Testing Concrete E‐modulus at Very Early Ages Through Several Techniques: An Inter‐laboratory Comparison

The design of concrete structures is based on calculation rules, which often do not take into account the very early age behaviour of the material. However, during this period, structural concrete is subjected to strains due to the hydration process of cement. If these strains are restrained by concrete itself or surrounding boundaries, stresses start to build up that can lead to the formation of cracks. Among the parameters involved in the stress build up, the stiffness evolution is of major importance. This paper reports the use of eight different techniques aimed at stiffness evolution assessment, applied on the same concrete mix, in a round robin experimental test within three laboratories. The observations are compared after having expressed the results at the same equivalent age. Both the loading stress rate and amplitude are observed to have an effect of limited importance on the determination of the quasi‐static elastic modulus, which might be explained by very short term creep. Ultrasonic measurements provide values of E‐modulus that are higher than the values provided by the quasi‐static tests at the time of the concrete setting. Similar mechanisms associated to very short term creep could explain the difference between the quasi‐static and high‐frequency elastic modulus.     

Effect of nonlinear response of concrete on its elastic modulus and strength

The purpose of this paper is to investigate the effect of nonlinear response of concrete on the relationship between modulus of elasticity at static and dynamic loading as well as on its strength. The obtained relationships are based on the thermofluctuation strength theory coupled with a nonlinear stress–strain material model. From the corresponding equations it was found that the ratio of the static to the dynamic modulus of elasticity depends on the strength of concrete, its temperature, rate of loading. Also it was confirmed that the dynamic modulus is greater than static modulus of elasticity. These equations explain the influence of the value of applied stress on the value of the static or dynamic modulus of elasticity. Comparative study shows substantial agreement with existing experimental results and the general equations given in standard BS 8110, Part 2:1985, and ACI documents. Based on the obtained relationships new methods for evaluating the static modulus of concrete and its strength from the results of dynamic tests are described subsequently.     

Experimental evaluation of fiber reinforced concrete fracture properties

Concrete is now universally recognized a construction material vital and essential for the regeneration and rehabilitation of the infrastructure of a country. The last few decades have now shown that high strength concrete, with a compressive strength of 100–120 MPa can be readily designed and manufactured. There have also been several advances made in the development of fiber reinforced concrete to control cracking and crack propagation in plain concrete, and to increase the overall ductility of the material. However, there are now many types of fibers with different material and geometric properties, and the exact fracture behavior of fiber reinforced concrete materials is not clearly understood. The overall aim of this paper is to establish the fracture properties and fracture behavior of concrete containing two widely used types of fibers, namely, steel (high modulus) and polypropylene (low modulus). The experimental investigation consisted of tests on cubes and notched prismatic specimens made from plain concrete and fiber concrete with 1% and 2% of steel or polypropylene fibers. The cube tests and the three point bending tests on notched specimens were carried out according to RILEM specifications, and extensive data on their compressive and flexural tensile behavior and fracture energy were recorded and analyzed. The results obtained from the tests are critically assessed, and it is shown that fibers contribute immensely to the structural integrity and structural stability of concrete elements and thereby improve their durable service life.     

影响铝合金弹性模量测量精度的因素

金属材料弹性模量的测量精度受很多试验因素的影响,将挤压成型的7N21铝合金加工成圆棒试样,采用静态拉伸试验方法测量了其弹性模量,并对控制模式、引伸计的使用及预拉伸对弹性模量测量的影响进行了分析.结果表明:弹性模量测量精度受控制模式与试样预拉伸的影响较小,主要由引伸计的选择与使用决定.采用单个引伸计测量同一试样多个方向上...     

