Concrete Zone Research Articles

Effect of multilayer casting technology of self-compacting concrete slabs on the load-bearing capacity of a layer-to-layer joint

The article summarizes research on the impact of the technology of casting multiple concrete layers on the load-carrying capacity of a layer-to-layer joint in the case of self-compacting concrete. The studies involved slab elements having dimensions of 1200 × 600 × 150 mm concreted from a single casting point. Two technological variants of practical realization of a joint between SCC layers were analysed: no interference in the first layer and mechanical interference into the first layer. Three delay times – 15, 30 and 60 min – were introduced between successive layers. The load capacity of the joint was found through tests of tensile strength by splitting cubic samples cut out of the slabs. X-ray computed tomography (CT) was employed to obtain images of the concrete layer contact zone. On the basis of the outcome of the strength tests and the analysis of the contact zone between the layers, it is concluded that there are three factors determining the reduction of the load-bearing capacity of the joint: delay between successive layers, distance to the casting point, first layer preparation technology. The present-day recommendations and standard guidelines concerning multilayer casting using self-compacting concrete seem to be insufficiently detailed. New recommendations are proposed aiming to limit the risk of cold joint formation and thereby the decrease of the load-bearing capacity of a joint.

Study on fracture parameters and fracture process zone of manufactured-sand recycled aggregate concrete

This paper studied the fracture parameters and fracture process zone (FPZ) of manufactured-sand recycled aggregate concrete (MSRAC) with different replacement rates of recycled coarse aggregate (RCA) by three-point bending test on single-edge notched beams. A threshold crack identification method (TCIM) was proposed to locate the crack tip and identify FPZ. Combined with TCIM and digital image correlation (DIC) technology, the fracture mechanism of MSRAC was investigated and the FPZ evolutions of MSRAC and manufactured-sand normal aggregate concrete (MSNAC) were comparatively analyzed. The results indicated that the crack tip and FPZ could be well identified by TCIM and it had good stability. The initial fracture toughness of MSRAC was not obviously different from that of MSNAC, while the unstable fracture toughness and fracture energy of MSRAC were significantly reduced. The brittleness of MSRAC increased with the RCA replacement rates. According to the difference of fracture behaviors between MSRAC and MSNAC, the FPZ evolutions of MSRAC were divided into three stages, including FPZ stable development, FPZ unstable development, and FPZ retraction development. The FPZ length increased with the RCA replacement rates while the FPZ width and FPZ area were opposite. The RCA reduced the aggregation interlocking effect of MSRAC, resulting in a faster crack growth rate and fewer secondary cracks and damage behaviors of MSRAC than MSNAC.

Experimental investigation on the mechanical and interfacial properties of fiber-reinforced geopolymer layer on the tension zone of normal concrete

This study aimed to understand the mechanical and interfacial properties of fiber-reinforced geopolymer (FRG) layer on the tension zone of normal concrete. A trial mix of FRG using local materials obtained in Hokkaido was investigated. Compression and splitting tensile tests on the FRG test pieces were conducted under different mixing procedures, constituents, volume fractions of the polyvinyl alcohol fiber, curing conditions, and ages. Flexural tests using FRG-normal strength concrete (NSC) composite specimens with different FRG layer thicknesses were carried out. By placing the FRG layer on the tension side of the composite specimen, the flexural strength and energy absorption were significantly increased. The flexural strength for the NSC alone was 3.5 MPa, while the FRG-NSC composite specimens showed higher flexural strengths of 6.1–6.6 MPa. Also, XRD, FTIR, and SEM analyses were carried out for the FRG samples.

The microstructure evolution of ballastless track high-strength concrete exposed to compressive and flexural fatigue loads

X-ray micro-CT method was applied to reveal the microstructure evolution of ballastless track high-strength concrete under fatigue loads. The CT image, porosity, pore volume distribution and aspect ratio of pores were analyzed. Results show that a visible crack is formed at stage II when concrete is under compressive fatigue loads. However, the pores around the interface transfer zone of compressive zone concrete are connected at stage III for concrete under flexural fatigue loads. As the microstructure of concrete is damaged and the single pores tend to be connected, the aspect ratio of pores is generally decreased during fatigue damage.

