The goal of this study is to investigate the effects of different grades of calcined clay on the extrudability and early-age strength development under ambient conditions. Four mix designs were proposed. Three of them contained high, medium, and low grades of calcined clay, respectively, and one was the reference without calcined clay. In terms of extrudability, an extrusion test method based on the ram extruder was introduced to observe the quality of extruded material filaments, and to determine the extrusion pressure of tested materials at different ages. For evaluating the very early-age strength development, the penetration resistance test, the green strength test, and the ultrasonic pulse velocity test were applied. Furthermore, the mechanical properties of the developed mix designs were determined by the compressive strength test at 1, 7 and 28 days. Finally, the main finding of this study was that increasing the metakaolin content in calcined clay could significantly increase the extrusion pressures and green strength, shorten the initial setting time and enhance the compressive strength at 1, 7, and 28 days.@en

A method is presented to model deformation and fracture behavior of 3D printed disordered lattice materials under uniaxial tensile load. A lattice model was used to predict crack pattern and load-displacement response of the printed lattice materials. To include the influence of typical layered structures of 3D printed materials in the simulation, two types of printed elements were considered: horizontally and vertically printed elements. Strengths of these elements were measured: 3 mm cubic units consist of lattice elements with two printing directions were printed and their strengths were tested in uniaxial tension. Afterwards, the measured element strengths and bulk material strength, respectively, were used as model input. Uniaxial tensile tests were also performed on the printed lattice materials to obtain their crack pattern and load-displacement curves. Simulations and experimental results were comparatively analyzed. For both levels of disorder considered, only when measured strengths were assigned to the elements with identical printing direction, are the predicted crack patterns and load-displacement curves in agreement with experimental results. The results emphasize the importance of considering printing direction when simulating mechanical performance of 3D printed structures. The influence of disorder on lattice material mechanical properties was discussed based on the experiments and simulations.@en

Adding extra functionality to cementitious materials will enhance the capabilities of understanding the use of structures where they are used and at the same time increase its life span. The construction industry is one of the most important economic sectors in many countries. Therefore, efforts to improve its productivity can result in significant developments in the economy of a region. Higher productivity can be achieved for instance by means of process automation, by adding value to the product generated with an activity or via the extension of the use of a given asset.....@en

Fibre reinforced cementitious composites (FRCC) may be characterized by their improved performance namely in terms of tensile ductility, accompanied by multiple cracking, and potentially lower permeability to liquid and gas in cracked state. Cracking, which is nearly inevitable, can occur due to applied structural loading, shrinkage, chemical attack, thermal deformations and restrained condition. Even though might not be a structural problem, cracking could be a durability issue, since it considerably modifies the transport properties of the cementitious composite and, as consequence, accelerates the deterioration process, which can significantly impair the long term service life of a structure or element. Literature indicates that, particularly for chloride penetration, the presence of cracks and/or load condition, causes a more deleterious attack compared to standard durability test on sound specimens composites. Contributing to that concern a methodology for accessing chloride attack in loaded and/or cracked FRCC is proposed. Cracking procedure and specimen geometry were selected, considering that cracks produced in laboratory should resemble those in structural elements (beams). Thus, FRCC specimens were firstly pre-loaded under four point bending up to a pre-defined crack width. The crack width was kept using special stainless steel frame. In addition, cracked but not loaded specimens were considered and sound specimens were used as reference. Then, specimens were exposed to wet-dry cycles in a chloride solution. It is argued that the chloride penetration is definitively influenced by the load and cracking conditions, which promoted a higher penetration depth leading to a severe fibres corrosion, which also compromised the mechanical performance of FRCC. @en

The subject of this paper is the extent to which, during wetting–drying cycles, chloride ions can penetrate Ultra-high-Performance Fibre Reinforced Cementitious Composites (UHPFRC) specimens subjected to combined mechanical and environmental load. Pre-cracking was obtained by subjecting prismatic specimens (40 × 40 × 60mm3) to four-point bending until a predefined crack opening displacement (COD) is reached, using a dedicated test setup. Three target CODs were studied: 300, 350 and 400 µm. Exposure to a concentrated chloride solution (3.5% NaCl) was used as an environmental load. Specimens we subjected to wetting–drying cycles for one year. After this exposure period, the chloride penetration was characterised both qualitatively (by colourimetric analysis with silver nitrate) and quantitatively (by determining the chloride profile). The effect of damage level on chloride penetration and its impact on structures durability is discussed in the current paper.@en

