摘要:

This paper presents theoretical and applied state-of-the-art information in the field of alkali-aggregate reactivity (AAR) in concrete. The aspects discussed include basic concepts of the reaction and expansion mechanisms, conditions conducive to the development and the sustainability of AAR in concrete, field and laboratory investigation programs for evaluating the potential alkali-reactivity of concrete aggregates, selection of preventive measures against AAR, and the management of structures affected by AAR. The management section includes the diagnosis of AAR in existing concrete structures, evaluation of the potential for future distress due to AAR, and mitigation and repair approaches used on such structures. This is an introductory paper and sets the stage for a special review of the current AAR situation in the various regions of Canada that is presented in seven papers as part of this issue.Key words: alkali-aggregate reaction, alkali-silica reaction, alkali-carbonate reaction, petrography, testing of concrete aggregates, performance of concrete, survey, remedial measures.

ISSN:0315-1468    

DOI:10.1139/l99-072

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Alkali-aggregate reactivity is a chemical reaction that occurs in some concrete structures. This reaction causes expansion and cracking of concrete. It may not be the main cause of premature deterioration; however, it is often the catalyst for other forms of deterioration. Since 1989, field work and laboratory testing for alkali-reactivity has been undertaken on selected aggregate, rock, and concrete core samples throughout Newfoundland. The field work and laboratory tests consisted of an assessment of the bedrock and aggregate sites to determine their quality and quantity for use as construction material; an examination of existing concrete structures to assess their durability with regard to alkali-aggregate reactivity. The site investigation of the concrete structures consisted of noting their date of construction, the rock and aggregate types used, the types of cracking, and the degree and type of deterioration of each structure. Results from laboratory testing, petrographic examination, and visual observation show that certain sedimentary, igneous, and metamorphic rock types may be potentially alkali-reactive, but the reaction may not always be deleterious to the concrete structure.Key words: alkali-aggregate reaction, concrete structures, laboratory tests, petrographic examination, rock types.

ISSN:0315-1468    

DOI:10.1139/l99-067

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Alkali-silica reactivity, a reaction between alkalies usually contained in the cement paste and siliceous aggregates, was positively identified in Nova Scotia in 1962. Powerhouse structures on the Mersey River constructed in 1928-1929 showed intolerable expansion by 1938, and this expansion continues to the present day. Extensive research conducted in the 1960s and 1980s has shown that widespread meta-sediments in Nova Scotia are the principal rock types responsible for the reaction. The rate and extent of reactivity is somewhat dependent on the degree to which the rock has been re-crystallized as a result of contact with intrusive granites. Supplementary cementing materials are effective in reducing or eliminating the excessive expansions caused by the reaction. Class F fly ash has been in general use in Nova Scotia since 1986 and used specifically for reducing alkali-aggregate reactivity since 1990.Key words: alkali-aggregate reaction, expansion, fly ash, meta-sediments, mortar bar test, pozzolans, prism test, quarries, supplementary cementing materials.

ISSN:0315-1468    

DOI:10.1139/l99-064

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

New Brunswick is located within the northeast-southwest trending Appalachian Region. The basement rocks consist largely of metamorphosed sedimentary types with some granitic intrusions and the composition of the natural gravels reflects the bedrock types. Research into alkali-aggregate reaction (AAR) problems in the province was sponsored initially solely by CANMET and more recently in association with the Department of Transportation. The research consisted of (i) petrographic studies of aggregates, (ii) petrographic studies and case histories of existing concrete structures, and (iii) laboratory expansion testing of concrete and mortar bar specimens. Alkali-aggregate reaction has been found to be one of the factors responsible for premature concrete deterioration in New Brunswick. Visual signs of Alkali-aggregate reaction require up to 10 years to appear and the reaction takes up to 30 years to fully develop. Principal reactive rock types are greywacke, argillites, and fine-grained volcanic rocks. The reactive component is thought to be fine-grained quartz less than 100 µm in size. Laboratory expansion test results on concrete specimens are sensitive to alkali levels. Concrete prism tests (CSA A23.2-14A) with 5.5-5.4 kg/m3Na2O equivalent have been used to predict aggregate performance. By comparison the water soluble alkali contents of field concretes constructed since about 1970 have been found to range from 3.5 to 5 kg/m3Na2O equivalent. Accelerated mortar bar expansion test results (CSA A23.2-25A) do not correlate well with concrete prism expansion tests and indicate a much higher percentage of deleterious aggregates. Use of supplementary cementing materials such as fly ash and silica fume have been found to be effective in long duration laboratory tests to control aggregate reactions.Key words: concrete, aggregates, alkali-aggregate reaction, standard and accelerated testing for AAR, fly ash, silica fume.

