Materials and Structures最新文献

This study investigates the effects of concrete pumping on air content, SAM Number, spacing factor, and freeze–thaw performance. This work focuses on how the air dissolves under pressure and then returns to the concrete at room (20 °C/68°F), cold (8 °C/46°F), and hot (40 °C/104°F) temperatures. The research reveals that concrete pumping leads to a significant reduction in air content, with cold mixtures experiencing higher air loss compared to room temperature and hot mixtures. Despite these changes, freeze–thaw performance remains satisfactory for mixtures with initial air content above 4% and SAM Number below 0.32. The study also observes that the dissolved air bubbles return to the concrete with a similar bubble distribution as was in the original mixture.

Asphalt pavements are constantly exposed to many destructive environmental factors including de-icing salts. The problem of the negative effect of salt ions on the performance and consequently the durability of road pavements occurs mainly in temperate climates and regions directly neighboring saline water areas. The salt ions react chemically with the bitumen components, which consequently changes their electronic structure and results in a weakening of the intermolecular interactions occurring between them. Therefore, this study focused primarily on an investigation into the potential for inhibiting the destructive erosion process of bitumen by its modification with chitosan. Studies involving changes in the acidity of the eroding solution as well as chemical and surface properties of the eroded bitumen were carried out for three different salts (NaCl, MgCl₂, CaCl₂) at varying concentrations, i.e. 5%, 10%, 15% (w/w) after 7 and 28 days of erosion process. Main findings demonstrate that chitosan prevents negative changes in the bitumen physico-chemical properties occurring during the salt erosion process. This effect is especially visible for the bitumen eroded with a solution of MgCl₂ and CaCl₂. For these salts, chitosan biopolymer reduces the introduction of Cl⁻ ions into the bitumen-building hydrocarbon structures and formation of C–Cl bonds, which is demonstrated by a reduction in the pH changes of the eroding solutions. In addition, chitosan biopolymer inhibits leaching of organic matter from the bitumen, prevents C = O groups formation and reduces the negative effects of de-icing salts on the cohesion energy of the bitumen.

The effect of agitation during the early-age hydration on thixotropy and morphology of cement paste prepared with and without superplasticizers (SP) is investigated by applying penetration test, small amplitude oscillatory shear sweep test (SAOS), isothermal calorimetric test, scanning electron microscopy (SEM) and energy dispersive X-ray analyses (EDX). The results show that the agitation of cement paste during the induction period increases the heat flow rate and destroys existing structures of samples without changing the mineral composition of samples. Yet, if the agitation is applied during the acceleration period, the heat flow rate is significantly lowered and the morphology and mineral composition of samples undergo irreversible change, freshly formed syngenite is destroyed and no longer restored. The penetration force and the static yield stress grow linearly during the induction period and exponentially during the acceleration period. Agitation during the induction period destroys the structure, which causes the static yield stress and the penetration force values becoming nearly equal to zero. However, during the acceleration period, even after agitation the static yield stress and the penetration force exhibit high residual values, which indicates the impact of hydration to the structural build-up.

Conventional impervious pavements occupy a large proportion of most cities, due to urbanization and the extensive development of transportation infrastructure. These pavements cause environmental problems such as flooding and urban heat islands. Pervious concrete (PC) is a special type of concrete, characterized by a porous structure that allows water infiltration and has the potential to reduce the effects of urban heat islands through cooling by evaporation. However, due to the low water absorption and retention properties of PC, it cannot effectively meet evaporative cooling needs. This study investigates the thermal behavior of PC in dry and wet laboratory conditions under controlled climatic conditions. Three types of PCs were fabricated: gravel-based PC (PCG) and Pozzolan-based PC (PCP1), having almost the same particle size distribution, and less coarse Pozzolan-based PC with a monodisperse particle size distribution (PCP2). The results show that the surface temperature of pozzolan-based PC is up to 4 °C higher than that of PCG in dry conditions. The partial immersion test shows that the use of pozzolan aggregates in PC mixtures improves the water-absorption properties compared to PCG. In addition, pozzolan-based PC can have a surface temperature up to 11.7 °C lower than PCG during the daytime and up to 3 °C lower during the nighttime. The use of lightweight aggregates with high water-absorption coefficients in the PC mix is only recommended in wet conditions.

It is well known that prolonged rainwater erosion can adversely affect the surface texture of asphalt pavements, leading to a rapid decline in their skid resistance. This study utilized a small-scale accelerated loading device, a high-precision 3D scanner, and digital image processing technology to investigate the surface texture wear process and skid resistance decay trends of basalt asphalt pavement and steel slag asphalt pavement under water erosion and traffic load. The results indicate that under submerged conditions, the skid resistance (BPN) of asphalt pavement declines rapidly during the first 500,000 load cycles, and the rate of decline gradually stabilizes after 500,000 cycles. After 1.2 million load cycles, the BPN of basalt pavement decreased by 28.10%, while that of steel slag pavement decreased by 21.18%, indicating that the skid resistance of steel slag pavement is significantly better than that of basalt pavement. Texture parameters—namely, root mean square height, peak material volume, core material volume, void volume of the core, and valley void volume—exhibited the same decay trend as BPN. The average correlation coefficients between BPN and texture parameters were 0.846, 0.848, 0.898, and 0.916, respectively, indicating that texture parameters can be used as evaluation indicators for skid resistance decay. Finally, the decay of pavement skid resistance was predicted using an exponential decay equation.