Mechanical Properties and Uniaxial Failure Behavior of Concrete with Different Solid Waste Coarse Aggregates.

To reveal the differences between the mechanical properties of solid waste coarse aggregate concrete and natural coarse aggregate concrete (NCAC) under equal strength, the basic mechanical properties of coarse aggregate concrete with seven different solid wastes (i.e., self-combusted coal gangue, uncombusted coal gangue, marble sheet waste, granite sheet waste, iron waste rock, recycled concrete, and self-combusted coal gangue ceramicite) were tested, and the trends in failure morphology, elastic modulus, and the stress–strain full curves of the different solid waste coarse aggregate concretes were analyzed and compared with NCAC. Finally, the interfacial structure of the concrete was characterized by SEM. The results showed that C30 strength grade concrete was prepared with different solid waste coarse aggregates; however, the 28 d compressive strength, split tensile strength, axial compression strength, flexural strength, and elastic modulus of the concrete was 35.26–47.35, 2.13–3.35, 26.43–42.70, 2.83–3.94, and 17.3–31.2, respectively. The modulus of elasticity of the solid waste coarse aggregate concrete was smaller than the NCAC under equal strength, with a maximum difference of 45%. The peak compressive strain and ultimate compressive strain were larger than the NCAC, with a maximum difference of 43%. The crushing value of the solid waste coarse aggregate affected the splitting tensile strength, flexural strength, and modulus of elasticity of the concrete to a greater extent than the compressive strength. The transition zone at the concrete interface of the coarse aggregates with different solid wastes varied widely. The porous micro-pumping effect of the self-combusted gangue and self-combusted gangue vitrified reinforced the concrete interface transition zone, and the polished surface of sheet waste, uncombusted gangue, and recycled concrete aggregate surface adhesion weakened the interface transition zone; Finally, the uniaxial compressive stress–strain curve model for concrete with different solid waste coarse aggregates was established based on the Guo Zhenhai model.

Evaluation of interface rapid bond strength between normal concrete and ternary system fast setting and rapid hardening self-compacting concrete

• The rapid bonding performance between NC substrate and TSSCC was investigated. • The bond strength meets the requirements of the executive standard within 2 hours. • Surface-milled revealed higher bond strength of NC/TSSCC composite. • The microstructure and bonding model of the NC/TSSCC interface were studied. • TSSCC is suitable for rapid repair of the reinforced concrete structures. The ternary system (Ordinary Portland cement-sulfoaluminate cement-anhydrite) fast-setting and rapid-hardening self-compacting concrete (TSSCC) is a new type of green and low-carbon material. In this study, the interfacial rapid bond strength and bonding mechanism between ternary system self-compacting concrete (TSSCC) as rapid repair material and normal concrete (NC) substrate was investigated at different ages. Two surface treatment methods, including milling and grooving, were employed to improve the bond properties. Meanwhile, the hydration products of TSSCC and the microstructure of the bonding interface were investigated using TGA-DSC and SEM. The results show that the bond strength of the surface-milled NC/TSSCC composite is about 1.2 times that of the surface-grooved composite. Due to the fast setting and rapid hardening of the ternary system, the 2 h bond strength of NC/TSSCC attained about 50 % of the 28d bond strength. Similarly, the 2 h slant shear bond strength of the NC/TSSCC composite satisfied the 7 d slant shear strength value recommended in the ACI 546R-04 Concrete Repair Guide. The composite NC matrix/TSSCC consists of three zones: the old concrete zone, the transition zone, and the new concrete zone. The NC/TSSCC rapid bond strength is derived from the main hydration product AFt of TSSCC to the pinning of pore cracks in the old concrete, the meshing between AFt, the penetration filling of the hydration product, and the van der Waals force between the old and new concrete hydration products. This study indicates that TSSCC has the potential application for the rapid repair of reinforced concrete structures.

