Response of high performance concrete plates to impact of non-deforming projectiles

The relatively recent technology, which enables the production of high strength concrete (HSC), makes HSC a prospective material for the construction of impact-resisting barriers. However, current penetration formulae are based on test data of normal strength concrete (NSC) and their extrapolation to higher concrete strengths is unsafe. The response of 80×80 cm high performance concrete (HPC) plate specimens to an impact of non-deforming steel projectiles was examined in an experimental study. The tests were planned with an aim to observe the influence of the concrete mix ingredients and amount and type of reinforcement on the performance of HSC under this type of loading. The variants that were examined were the aggregates (different types and maximum size), addition of micro-silica (MS) and steel fibers, and reinforcement details. The main findings show that design of HPC barriers to withstand impact loads involves several aspects. These are aimed at achieving enhanced properties of the structural element, where only one of which is the concrete's compressive strength.     

Influence of concrete strength and confinement method on axial compressive behavior of FRP confined high- and ultra high-strength concrete

This paper presents an experimental investigation on the effect of concrete compressive strength and confinement method on confined high and ultra high-strength concrete (HSC and UHSC) specimens. A total of 55 fiber reinforced polymer (FRP) confined concrete specimens were tested under monotonic axial compression. All specimens were cylinders with 152 mm diameter and 305 mm height and confined by carbon FRP (CFRP). Three different concrete mixes were examined, with average compressive strengths of 35, 65 and 100 MPa. The effect of the confinement method was also examined with FRP-wrapped specimens compared to FRP tube-encased specimens. Axial and lateral behavior was recorded to observe the axial stress–strain relationship and lateral strain behavior for concentric compression. Ultimate axial and lateral conditions are tabulated and the complete stress–strain curves have been provided. The experimental results presented in this paper provide a performance comparison between FRP-confined conventional normal-strength concrete (NSC) and the lesser understood area of FRP-confined HSC and UHSC. The results of this experimental study clearly indicate that above a certain confinement threshold, FRP-confined HSC and UHSC exhibits highly ductile behavior, however for the same normalized confinement pressures, axial performance of FRP-confined concrete reduces as concrete strength increases. The results also indicate that ultimate conditions of FRP-wrapped specimens are similar to those confined by FRP tubes, however a performance difference is evident at the transition region. The performance of 10 existing stress–strain models were assessed against the experimental datasets and the performance of these models discussed. The results of this model assessment revealed the need for further development for stress–strain models developed specifically for FRP-confined HSC or UHSC.     

Flexural ductility of high-strength concrete columns with minimal confinement

The use of high-strength concrete (HSC) instead of normal-strength concrete (NSC) in columns has the advantage of allowing the column size to be reduced and is thus becoming popular. However, since HSC is more brittle than NSC, its use could result in undesirable brittle failure. To evaluate the ductility of columns, nonlinear moment–curvature analysis taking into account the stress-path dependence of the steel reinforcement is required. Based on such analysis, a parametric study has been conducted to investigate the effects of various factors on the ductility of columns. The results revealed that the effect of concrete strength is dependent on the axial stress level (axial load to area ratio) and axial load level (axial load to capacity ratio). At the same axial stress level, the use of HSC has little or basically no adverse effect on the ductility but if the same axial load level is maintained to reduce the column size, the use of HSC would significantly reduce the ductility. Finally, two formulas for direct evaluation of the ductility of columns are developed.     

