Today, the majority of research in 3D concrete printing focuses on one of the three methods: firstly, material extrusion; secondly, particle-bed binding; and thirdly, material jetting. Common to all these technologies is that the material is applied in horizontal layers. In this paper, a novel 3D concrete printing technology is presented which challenges this principle: the so-called Injection 3D Concrete Printing (I3DCP) technology is based on the concept that a fluid material (M1) is robotically injected into a material (M2) with specific rheological properties, causing material M1 to maintain a stable position within material M2. Different to the layered deposition of horizontal strands, intricate concrete structures can be created through printing spatially free trajectories, that are unconstrained by gravitational forces during printing. In this paper, three versions of this method were investigated, described, and evaluated for their potential in construction: A) injecting a fine grain concrete into a non-hardening suspension; B) injecting a non-hardening suspension into a fine grain concrete; and C) injecting a fine grain concrete with specific properties into a fine grain concrete with different properties. In an interdisciplinary research approach, various material combinations were developed and validated through physical experiments. For each of the three versions, first architectural applications were developed and functional prototypes were fabricated. These initial results confirmed both the technological and economic feasibility of the I3DCP process, and demonstrate the potential to further expand the scope of this novel technology. While additive manufacturing (AM) is already a well-established technology and integral part of the production in many industrial sectors, the potential of AM to also become a key technology in the construction industry is clearly evident today. Compared to traditional concrete construction, AM offers a number of advantages: through the use of computer-controlled machines, building components can be mass-customized and produced individually in series, thus combining the benefits of traditional craftsmanship and industrialized production. In addition, digital control of 3D printing hardware allows the geometric complexity of components to be increased in order to improve component efficiency, for example by means of structural optimization. As such, material can be placed exclusively where it is structurally required. A further advantage of this technology is the complete elimination of conventional formwork, therefore minimizing the amount of work required for the formwork assembly, as well as the amount of construction waste caused by the disposal of the used formwork . CurrentParticle bed printing is a process where fine particles are selectively bound through the localized application of binder. For this, a thin layer of particles is evenly distributed inside a given building envelope. In a subsequent step a binder is deposited onto the particle layer, which selectively bonds the particles. Next, the particle bed is lowered, a new layer of particles is applied, and the binding process is repeated. In a layered manner, complex geometries can be fabricated, resting loosely inside the bed of unbound particles. This technology is used, for example, for the production of complex molds in metal foundry , but alsIn the extrusion-based approach to additive manufacturing, strands of fine concrete are printed layer-wise on top of each other, progressively creating a three-dimensional object. Here, predominantly two different concepts are pursued, firstly the contour crafting approach , and secThe third approach, Shotcrete 3D Printing, differs from extrusion-based concrete printing in the respect that the concrete is not deposited in strands, but is sprayed by means of compressed air . This reEach of these techniques features a distinct set of advantages, as well as process inherent limitations. 3/h. Even higher volume flows of currently up to 1 m3/h can be achieved with the Shotcrete 3D Printing method. However, printing speed is inversely related to geometric resolution and surface quality. Accordingly, particle bed printing is, for example, the most geometrically versatile, but also the slowest additive manufacturing process.Material extrusion, for example, offers high building rates. Depending on the printing system, state-of the-art concrete printers reach building speeds of up to 16 cm/s , correspA common feature of all three approaches is that the printed object is always built-up in horizontal layers. In this paper a novel 3D printing technology with concrete is introduced, which challenges the layered build-up and proposes a printing approach enabling more complex spatial printing trajectories. In the so-called Injection 3D Concrete Printing (I3DCP) method, one fluid material (M1) is robotically injected into another fluid material (M2) with similar rheological properties. In consequence, the extruded material M1 maintains in a stable position within material M2 without sinking or buoying. Accordingly, this process combines the advantages of the two techniques described above: firstly, the material M1 can be extruded at high building rates, and secondly, the surrounding liquid enables omnidirectional printing and thus comprehensive spatial freedom without restriction by gravity.3D printing of a fine grain concrete into a non-hardening suspension called “CiS” (Concrete in Suspension);3D printing a non-hardening suspension into a fine grain concrete called “SiC” (Suspension in Concrete);3D Printing a fine grain concrete with specific properties into another fine grain concrete with different properties, the so called “CiC” process (Concrete in Concrete).In this paper the systematic investigation of three variations of Injection 3D Concrete Printing technique are described:With the first process version, CiS, a fine grain concrete is printed into a non-hardening suspension. As such, intricate truss structures and filigree concrete space frames can be fabricated, which are otherwise difficult or impossible to manufacture. The second process version SiC (suspension in concrete) is the reversal of the CiS principle: in the SiC process, a non-hardening suspension is printed into a vessel filled with fresh concrete. Subsequently, the suspension is removed, leaving cavities or channels. This can be used to functionally grade concrete components through differentiated density or to utilize them for physical element activation. For the third version of this concept, concrete is used for both the extrusion and the supporting material, whereas the material properties differ according to functional needs CiC, . One posThe underlying concept, to continuously extrude material in spatial trajectories, was previously investigated with other materials, with and without a supporting medium. The Mataerial research project of the Institute for Advanced Architecture of Catalonia (IAAC) investigated the usage of a two-component thermosetting polymer in order to robotically print material in mid-air . ConcurrThese initial successes using polymers triggered testing of the concept with cement-bonded materials as well. For example, the French start-up Soliquid recently published a video of extruding a concrete in a container filled with gel . Anotherthe identification of process inherent applications in construction;the use and design of materials suitable for construction in terms of robustness, ecological, and economic factors;fundamental interactions between material behavior and fabrication process.From the outset, the concrete-based I3DCP research at TU Braunschweig aimed to systematically investigate the method in its three conceptual variations—CiS, SiC, and CiC—in order to create a coherent system and to develop material combinations that make all three versions applicable for the construction industry. Here, a particular focus was directed to: In the following sections, the fundamental and systematic exploration of the three versions of I3DCP is described. The methods were developed in the research seminar “Digital Building Fabrication Studio” (DBFS) at TU Braunschweig and were validated through a manifold of physical experiments. The following sections describe the robotic setup used, the digital workflows, and the materials developed. This is followed by a description of the experiments on each of the three variations of the I3DCP technique. All experiments were carried out on an UR 10 robot  using onFor design, path planning, simulation, and robot control an integrative design-to-fabrication approach was developed. For this purpose, the modeling software Rhino 3D  with itsThere were three materials used in the present experiments: (a) fine grain concrete with an Ordinary Portland Cement  and a quartz sand with a maximum grain size of 2 mm; (b) ground limestone-sand suspension with a maximum grain size of 0.5 mm as non-hardening suspension; and (c) sand-gel suspension as non-hardening suspension with a maximum grain size of 0.5 mm. A detailed overview of all components used is given in Printing concrete in a suspension offers the potential to create lightweight, intricate spaceframe structures, with a high strength to weight ratio, which are difficult to manufacture in other ways. The focus of these investigations was directed firstly toward the design potentials of this approach, and secondly to develop material formulations that are technically and economically feasible, also for the harsh environment of the construction site. For calibration purposes a series of small-scale experiments was conducted. In order to allow a visual examination of the extrusion process, the fine grain concrete was printed into a transparent vessel (37.6 × 26 × 18.9 cm) filled with gel. The pneumatically actuated end effector was filled with 3 liters of fine grain concrete, and the air pressure was initially set to 2 bar. A simple, yet spatial, zig zag geometry was chosen for the initial experiment.The printed structure was removed from the gel after 48 hours of curing. Already during this time, the following observations were made: only a few hours after printing, a white, dense, gel-skin developed around the concrete structure . At the As the extrusion appeared very bulky and undefined, in the subsequent calibration steps, the air pressure was gradually reduced, while the robot speed was kept constant. This was repeated until the robot speed and extrusion rate resulted in a constant extrusion diameter corresponding to the 16 mm diameter of the nozzle. For this, an air pressure of 0.5 bar was found to be a suitable. Different space filling geometries were tested .In order to prevent the decomposition of the gel, as well as the formation of the white skin, and to counteract progressive sinking of the printed structure, two alternative support mediums were developed and tested. Firstly, in an effort to achieve greater overall stability , the same bulk volume of quartz sand was added to the pure gel. Additionally, a less sensitive and more cost-effective limestone-sand suspension was developed, which did not require the use of expensive gel at all. Both mediums were tested and compared in printing experiments .For the gel-sand suspension no formation of a white skin was observed, and only a very small amount of water accumulated on the surface of the printing container. When extruding within the limestone-sand suspension, it was observed that furrows were created behind the printing nozzle, when moving through the gel. Those furrows saturated with concrete, leading to the creation of ridges above the print trajectory b. In furTo validate the CiS principle, a medium-sized object was developed for production in a 50 × 50 × 50 cm container. For this, a formwork was filled with approximately 110 liters of the non-hardening limestone-sand suspension. The object represents a 14-sided polyhedral spaceframe structure derived from a hexagonal base a. The printing path and sequence, as well as the robot simulation, are depicted in After three days the object was removed from the limestone-sand suspension a. The vaThe potential of the SiC approach is to create concrete elements which are graded in porosity, or which can integrate geometrically