Ductility of 3D printed concrete reinforced with short straight steel fibers

ABSTRACT

With the number of 3D printed concrete structures rapidly increasing, the demand for concepts that allow for robust and ductile printed objects becomes increasingly pressing. An obvious solution strategy is the inclusion of fibers in the printed material. In this study, the effect of adding short straight steel fibers on the failure behaviour of Weber 3D 115-1 print mortar has been studied through several CMOD tests on cast and printed concrete, on different scales. The experiments have also been simulated numerically. The research has shown that the fibers cause an important increase in flexural strength, and eliminate the strength difference between cast and printed concrete that exists without fibers. The post-peak behaviour, nevertheless, has to be characterised as strongly strain-softening. In the printed specimens, a strong fiber orientation in the direction of the filament occurs. However, this has no notable effect on the performance in the tested direction: cast and printed concrete with fibers behave similarly in the CMOD test. For the key parameters, no scale effect was found for the specimens with fibers, contrary to the ones without. Numerical modelling of the test by using the Concrete Damage Plasticity material model of Abaqus, with a Thorenfeldt-based constitutive law in compression and a customised constitutive law in tension, results in a reasonable fit with the experimental results.     

Determination of initial degree of hydration for improvement of early-age properties of concrete using ultrasonic wave propagation

During the first few hours after mixing, the properties of concrete change between different types of material behaviour. Fresh concrete is during mixing a Bingham material, gradually attaining solid body properties with considerable compressive strength and stiffness. The development of mechanical properties can be described by the degree of hydration. For the prediction of mechanical properties of early-age concrete as well as for the prediction of stresses caused by differences of temperature and autogenous shrinkage, it is essential to know the initial degree of hydration, from which on the development of strength and stiffness can be assumes to begin. This paper deals with the determination of the end of the dormant phase by using ultrasonic pulse velocity techniques. Using compression wave and shear wave transducers the hardening of concrete is observed under adiabatic curing conditions. From the development of dynamic Young’s modulus and Poisson’s ratio a model of the initial degree of hydration is derived to improve existing models of the development of tensile strength and modulus of elasticity for very early-age concrete. A procedure to determinate an upper and lower bound for the end of setting time is presented. Typical results are presented for different concrete compositions, especially for high strength concrete.     

Roller compacted concrete-tensile strength of horizontal joints

Roller Compacted Concrete is a very dry material consolidated by the use of a powerful external vibatory compaction. This material is used mainly in gravity or arch/gravity dams, and pavements. RCC dam construction methodology, by concrete layers, involves a high number and an extensive area of horizontal joints. These joints are the weakest parts of RCC in terms of strength and permeability, being one hindrance to more widespread use of RCC. This study analyses the influence of different parameters on the direct tensile strength of the joints, namely the relative humidity in the concrete surface, the setting and hardening state of concrete, and the cold joint treatment. The results obtained indicated that the decrease in the joint quality with the time of exposure is continuous being important the compaction of the upper layer before the initial setting time of the lower layer. Cold joint treatments should expose aggregate particles.     

Steel-fibre-reinforcement and hydration coupled effects on concrete tensile behaviour

This paper presents the effect of hydration development on the tensile behaviour of steel-fibre-reinforced concrete. Tensile tests were performed on plain and fibre-reinforced concretes at 2, 7 and 28 days in order to determine the response of the composites, in particular to establish the post-peak behaviour and the evolution of the residual post-peak strength with hydration development and the associated improvement in the fibre–matrix bond.From the laws governing the tensile behaviour of plain concrete on the one hand and the residual strength capacity due to fibre reinforcement on the other, parameters fitting an analytical model of the uniaxial tensile response of fibre-reinforced concrete were determined. The proposed model takes into account the tensile damage to the concrete and the development of hydration. An original aspect of the model is that it also integrates the damage to the fibre–matrix bond.     

