Strengthening of stirrup-reinforced slabs against impact
Table of contents
Project data
| Titel | Title Promotionsprojekt A5/II: Verstärkung auf der Rückseite von flachen, bügelverstärkten Massivbauelementen gegen Impakt als Teilprojekt des GRK 2250 | Doctoral projekt A5/II: Strengthening on the rear side of flat, stirrup reinforced solid construction elements against impact as part of RTG 2250 Förderer | Funding Deutsche Forschungsgemeinschaft (DFG) / GRK 2250 Zeitraum | Period 05/2020 – 04/2023 (2. Kohorte | 2nd cohort) Leitung | Project managers Prof. Dr.-Ing. Dr.-Ing. E.h. Manfred Curbach, Dr.-Ing. Birgit Beckmann Bearbeiter | Contributor |
Report in the annual report 2023
Strengthening on every front
Impact test on a plate with a combinatin of damping system on top and rear side strengthening layer
In general, existing reinforced concrete structures only have limited resistance to impact loading. For this reason, the Research Training Group (RTG) 2250 “Mineral-bonded composites for enhanced structural impact safety” focused on developing solutions to optimize the behavior of components under impact loading.
In the first two cohorts of doctoral project A5, the research focus was on thin mineral-bonded strengthening layers applied to the rear side of reinforced concrete slabs which were designed both with and without shear reinforcement. In the current third cohort, the research approach is complemented by the consideration of an additional mineral-bonded damping layer applied to the impacted plate side. This research should result in an engineering model that describes the behavior of such reinforced concrete slabs.
At the end of the second cohort, a final series of tests was conducted in the institute’s drop tower facility to combine the research of several doctoral projects of RTG 2250. For this purpose, specimens with a rear side carbon-reinforced concrete layer (topic in projects A5/I and II) as well as plates with additional top side damping layer (topic in project A6/II) were tested. It was shown that the multi-layer damping system developed in project A6/II effectively reduced the impact load applied to the test specimen and completely prevented spalling on the top side. Fragmentation on the rear side due to scabbing was also reduced. In this respect, the strengthening layers showed their efficiency, as they were able to completely prevent the detachment of the fracture body due to their membrane effect. The combination of these two positive effects led to a significant reduction in plate damage in the velocity range tested and to the successful completion of the work of the second cohort.
In the third cohort, the system of plates strengthened on both sides is considered using a three-mass oscillator. Individual springs within the model describe the global bending properties, the local punching characteristics and the (damped) effect of the impactor. The behavior of the simplified dynamic model will be verified and improved by further tests.
Report in the annual report 2022
Thanks strengthening, perforation banned
Top (left) and rear side (rigth) of a tested plate in the drop-tower
Existing structures made of steel reinforced concrete usually have only low resistance regarding impact loading. Due to this, the doctoral candidates of Research Training Group (RTG) 2250 “Mineral-bonded composites for enhanced structural impact safety” investigate the possibility of improvements in the structural behaviour under impact loading. Within doctoral project A5, thin, rear side added, mineral-bonded strengthening layers are in the focus of research whose principal effect was already investigated within the RTG’s first cohort. For the currently ongoing second cohort, the focus is on the quantification of the resistance increase obtained.
For this purpose, first of all non-strengthened steel reinforced concrete plates were investigated under different impact velocities. The intensity was chosen in such a way that, in addition to the rebound of the impactor as an indication of a mainly global behavior of the specimen, also the perforation of the plate due to the impactor occurred as an indicator of exceeding local capacity. Different flexural and shear reinforcement configurations within the specimens were experimentally investigated and the perforation behavior was characterized by determining the perforation velocity. From the tests conducted so far, it was found that the additional usage of well-anchored stirrups, the perforation limit could be increased by at least 12 %.