complex systems of channels or ducts. These features could, for example, be used for aspects of structural performance, material savings, technical installations, or for component activation and building physics. In this approach the voids are created by printing a non-hardening suspension acting as displacement bodies directly into the fresh concrete. The challenges here were firstly, calibrating the material properties in such a way that the material remained stable within the concrete and generated well-defined voids, and secondly, designing the channel layout in a manner that the displacement material could drain off after the concrete had cured. A preliminary setup was developed for material and process calibration that allowed immediate visual feedback. For this purpose, a 30 × 30 × 4 cm³ formwork was built, in which the 30 × 30 cm² sides were closed with Plexiglas. For an initial test a centrically placed regular 6 × 6-point grid was generated, and at each point gel was injected into the fine grain concrete for a certain amount of time. The electromechanic piston extruder was used, due to the advantage that the material volume can be controlled more precisely. Compared to the pneumatic extruder, the material flow could be stopped and started at any time, whereas the pneumatic extruder continually builds up or releases pressure. This is particularly important if only material is to be injected selectively, for example as discretely enclosed volumes or voids. However, the initial experiment showed that simply stopping the piston does not automatically stop the material from flowing. Therefore, as a side-effect, channels were created when moving the nozzle from one point to the next.Moreover, it was recognized that the gel did not have the capacity (density) to displace the concrete evenly b and tha2 was chosen, whereas the thickness of the panel was maintained. In contrast to the preliminary tests, in this series a differentiation of the size of the displacement bodies was deliberately made and realized by varying the duration of the extrusion at each point. Moreover, the pattern was changed from a regular 6 × 6 configuration to a diagonal pattern, where one row of points was shifted horizontally by half the distance between two points  with a limestone-sand suspension (<0.08 €/kg), the costs were reduced by almost two orders of magnitude. Moreover, all the methods presented in this paper are based on standard concrete technologies, for example, standard pumps and formwork systems. Therefore, all processes are considered scalable and, more importantly, economically feasible on the large scale of construction. In addition, the dimensional limitations are comparable to current precast concrete production or in-situ concrete constructions.The unique potentials of each of the process variation and specific applications in construction are outlined in the following paragraphs. The challenges associated with up-scaling must also be identified and addressed in future research. The CiS process offers the potential to radically expand the design space of concrete constructions. With contemporary techniques, be it conventionally casted or 3D printed, concrete elements are predominantly conceived as flat or curved surface structures (with the exception of columns). The CiS process, however, makes it possible to dissolve concrete structures into highly differentiated intricate spatial structures. This can be of practical use when, for example, structural performance is the main requirement, and other features, like, for example, the enclosure of a thermal envelope, are of secondary importance. In such cases the structure can be reduced to solely loadbearing members, and locally differentiated spaceframe structures can be printed with a minimum of material used. Moreover, and in addition to the potential of globally differentiating the spatial structures, the process enables the local differentiation of each individual truss member by altering the robot speed and extrusion rate during printing. First experiments have verified this approach, and are depicted in Nevertheless, the structural performance of the systems depends not only on mere geometry but also on other parameters, like node connectivity, and on the integration of reinforcement. With the former, it must be ensured in each node that the materials bind monolithically to each other and that no separating layer is formed, as occasionally observed in the experiments. Regarding the integration of reinforcement, one major advantage here compared to the layered build-up of conventionally 3D printed structures, is the possibility to extrude material in line with the prevailing force trajectories. This unique feature makes it possible to co-extrude reinforcement, such as a continuous fiber or steel cables within the concrete strand. Initial tests have been carried out and are currently being developed further b.Finally, a future challenge is to investigate the process on a larger scale. In this respect, two scenarios are considered possible. Firstly, to scale up the entire system and to print in very large containers filled with suspension, and secondly, to modularize the structures and build large constructions from smaller components. As the CiS process is believed to be most efficient in prefabrication, a modularization based on the maximum transportation size seems to be the most feasible fabrication scenario. Nevertheless, particularly as the limestone-sand suspension is very robust (compared to gel), this process would be feasible for onsite production as well. In that case larger spaceframes would be printed in an onsite factory, and be lifted into place using cranes. The two main features which were explored in the SiC process were, firstly, the gradation of concrete by locally injecting discrete displacement bodies, and secondly, the integration of continuous channels within a solid building element. Both features offer a vast potential for practical application in construction. In addition to the possibility of making buildings generally lighter, the production of lighter prefabricated parts also offers considerable potential for saving energy during transportation to the building site. Besides saving weight, and hence resources, SiC structures also facilitate the integration of additional component functionalities. For example, through the integration of channels and ducts, active cooling or heating of building elements can be facilitated. Moreover, integrated channels may be used for structural purposes, for example, for the integration of post-tensioning cables. With regard to the location of production, both prefabrication and in situ fabrication seem possible. In further development, however, research will concentrate on prefabrication. This is due to the fact that the release and reuse of the suspension from the hardened concrete can be better controlled in a factory setting. The CiC principle fosters the fabrication of high-performance components using mostly lower grade materials. For example, a normal concrete can be locally reinforced by injecting high-performance concrete in exceedingly stressed areas. This shows potential for a variety of practical applications. For example, the load paths in a component could be traced using a higher-quality material, so that the load-bearing capacity of the component increases along these paths. In the case of walls, for example, these would be areas in the immediate vicinity of doors or window openings. In addition, this method could also be used to locally reinforce areas of direct load introduction, for example in the supports where beams rest on a wall. While the CiC experiments described in this paper essentially used the same concrete, merely with the addition of black pigments, the challenge in further investigations will be the use of concrete with vastly differing mechanical properties. The challenge here will be maintaining their printing and bonding compatibility. In addition to using lightweight concrete as a support medium, the use of recycled concrete will also be subject to future investigation. In terms of structural design, currently predominantly continuous geometries have been printed into the formwork. This approach could be extended towards printing strategies in which the properties of a building element change gradually from property A to property B. For this, an injection strategy similar to the one described in the SiC approach will be investigated.Of all three process variations, the CiC process seems to be most likely to be implemented in situ, for example, by mounting a lightweight robot onto standard concrete formwork, in order to locally inject high performance mortar into cast-in place concrete constructions. However, here too there is great potential for prefabrication, especially in view of the fact that this process can be combined and hybridized with the SiC process. In this paper, an overall concept for a novel, so-called Injection 3D Concrete Printing technology was presented, which is based on the injection of a fluid material into another fluid material with specific rheological properties. Three variations of this method were described and evaluated: A) Concrete in Suspension (CiS): injecting a fine grain concrete into a non-hardening suspension; B) Suspension in Concrete (SiC): injecting a non-hardening suspension into a fine grain concrete; and C) Concrete in Concrete (CiC): injecting a fine grain concrete with specific properties into a fine grain concrete with different properties. Each of these variations of the Injection 3D Concrete Printing concept was verified through numerous pre-studies as well as a final demonstration object for each approach. Within this framework a particular focus was directed to the scalability and economic feasibility of the 3D printing processes. For this, material combinations were developed which meet construction requirements in terms of economy and robustness. While each of the three variants is associated with specific potentials and challenges, there are characteristics in common between the different processes that need to be investigated in the further course of research. Most importantly, the fabrication process needs to be scaled from lab environment to real scale. In particular, the extruder technology needs to be upgraded to meet reliable and robust industrial standards, and to be able to extrude larger quantities of concrete without interruptions. Moreover, the digital geometry generation and path planning should be extended to include a structural analysis of the generated geometry, as well as an automated feasibility check of the robotic trajectory, in order to avoid intersections of the already printed geometries. The rheological properties (yield stress and viscosity) of the materials need to be specified to enable geometric precision of the printed objects. In addition, the amount of creep and shrinkage of the extruded material has to be investigated in future research. Another aspect for further investigation is precise control over the increasing volume during the printing progress: the more material is injected into the other, the more the volume increases. The mechanisms of the rising material level and the associated positional changes of the already injected material will have to be investigated more closely in future research. Following these initial proof of concept studies, in future research the structural properties will have to be quantified. For all versions of the I3DCP process, specific benchmarking, e.g., regarding the weight/performance ratio, must be developed and performed. The implementation of the process in the construction industry can only be realized in the medium term through quantified comparison.Finally, it should be stressed that the Injection 3D Concrete Printing method depends on a close interaction between the fabrication process and material parameters, and that these aspects cannot be considered separately. Hence, a close collaboration of material science, structural design, and process engineering is required to successfully advance the research. https://www.ezn.de/ezn-patent/injection-3d-printing/.A patent is filed under: N. Hack, D. Lowke, H. Kloft, Injection 3D Printing, DE 10 2019 105 596.2, 2019.