高强箍筋高强混凝土柱抗震性能研究

为研究高强箍筋高强混凝土柱的抗震性能,首先进行了6根配置高强箍筋的高强混凝土柱和3个普通强度混凝土柱(作为对比)抗震拟静力试验,并对其破坏形态、滞回曲线、延性及耗能指标以及抗剪强度等进行了对比分析。结果表明:高强混凝土试件与普通混凝土试件破坏过程相似,均呈弯剪破坏形态,采用高强混凝土可有效降低试件轴压比,对其延性和耗能能力有利。将国内外进行的高强箍筋高强混凝土柱抗剪承载力试验结果与美国ACI318规范、我国混凝土结构设计规范(GB50010-2002)的抗剪公式进行了对比,认为ACI规范及我国规范在计算高强箍筋高强混凝土柱抗剪承载力时均有不安全因素,宜在设计时注意。采用Mander建议的约束混凝土本构关系和纤维单元程序USC_RC仍可以对高强箍筋高强混凝土柱的受弯承载力进行较为准确的模拟分析。     

Design shear strength formula for high strength concrete beams

After a brief review on the concrete shear strength mechanisms, two very reliable expressions for predicting the shear strength of beams without transverse reinforcement are reported: the one proposed by Ba?ant and Kim [7], which is valid for Normal strength Concrete (NSC) beams, and the other recently proposed by the authors, which is valid for High Strength Concrete (HSC) beams. Hence a summary of a shear strength model for beams with stirrups is provided, which was derived [27] on the basis of the Ba?ant and Kim expression and therefore is adequate only for NSC beams.On the basis of the expression obtained for HSC without stirrups and of the model already proposed for NSC with stirrups, a shear strength expression for HSC beams with stirrups is derived. The obtained expression is applied to an experimental program of 116 HSC beams with stirrups, and is found to predict the test results better than ACI Code [1], Eurocode [12] and CEB/FIP Model Code [9].A design formula is hence proposed, which is adequately conservative and accurate.A design example of a HSC beam with stirrups is carried out, and the various design expressions previously considered are compared.     

Moment redistribution in two-span prestressed NSC and HSC beams

This paper presents a numerical investigation into two-span continuous prestressed normal-strength concrete (NSC) and high-strength concrete (HSC) beams, focusing on aspects of behavior related to moment redistribution. A comparative study is performed by using an experimentally validated computer model. The concrete cylinder compressive strength of investigated beams covers from 40 to 90 MPa, while the prestressing reinforcement ratio (ρ _p) ranges from 0.15 to 1.29%. The results show that the tendon tensile strength can be better exploited by HSC than by NSC at moderate to high ρ _p levels. At a given level of ρ _p, HSC generally mobilizes smaller neutral axis depth, higher strain in nonprestressed steel and a bit larger moment redistribution at ultimate than NSC. Typical code recommendations (ACI, CSA and EC2) for permissible moment redistribution are examined. The effect of concrete strength on moment redistribution in prestressed beams is well reflected in ACI but inadequately reflected in CSA and EC2. A simplified equation for calculating moment redistribution in continuous prestressed beams is proposed.     

Comparison between high strength concrete and normal strength concrete subjected to high temperature

Two normal strength concretes and three high strength concretes, with 28-day compressive strengths of 28, 47, 76, 79 and 94 MPa respectively, were used to compare the effect of high temperatures on high strength concrete and normal strength concrete. After being heated to a series of maximum temperatures at 400, 600, 800, 1000 and 1200°C, and maintained for 1 hour, their compressive strengths and tensile splitting strengths were determined. The pore size distribution of hardened cement paste in high strength concrete and normal strength concrete was also investigated. Results show that high strength concrete lost its mechanical strength in a manner similar to or slightly better than that of NSC. The range between 400 and 800°C was critical to the strength loss of concrete with a large percentage of loss of strength. Microstructural study carried out revealed that high temperatures have a coarsening effect on the microstructure of both of high strength concrete and normal strength concrete.     

Permeability of cracked concrete

The goal of the research presented here was to study the relationship between cracking and water permeability. A feedback-controlled test was used to generate width-controlled cracks. Water permeability was evaluated by a low-pressure water permeability test. The factors chosen for the experimental design were material type (paste, mortar, normal and high strength concrete), thickness of the sample and average width of the induced cracks (ranging from 50 to 350 micrometers). The water permeability test results indicated that the relationships between permeability and material type differ for uncracked and cracked material, and that there was little thickness effect. Permeability of uncracked material decreased from paste, mortar, normal strength concrete (NSC) to high strength concrete (HSC). Water permeability of cracked material significantly increased with increasing crack width. For cracks above 100 microns, NSC showed the highest permeability coefficient, where as mortar showed the lowest one.     

