A sophisticated simulation for the fracture behavior of concrete material using XFEM

The development of a powerful numerical model to simulate the fracture behavior of concrete material has long been one of the dominant research areas in earthquake engineering. A reliable model should be able to adequately represent the discontinuous characteristics of cracks and simulate various failure behaviors under complicated loading conditions. In this paper, a numerical formulation, which incorporates a sophisticated rigid-plastic interface constitutive model coupling cohesion softening, contact, friction and shear dilatation into the XFEM, is proposed to describe various crack behaviors of concrete material. An effective numerical integration scheme for accurately assembling the contribution to the weak form on both sides of the discontinuity is introduced. The effectiveness of the proposed method has been assessed by simulating several well-known experimental tests. It is concluded that the numerical method can successfully capture the crack paths and accurately predict the fracture behavior of concrete structures. The influence ofmode-Ⅱ parameters on the mixed-mode fracture behavior is further investigated to better determine these parameters.

Effects of an AMPS-modified Polyacrylic Acid Superplasticizer on the Performance of Concrete Materials

A self-made 2-acrylamide-2-methyl propylene sulfonic （AMPS）-modified polyacrylic acid superplasticizer and two other commercially available superplasticizers with different molecular structures are used in this study to investigate the effect of an AMPS-modified polyacrylic acid superplasticizer on the properties of concrete materials. In the experiments, initial and 1.5 h slumps over time after admixtion are determined by adding different dosages of three superplasticizers into the premixed concrete to characterize the slump loss resistance of the premixed concrete. The water-reducing rates of three different types of concrete are determined to characterize the water-reducing capacity of the concrete with each superplasticizer. The 3, 7 and 28 d compressive strength is determined to characterize the mechanical properties of the concrete with each superplasticizer. In the meanwhile, 1, 1.5 and 2.0 h slump loss rates over time after admixtion are determined by adding different dosages of the three superplasticizers into the high-performance concrete （HPC） to characterize the slump loss resistance of HPC. The 7, 28, 60 and 90 d compressive strength is determined to characterize the compressive properties of HPC with each superplasticizer. The dry shrinkage rates of three different types of HPC are determined with each superplasticizer. Electric flux after standard curing for 56 d and chloride ion diffusion coefficient after curing for 28 d of I-IPC are determined to characterize the impermeability of HPC with each superplasticizer. The cross-section was examined using a scanning electron microscopy （SEM） system. Results demonstrate that the AMPS-modified polyacrylic acid superplasticizer has better water-reducing effect and slump than the two commercially available polyacrylie acid superplasticizers. The AMPS-modified polyacrylic acid superplasticizer also shows significant improvement of the compressive strength, especially in comprehensive performance of HPC. In conclusion, the AMPS-modified polyacrylic acid superplastieizer is particularly suitable for the preparation of HPC.

Nondestructive testing algorithm of building concrete material defects based on machine learning

In order to pay more attention to the quality of construction concrete and accurately judge whether concrete material meets the standard,a nondestructive testing algorithm of building concrete material defects based on machine learning is proposed.Through the ray tracing algorithm of Snell’s theorem,the shortest path between two random punctuation marks of building concrete is calculated.The original coordinate system and grid size were set,the trend and length of the line in the grid were calculated,and the coordinates between the grid corner points and the transmitting probe were calculated so as to obtain the position of the intermediate refractive points of the two probes.Finally,the vector dot product of the local defects is obtained by the optimal hyperplane calculation of the binary classification in the support vector machine.Experimental results show that the proposed method has the advantages of high precision.

A critical review of preparation design and workability measurement of concrete material for largescale 3D printing

In recent few years, significant improvement has been made in developing largescale 3D printers to accommodate the need of industrial-scale 3D printing. It is of great feasibility to construct structural components and buildings by means of 3D concrete printing. The major issues of this innovative technique focus on the preparation and optimization of concrete materials which possess favourable printable properties as well as the measurement and evaluation methods of their workability. This paper firstly introduces three largescale 3D printing systems that have been successfully applied in construction industry. It then summarizes the commonly used raw materials in concrete manufacturing. Critical factors that should be particularly controlled in material preparation are specified. Easy-extrusive, easy-flowing, well-buildable, proper setting time and low shrinkage are significant for concrete mixture to meet the critical requirements of a freeform construction process. Thereafter, measuring methods that can be employed to assess the fresh and hardened properties of concrete at early stages are suggested. Finally, a few of evaluation methods are presented which may offer certain assistance for optimizing material preparation. The objective of this work is to review current design methodologies and experimental measurement and evaluation methods for 3D printable concrete materials and promote its responsible use with largescale 3D printing technology.

MULTI-SCALE NUMERICAL MODEL FOR SIMULATING CONCRETE MATERIAL BASED ON FRACTAL THEORY

A new multi-scale numerical model is presented using the fractal theory and adopting FEM to simulate the failure of concrete.The relation between the fractal box dimension in large scale and the damage to concrete in small scale is deduced.And the evolutionary process of elastic modulus and strength in small scale is given.Consequently,the multi-scale numerical model is proposed to describe the constitutive relation of concrete between small scale and large scale.A two-dimensional static analysis of a concrete block is performed by using this model and the calculation result is discussed.The propagation of cracks of the concrete block is also studied.

Study on Quality Control of Concrete Raw Materials in Road and Bridge Construction

The main material of concrete is a construction building material composed of water and mineral mixture and cement and chemical additives in the corresponding proportion and below the standard.In the process of making concrete material,slurry and cement are needed to mix,then cement slurry and sand are mixed into mortar according to the corresponding proportion,and aggregate is added to mortar to form concrete building material.In the process of concrete preparation,the most important construction link is mixing,which needs to be fully stirred to make the performance of concrete meet the construction needs.In the process of concrete construction technology development,both mix ratio and production technology have become more and more mature,but there are still some problems,which have an impact on the quality of concrete[1].Therefore,this paper discusses the quality control of concrete raw materials according to the construction process of road and bridge.

Further numerical investigation on concrete dynamic behaviors with considering stress non-equilibrium in SHPB test based on the waveform features

In this study,with the meso-scale model reliably validated in our previous work(Construction and Building Materials,2018),the waveform features of plain concrete under various loading conditions and especially with considering stress non-equilibrium are reliably reproduced and predicted.Associating with waveform features,the violation indicator of the specimen stress equilibrium in the split Hopkinson pressure bar test is identified for concrete-like damage softening materi-als.The concrete material behaviors for stress non-equilibrium are further analyzed,e.g.the dynamic increase factor(DIF)and damage development,etc.The conception of“damage failure volume”is introduced,and a new method of defining the development of concrete dynamic damage is given in the nimierical study.What’s more,the“compression wave”and“double peak”phenomena observed in the experiment are further interpreted based on the means of numerical simulation.Waveform features how to reflect the concrete material properties is also concluded.The results show that,the disappearance of the“double peak” phenomenon of reflection curve under high strain rate can be regarded as the indicator of the violation of stress equilibrium.After the violation of the stress equilibrium,the relevant DIFs of the concrete specimen will not change significantly.Especially,the concrete specimen will turn into structural response from material response.The conception of“damage failure volume”can well explain the generation of the“double peak”phenomenon of the reflection curve.The “compression wave” phenomenon of reflection curve under lower strain rates is derived from the unloading expansion recovery of the concrete specimen.Furthermore,under the same loading condition,the amplitude of the first peak of the reflection curve can be used as the evaluation standard of the bonding quality between mortar and aggregates.