Crystalline materials generally show small positive thermal expansion along all three crystallographic axes because of increasing anharmonic vibrational amplitudes between bonded atoms or ions pairs on heating. In very rare cases, structural peculiarities may give rise to negative, anomalously large or zero thermal expansion behaviors, which remain poorly understood. Host–guest composites may exhibit such anomalous behavior if guest motions controllable. Here we report an anionic framework of helical nanotubes comprising three parallel helical chains. The anisotropic interaction between the guest and the framework, results in anisotropic thermal expansion in this framework. A series of detailed structural determination at 50 K intervals enable process visualization at the molecular level and the observed guest-dependent phases of the framework strongly support our proposed mechanism. Yield: 1.12 g 72.2%. Anal. Calcd (Found) for C16O8H10: C, 58.19 (58.10); H, 3.05 (3.09) %.H2NH2]·[Mg2(BPTC)(NO3)(H2O)] (1) A mixture of Mg(NO3)2·6H2O , H4BPTC , 4,4′-azopyridine  and HCl (1 mL 1mol/L) in DMF (5 mL) was heated at 120°C in a sealed 20 mL glass via for 1 day, and cooled to room temperature. Colorless needle crystals of compound 1 were filtrated, washed using methanol and air-dried . Anal. Calcd (Found) for MgNC9O6H8: C, 43.12 (43.03); H, 3.19 (3.11); N 5.59 (5.63)%.[Me3]·[Mg2(BPTC)(NO3)(H2O)] (2) A mixture of Mg(NO3)2·6H2O , and H4BPTC  in NEF (5 mL) was heated at 120°C in a sealed 20 mL glass via for one day, and cooled to room temperature. Colorless needle crystals of compound 2 were filtrated, washed using methanol, and air-dried . Anal. Calcd (Found) for MgNC9O6H8: C, 43.12 (43.06); H, 3.19 (3.09); N 5.59 (5.51)%.·[Mg2(BPTC)(NO3)(H2O)] (1) as colorless needle-shaped crystals. Single-crystal X-ray crystallography revealed that compound 1 consists of a three dimensional (3D) chiral anionic framework assembled from nanotubes comprising parallel triple helices. Charge neutrality is achieved by protonated dimethylamine cations [Me2NH2]+. NO3− anions and H2O molecules present a statistical distribution in the anionic framework  and elemental analysis, results. Compound 1 crystallizes in the chiral orthorhombic space group I212121 and the asymmetric unit contains a Mg2+ ion, half a BPTC4− ligand, half a μ-NO3− anion and μ-H2O in statistical distribution and half a [Me2NH2]+ cation. Each Mg2+ ion is coordinated with six O atoms in an octahedral geometry, four of which from four different BPTC4− ligands and the other two from two different μ-NO3− or μ-H2O , or the angle ϕ and the distance between MgD and MgE (d2), respectively  the torsion angle between the two phenyl groups in BPTC4− ligand has been shown to be variable  showed that compound wn phase , S6, S7.β, and γ  are key ectively . Therefo1 from 123 K to 373 K at 50 K intervals. All lattice parameters change almost linearly with increasing temperature. Interestingly, b- and c-axes change by +1.52% and −0.52% over a 250 K temperature range, while the a-axis remains relatively unchanged (NO3)(H2O)] (2) with [EtNH3]+ cations confined in the tetragonal nanotube. Schröder et al. Hong et al., and Feng et al. have recently reported neutral frameworks with similar structure  showed that compound 2 can retain framework integrity up to 443 K  and obtained the tetragonal phase of the anionic framework [EtNHto 443 K . In compmpound 1 . On the mpound 1 . As expeeriments . The theto 363 K . Comparenchanged . These rIn conclusion, an anionic framework exhibiting guest dependent phases was designed. The anisotropic thermal expansion observed in the orthorhombic phase stems from the anisotropic hydrogen bonding between the guest and the framework. This is strongly supported by the observation of normal thermal expansion of the tetragonal phase in the absence of notable hydrogen bond interaction between guest and framework. This mechanism offers a new way to transfer guest motions to host in host–guest composites. A detailed understanding of the mechanisms governing this unusual thermal expansion behavior at the molecular level may provide key information on new design principles for sensitive thermo-mechanical actuators.www.ccdc.cam.ac.uk/data_request/cif.The X-ray crystallographic coordinates for structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC 1021857-1021862 for compound 1 and 1021863-1021869 for compound 2. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via ZZ: synthesized the compounds and performed PXRD measurements. Y-GH: designed this study, conducted the experiments, and wrote the manuscript. KW: designed this study. OS: designed this study and wrote the manuscript. All authors read and approved the final manuscript version to be submitted.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


The analysis of the results for four lightweight aggregate concretes revealed no scale effect in compressive strength tests determined on cored specimens. Neither the slenderness, nor the core diameter seemed to affect the strength results. This fact should be explained by the considerably better structural homogeneity of the tested lightweight concretes in comparison to normal-weight ones. Nevertheless, there were clear differences between the results obtained on molded and cored specimens of the same shape and size.The aim of this paper is to discuss the unrecognized problem of the scale effect in compressive strength tests determined for cored specimens of lightweight aggregate concrete (LWAC) against the background of available data on the effect for normal-weight concrete (NWAC). The scale effect was analyzed taking into consideration the influence of slenderness ( Higher thermal insulation and better sound absorption ,2,3;The possibility of building constructions with longer spans and/or higher height and/or smaller cross-sections of structural members ,5,6;The possibility of autogenous shrinkage elimination ,8,9;Better durability: higher fire resistance, possible higher freeze–thaw resistance, possible lower carbonation, and possible lower water permeability ,14,15,16Less likely to crack, resulting from shrinkage, creep, thermal deformation, or loads ,18,19,20Lightweight aggregate concrete (LWAC) has been one of the most popular and versatile building materials in the world for decades. The most important advantages of its application in comparison to normal-weight concrete (NWAC) of the same strength class are the following: The better durability and lesser likelihood of cracking of LWAC result mostly from the better homogeneity of the LWAC structure. Nevertheless, lightweight aggregate concrete is rarely used as a structural material compared to the most popular option—normal-weight concrete. The most important reasons for this situation are some technological problems with LWAC structure execution, i.e., higher risk of workability loss and concrete segregation, as well as a usually higher price per volume unit, and mainly a lack of versatile procedures for designing, execution, testing, and assessment. Meanwhile, the use of structural lightweight concrete, made of manufactured or recycled aggregates, in the near future should become widespread due to the depletion of natural aggregate deposits and the emphasis on sustainable, less energy-consuming constructions.λ = height (h)/cross-section dimension (d)) are not insignificant. The circular cross-section provides a more uniform stress distribution compared to the square one because its failure is less affected by the end restraint of the specimen. Moreover, the strength of the cylinders is less influenced by the properties of the coarse aggregate due to the more uniform composition of concrete along the circular edge in comparison to specimens of a square cross-section revealing a higher content of cement paste in the corners. Hence, at the same slenderness and cross-sectional area, cylindrical specimens may exhibit higher strength than cubes [λ = 2.0 and 1.0 is ca. 0.85–0.95 and is lower for lower strength concrete. The scale effect in the case of normal-weight concrete of different types—plain, ordinary, self-compacting, high strength and ultra-high strength (reactive powder concrete), fiber reinforced—was proven in numerous studies, e.g., [The size and shape effects of test specimens on the assessment of LWAC’s properties are some of the less qualitatively and quantitatively recognized problems. Generally, according to Griffith’s and Weibull’s theory ,21, the an cubes . Lowerins, e.g., ,32,33,34The more regular shape and size of manufactured aggregates;The smaller difference between the values of the strength and modulus of elasticity of porous aggregate and cement matrix;The better bond between porous aggregate and cement paste, resulting from better adhesion, absorption of mixing water by the porous aggregate, and in some cases, the pozzolanic reaction.In general, the scale effect of LWAC should be expected to be less pronounced in relation to NWAC because the structure of lightweight aggregate concrete is usually more homogenous in comparison to normal-weight concrete. The main reasons for the better homogeneity of LWAC are the following:ckfcyl,/ckfcube,), resulting from the strength classes specified in EN 206 [ckfcyl,/ckfcube, ranges from 0.78 up to 0.87 and is higher for higher strength classes. Nevertheless, there are some reports indicating the opposite trends. It was shown in [λ. Additionally, in the case of LWAC, the aggregate size was not of importance for the scale effect. The probable reason for such a discrepancy in the qualitative assessment of the scale effect of LWAC presented in [Confirmation of the less pronounced scale effect of LWAC was found in some research ,35,36,37n EN 206 , ranges shown in ,40 that ented in ,40 and [ented in ,35,36,37ented in ,40 declaThe basic difference in the scale effect determined for molded and cored specimens consists of lack of the “wall effect” in this last case. Besides, the specimens taken from a structure usually have different, less favorable, conditions of compaction and curing in comparison to molded specimens. Moreover, the process of specimen drilling may itself cause some microcracks in cored specimens. As a result, it is assumed in EN 13791  that forλ = 2.0 and 1.0 may be assumed as 0.82, while for lightweight concrete, there is no relevant information. For LWAC, EN 13791 [Although the European Standard EN 13791  providesEN 13791  recommenSince there are no specific guidelines for the testing and estimation of lightweight concrete strength in a construction or in precast members, the main purpose of the research was to assess the unrecognized scale effect in compressive strength tests carried out on cored LWAC specimens. An additional aim of the research was to verify whether the assumed strength decrease of 15% for cored specimens in relation to molded ones was valid also for LWAC. For these purposes, four series of lightweight aggregate concrete with a closed structure of different compositions were prepared, and for each series of concrete, both standard molded specimens, as well as 12 types of cored cylinders were tested to determine the compressive strength. The research program carried out enabled quantitative and qualitative assessment of the scale effect of cored LWAC specimens against the background of available data on the effect for normal-weight concrete. It also gave some