This work aims to provide a method for numerically and experimentally investigating the fracture mechanism of cement paste at the microscale. For this purpose, a new procedure was proposed to prepare micro cement paste cubes (100 × 100 × 100 µm3) and beams with a square cross section of 400 × 400 µm2. By loading the cubes to failure with a Berkovich indenter, the global mechanical properties of cement paste were obtained with the aid of a nano-indenter. Simultaneously the 3D images of cement paste with a resolution of 2 µm3/voxel were generated by applying X-ray microcomputed tomography to a micro beam. After image segmentation, a cubic volume with the same size as the experimental tested specimen was extracted from the segmented images and used as input in the lattice model to simulate the fracture process of this heterogeneous microstructure under indenter loading. The input parameters for lattice elements are local mechanical properties of different phases. These properties were calibrated from experimental measured load displacement diagrams and failure modes in which the same boundary condition as in simulation were applied. Finally, the modified lattice model was applied to predict the global performance of this microcube under uniaxial tension. The simulated Young’s modulus agrees well with the experimental data. With the method presented in this paper the framework for fitting and validation of the modelling at microscale was created, which forms a basis for multi-scale analysis of concrete.@en

Recent studies have shown promising potential for using Glass Pozzolan (GP) as an alternative supplementary cementitious material (SCM) due to the scarcity of fly ash and slag in the United States. However, comprehensive studies on the freeze-thaw (FT) resistance and air void system of mixtures containing GP are lacking. Therefore, this study aimed to evaluate GP’s effect on FT resistance and characterize mixtures with different GP contents, both macro- and microscopically. In this study, six concrete mixes were considered: Three mixes with 20%, 30% and 40% GP as cement replacements and two other comparable mixes with 30% fly ash and 40% slag, as well as a mix with 100% Ordinary Portland cement (OPC) as a reference. Concrete samples were prepared, cured and tested according to the ASTM standards for accelerated FT resistance for 1000 cycles and corresponding dynamic modulus of elasticity (Ed). All the samples showed minimal deterioration and scaling and high F/T resistance with a durability factor of over 90%. The relationships among FT resistance parameters, air-pressured method measurements of fresh concretes and air void analysis parameters of hardened concretes were examined in this study. X-ray micro-tomography (micro-CT scan) was used to evaluate micro-cracks development after 1000 freeze-thaw cycles and to determine spatial parameters of air voids in the concretes. Pore structure properties obtained from mercury intrusion porosimetry (MIP) and N2 adsorption method showed refined pore structure for higher cement replacement with GP, indicating more gel formation (C-S-H) which was verified by thermogravimetric analysis (TGA).@en

The high cost of UHPFRC is a limitation on the practical application in real construction projects. However, a very competitive UHPFRC approach is the hybrid structural elements, where thin layers of UHPFRC are employed to rehabilitate/strengthen damage cover concrete. New layers subjected to harsh conditions (loads and/or environmental) can eventually crack under service conditions, changing the local transport properties and thus, a faster ingress of detrimental substances occur, such as chlorides ions. Most of the studies on chloride penetration in UHPFRC have focused on determining the transport properties of sound, non-cracked specimens. Thus, an experimental campaign was carried out to assess chloride ingress in loaded and/or cracked UHPFRC and the effect of such ions on mechanical performance. Typical service cracks patterns were imposed on UHPFRC specimens and then exposed to wetting–drying cycles in a chloride solution. After 1-year chloride exposure, UHPFRC specimens were in good condition with no significant losses in flexural strength; however, stiffness might be affected. The chloride contents up to 20 mm depth were superior to the European standards critical chloride content. A minimum cover depth of 20 mm of new UHPFRC is recommended to protect a concrete substrate in hybrid structures for exposure classes XS3.@en

