Testing of concrete under closed-loop control (original) (raw)
Related papers
3action: A Viable Setup For Direct-Tensile Testing Of Concrete
2023
The experimental characterization of concrete's post-peak stress vs crack opening curve is a key aspect in many practical problems involving crack stability, like size-effect and explosive spalling of High-Performance concrete in the fire. The direct-tension test is recognized as the most straightforward solution, though the strain-softening material behaviour entails tricky issues concerning the axial and flexural stability of the test. Axial stability is impaired by the deformability of the loading frame since the traditional scheme of universal testing machines entails compression of rather long columns and bending of the crosshead and the table. The consequent deformation energy largely exceeds the dissipation capacity of the tested sample, requiring responsive closedloop control systems for smoothly driving the descending branch of the test. As regards flexural stability, several solutions have been proposed to improve the bending stiffness of the test rig, mostly based on ball bushing guiding systems or adjustable tie-rods secured to the loading platens. However, the transversal restraint to the sample may trigger parasitic shear stress, which translates into inclined fracture propagation paths and overlapping cracks. To address these multifaced issues, an innovative frameless test rig has been designed, based on three symmetrically arranged electro-mechanical jacks directly pushing the top of the sample up. The samples are short, notched cylinders (Ø=100mm, h=150mm) preliminarily glued to thick steel platens and then bolted to the bottom table and the moving head of the machine, speeding up their installation and later removal. The load is exerted through stiff load cells and pendular struts, preventing any transversal load component. The actuators are optimized for stiffness and promptness, and three parallel control loops are implemented to drive the mean opening and two orthogonal rotation components of the crack mouth. At the present stage, the prototype has been assembled and the control routines are being tested. A first series of experimental results will be included in the final presentation at the conference.
Large scale high strain-rate tests of concrete
EPJ Web of Conferences, 2012
This work presents the stages of development of some innovative equipment, based on Hopkinson bar techniques, for performing large scale dynamic tests of concrete specimens. The activity is centered at the recently upgraded HOPLAB facility, which is basically a split Hopkinson bar with a total length of approximately 200 m and with bar diameters of 72 mm. Through pre-tensioning and suddenly releasing a steel cable, force pulses of up to 2 MN, 250 µs rise time and 40 ms duration can be generated and applied to the specimen tested. The dynamic compression loading has first been treated and several modifications in the basic configuration have been introduced. Twin incident and transmitter bars have been installed with strong steel plates at their ends where large specimens can be accommodated. A series of calibration and qualification tests has been conducted and the first real tests on concrete cylindrical specimens of 20cm diameter and up to 40cm length have commenced. Preliminary results from the analysis of the recorded signals indicate proper Hopkinson bar testing conditions and reliable functioning of the facility.
Measurement of time-dependent strains of concrete
Materials and Structures, 1998
1359-5997/98 © RILEM tion of long-term behaviour. At the same time, creep and shrinkage values appear in certain cases to be a material selection criterion. Shrinkage and creep tests thus have two possible purposes: • determination of the properties of the concrete of a given mix design, aimed to provide the input data for the time-dependent constitutive law of the material; in this case, the aim is to determine the influence of the main parameters by varying the test conditions (loading age, ambient environment, compression stress magnitude, age at unloading); • concrete mix design optimization (generally in relation with other mechanical or physical properties, and in particular compressive strength); in this case, it is normally feasible to test only some representative conditions (early age and 28-day loading, with and without drying, for example). In both situations, the pronounced hereditary behaviour of the material (i.e. the fact that the behaviour at a given instant depends on the entire history of preceding stress) calls for a rigorous definition of the creep test, which does not require any correction in the case of non-constant loading. The quality of the test consequently depends primarily on the ability of the device to keep the applied load constant, particularly at the very beginning of the test.
Technological Control Assessment and Concrete Receiving
International Journal for Innovation Education and Research
With the growth in the area of construction, a greater demand is obtained from both the consumer and the current standards, due to this there is the need for a refined assessment on the process of technological control of concrete. Concrete is one of the most important materials in construction, this material is the most tested and properly controlled, thus bringing the need for the development of this article. Upon receiving the concrete on site, according to the NBR technical standard [1], a series of conferences and preliminary test were analyzed, from which there is the first evaluation of the material in the fresh state, with the slump test, to deepen the Diagnosis of concrete evaluation A laboratory was hired to perform tests and ensure quality and reliability. In the laboratory, specimen ruptures were made through the compressive strength test, which is essential for the final verification of structural strength. The objective of this study is to evaluate the technological ...
