Sustained Compressive Loading Research Articles

Frost resistance of recycled aggregate concrete subjected to coupling sustained compressive load and freeze-thaw cycles

Recycled aggregate concrete (RAC) structures situated in cold regions are required to withstand the combined effects of mechanical load and environmental factors throughout their operational lifespan. The influence of sustained load introduces complexity to the degradation mechanism of RAC. This research examines the frost resistance of RAC across varying levels of recycled coarse aggregate (RCA) replacement (0, 50 %, 100 %) and sustained compressive loading (0, 0.3 fc, 0.5 fc). The frost resistance was quantified by the indices of macroscopic morphology change, mass loss, relative dynamic modulus of elasticity, and mechanical properties. Results show that 0.3 fc can effectively improve the frost resistance of RAC, a trend that becomes more pronounced with increasing replacement rates of RCA. Excessive stress (0.5 fc) was found to be unfavorable to the frost resistance of RAC and increase the compressive strength variability of RAC after freeze-thaw cycles. Analysis of meso/micro-structure characteristics using optical and scanning electron microscopes, along with the mercury intrusion porosimetry, reveals that optimal compressive loading (0.3 fc) enhances mortar integrity, reduces porosity, and improves compactness, thereby bolstering RAC’s frost resistance.

Improving the salt frost resistance of recycled aggregate concrete modified by air-entraining agents and nano-silica under sustained compressive loading

This paper systematically investigated the combined effects of sustained compressive loading and the salt frost resistance for recycled aggregate concrete (RAC) incorporating air-entraining agents (AEA) and nano silica (NS). The effects of different RCA replacement ratios, freeze-thaw cycles, AEA dosage and compressive stress level on the freeze-thaw damage properties including the apparent morphology, mass loss rate and relative dynamic elastic modulus (RDEM) were discussed. The microstructure evolution of RAC samples before and after freeze-thaw cycles were characterized and analyzed through the SEM observation, microhardness measurement and X-CT tests. The synergistic modification mechanism of NS and AEA on RAC under salt frost action were investigated, and the link between the macroscopic and microscopic properties of air-entrained NS-modified RAC was further explored. The results showed that the RAC mixed with a certain amount of AEA and NS dosages exhibited the stronger salt frost resistance compared to that of control group. The effect of AEA dosage and compressive stress level exhibited similar changing trends on the salt frost resistance of RAC. When adding a moderate amount of AEA and applying a certain sustained compressive loading, the RDEM loss of RAC was reduced so that the salt frost resistance was partly improved. Similarly, the moderate AEA dosages can reduce the deterioration at ITZs of RAC. The compound use of NS and AEA proved to be as an efficient method to strengthen the microstructure of RAC, further improving the frost resistance under the coupled actions of sustained compressive loading and chloride salt attack. This study provides a feasible approach to improve the salt frost resistance of RAC under the coupled actions of sustained compressive loading, which is expected to promote the application and extension of RAC in complex environmental regions.

Modulation of TRPV4 protects against degeneration induced by sustained loading and promotes matrix synthesis in the intervertebral disc.

While it is well known that mechanical signals can either promote or disrupt intervertebral disc (IVD) homeostasis, the molecular mechanisms for transducing mechanical stimuli are not fully understood. The transient receptor potential vanilloid 4 (TRPV4) ion channel activated in isolated IVD cells initiates extracellular matrix (ECM) gene expression, while TRPV4 ablation reduces cytokine production in response to circumferential stretching. However, the role of TRPV4 on ECM maintenance during tissue-level mechanical loading remains unknown. Using an organ culture model, we modulated TRPV4 function over both short- (hours) and long-term (days) and evaluated the IVDs' response. Activating TRPV4 with the agonist GSK101 resulted in a Ca2+ flux propagating across the cells within the IVD. Nuclear factor (NF)-κB signaling in the IVD peaked at 6 h following TRPV4 activation that subsequently resulted in higher interleukin (IL)-6 production at 7 days. These cellular responses were concomitant with the accumulation of glycosaminoglycans and increased hydration in the nucleus pulposus that culminated in higher stiffness of the IVD. Sustained compressive loading of the IVD resulted in elevated NF-κB activity, IL-6 and vascular endothelial growth factor A (VEGFA) production, and degenerative changes to the ECM. TRPV4 inhibition using GSK205 during loading mitigated the changes in inflammatory cytokines, protected against IVD degeneration, but could not prevent ECM disorganization due to mechanical damage in the annulus fibrosus. These results indicate TRPV4 plays an important role in both short- and long-term adaptations of the IVD to mechanical loading. The modulation of TRPV4 may be a possible therapeutic for preventing load-induced IVD degeneration.

