A review of 3D printed concrete : performance requirements, testing measurements and mix design

As one of the ways contributing to the progress of the industrialization of the construction industry, 3D printed concrete (3DPC) has attracted more and more attention in recent years. The utilization of 3DPC can accelerate the construction speed, save the labor and raw materials, as well as improve the design freedom of construction without formworks. However, one of the most significant challenges for the application of 3DPC is the printing materials. These is a significant difference in the mixtures and performance between 3DPC and normal concrete. In order to provide an intensive reference for future studies to satisfy the performance requirements of 3DPC structures, this study firstly reviews the performance requirements of 3DPC, including the printability, fresh and hardened mechanical properties, and durability. Based on this, the specialized test methods for 3DPC are reviewed for the effective quality evaluation of 3DPC. The last part presents a review of mix design from the point of view of different materials and mix design approaches. The results show that 3DPC needs to meet the printability that it has higher requirements for rheology, hydration, and green strength than normal concrete. The interlayer bond is the key to study the anisotropic strength and durability degradation. More accurate test methods and testing standards should be developed. Besides, coarse aggregates and recycled materials need to be considered in the mix design of 3DPC

Comparative investigation on nanomechanical properties of hardened cement paste

Three types of nanomechanical methods including static nanoindentation, modulus mapping and peak-force quantitative nanomechanical mapping (QNM) were applied to investigate the quantitative nanomechanical properties of the same indent location in hardened cement paste. Compared to the nanoindentation, modulus mapping and peak-force QNM allow for evaluating local mechanical properties of a smaller area with higher resolution. Beside, the ranges of elastic modulus distribution measured by modulus mapping and peak-force QNM are relatively greater than that obtained from nanoindentation, which may be due to a result of the shaper probe and local confinement effect between multiple phases. Moreover, the average value of elastic modulus obtained using peak-force QNM were consistent with those obtained by modulus mapping, while the different in modulus probability distribution could be related to the different nanomechancial theories and contact forces. The probability distributions of elastic modulus measured using nanomechanical methods to provide a basis for the different types of phases existing in cement paste. Based on the observation with high spatial resolution, cement paste can be likely found as nanocalse granular material, in which different submicron scale or basic nanoscale grain units pack together. It indicates that the peak-force QNM can effectively provide an effective insight into the nanostructure characteristic and corresponding nanomechanical properties of cement paste

Performance and durability of self-compacting mortar with recycled sand from crushed brick

peer reviewedThe demolition of brick masonry structures and the rejected non-conform bricks are generating a great volume of brick residues. The use of recycled sand from brick residues in the production of mortar could decrease the amount of waste going into landfills and reduce the consumption of natural resources. This paper investigated the feasibility of using recycled sand from crushed brick (RBS) in the self-compacting mortar (SCM). The crushed limestone sand was partially replaced with RBS at different levels (0, 5, 10, 25 and 50%). The properties at fresh state, mechanical behavoir, drying shrinkage and durability of SCM were discussed. As the substitution of limestone sand by RBS increased, the compressive strength of mortars slightly reduced at the age of 28 days (3.3% and 16.9% lower than the reference mortar, respectively for 25% RBS and 50% RBS content); however, which is within the compressive strength requirement in European standard EN 998-2 for masonry mortars. The incorporation of RBS in SCM showed better resistance to chloride diffusion, whereas more attention should be paid to carbonation and sulphate attack. The results indicate that it is possible to manufacture SCM by partially replacing the crushed limestone sand with RBS up to 25% replacement level.ECOLISER - ÉCOliants pour traitement de Sols, Etanchéité et Routes

QM/MM MD and Free Energy Simulations of G9a-Like Protein (GLP) and Its Mutants: Understanding the Factors that Determine the Product Specificity

Certain lysine residues on histone tails could be methylated by protein lysine methyltransferases (PKMTs) using S-adenosyl-L-methionine (AdoMet) as the methyl donor. Since the methylation states of the target lysines play a fundamental role in the regulation of chromatin structure and gene expression, it is important to study the property of PKMTs that allows a specific number of methyl groups (one, two or three) to be added (termed as product specificity). It has been shown that the product specificity of PKMTs may be controlled in part by the existence of specific residues at the active site. One of the best examples is a Phe/Tyr switch found in many PKMTs. Here quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) and free energy simulations are performed on wild type G9a-like protein (GLP) and its F1209Y and Y1124F mutants for understanding the energetic origin of the product specificity and the reasons for the change of product specificity as a result of single-residue mutations at the Phe/Tyr switch as well as other positions. The free energy barriers of the methyl transfer processes calculated from our simulations are consistent with experimental data, supporting the suggestion that the relative free energy barriers may determine, at least in part, the product specificity of PKMTs. The changes of the free energy barriers as a result of the mutations are also discussed based on the structural information obtained from the simulations. The results suggest that the space and active-site interactions around the ε-amino group of the target lysine available for methyl addition appear to among the key structural factors in controlling the product specificity and activity of PKMTs