information on the selection of the types of cored specimens to achieve reliable results of compressive strength of lightweight concrete built into a construction or a precast element. Such information may be of practical importance in the case of the estimation of compressive strength for structural assessment of an existing structure or the assessment of the compressive strength class of LWAC in case of doubt.The compositions of prepared LWAC differed in the type of lightweight aggregate (LWA) and the strength of cement matrix, as well as their volume share. Two types of coarse lightweight aggregate were selected: expanded clay (EC) and sintered fly ash (SFA) . These tThe rest of the constituent materials for concrete mixtures were as follows: Portland cement CEM I 42.5 R, natural sand 0/2 mm as a fine aggregate, tap water, and superplasticizer. Cement mortars, being cement matrices for the prepared lightweight concretes, were characterized by significantly different water /cement ratio (w/c) of 0.55 and 0.37. The share of coarse lightweight aggregate in prepared concretes ranged from 52 up to 55%, respectively, for w/c = 0.37 and 0.55. The concrete compositions are presented in From each concrete series, 6 standard cubes (d = 150 mm) and 6 cylinders (d = 150 mm and h = 300 mm) were molded as reference specimens. Additionally, for comparative purposes, standard cubes with mortars of compositions corresponding to those used in the concretes were molded. Besides, 4 big concrete blocks with dimensions of 400 × 600 × 1000 mm were cast for drilling cored specimens . SpecimeAfter 28 days of curing, cores were drilled from the blocks and cut into specimens according to EN 12504-1 . Four drThe total number of cored specimens to be tested was 336. The density and compressive strength of the moist molded and cored specimens were tested at the age of 28 days according to EN 12390-7  and EN 1The results of the tests carried out on molded specimens are presented in cmf), calculated as the averages of six cores of the same type, are presented in CMf) was calculated as an average of the mean values of all core types. Meanwhile, strength results presented in The results of the compressive strength tests determined for cored specimens are presented in 3, and in wet condition, the corresponding range was 1290–1880 kg/m3. The “wall effect” seemed to have a negligible influence on concrete density; therefore, there were almost no differences between the results determined for molded and cored specimens. Moreover, the similar results of density tests carried out on molded specimens cured in water and cored specimens indicated that the condition of the cores was similar to the saturated due to external curing, but primarily owing to inner curing with water accommodated in porous aggregate. The values of moisture content of concretes were particularly interesting. Despite expanded clay being characterized by water absorption almost twice as high as for sintered fly ash, the moisture content of the tested lightweight concretes seemed to depend mainly on the tightness of the cement matrices. If the aggregates were used initially saturated, their water absorption would certainly affect the water absorption/moisture content of the composites. In the case of the tested concretes, the aggregates were only initially moistened to the moisture content, ensuring a good bond and sealing the aggregate structure with the cement paste. Such an effect was proven in [Analysis of the results showed, as assumed, significantly different levels of compressive strength and density of the four concrete series. The concrete strength ranged from 14.5 to 49.5 MPa when determined for molded cube specimens and from 13.8 to 47.6 MPa for molded cylinders. The concrete oven-dried density ranged from 1140 to 1680 kg/mroven in .Generally, concretes made with stronger, sintered fly ash aggregate (I SFA and II SFA) achieved higher density and compressive strength  than those made of expanded clay (I EC and II EC). The strength improvement by the application of a stronger mortar (II of w/c = 0.37) as a cement matrix was also much more effective in the case of SFA concretes than for EC concretes . In the cmfcyl,/cmfcube,) was dependent on concrete homogeneity: The smaller the difference in aggregate and cement matrix strength, the higher the ratio. The mean ratio values were 0.95, 0.93, 0.99, and 0.96, respectively, for concretes I EC, II EC, I SFA, and II SFA. Therefore, these values were clearly higher than those resulting from EN 206 [cmfcyl,/cmfcube,: assessment of lightweight aggregate concrete strength determined for standard cylinders may lead to a higher class than in the case when it is determined for standard cubes.The ratio of strength determined on standard cubes and cylinders (m EN 206  and confcmf) was slightly bigger in comparison to concretes with sintered fly ash. To confirm these observations, a more detailed analysis was carried out. The analysis covered the results’ dispersion for a particular cored specimen type, as well as the ratio of mean strength values determined for the reference cored cylinder  and a particular cored specimen type.In the case of the cored specimens, the size effect turned out to be basically imperceptible . This tefσ) and coefficient of variation (vf = σf /fc) were rather independent of the volume and slenderness of cored specimens. The rule of greater dispersion of the strength test results for specimens of smaller volume was not confirmed here. The coefficients of variation for a particular cored specimen type are presented in fv ranged from 0.01 to 0.15, and their average values were 0.07, 0.08, 0.05, and 0.03, respectively, for concretes I EC, II EC, I SFA, and II SFA. The values of fσ for a particular type of cored specimen ranged from 0.3 to 2.2 MPa, and their average values were 1.1 MPa, 0.9 MPa, 1.5 MPa, and 1.2 MPa, respectively, for concretes I EC, II EC, I SFA, and II SFA. These values were almost the same as the standard deviations of the values of mean strength (cmf) in relation to the global average (CMf), presented in fv proved the excellent structural homogeneity of the tested lightweight concretes, especially of composites with sintered fly ash aggregate. The results also indicated the possibility of using even the smallest core specimens (within the considered range) to assess strength in a lightweight concrete structure without increasing the number of specimens.The research of strength results dispersion showed that for all tested concretes, the values of standard deviation  are presented in cm 300:150 core/fcm 150:150 coref) was considerably higher (on average 0.98) than the 0.82 assumed by EN 13791 [R equaled exactly 1.00, and no influence of slenderness or core diameter was observed. This means that in the case of such concretes, the type of cored specimens may be assumed as not relevant to in situ strength results. However, in the case of expanded clay concretes, the interpretation of the strength ratio results was not so clear. The mean ratio value was 1.06 and 0.94 for concrete I EC and II EC, respectively, and generally, the dispersion of the ratio values was much bigger in comparison to concretes with SFA. To determine the reliable value of the strength ratio for such weak concretes, some additional verification tests should be carried out.The results of the analysis of the ratios of mean strength values determined on the reference cored cylinder (d = 150 mm and h = 300 mm) and on cored specimens of a particular type , which were examples of typical LWAC used for precast member manufacturing or construction execution, revealed a higher ratio cmfcore, cm/fcyl,  (on average 0.90) than that assumed in the standard (0.85). Generally, due to different technologies of LWAC production and various types of structure of lightweight aggregate applied in the world, the ratio value of 0.85 may be retained in the common guidelines for assessment of concrete strength in a structure or a precast element. Nevertheless, in the case of lightweight aggregate concrete of a more homogeneous structure, the overestimation of the strength class of LWAC built in a construction or precast elements should be taken into account. Therefore, the standard recommendation to form provisions valid in the place of LWAC use was fully justified. For tested LWAC, excluding concrete II EC, the “wall effect” and different curing temperature seemed to be the dominant factors determining the difference between strengths specified on cored and molded specimens. Concrete moisture condition (due to inner curing) and susceptibility to microcracking resulting from the drilling process or high temperature probably were of less importance here than in the case of NWAC.Despite the demonstrated lack of size and shape effect in the compressive strength tests of lightweight concretes, there were clear differences between the results obtained for molded and cored specimens. The ratio of strength values determined on cored and molded cylinders The research program carried out and the analysis of obtained results revealed no scale effect in compressive strength tests determined on cored specimens of four types of lightweight aggregate concretes with a closed structure. Neither the slenderness, nor the core diameter seemed to affect the strength results. This fact should be explained by the incomparably better structural homogeneity of the tested lightweight concretes in comparison to normal-weight ones. Moreover, the rule of the greater dispersion of the strength test results for specimens of a smaller volume was not confirmed here. This means that, in contrast to NWAC, it was possible to reliably assess the compressive strength of such LWAC types built in a construction or precast components using even the smallest cores (within the considered range) without increasing the number of specimens. Besides, in the case of such concretes, it seemed sufficient to use cores with a slenderness of 1.0 instead of the required 2.0 if the strength test results were to be related to 2:1 molded cylinders. Nevertheless, it should be supposed that in the case of lightweight concrete prepared with initially saturated aggregate or with aggregate particles of tighter and/or smoother external shale, the size effect may be more pronounced. Therefore, the quantitative findings of this research could not be generalized for all types of LWAC.cmfcore, cm/fcyl,  was slightly higher (on average 0.90) than the 0.85 assumed in the standards. As a result, the application of the standard ratio for compressive strength assessment of an existing structure made of such types of LWAC may lead to overestimation. Therefore, the standard recommendation to form provisions valid in the place of LWAC use was fully justified.Despite the demonstrated lack of scale effect in the compressive strength tests of lightweight concretes, there were clear differences between the results obtained on molded and cored specimens. However, for tested LWAC, excluding the concrete II EC, the ratio The analyze of the relationship between strength specified on standard molded specimens showed that, due to the much less pronounced scale effect of LWAC in relation to NWAC, assessment of lightweight aggregate concrete strength determined on standard cylinders may lead to a higher strength class than in the case when it is determined on standard cubes.