Water transport in recycled aggregate concrete under sustained compressive loading: Experimental investigation and mesoscale numerical modelling

The durability properties of load-induced damaged concrete incorporating recycled concrete aggregates (RCAs) have previously been reported, but the investigation on water transport in recycled aggregate concrete (RAC) subjected to sustained loading remains to be limited, especially involving both numerical simulation and experimental aspects. This paper presents a comprehensive exploration on capillary water absorption in RAC under various sustained compressive stress levels (λc = 0, 0.1, 0.3, 0.5, 0.7). An improved experimental set-up was self-designed for realizing the coupled action of sustained compressive loading and water absorption by RAC, and then a series of water absorption tests for specimens with different replacement ratios of RCAs (0, 30%, 50%, 100%) were experimentally conducted. The coupled effects of sustained stress level and replacement ratio on the sorptivity of RAC were further discussed. The quantitative correlation between the sorptivity and stress level was obtained to derive the formula of hydraulic diffusivity. Based on the unsaturated flow theory and Mazar's damage variable, the prediction model of water transport in load-induced damaged concrete was established. A mesoscopic model of RAC modelled as a five-phase composite material by considering the old and new interfacial transition zones (ITZs) as interphases was developed and employed to numerically simulate the process of water transport in load-damaged concrete. The mechanical and hydraulic parameters for multiple ITZs of RAC were determined by summarizing the previous studies. The visual numerical results indicate that the distribution of water content and wetting front of penetration depth were remarkably affected by the stress level and the content of RCAs. The comparison of cumulative amount of water absorption indicates that the numerical results agreed well with the obtained experimental measurements, fairly validating the feasibility and accuracy of the developed models. It further illustrates the mesoscopic numerical modelling can be practically adopted to simulate water transport process coupled with sustained loading and also visually exhibit the wetting front of water penetration in RAC.

Combined Effect of Stray Current and Sustained Compressive Loading on Chloride Transport in Concrete

As with the leakage of stray current in the surrounding medium, the chloride transport in concrete is influenced by the stray current and loading of the subway structure. This paper presents the results of the experimental study on the chloride transport properties of concrete under the combined action of stray current and sustained compressive loading. First, an experiment was setup to explore the chloride transport in the subway structure as the concrete specimen embedded with steel under test current and study the influence of the existence of steel on the chloride transport profiles in concrete under stray current. Then, to investigate the combined effect of stray current and loading on the chloride transport properties, an improved experiment was designed with stray current and sustained compressive loading. The chloride transport profiles were measured, respectively, subjected to different stray currents and compressive stress levels. The experimental results indicated that stray current and sustained compressive loading have a significant influence on the chloride transport properties of concrete, and the loading threshold existed as the turning point of the chloride transport rate. Based on the experimental data and migration theory, the prediction model of chloride transport in concrete under stray current and sustained compressive loading was established and verified by the experimental measurements, and the steel corrosion‐induced cover cracking was studied, and the comparison indicated that the numerical results were consistent with the experimental results.

One-dimensional (1D) immediate compression and creep in large particle sized Tire Derived Aggregate (TDA) for leachate collection and removal systems (LCRS)

Potential cost savings and environmental benefits are two reasons for using Tire Derived Aggregate (TDA) in place of gravel, or drainage geosynthetics, as drainage material in leachate collection and removal systems (LCRS) of waste disposal sites. As a polymeric material, TDA is expected to undergo both immediate and time dependent compression (creep) under sustained compressive loading, and these may influence performance. The response of large particle sized TDA – TDA with individual particle sizes generally greater than 50 mm – to large compressive loads up to 300 kPa was studied to assess void volume reduction from the individual and combined effects of immediate compression and creep. The results showed void volume reduction from creep, but this was considerably less than that from the immediate compression. Isotropic solid volume compression in individual TDA particles was found to be negligible. The final void ratio appeared to be dependent on the initial void ratio, as well as on the applied load, but did not appear to be dependent on the loading rate or to be influenced by elevated temperatures (up to 58 C). Presented here are the details of the study, findings and implications for practice.

