Materials and Structures最新文献

The design of post-installed lap splices typically relies on provisions derived from cast-in-place reinforcing bars (rebars). However, some bond characteristics of post-installed rebars show significant differences compared to cast-in-place rebars, especially when using high-performance mortars. To quantify the bond strength for design purposes, a sound understanding of differences in bond stress distribution, splitting failure mode, and load transfer mechanisms is crucial. This study offers new experimental evidence on these aspects, focusing on the impact of different high-performance injection mortars, lap lengths, and the incorporation of hooked steel fibers in the concrete. Direct tension tests were conducted on spliced post-installed and cast-in-place rebars. Fiber-optic sensors were used to measure strains quasi-continuously on the rebars, minimizing interference on the bond. The findings reveal that post-installed lap splices yield slightly higher bond strength than their cast-in-place counterparts, mainly due to the higher bond stiffness of the mortars. However, this advantage is limited by the bond behavior of the cast-in-rebar within the post-installed lap splice, particularly in conditions of poor confinement. In ordinary strength concrete, cast-in-place rebars exhibit an approximately constant bond stress distribution, as typically assumed for design purposes; by contrast, post-installed rebars show a pronounced non-linear distribution. Furthermore, an addition of steel fibers alters the bond stress of the rebars, resulting in a non-linear distribution in all cases. The study reveals a 20% increase in bond strength of lap splices in concrete reinforced with 80 (hbox {kg}/hbox {m}^3) of steel fibers. Moreover, they improve the ductility of bond failure for post-installed lap splices.

The structural performance of asphalt concrete is highly dependent on its thermal properties, especially in regions where daily or seasonal temperature variations are significant. In mechanistic design methods, thermal properties (specific heat, thermal conductivity, and coefficient of thermal expansion) are necessary to estimate the thermal behavior of asphalt concrete. However, the measurement of these properties is still a challenge, not only because of the heterogeneous structure of asphalt concrete but also because of the limitations imposed by the size of the test samples and the reliability of the measurement methods. In this study, a practical method, the Transient Plane Source (TPS), is proposed to measure the thermal conductivity of laboratory-produced asphalt concrete samples. To determine how thermal conductivity is affected by the maximum aggregate size, air voids, and aggregate source, a series of asphalt mixtures are compacted using the Superpave gyratory compactor to produce test samples. To determine the possible relationship between microstructural and thermal properties, which has not been investigated in previous studies, an image analysis is also performed to calculate the number of contact points and the total aggregate area in each sample. The statistical analyses show that all mixture properties, i.e., maximum aggregate size, air void content, and aggregate source, are significant, with the aggregate source having the greatest influence on the thermal conductivity of the samples. It is also shown that the TPS method is sensitive to the properties of the contact area, which significantly affects the reliability of the measurements.

In a variety of applications, such as in tidal zones, abutments of bridges and concrete tunnel linings, reinforced concrete is exposed to both carbonation and chloride ingress. The exposure can be either simultaneous or sequential. However, durability design rarely considers synergistic effects due to carbonation and chloride ingress, even though this may have detrimental consequences for performance. Comparative implications of exposure sequence across different concrete compositions are also unknown. In this study, an experimental investigation on the effects of the sequence of carbonation and chloride ingress was conducted, using two concretes which differ by 50% cement replacement with ground granulated blast furnace slag (GGBS). Specimens were exposed to a combination of 10% CO₂ accelerated carbonation and immersion in 3% sodium chloride solution, in either sequence, and compared with companion samples subjected to only one of these aggressive environments. The extent of carbonation was measured using phenolphthalein indicator solution, while silver nitrate and Rapid Chloride Testing provided indicators of the chloride ingress. For both concrete mixes, specimens with prior chloride ingress exhibited a decreased rate of carbonation when compared to specimens with no prior exposure. Conversely, specimens with prior carbonation displayed an increased rate of chloride ingress compared to non-carbonated counterparts and a step in the acid soluble chloride content in the region of the carbonation front. The concrete composition appeared to play a role since a greater increase in chloride diffusion coefficient due to prior carbonation was observed in the mix with 50% GGBS replacement than the mix without. These findings suggest that in concrete structures exposed to air and saline environments, the effects of sequential exposure should be characterised.

The use of ice as structural material has two main concerns: the low strength and the brittle failure of the structures. With the aim of finding a solution to these problems, an experimental campaign, performed on fiber-reinforced ice (FRI) samples, made with plain water and bio-fibers, is presented in this paper. In total, 12 ice prisms were cast at − 18 °C with a different content of fibers, and then tested in three-point bending and uniaxial compression. Test results indicate that the presence of a reinforcement increases both flexural and compressive strength with respect to plain ice. Moreover, FRI is a tougher material, as multiple cracking and deflection hardening behavior can be observed in the flexural tests. However, the mechanical performances of plain ice are not always enhanced by the fiber-reinforcement. Therefore, an empirical model, capable of predicting the optimal content of bio-fibers, is also proposed.

Bitumen-rubber composite binder (BRCB) has great potential to construct durable road pavement infrastructures that can withstand the environmental aging. However, the aging behavior of BRCB has not been fully investigated so far, especially from the perspective of biphase system with the phase interactions consideration. Therefore, this study investigated the progressive aging behavior of BRCB in terms of separate bitumen phase and rubber phase as well as the biphase interactions, to further understand the mechanisms behind. The results showed that the bitumen phase gradually dominated rheological performance of BRCB with the progressive aging. On the contrast, the fatigue resistance of BRCB was constantly controlled by its rubber phase in the aging process. Secondly, the rubber phase gradually dissolved during the aging with a decrease in the crosslinking density, although the change rate slowed down with the aging duration. The breakdown pattern of rubber structure was further identified as the simultaneous scission of crosslinking bonds and main chains. Besides, biphase interactions during progressive aging primarily included the absorption of light components from bitumen phase into rubber phase and the release of long rubber molecular chains and fillers from rubber phase into bitumen phase. Overall, the progressive aging of BRCBs can be considered as the combined effect of the secondary bitumen-rubber biphase interactions after the first-stage production and thermal oxidation of the bitumen phase.