Hot weather concreting has gained significant attention in recent years due to the increasing annual mean temperature. The accelerated hydration process under high temperature curing conditions can lead to premature hydration products, therefore, retarding admixtures are recommended to control the setting time. Various non-destructive methods were analyzed to estimate the setting time of cement-based materials. However, the evaluation of delayed cement hydration with added retarding admixtures has not been reported. This study aims to monitor the two non-destructive methods, electrical resistivity and ultrasonic pulse velocity, during the initial 24 h in cement pastes with added calcium lignosulfonate, the most common retarder. The setting time of cement pastes, cured at temperatures of 20, 30, and 40 ℃, was evaluated based on the rise time of these non-destructive measurements. Further, the effect of added retarder on the setting delay in cement paste was discussed and compared with the Vicat needle test. The results of X-ray diffraction and thermogravimetric analysis at the rising time of electrical resistivity revealed that the use of the retarding admixture induces delayed hydration reaction of C₃S, C₂S, and C₃A, key hydration products influencing the setting delay.

The aim of this study is to clarify the effect of roughening the surface of normal concrete (NC) substrates on the interfacial bonding performance, and the effect of substrate surface roughness and coarse aggregate area on the interfacial bonding performance was investigated by digital image technology. The results showed that the shear damage model of the roughened ultra-high performance concrete (UHPC) and NC composite specimens was divided into linear elasticity, yielding, and damage stages; the failure loads of the gouge interface group and the high-pressure water jet interface group were elevated by 18.3–33.9% and 43.0–140.0%, respectively, compared with those of the smooth interface group. In addition, the chiselled interface group and high-pressure water jetting interface group obtained an overall failure load of 44.2–50.0% and 53.4–90.0%, respectively; the exposed area of coarse aggregate on the substrate surface and the roughness of the concrete surface of the substrate showed a linear correlation with the interfacial shear strength, and the presence of coarse aggregate provided more mechanical anchorage points at the interface and increased the contact area with the UHPC. Substrate surface roughness increased the probability of steel fibre insertion into the groove of the substrate, and the formation of pin effect and bridge effect at the interface improved the shear slip capacity and bond strength at interface failure; substrate concrete surface roughness is a key parameter affecting the secondary damage of the UHPC-NC interface, and the degree of interface damage increased linearly with the increase of roughness.

The paper focuses on investigating a novel sandwich panel, which incorporates extruded polystyrene foam boards placed between the inner and outer concrete panels. These are connected using various types of shear connectors to achieve a composite action, meeting the bending resistance requirements. To study the structural performance of the sandwich panels, three representative specimens were designed. These specimens employed different types of shear connectors, namely, steel truss connectors with inclined bars at 45°, and GFRP connectors with inclined bars at 45° and 30°, respectively. The specimens underwent full-scale testing using a step-by-step four-point loading method. The test results indicated shear-compression failure of the specimens, exhibiting consistent failure modes across all specimens, and a composite action in resisting bending moments. Based on theoretical stress–strain diagrams and considering the slip phenomenon between the concrete panel and the connectors, this paper establishes an analytical model. It can reasonably estimate the ultimate load-carrying capacity of sandwich insulation panels under shear-compression failure, which has implications for subsequent engineering applications.

Kenaf geopolymer for 3D printing is a promising representative for comprehensive and intelligent utilization of industrial and agricultural wastes. Aiming at exploring the effect of raw materials’ proportion on printing performances, alkali activator dosage (10 wt.%, 15 wt.%, 20 wt.%) and mass ratio of ground granulated blast furnace slag (GGBFS) to fly ash (FA) (15:85, 20:80, 25:75) were adopted as main variables in this paper. Results have shown that increasing the activator dosage and decreasing the mass ratio of GGBFS to FA can improve the flowability, and adjusting these two parameters can tailor the fluidity to a suitable range. Moreover, dry density of kenaf geopolymer was more significantly affected by alkali activator dosage compared with mineral powder ratio, and lightweight characteristic due to kenaf participation effectively improved shape retention ability of printed specimens. Besides, microstructure analysis further confirmed that with appropriate alkali activator dosage and mineral powder ratio, high level of geopolymerization reaction can be achieved to produce enough gel product for a satisfactory internal structure, which externally manifested as excellent printability and mechanical strength. Finally, formula with alkali activator dosage of 15 wt.% and GGBFS to FA mass ratio of 25:75 was recommended for satisfactory printing performance and mechanical properties.