The issue of chloride induced corrosion of reinforced concrete is a serious problem affecting infrastructure globally and causing huge economic losses. As such this issue has gained a considerable attention in the scientific community in the recent past. Layered Double Hydroxides (LDHs) have recently emerged as a new class of concrete-additives with a potential to increase the chloride resistance of concrete and mitigate corrosion. LDHs are clay like structures consisting of positively charged layers of cations with associated hydroxides and exchangeable anions in between the layers. Due to this charge balanced structure, LDHs possess the property of encapsulating an anion from the environment and replacing it with an exchangeable anion present in its layers. Potential applications include chloride entrapment in concrete and delivery of corrosion inhibiting anions. However, many versatile compositions of LDHs can be easily synthesized and their application as cement additives reach far beyond corrosion mitigation in concrete. This review presents a summary of recent advances on the applications of LDH in concrete. An extensive set of recently published literature has been critically reviewed and trends have been identified. Our planet has experienced an unprecedented population growth in the last century . The rismissions . The carmissions ,5,6. To missions ,12,13,14missions ,16, ice missions ,18,19, amissions ,21 etc.,Out of the above mentioned degradation mechanisms, reinforced concrete structures are particularly susceptible to chloride induced corrosion ,22. Chlo− ions. Potential applications include chloride ion and carbonate ion entrapment in concrete. This work provides an overview on the state-of-the-art on the applications of LDHs in concrete technology, based on a critical review of recently published reports and articles. One of the first reviews on application of LDHs in concrete was provided by Raki et al. [In recent times, layered double hydroxides (LDHs) have emerged as a new class of engineering materials ,25 whichi et al.  in 2004 i et al. ,28 in 20Concrete is the most widely used engineering material ,30 whichConcrete is an alkaline environment ,32,33 anOnce the critical threshold is reached at the steel-concrete interface, the onset of corrosion is said to take place. This leads to the formation of active anodic regions, and in the presence of moisture and oxygen lead to the formation of a corrosion cell between anodic and cathodic parts of the rebar. As corrosion progresses, rust products are generated as a result of corrosion reactions. Rust products are more expansive than parent steel  and theiThe transport of chloride ions in concrete matrix occurs through the pore network ,46. It i− level.A significant contribution of bound chlorides is attributed to tricalcium aluminate  interlayers by replacement of loosely bound OH− ion by Cl− thereby maintaining electroneutrality in the system. This physical adsorption mostly proceeds on the external layers of CSH layers [On the other hand, the physical binding of ClH layers . Tang etH layers  reportedH layers  whereas H layers  considerH layers .Additionally, supplementary cementitious materials (SCM) are added to concrete mixes to replace cement binder, usually as a replacement. One of the motivations behind replacement of cement binder by SCM is to reduce the carbon footprint of concrete industry and also to impart additional functionality to concrete . As such−) from the environment and release a tailored anion in its place (like a corrosion inhibiting anion such as NO−) via a controlled release mechanism. This property has been termed as “Self-Protection” of concrete and can result in an extended corrosion initiation stage as well as a reduced corrosion degradation in the propagation stage as shown schematically in In the last two decades, Layered Double Hydroxides have emerged a new class of nanoscale engineered concrete additives which can impart additional functionality to concrete. Although the role of LDHs in concrete technology is widespread in different areas, the most outstanding application of LDH is its ability to trap anions  as SCM also leads to the production of hydrotalcite like phases [LDHs are not strange to concrete chemistry either. The hydration of cement binder is accompanied by the formation of a family of hydrated calcium aluminate phases (AFm) which belong to the broader class of the LDH family. According to Matschei et al. , in somele anion . AFm phae phases ,91 whiche phases . Studiese phases  revealed− containing solution at time (t = 0 s) under stirring conditions. Chloride ion concentration was constantly monitored for 300 s after LDH immersion. It is evident that LDH was able to reduce the Cl− concentration from the surrounding solution. − ion from Zn-Al-NO− ions is presented. This confirms the ion release property of LDH. An in-depth explanation on the ion-exchange property is presented later. Apart from the ion exchange property, LDHs posses numerous other interesting properties which are of vital importance in other branches of science and technology. The interested reader is referred to the extended literature on LDH by Evans et al. [As mentioned previously, the ion exchange property is the most vital property concerning the application of LDH in concrete. s et al. , Forano s et al.  and Kuans et al. .− by LDH in cement/concrete, it is important to understand the compatibility of LDH with cement. In this section, compatibility with cement, influence on mechanical properties and the effect of dosage on the microstructure are discussed. The discussion is based on a critical analysis of recently published literature in this field.LDHs have been mostly used in concrete in order to improve its chloride resistance by harnessing the ion exchange property of LDH. However, prior to discussing the ion exchange property and chloride encapsulation of ClLDH are nanoparticles which are reported to refine the microstructure of concrete without causing a substantial increase in porosity. As LDH is usually added in its powdered form to concrete mixes, these nanoparticles are uniformly distributed in the matrix and in the course of cement hydration process, LDH can provide additional nucleation sites for cement hydration. This in turn can promote growth of CSH gel into the voids, thereby refining the microstrucutre. Additionally, the particles themselves act as micro-fillers or micro-aggregates inside the cementitious matrix. Depending on the type of LDH in use, it can have a positive effect on the mechanical properties of concrete especially its compressive strength, see From Contrary to the studies of Qu and colleagues , Yang etDifferent authors who have worked with LDH have resorted to different dosages of LDHs for applications in concrete. Usually a range of values is evaluated and higher dosages are used with the aim of improving a desired property of concrete or to check the ultimate threshold of the dosage in relation to the desired property. However, while one property might improve, addition of LDH can affect other important properties of mortars and concrete. One of the predominant effects is on porosity as found by Yang et al.  where poPrevious studies by Qu et al.  have rev− are the most accelerating cation and anion respectively [− ions. The underlying mechanisms regarding effect of various ion on the hydration kinetics are not fully understood and the reader is directed to the works of Myrdal [LDHs have also been used to modify the hydration kinetics of concrete. It is generally believed that the addition of nano-particles can lower the energy barrier for precipitation reactions taking place in liquid state and also provide additional nucleation sites for cement hydration due to their very small size and very high surface area ,108,109.ectively . One canf Myrdal  for furtAdditionally, Xu et al. conducted in situ XRD analysis of cement mortars with and without Ca− in the pore solution thereby retarding the hydration reactions [On the other hand, LDH can also decelerate or retard cement hydration. Recent studies by Gomes et al.  reveled ydration ,113. Theeactions . Gomes eeactions  also obsself-protection process in concrete.LDHs by their nature and unique chemical structure are able to exchange anions present in the external environment by an anion from their inter layers. This process is governed by the difference of chemical potential in the direction of decrease of Gibbs free energy. In order to maintain electro-neutrality, a charge balance is maintained before and after the ion exchange ,123,124.The ion exchange property of LDH is also used to produce LDH with a desired anion by exposing LDH to a solution of this anion. The desired anion is captured by LDH by replacing the anion originally present in its galleries. The LDH is then usually centrifuged and separated from the filtrate, washed and dried. It must be noted that anion uptake is governed by many factors such as temperature , particl−/HCalcined LDH has also been used to conduct ion capture in concrete . Calcina− ions present in the pore solution at equilibrium. Studies report that higher the external Cl− ion concentration, higher is the Cl− binding due to LDH. As such, the binding behavior can be represented by Freundlich or Langmiur type isotherms  and the Clsotherms  right. Ia et al.  computati et al.  concludei et al.  was not i et al.  as they − but on the other hand, higher dosages can certainly increase the porosity of concrete which facilitates Cl− ingress.The ion exchange property of LDH has been used extensively in the corrosion protection of concrete. Many authors have used LDH to conduct additional chloride binding in concrete as well as controlled delivery of corrosion inhibiting anions. Numerous applications also report on the use of LDH for mitigation of carbonation induced steel corrosion in concrete. − ions from the pore solutions lessening its concentration. Zhonghe et al. [LDH is able to capture Cle et al.  studied n et al.  used com− ion following a mechanism of dissolution and recrystallisation, similar in behavior to AFm phases in concrete. The authors concluded that the ion exchange mechanism is the major contributor but it is hard to distinguish between these mechanisms in cementitious environment. The selectivity series were demonstrated to be Cl−> OH−> NO−. Surface adsorption of ions on LDH surface has also been reported as an uptake mechanism for LDHs. Ke et al. [Although chloride capture due to LDHs has been mostly due to the ion exchange property, other binding mechanisms do exist. Chen et al.  suggestee et al.  concludeOne of the best real life tests on the use of LDHs in concrete structural members was carried out by Tatematsu et al. . In thei− uptake was observed to be followed by a delivery of OH− ion from AFm phases [As mentioned previously, AFm phases in concrete also belong to LDH family and are responsible for the majority of the chloride binding ,135,136.m phases . Hirao em phases  experimem phases .