不同初始静载混凝土轴向拉伸试验研究

为研究混凝土材料的动态性能,利用MTS-810NEW液压伺服试验机对尺寸为100 mm×100 mm×510 mm棱柱体混凝土材料试样进行了初始静态荷载为0~20 k N的动态轴向拉伸试验,研究了混凝土材料经历不同初始静态荷载后的动态拉伸破坏特征、应力应变关系和动态抗拉强度。结果表明:荷载值由静态过渡到动态荷载时,混凝土材料的动弹性模量发生较大变化,且随着初始静态荷载值的增加,混凝土材料动弹性模量有增大趋势;混凝土材料动态应力应变关系曲线中,峰值应力所对应的应变值与初始静态荷载值无关;随着初始静态荷载的增加,混凝土材料动态拉伸破坏断面面积逐渐增大,且粗骨料被拉断的数目随着初始静态荷载的增加而先增加,后趋于平稳;随着初始预加静态荷载值的增加,混凝土材料的动态轴向拉伸强度先增加,然后趋于稳定。     

Mechanical evolution of concrete during setting

This paper deals with the rheological behaviour of fresh concrete during setting. An experimental non-destructive device based on the propagation of acoustic waves (compression and shear waves) at low frequency (20Hz–800Hz) is used to continuously characterise the setting and hardening of hydraulic concrete. The results are presented for various concrete formulations involving various initial temperatures and water/cement ratios. The characteristics of wave propagation (velocities, damping coefficients,...) enable to monitor material setting and hardening phases. By means of an inverse analysis, the evolution of rheological parameters is presented from the fluid to the solid state. It is shown that the mechanical evolution of fresh concrete exhibits a characteristic time (τ). The wave velocity presented as a function of normalized time (t/τ) follows a master curve which depends only slightly on the formulations tested.     

Using multi-axis material extrusion to improve mechanical properties through surface reinforcement

ABSTRACT

Due to the layer stacking inherent in traditional three-axis material extrusion (ME) additive manufacturing processes, a part's mechanical strength is limited in the print direction due to weaker interlayer bond strength. Often, this requires compromise in part design through either adding material in critical areas of the part, reducing end-use loads or forgoing ME as a manufacturing option. To address this limitation, the authors propose a multi-axis deposition technique that deposits material along a part's surface to improve mechanical performance. Specifically, the authors employ a custom 6 degree of freedom robotic arm ME system to create a surface reinforcing ‘skin’, similar to composite layup, in a single manufacturing process. In this paper, vertical tensile bars are fabricated through stacked XY layers, followed by depositing material directly onto the printed surface to evaluate the effect of the skinning approach on mechanical properties. Experimental results demonstrate that surface-reinforced interlayer bonds provide increased yield strength.     

Additive manufacturing of mechanical testing samples based on virgin poly (lactic acid) (PLA) and PLA/wood fibre composites

3D printing in additive manufacturing is considered as one of key technologies to the future high-precision manufacturing in order to benefit diverse industries in building construction, product development, biomedical innovation, etc. The increasing applications of 3D printed components depend primarily on their significant merits of reduced weight, minimum used materials, high precision and shorter production time. Furthermore, it is very crucial that such 3D printed components can maintain the same or even better material performance and product quality as those achieved by conventional manufacturing methods. This study successfully fabricated 3D printed mechanical testing samples of PLA and PLA/wood fibre composites. 3D printing parameters including infill density, layer height and the number of shells were investigated via design of experiments (DoE), among which the number of shells was determined as the most significant factor for maximising tensile strengths of PLA samples. Further, DoE work evaluated the effect of material type (i.e., neat PLA and PLA/wood fibres) and the number of shells on tensile, flexural and impact strengths of material samples. It is suggested that material type is the only predominant factor for maximising all mechanical strengths, which however are consistently lower for PLA/wood fibre composites when compared with those of neat PLA. Increasing the number of shells, on the other hand, has been found to improve almost all strength levels and decrease infill cavities. The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-018-0211-3     

骨料粒径对混凝土动态拉伸强度及尺寸效应影响分析

骨料粒径是影响混凝土力学性能及破坏机理的重要因素。从细观角度出发,将混凝土看作由骨料颗粒、砂浆基质及界面过渡区组成的三相复合材料,考虑细观组分的应变率效应,建立了混凝土动态拉伸破坏行为研究的细观力学分析模型,模拟研究了不同骨料粒径下混凝土动态拉伸破坏行为,并揭示了动态拉伸强度的尺寸效应规律。研究表明:低应变率下骨料不发生破坏,骨料粒径对混凝土动态拉伸破坏模式及拉伸强度影响显著,且拉伸强度的尺寸效应随骨料粒径的减小而削弱;高应变率下裂缝将贯穿骨料,骨料粒径的大小对混凝土动态拉伸强度及尺寸效应影响可忽略。最后,结合应变率效应的影响机制,建立了混凝土拉伸强度的"静动态统一"尺寸效应理论公式,该公式可以较好描述各骨料粒径下混凝土动态拉伸强度与试件尺寸的定量关系。     