Reinforced Concrete Posts Made by Long-Line Formwork-Free Shaping for 0.4–10 kV Overhead Line Supports

When using the technology of long-line formwork-free shaping for the manufacture of reinforced concrete structures, industrial engineering can be conducted in accordance with the requirements of modern regulatory documents. This allows projects to be developed individually, and production lines can be readjusted in a short time in accordance with emerging needs. In this regard, the possibility of using reinforced concrete posts produced by the technology of long-line formwork-free shaping for 0.4-10 kV overhead line supports is being considered. The designs of such supports are developed on the standard basis of the product range. The problem of establishing the minimum number (two) of cross-sections with different geometric dimensions for all brands of the offered posts is solved, subject to the operational requirements and technological specifications of production. The cross-sections of the proposed posts represent trapeziums, with the dimensions of the upper bases smaller than the dimensions of the lower bases in order to maintain the shape of the freshly formed concrete body of the posts made using long-line formwork-free shaping technology. For posts up to 11.0 m long, a solid cross-section with trapezoid edges is proposed: h = 245 mm; b = 150 mm (an upper base); b1 = 180 mm (a lower base). In the middle part of the cross-section of posts with a length of 11.0 to 16.4 m there is a cavity along the entire length of the posts. Cross-sectional dimensions of such posts are h = 300 mm; b = 205 mm, b1 = 235 mm. Long-line formwork-free shaping technology provides for the reinforcement of prestressed structures with high-strength wire or rope reinforcement. The proposed posts are rein-forced with 5Вр1400 rods, therefore, during their operation, the formation of cracks in the tensile zone of concrete is not fore-seen. The rods are located in groups at all corner sections along the cross-section of the post with an equal number of rods in each group, taking into account the technological features of the long-line formwork-free shaping. For the proposed cross-section of hollow posts, produced by long-line formwork-free shaping for 0.4–10 kV overhead line supports, a patent for autility model has been obtained from the Agency for Intellectual Property of Uzbekistan. Qualitative characteristics of the posts for overhead line supports are low material consumption, versatility, manufacturability, innovation, which lies in the fact that their implementation is associated with the design, manufacture and testing of experimental products for use by specific consumers, i. e. with the commercialization.

Influence of thermally activated coal gangue powder on the structure of the interfacial transition zone in concrete

Coal gangue is a kind of solid waste produced in the coal mining and preparation. Abundant SiO 2 and Al 2 O 3 are contained in coal gangue, which has potential cementitious activity. Therefore, the preparation of cementitious materials from coal gangue powder has broad application prospects. The effect and mechanism of the cementitious activity of thermally activated coal gangue powder on the pore structure, microscopic morphology , and mineral composition of the interfacial transition zone in concrete were studied. The results showed that the flexural and compressive strengths of concrete prepared with cement replaced by 15 wt% thermally activated coal gangue powder at 800 °C were 0.39% and 5.94% lower than those of pure cement concrete and 7.47% and 20.17% higher than those of raw coal gangue powder concrete. Mainly because abundant active Al 2 O 3 and SiO 2 released by thermally activated coal gangue powder at 800 °C reacted with the cement hydration product calcium hydroxide, calcium silicate hydrate gel was produced, and then the pores in the interfacial transition zone were filled and transformed into smaller pores, resulting in the pore structure and microscopic morphology of the interface transition zone being optimized, and the bond between the matrix and aggregate becoming closer.