− ion and simultaneous release of a corrosion inhibiting ion. LDHs can be easily loaded with corrosion inhibiting ions. The use of NO− as a corrosion inhibitor has been well documented [− can be easily incorporated into LDH. However, it is important to note that the inhibitive action takes place only at the steel-concrete interface. The nitrite ions released in the bulk will have to transport themselves to the steel-concrete interface. On the other hand, LDH loaded with inhibiting ions can also be applied as a cement slurry coating on the rebar surface prior to embedment. Yang et al. [The dual benefit of self-protection of reinforced concrete can be achieved by the capture of Clcumented ,144 and g et al. , applied− ion in corrosion inhibition was previously tested by Dry [− ion with the aim of capturing Cl− ion and releasing NO− ion and reported positive results.Use of NOd by Dry  who useds et at.  used LDHMany authors ,100 haveChloride diffusion test were also conducted by Qu et al.  with 0.5It is well understood that the COent ions . Due to ent ions ,148. In ent ions , cement ent ions . This hi− ions and has bound Cl− ions, it might release these bound chlorides upon exposure to carbonates, since uptake of carbonate ions is preferred over chlorides [− can be replaced with CO− with CO−. The study was extended to other anions as well but it was observed that only small ions such as Cl− and Br− and ClOThe high affinity of LDH towards carbonate ions can have a negative impact on chloride binding capacity of LDH. It is important to note that, if the LDH was previously exposed to Clhlorides . Ke et ahlorides  studied hlorides  concludei et al.  was abley Miyata .In this study a review on the applications of LDH in concrete is presented. The main advantage of using LDH in concrete is to improve the chloride and carbonation resistance of concrete. This can consequently extend the service life of concrete structures exposed to corrosive environments. In this aspect, this review presents many recent studies reporting on the application of LDH not only in pore solutions but also in concretes, mortars and pastes. The review also showcases that even though LDH addition is primarily aimed to improve chloride/carbonation resistance, its addition can also affect hydration, strength, microstructure and other properties of concrete. Additionally, recommendations on LDH dosage are presented. In general, this review should aid scientists and engineers to develop a basic understanding on the use of LDH as an engineering material and help in the design of experiments and be able to foresee main results. Depending of the type of LDH as starting material, quantitative and qualitative effects on desired property as well as associated properties of concrete are presented, trends are identified and selected results are discussed. A compilation of focused results are presented in text as well as in tables, highlighting the benefits of LDH as well as details of the experiments and main results.The ion exchange property of LDH has been widely exploited in concrete technology with a majority of applications aimed towards increasing the chloride and carbonation resistance of concrete. Although the underlying mechanics for these applications are well researched and thoroughly discussed in this text, many other mechanisms are still not fully clear and vary with the type of LDH, such as, the effect of increase of compressive strength and flexural strength upon addition of LDH, as well as acceleration of hydration kinetics. Moreover, there is no general consensus on the optimum dosage of LDH in concrete, although a 2% dosage has been identified as the threshold of LDH addition from the presented literature. A higher dosage can cause agglomeration of particles and have a profound effect on key properties such as chloride durability and compressive strength.Despite the recent advances in the application of LDH in concrete technology, many issues stay unresolved and need further understanding. Firstly, the stability of LDH particles inside concrete is not well understood and needs more research in the future. Higher pH of fresh concrete can potentially cause partial dissolution of LDH particles which can in turn reduce their functionality. Apart from that, particle size is a very important parameter as finer particles exhibit more surface area than coarse particles. It has been reported that particle size effects LDH functionality as well as some vital concrete properties. Very few studies have reported on these issues and more research is required in this area. Additionally, one important factor is the economic aspect of using LDH as a concrete additive and not much information is available on the cost of LDH and its carbon footprint. These two factors are very important for acceptance of LDH in the concrete infrastructure market. Dismantling buildings made with nano-particles can be dangerous as there is the chance that these nano particles might be released in the environment. Therefore, proper guidelines should be made available to building engineers and these buildings should be classified for easy recognition in the distant future. More research must be conducted on the interaction of humans against long term exposure to LDH. All these points need more efforts and extensive research.However, the authors strongly believe that LDH due to its unique chemistry, versatile combinations, facile preparation and ease of incorporation into concrete, will be used more in the future to produce smart concrete structures. To make LDH a promising concrete additive, more research should be focused towards their application in concrete. One of the biggest foreseeable benefit of using LDH in concrete is the enhanced chloride/carbonate binding effect, although other notable benefits have been presented. This class of additive can result in smart structures which can potentially show extended service life and can directly result in a substantial economic benefit in the global infrastructure market.


We report novel pseudorotaxanes based on the complexation between pillar[4]arene[1]quinone and 1,10-dibromodecane. The complexation is found to have a 1:1 host–guest complexation stoichiometry in chloroform but a 2:1 host–guest complexation stoichiometry in the solid state. From single crystal X-ray diffraction, the linear guest molecules thread into cyclic pillar[4]arene[1]quinone host molecules in the solid state, stabilized by CH∙∙∙π interactions and hydrogen bonds. The bromine atoms at the periphery of the guest molecule provide convenience for the further capping of the pseudorotaxanes to construct rotaxanes. Relying on the research of basic science, supramolecular chemistry has become an important mean for constructing functional materials from the bottom up as well as an important way to create new substances with functions –5. ThrouAs a new class of supramolecular macrocyclic hosts, pillararenes have received extensive attention in recent years due to their unique pillar structures and rich environmental responsiveness –29. TherH and 1,10-dibromodecane  was conducted to investigate the complexation properties. However, no signal related to the complex but only peaks of H and G. The formation signified a 1:1 binding stoichiometry in chloroform-d at room temperature  calculated by the nonlinear curve fitting method was 20.0 ± 2.4 M−1 (Figures S7 and S8, A Job plot based on the proton NMR data was made to determine the complexation stoichiometry between 1 of H and Ha of G, which agreed with the formation of the complexation; the alkyl chains of G were encapsulated in the cavity of H. This further indicated that the interactions between H and G were weak.To further explore the geometry of the complexation in solution, we performed a NOESY study . Only onH, we wondered the influence of G on the optical properties of H, so an UV–vis spectroscopy experiment was carried out. We investigated the changes in the UV–vis absorption of the complex at different guest concentrations corresponding to 0.5 and 1 equiv. As shown in G did not affect the absorption maximum. This also indicated that the interactions between H and G were weak.Based on the UV–vis properties of 1H NMR experiments revealed that the pillar[4]arene[1]quinone encapsulated the guest molecule with a 1:1 stoichiometry to form a [2]pseudorotaxane in solution. One possible reason may be that the interactions between H and G were weak and that there was a complexation competition with solvent molecules. Furthermore, the addition of G did not change the maximum UV–vis absorption wavelength of H. The bromine atoms at the periphery of the guest molecule provide convenience for the further capping of the pseudorotaxanes to construct rotaxanes, which will broaden the range of potential applications of pillararene derivatives for the manufacture of sophisticated supramolecular architectures and functional supramolecular systems.In summary, we constructed novel pseudorotaxanes based on a pillar[4]arene[1]quinone and 1,10-dibromodecane. Single crystal X-ray diffraction analysis showed an alkane molecule threaded into the cavities of two pillar[4]arene[1]quinone molecules, forming a [3]pseudorotaxane in the solid state. However, File 1H and G, crystallographic data, and characterization studies on the complexation between H and G in solution.Materials and methods, characterizations of File 2H and G.CIF file for the complex between


The presence of embedded steel affects the estimates obtained for the compressive strength of concrete during ultrasonic testing, as it increases the ultrasonic wave velocity. Thus, if the presence of steel in concrete is inevitable, then a correction factor is required for an accurate estimation of the concrete strength. While previous studies focused on the effect of steel reinforcing bars on the speed of ultrasonic waves in concrete, this work expands on the significance of embedded steel from steel bars to include steel plates. The wave velocity was measured for varying dimensions of embedded steel plates from 15 mm to 150 mm using 54-kHz ultrasonic testing equipment. Through experiments, the effect of steel plates on the ultrasonic testing of concrete was quantified to derive proper correction factors. It was found that the thickness, depth, and height of the steel plates significantly affected the test results. These findings can be applied to ultrasonic testing to estimate the compressive strength of concrete consisting of a significant volume of steel, such as in steel-reinforced concrete structures. To determine the compressive strength of concrete, destructive coring tests as well as nondestructive testing (NDT) methods such as the rebound hammer, ultrasonic tests, and radar are widely used . HoweverTest methods are also categorized into penetration and nonpenetration modes. Because concrete typically exhibits variations in its material property even within a single member, concrete strengths are better obtained via penetration mode tests such as in ultrasonic testing by passing through the entire cross-section of a member rather than part of it, as in the nonpenetration or reflection mode. However, ultrasonic waves are known to be affected by the presence of embedded steel inside the concrete. Thus, it is necessary to quantitatively investigate the significance of the effect of steel.When dealing with the effect of steel in concrete on the estimation of the compressive strength of concrete, two scenarios need to be considered. First, it has been reported that when an ultrasonic wave crosses the steel bars in a direction that cuts through the cross-sectional areas, the summation of the cross-sectional areas of steel causes the wave velocity to increase ,5. SeconCurrently, steel-reinforced concrete (SRC) structures are used extensively worldwide. In SRC structures, in addition to traditional steel reinforcing bars, steel plates are incorporated to increase the load-carrying capacity ,10. ThusIn this study, the effect of steel plates on ultrasonic testing for estimating the compressive strength of concrete was quantified through a series of experiments. The results obtained indicate that the thickness, depth, and height affect the test results significantly. The findings can be applied when ultrasonic testing is utilized to estimate the compressive strength of concrete consisting of a significant volume of steel, such as in SRC structures. The work presented in this paper can be applied to estimate the compressive strength of concrete in the field when the exact inside layout of the section is known. Unlike metal or metal-based materials, the testing of concrete has an intrinsic uncertainty owing to its highly heterogeneous nature. The proper estimation of concrete strength in the field during construction is always an important task to be performed. Usually, control cylinders are made on-site as ready-mixed concrete is delivered by trucks, and the cylinders are tested to ensure the strength of the concrete cast inside the formworks. However, the curing conditions of these cylinders and those of actual cast concrete as building members are different. Again, because of their heterogeneous nature, it is helpful to have some data from the cylinders, but the results cannot guarantee the quality of the actual cast concrete. Thus, it would be helpful if the curing conditions of cast concrete can be more accurately monitored in the field. In addition to the heterogeneous problem of concrete, reinforced or steel-reinforced concrete structures have steel reinforcing bars and/or steel plates. This further complicates the concrete strength estimation. The effect of steel on the wave velocity inside concrete has been reported in many previous studies; however, the effect of steel plates on the velocity inside an actual steel-reinforced concrete member has never been reported. With the increased use of steel plates inside concrete structures, it is now necessary to address this situation in terms of concrete strength tests in the field.In the field, a concrete member under testing is covered with a formwork, as shown in A proper location for measurements can be identified, as shown in (a)The reference specimen is made on-site with the same inside configuration as that to be built for a structure under construction.