Recording of material strains

Recording of strains on ultra-high strength concretes under impact loading at the Technical University of Munich (cbm), Chair of Building Materials Science and Materials Testing, Prof. Dr.-Ing. C. Gehlen BAUER Spezialtiefbau GmbH, Dr.-Ing. K. Beckhaus, Dr.-Ing. H. Ibuk.

The strength of cementitious building materials has been steadily increased in recent years. The latest stage in this development is ultra-high performance concrete (UHPC), which has been the subject of intensive research at cbm since 1998. This concrete enables larger spans or slimmer structures due to the higher loads that can be absorbed. This opens up completely new possibilities for the use of this innovative concrete.

As part of a project funded by the German Federal Ministry of Education and Research, the use of the nano-material ultra-high performance concrete (UHPC) in foundation engineering is being researched for cost- and material-oriented optimization together with the industrial partner BAUER Spezialtiefbau GmbH. Among other things, driven piles made of UHPC are being investigated, as both ecological and economic advantages are expected, especially in exchange for steel.

Driven piles are driven into the ground with the aid of a hydraulic excavator to improve the foundation soil. In order to be able to reproduce the stress during an impact on the pile head on a laboratory scale, UHPC cylinders of different compositions with a diameter of 100 mm and a height of 200 mm were manufactured.

The tests were performed in an impact device commonly used to determine the resistance to crushing of aggregate. The drop hammer impacts a punch with a weight of 50 kg from a selected drop height, which transmits the force into the UHPC cylinder.

The high loading velocity expected during the impact tests as well as the explosive material failure of the UHPC made a data logger with a correspondingly high measuring frequency and recording capability indispensable. During a preliminary test at the cbm with the LTT24, it was quickly shown that its measuring rate of 500 kHz per channel is optimal for these tests.

The SensorCorders from LTT Labortechnik Tasler offer 8, 16 or 32 galvanically isolated input channels including integrated combination amplifiers for volt, strain gauge and ICP signals. The circuitry for the respective sensors is configured via software - no hardware modification is necessary. In strain gauge operation, full, half and quarter bridge circuits are supported. An optional constant current supply enables highly accurate and extremely low-noise strain gauge measurements for high-temperature applications.

Connection to the PC is via USB or Ethernet - if required for longer distances also via fiber optic cable. With the PC connected, the signals can be displayed and monitored online, even remotely. The LTTpro measurement software considerably simplifies the acquisition, playback and analysis of measurement data. The extensive trigger function defines the start of the data acquisition. The online mathematics allows an initial evaluation of the measurement while it is still being recorded. The direct saving of the measurement data in many common file formats allows direct further processing of the measurement data.

In the following investigations, the transverse and longitudinal strains during an impact were recorded on the cylinders by opposing strain gages (DMS) bonded in a two-quarter bridge circuit using the LTT24. With a total of 8 channels available, additional strain gages could be applied distributed over the height of the cylinders to obtain information about differences in the amounts of strain and the strain curves. By defining a suitable trigger, it was possible to keep the amount of data low after application of the blows. A comparison of the strains with cylinders of other concrete compositions is a prerequisite for further optimization steps with regard to a suitable formulation for UHPC driven piles.

The investigations for the presented research project are currently still ongoing. In particular, strain measurements on demonstrators at the construction site are planned. However, it has already been shown that the acquisition of the LTT24 can provide important insights into the material behavior of UHPC under impact loading.