absorption properties of LF, coarse slag (CS), and fine slag (FS) were characterized by scanning electron microscopy, X-ray fluorescence tests, and electromagnetic measurement. Afterward, an infrared camera examined the microwave-heating behaviors of different asphalt mixtures under microwave irradiation. Furthermore, semicircular bending fracture–healing–fracture tests were carried out to evaluate the self-healing properties of the asphalt test samples. Finally, the economical aspects were calculated by the cost analysis. Results indicated that CS and FS exhibited excellent microwave absorption performance. However, the excessive use of CS or FS could potentially weaken the thermal diffusion behavior of asphalt mixtures. Moreover, the microwave-heating capacity and self-healing properties of asphalt mixtures containing coal gasification slag were improved compared to typical specimens. It was possible to find that 50% CS or 40% FS could be considered as the suitable alternative because this rate obtained excellent microwave-heating efficiency, while significantly increasing the self-healing ratio. Moreover, the statistical results demonstrated significant differences in microwave-heating capacity and self-healing properties between CS and FS asphalt mixtures. And the results of cost analysis showed that the utilization process can lead to substantial economic advantages. In general, this study proves the feasibility of utilizing coal gasification slag as a microwave absorber to accelerate the efficiency of microwave healing of asphalt mixture. This finding provides a new methodology for consuming coal gasification slag, enabling the reuse of waste resources.

Adding graphene microflakes with excellent mechanical properties to asphalt materials can promote the development of sustainable transportation infrastructure. Recently, graphene oxide–modified asphalt has gained popularity due to its enhanced storage stability, ease of construction, and high-temperature stability. However, the modification mechanism of graphene oxide and polymer modifiers within asphalt remains unclear. This study aims to investigate the mechanism of action of aminated graphene oxide and styrene-butadiene-styrene (SBS) within asphalt and elucidate their influence on the properties of composite-modified asphalt. This research utilized X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), dynamic shear rheometer (DSR), multiple stress creep recovery (MSCR), bending beam rheometer (BBR), and thermogravimetry analysis (TGA) to explore the performance of composite-modified asphalt and the modification mechanism of modifiers. X-ray diffraction and Fourier transform infrared spectroscopy showed that the modification effect was better, the surface wrinkles of modified graphene oxide increased, and the interlayer spacing expanded, which was favorable to its compatibility with asphalt. Conventional test and Brookfield viscosity revealed that composite-modified asphalt possessed favorable high-temperature resistance and plasticity compared to the original asphalt. Additionally, dynamic shear rheological and storage stability tests indicated that the addition of aminated graphene oxide not only improved the viscoelastic properties of asphalt but also enhanced the compatibility between various substances. Multiple stress creep recovery and bending beam rheometer tests measurements confirm that the composite-modified asphalt exhibits superior high-temperature rutting resistance and low-temperature crack resistance. Fluorescence microscopy analysis demonstrated the uniform distribution of the modifier and SBS within the asphalt, while thermogravimetry analysis revealed that composite-modified asphalt exhibited higher thermal stability compared to SBS-modified asphalt. This study holds significant importance in advancing the development and practical application of road modification materials.

There is a noticeable research gap concerning the durability of recycled concrete, a pivotal resource for fostering urban sustainable development. This paper presents the results of an investigation into the bonding characteristics between complete carbonation recycled aggregate concrete (RAC) and reinforcing steel. Test specimens were meticulously prepared, incorporating five distinct replacement ratios (0%, 30%, 50%, 70%, and 100%) of recycled coarse aggregate (RCA). These specimens were subsequently categorized into two groups: one featuring stirrup reinforcement and the other devoid of it. Following accelerated carbonation, pull-out tests were diligently conducted. The outcomes of these tests revealed that specimens with stirrup reinforcement exhibited pull-out failure, whereas those without stirrup reinforcement underwent splitting failure. As the replacement ratio of RCA increased in complete carbonation pull-out specimens, there was an observed decrease in bond strength, slip value, and steel strain. Relative to natural concrete (NC), RAC70 exhibited a decrease in ultimate bond strength of 1.46 MPa, whereas RAC100 showed a decrease of 1.63 MPa. Additionally, the bond-slip curve displayed a diminished slope with higher replacement ratios. Furthermore, the peak value of the bond stress distribution curve shifted toward the free end with an increasing RCA replacement ratio. Complete carbonation recycled concrete exhibited heightened compressive strength, improved bond strength, and a steeper ascending segment in the bond-slip curve. The ultimate bond strength of NC-confined specimens increased by 26.76%. Conversely, for specimens with 70% and 100% replacement rates, the ultimate bond strength only increased by 9.55% and 3.89%, respectively. Furthermore, a constitutive relationship model for the bond slip between complete carbonation recycled concrete and reinforcing steel was formulated based on the empirical findings. Ultimately, the bond stress distribution curve reveals two peak points, observed near the loading end and the free end, respectively, with a more pronounced gap between the two peaks after complete carbonation. As the replacement rate of recycled aggregates increased, the maximum bond stress tended to shift toward the free end. When the replacement rate reached 100%, the gap between the two peak points significantly diminished.

