Short-term Shrinkage Research Articles

Short-term shrinkage stress in deck concrete of rail-cum-road truss bridge

Rail-cum-road truss bridges are rapidly developing, and frequent cracking in freshly poured concrete is of great concern. For the performance of such a load-bearing deck, it is necessary to understand its stress status due to short-term shrinkage. In this study, a newly constructed railway-cum-highway truss bridge in Guangdong Province was used to analyze the stress distribution in the concrete deck under short-term shrinkage. In-situ tests were performed to measure the shrinkage of one deck panel and two companion specimens shortly after the concrete pouring. It was found that the free shrinkage of companion specimens at 42 days was 140–150 microstrains, which was significantly higher than that indicated by current codes or standards. An exponential form of shrinkage development curve was proposed based on shrinkage test results. The measured shrinkage of the deck panel varied with locations, with a relatively large concrete shrinkage between grillages, reaching 45–55 microstrains at the end of the shrinkage test. The resulting mechanical strain due to restraints, taken as the difference in measured strains between the deck panel and companion specimen, was consistent with numerical simulation results, demonstrating the validity of the analysis method adopted. Under short-term shrinkage, the secondary tensile stress of 1.5–3.5 MPa was present in the deck above the main trusses and cross beams, triggering potential cracking. The stress is primarily attributed to the external restraint from connecting steel members, whose degree of restraint dramatically exceeds that of internal restraint from reinforcement. Although creep tends to reduce such shrinkage stress slightly, the effect of short-term shrinkage requires significant attention.

A study on the spatial and temporal evolution of urban shrinkage and its influencing factors from a multidimensional perspective: A case study of resource-based cities in China.

With social and economic environment changes occurring in the world and deepening of the urbanization process, China’s urban development exhibits a new phenomenon of growth and shrinkage fluctuations. The resource-based city shrinkage phenomenon is particularly prominent. Research on the commonalities and patterns of similar groups should be enhanced. We constructed an urban shrinkage evaluation index system from the three dimensions of population, economy and space. Accordingly, we explored the spatiotemporal evolution characteristics of 175 resource-based cities in China from a multidimensional perspective with the entropy method, shrinkage model and transfer matrix method. The results indicated that most resource-based cities in China occurred in the non-shrinking state, but their development speed gradually decreased or even presented stagflation. The shrinkage measure-related results in the different dimensions revealed that the number of shrinking cities is increasing. The population, economic and comprehensive shrinkage levels were mainly slight and remained stable. The number of cities experiencing moderate and severe shrinkage was relatively small and mostly encompassed short-term shrinkage. Spatial shrinkage demonstrated a clear administrative hierarchy difference. Moreover, the spatial distribution range of shrinking cities in each dimension expanded and exhibited obviously similar characteristics, i.e., shrinking cities were relatively concentrated in Northeast China, while they were more scattered in other regions. Furthermore, the geodetector technique was applied to reveal the influencing factors of resource-based city growth and shrinkage. Based on the results, the change in the secondary industry output value share at the start of the study was the primary factor. The impact of each employment structure indicator from 2014 to 2018 was particularly significant. Comprehensive exploration of the shrinkage characteristics of this particular group of cities and their development behavior from a multidimensional perspective can provide an important reference for the transformation and high-quality development of resource-based cities.

Impact of Rapeseed Press Cake on the Rheological Properties and Expansion Dynamics of Extruded Maize Starch.

Rapeseed press cake (RPC), an oil pressing side product rich in protein and fiber, can be combined with starch and valorized into directly expanded products using extrusion technology. The mechanism of starch expansion has been studied in detail, but the impact of RPC on expansion behavior is poorly understood. However, it can be linked to rheological and physicochemical properties and is a key product quality parameter. Blends with different amounts of RPC (0, 10, 40 g/100 g) were extruded at different barrel temperatures (100, 120, 140 °C) and moisture contents (24 or 29 g/100 g). The initial, intermediate and final sectional, longitudinal and volumetric expansion indices (SEI, LEI, VEI) were monitored directly, 10 s and 24 h after die exit to measure extrudate growth and shrinkage. The viscous and elastic properties of the extruded blends were investigated in a closed cavity rheometer. Starch and blends with 10 g/100 g RPC achieved a high initial SEI followed by significant short-term shrinkage. Blends containing 40 g/100 g RPC did not show any initial expansion. With increasing RPC content, the intermediate SEI decreased, but all samples reached a similar final SEI due to time-dependent swelling of the RPC blends. With increasing RPC content, the elasticity of the starch-based extruded samples significantly increased. Our study shows that comprehensive control and understanding of expansion mechanisms can be achieved only by investigating all stages of extrudate growth and shrinkage. We also found that the closed cavity rheometer is a powerful tool to correlate the rheological properties and expansion mechanisms of biopolymers.

