Strengthening of the impacted side

Table of contents

1. 1. 1. Project data
      2. Report in the year book 2023
      3. Report in the year book 2022
      4. Report in the year book 2021
      5. Report in the year book 2020
      6. Report in the year book 2017

Project data

Titel | Title Promotionsprojekte A6/I und II: Verstärkung von flächigen Massivbauelementen gegen Impakt auf der impaktzugewandten Seite als Teilprojekt des GRK 2250 | Doctoral projects A6/I and II: Strengthening of plane RC elements against impact on the impacted side as part of RTG 2250 Förderer | Funding Deutsche Forschungsgemeinschaft (DFG) / GRK 2250 Zeitraum | Period 05/2017 – 04/2020 (1. Kohorte | 1st cohort) 05/2020 – 04/2023 (2. Kohorte | 2nd cohort) Projektleitung | Project managers Prof. Dr.-Ing. Dr.-Ing. E.h. Manfred Curbach Dr.-Ing. Silke Scheerer Bearbeiter:in | Contributors Dipl.-Ing. Lena Leicht (2. Kohorte | 2nd cohort) Saeid Zabihi Moghaddam M. Sc. (1. Kohorte | 1st cohort) Co-Betreuung | Co-mentoring Lehrstuhl für Betriebliche Umweltökonomie, TU Dresden Partner Shock-Tube-Tests | Partner shock tube trials Politectnico di Milano (Italy) Homepage des GRK 2250 | Website of RTG 2250

Report in the year book 2023

Shock waves due to an explosion

How do reinforced concrete (RC) structures behave under extreme load scenarios such as explosions inside a tunnel? Under such loads, pressure waves occur on the one hand, and on the other hand, fragments can arise that can impact surrounding structures after the pressure wave. Such impact scenarios can be studied in the drop tower at TU Dresden. On the other hand, the shock tube facility at the Politecnico di Milano allows loading slabs with a planar shock wave with varying amplitudes.

The second question was: How can the structures be protected against such extreme events? For this purpose, layered strengthening structures were under investigation. Impact tests in the drop tower with a steel impactor of 22 kg mass that was accelerated to up to 220 kilometers per hour proved that the developed strengthening layers, composed of a thin cover layer consisting of short-fiber concrete and a 4 cm thick infra-lightweight concrete damping layer underneath, significantly reduced the impact energies and forces acting on the structure. The layer prevented damage to fully supported reinforced concrete cuboids and reduced considerable damage to large-scale RC slabs. This strengthening solution was then tested under two different pressure waves in the shock tube.

In order to be able to classify the behavior of the strengthened concrete slabs, it was compared to that of two unstrengthened reinforced concrete slabs. One of them, with a thickness of only 4 cm, corresponded to the base slab of the strengthened solution. The 10 cm thick reinforced concrete slab was a reference with a similar natural circular frequency as the strengthened, also 10 cm thick, slab. In the case of the 4 cm thick reinforced concrete slab, cracks on the tensile side of the slab already formed under low pressure, and the slab was even completely destroyed under high loading pressure. By contrast, the measured deformations of the 10 cm thick reinforced concrete slab and the strengthened solution were very similar. The good performance of the strengthened solution was partly due to the excellent bond between the layers, which results in rigid body behavior.

Report in the year book 2022

Impact softener

How can we change a hard to a soft collision? It is simple as that: insert an energy-absorbing interlayer between the impacting and the impacted object. The remaining question, however, is: what type of material best fulfills this task? The research project presented in the following deals with this question.

For this reason, damping layers consisting of different mineral-bonded materials were compared to each other. The interlayer is supposed to absorb impact energy and thus reduce the transferred forces and energies to the underlying concrete structure. Therefore, it prevents strong fracturing of the underlying concrete element. Among the investigated materials were two different lightweight concretes, one with expanded clay and one with expanded shale aggregates, an infra-lightweight concrete, and a concrete which contained waste tire rubber. The materials have different strengths, elastic moduli, and densities. The infra-lightweight concrete possesses the lowest values, while the lightweight concretes achieved the highest ones.

A cover layer placed on top of the damping layer distributes the impact force to wider regions. For the tests, steel reinforced concrete blocks were coated with the above-mentioned damping layers and an overlying layer of carbon-reinforced concrete. A steel impactor hits the fully supported reinforced concrete cuboid with velocities of up to 200 km/h in the experiments.

The comparison of different experiments showed that a 4 cm thick infra-lightweight concrete layer with a fine grained concrete cover layer of 2 cm thickness, containing one sheet of carbon-fiber textile, was the best option. This layered structure almost completely prevented concrete damage during the impact experiment. In return, that means that the additional applied layers absorbed a high amount of energy. By contrast, the interlayer’s nonexistence or inadequacy led to substantial damage to the reinforced concrete specimen.

