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As a high-performance composite material, polyvinyl alcohol (PVA) fiber-reinforced engineered cementitious composites (PVA-ECCs) are often used in sub-elevated temperature environments (< 200°C) for long-term service. However, the effect of exposure time in sub-elevated temperature on the performance of PVA-ECCs is not still clear. In this study, the properties of PVA-ECCs in sub-elevated temperature environments were examined, with emphasis on the effect of exposure time (1, 3, 6, and 9 h). Furthermore, the microstructure and tensile behavior of single PVA fibers exposed to a sub-elevated temperature environment was investigated. Results showed that PVA-ECCs have enhanced tensile ductility at different sub-elevated temperature exposure times below 100°C. However, the tensile properties of PVA-ECCs at 150°C showed a strain retraction phenomenon at 6 and 9 h. The strain-hardening characteristics of PVA-ECCs gradually disappeared with the increase of the sub-elevated temperature exposure time at 200°C. The results of thermogravimetric-differential scanning calorimetry (TG-DSC), Fourier transformation infrared spectroscopy (FTIR), and scanning electron microscope (SEM) revealed that the degradation of PVA fibers was accelerated by exposure time in sub-elevated temperature, which caused the reduction of mechanical properties of PVA-ECCs. [ABSTRACT FROM AUTHOR]

The application of waste tire rubber in the concrete industry is one of the most effective and environmentally friendly ways to solve the problem of so-called black pollution. However, the application of rubber particles (RP) in the concrete industry is hindered by their low stiffness and poor bonding performance with the cement matrix. In this paper, a novel modification method of polydopamine (PDA) modification was used to enhance the hydrophilicity of the RP surface. The surface morphology, element content, and hydrophilicity of RP treated with PDA and KMnO4 were analyzed through scanning electron microscope and energy dispersive spectrometer (SEM-EDS) and sedimentation methods. The unit weight, ultrasonic pulse velocity (UPV), compressive strength, compressive toughness, toughness index, and flexural strength were taken to determine the mechanical properties of ultrahigh-performance rubber concrete (RUHPC). The results showed that the adhesion between paste and RP and the sedimentation rate of RP with PDA or KMnO4 treatment methods exhibited a remarkable improvement. Compared with unmodified RUHPC, the basic mechanical properties of KMnO4 -modified RUHPC were improved slightly, and its toughness index was improved greatly. SEM and EDS results confirmed that PDA was deposited on the surface of RP, eliminating the gap of interface transition zone and significantly enhancing UPV, compressive strength, and flexural strength of RUHPC by 4.1%–5.8%, 17%–20%, and 6.1%–9.8%, respectively. In addition, the compressive toughness of unmodified and KMnO4 -modified RUHPC decreased, whereas that of PDA-modified RUHPC increased significantly. It can be concluded that the mechanical properties of RUHPC with treated RP have improved significantly and the application of RP in ultrahigh-performance composites (UHPC) offers more opportunities for recycling RP. [ABSTRACT FROM AUTHOR]

MgO-based cementitious materials have attracted increasing attentions due to their advantages of low energy consumption, fire resistance, and low CO2 emission. This study develops a new reactive MgO cement (RMC)-silica fume (SF) engineered cementitious composite (ECC). The effects of RMC∶SF mass ratio, fiber type, fiber content, and curing conditions on the mechanical properties and strain-hardening behavior of the RMC-SF ECC are investigated. The hydration characteristics of RMC-SF ECC under different curing conditions were studied through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) analysis. It was found that the RMC-SF ECC obtained a tensile strain capacity of 5.7%. Polyethylene (PE) fibers achieved higher compressive strength, and polyvinyl alcohol (PVA) fibers yielded better tensile strain capacity. A RMC-SF mass ratio of 1∶1 obtained a higher ductility under high temperature curing at 50°C. Although carbon dioxide curing improves the mechanical properties of RMC-SF ECC due to the formation of MgCO3 , it greatly reduces its ductility. A large amount of brucite and M─ S─ H gels were generated under room temperature curing with plastic wrap. [ABSTRACT FROM AUTHOR]