Current research into the use of supercritical CO2 technology in shale gas exploitation

The use of supercritical CO2 for shale gas extraction is a promising new technology. This paper explores current research into this process, looking at analysis of the mechanism of CH4 displacement in nanoporous shale, the positive and negative effects accompanying its use for sequestration as well as organic extraction, the migration of elements and the swelling process, and the macro and micro control mechanisms involved in permeability enhancement in reservoirs. Fruitful directions for future research are also considered through comparison with hydraulic fracturing. The research findings indicate that ScCO2 fluid replacement can be used to increase gas production and seal up greenhouse gases as an effective, clean and safe method of shale gas exploitation. It is particularly effective for promoting the desorption of CH4 in shale reservoirs that have developed fine neck-wide body pores, and the subtle structural changes effected by ScCO2 fluid in sensitive minerals in reservoirs with a high brittle mineral content also have a positive effect on permeability and storage capacity. The adsorption process has been characterized as consisting of three stages: short-term shrinkage, slow swelling, and stability; an expansion equation has been proposed for CO2/CH4 that incorporates competitive adsorption, collision desorption, and impingement re-adsorption. ScCO2 fracturing has been found to be more effective than hydraulic fracturing for dense reservoirs and more effective at linking up pore-micro-fissure-fracture systems.

Unrestrained Short-Term Shrinkage of Calcium-Hydroxide- Based Alkali-Activated Slag Concrete

A total of 34 concrete mixtures were tested to examine the unrestrained shrinkage behavior of calcium-hydroxide (Ca(OH)2)-based alkali-activated (AA) ground-granulated blast-furnace slag (GGBS) concrete. The main parameters investigated were water-binder ratio (w/b), unit water content (Wc), and fine-aggregate-to-total-aggregate ratio (S/A). From the regression analysis of test results, the comprehensive basic design equations were formulated to predict the 28-day shrinkage strain and time function, which can be expanded to long-term shrinkage strain or ultimate results of Ca(OH)2-based AA-GGBS concrete. The test results showed that the increasing rate of shrinkage strain against age and the amount of 28-day shrinkage strain of Ca(OH)2-based AA-GGBS concrete were significantly affected by the w/b, Wc, and the type of auxiliary activators, while they were minimally influenced by the S/A. When the w/b was higher than 0.35, a higher 28-day shrinkage strain was observed in Ca(OH)2- and Na2CO3 activated GGBS concrete than in Ca(OH)2- and Na2SiO3-activated GGBS concrete. The proposed equations showed a significantly better accuracy in the test results than the existing empirical models of which agreement was dependent on the mixing proportions of the concrete tested.

Investigation of Growth and Shrinkage Mechanisms in Vapor-Induced Expansion of Extrusion-Cooked Corn Grits

The development of expansion models with good predicting capabilities requires experimental data and a more fundamental understanding of the vapor-induced expansion behavior in extrusion cooking. In this study, we used a conventional and also a high-speed camera to observe the radial and axial expansion behavior of extrusion-cooked corn grits. Image processing was performed with an automated evaluation code. The experimental setup consisted of a novel high-speed extruder, which enabled the determination of the influence of high screw speeds of up to 1,200 rpm on extrudate growth and shrinkage. In addition to optical measurement of the expansion, moisture content and temperature development during growth and shrinkage were determined. Three distinct stages of expansion were observed: extrudate growth, short-term shrinkage, and long-term shrinkage. The initial extrudate growth rate fitted well with a linear regression. Especially the second stage of shrinkage occurring in a time range of 24 h after the experiment showed that shrinkage is often underestimated in existing models. Furthermore, total shrinkage increased at higher screw speeds. The data obtained deliver new insight into expansion phenomena and can support the development of predicting models.

Study of short term shrinkage and creep of lightweight concrete

In the present paper, short term shrinkage and creep of lightweight concrete up to 100 days of designed strengths 25 and 35 MPa were investigated. Results indicated that the autogenous and total shrinkage of lower strength lightweight concrete were higher. The total shrinkage of the G35 mix was 320 × 10–6 comparable to 415 × 10–6 of the G25 mix after 3 months. Though the total creeps were the same at 590 × 10–6, the basic creeps per MPa of the G35 concrete mix are 16˙4% lower than the G25 mix after 3 months. The specific creeps of G35 and G25 mixes were 42˙8 × 10–6 and 60˙9 × 10–6 respectively after 3 months. The larger shrinkage and creep values of the lower grade lightweight concrete mixes were caused by the higher porosity ratio of the lightweight aggregates used and the dissipation of moisture kept inside the aggregate due to prewetting of aggregate during concrete mixing.

Behaviour of post-tensioned concrete box girders

This paper describes the results obtained to date from a study of the strains developed in two post-tensioned, prestressed concrete box girder bridges both during their construction and subsequent service lives. The instrumentation that was used is described and the simplification and representation of the material properties that were required for the subsequent analysis of the data are discussed. Results are presented for both short-term shrinkage strains developed during the construction of the concrete segments, short-term creep strains developed during the construction of the viaducts and long-term creep and service life strains developed over the 17-year period since their construction. It is shown that the assumptions, which are generally accepted for the design of such superstructures, are valid and that satisfactory predictions of the long-term behaviour can be obtained from analyses based on short-term properties, despite the variable environmental conditions that are likely to be experienced by the structures.

Behaviour of post-tensioned concrete box girders

This paper describes the results obtained to date from a study of the strains developed in two post-tensioned, prestressed concrete box girder bridges both during their construction and subsequent service lives. The instrumentation that was used is described and the simplification and representation of the material properties that were required for the subsequent analysis of the data are discussed. Results are presented for both short-term shrinkage strains developed during the construction of the concrete segments, short-term creep strains developed during the construction of the viaducts and long-term creep and service life strains developed over the 17-year period since their construction. It is shown that the assumptions, which are generally accepted for the design of such superstructures, are valid and that satisfactory predictions of the long-term behaviour can be obtained from analyses based on short-term properties, despite the variable environmental conditions that are likely to be experienced by the structures.