虚拟裂缝尖端拉应力大小的确定

针对单边切口的混凝土轴心受拉构件,基于虚拟裂缝模型提出一种计算极限承载力的解析模型,并在此基础上确立了虚拟裂缝尖端拉应力与混凝土轴心抗拉强度之间的关系。结果表明:二者的比值随初始缝高比的增大呈线性增加,但对混凝土强度等级的变化不敏感。其原因是由于所有的混凝土试件都存在初始缺陷,导致截面上存在明显的应力梯度,因而得到的混凝土轴心抗拉强度值是截面应力的平均值,而虚拟裂缝尖端拉应力为截面上的最大应力。很显然,轴心受拉构件的初始缺陷越长,截面的应力梯度越大,虚拟裂缝尖端拉应力与平均应力的比就越大。通常情况下,虚拟裂缝尖端的拉应力大小约为混凝土轴心抗拉强度值的1.22倍,约等于混凝土的抗折强度。     

Three‐dimensional finite‐element simulation of the dynamic Brazilian tests on concrete cylinders

We investigate the feasibility of using cohesive theories of fracture, in conjunction with the direct simulation of fracture and fragmentation, in order to describe processes of tensile damage and compressive crushing in concrete specimens subjected to dynamic loading. We account explicitly for microcracking, the development of macroscopic cracks and inertia, and the effective dynamic behaviour of the material is predicted as an outcome of the calculations. The cohesive properties of the material are assumed to be rate‐independent and are therefore determined by static properties such as the static tensile strength. The ability of model to predict the dynamic behaviour of concrete may be traced to the fact that cohesive theories endow the material with an intrinsic time scale. The particular configuration contemplated in this study is the Brazilian cylinder test performed in a Hopkinson bar. Our simulations capture closely the experimentally observed rate sensitivity of the dynamic strength of concrete in the form of a nearly linear increase in dynamic strength with strain rate. More generally, our simulations give accurate transmitted loads over a range of strain rates, which attests to the fidelity of the model where rate effects are concerned. The model also predicts key features of the fracture pattern such as the primary lens‐shaped cracks parallel to the load plane, as well as the secondary profuse cracking near the supports. The primary cracks are predicted to be nucleated at the centre of the circular bases of the cylinder and to subsequently propagate towards the interior, in accordance with experimental observations. The primary and secondary cracks are responsible for two peaks in the load history, also in keeping with experiment. The results of the simulations also exhibit a size effect. These results validate the theory as it bears on mixed‐mode fracture and fragmentation processes in concrete over a range of strain rates. Copyright © 2000 John Wiley & Sons, Ltd.     

Non-monotonic radial distribution of tensile yielding strength in cold-drawn pearlitic wire

It is commonly accepted that the surface layer in the cold-drawn pearlitic steel wire has a higher strength than the centre. In the present work, via testing the cold-drawn wire after removing the surface layer by an electrochemical method, it is discovered that the tensile yielding strength distributes non-monotonically from the surface to the centre. A valley is observed at the sub-surface. It is found that the maximum accumulative strain during drawing occurs at the sub-surface which leads to more hardening. The presence of residual stress after drawing reduces the tensile yielding stress. With increasing thickness of the removed layer, the tensile yielding stress increases monotonically. Finally, the only reason for the non-monotonic distribution of the tensile yielding stress is the strain path which the material at the sub-surface experiences in drawing and subsequent tension. It actually belongs to a reverse and a cross loading, which result in softening.     

Splitting tensile test for fibre reinforced concrete

The splitting tensile test is a much used method to determine the tensile strength of concrete. The conventional test procedure is known to have a number of limitations related to size effect and boundary conditions. Furthermore, it has been reported to be impossible to determine the tensile strength of Fibre Reinforced Concrete (FRC) using the standard splitting tensile test method. The objective of this paper is to present a methodology to obtain a close estimate of the true tensile strength of FRC from an adjusted tensile splitting test procedure. Splitting tests were performed on cylindrical specimens of four FRC mixes. The transversal deformation perpendicular to the load direction was recorded during the tests. The experimental load-deformation curves thus obtained have two peaks, an initial one as a result of the tensile stresses at the centre of the specimen and a second peak due to secondary cracking outside the loading axis. The tensile strength can be calculated from the first peak which represents the elastic limit state for the material. The method is validated through numerical simulation of the splitting tests using a cohesive crack approach. It is concluded that it is possible to obtain a close estimate of the true tensile strength of FRC using the procedure developed in the paper.     