Graphical abstract

Synthetic fibers deforming over time can be a concern in structural design, particularly in serviceability limit states. Short-term pullout tests are commonly used to predict fiber–matrix interactions, but even in this case, an individualized evaluation of the pullout behavior of single fibers oriented parallel to the load direction may not be sufficient to predict the efficiency of the composite. In the present work, short- and long-term pullout tests were performed with fibers oriented at angles of 15°, 30°, and 45° to the direction of the load to investigate the influence of macro synthetic fibers orientation on fiber–matrix interactions. In short-term tests, optical microscopy images were obtained on the pulled-out fibers to correlate the surface degradation of the fibers with the stress versus strain curves. In quasi-static pullout (short-term), small reductions in pullout strength were observed for all fibers and angles, in addition to an intensive degradation of their surfaces owing to the significant snubbing effect of this type of fiber. In contrast, for the long-term tests, a creep reduction was observed with increasing fiber inclination angle caused by the creep reduction of the fiber due to non-axial loading and additional force components produced by the deviation of the axial force. The parameters of Burgers rheological model were written as a function of the fiber orientation angle, with excellent adjustment to the experimental data.

The use of biochar as a concrete constituent has been proposed to reduce the massive carbon footprint of concrete. Due to the low density and complex porosity of biochar, microstructural analysis of Portland cement-biochar composites is challenging. This causes challenges to the improvement of the micro-scale understanding of biochar composite behavior. This work advances the microstructural understanding of Portland cement composites with 0, 5, and 25 volume percent (vol%) of cement replaced with wood biochar by applying common characterization techniques of mercury intrusion porosimetry (MIP), gas sorption, scanning electron microscopy, and isothermal heat flow calorimetry (HFC) in conjunction with ¹H nuclear magnetic resonance (NMR) and micro-X-ray computed tomography (XCT) analysis techniques. The combination of these techniques allows a multi-scale investigation of the effect of biochar on the microstructure of cement paste. NMR and XCT techniques allow the observation and quantification of the pore space. HFC and MIP confirmed that biochar absorbs moisture and reduces the effective water-cement ratio. Gas sorption, MIP, and NMR shows that 5 vol% replacement does not significantly affect the gel and capillary pore structures. Results from XCT (supported by MIP and NMR) show that biochar can reduce the formation of larger pores. Importantly, XCT results suggest that biochar can act as a flaw in the microstructure which could explain reductions in the mechanical properties. Overall, the mechanical properties already analyzed in the literature are consistent with the microstructural changes observed, and these results highlight the need to carefully tailor the volume fraction of biochar to control its effect on the paste microstructure.

The chemical reaction between CO₂ and a blended Portland cement concrete, referred to as carbonation, can lead to reduced performance, particularly when concrete is exposed to elevated levels of CO₂ (i.e., accelerated carbonation conditions). When slight changes in concrete mix designs or testing conditions are adopted, conflicting carbonation results are often reported. The RILEM TC 281-CCC ‘Carbonation of Concrete with Supplementary Cementitious Materials’ has conducted a critical analysis of the standardised testing methodologies that are currently applied to determine carbonation resistance of concrete in different regions. There are at least 17 different standards or recommendations being actively used for this purpose, with significant differences in sample curing, pre-conditioning, carbonation exposure conditions, and methods used for determination of carbonation depth after exposure. These differences strongly influence the carbonation depths recorded and the carbonation coefficient values calculated. Considering the importance of accurately determining carbonation potential of concrete, not just for predicting their durability performance, but also for determining the amount of CO₂ that concrete can re-absorb during or after its service life, it is imperative to recognise the applicability and limitations of the results obtained from different tests. This will enable researchers and practitioners to adopt the most appropriate testing methodologies to evaluate carbonation resistance, depending on the purpose of the conclusions derived from such testing (e. g. materials selection, service life prediction, CO₂ capture potential).

The quality of the air void system is essential for the frost resistance of concrete. Therefore, it is crucial to assess the quality of the air void system in hardened concrete in an appropriate way. In this study, the shape from focus (SFF) method is applied to identify the focused image of each point on the polished surface of hardened concrete and thereby acquire the depth information of the entire surface. The performance of various focus measure operators and window sizes is evaluated. Thereafter, the selected focus measure operator and window size are applied for the SFF analysis of all samples. Based on the obtained depth map, the voids are identified, and the void parameters of the hardened concrete are determined by an automated procedure. The main advantage of the SFF method is that it is possible to carry out an automated air void analysis without contrast enhancement.

This paper presents the results of experimental investigations on six-layered, homogeneous glulam beams made of Italian short supply chain beech (Fagus sylvatica L.). At first, the beams were produced and mechanically characterized for bending, then, they were employed to realize timber-timber composite joints and tested under quasi-static monotonic loading. The test configurations were adopted to reproduce connections used in timber-to-timber composite structures for applications in new constructions. Outcomes in terms of connection stiffness, strength, static ductility and failure modes are presented and discussed. Moreover, the experimental stiffness were used to carry out analytical verification at the serviceability and ultimate limit states to extend the validity of the proposed screw and specimen’s configurations.