A testing technique for concrete under confinement at high rates of strain
International Journal of Impact Engineering, 2008
A testing device is presented for the experimental study of dynamic compaction of concrete under high strain-rates. The specimen is confined in a metallic ring and loaded by means of a big hard steel Hopkinson pressure bar (80 mm diameter, 6 m long) allowing for the testing of specimens large enough regarding the aggregates size. The constitutive law for the metal of the ring being known, transverse gauges glued on its lateral surface allow for the measurement of the confining pressure. The spherical and the deviatoric response of the specimen can then be computed. The proposed method is validated by several numerical simulations of tests involving a set of 4 different concrete-like behaviours and different friction coefficients between the cell and the specimen. Finally, 3 tests
An experimental method for dynamic tensile testing of concrete by spalling
International Journal of Impact Engineering, 2001
A new application of the spalling phenomenon in long specimens is reported in this paper. The new experimental technique is based on an experimental setup which consists of an air launcher of cylindrical projectiles with a Hopkinson bar as a measuring tool and a relatively long concrete specimen in contact with the bar. The incident compression wave transmitted by the Hopkinson bar into the specimen is re#ected as a tensile wave causing spalling. Although such con"gurations have been reported in the past, the main advantage of the present approach lies in the application of the detailed analysis, based on the wave mechanics with dispersion, to extract the specimen behaviour. Such an approach leads to an exact estimation of the local failure stress in tension at high strain rates, even above 100 s\. This paper demonstrates, using two series of tests on concrete, that this experimental setup can cover one decimal order of strain rates, from &10 to &120 s\. The tests performed at high strain rates on wet and dry concrete have indicated that the tensile strength is substantially in#uenced by the loading rate or strain rate. The absolute value of the failure stress for wet and dry concrete is almost the same for a particular strain rate, which does not occur when subject to low strain rates in tension or compression. A brief discussion is o!ered on a high rate sensitivity of concrete strength in tension at high strain rates.
Destructive and Non-destructive Testing of Concrete Structures
The estimation of mechanical properties of concrete can be carried out by several methods; destructive and non-destructive. In this context, the crushing of the samples is the usual destructive test to determine the concrete strength. The rebound hammer test and the ultrasonic device are used in the field of non-destructive tests to determine respectively the compression strength and the ultrasonic pulse velocity (UPV) in the concrete. In this work, eight concrete compositions were used to prepare cylindrical specimens (16 cm x 32 cm) by varying the water/ cement ratio and the cement dosage. An experimental study was conducted to determine the compressive strength of concrete by destructive (compression) and non-destructive (rebound hammer) tests at different ages (7, 14 and 28 days). In addition, the influence of several factors on the modulus of elasticity determined by pulse velocity test was investigated. These factors mainly included the age of concrete and the water/ cement ratio. The results showed that the difference between the resistance values obtained by destructive and non-destructive methods decreases with increasing age of concrete. The dynamic modulus of elasticity increases with the curing time of the concrete until the age of three months. In addition, a simplified expression has been proposed to estimate the rebound number from the value of the dynamic modulus of elasticity determined by pulse velocity test.
An Experimental Method to Determine the Tensile Strength of Concrete at High Rates of Strain
Experimental Mechanics, 2010
In the present work, dynamic tensile strength of concrete is experimentally investigated by means of spalling tests. Based on extensive numerical simulations, the paper presents several advances to improve the processing of spalling tests. The striker is designed to get a more uniform tensile stress field in the specimen. Three methods proposed in the literature to deduce the dynamic strength of the specimen are discussed as well as the use of strain gauges and a laser extensometer. The experimental method is applied to process data of several tests performed on wet micro-concrete at strain rates varying from 30 to 150/s. A significant increase of the dynamic tensile strength with strain-rate is observed and compared with data of the literature. In addition, post-mortem studies of specimens are carried to improve the analysis of damage during spalling tests.