Two-layer pre-stressed beams consisting of normal and steel fibered high strength concrete

The paper is focused on analysis of two-layer bending pre-stressed beams consisting of steel fibered (SF) high strength concrete (HSC) in compressed zone and normal strength concrete (NSC) in tensile zone. Investigation of such beams is important for RC structural design, because calculation of fibers volume ratio is significant, like that of reinforcing steel bars for usual RC elements. In other words, such elements are made of high performance concrete (HPC). There is a growing tendency that more effective HPC structures replace NSC ones, first of all in pre-stressed elements. Definition of the HSC class lower limit, to be used in the compressed zone of a two-layer pre-stressed beam, is given. It was demonstrated that SF have little effect on the beam elastic deflections. However, the ultimate deflections of the section increase because additional potential for plastic energy dissipation (PED) in the bending element. NSC, used in the section tensile zone, contributes additionally about 20% to the section’s PED potential compared to one-layer HSC beams. In order to guarantee sufficient section’s ductility of the pre-stressed beams, required to withstand dynamic loadings, a minimum SF ratio is proposed to be considered. The fibers take the tensile stresses, yielding cracks in the concrete matrix. A design method for calculation of the SF volume ratio, as a function of required ductility, is proposed. A numerical example, illustrating the efficiency of this method is presented.     

高温下钢管混凝土SHPB动态力学性能试验研究

采用霍普金森压杆(Split Hopkinson pressure bar,SHPB)试验装置和特制高温试验炉进行了高温下钢管混凝土的抗冲击性能试验研究,通过测试高温下钢管混凝土的动态强度和应力-应变曲线,揭示温度和冲击速度(应变率)对高温下钢管混凝土动态力学性能的影响规律。试验结果表明,高温下钢管混凝土仍具有良好的抗冲击性能、延性和耗能能力。在该文试验参数范围内,温度作用相比冲击速度的影响更加显著。高温下钢管混凝土动态强度受试件尺寸影响显著,后续研究工作应关注钢管混凝土动态冲击荷载作用下的尺寸效应研究。     

Self-desiccation and its importance in concrete technology

In this article, an extensive experimental and numerical study of self-desiccation and its importance in concrete technology is outlined. For this purpose, cubes and cylinders of eight qualities of concrete were studied at 4 different ages for each one. Parallel studies of autogeneous shrinkage, basic creep, hydration, internal relative humidity and strength were carried out. Previous research in the field is summed up. Finally, the article presents the mechanisms of self-desiccation as well as its importance in concrete technology by modelling the autogenous shrinkage and its effect on basic creep. This project was carried out between the years 1992 and 1995.     

Transient strain of high strength concrete at elevated temperatures and the impact of polypropylene fibers

This paper presents the results of an experimental study on the transient strain of high strength concrete (HSC) under heating up to 750°C and the impact of polypropylene (PP) fibers. Concerning this topic only few results are available in the literature and systematic investigations are missing. However, basic knowledge is necessary for the understanding of the internal damage processes in the material as well as for heated structures. The transient strain during heating can be separated in two basic components: the free thermal strain and the mechanical strain. They were experimentally determined exemplarily for one HSC. For the determination of the mechanisms of transient strain and particularly the influence of PP fibers different techniques were applied. In this context the monitoring of the microcracking was done for the first time with acoustic emission analysis in combination with ultrasonic measurements. This new approach helps fundamentally to explain the impact of PP fibers on free thermal strain and mechanical strain during heating up. Furthermore weight loss measurements were carried out to characterize the moisture transport. It was shown that the PP fibers cause an acceleration of the moisture transport in the temperature range from 200 to 250°C which leads to drying shrinkage in opposite direction to the free thermal strain. Hence this paper is a contribution to the general understanding of the impact of PP fibers in HSC at high temperatures and emphasizes the important influence of PP fibers on the thermal and mechanical induced strain of HSC.     