The traditional viscoelastic-plastic constitutive model of asphalt mixture is mostly suitable for the case of small deformation at room temperature, which may have difficulties in the evaluation of the large plastic deformation during the compaction process. In order to evaluate the viscoelastic-plastic rheological characteristics of hot-mix asphalt (HMA) during construction compaction, in this research, the Bodner–Partom (BP) model was revised by considering the strain after the total strain minus the instantaneous elastic strain, and the viscoelastic rheological parameters of the revised BP model were obtained through uniaxial compression creep tests. Also, the creep behavior of the BP model was verified through numerical simulation of the finite-element method. The results found that in the initial creep stage, with the increase of initial compaction density, the instantaneous deformation decreased, and the deformation recovery rate increased after unloading. Correspondingly, the pure viscosity parameter ($\eta$) increased, the hardening constant of the load ($Z$) increased, and the limit value ($D_0$) decreased, indicating that the viscous properties of HMA were enhanced, and the plastic and viscoplastic strain rates decreased. In the stable creep stage, the strain value was the largest under the initial loading. As the number of cycles increased, the HMA strain rate decreased and tended to be stable, and the compaction was basically formed. Correspondingly, $\eta$ gradually increased, $Z$ increased, and the plastic and viscoplastic strain rates decreased. $D_0$ decreased first and then tended to be stable, indicating that the proportion of plastic deformation of single cycle load to total deformation gradually decreased and tended to be stable. With the increase of initial compaction density, the plastic strain of the same loading cycle decreased first and then tended to be consistent. Finally, it was found that the simulated calculation curve further verified the rationality of the BP model. The relevant results can lay a theoretical foundation for analyzing the rheological mechanical properties of HMA in the construction stage.

Traditional methods for repairing concrete cracks using inorganic and organic polymer materials have shown unsatisfactory performance in engineering practice and have also led to significant environmental pollution. However, microbial induced calcium carbonate precipitation (MICP) has been proven to be a clean technology for crack repair in concrete, with the added benefit of improving its mechanical properties. One crucial aspect to consider is the impact of the freeze–thaw process on the durability of concrete. Therefore, it is vital to assess the freeze–thaw durability of MICP-repaired concrete. This study first evaluated the efficiency of crack repair using MICP through water permeability and electrical flux tests. Subsequently, the changes in the apparent morphology, mass, and permeability of MICP-repaired concrete specimens were investigated after undergoing freeze–thaw process in order to assess their durability. The results indicated that mineralized calcium carbonate effectively filled the cracks, enhancing the compactness of the concrete and significantly improving its resistance to freeze–thaw process. The filling of cracks with calcium carbonate alerted the freeze–thaw erosion path in the concrete specimens. Additionally, it was found that increasing the initial width and depth of cracks weakened the impact of MICP on the freeze–thaw erosion resistance of cracked concrete.