Modifications of RHT material model for improved numerical simulation of dynamic response of concrete

Sophisticated numerical models are increasingly used to analyze complex physical processes such as concrete structures subjected to high-impulsive loads. Among other influencing factors for a realistic and reliable analysis, it is essential that the material models are capable of describing the material behaviour at the pertinent scale level in a realistic manner. One of the widely used concrete material models in impact and penetration analysis, the RHT model, covers essentially all macro features of concrete-like materials under high strain rate loading. However, the model was found to exhibit undesirable performance under certain loading conditions and some of the modeling issues have been discussed within a recent review paper by the authors. The present paper provides a more in-depth evaluation of the RHT model and proposes modifications to the model formulation to enhance the performance of the model as implemented in the hydrocode AUTODYN. The modifications include Lode-angle dependency of the residual strength surface, tensile softening law and the dynamic tensile strength function. The improvement of the performance of the modified RHT model is demonstrated using numerical sample tests, and further verified via simulations of two series of physical experiments of concrete penetration/perforation by steel projectiles. The results demonstrate an overall improvement of the simulation with the modified RHT model. In particular, the depth of penetration, projectile exit velocity and the crater size are predicted more favourably as compared to the test data. It is also shown that the modeling of the concrete tensile behaviour can affect sensibly the predicted perforation response (e.g., the projectile exit velocity), as is generally expected when the impact velocity exceeds the ballistic limit.     

Ultra‐High‐Speed Full‐Field Deformation Measurements on Concrete Spalling Specimens and Stiffness Identification with the Virtual Fields Method

Abstract: For one decade, spalling techniques based on the use of a metallic Hopkinson bar in contact with a concrete sample have been widely employed to characterise the dynamic tensile strength of concrete at strain rates ranging from a few tens to hundreds of s^?1. However, the processing method based on the use of the velocity profile measured on the rear free surface of the sample (Novikov formula) remains quite basic. In particular, the identification of the whole softening behaviour of the concrete material is currently out of reach. In the present paper, a new processing technique is proposed based on the use of the virtual fields method (VFM). First, a digital ultra‐high‐speed camera is used to record the pictures of a grid bonded onto the specimen. Then, images of the grid recorded by the camera are processed to obtain full‐field axial displacement maps at the surface of the specimen. Finally, a specific virtual field has been defined in the VFM equation to use the acceleration map as an alternative ‘load cell’. This method applied to three spalling tests with different impact parameters allowed the identification of Young's modulus during the test. It was shown that this modulus is constant during the initial compressive part of the test and decreases in the tensile part when microdamage exists. It was also shown that in such a simple inertial test, it was possible to reconstruct average axial stress profiles using only the acceleration data. It was then possible to construct local stress–strain curves and derive a tensile strength value.     

A high performance fibre reinforced cement based plaster for retrofitting RC members

A high performance fibre-reinforced cementitious composite (HPFRCC) material is developed to be used for retrofitting reinforced concrete members. It can be applied to the face of a concrete member to the desired thickness as a wet mix or as an adhesively-bonded prefabricated slab or strip. The material is compatible with concrete and possesses favourable strength and ductility properties, desirable for seismic retrofit. It overcomes some of the problems associated with the current techniques based on externally bonded steel plates and fibre-reinforced polymer (FRP) laminates caused mainly by the mismatch of their tensile strength and stiffness with that of the concrete member being retrofitted. An extensive rheological analysis is undertaken to develop the appropriate mixes using different types and mix proportions of constituent materials including; fine steel fibres, fine quartz sand, silica fume, cement and superplasticizer. Much reduced amounts of steel fibres are used compared to the previous studies so that ordinary mixing procedures could be applied and a more cost-effective retrofitting material could be developed. Samples made of the optimum mixes are shown to posses very high compressive and tensile strengths and sufficient ductility for the composite plaster to be used externally for strengthening and seismic retrofitting of concrete members.     