(b)Control cylinders were made to measure the compressive strength of concrete over the curing time as often as needed.(c)Ultrasonic measurements were conducted at a predesignated place for a given curing age. This was repeated for up to four weeks, which is considered when 100aconcrete strength of is achieved.(d)At each ultrasonic measurement time in Step (c), the compressive strength of the control cylinders is measured.(e)Finally, a database or graph is obtained for the ultrasonic velocity vs. the compressive strength of concrete.(f)Now, it is time to measure the velocity of a real steel-plate-reinforced concrete member. Once the value is obtained, it is compared to the graph in Step (e) so that the compressive strength of the concrete in the member can be estimated.To measure the compressive strength of the concrete of a steel-plate-embedded concrete member in the field, the following procedure is proposed, as shown in Ultrasonic waves are used during the NDT of concrete to determine the thickness of the concrete , the locThe velocity of an ultrasonic wave can be determined using Equation (1) . As the 3). In concrete, the elastic modulus of cE is directly related to the strength of the concrete, The pulse velocity V (m/s) of longitudinal stress waves in a concrete mass is related to its elastic properties and density according to the following relationship:This study investigated how the volume of steel affects the wave velocity, with the objective of applying the quantitative results obtained to the ultrasonic NDT of concrete with steel plates inside. A mortar mix was used to avoid the wave propagation complexity that arises when a concrete mix with gravel is used. The mix ratio of the mortar was water:cement:sand at 1:2:4. Its28-day compressive strength was 19.85 MPa. For the ultrasonic measurements, 54-kHz ultrasonic testing equipment(Pundit Lab Ultrasonic Instrument by Proceq in Switzerland) in the form of a transmitter and receiver was used. The probe was50 mm in diameter and 46 mm in height, and weighed287 g. The contact surface diameterwas36.77 mm. It was a single-crystal type with a bandwidth of less than 10 kHz. The piezoelectric electrical cell capacity was2 nF. The receiver gain was selected 100 times, and the excitation voltage was set to 125, 250, or 350 V. To examine the effect of steel plates on the velocity of ultrasonic waves in concrete, the parameters of thickness (t), depth (d), and height (h) of the steel plates were varied .Thus, seven steel plates of different dimensions were obtained. For each set of measurements, one parameter was varied, and the other two were kept constant. Each plate was placed inside a mortar specimen with dimensions of 150 mm × 150 mm × 225 mm . a (mm) that was varied from the centerline of the plate, as shown in The probes were placed along the depth of a plate at a distance The first set of measurements was made while varying the thickness of the plate. With the depth and height fixed at 75 mm and 150 mm, respectively, the thicknesses of the plates were varied, with values of 15 mm, 20 mm, and 30 mm, as shown schematically in For each measurement point, a total of 15 measurements were taken, and the average value and standard deviation of each measurement are shown in First, the velocity through a mortar specimen was 4150 m/s on average with a standard deviation of 25 m/s, which is 0.6% of the average value. This is regarded as quite stable, and is expected owing to the homogeneity of the mortar. Using this as a reference, the measured velocity with different thicknesses of the steel plates is plotted in In general, the specimens with the steel plates showed a significant difference in the measured velocity compared to the mortar. The 30-mm-thick plate specimen showed the highest velocity of 4871 m/s, followed by the 20-mm-thick plate specimen, which showed a velocity of 4827 m/s. Finally, the 15-mm-thick plate specimen showed a velocity of 4753 m/s along the centerline. The standard deviations for the 30-mm-, 20-mm-, and 15-mm-thick plate specimens were 14 m/s, 33 m/s, and 45 m/s, respectively. Therefore, the results were stable. Because the probes were placed farther away from the centerline of the steel plate, it is clear that the effect of the presence of the steel plate on the wave velocity diminished, as shown by the decreasing lines over the a axis in Even though the results with the steel plates showed a clear difference from the mortar results, the distinctions between different thicknesses overlapped somewhat, especially considering the band created by the standard deviation. The measurement results showed that it was quite possible to consider the effect of the steel plate when determining the compressive strength of concrete using ultrasonic testing, as shown in a in The second set of measurements was obtained while varying the depth of the plate inside the mortar specimen. Four specimens were used for the measurements. With the thickness and height fixed at 20 mm and 150 mm, respectively, the depth of the plates was varied over the values of 37.5, 75, and 150 mm, which corresponded to 25%, 50%, and 100% of the maximum depth, respectively. The depth denotes a direction parallel to the wave propagation. This depth change will significantly affect the wave velocity. Eight ultrasonic measurements were made at the centerline of the plate and at certain distances away from the centerline, which are denoted as a variable distance The results obtained showed a significant distinction in the measured velocity as the depth of the steel plate was varied, as shown in The average velocity of the wave in the mortar was 4180 m/s. It increased to 4392 m/s for the path along the 37.5-mm-deep steel plate, 4696 m/s for the path along the 75-mm-deep steel plate, and finally, 5421 m/s for the path along the 150-mm-deep steel plate. The wave velocity was significantly affected by the length of the steel located parallel to the direction of wave propagation. The results can be used as a basis for considering the effect of steel on the ultrasonic testing of reinforced concrete. The standard deviation for the 37.5-mm-deep plate specimen was 55 m/s; for the 75-mm-deep plate specimen,21 m/s; and for the 150-mm-deep plate specimen, 27 m/s, along the centerline of the plates. Similar to the thickness variation results in a in The third set of measurements was made while varying the height of the plate. Four specimens were used for the measurements. With the thickness and depth of the steel plate fixed at 20 mm and 75 mm, respectively, the height of the plates was varied over the values of 37.5 mm, 75 mm, and 150 mm, which corresponded to 25%, 50%, and 100% of the maximum height, respectively. Eight ultrasonic measurements were made at the centerline of the plate and at several distances away from the centerline, denoted as variable distance In contrast to the previous set of measurements, only marginal differences were observed among the measurements. This implies that ultrasonic measurement is not very sensitive to height. However, the measured velocity was greater than that in concrete with no plate inside. The average velocity of the wave for the 37.5-mm-high plate was 4893 m/s; for the 75-mm-high plate, 4895 m/s; and for the 150-mm-high plate,4924 m/s. The standard deviation for the 37.5-mm-high plate specimen was 15 m/s; for the 75-mm-high plate specimen, 10 m/s; and for the 150-mm-high plate specimen, 13 m/s, along the centerline of the plates. Similar to the previous results in The average standard deviation of the combined results with varying heightsof the steel plate was 18 m/s. It seems to be acceptable to use The purpose of conducting these experiments was to investigate the effect of steel on the measurement velocity of an ultrasonic wave so that the variation in wave velocity owing to different types of steel can be quantitatively studied. In practice, steel is inevitably present inside concrete. Ultrasonic test measurements are required to relate the measured velocity to the compressive strength of concrete, which is the most important property of concrete as it is being cured. The findings from the results of these measurements can provide useful relationships to determine the velocity of the wave in concrete with steel. This can be achieved by subtracting the value of the velocity owing to steel from the velocity measurement of the combined steel and concrete. Finally, the corresponding velocity of only concrete is obtained and is related to the compressive strength of concrete. The entire procedure is explained as follows:In the laboratory, a database contains different steel shapes that are used practically and at different offset distances from the steel (as expected in the field). Then, we obtain the velocity difference data, as shown in From the laboratory measurements, the database for Practically, for the actual application of this method, the ultrasonic velocity is measured with known steel shapes and at known offset distances from the steel Finally, the compressive strength of concrete was evaluated from the premade velocity-vs.-strength graph. This graph can be easily obtained by making concrete specimens with different strengths and corresponding ultrasonic measurements. An example of such a graph is shown in The results of this study showed that the thickness, length, and height of the steel plates significantly affect the ultrasonic wave velocity, depending on the size and orientation of the plates. This paper proposed a method for determining the compressive strength of concrete in the field when there is a steel plate inside. By preestablishing a database in the laboratory with the same conditions of the steel layout as in the field, it is possible to exclude the effect of the increased portion of the velocity owing to the steel. The findings from this study can be applied when ultrasonic testing is utilized to estimate the compressive strength of concrete consisting of a significant volume of steel, such as in SRC structures. The work presented here applies to fresh concrete with an inside steel layout that is known beforehand. For already hardened and existing concrete members, a different approach needs to be studied.