Quasi-static test of the lattice-type railway bridge pier with replaceable connection component

Replaceable components are beneficial to functionality rapid restoration of structures after earthquakes, which is a good option for seismic design of resilient bridges. In this study, a lattice-type bridge pier with replaceable connection component was proposed for railway bridges. Large initial stiffness was provided by using steel truss connection system as replaceable connection component between two concrete columns. A 1/12-scaled bridge pier model was designed and fabricated to investigate its cyclic responses by quasi-static testing. It was proved that the replaceable connection component can be easily assembled between concrete columns and work well together with them. During testing, cracks occurred at the concrete columns and crack zone extended with the increase of loading displacement. At the later loading stage, some of steel rods of the connection component between the concrete columns began to yield, the hysteretic loop of the bridge pier showed obvious pinching effect. However, concrete columns were prevented to be severely damaged due to the sacrifice of replaceable connection components. There were differences in the strain distribution of the replaceable connection component and not all the replaceable components yielded during testing. Thus, steel rods with varied strength should be used in replaceable components for the lattice-type railway bridge pier. Otherwise, it should balance the stiffness and ductile in seismic design for the lattice-type railway bridge pier with replaceable connection component.

DEFORMATION MODEL AND ALGORITHM FOR CALCULATION OF REINFORCED CONCRETE STRUCTURES OF ROUND CROSS-SECTION UNDER TORSION WITH BENDING

Despite a fairly long period of research and a significant number of publications around the world on the problem of the complex resistance of reinforced concrete, the existing calculation models still remain far from perfect. This is especially true for structures with a non-rectangular cross section. The article presents a version of the model and an algorithm for the analytical calculation of reinforced concrete structures of a circular cross section in torsion with bending, which most fully reflects the specifics of the power resistance of such structures. The model takes into account all the components of external forces in a rod element of a circular cross section, the spatial nature of cracks, with the combined action of moments, various cases of the location of the compressed concrete zone, depending on the ratio of the acting forces in the calculated structure. For a spatial crack, calculated sections are taken in the form of diagonal large and small ellipses and a spatial surface bounded by concave and convex spatial parabolas. In compressed and stretched concrete, a broken section of three sections is considered, two in the form of longitudinal trapezoid and the third, middle section in the form of a small ellipse rotated at an angle to the longitudinal axis of the structure. The obtained analytical dependencies allow one to determine interconnected design parameters, such as stresses in the concrete of the compressed zone, the height of the compressed concrete, stresses in the longitudinal and transverse reinforcement, deformations in concrete and reinforcement, the length of the projection of a spatial inclined crack, and others. The deformation model and algorithm can be used in the design of reinforced concrete structures of circular and annular cross-section, working in bending with torsion.

Computational Analysis of Concrete Flow in a Reinforced Bored Pile Using the Porous Medium Approach

In this paper, the flow of concrete in a reinforced bored pile is analysed using computational simulations. In order to reduce the computational time, a porous medium that equally mimics the presence of the reinforcement is used. Experimental measurements are used as bounds on the material parameters describing the flow of fresh concrete. The influence of rheological properties of fresh concrete and the thickness of the porous medium that represents the reinforcements is analysed with a classical U-box simulation. Finally, casting of a bored pile is analysed using computational simulation implementing a porous medium representing the reinforcement cage. The concrete flow behavior and especially the filling of the concrete cover zone is analyzed for casting scenarios with different concretes varying in their rheological behavior. Simulations using the porous medium approach is 10x faster than simulations that explicitly model the reinforcements. Simulation results show that a good workability (low viscosity and low yield stress) of the initial batches of concrete must be maintained throughout pouring to avoid the risk of defect formation in the cover zone.