早龄期水泥混凝土拉伸徐变实测与模型

该研究研发了混凝土早龄期拉伸徐变测量装置,并能够测量直接拉伸强度和拉伸弹性模量。对测得的拉伸徐变进行模拟,采用现存模型对数据进行验证,认为现存模型不能够精确预测混凝土早龄期拉伸徐变。根据实测拉伸徐变数据对现存模型进行修正,建立了更能代表实际工程情况、用于混凝土结构物应力计算的拉伸徐变模型,提供了可用于结构物应力计算的松弛模量。     

A stereological analysis of aggregate grading and size effect on concrete tensile strength

One of the most important topics in solid mechanics is the study of the so-called size effects, whose importance has been widely recognised during the last decades. Size effects are particularly strong in quasi-brittle (i.e., concrete-like) materials. In this paper we focus our attention on the tensile strength decrease associated with the size of concrete structures. An original explanation of this well-known size effect was proposed by the first Author based on the assumption of a fractal-like damage localisation at the mesostructural level. This hypothesis leads to a multifractal scaling law (MFSL) for concrete tensile strength. The present contribution provides a scaling law for concrete tensile strength based on its aggregate size distribution. Since the weakest link in normal strength concrete is represented by the interface between the cementitious matrix and the aggregates, it seems reasonable to look for a relationship between the aggregate grading and the material strength. Based on the hypothesis that the strength depends on the largest flaw, we compute the strength of a concrete specimen as a function of its size. Differently from other statistical approaches, we use a truncated distribution (namely the Füller distribution) in order to describe realistically the flaw population inside the specimen. Calculating the distribution of the largest flaw size by means of statistics of extremes, and relating it to the specimen size, we obtain a scaling law for concrete tensile strength whose trend strictly agrees with the MFSL. Finally, we pay particular attention to the computation of the power law exponent characterising the strength scaling at the smallest sizes and present a comparison with available experimental data.     

A stereological analysis of aggregate grading and size effect on concrete tensile strength

One of the most important topics in solid mechanics is the study of the so-called size effects, whose importance has been widely recognised during the last decades. Size effects are particularly strong in quasi-brittle (i.e., concrete-like) materials. In this paper we focus our attention on the tensile strength decrease associated with the size of concrete structures. An original explanation of this well-known size effect was proposed by the first Author based on the assumption of a fractal-like damage localisation at the mesostructural level. This hypothesis leads to a multifractal scaling law (MFSL) for concrete tensile strength. The present contribution provides a scaling law for concrete tensile strength based on its aggregate size distribution. Since the weakest link in normal strength concrete is represented by the interface between the cementitious matrix and the aggregates, it seems reasonable to look for a relationship between the aggregate grading and the material strength. Based on the hypothesis that the strength depends on the largest flaw, we compute the strength of a concrete specimen as a function of its size. Differently from other statistical approaches, we use a truncated distribution (namely the Füller distribution) in order to describe realistically the flaw population inside the specimen. Calculating the distribution of the largest flaw size by means of statistics of extremes, and relating it to the specimen size, we obtain a scaling law for concrete tensile strength whose trend strictly agrees with the MFSL. Finally, we pay particular attention to the computation of the power law exponent characterising the strength scaling at the smallest sizes and present a comparison with available experimental data.     

Ultrasonic testing of reactive powder concrete

Concrete is a critical material for the construction of infrastructure facilities throughout the world. Traditional concretes consist of cement paste and aggregates ranging in size from 6 to 25 mm that form a heterogeneous material with substantial compressive strength and a very low tensile strength. Steel reinforcement is used to provide tensile strength for reinforced concrete structures and as a composite the material is useful for structural applications. A new material known as reactive powder concrete (RPC) is becoming available. It differs significantly from traditional concrete; RPC has no large aggregates, and contains small steel fibers that provide additional strength and, in some cases, can replace traditional steel reinforcement. Due to its high density and lack of aggregates, ultrasonic inspections at frequencies 10 to 20 times that of traditional concrete inspections are possible. This paper reports on the initial findings of research conducted to determine the applicability of ultrasonic testing techniques for the condition assessment of RPC. Pulse velocities for shear and longitudinal waves and ultrasonic measurement of the modulus of elasticity for RPC are reported. Ultrasonic crack detection for RPC also is investigated.