This study aims to compare the microstructural and mechanical properties of lightweight polymer concrete (LPC) and lightweight ordinary cement concrete (OCC) before and after exposure to a corrosive acidic environment. In this regard, LPC mixes were prepared with five different unsaturated polyester resin contents (15%–27%) and combinations of lightweight expanded clay aggregate (LECA) and natural river sand. Thereafter, specimens were exposed to sulfuric acid solution for up to 90 days. In addition to visual inspections, physical and mechanical properties [mass and volume changes, compression strength, and modulus of elasticity (MOE)], petrographic inspections, and scanning electron microscope investigations were carried out. Mechanical results showed that LPCs had by far better mechanical features compared with OCC, where at the optimum mix comprised of 24% polymer, reduction of 6% for compression, and 7% for MOE was observed, whereas these values were respectively 70% and 55% for OCC at the same conditions. Outcomes of microscopic observations indicated that increasing the polymer content resulted in strengthening the bond between binder and aggregates where pores were filled by resin. In addition, the impact of the acidic solution on the LPC mixes was less pronounced than for OCC mixes, and lamination of the matrix and affected polymer areas were reduced in LPC mixes with a resin content of 24% and 27%. After 90 days, a slight reduction in properties of LPC specimens and degradation was observed due to the reaction of the acid with the surface of LPC specimens. [ABSTRACT FROM AUTHOR]

This paper is based on the basic mechanical properties and volume deformation test, as well as the characterization of hydration products and microstructure properties such as x-ray diffraction (XRD), scanning electron microscope (SEM), and mercury injection method (MIP). The effects of CO2 curing on the mechanical properties, hydration product composition, and microstructure of magnesium oxysulfate (MOS) cement were studied. The results show that the carbonation properties of MOS cement are closely related to the standard air curing age after demolding. The CO2 curing immediately after demolding is not conducive to the development of compressive strength of MOS cement, while the compressive strength of MOS cement in standard air curing for 14 or 28 days after demolding is increased during CO2 curing. The CO2 can prevent the continuous hydration of residual MgO to form Mg(OH)2 or 5·1·7 phase, which affects the development of the compressive strength of MOS cement. The appropriate addition of low-calcium fly ash (L-FA) can compensate the effect of CO2 on the mechanical properties of MOS cement. During CO2 curing, some Mg(OH)2 is transformed into an amorphous phase to increase the compactness of its microstructure and a small amount of Mg(OH)2 reacts with CO2 to form MgCO3·3H2O. The results provide an important theoretical and experimental foundation for the further study of the effects of CO2 on the properties of MOS cement and the use of CO2 curing modification. [ABSTRACT FROM AUTHOR]

Rubber powder is widely used to modify base asphalt binder to improve the low-temperature performance and crack resistance of asphalt mixtures. This paper investigates the aging mechanism and performance evolution of rubber-modified asphalt in terms of three levels: rubber powder, rubber-modified asphalt, and rubber-modified asphalt mixture. The multilevel analysis results show that mass loss, phase transition, and melting occur during the process of rubber powder aging, and scanning electron microscope images indicate that the surface morphology of the rubber powder becomes smoother. The aging of base asphalt leads to the volatilization and oxidation of light components, resulting in the production of carbonyl and sulfoxide groups and a reduction in the swelling between the rubber powder and aged base asphalt. The aging of rubber powder leads directly to a significant decrease in the modification effect of the rubber; thus, rubber powder aging plays a more decisive role in the degradation of rubber-modified asphalt than base asphalt aging. By contrast, the performance indicators of aged rubber-modified asphalt are able to reach their highest values due to the effective blending between the rubber powder and base asphalt, which results in an enhanced antiaging capability. The sensitivity of all performance indicators of rubber-modified asphalt to the aging temperature is relatively low, except for the 5°C ductility index. Therefore, the mixing temperature needs to be strictly controlled to a specified value when rubber-modified asphalt is applied in cold areas that require low-temperature stability of the mixture. As for the aged rubber-modified asphalt mixture, the dynamic stability improved significantly, the ultimate tensile strain at low temperature decreased, and the antifatigue performance decreased to different extents under different stress ratios. [ABSTRACT FROM AUTHOR]

Service life of concrete structures depends on the structure design, construction management, and the durability of concrete in aggressive exposure conditions. The old Fengman Dam is an 80-year-old gravity dam, and due to design and construction defects, it has worked with leakage and freeze-thawing problems for more than 70 years. In this study, the durability of the dam concrete is investigated, and a coring program is conducted. The static and dynamic mechanical properties of aging dam concrete (ADC) are tested and the micropores are also observed by scanning electron microscope. Young laboratory concrete (LC) with similar compression strength of the ADC is prepared for comparison. The experimental results show that despite the harsh working conditions, the ADC exhibits good mechanical characteristics in terms of static and dynamic strength and microporosity of interfacial transition zones. The dam concrete is proved to exhibit good durability, and the service life of old Fengman Dam was limited by the poor construction management. [ABSTRACT FROM AUTHOR]