Coupled effects of chemical environments and freeze–thaw cycles on damage characteristics of red sandstone

To study the influence of freeze–thaw cycles under complex chemical environments on the red sandstone, nuclear magnetic resonance (NMR) technology was used to analyze the damage evolution law of red sandstone. The freeze–thaw cycles were carried out on four groups of samples soaked in sulfuric acid solution, sodium hydroxide solution, sodium chloride solution and pure water, respectively. Further, NMR tests were performed on the samples and the porosity change of samples, T₂ distribution and nuclear magnetic resonance images were analyzed. Results show that: (1) chemical environments have significant effects on the freezing-thawing damage, and the porosity increases linearly with increasing number of freeze–thaw cycles. (2) Under the frost-heaving force and ionic reactions, the T₂ spectrum will successively shift towards the left and then to the right with increasing number of freeze–thaw cycles. Micropores will constantly appear, and small pores will dynamically expand into macropores. (3) With increasing number of freeze–thaw cycles, the areas of light spots in NMR images will increase gradually, which means that the damage degree is aggravated with continuous development of internal pores. (4) The samples soaked in sodium hydroxide solution and sodium chloride solution are damaged more seriously than those soaked in sulfuric acid solution and pure water. By analyzing and comparing the NMR characteristics of rock under the coupled effects of chemical environments and freeze–thaw cycles, more reliable test data will be available to study the damage mechanism of rock.     

Evaluation of prestress losses in prestressed concrete specimens subjected to freeze–thaw cycles

Prestressed concrete structures are considered to be reliable and durable. However, their long-term performance when subjected to frost attack is still unclear. In this work, experiments were carried out to evaluate the prestress losses in post-tensioned prestressed concrete specimens subjected to freeze–thaw cycles (FTCs). Two cases were considered: in one case, a series of specimens were prepared and tested in a freeze–thaw chamber; in the second case, the same series of specimens were tested in an indoor environment (outside the chamber). The difference between the prestress losses of the specimens inside the freeze–thaw chamber and those outside the chamber equalled the prestress losses due to FTCs. When using mathematical models to predict the prestress losses due to the FTCs, it was found that they were relatively small when the concrete was slightly damaged. However, they increased rapidly when the FTCs were repeated. The eccentricity of the prestress wires led to larger prestress losses when subjected to FTCs. Moreover, the same cross section and eccentricity resulted in similar prestress losses due to the FTCs, and the relatively high-strength concrete could withstand more FTCs.     

Correction to: Shear properties and pore structure characteristics of soil–rock mixture under freeze–thaw cycles

Bulletin of Engineering Geology and the Environment - A Correction to this paper has been published: https://doi.org/10.1007/s10064-021-02169-7     

Chloride ion diffusivity of fly ash and silica fume concretes exposed to freeze–thaw cycles

This research focuses on investigating the durability of concretes containing fly ash and silica fume exposed to combined mode of deterioration. For this purpose, the chloride ion diffusivity of concrete was evaluated before and after 300 freeze–thaw (F–T) cycles. It was found that the coefficient of chloride ion diffusivity (CCID) increased as water to cementitious material ratio (w/cm) and air content increased. Test results clearly showed that CCID for all concretes increased after F–T cycles. In addition, concrete incorporating silica fume showed the lowest CCID and highest durability factor (DF), regardless of curing regime, air content, and w/cm. However, fly ash concrete showed good resistance to chloride ion diffusivity before and after F–T cycles when low w/cm as well as a proper curing and air content were provided.     

Effect of freeze–thaw and thermal shock weathering on the physical and mechanical properties of an andesite stone

Natural stones are exposed to physical weathering due to freeze–thaw (F–T) and thermal shock (TS) when they are used as pavement, cladding and masonry material. In this study, the deterioration of andesite was investigated by determining the physical and mechanical properties of andesite samples after each 10 cycles of F–T and TS up to 50 cycles. It was found that the P-wave velocity, Schmidt hardness and compressive strength decrease to different extents with F–T and TS while porosity and water absorption increase with F–T cycles but decrease with TS cycles. The results showed that F–T has a more destructive effect on the studied material than TS, although abrasion loss measurements suggest that the effect on the surface of the material is greater with TS. An exponential model is proposed to predict the variation of material properties with F–T and TS cycles.     