2015
The early age cracking of concrete, which includes plastic shrinkage cracking (PShC) and plastic settlement cracking (PSeC), commonly occurs in flat concrete elements such as bridge decks and slabs or at the change of a concrete section depth. These cracks typically occur once the concrete has been cast and consolidated up to the final setting time, and initiate when the tensile stresses developed in the concrete exceeds its ultimate tensile strength or, alternatively phrased, when the restrained shrinkage induced strain in the concrete exceeds its tensile strain capacity. These cracks have a premature detrimental effect on the durability and strength of concrete structures as they allow deleterious materials to penetrate the concrete, which could cause the corrosion of steel reinforcing. With this in mind, the objective of this study is to gain a fundamental understanding of the tensile properties of early age concrete, up to the point of final setting, as well as the variables that affect these properties. This is done to better understand, and ultimately reduce the risk of early age cracking. To achieve this, experimental assemblies found in literature were evaluated and built upon to create a multi-component uniaxial tensile testing setup that is able to capture the complete stress-strain behaviour of early age concrete, while still in a plastic state. The following significant findings were attained from this study: Reducing the coarse aggregate size in a concrete mix increases both the tensile strength and Young's modulus of early age concrete, while reducing both its fracture energy and fracture process zone (FPZ) characteristic length. The low volume addition of microfibres to a conventional concrete mix increases both the fracture energy and the FPZ characteristic length of early age concrete. The low volume addition of microfibres to a conventional concrete mix increases the strain capacity of early age concrete shortly before and after the initial setting time. This increased strain capacity is believed to be of great significance for the prevention of PShC. The addition of an accelerator to a conventional concrete mix accelerates the development of the tensile properties of early age concrete, while a retarder reduces it. The addition of a retarder to a conventional concrete mix increases the strain capacity of early age concrete shortly before and after the initial setting time. This provides a reason for the reduced PShC severity observed in retarded mixes in certain instances. From this study it is concluded that the results from the tensile tests provide a greater understanding of the tensile properties of early age concrete as well as the variables that affect them. When interpreting these results in combination with those obtained from PShC experiments, it is suggested that it is possible to determine when and if PShC will occur. Stellenbosch University https://scholar.sun.ac.za I would like to thank the following people for their assistance and support during this study: My supervisors, Riaan Combrinck and Prof Billy Boshoff for their guidance and support throughout this study. The staff in the workshop and laboratory at the Civil Engineering Department of the University of Stellenbosch for their assistance in the construction of the tensile test setup and support during the period of experimental work. Gustav Andrag and Juan Louw, for their assistance in the construction of the tensile test setup. Lohan Jacobs, for his assistance in the carrying out the experimental work. Kevin Neaves, at the Mechanical and Mechatronic Engineering Department of the University of Stellenbosch for his time and assistance with setting up the motor and motion controller assembly of the linear actuator used in this study. My friends, for their encouragement and refinement during this study. My family, for their love and continual encouragement throughout this study. God, for giving me life and blessing me with the opportunities and abilities to complete this study.
Static and dynamic testing of a damaged post tensioned concrete beam
MATEC Web of Conferences, 2015
In this paper are reported the results of an experimental campaign carried out on a post tensioned concrete beam with the aim of investigating the possibility to detect early warning signs of deterioration basing on static and/or dynamic tests. The beam was tested in several configurations aimed to reproduce several different phases of the 'life' of the beam: the original undamaged state, increasing loss of tension in the post tensioning cables, a strengthening intervention carried out by means of a second tension cable, formation of further cracks on the strengthened beam. Responses of the beam were measured by an extensive set of instruments consisting of accelerometers, inclinometers, displacement transducers, strain gauges and optical fibres. The paper discusses the tests program and the dynamic characterization of the beam in the different damage scenarios. The modal properties of the beam in the different phases were recovered basing on the responses recorded on the beam during sine-sweep and impact hammer tests. The variation of the first modal frequency was studied to investigate the sensitivity of this parameter to both the cracking of the concrete section and the tension in the cables and also to compare results given by different types of experimental tests This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.