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。     

立体视觉方法在混凝土表面裂缝检测中的应用

针对混凝土结构表面裂缝宽度评定需要,研究了立体视觉测量方法的三维数学模型,提出了将裂缝边缘曲线投影到空间平面,用来恢复裂缝边缘的三维坐标的方法,并利用最小距离法和裂缝图像识别算法完成了对裂缝宽度的评定。使用该文集成的立体视觉测量系统完成了在破坏性静载试验中产生表面裂缝的三根混凝土梁的检测试验,得到了每根混凝土梁2个检测区域的裂缝宽度曲线,并与微显微观测仪的数据进行比较分析。试验结果表明：基于立体视觉的裂缝宽度检测方法准确地评定了裂缝宽度,验证了该方法用于混凝土表面裂缝宽度检测的可行性。     

Quantitative estimates of gas maldistribution and methods for their localization in absorption columns

In the present work, we carry out evaluation of gas flow maldistribution in different types of gas distributing devices (GDD) and packings by using experimental data for the output velocity profiles for both the absorption pilot column and industrial column. Two GDD construction types—with circular and with local tube gas feed, together with their modifications—are investigated. Two pilot columns—with random packing RSRM 1.5″ and with structured ceramic Honey-comb packing, and one industrial contact economizer with Honey-comb and inclined rings—have been studied. Six types of quantitative estimations of GDD and packings maldistribution are determined by the help of MATHCAD. Then, their sensitivity toward several factors, such as the type of GDD and packing, the initial gas flow velocity, the number and dimension of measuring cells on the column cross-section, the measurement error, and so on, is investigated. It is found that only two of the quantitative maldistribution estimations take into account the formation of maldistribution clusters, which deteriorates the uniform gas distribution. A novel method for quantitative determination of these clusters and also for identification of the zones at column cross-section, in which they appeared, has been developed. It is established that the measurement error and the dimension of measuring cell deeply influence the number, the area, and spatial distribution of these zones on the cross-section, as well as the values of maldistribution estimates.     

An acoustic tensile load cell able to operate under high water pressures

The principles of operation and the experimental equipment used during an investigation into the role of percolating water within concrete and rock, have been outlined. Attention has been drawn to a problem of measuring a friction force which, if not resolved, would have compromised the investigation. The design requirements of a simple acoustic tensile load cell, capable of resolving the problem, are listed with details of its design, construction and satisfactory performance.     

静载和动载下不同龄期混凝土力学特性的试验研究

利用杆径为75mm的SHPB试验装置对5种不同龄期下的混凝土分别进行了冲击压缩试验,系统了解了冲击载荷对不同龄期支护混凝土力学特性的影响。为了进行对比,利用INSTRON系统也进行了相应龄期下的静载压缩试验。试验研究表明：静载下混凝土强度、割线弹性模量随龄期增长而增长,其中强度增长主要集中在龄期7d以前,割线弹性模量增长则集中在龄期14d以后,而峰值应变随龄期增长整体上呈减小的趋势;动载下混凝土强度、峰值应变以及单位体积吸收能随着龄期增长而增长,在各个龄期都表现出对应变率具有一定的敏感性,其中不同龄期混凝土的动态强度随应变率增加呈现指数函数增长趋势。不同龄期的混凝土在动载下以拉伸破坏为主,静载下基本呈现剪切破坏形式。     

Damage evaluation in freezing and thawing test of concrete by elastic-wave methods

In the freezing and thawing test of concrete, elastic-wave methods are applied to estimate the frost damage nondestructively. Recently, two test methods are proposed in the RILEM recommendation. The alternative method A is based on the ultrasonic test, which measures the transmission time of the longitudinal wave (P wave), while the alternative method B measures the resonant frequencies of vibrations. With relation to these methods, evaluation of the frost damage by the elastic-wave methods is studied, focusing on the dynamic modulus of elasticity. The freezing and thawing tests were conducted by employing concrete samples after 1 year curing. As a result, the samples were not heavily damaged even after 300 cycles. Concerning the resonant frequencies of concrete samples, a three-dimensional analysis is performed by the boundary element method to identify actual vibration modes. It is clarified that an assumption of the one-dimensional resonant vibration is not applicable, resulting in a false discrepancy between the dynamic modulus and the static modulus of elasticity. It is found that relative modulus obtained from P-wave velocity is comparable to that from the tangential modulus of elasticity in the compression test, and is reasonably recommended to estimate the frost damage nondestructively.     