Since the advent of three-dimensional concrete printing (3DCP), several studies have shown the potential of strain hardening cementitious composites (SHCC) as a self-reinforcing printable mortar. However, only a few papers focus on achieving sufficient buildability when developing printable SHCC. This study investigates the role of the particle size distribution (PSD) in relation to the buildability properties of the mixture in the fresh state and strain hardening properties in the hardened state. To this end 6 mixtures were designed based on optimal particle packing with the application of the Modified Andreasen and Andersen Model. The two mix designs showed the highest displacement at maximum stress were selected for further development of their fresh state rheological properties. This was achieved by addition of a viscosity modifying agent (VMA) and a super plasticizer (SP) and through material analysis by means of ram extrusion tests. Further fresh material characterization on the final two 3DP-SHCC mix designs was attained by the deployment of uniaxial unconfined compression tests (UUCT), Vicat tests and Buildability tests. After successful printing of the two SHCC composites, the compressive strength, the 4-point bending strength and the uniaxial tensile strength and strain were determined at an age of 28 days. The research shows that optimization of the PSD in a 3DP-SHCC mix design results in an improvement of the buildability, but can introduce decreased pumpability and strain hardening capacity.@en

Cracking in concrete structures compromises the durability and functionality of the structures themselves. Different kinds of self-healing concretes, less or more sophisticated, have been developed in the past ten years to overcome early cracks in structures. An experimental study of a novel self-healing concrete is presented. Bitumen, used as the healing agent, is introduced in fresh fibre-reinforced concrete as the coating of steel fibres. The mechanism exploits induction energy to heat up the steel fibres inside the cracked concrete matrix; the bitumen then melts and finally flows into the cracks, sealing them. The aim of the research is to set up the main parameter affecting the performance of the healing mechanism as well as its efficiency. In order to achieve this goal, the microstructure of healed specimens has been studied through Light Microscope. Mechanical behaviour and permeability of the samples, before and after healing, were also checked. Fiber content is studied in the paper amongst the many parameters affecting the mechanism. Results point out the potential of the proposed self-healing mechanism to contrast early cracking (i.e. due to shrinkage). Presence of a certain amount of fibres bridging the crack highly influenced the healing efficiency, and so a uniform distribution inside the concrete matrix, which was directly related to fibre amount and its optimum concrete matrix. @en

Over the last few years, the United States has experienced a shortage of fly ash and slag that consequently created a need for an alternative material that is locally available, sustainable, and provides desirable concrete properties. Recent studies have shown that Ground Glass Pozzolan (GGP) offers favorable attributes as a supplementary cementitious material (SCM) for concrete. However, there are limited studies demonstrating freeze-thaw (FT) resistance of concrete with GGP, as well as assessing the FT resistance in relation with the air-void system of GGP mixtures. In response, this study aimed to evaluate both macro- and micro-level behavior of GGP on FT resistance, and characterize mixtures with different contents of GGP. Six concrete mixtures were evaluated: three mixtures with 20, 30, and 40% GGP as cement replacements and three other reference mixtures with 30% fly ash and 40% slag and 100% Ordinary Portland cement (OPC). Following ASTM standards, concrete beam samples were tested for accelerated FT resistance and dynamic modulus of elasticity up to 1000 cycles. All concretes showed high FT resistance with a durability factor over 90% and, consequently, minimal deterioration and scaling. Core samples extracted from the FT conditioned beams were scanned with the X-ray micro-tomography (CTscan) to identify air-void parameters. Through image analysis a quantification of air-void parameters was obtained, and their relationship to FT resistance was established. Using CT scan analysis, we demonstrated that concretes with the highest cement replacement with GGP and slag developed the most desirable spacing factor and specific surface for FT resistance.@en