The frictional properties of aggregates play a significant role in determining pavement skid resistance. To preserve natural aggregates, reclaimed asphalt pavement (RAP) has been widely used for decades. However, its use in pavement surface courses is often limited due to its unknown frictional properties. Therefore, this research aimed to quantitatively investigate both aggregates and RAP to develop a quick evaluation method for frictional properties. Nine types of aggregates from Tennessee were selected for relative frictional tests. X-ray fluorescence was utilized to quantify the silica dioxide (${\mathrm{SiO}}_2$) contents, establishing correlations with polish stone value (PSV), micro-Deval (MD) abrasion loss, and surface morphological characteristics of aggregates tested via the aggregate image measurement system. Subsequently, to develop a quick way to measure the ${\mathrm{SiO}}_2$ of RAP, laboratory-produced RAP and field-sampled RAP were compared. Laboratory-produced RAP was fabricated and compared with chemical trichloroethylene (TCE) and a hammer-crushed processing method to accurately measure the ${\mathrm{SiO}}_2$ content for the application of field-sampled RAP. Field pavement friction tests were conducted to correlate the laboratory findings to field measurements. Results show that the coefficients of determination (${\mathrm{R}}^2$) of ${\mathrm{SiO}}_2$ from raw aggregates with PSV, MD loss, and texture loss are 0.81, 0.60, and 0.78, respectively. Field-sampled RAP washed by TCE three times could provide more accurate ${\mathrm{SiO}}_2$ content compared with hammer crushing. The field pavement friction test demonstrates that pavements with lower ${\mathrm{SiO}}_2$ contents exhibit reduced skid resistance due to texture loss caused by traffic polishing. These findings offer a straightforward method for evaluating pavement friction and selecting RAP with high skid resistance, where higher ${\mathrm{SiO}}_2$ content provides higher friction properties. As a result, this research provides a valuable strategy for the rapid evaluation of aggregates and RAP for skid resistance.

Abstract

Numerous applications in civil and transportation engineering require concrete with high early strength and good durability properties. Limited studies have revised the performance of high early-strength concrete prepared with calcium sulfoaluminate (CSA) cement and internal curing technology. This study evaluates the strength and durability properties of four concrete mixtures designed with low and high cement contents, incorporated with and without internal curing. Compressive and split tensile strength, autogenous and drying shrinkage, and freezing and thawing resistance were evaluated. High early-strength concrete mixtures with internal curing achieved lower strength and improved durability (shrinkage and freezing and thawing resistance) in comparison with their analogous control mixtures. All concrete mixtures obtained compressive strength higher than 17 MPa (2,500 psi) within the first 4 h. Concrete mixtures with high cement content showed less contraction ($<200\mu \epsilon$) than their analogous control mixtures. The internally cured concrete mixture with low cement content exhibited slight expansion instead of shrinkage. Finally, the freezing and thawing resistance showed great improvement because no or little damage was observed in the internally cured concrete mixtures. The internally cured concrete specimens overcame 300 cycles, whereas the control mixtures reached maximum 164 cycles. The internally cured mixtures with low and high cement contents showed a final relative dynamic of elasticity of 79.7% and 99%, respectively. Based on these results, using CSA concrete for repair works could be feasible with internal curing technique.

Practical Applications

Extensive damage is observed in concrete pavements due to several factors, primarily due to freeze–thaw action and shrinkage. Given that replacing the damaged pavement is expensive and time-consuming, several transportation agencies prefer to restore/repair the degraded portion of pavement using high early-strength concrete. Using high early-strength concrete ensures the pavement achieves its design strength at an early age, which ultimately allows opening the roadway to oncoming traffic in a shorter duration. The use of calcium sulfoaluminate cement instead of ordinary portland cement in high early-strength concrete helps to reduce the overall carbon footprint due to lower energy consumed during calcium sulfoaluminate’s clinkering process, along with relatively faster strength development. Previously, researchers have demonstrated the effectiveness of internal curing to improve durability of high early-strength concrete by better freeze–thaw and shrinkage resistance. Overall, the given study focuses on understanding the benefits of internal curing in high early-strength concrete mixtures prepared with a low and high calcium sulfoaluminate cement content. Irrespective of the cement content, high early-strength concrete mixtures with internal curing had superior freeze–thaw and shrinkage resistance, proving that internal curing effectively improves the durability properties without compromising on its design strength.