The corrosion by severe sulfuric acid attack at pH 2 of two self-compacting concrete (SCC) types that are based on ordinary Portland cement (OPC) and granulometrically optimized blast-furnace slag cement was evaluated by three complementary tests that were performed in different research institutes. The use of SCC is a smart and promising solution to improve the performance of concrete in an aggressive environment, especially regarding ready-mixed concrete applications, since good compaction is less dependent on workmanship. The relevance and practical advantages of the different test protocols and the influence of the experimental parameters are discussed. It appears that the frequency of renewing the acid solution during the exposure period is the main parameter that influences the mass loss and the rate of degradation, while the sample geometry and the ratio between the volume of solution and concrete surface area had no clear influence. Nevertheless, there was reasonable agreement between the methods regarding the magnitude of the concrete degradation (resulting in a mass loss of about 2.5 kg/m² in six months time). The use of granulometrically optimized slag cement provided a moderate increase of the concrete resistance against acid attack, and this practice might be recommended in order to increase the durability of structures exposed to sulfuric acid media. The fact that the difference in comparison with SCC-OPC was rather limited shows that the influence of the cement type becomes less relevant in the case of concrete with low w/c ratio and optimized concrete technology. The evaluation of the durability of concrete infrastructures that are exposed to aggressive environments needs a deep investigation of the degradation mechanisms in severe conditions, such as mechanical fatigue, thermal stresses, freeze/thaw cycles, carbonation, soft-water leaching, and chemical attack. Among chemical attacks reducing the service life, acid attack is particularly detrimental for cementitious material due to its alkaline matrix. During an acid attack, the dissolution of the hardened matrix and, if soluble, the aggregate results in the formation of a visible degraded layer, which affects the life cycle performance and the maintenance costs of the infrastructures. Acid attack can be extremely diverse and it is related to specific applications . The sevAcidic media with pH lower than 4.5 are considered very aggressive. Nevertheless, the aggressiveness cannot be determined by the pH alone. Degradation by sulfuric acid is considered to be one of the most severe deterioration mechanisms and usually originates from industrial processes, for example: flue gas condensation in cooling towers, geothermal energy generation, mining production processes, wastewater treatment in sewage plants, and transport in sewer pipes.4 2H2O) on and below the concrete surface, which can induce tensile stresses in concrete, resulting in cracking and spalling high [SO → Ca(OH)2 + H2SO4→ CaSO4 2H2O (gypsum) → dormant period with precipitation → expansion42-]low [SO → 3CaO Al2O3 12H2O + 3(CaSO4 2H2O) + 14H2O → delayed ettringite formation → expansionIn this method, the concrete prims are immersed in sulfuric acid solution at a constant pH of 2. During the first weeks, a high precipitation of gypsum occurs on the concrete surface due to the continuously increasing sulfuric acid concentration that is necessary for maintaining the pH constant and, consequently, increasing sulfate concentration. The prismatic shape crystals of gypsum induce an expansive reaction with a consequent cracking. Anyway, especially during the first week, the precipitated gypsum creates a sort of passivation layer that induces a dormant period . The cryIt was observed that the kinetics of the degradation were strongly dependent on the frequency the solution was changed. During the first months, the solution was renewed every week, whilst afterwards every months, which explains the faster kinetics of loss of mass in the initial period ʋ1, in comparison with the subsequent period ʋ2, which is clearly evidenced in Except for the slightly lower loss of mass of the concrete SCC-OPC in the initial period, which was probably due to the higher gypsum precipitation of this concrete at early stage, no significant differences are observed between the two concrete types. The evaluation of the corrosion by means of microscopy is useful for understanding the mechanism of sulfuric acid attack, although it is difficult to obtain a quantitative assessment of the damage. The preparation of a thin section is a suitable solution to obtain good scanning electron microscopy images. Nevertheless, it would be necessary to have much more than one thin section per concrete type to guarantee the reproducibility of the corrosion depth measurement along the surface in contact with the acid. Furthermore, the depth of penetration is strongly related to the size of the aggregates. The precipitation of gypsum crystals at the interface between the aggregates and the matrix was clearly observed . This meThe sulfuric acid solution was initially set at pH 2, but, after the contact with the cementitious matrix, the pH increased due to the alkaline character of the concrete, as shown in The brushing surface cleaning method showed higher pH changes, especially in the case of the Portland cement based SCC-OPC; this could be the consequence of higher portlandite content and, hence, an increased hydroxide leaching. Furthermore, this could induce more gypsum precipitation, due to intensive decalcification of the C-S-H and portlandite, as observed in the “constant pH method”. 2+ ions in the solution confirmed the increase in pH of acid solutions in contact with the OPC after 60 days of exposure due to portlandite dissolution. 2+ ions were released. The brushing eliminated a larger amount of this reacting layer, exposing new alkaline concrete surface and accelerating the pH increase of the acid solution. The increased amount of Ca2+ present in portlandite, C–S–H gel and AFm/AFt compounds can be an index that represents the corrosion rate. During the first 20 days of acid attack, the Ca2+ released is very similar in the two concrete types; no differences are expected from the method whatever the ratio between the acid volume/concrete surface, the concrete size and geometry, or surface cleaning system used. After the first 20 days of testing, the amount of Ca2+ ions released from brushed concretes is greater, this explains the increase of pH at longer ages  than for the other acids. Furthermore, in sulfuric acid attack, the decrease in density and the increase in volume of the concrete due to the sulfuric acid reaction with the hydration products can be even larger at higher pH. This means that a severe sulfuric acid attack (very low pH) could produce a smaller weight of loss in a concrete specimen than that produced by a weaker solution (i.e. at pH 4), as observed in previous studies of Attiogbe and Rizkalla . Indeed,3A  leads to a minor influence of the type of cement used.The influence of the cement type and, in particular, the content of C2 instead of loss of mass by % related to the initial weight, as often reported in literature, allows for the comparison between concrete samples with different geometry, since the acid attack is related to the surface that is exposed to the solution and not the bulk. Each test method presented in this study had some practical advantages, as summarized in The brushing or not brushing procedure of the surface is often a matter of discussion in literature; however, many researchers agreed that the brushing is recommended to obtain a reliable comparison between the behavior of different concretes that are subjected to acid attack .2, regardless the sample geometry and the different ratio Va/S. Afterwards, the frequency of the solution renewal in the “constant pH method” is reduced to one time per month; this can easily explain why after seven months the loss of mass is lower than the one that is obtained in the “constant sulfate method”. It can be concluded that the frequency of the solution renewal has the biggest influence on the results, whilst the influence of sample geometry and the ratio between solution volume/specimen surface is less relevant.The “constant pH method” and “constant sulfate method” use a similar approach, which is based on the immersion of concrete specimens, even if with different geometry, in sulfuric acid solution at pH 2, and a manual brushing of the surface. The direct comparison of the loss of weight obtained after seven months of exposure from the two methods is strongly affected by the differences in the experimental parameters, mainly the frequency of renewal of the solution and the sulfate concentration. In the “constant pH method”, the acid solution is added by a titration mechanism to keep the pH constant, which implies a higher concentration of sulfate when compared to the “constant sulfate method” in which the pH is evaluated manually and adjusted when it goes above pH 4. During the first months, the solution is renewed in both methods weekly and in both cases the brushing is done manually, even if with different brushes and by different operators. This explains the good agreement regarding the loss of mass measured during the first 30 days obtained from the two methods, circa 0.4 kg/mActually, a comparison with the amount of material that is removed by the TAP method can also be made, under the assumption that the swelling of the remaining concrete matrix after brushing is limited. Subsequently, a change in radius of about 1 mm corresponds to a mass loss of 2.338 kg/m² for the SCC-OPC and 2.381 kg/m² for the SCC-Slag . The impact of the experimental conditions were evaluated and discussed. The quantification of the degradation by the loss of mass or volume change of concrete is mainly influenced by the frequency of the acid solution renewal, in which the concretes are immersed and the mechanical brushing process used to remove the degraded layer. The sulfuric acid attack at very low pH is strongly influenced by the precipitation of gypsum crystals, which can induce a long dormant period and act as self-protecting layer that is limiting the diffusion of the acid inside the matrix. For this reason, the two experimental parameters mentioned before were identified as the most relevant for a data comparison between the different methods.The use of fly-ash might give a positive contribution against acid attack, since it was observed by electron microscopy that the fly ashes withstand the acid attack. The resistance of SCC that was made with OPC and with granulometrically optimized slag cement to sulfuric acid attack at pH 2 was almost comparable. In the case of low w/c ratio and optimized concrete technology, the influence of the cement type becomes less relevant. Nevertheless, a common result that emerges from the different test procedures is that the granulometrically optimized slag cement moderately improves the resistance against sulfuric acid. This observation implies that the use of granulometrically optimized slag cement is recommended to increase the durability of structures exposed to acidic media.