Abstract In the past two decades, much research has been devoted to overcoming the inherent brittleness of cementitious materials. To that end, several solutions have been proposed, mainly utilizing fibres. One of the most promising classes of materials is strain hardening cementitious composite (SHCC). It utilizes PVA fibres, and it is relatively costly compared to regular concrete, so it is commonly used only in surface layers. In this paper, a multi-functional ductile cementitious composite based on SHCC has been developed. It uses microencapsulated phase change materials (PCMs), capable of reducing temperature fluctuations in the material due to their high heat of fusion. It is shown that, although addition of microencapsulated PCMs are detrimental to compressive strength, they have very little effect on the flexural strength and deflection capacity. In the future work, mixtures with higher PCM contents will be developed in order to exploit their heat storage capability better. This material has potential to reduce temperature effects on concrete surfaces, while at the same time being extremely ductile. @en

Recent years have seen a rapid growth of additive manufacturing methods for concrete construction. Potential advantages include reduced material use and cost, reduced labor, mass customization and CO2 footprint reduction. None of these methods, however, has yet been able to produce additively manufactured concrete with material properties suitable for structural applications, i.e. ductility and (flexural) tensile strength. In order to make additive manufacturing viable as a production method for structural concrete, a quality leap had to be made. In the project ‘3D Concrete Printing for Structural Applications’, 3 concepts have been explored to achieve the required structural performance: applying steel fiber reinforcement to an existing printable concrete mortar, developing a strain-hardening cementitious composite based on PVA fibers, and embedding high strength steel cable as reinforcement in the concrete filament. Whereas the former produced only an increase in flexural tensile strength, but limited post-peak resistance, the latter two provided promising strain hardening behavior, thus opening the road to a wide range of structural applications of 3D printed concrete.@en

Recent studies have shown that concrete containing Phase Change Materials (PCM) with low transition temperatures may reduce the number of freeze/thaw cycles suffered by the cementitious composite in temperate climates. Nevertheless, the positive influence of such admixtures on the frost resistance of cement-based materials has not been directly shown, nor the negative. In this study, mortars with different contents of microencapsulated PCM by volume of cement paste were studied with regard to the progression of their internal and salt scaling damages during freeze/thaw cycles. X-ray micro tomography was used to monitor damage development and spatial distribution in the mortars. Furthermore, the pore system and microstructure of the PCM-modified mortars were characterized to unveil the causes of the observed macroscopic behavior during frost weathering. The results show that limited amounts of PCM in mortar, namely 10% by volume of cement paste, results beneficial for the frost and scaling resistance of the composite. Whereas, for larger PCM additions, like 30% by volume of paste, the changes in microstructure, porosity and mechanical strength brought in by these admixtures resulted in worsened performance against freeze/thawing cycles.@en

To investigate the effects of viscosity-modifying admixture (VMA) on the extrudability of limestone and calcined clay-based cementitious materials, three mix designs with different dosages of VMA were proposed in this study. The ram extrusion was utilized as an extrusion model for exploring the fresh properties of printable materials. Two methods were used, based on the ram extruder setup—(a) extruding materials with the same extrusion speed at different rest times to determine how the pressure changes with time; (b) extruding materials with different extrusion speeds at the same rest time to investigate the material flow parameters using the Basterfield et al. model. The main findings of this study could be summarized as—(1) the extrusion pressure of all mix designs exhibited an increasing trend with time. At the same tested age, the extrusion pressure under 0.25 mm/s of piston speed was increased and the shape retention of the extruded filaments was enhanced by increasing the dosage of VMA; (2) the correlation between the experimental results and the Basterfield et al. model was excellent (R-squared value: 0.99). The mixture with a higher content of VMA showed an increased elongational yield stress, flow consistency, and shear yield stress.@en

New additive manufacturing methods for cementitious materials hold a high potential to increase automation in the construction industry. However, these methods require new materials to be developed that meet performance requirements related to specific characteristics of the manufacturing process. The appropriate characterization methods of these materials are still a matter of debate. This study proposes a rheology investigation to systematically develop a printable strain hardening cementitious composite mix design. Two known mixtures were employed and the influence of several parameters, such as the water-to-solid ratio, fibre volume percentage and employment of chemical admixtures, were investigated using a ram extruder and Benbow-Bridgwater equation. Through printing trials, rheology parameters as the initial bulk and shear yield stress were correlated with variables commonly employed to assess printing quality of cementitious materials. The rheology properties measured were used to predict the number of layers a developed mixture could support. Selected mixtures had their mechanical performance assessed through four-point bending, uni-axial tensile and compressive strength tests, to confirm that strain hardening behaviour was obtained. It was concluded that the presented experimental and theoretical framework are promising tools, as the bulk yield stress seems to predict buildability, while shear yield stress may indicate a threshold for pumpability.@en