Relative damage evaluation of concrete in a road bridge by AE rate-process analysis

It is widely reported and realized that durability of concrete decreases due to such effects, as salt and chemical attack, freezing and thawing, and carbonation and fatigue. In existing structures mechanical damage of concrete has been practically estimated from the strength by a compression test and a rebound hammer test. For effective maintenance of concrete structures, it is necessary to evaluate quantitatively not only the strength of concrete but also the degree of damage. Quantitative damage evaluation of structural concrete is proposed by applying acoustic emission (AE) technique and damage mechanics. Consequently, damage evaluation is conducted on concrete cores of an existing structure. Core-samples were collected from an arch fragment of a reinforced concrete road bridge that had been in service for 87 years. Compressive strengths and Young’s moduli were measured during the compression test along with AE measurement. Dynamic Young’s moduli were also calculated from the longitudinal wave velocity. Thus the relative damages are quantitatively evaluated from static and dynamic Young's moduli and compared with results of AE rate-process analysis.     

Angle Encoding Disk Surface Parameters Evaluation Using Conventional Laser Head: Initial Study

Surface quality is one of the most important parameters in mechanical treatment. In recent years a great number of contact and non-contact surface quality measurement methods have been created. This article covers a non-contact laser method of surface parameter evaluation, intended for the evaluation of the surface quality of an angle encoder disk. For such a measurement, this paper proposes using a conventional compact disc laser-reading unit. The proposed system is relatively simple and inexpensive. The device allows measuring unevenness in the surface thickness of an encoder disk, as well as its surface roughness and profile details. The structure of the measurement system, the static and dynamic calibration results and the experimental surface measurement results for an angle encoder disc are presented.     

A comprehensive investigation into the effect of aging and coarse aggregate size and volume on mechanical properties of self-compacting concrete

The popularity of self-compacting concrete (SCC), as an innovative construction materials in concrete industry, has increased all over the world in recent decades. SCC offers a safer construction process and durable concrete structure due to its typical fresh concrete behavior which is achieved by SCC’s significantly different mixture composition. This modification of mix composition may have significant effect on the hardened mechanical properties of SCC as compared to normal vibrated concrete (NVC). Therefore, it is necessary to know whether the use of all rules and relations that have been formulated for NVC in current design codes based on years of experience are also valid for SCC. Furthermore, this study represents an extensive evaluation and comparison between mechanical properties of SCC using current international codes and prediction equations proposed by other researchers. Thus, in this experimental study, major mechanical properties of SCC are investigated for twelve SCC mixes with wide spectrum of different variables i.e. maximum coarse aggregate size, coarse aggregate volume and aging. In the present study, an extensive body of data reported by many researchers for SCC and NVC has been used to validate the obtained results.     

Experimental Investigation of Bond Stress and Deformations in LWAC Ties Reinforced with GFRP Bars

The structures' durability is an engineering concern for a long time but has been increased in the last years. Lightweight aggregate concrete (LWAC) combined with glass fibre reinforced polymer bars allows to create structures with high performance in terms of durability. The glass fibre reinforced polymer (GFRP) bars have different ribs from those of steel bars, and consequently, its bond to concrete is affected. Moreover, the Young's modulus of GFRP is much below compared with that of steel, and this influences significantly the behaviour of structural elements reinforced with this material. This paper presents an experimental study focused on bond between LWAC and reinforcing bars of GFRP. Thirty‐six pull‐out tests were carried out using steel and GFRP bars. These reinforcements were combined with three types of concrete, all with the same design density 1900 kg m^?3 but with different values of compressive strength: 35, 55 and 70 MPa. Furthermore, 12 reinforced ties were tested, combining different types of bars (steel and GFRP), two different diameters (12 and 16 mm) and the three types of LWAC. Based on experimental results, several relations were established to understand the behaviour of LWAC structures reinforced with GFRP bars, mainly in the serviceability conditions. These results point out that ties deformation and crack width are very affected by the reduced Young's modulus of GFRP: deformations and crack width of ties reinforced with GFRP are significantly higher, approximately three times greater, compared with those of ties reinforced with steel. The tension stiffening effect was also analysed in detail, and it was found that it is slightly influenced by the concrete compressive strength but is highly dependent of the Young's modulus of the reinforcing material.     