Cyclic loading test for RC bridge piers strengthened with UHPC jackets in the corrosive environment

Ultrahigh-performance concrete (UHPC) jacketing has been proposed to improve the mechanical performance of the new or existing RC bridge piers, though little is known about their seismic behavior in the corrosive environment. To address this issue, this study presents the quasi-static cyclic experiments of four piers locally strengthened with UHPC jackets. The investigated variables of these piers are UHPC height and corrosion level. Based on the test results, the influence of corrosion level and height of UHPC jacket on the hysteretic curve, strength, ductility, stiffness and strength degradation, energy dissipation capability, and residual displacement, are analyzed and discussed. The results show that the corrosion level of reinforcements in the UHPC zone was smaller than that in the normal strength concrete zone of specimens. Compared to the uncorroded specimen, the joint effect of steel fibers corrosion and reinforcement corrosion led to a poor hysteresis performance of the corroded structure in terms of elastic stiffness, displacement ductility, and load-bearing capability. Furthermore, under the similar corrosion degree of reinforcement, the smaller height of UHPC jackets resulted in more significant degradation of stiffness and strength, and weaker energy dissipation capability for the pier.

Effects of concrete materials on the vibration of the adjacent tunnel and surroundings by DEM and experiment

Concrete material can reduce the disturbance of train vibration to adjacent tunnels during the operation of near-distance twin tunnels. This paper adopts the discrete element method (DEM) and experimental model (scaled by 1:20) to study the influence of the train vibration at the speed of 120 km/h on the static and dynamic characteristics of the adjacent tunnel's liner and sleeper. The region between the twin tunnels was set as the concrete zone (CZ) to explore the effect of train vibration waves. The distance between twin tunnels is set to 0.25 D, where D (6.2 m) is the diameter of the tunnel. Foam brick (FB) is the similar materials of CZ in similar experiment. The DEM and similar experiments obtained similar results. The CZ (FB) can effectively block the propagation of the train vibration waves in T1, thus reducing the disturbance to the T2 liner and sleeper. Next, the CZ with four thicknesses and six heights are discussed with DEM, and it is found that thickness and height have a significant influence on the isolation effect of CZs, so the size should be selected according to actual engineering needs. This study provides a reference for studying the vibration reduction effect of CZs on adjacent tunnels during train operation.

Mechanical properties measurement and micro-damage characterization of ITZ in concrete by SEM-DIC method

Due to the limitation of current experimental measurements, there is an almost insurmountable challenge to determine the mechanical tensile properties of interfacial transition zone (ITZ) in concrete. The present work aims to experimentally characterize micro-mechanical properties of ITZ under tensile load with the combined strategy of scanning electron microscope (SEM), digital image correlation (DIC), nanoindentation and wavelet packet technology. A bi-material sample composed of aggregate and mortar in the same volume and shape is designed for convenience to conduct tensile experiments using the proposed SEM-DIC method. By analyzing the stress obtained from the loading device and full-field strain calculated through DIC technique, the mechanical properties including tensile strength, nominal fracture energy (GNF) and nominal tensile elastic modulus (ENT) of ITZ are evaluated. Nanoindentation tests are also conducted, and their results are briefly compared with that of SEM-DIC method. The micro-damage distribution in ITZ area is further evaluated by wavelet packet analysis. Results reveals that a region with apparently lower ENT values can be characterized as the ITZ area, in which the average ENT value is about 55% of that of the mortar. In addition, growing path of microcracks can be estimated by the direction of maximum principal strain, and the degree of micro-damage could be well characterized by energy change rate (ECR) through wavelet packet analysis.

