Abstract An increase in the strength of concrete when loaded dynamically has been noted in the testing literature since the early twentieth century. The origins or mechanisms leading to this increase, despite having been observed in a variety of tests, are not satisfactorily established. Aspects of test setup, specimen design, etc., have been shown to influence the outcome of any given test. More recently, computer representations of concrete have been tasked with analyzing or predicting the dynamic behavior of structures. Computers have also enabled an inward look at the same empirical tests, showing that some strength increase in compression can be captured by implementation of the proper plasticity model. The major factor touted for strength increase is the well known pressure sensitivity of concrete and a mechanism known as ‘inertial confinement’. The present work proposes a new mechanism for dynamic strength increase, focusing on the failure mechanism of concrete in compression known as shear faulting. The faulting process and its associated plastic deformation mode is compared using several material models. Adjustments are made to some parameters within these models to study their effect on dynamic and inertial plastic response. Shear dilation, which does little to increase dynamic strength at moderate strain rates, is identified as a key component of a concrete material model subject to high strain rates. Shear dilation’s effects can be seen in the range of strain rates that are practically attainable in a laboratory by using the split Hopkinson pressure bar apparatus. They may also have an increasingly important effect on problems featuring even higher strain rates, such as blast, impact, and penetration through concrete slabs.

Dynamic Strength Increase of Plain Concrete from High Strain Rate Plasticity with Shear Dilation

The time-dependent plastic deformation response of magnesium/carbon nanotube (CNT) nanocomposites containing 0.25, 0.5, and 0.75 vol% of carbon nanotubes is investigated through depth nanoindentation tests against monolithic pure magnesium in the present study. The Mg-CNT nanocomposite materials were successfully synthesized via a powder metallurgy technique coupled with microwave sintering followed by hot extrusion to produce 8-mm diameter, long solid bars. All depth-sensing indentation creep tests were conducted at ambient (room) temperature employing a diamond Berkovich pyramidal indenter. These tests are dual-stage, i.e., loading to a prescribed peak load of 50 mN, holding the peak load constant for a dwell period of 500 s, and finally unloading. Various strain rates of 0.01, 0.1, 1, and 10 s−1 were performed to assess the effects of strain rate and dwell time on the ambient temperature creep response of the Mg-CNT nanocomposites. The outcomes of these tests are explained through material hardness, microstructure, the extent of CNT content in each material, and strain rate sensitivity. Upon analyzing the nanoindentation creep tests, the dominant creep mechanism at room temperature was found to be a dislocation creep mechanism. It is also found that CNTs increase the creep resistance of magnesium. Findings of this study can be used as a starting point for a high-temperature creep study on Mg-CNT nanocomposites. This paper is a continued study from our group on time-dependent plastic deformation of Mg nanocomposites (i.e., see Haghshenas et al., Journal of Composite Materials, https://doi.org/10.1177/0021998318808358
 ). The short-term goal is to provide a compressive picture of the controlling creep mechanisms and their dependency upon, time, temperature, strain rate, volume fraction of the nanoparticles, and the type of the nanoparticles. Mg, in general, is a notorious material for high-temperature application; therefore, the long-term objective is to propose Mg nanocomposite as reliable replacements for Mg when lightweight and creep resistance are needed. However, to be able to confidently suggest such a replacement detailed understanding on the controlling phenomena, mention as short-term goals, are required.

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Indentation-based characterization of creep and hardness behavior of magnesium carbon nanotube nanocomposites at room temperature

Embedded PZT aggregates for monitoring crack growth and predicting surface crack in reinforced concrete beam

Embedded Pzt Aggregates for Monitoring Crack Growth and Predicting Surface Crack in Reinforced Concrete Beam

Different strains of bacteria capable of calcite precipitation can be incorporated into fresh concrete or cement mortars to efficiently promote self-healing of microcracks after hardening. To date, all bacterial strains investigated have required protection against the highly alkaline environments and high pressures in cementitious matrices. The core objective of this study was to eliminate the inconveniences related to protecting (typically encapsulating) bacteria. Poly(vinyl alcohol) (PVA) doped with spores of Bacillus pseudofirmus and nutrients were added to mixing water when cementitious mortars were prepared. The hardened mortar specimens were cracked and studied using microscopy, resonance measurements, X-ray microtomography (μ-CT), and destructive mechanical tests to assess the rate and efficiency of the healing processes. PVA was efficient in promoting remediation in all mortar specimens, both with and without bacteria. Specimens containing PVA plus bacteria showed the most efficient self-repair processes, while autogenous remediation in reference samples lacking PVA or bacteria were least efficient in healing microcracks. 

PVA increases efficiency of bacterially-induced self-healing in cement mortars

X-ray images can be used to nondestructively monitor the initiation, extension, and combination of cracks in concrete. In this study, real-time X-ray computed tomography (CT) scanning of concrete specimens under static and dynamic loadings was done. The CT images showed the growth, propagation, and penetration of the cracks and showed the ultimate failure of the concrete samples. Analysis of the CT images and CT numbers showed that the failure followed the structure’s areas of weakness under the static load, but for dynamic loading, the cracks formed very rapidly along straight lines through the aggregate.

https://downloads.hindawi.com/journals/amse/2018/3019158.pdf

Guangyu Lei

Papers

Using X-Ray CT Scanning to Study the Failure Mechanism of Concrete under Static and Dynamic Loadings