混凝土循环加卸载动态损伤特性研究

利用大型多功能动静力三轴仪对混凝土试件进行了5种应变速率下的动态循环加卸载压缩试验。对混凝土的物理力学参数的变化规律进行了统计分析。结果表明：峰值应力和弹性模量随加载速率的提高而增大,但峰值应变随加载速率的变化表现出较大的离散性。在此基础上,进一步研究了混凝土在不同加载速率下的刚度退化规律。最后,选用基于Weibull统计理论的分段式动态损伤本构模型对试验数据进行拟合。经验证,此模型能够较好的模拟混凝土材料的本构特性。     

用于结构减振的聚合物混凝土试验研究

摘 要：对于目前工程中新出现的桥梁-建筑合一的大型结构,如何减小运行荷载产生的振动是颇受关注的问题。本研究通过在混凝土中掺入高分子聚合物以提高其阻尼性能达到减振的目的。采用矩形截面简支梁和以实际大跨复杂箱梁为原型的1/6缩尺模型,分别进行振动测试、静载试验和疲劳试验,对比普通混凝土和聚合物混凝土的阻尼性能、静力性能和疲劳性能。同时测试基准混凝土和各聚灰比的聚合物混凝土试块的抗压强度和弹性模量。试验结果表明,随聚灰比的增大混凝土梁的阻尼比大幅增大,试块抗压强度和弹性模量略有下降,但满足规范的要求。聚合物混凝土梁的静力性能比普通混凝土梁更好,疲劳性能与之相当。聚合物混凝土的施工性能指标如塌落度和扩展度良好。     

Experiments on concrete under uniaxial impact tensile loading

A problem of practical importance for designing of structural elements is discussed in this paper—the behaviour of concrete subjected to uniaxial impact tensile loading. The “Split Hopkinson Bar” technique was adopted for testing concrete in uniaxial tension at stress rates between 2 and 60 N/mm²/ms. A remarkable increase in tensile strength was observed due to high stress rate. The ratio of impact and static tensile strength varied between 1.33 and 2.34 for various concrete mixes. The influence of maximum aggregate size, water-cement ratio, cement content, cement type and quality, specimen humidity, static compressive strength and loading/casting direction upon the uniaxial impact tensile strength was studied. The high stress rate resulted in an increase of the modulus of elasticity of concrete in uniaxial tension. An explanation for the observed phenomena is suggested.     

Compressive stress-strain model for low-calcium fly ash-based geopolymer and heat-cured Portland cement concrete

This research focuses on elucidating the present knowledge gaps in geopolymer concrete's engineering properties, specifically its stress-strain behaviour. Geopolymer concrete (GPC) is an emerging alternative to ordinary Portland cement concrete (OPCC), and is produced via a polycondensation reaction between aluminosilicate source materials and an alkaline solution. As a relatively new material, many engineering properties of geopolymer concrete are still undetermined. In this paper, the compressive strength, modulus of elasticity and stress-strain behaviour of ambient and heat-cured GPC and OPCC have been studied experimentally. A total of 195 geopolymer concrete cylinders and 210 Portland cement concrete cylinders were tested for the above mentioned characteristics. Based on the experimental results, constitutive models describing the complete stress–strain behaviour in uniaxial compression have been developed for the low-calcium fly ash-based geopolymer concrete and the heat-cured Portland cement concrete.     

On the effects of a crack propagating toward the interface of a bimaterial system

This paper deals with the influence of matrix cracks on the failure mode of bimaterial systems and composite materials. In order to investigate such an influence, the stress field near a crack embedded into the more yielding material and propagating perpendicularly to the interface, has been analyzed by using systematic numerical simulations. Such analysis has shown that the crack propagation give rises to transversal stresses that can damage the reinforcing materials when this has low modulus, as glass fibers, or low transversal strength, such as carbon fibers. Moreover, the longitudinal stress concentration can damage the reinforcing material only if this has high stiffness, as in the case of aramid and carbon fibers.Also, the numerical results have permitted to implement simple formulas that allows the user an accurate evaluation of the SIF as well as to predict possible debonding or fiber splitting phenomena. Finally, the SIFs evaluated numerically have been corroborated by experimental tests carried out by using an efficient procedure based on RGB digital photoelasticity.