Effects of experimental frost–thaw cycles on sandstones with different weathering degrees: a case from the Bingling Temple Grottoes,China

Numerous ancient sandstone grottoes remain in northwest and central China, and weathering issues have significantly influenced their preservation conditions. The dramatic naturally cyclic changes in water content and temperature in the environment have been considered to be the main drivers of the physical weathering that commonly occurs at these archaeological sites. Therefore, comparing and understanding the behaviors of sandstone with different weathering degrees under variable environmental conditions would be helpful for further study on predicting the type, location and extent of deterioration of sandstone relics in a small region (such as the surrounding rock of grottoes). This study examines Cretaceous sandstones with two weathering degrees from Bingling Temple Grottoes, China. Standard thin section photomicrographs provide petrographic and mineralogical data and show that the sandstones have identical lithologies. Three types of specifically designed frost weathering tests are then conducted on the samples. After every six weathering cycles, the weathering processes are suspended, and the corresponding parameters, such as dry weight loss, dry density, effective porosity, porosity, P wave velocity, surface hardness and drilling resistance, are measured. At the end of the weathering cycles, the sample variations in grain size distributions are compared, and statistical tests are performed to show the statistical significance of the results. The results indicate that similar deterioration patterns occur on the samples with two weathering degrees under the same weathering tests. The increase in effective porosity surpasses that of the porosity only when the weathering effect is large enough. Furthermore, sandstone with a high degree of weathering might be more susceptible to changes in the internal pores due to its greater initial interconnectivity. In an open system, physical weathering (frost–thaw and dry–wet cycles) would cause the superficial grain size distributions of different rocks to become relatively uniform. Finally, when exposed to the same weathering process, the decreases in the overall mechanical strength in the two sandstones do not differ significantly, but the loss of superficial strength may be different.

     

Determinants of black interstate migration, 1965–70 and 1975–80

This paper presents and tests a regression-based model of black interstate migration. Explanatory variables include characteristics of origins and destinations, distance, and two migrant stock measures. The model is tested using black interstate migration flows published by the U.S. Bureau of the Census for 1965–70 and 1975–80. Three findings stand out. Firstly, the stock measures are strong determinants of black migration. They tap behavioral processes that channelize black migration streams, including information flows through familial and social networks and return migration. Secondly, the migrant stock measures attenuate effects of other explanatory variables indicating that other variables influence current migration both directly and indirectly through the stock measures. Thirdly, changes in coefficients of explanatory variables between the two periods reflect shifts in black migration patterns that occurred during the 1970s.     

A chronology of the field of modern Chinese architectural history, 1986–2012

The research of modern Chinese architectural history formally started in the mid-1980s and the first conference held in 1986 in Beijing marks the establishment of the field. Over the past 26 years, this emerging field has developed fast and steadily. As a result, thirteen biennial conferences have been held since 1986, and academic products of various forms with over ten million characters have been published. This article surveys the development of modern Chinese architectural history as a field of scholarly inquiry in China and outlines some of keystone events in the past 26 years. It also charts out how some key concepts of the field, such as timeline, geography and research approaches have been evolving over time. The article introduces some of the most significant studies in modern Chinese architectural history from the middle 1980s to the present.     

The economics of housing programmes in Ghana, 1929–66

Researchers have generally assumed that housing policies of the colonial and the immediate post‐colonial governments were shaped solely by social and political considerations. From a social perspective, some have argued that governments intervened in the housing field purely on health grounds to create good sanitary conditions and prevent the spread of diseases, especially amongst the colonizers and indigenous educated elites. From a political perspective, writers have argued that the key goal for most housing programmes was to prevent unrest and ensure political longevity. Even though each of these arguments has some merit, little, if any, consideration has been given to the economic logic of housing policies and programmes. Indeed, researchers have generally assumed that colonial and immediate post‐colonial governments never considered the economic significance of housing. This assumption is incorrect. Through surveying published and archival sources, this paper aims to rectify the neglect of the economic logic of housing policies by demonstrating that economic implications were considered in the implementation of housing policies and programmes. As will be shown, housing was seen not only as a necessary tool to secure labour and improve productivity, but also as an essential element for the success of economic development projects, especially industrialization programmes.     

Axial capacity and crushing of thin-walled metal,fibre–epoxy and composite metal–fibre tubes

Recent investigations of square hollow section (SHS) metal tubes with externally bonded carbon fibres have shown significant increases in the axial capacity and mean crushing load, compared with the metal SHS. The composite metal–fibre tubes employed carbon fibre reinforced polymer (CFRP) matrix layouts of two and four layers of carbon fibres. In this paper the same sized two and four layer CFRP SHS were manufactured independent of the metal SHS, and the axial capacity and crushing behaviour were determined experimentally. Four different tube sizes were tested, resulting in tube width to thickness ratios between 32 and 144. A photogrammetry system was employed to accurately determine the buckling and post-buckling behaviour. It is shown that the capacity and mean crush load of the composite metal–CFRP SHS exceed the sum of those for the individual metal SHS and CFRP SHS, by up to 1.8 times. This composite action results from the bond between the metal and the carbon fibres, and the mechanics with respect to buckling, capacity and crushing is discussed. The strength of metal, composite metal–CFRP and CFRP tube walls are determined using the effective width approach, and are shown to compare well with the experimental results.     