ВИЗНАЧЕННЯ НЕСУЧОЇ ЗДАТНОСТІ ДВОТАВРОВИХ ЗАЛІЗОБЕТОННИХ КОЛОН ПРИ ПРЯМОМУ ПОШКОДЖЕННІ

Based on the field studies and determination of the parameters of the stress-strain state, as well as the nature of the operation of reinforced concrete I-beam columns damaged during operation and during hostilities, a general method for determining the residual bearing capacity of elements was created. The article presents a method for determining the residual bearing capacity of damaged compressed reinforced concrete columns with direct damage, when the damage front is parallel to one of the main axes of the section. The prerequisites for the calculation of damaged reinforced concrete I-columns are proposed: the hypothesis of flat sections is accepted; stresses in the compressed zone are distributed uniformly with intensity ηfcd; the stresses in the reinforcement are taken depending on the height of the compressed zone of concrete, the forces in the tension zone are perceived by the reinforcement and are taken no more than the design tensile strength ft; the work of tensioned concrete is not taken into account; the condition of parallelism of force planes is accepted (the plane of action of external and internal forces are the same or parallel, depending on the design case); the reduced bearing capacity of exposed reinforcing bars is taken into account; damage front is a straight line. Equilibrium equations are composed. The proposals set forth in the article are based on the main provisions of the current norms and expand the scope of their use. The proposed method for determining the residual bearing capacity of reinforced concrete compressed elements of the tee profile, damaged during operation, is statistically justified and reliable. This allows the calculation method to determine the possibility of further trouble-free operation of structures or the need for their strengthening or reconstruction. A system of equations has been created that takes into account all the variety of shapes and sizes of the cross section of the element as a whole, the shapes and sizes of the compressed concrete zone. The reliability of these proposals is confirmed by comparison with data obtained experimentally, and statistical processing of such a comparison. The deviation variation coefficient is 0,125.

Physical and mechanical properties of expanded vermiculite (EV) embedded foam concrete subjected to elevated temperatures

Expanded vermiculite (EV) is an innovative alternative to conventional fire-resistance material due to exceptional thermal performance and non-combustible property. In this study, the effect of elevated temperatures on microstructure, mechanical and physical properties of foam concrete containing EV as lightweight aggregates (LWA) was investigated. Foam concrete samples with 10%, 15% and 20% EV as sand replacement were developed and exposed to 300 ℃, 600 ℃, and 900 ℃. Scanning electron microscopy (SEM) test was conducted to identify microstructure changes in the cement matrix and fine aggregates after elevated-temperature exposure. The enhanced close-fitted bond in the interfacial transition zone (ITZ) of EV-based foam concrete was observed in SEM, which might be due to the higher water absorption of EV granules. Residual compressive and flexural strength, relative porosity and water sorptivity coefficient of foam concrete were also obtained. The 15% EV reduced compressive strength loss of samples by 1.84%, 11.68% and 69.1% at 300 ℃, 600 ℃ and 900 ℃, respectively, compared with that of the reference as 9.81%, 33.7%, and 72.28%, respectively. Enhanced sorptivity-related durability and lower residual porosity were also obtained at elevated temperatures when 15% EV was added.

The micro-facial characteristics and mechanical behavior of metakaolin and steel fibers modified concrete at high fluidity

The pumped concrete, which being used in driven cast-in-place piles, demand lower cement usage, higher fluidity, preferable mechanical properties at pre-cracking stage and higher capacity of deformation resistance at the post-cracking stage. To develop the above properties, the metakaolin (MK) modified steel fiber (SF) reinforced concrete at water/binder = 0.6 were used in this case, and its microstructure and mechanical behavior after 7 and 28 days curing were investigated. The compression and splitting tests, Scanning Electron Microscope (SEM), X-ray Energy Dispersive Spectroscopy (EDS), X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM) tests were performed. The results showed that the coupling effects of SF and MK on the fluidness reduction were evident. Without incorporation of SF, the compressive strengths, the tensile strengths, the deformation modulus E50 and elastic modulus E were improved by 140–162%, 171–178%, around 150% and 130–140% respectively by MK. At post-cracking stage, SF and MK combination promoted residual strengths and prevented the further development and connection of the crack. The portlandite (CH) were massively consumed by MK, and more stable Hydrated Calcium Silicate (CSH) gel and ettringite phase matters were produced to fill the porosity and defects. A large amount of hydration products in the form of acicular and fibrous particles enhanced the bonding strength on interfacial transition zone of MK concrete. The steel fibers were significantly stretched during pulling out with MK incorporation. Based on AFM tests, the average roughness Ra and mean square roughness Rq of microstructural faces both decreased with curing age and MK contents. The conclusions would be used to proportioning design of MK-SF concrete in an economic way, and understand the mechanisms of micro cracking between geopolymer and inert matters.