High strength concrete response to hard projectile impact

High strength concrete (HSC) becomes more common in practice and may have advantageous implementations. According to existing penetration formulae HSC is expected to enhance the performance of structural elements that are designed to resist projectile impacts. However, scabbing at the rear face is expected to be more severe in elements that are made of HSC, because of the relatively high material brittleness. Therefore, it is important to enhance the ductility of HSC elements, and one possible direction is to use fibers or wire mesh reinforcement. In order to study the influence of the concrete strength and of the reinforcement type on the elements response, penetration tests were conducted on regular strength concrete (RSC) and on HSC plates, with the following types of reinforcement: 5 mm steel mesh, steel fibers, small diameter steel wire mesh, and woven steel fence mesh of various diameters. The plates were subjected to an impact of a cylindrical hard steel projectile, weighing 120 g, with a conical nose and a 1.5 aspect ration. The projectiles were accelerated by a laboratory gas gun to velocities that ranged between 85 and 230 m/sec, which were measured by an electro-optical device. By comparing the response of these plates to an impacting projectile, the effects of concrete strength and of the reinforcement were studied. Major trends of the elements behavior were studied, their responses were compared and are described herein.     

大尺寸RPC管-HSC组合柱轴压性能试验研究

在活性粉末混凝土(RPC)预制管内部浇筑高强混凝土,形成RPC管-高强混凝土(HSC)组合柱,扩展了RPC管-混凝土组合柱(CFRT)这一新型组合结构的范围。开展了RPC管离心法成型的配合比与工艺研究,成功试制了RPC管。对4组大尺寸CFRT和1组箍筋约束高强混凝土柱试件进行轴压试验,试验参数为内部混凝土和纵筋配置率。结果表明:内部填充HSC的CFRT柱在整个受力过程中,预制管身基本完整,抗压性能显著优于用于对比的箍筋约束高强混凝土柱;CFRT柱的抗压强度随内部混凝土强度的提高而提高,但约束效应逐步降低,对CFRT柱的内部混凝土的强度应该有所限制;在内部混凝土中配置纵向钢筋,对CFRT的延性影响不大;基于Mander(1988)约束模型,提出了内部填充高强混凝土的CFRT柱轴向承载力计算方法,模型的预测结果与试验结果吻合较好。     

Critical parameters for the compressive strength of high-strength concrete

This study investigates the influence of several material properties underlying the failure mechanism of high-strength concrete (HSC) under uniaxial compression. An experimental-numerical characterization of a single inclusion block (SIB) – an idealized composite comprising of a granite cylindrical core embedded within a high-strength mortar (HSM) matrix – is first carried out. Parametric studies are next conducted with the calibrated SIB model, to identify the critical parameters governing the failure of the idealized composite. The qualitative understanding obtained from the SIB is then utilized to design a series of experiments, exploring the extent of influence of the identified critical parameters on the compressive strength of HSC. Complementary experimental data in literature are also examined. For the range of specimens considered, it is found that the lateral strain capacity of mortar matrix has the most influence on the compressive strength of HSC.     

The nitrogen gas tension test. Part 2: failure mechanism

In this study, the mechanism of concrete failure in the nitrogen gas tension test was investigated through a series of experiments. First, the nitrogen gas tension test was carried out two types of specimens: solid cylinders and hollow cylinders. The test results clearly showed that there was no significant difference in the gas pressure at failure between the solid specimen and the hollow specimen. Since a tension crack occurring on the surface of the concrete specimen at a gas pressure almost equal to the tensile strength of the concrete might play a key role in understanding the failure mechanism, a failure criterion based on linear elastic fracture mechanics (LEFM) was consequently developed. The nitrogen gas tension test was newly carried out on cylindrical specimens with circumferential notches of various depths. Though LEFM was found to be useful in developing an understanding of the mechanism of concrete failure, the experimental results indicated that it was not really valid for specimens with notch depths deeper than some critical size (critical notch depth). However, based on the experimental observation that the concrete specimen failed at its tensile strength at notch depths smaller than the critical notch depth, a modified LEFM based failure mechanism was proposed taking into account the notch sensitivity of the concrete.     

