Biaxial compressive strength of concrete under high loading rates
Table of contents
Project data
| Titel | Title Zweiaxiale Betondruckfestigkeit unter hohen Belastungsgeschwindigkeiten | Biaxial compressive strength of concrete under high loading rates Förderer | Funding Bundesministerium für Wirtschaft und Energie (BMWi) / KEK Zeitraum | Period 10/2014 – 09/2017 Leiter | Project manager Prof. Dr.-Ing. Dr.-Ing. E.h. Manfred Curbach Bearbeiter | Contributor Dipl.-Ing. Matthias Quast |
The presented project was funded by the German Federal Ministry of Economic Affairs and Energy (BMWi, project no. 1501483) on basis of a decision by the German Bundestag.
Report in the year book 2017
CONCRETE QUICKLY LOADED
Specimen in uniaxial static test setup
Structures that play a major role in society belong to the so-called ‘critical infrastructure’. Among them are health care facilities, transportation, and energy supply infrastructure. These structures are exposed to external natural and anthropogenic risks. In most cases, a reinforced concrete structure acts as a barrier to protect the users. Therefore, it is vital to study the material behaviour of concrete under extraordinarily load cases, such as impact or explosion.
The effect of an impact load on the uniaxial and biaxial compressive strength of a concrete type C20/25 was extensively investigated in various series of tests. The static reference tests were carried out in hydraulic testing machines with a strain rate of about 3 ∙10-5 1/s. This loading rate is in the range of conventional static laboratory tests. The impact load occurred at a strain rate in the range of 75 to 150 1/s, which roughly corresponds to the load sustained by an aircraft during a crash. These dynamic experiments were carried out using the classic experimental principle of the split-Hopkinson bar, in which the concrete sample is loaded and destroyed by means of a shock wave.
For the different loading rates, both uniaxial tests and biaxial tests with different stress ratios were conducted. Comparative calculations were made by the individual test results. From this, the influences of the dynamic load on the uniaxial case and various biaxial stress states can be derived. On the other hand, the influence of the stress ratio can be determined at a certain load rate. In summary, it can be stated that increasing the compressive strength of concrete affects both, the biaxial stress state and the response to an impact load; the increase of the dynamic strength has a clear effect on the multi-axial behaviour of the concrete sample within the examined velocity range.
Report in the year book 2016
Fractured is not equal to almost fractured
Fractures of cubic specimens after uni- (a, b) resp. biaxial (c, d) compressive tests with lower (a, c) resp. higher (b, d) impact velocity
Whether it is a rock fall shelter in the Alps or a wave breaker in the North Sea – in many cases, concrete structures form a protective barrier between people and exterior conditions. To ensure that these structures can adequately fulfill their protective role, the behaviour of concrete under various loading conditions needs to be well-documented. A possible, but not well-researched, case is the multiaxial dynamic compressive load, which can occur, for example, in the event of a rock or of a vehicle impact. With a biaxial split-Hopkinson bar, the only one of its kind worldwide, concrete cubes can be examined under uni- and biaxial dynamic pressure loads in a variety of experiments. For comparison, corresponding uni- and biaxial static pressure tests are also carried out.
It is found that the height of the transverse pressure or the ratio of the transverse to the longitudinal pressure has a decisive influence on the fracture type – in the uniaxial case, the main cracks occur parallel to the loading direction; in the biaxial case, a rhombic crack pattern can be seen between the loaded surfaces. The rate of loading, in turn, influences the degree of fragmentation. The faster the specimen is loaded, the more and the smaller the fragments are.
The formation of the crack pattern can be recorded with the help of high-speed cameras with a rate of 100.000 frames per second, and thus the crack development can be made visible to the human eye. For a more detailed analysis of the recorded frames, the surface of the samples was painted white and patterned with black dots. The displacement of these points relative to each other can be evaluated from image to image by the method of digital image correlation. So the concentration of strains can be seen before the macroscopic crack opens.
Concerning the compressive strength, it can be shown that both the biaxial stress state as well as the increased loading rate contribute to a strength increase compared to an uniaxial static loading; both influences interact with each other. In the case of the tests carried out at our institute, the main influence on the compressive strength was the increase of the loading rate.
Report in the year book 2015
When waves propagate...
Failed concrete specimen in the SHB after a compression (top) and after a tensile test (bottom)
Who has not ever sat on the shore of a lake and daydreamed while watching the uniform circles of propagating waves on the water surface? But who worries about this and the strength of concrete? And yet, there is a relationship; the waves propagate not only on water surfaces but also in all solid, liquid and gaseous materials as longitudinal or transverse waves or a mixture of both. Here, waves are reflected or transmitted within the material interfaces to varying degrees.
Understanding of this wave behaviour can be used in several ways, for example, in civil engineering for non-destructive structural inspections or for research in the investigation and description of impact loading on concrete or reinforced concrete structures. At our institute, the behaviour of concrete under high loading rates is studied by the use of two different split-Hopkinson bars (SHB). This experimental set-up is based on the transmission of a shock pulse by a cylindrical aluminium bar into a test specimen. Until the concrete fails, a part of the induced shock pulse is transmitted through the specimen to a second aluminium bar. Amplitude, wave length and wave shape can be measured in the bars before and behind the test specimen, allowing to draw conclusions regarding the behaviour of the concrete. Depending on the experimental configuration, both short-term high dynamic compressive and tensile stresses can be generated in the concrete specimen.
Generally, it is well known from past research that the bearable tensile and compressive stresses of concrete increase with the loading rates. In addition, a multi-axial state of compressive stress causes an increased concrete strength. This leads to the question, currently under investigation, of whether and how these two effects interfere when a concrete specimen is stressed by an impact load acting simultaneously on multiple directions. For these studies, a biaxial SHB was developed at our institute. Current results show that the dynamic concrete compressive strength increases with increasing static lateral pressure. Furthermore, an additional increase of the compressive strength can be caused by a lateral dynamic instead of the static loading.
Report in the year book 2014
When is concrete really broken?
Remains of concrete after failure in a biaxial split-Hopkinson bar
Looking at concrete, for example, in uniaxial static compression tests, this question seems to be easily and clearly answered: when you hear it crack, the concrete has been destroyed. Depending on how brittle the tested concrete is, it fails more or less suddenly when the maximum bearable load or stress is reached. This is also reflected by significant cracking in the specimen and a descending branch in the load deformation curve. But what seems so clear in the static case, is no longer so clear under a dynamic load.
For several years dynamic impact trials were performed in a one- and two-axial split-Hopkinson bar on various concretes at our institute. In these tests, the load was applied on the specimen very quickly, within 20 to 30 microseconds. This highly dynamic loading has the effect that the sample is subject to a further increase in load, which occurs after exceeding the concrete strength and after starting of the crack propagation, until the specimen is finally fragmented few microseconds later. The reasons for this delayed damage are the friction and the inertia of the crack’s faces and the thereby limited crack velocity.
In the two-axial dynamic compression tests, the type of loading can be derived from this phenomenon and from the time difference between the arrival of the first and second load pulse. In trials with a time difference of less than 20 microseconds between the two loading pressure pulses, a biaxial state of stress is formed, from which a higher material resistance of the sample results. For tests with a time difference of 20 to 400 μs, the load pulse from the earlier axis passes first through the specimen reaching a maximum stress, which already loads the sample to failure. Due to the limited crack velocity and the inertia of the material, the specimen is not fragmented completely when the delayed impulse in the second direction of loading arrives. This means that even after the strength of the concrete has already been exceeded, at which the sample should had been destroyed, a remaining transverse strength of the sample can be found within 400 microseconds after the first loading pulse.