Aquifer characteristics and water quality of Miocene–Pleistocene sediments, Kuwait

Al-Atraf is one of the water well fields of Kuwait supplying Kuwait City with the brackish groundwater obtained from the Kuwait Group aquifer of Miocene–Pleistocene age. The study determined the hydrogeological and hydrochemical characteristics of the groundwater in order to identify the major chemical processes that influence the groundwater quality of the study area. The results of the aquifer test analyses indicate that the Kuwait Group is a confined to semi-confined aquifer, with a transmissivity ranging between 62 and 321 m²/day. The flow net analysis implied that the groundwater moves from southwest to northeast. The estimated transmissivity values agree well with those calculated from the aquifer test. The results of the chemical analyses data of the Al-Atraf field show that the groundwater is mainly brackish, of NaCl and Na₂SO₄ water types. The groundwater is undersaturated with respect to halite, gypsum and anhydrite and supersaturated with respect to quartz, dolomite and calcite in the direction of groundwater flow. The average Pco₂ of the groundwater is higher than the Pco₂ of the earths atmosphere indicating that the groundwater is supplied with CO₂ during the infiltration processes and dissolves the carbonate minerals under closed-system conditions.     

Dutch urban renewal, transformation of the policy discourse 1960–2000

Since the sixties of the twentieth century, thepolicy of the Dutch government on urban renewalhas been subject to three approaches, each onedifferent from the others. Up until thebeginning of the seventies, the accent was onthe expansion of the function of the largercities as economic centers. The expansion ofthe inner city for that purpose proceeded atthe expense of the residential function of thebuilt-up area. Later, the main goal was justthe opposite; attention was turned to thequantitative and qualitative reinforcement ofthe urban residential function of the citycenter and its surrounding urban residentialneighborhoods. Under that approach, the accentwas placed on improving the housing conditionsof the `sitting' residents. Accordingly, theconstruction program consisted for the mostpart of social housing that was extrainexpensive. In the course of the nineties,this so-called classic urban renewal approachwas displaced by urban revitalization. The newapproach placed the accent on strengthening thecompetitive position of cities as locations forpromising economic sectors and households withhigher incomes. This article attempts to characterize these`shifts' in policy on the grounds of a methodderived from discourse theory, which is brieflyexplained in the second section. The thirdsection typifies the policy philosophy of eachof the approaches and attempts to characterizethe changes in the policy discourse in relationto the continually changing combinations ofsocietal events and situations that, from theperspective of the policy sector, were eithernot foreseen or difficult to influence.Looking at the way policy has developed over alonger period, it seems to be less of arational learning process than a`merry-go-round' of fixed themes, visions, andsolutions that jostle for priority and keepcoming back only to disappear again. Furthermore, the logical consistency of apolicy philosophy – for instance, betweennormative and causal assumptions and measures proves to be more the exception than therule.     

Muslim Prayer on Picture Postcards of French Algeria, 1900–1960

From 1900 to 1960 French soldiers and visitors to North Africa sent home striking postcards showing Muslims in prayer. As inscribed objects they linked separated family and friends, and were collected, compiled and stored. Based on 2500 of these cards offered on an Internet auction site, this study examines the senders, the receivers, and the attitudes to the prayers in the messages. The latter range from indifference, apprehension, and ridicule to respect and a profound fascination that points to a desire to imagine a shared humanity (“union in prayer”). Like the widely popular Angelus images in France at the time, the Muslim prayer images fed a sustained interest in piety in the natural, public space and offered a visual counterpoint to France’s emergent secularism.     

The illicit consumption of military uniforms in Britain, 1914–1918

Focusing on the British home front during the First World War, this article explores civilians’ motives for acquiring and wearing military garments and accoutrements to which they were not entitled. It suggests that uniforms could be donned either to avoid the attentions of recruiting sergeants, or to perpetrate criminal deceptions. That said, individuals did not always wear illicit uniforms in order to ‘disguise’ their civilian identity. Rather, many men claimed a sense of entitlement to such items, either on the basis of previous war service, or, more often, on the basis of their contributions to the war effort on the home front. The acquisition of military items could also reflect men's roles as consumers: for many civilians, acquiring and wearing the newly glamorous uniforms was a consumer choice that could also open the door to further leisure and consumer opportunities. Overall, illicitly wearing military items undermined the uniform's link with service and sacrifice on the battle fronts: it allowed individuals to assume the appearance of combatants or to assert their patriotic identities without actually exposing themselves to military duties or dangers. It also reflected (some) men's continued perception of themselves as consumers, keen, even in wartime, to adopt what they saw as the most desirable sartorial option.