In the second part, plates with a 2 cm rear side strengthening layer were manufactured, using a matrix of fine-grained concrete as well as two crosswise laminated carbon-fiber textiles. The specimens were also tested until the perforation damage occurred. From the previous results of strengthened plates without shear reinforcement, an increase of at least 12 % in perforation resistance can also be observed. It can be concluded, that missing or to less shear reinforcement can be compensated by adding a rear side strengthening layer of carbon-reinforced concrete.
Report in the annual report 2021
Membrane effect against perforation
Stereo image of a small scale impact test for material characterisation
As a consequence of steadily rising fear of natural and human caused disasters, there is a worldwide interest to increase the impact safety of building constructions. A promising approach is to strengthen the existing structures by using mineral-bonded composites. Appropriate material combinations are currently investigated and further developed in the frame of Research Training Group (RTG) 2250: “Mineral-bonded composites for enhanced structural impact safety”. In this context, a central role is on the mechanism on the structural level, that is investigated in doctoral project A5.
In the framework of first cohort, concrete plates with bending reinforcement were strengthened by adding a layer of carbon-fiber textile-reinforced concrete respectively a layer of high-strength polymer fiber concrete matrix (strain hardening cementitious composite, SHCC) on the impact rear side. Among others, due to the pronounced membrane effect of the additional strengthening layers, the perforation resistance could be increased.
Following this work, in second cohort the focus is currently on the enlargement of the experimental programme. Besides the systematic investigation of the already used plates with bending reinforcement, additional plates with shear reinforcement will be tested. Due to the experimental realisation of different damage levels, the already existing damage description is further developed as well as a quantitative evaluation of the perforation limit-shifting should be realised. From the previous work of the fracture body analysis, a possible correlation between damage level of the base plates and the performance of strengthening layers, applied at a later stage of the project, could be already derived. For that, the size of the punching cone angle and the extent of the damaged area underneath the plate leads a central role.
During the evaluation of a large number of other impact experiments on the structural level, various factors influencing these characteristic values were identified in the frame of a literature research. With colleagues from other departments, additional small-scale impact tests take place for a further characterisation of the strengthening materials.
Report in the annual report 2020
Carbon-reinforced concrete and hybrid reinforcement vs. impact
Strengthening of a reinforced concrete slab for an impact test
In the first cohort of the Research Training Group (RTG) 2250, subsequently applied strengthening layers made of carbon-reinforced concrete, as well as carbon-fiber fabrics in a high-strength polymer fiber concrete matrix (SHCC – strain hardening cement composite), were used on the component side facing away from the impact. Both strengthening layers showed a very good effect by significantly increasing the perforation resistance of the reinforced
concrete plate. Furthermore, the stiffness of the structural element was improved by the
application of the strengthening layers.
Within the scope of the investigations carried out so far, a select group of reinforced concrete plates were considered for strengthening, in which only flexural reinforcement was used. Based on these specimens and a couple of reference plates made of reinforced concrete, the development of a damage description started, which allows the quantification of the structural damage caused by an impact event. Furthermore, the continuing development of an existing engineering model for the description of impact events on reinforced concrete structures was worked on. For this purpose, a simplified mechanical substitute model consisting of masses, springs and dampers was used, which was extended by the component of the strengthening layer. Experimental results as well as own numerical simulation data could be used as a basis for this.
The more detailed examination and systematic expansion of these results are planned within the second cohort of the Research Training Group 2250. One main focus of the investigation focuses on the question, in which way the perforation resistance can be explicitly increased by a subsequently applied strengthening layer? Considering that before only a qualitative increase could be determined. In addition to the strengthening of reinforced concrete plates with bending reinforcement exclusively, the experimental test program is also systematically extended to base plates with additional stirrup reinforcement. As in the previous cohort, a hybrid strengthening layer consisting of carbon-fiber fabric and an SHCC matrix is also used in addition to carbon-reinforced concrete. Finally, the existing damage description and the engineering model are supplemented and validated, especially concerning the additional effect of the shear reinforcement.