Report in the year book 2021

Well protected

Protections against aggressors, weapons or collision in nature but also in technical applications are often layered. The outer layer distributes the impact load and prevents penetration to underlying materials. In nature, this layer is often lamellar and in the field of armors, carbon-fiber-reinforced epoxy matrices are widely used. The high stiffness and strength characterizes this layer. This layer is backed by a softer layer which is crushed and thus absorbs impact energy or it otherwise damps the impact for the being or structure it surrounds. Elastomers or foam-like structures are well-suited for this task.

However, the mineral-bonded composites studied in this research project are good candidates as well. Different types of lightweight concretes and a concrete with waste tire rubber aggregates were studied in a split Hopkinson bar.

A split Hopkinson bar consists of at least two slender bars which sandwich a specimen. A tensile or compressive wave is introduced in the first (incident) bar and protrudes towards the specimen. The specimen transfers one part of the wave into the second (transmission) bar and the other part of the wave is reflected back into the incident bar by the specimen. Strain measurements on the incident and transmission bar allow for the evaluation of the specimen’s response.

The goal of the small-scale specimens was to compare the material behavior under different loading rates. The energy absorption capacity, which describes the absorbed energy compared to the introduced energy, was of special interest. Above that, the failure mode of the specimens is an important criterion. After finishing the series of tests in the SHB, the most promising materials were chosen and tested on a larger scale. The protection layer was applied on a concrete cuboid specimen that was subjected to hard impact in the drop tower.

Report in the year book 2020

Soft, softer, damping

Impact damage is known in many engineering fields. Therefore, it is a normal task to prevent humans and goods from this type of threat. The protection can be granted by personal armour, but there are also protection systems in cars, stone fall protection systems, and systems to prevent damage of bridge piers due to ship collision. The research question in this project is similar to the aims of other engineers: how to protect humans and goods from the flying debris and other threats? As well as how to reduce the impact damage of reinforced concrete structures? A possibility to reach this goal is the damping of the impact load. For this sake, an energy-absorbing layer needs to be applied to the structure. The question is, how such a layer should be composed. Inspiration from nature and other engineering fields can be helpful to answer this question. In many cases, layered structures are used. They consist, for example in the case of armours, of a hard outer layer backed by other layers that can either behave softly and elastically or crush and thus absorb energy. Ceramics and fiber-reinforced polymers are typically used as outer layers and elastomers or foam structures with a high pore volume and generally applied underneath. Finding cementitious materials with comparable damping properties is the goal of the research project.

The dynamic material properties are examined with the help of a so-called split-Hopkinson bar. This is a facility consisting of two bars that sandwich a specimen. A compressive or tensile wave is generated in the first bar and transferred to the second bar via the specimen. The stress-strain-condition within the specimen can be derived from strain measurements on the bars. Suitable materials for the different layers need to be found in small-scale experiments but the efficiency and the damping properties of the layers and composites also need to be studied on a large scale and compared to one another. Damage criteria are necessary to compare the performance of the different materials by evaluating the damage of the structure beneath. The large-scale tests are carried out on cuboids and plates in the drop tower.

Report in the year book 2017

Bio-inspired impact-resistant structures

This project aims to increase the resistance of reinforced concrete (RC) elements against impact load using an energy-absorbing strengthening layer on the impacted side. In conventional approaches, the strain energy capacity of a structure, or its inertia, plays an important role to dissipate the energy of an impactor. Current methods of protecting structures against impact loading increase the energy absorbing potential of the elements, by using materials with high density or rigidity, which in turn increases not only a structure’s impact resistance but also its size and weight.

However, depending on the load intensity and strain rate some alternative materials or structures with specific properties can be utilized to protect the buildings against impact without a drastic increase in its weight. Therefore, it is necessary to investigate approaches that, in addition to energy dissipation, can fulfill the other requirements such as the lowest possible dead weight, adaptability to different geometries, cost and resource efficiency, and so on. For example, applications of foam-like materials or light honeycomb-like structures are known solutions. In different fields of science, in nature or industry, e.g., automotive or packaging industry, there are many different solutions established for damping elements or mechanisms. A systematic investigation and in-depth analysis of these approaches is the basis for developing impact-resistant layers which are suitable for use in reinforced concrete structures.

In a first step, the solutions found are evaluated regarding their efficiency for dynamic load scenarios and their applicability to energy-absorbing strengthening layers for RC structures to identify the most appropriate approaches. In a second step, the most promising solutions will be adapted for reinforced concrete elements and experimentally investigated, accompanied by numerical simulations.