Coupled effects of sustained compressive loading and freeze–thaw cycles on water penetration into concrete

AbstractThe combined action of sustained loading and freeze–thaw cycles (FTCs) plays an important role in the durability and service life of concrete exposed to the cold coastal area. Water transport is a primary factor closely pertaining to freeze–thaw damage in concrete, and the microcracks induced by sustained loading could provide a flow path to facilitate water penetration into concrete. This paper presents the results of an experimental investigation on water penetration behavior of ordinary and air‐entrained concrete (OC and AC) subjected to sustained compressive loading and FTCs. Aiming at the combined effects of low compressive stress ratio and FTCs on water transport, a series of water absorption and permeability experiments with the stress ratio of 0, 0.3, and 0.5 after various FTCs were conducted. The relative dynamic elastic modulus and mass loss were afterwards measured to characterize the frost damage within concrete. The experimental results indicated that the sustained compressive loading and frost action have significant influences on capillary water absorption and permeability for the OC and AC specimens. Both the initial coefficient of capillary absorption and water permeability coefficient increase with the increase of FTCs, while they initially decrease and later increase with the increase of compressive stress ratio.

Finiteelement analysis reveals an important role for cell morphology in response to mechanical compression.

Mechanical loading naturally controls cell phenotype, development, motility and various other biological functions; however, prolonged or substantial loading can cause cell damage and eventual death. Loading-induced mechanobiological and mechanostructural responses of different cell types affect their morphology and the internal architecture and the mechanics of the cellular components. Using single, mesenchymal stem cells, we have developed a cell-specific three-dimensional finite-element model; cell models were developed from phase-contrast microscopy images. This allowed us to evaluate the mechanostructural response of the naturally occurring variety of cell morphologies to increase sustained compressive loading. We focus on the morphology of the cytoplasm and the nucleus, as the main mechanically responsive elements, and evaluate formation of tensional strains and area changes in cells undergoing increasing uniaxial compressions. Here, we study mesenchymal stem cells as a model, due to their important role in tissue engineering and regenerative medicine; the method and findings are, however, applicable to any cell type. We observe variability in the cell responses to compression, which correlate directly with the morphology of the cells. Specifically, in cells with or without elongated protrusions (i.e., lamellipodia) tensional strains were, respectively, distributed mostly in the thin extensions or concentrated around the stiff nucleus. Thus, through cell-specific computational modeling of mechanical loading we have identified an underlying cause for stiffening (by actin recruitment) along the length of lamellipodia as well as a role for cell morphology in inducing cell-to-cell variability in mechanostructural response to loading.

Model of time-dependent and stress-dependent chloride penetration of concrete under sustained axial pressure in the marine environment

The effect of sustained compressive loading on the chloride penetration of concrete was investigated and a self-monitoring loading device was developed. A series of prisms were loaded under five different sustained axial stress levels and four different numbers of wet/dry chloride cycles. Both the apparent chloride diffusion coefficient Da and the surface chloride concentration Cs values were time and stress dependent with a sudden change threshold value of approximately 0.3 fc. A model was proposed to predict the chloride penetration profiles of concrete under sustained axial pressure, taking both the time-dependent and the stress-dependent characteristics of Da and Cs into account.

Empirical Estimation of the Hydraulic Diffusivity and Sorptivity of Unsaturated Concrete Subjected to Sustained Compressive Loading

AbstractThe hydraulic diffusivity and sorptivity of cement-based materials are commonly regarded as two important properties to characterize the durability potential of building structures, especia...