Abstract

To mitigate the impact of vehicular emissions on air quality and reduce the concentrations of atmospheric pollutants, this study developed pavement materials with purification functions using ${\mathrm{TiO}}_2$ as a photocatalyst and construction waste as an aggregate. Polymethyl methacrylate (PMMA) acrylic particles were added to enhance the light transmittance of the concrete, and the volume replacement percentages of PMMA replacing aggregate (sand) were 20%, 40%, 60%, and 80%. The physical, mechanical, and photocatalytic properties of the photocatalytic recycled concrete were characterized through testing the water absorption, drying shrinkage, compressive strength, flexural strength, splitting tensile strength, and degradation efficiency. The results showed that the addition of PMMA increased the water absorption, reduced the drying shrinkage, and decreased the mechanical strength (compressive strength, flexural strength, and splitting tensile strength) of the photocatalytic recycled concrete. Notably, PMMA significantly improved the photocatalytic efficiency by increasing the light transmittance, but the promotion effect was weakened when the PMMA replacement percentages exceeded 60%. When the PMMA replacement percentages ranged from 20% to 40%, the photocatalytic recycled concrete maintained good mechanical strength and photocatalytic efficiency. This study presents an effective approach for improving the catalytic efficiency of photocatalytic recycled concrete while concurrently enhancing the utilization rate of construction waste.

Practical Applications

The photocatalytic recycled concrete developed in this study has significant potential in practical applications, especially in urban environments. It can serve as a material for roads, sidewalks, and building surfaces. Due to its air purification function, it can break down harmful pollutants in the atmosphere, such as nitrogen oxides and volatile organic compounds, thereby aiding in the improvement of urban air quality. In addition, the use of construction waste as aggregate promotes the recycling of construction waste and reduces dependence on natural resources.

Chloride transport is one of the most serious problems facing reinforced concrete structures, and coatings can effectively block the intrusion of chloride ions. In order to evaluate the resistance of coatings to chloride ion erosion more quickly and accurately, based on the transport mechanism of chloride and water in coated concrete, a two-dimensional mesoscale model of concrete containing coating, aggregate, and matrix was established in this paper. In response to the transport mechanism of chloride ions in coated concrete, a coupled convection–diffusion numerical model considering the binding effect of chloride, temperature effect, and hydration effect is established. The idealized service life conditions of the coating are introduced, and the influence of coating type, coating thickness, and coating service life on the distribution of erosive agents inside the coated concrete is analyzed. After analysis and research, it is recommended that coating concrete exposed to 3.5% NaCl erosion use a film-forming coating with an expected life of more than 10 years and a coating thickness of at least 1.5 mm, preferably chlorinated polyvinyl chloride (CPVC) and chlorinated polyethylene (CPE) coatings.

Saline–alkaline solutions have a detrimental impact on the durability of bituminous pavements. Thus, it is important to understand the interaction between saline–alkaline environments and bitumen and investigate the corresponding interfacial characteristics. In this study, molecular dynamics simulations were carried out to evaluate the interfacial interaction behavior between different saline–alkaline solutions and bitumen. The interaction energy, mean square displacement, interfacial molecular arrangement, and ${\mathrm{Na}}^{+}$ distribution were analyzed for different bitumen-solution systems. The results revealed that the interaction between bitumen and alkaline salt solutions is more powerful than between bitumen and neutral salt solutions. The damaging impact of different solutions on bitumen can be written as: alkaline salt solution > neutral salt solution > deionized water. Oxidative aging can further strengthen the interaction between saline solutions and bitumen. Bitumen molecules diffusion is the fastest in an alkaline salt solution, as indicated by the high diffusion coefficient. Furthermore, adsorption occurs at the interface between a neutral salt solution and bitumen, leading to interspersing of the salt and bitumen molecules. The water molecules and ${\mathrm{Na}}^{+}$ ions in an alkaline salt solution can penetrate the bitumen surface, leading to a more chaotic molecular arrangement at the interface. The interfacial film formed in the bitumen-alkaline solution system has the highest thickness owing to the extremely strong interaction. Furthermore, ${\mathrm{Na}}^{+}$ ions aggregate at the bitumen surface, and aging accelerates the diffusion of ${\mathrm{Na}}^{+}$ ions toward the bitumen surface.