Advances in studying of recycled aggregate concrete in China

Abstract only. - This report firstly presents the current state of construction and demolition (C&D) waste generation in PR China. It is found that the composition of C&D waste in mainland China is much different from other countries, leading to some difficult on the aspect of waste concrete recycling. As a result, in China, some reclamation chains have been well-established, which are suitable for local conditions. Secondly, the research work on recycled aggregate concrete (RAC) in mainland China is introduced, including mechanical property of RAC material, structural behavior of RAC load-bearing elements, and seismic performance of RAC frame structures. The experimental study results prove that it is feasible to apply RAC as a structure material in building structures. Lastly, this report presents an outline of Chinese technical codes for RAC organized and edited by the speaker. It also puts forward some successful applications of RAC in building structures in the mainland of China which will be helpful to promote and popularize RAC as one kind of ecological structural materials

Numerical analysis of hysteretic behavior for RAC structure under earthquake loading

The hysteretic properties of RAC structure subjected to different strain rates are analyzed, and the displacement ductility is assessed. It is revealed that, under earthquake wave excitation with the increasing strain rate, the lateral stiffness, the bearing capacity, and the energy dissipation increases progressively. However, the ductility of RAC structure decreases gradually with the increasing strain rate. The rate-dependent capacity curve model of RAC structure is suggested. The damage evolution of RAC structure subjected to different strain rates is investigated. Furthermore, the dynamic damage model of RAC structure in which the strain rate effect is taken into account is proposed

Discrete Element Simulation Analysis of Biaxial Mechanical Properties of Concrete with Large-Size Recycled Aggregate

Concrete made with large-size recycled aggregates is a new kind of recycled concrete, where the size of the recycled aggregate used is 25–80 mm, which is generally three times that of conventional aggregate. Thus, its composition and mechanical properties are different from that of conventional recycled concrete and can be applied in large-volume structures. In this study, recycled aggregate generated in two stages with randomly distributed gravels and mortar was used to replace the conventional recycled aggregate model, to observe the internal stress state and cracking of the large-size recycled aggregate. This paper also investigated the mechanical properties, such as the compressive strength, crack morphology, and stress–strain curve, of concrete with large-size recycled aggregates under different confining pressures and recycled aggregate incorporation ratios. Through this research, it was found that when compared with conventional concrete, under the confining pressure, the strength of large-size recycled aggregate concrete did not decrease significantly at the same stress state, moreover, the stiffness was increased. Confining pressure has a significant influence on the strength of large-size recycled aggregate cocrete

Plastic Shrinkage and Cracking Behavior of Mortar Containing Recycled Sand from Aerated Blocks and Clay Bricks

This research assessed the shrinkage behavior of mortar containing recycled sand generated from aerated blocks and clay bricks by replacing natural sand with a volume ratio of 30%. Five mix proportions were prepared to cast mortar plate specimens. They were named as the control, ABs1, ABs2, CBs1 and CBs2. Among them, ABs1 and CBs1, containing recycled sand from aerated blocks and clay bricks respectively, have the same water/cement ratio as the control. ABs2 and CBs2 had the same workability as the control by adjusting the required amount of water. Digital image correlation (DIC) was adopted to determine the strain distribution during the plastic shrinking period. The results showed that, when no extra water was added, the mortar containing the aerated blocks sand had a lower risk of cracking, because of the internal curing effects brought by the low–weight fine aggregates. However, when the extra water was added to ensure the same fluidity with the control, there were obvious visible cracks in group ABs2, and the average distance between cracks reached 25 mm. On the contrary, the brick sand has no obviously higher water requirement, as a result, it did not lead to higher risk of cracking. The aerated block sand particles contain 2–50 μm pores, which is the reason why aerated block sand tremendously reduced the fluidity of the mortar

On statistical characteristics of the compressive strength of recycled aggregate concrete

status: publishe