Recently the concept of crack self-sealing has been investigated as a method to prevent degradation and/or loss of functionality of cracked concrete elements. To obtain self-sealing effect in the crack, water swelling admixtures such as superabsorbent polymers (SAP) are added into the cementitious mix. In order to design such self-sealing systems in an efficient way, a three-dimensional mesoscale numerical model is proposed to simulate capillary absorption of water in sound and cracked cement-based materials containing SAP. The numerical results yield the moisture content distribution in cracked and sound domain, as well as the absorption and swelling of SAP embedded in the matrix and in the crack. The performance of the model was validated by using experimental data from the literature, as well as experimentally-informed input parameters. The validated model was then used to investigate the role of SAP properties and dosage in cementitious mixtures, on the water penetration into the material from cracks. Furthermore different crack widths were considered in the simulations. The model shows good agreement with experimental results. From the numerical investigation guidelines are suggested for the design of the studied composites.@en

In 3D concrete printing (3DCP), it is necessary to meet contradicting rheological requirements: high fluidity during pumping and extrusion, and high stability and viscosity at rest to build the layered structure. In this paper, the impact of the hydroxypropyl methylcellulose (HPMC)-based viscosity-modifying admixture (VMA) on the 3D printability and mechanical performance of a limestone and calcined clay based cementitious material is investigated. A combination of VMA and superplasticizer was used for that purpose. In this case, controlling the competitive effects between VMA and superplasticizer becomes critical. The main strategy for 3D printing in this study was to add an optimal dosage of VMA in the solid suspension that was already mixed with water and superplasticizer. A lab-scale 3DCP setup was developed and demonstrated as well. A series of tests was performed to characterize the effects of VMA on flowability, extrudability, open time, buildability, green strength, hydration, compressive strength, and air void content and distribution. Experiments performed in this study showed that the mixture containing 0.24% (of the binder mass) of VMA exhibited satisfactory 3D printability and optimal mechanical performance. Finally, the results, limitations, and perspectives of the current research were discussed.@en

Extrusion based additive manufacturing of cementitious materials has demonstrated strong potential to become widely used in the construction industry. However, the use of this technique in practice is conditioned by a feasible solution to implement reinforcement in such automated process. One of the most successful ductile materials in civil engineering, strain hardening cementitious composites (SHCC) have a high potential to be employed for three-dimensional printing. The match between the tailored brittle matrix and ductility of the fibres enables these composites to develop multiple cracks when loaded under tension. Using previously developed mixtures, this study investigates the physical and mechanical performance of printed SHCC. The anisotropic behavior of the materials is explored by means of mechanical tests in several directions and micro computed tomography tests. The results demonstrated a composite showing strain hardening behavior in two directions explained by the fibre orientation found in the printed elements. Moreover, the printing technique used also has guaranteed an enhanced bond in between the printed layers.@en

Significant attention has been given to the development of new materials and techniques to be employed in the construction market. One of the techniques which has drawn noticeable attention is the additive manufacturing process (a.k.a. 3-dimensional printing (3D printing)). One of the approaches of this construction technique is the extrusion of cementitious composites to form contour of a desired geometry. To achieve high viscosity in cementitious materials, usually viscosity modifying admixtures (VMA) are employed. However, the consequences of using these admixtures at high dosages is still not fully understood. This study characterized the influence of different VMA dosages on Portland cement paste, through a microstructure analysis. Hydration development was assessed, and effect of the admixture was quantified at different curing ages. Techniques such as thermogravimetric analysis, optical and electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, micro computed tomography scan and nanoindentation were employed. Important negative side effects were found such as: VMA increasing the cement setting time, anomalous dispersion of hydration products in the bulk and increasing the void content. On the other hand, positive effects were also found such as: evidence of internal curing, higher degree of hydration and lack of undesired hydration products.@en