The Effect of Size on the Splitting Strength of Cubic Concrete Members

In the study of concrete fractures, split‐tension specimens, such as cylinders, cubes and diagonal cubes, are frequently preferred to beams. However, experimental investigations on concrete reveal that for the same specimen geometry, the nominal strength of specimen decreases with increasing specimen size. This phenomenon is named as the size effect in the fracture mechanics of concrete. Although nominal strength is also highly affected by the width of the distributed load in the split‐tension cylinder and cube specimens, this effect can be negligible within the practical range of the load‐distributed width in the diagonal cubes. However, the number of theoretical and experimental studies with diagonal split‐tension specimens is limited. Besides, a size effect formula for estimating the split‐tensile strength of the diagonal cube specimens has not been proposed. In this study, nine series of cube and diagonal cube specimens, with three different sizes but similar geometries, were tested under different load‐distributed widths. The ultimate loads obtained from the test results are analysed by the modified size effect law. Subsequently, prediction formulas are proposed, and they are compared with historical test data from the split‐cylinder specimens.     

Behavior of normal and high-strength concrete cylinders confined with E-glass/epoxy composite laminates

This paper is aimed at studying the behavior of concrete cylinders with varying compressive strength wrapped with E-glass/epoxy fiber reinforced polymer (GFRP) jackets and subjected to uniaxial compressive loads. A comprehensive experimental program which involves 54 plain concrete cylinders was conducted in this study. The cylinders evaluated in this study, were divided into six groups, and each group contain a control cylinder without confinement to quantify the amount of gain obtained using the GFRP laminates. Experimental results indicated that the use of GFRP jackets substantially increases both the compressive strength and ductility of unreinforced concrete cylinders. In this paper, the influences of two parameters influencing the behavior of the GFRP confined cylinder is investigated. These parameters are: the number of composite plies (i.e. composite thickness) and concrete compressive strength. The results of this study showed that: (i) compressive strength and ductility of the concrete cylinders increases with number of composite layers; and (ii) effect of confinement is substantial for normal strength concrete and marginal for high-strength concrete. A semi-empirical theoretical model is also presented in order to predict stress–strain relationship of GFRP confined concrete cylinders. The model results showed an excellent agreement with experimental values.     

Effects of stirrup,steel fiber,and beam size on shear behavior of high-strength concrete beams

This study investigates the effectiveness of steel fibers and minimum amount of stirrups on the shear response of various sized reinforced high-strength concrete (HSC) beams. For this, six large reinforced HSC beams with a shear span-to-depth ratio (a/d) of 3.2 were manufactured. Three of them contained 0.75% (by volume) steel fibers without stirrups as per ACI Committee 318, while the rest were reinforced with the minimum amount of stirrups without fibers. Test results indicate that, with increasing beam size, significantly lower shear strength was obtained for steel fiber-reinforced high-strength concrete (SFR-HSC) beams without stirrups, than for the plain HSC beams with stirrups. The inclusion of steel fibers effectively limited crack propagation, produced more diffused initial flexural cracks, and led to higher post-cracking stiffness, compared to plain HSC. On the other hand, the use of minimum stirrups gave better shear cracking behaviors than that of steel fibers, and effectively mitigated the size effect on shear strength. Therefore, a large decrease in shear strength, with an increase in the beam size, was only obtained for SFR-HSC beams without stirrups. A shear strength decrease of 129% was obtained by increasing the effective depth from 181 mm to 887 mm. The shear strengths of reinforced steel fiber-reinforced concrete beams were not accurately predicted by most previous prediction models. Therefore, a new shear strength formula, based on a larger dataset, that considers the size effect, is required.