To enhance the mechanical properties of concrete and inhibit the development of cracks, this study designed and prepared four kinds of shape memory alloy fiber–reinforced concrete (SMAFRC) with different volume contents (0%, 0.3%, 0.6%, and 0.9%), and carried out cube compression, splitting tensile, and bending tests at two temperatures (20°C and 100°C). The SMAFRC failure samples were analyzed by scanning electron microscopy (SEM) and the failure mechanism was explored. The results showed that the SMAFRC specimen with 0.9% SMA fiber content (S-0.9) had the best mechanical properties at 20°C, and the cube compressive strength, splitting tensile strength, and bending strength were increased by 18.22%, 32.34%, and 33.20%, respectively, compared with the pure specimen (S-0). At 100°C, the strength of concrete decreased compared with that at 20°C and the SMAFRC sample with an SMA fiber content of 0.6% (S-0.6) decreased slightly at 100°C, indicating stable mechanical properties. SEM shows that the presence of SMA fibers not only acts as a bridge in concrete, but also blocks the generation and development of microcracks.

Using steel slag in architectural three-dimensional (3D) printing not only enhances its utilization efficiency but also significantly reduces cement consumption, thereby mitigating carbon emissions. This study evaluated the substitution of steel slag for cement in 3D printing and examined its impact on rheological properties, fluidity, green strength, and early hydration microstructure based on alkalinity level, particle-size distribution, and blending quantity. The relationship between the early properties of 3D-printed steel slag cementitious materials and their pore microstructures was investigated using low-field nuclear magnetic resonance (NMR) tests. The results showed that high-alkalinity steel slag has superior rheological characteristics and generates a larger amount of gel water within the initial 30 min of hydration. Steel slag particles ranging from 5 to $20\mathrm{\mu m}$ had the most significant influence on enhancing the rheological properties of 3D-printed steel slag cementitious materials by facilitating the formation of a flocculated mesh structure during early hydration. Optimal rheological performance was achieved with a dosage level of 10% steel slag, effectively reducing porosity and improving compactness in the mortar.

To solve the problem of carbon emissions and pollution of traditional cement, composite geopolymer based on alkali-stimulated slag powder and fly ash was used to solidify loess. The effects of the alkali activator content, the ratio of NaOH to alkali activator, and the ratio of slag powder to silicon-aluminum (Si-Al) raw material on the unconfined compressive strength (UCS) of solidified loess were investigated through different tests. Then, the results from microscopic tests such as scanning electron microscopy (SEM), x-ray diffraction (XRD), and mercury intrusion porosimetry (MIP), as well as the variation results of the UCS, were used to analyze the influencing mechanism of the three factors on the solidified loess. The test results showed that the UCS of the slag powder– and fly ash–based composite geopolymer solidified loess first increased and then decreased with the increasing alkali activator content, and the UCS of the solidified loess also first increased and then decreased with the ratio of NaOH to alkali activator. The UCS increased as the ratio of slag powder to Si-Al raw material increased. The reaction of composite geopolymer solidified loess produces amorphous hydrated calcium silicate hydrates (C─ S─ H), calcium aluminum silicate hydrates (C─ A─ S─ H), and sodium-based aluminosilicate (N─ A─ S─ H) gels. These gels effectively bond and fill the soil particles, leading to a decrease in the number of macropores and total pores in the soil. As a result, the soil becomes more compact and increases the UCS of solidified loess. The results of this study provide a certain basis for engineering applications of the composite geopolymer to solidify loess based on slag powder and fly ash.

Basalt fiber–reinforced polymer (BFRP) is widely used to reinforce concrete due to its high strength, lightweight nature, good corrosion resistance, and low cost. Previous studies have shown that the double-helix BFRP macrofiber has better bond behavior with concrete compared with other types of BFRP fibers. This is attributed to its irregular geometry. The bond-slip behavior between double-helix BFRP macrofiber and concrete is further numerically studied in this study. The corresponding finite-element model is established, and the accuracy of the numerical method is validated by the experimental results based on fiber-matrix pullout tests. The effects of twisted pitches, bundle numbers, and cross-section shapes of the fiber on the bond-slip behavior are extensively investigated and discussed. It is shown by the numerical results that the bond stress and energy-dissipating capacity increase with the decrease of twisted pitches (30, 20, 10, and 5 mm). The bond stress of the fiber with a twisted pitch of 5 mm can be increased by 17.0% at most compared with the fiber with a twisted pitch of 30 mm. Furthermore, it is found that the double-helix BFRP fiber has higher bond stress than the fiber with three or four bundles, with corresponding increases of 11.9% and 16.9%, respectively.