Particle size distribution of aggregate effects on the reinforcing roles of carbon nanotubes in enhancing concrete ITZ

By virtue of their superior mechanical properties and huge surface area, carbon nanotubes (CNTs) have become a potential representative in the field of the nanoengineered concrete, which provides an excellent opportunity to reinforce concrete interfacial transition zone (ITZ) characteristics. Adjusting the most reasonable aggregate gradation of concrete to maximize the enhancement effects of CNTs to ITZ is the key to popularizing the application of nano reinforcement in practical engineering. Hence, in this work, we investigated the reinforcing roles and mechanisms of CNTs on the concrete ITZ, and the corresponding influence on the mechanical performance and impermeability properties under different aggregate particle size distributions. The results reveal that CNTs can promote nucleation effects to generate hydration products growth and act as the refined templates to produce finer microstructure in concrete ITZ. Compared with plain concrete, the reinforcing ratios of CNTs on the ITZ characteristics, mechanical performance and impermeability present a quadratic function correlation with the Talbot index. The optimal effects of CNTs on the reinforcement of concrete ITZ characteristics appear at 0.8 Talbot index, reducing about 30% ITZ width and 22.8% abrasion crack width in ITZ. The corresponding mechanical properties and impermeability improve 20.7–35.6% and 38.1–43.6%, respectively. Low Talbot gradation index would result in the uneven dispersion of CNTs among the capillary pores. In contrast, high Talbot gradation index may enlarge the ITZ range, increase the amount and width of abrasion cracks in ITZ, and further weaken the reinforcing effects of CNTs.

Investigating high-temperature deformation evolution of concrete under sustained loading using DIC technology and a temperature-mechanical coupled damage constitutive model

Based on digital image correlation (DIC) technology and uniaxial compression tests, the deformation evolution characteristics of concrete were analyzed in the heating process under a sustained load and in the failure process under uniaxial compression. The effect of the sustained load on the mechanical properties of concrete at high temperatures was explored. A damage constitutive model of concrete under the combined action of high temperature and the sustained load was established. The research results show that a sustained load can effectively restrain the vertical thermal expansion of concrete and compact loose and weak zones of concrete. However, when the load level or temperature is too high, excessive lateral deformation of concrete will cause local strain concentration and cracking failure. In the failure process under uniaxial compression, both high temperature and sustained load can make the macroscopic cracks more smeared and branched. When the temperature is 200 °C and 400 °C, the sustained load has only a little enhancement effect on the high-temperature mechanical properties of concrete. When the temperature is 600 °C and 800 °C, the sustained load can significantly enhance the mechanical properties of concrete at high temperatures. However, at higher temperatures, excessive sustained loads can cause premature failure of the specimen. Based on the Weibull distribution, a statistical damage constitutive model of concrete under the combined action of sustained load and high temperature is established, which is in good agreement with the test curve. The curve of the degree of damage of concrete Dm shows that the sustained load causes the damage Dm to start earlier, the damage rate to increase, and the degree of damage at the peak stress to increase in the failure stage under uniaxial compression.

Microstructural examination of carbonated 3D-printed concrete.

The recent interest in 3D printing with concrete has generated great interest on how inhomogeneities arise and affect performance parameters, in particular strength and durability. With respect to durability, of particular interest is how 3D‐printed layer interfaces can impact transport of species of interest, such as moisture, chlorides or carbon dioxide in carbonation processes. This is of particular interest considering that the primary use case of 3D‐printed concrete has been as a lost formwork for a cast structural concrete, and thus it is of interest to determine the carbonation resistance. This study consists of a preliminary look at the microstructure after accelerated carbonation of a 3D‐printed concrete used as a lost formwork. Preferential carbonation is observed in the layer interfaces compared to the bulk of the printed filaments, possibly related to porosity from air voids or a locally high capillary porosity corresponding to the lubrication layer.