Combined effect of water and sustained compressive loading on chloride penetration into concrete

This paper presents the results of an experimental investigation on the water and chloride transport properties of unsaturated concrete under combined short-term sustained compressive loading and chloride attack. Aiming for the coupled effect of sustained compressive loading and chloride penetration, an improved test apparatus, which is compared with the traditional gravimetric measurement, was designed to real-timely and continuously measure the amount of water solution absorbed by the cylindrical hollow concrete specimen under sustained loading. A series of chloride transport experiments on water/chloride diffusivity and penetration profiles of chloride were conducted on the saturated, half-saturated and fully dried concrete respectively subjected to several compressive stress levels (ranging in 0–50% of the corresponding compression loading capacity). The experimental results showed that the saturation level and sustained compressive loadings have significant influences on water and chloride transport properties of the unsaturated concrete. The quantitative relationship between water/chloride diffusivity and compressive stress level was developed by fitting the obtained experimental data. Finally, on the basis of convection-diffusion equation, the prediction model of chloride transport in unsaturated concrete under sustained compressive loading was proposed and validated by the experimental results. The comparison of chloride profiles indicates that the numerical results were in good agreement with the experimental measurements.

Time-Dependent and Stress-Dependent Chloride Diffusivity of Concrete Subjected to Sustained Compressive Loading

This paper studies the chloride diffusivity of concrete subjected to long-term exposure in a simulated marine environment with sustained compressive loading for up to 224 days. A series of concrete prisms were loaded under five different compressive stress levels (0, 0.2, 0.3, 0.5, and 0.7fc) and were subjected to different numbers (4, 8, 12, and 16) of wet/dry seawater exposure cycles. At the end of each exposure period, the concrete specimens were analyzed to determine their chloride penetration profile, the apparent chloride diffusion coefficient Da of the concrete, and the surface chloride content Cs. It was found that the Da and Cs values are both time and stress dependent and that these two dependencies have complex interactions. The value of Da decreases with time due to the hydration of the cement matrix, whereas the value of Cs increases over time. The effects of compressive stress on the values of Da and Cs strongly depend upon the applied stress level, which has a threshold value at approximately 30% of the ultimate compressive strength. Below this threshold, the value Cs remains constant; after this threshold, Cs increases linearly as the applied stress increases. Below the threshold, the value of Da decreases marginally as the stress level increases. However, above the threshold, Da increases rapidly with the stress level as a result of microcracking. Through regression of the chloride penetration profiles, empirical models are proposed to quantify the dependence of Da and Cs on the exposure time and stress level. The validity of these models is evaluated through comparisons with test results.

Correction: Biological responses of three-dimensional cultured fibroblasts by sustained compressive loading include apoptosis and survival activity.

Correction: Biological responses of three-dimensional cultured fibroblasts by sustained compressive loading include apoptosis and survival activity.

Biological responses of three-dimensional cultured fibroblasts by sustained compressive loading include apoptosis and survival activity.

Pressure ulcers are characterized by chronicity, which results in delayed wound healing due to pressure. Early intervention for preventing delayed healing due to pressure requires a prediction method. However, no study has reported the prediction of delayed healing due to pressure. Therefore, this study focused on biological response-based molecular markers for the establishment of an assessment technology to predict delayed healing due to pressure. We tested the hypothesis that sustained compressive loading applied to three dimensional cultured fibroblasts leads to upregulation of heat shock proteins (HSPs), CD44, hyaluronan synthase 2 (HAS2), and cyclooxygenase 2 (COX2) along with apoptosis via disruption of adhesion. First, sustained compressive loading was applied to fibroblast-seeded collagen sponges. Following this, collagen sponge samples and culture supernatants were collected for apoptosis and proliferation assays, gene expression analysis, immunocytochemistry, and quantification of secreted substances induced by upregulation of mRNA and protein level. Compared to the control, the compressed samples demonstrated that apoptosis was induced in a time- and load- dependent manner; vinculin and stress fiber were scarce; HSP90α, CD44, HAS2, and COX2 expression was upregulated; and the concentrations of HSP90α, hyaluronan (HA), and prostaglandin E2 (PGE2) were increased. In addition, the gene expression of antiapoptotic Bcl2 was significantly increased in the compressed samples compared to the control. These results suggest that compressive loading induces not only apoptosis but also survival activity. These observations support that HSP90α, HA, and, PGE2 could be potential molecular markers for prediction of delayed wound healing due to pressure.

Bone creep can cause progressive vertebral deformity

Introduction Vertebral deformities in elderly people are conventionally termed “fractures”, but their onset is often insidious, suggesting that time-dependent (creep) processes may also be involved. Creep has been studied in small samples of bone, but nothing is known about creep deformity of whole vertebrae, or how it might be influenced by bone mineral density (BMD). We hypothesise that sustained compressive loading can cause progressive and measurable creep deformity in elderly human vertebrae. Methods 27 thoracolumbar “motion segments” (two vertebrae and the intervening disc and ligaments) were dissected from 20 human cadavers aged 42–91 yrs. A constant compressive force of approximately 1.0 kN was applied to each specimen for either 0.5 h or 2 h, while the anterior, middle and posterior heights of each of the 54 vertebral bodies were measured at 1 Hz using a MacReflex 2D optical tracking system. This located 6 reflective markers attached to the lateral cortex of each vertebral body, with resolution better than 10 μm. Experiments were at laboratory temperature, and polythene film was used to minimise water loss. Volumetric BMD was calculated for each vertebral body, using DXA to measure mineral content, and water immersion for volume. Results In the 0.5 h tests, creep deformation in the anterior, middle and posterior vertebral cortex averaged 4331, 1629 and 614 micro-strains respectively, where 10,000 micro-strains represents 1% loss in height. Anterior creep strains exceeded posterior ( P < 0.01) so that anterior wedging of the vertebral bodies increased, by an average 0.08° (STD 0.14°). Similar results were obtained after 2 h, indicating that creep rate slowed considerably with time. Less than 40% of the creep strain was recovered after 2 h. Increases in anterior wedging during the 0.5 h creep test were inversely proportional to BMD, but only in a selected sub-set of 20 specimens with average BMD < 0.15 g/cm 3 ( P = 0.042). Creep deformation caused more than 5% height loss in four vertebrae, three of which had radiographic signs of pre-existing damage. Conclusion Sustained loading can cause progressive anterior wedge deformity in elderly human vertebrae, even in the absence of fracture.

Compression-induced deep tissue injury examined with magnetic resonance imaging and histology

The underlying mechanisms leading to deep tissue injury after sustained compressive loading are not well understood. It is hypothesized that initial damage to muscle fibers is induced mechanically by local excessive deformation. Therefore, in this study, an animal model was used to study early damage after compressive loading to elucidate on the damage mechanisms leading to deep pressure ulcers. The tibialis anterior of Brown-Norway rats was loaded for 2 h by means of an indenter. Experiments were performed in a magnetic resonance (MR)-compatible loading device. Muscle tissue was evaluated with transverse relaxation time (T2)-weighted MRI both during loading and up to 20 h after load removal. In addition, a detailed examination of the histopathology was performed at several time points (1, 4, and 20 h) after unloading. Results demonstrated that, immediately after unloading, T2-weighted MR images showed localized areas with increased signal intensity. Histological examination at 1 and 4 h after unloading showed large necrotic regions with complete disorganization of the internal structure of the muscle fibers. Hypercontraction zones were found bilateral to the necrotic zone. Twenty hours after unloading, an extensive inflammatory response was observed. The proposed relevance of large deformation was demonstrated by the location of damage indicated by T2-weighted MRI and the histological appearance of the compressed tissues. Differences in damage development distal and proximal to the indenter position suggested a contribution of perfusion status in the measured tissue changes that, however, appeared be to reversible.

Delayed failure in rock loaded in uniaxial compression

The long-term response to sustained compressive loading of two crystalline hard rocks of the Canadian Shield has been investigated. Static fatigue tests conducted on granite and anorthosite have shown that in a humid environment the long-term strengths of these crystalline igneous rocks could be less than 60 percent of their dry instantaneous strengths. Such reduction in strength has implications for the design and construction of deep tunnels, mines and other underground installations. The particular case of a nuclear fuel waste vault located at a depth of one kilometer is considered. Considering a service life of 1,000 years, a vault one kilometer deep in granite could suffer time-dependent spalling in highly stressed regions of the rock where the maximum principal stress exceeds about 130 MPa. Anorthosite has a lower instantaneous strength than granite and it is sensitive to static fatigue as well. A vault in anorthosite would be subject to time-dependent spalling of the perimeter rock in places where the maximum principal stress is above 80 MPa.