ISSN:0315-1468    

DOI:10.1139/l99-077

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

In the province of Québec, due to the particular geology and the historic use of high-alkali cements, a large number of concrete structures are affected by alkali-silica reactivity (ASR). Consequently, tremendous effort was made in this province during the last 20 years on (1) the determination in the laboratory (testing methods) as well as in the field (inspection of structures) of the potential alkali-aggregate reactivity (AAR) of concrete aggregates, (2) the prevention of AAR in new structures, and (3) the management of existing structures affected by this problem. For new structures, the most popular measure used in Québec in the presence of potentially reactive aggregates consists of limiting the alkali contribution by the cement to 3 kg/m3of concrete (Na2Oeq). Also, blended silica fume cements were used on many occasions against ASR; ternary cements containing fly ash and silica fume, which proved in the laboratory to be effective against ASR, are presently available. As concerns the existing structures affected by ASR, many of them were repaired using various techniques ranging from the simple application of a penetrating sealer to the application of post-constraints or slot cutting.Key words: aggregate, alkali-aggregate reaction, cement, concrete, diagnosis, management, preventive measures, prognosis, Québec, testi

ISSN:0315-1468    

DOI:10.1139/l99-076

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

In Ontario, two types of alkali-aggregate reaction exist. Each type is evaluated using different tests. Over the past few years, new tests have been introduced to replace some existing test methods. The new tests are faster and more reliable. Preventive measures such as the use of low-alkali cement and supplementary cementing materials, while they are effective, have not been extensively used with reactive aggregate in Ontario. Beneficiation or selective extraction is used with some potentially reactive aggregates.Key words: alkali-aggregate reaction, concrete, cracking, Ontario, structures.

ISSN:0315-1468    

DOI:10.1139/l99-073

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

This paper summarizes alkali-aggregate reaction (AAR) experience in the prairie region to date. Information is presented on reactive rock types and their geological significance, laboratory test data, field evidence of affected structures, and industry practices for preventive measures and management of affected structures. The frequency and severity of documented AAR-affected structures in Manitoba, Saskatchewan, and Alberta is low, Alberta having the highest incidence. A significant number of aggregate sources in the region show potential for excessive expansion when subjected to laboratory testing. The lack of field evidence is generally attributed to the lower alkali contents of cements that have historically been produced and used in the region. Limited number and extent of searches for affected structures, inadequate diagnosis of the phenomenon, and client confidentiality are also factors. It is imperative that specifiers, owners, researchers, suppliers, and producers be aware of the potential reactivity of local aggregates. The occurrences of AAR-related deterioration in the field demonstrates that the phenomenon is not limited to the laboratory. The effectiveness of low to moderate alkali contents of concrete mixtures in controlling the AAR phenomenon demonstrates the need to specify low-alkali cement and the importance of measuring the effectiveness of supplementary cementing materials in preventing AAR. There is a need to conduct extensive field searches for affected structures and it is important that suspected cases of AAR be properly diagnosed.Key words: alkali-aggregate reaction, concrete, Manitoba, Saskatchewan, Alberta, laboratory testing, field evidence, pattern cracking, prisms, mortar bars, prevention, management.

ISSN:0315-1468    

DOI:10.1139/l99-068

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

In British Columbia, alkali-aggregate reaction (AAR) in concrete has not historically been a well-studied, and thus well-understood, mechanism. This paper provides a synopsis of the current state of what is known of AAR in British Columbia. The foci of the paper are (i) locations of AAR in concrete, (ii) the AAR characteristics of aggregates in British Columbia, (iii) the relationship between geology and AAR, and (iv) preventative methods and prescriptions used to prevent or mitigate AAR in British Columbia.Key words: alkali-aggregate reaction, British Columbia, concrete, aggregate, expansion test.

ISSN:0315-1468    

DOI:10.1139/l99-058

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Flow in compound (or two-stage) channels is very complex and different energy loss mechanisms operate under different geometric and flow conditions. Neither theoretical analyses nor current empirical approaches are sufficiently developed for practical calculation of conveyance for all conditions experienced in practice. An alternative approach, using artificial neural network modelling, has been successfully applied to predict conveyance under a wide range of conditions. The model proposed uses a feed-forward system with one hidden layer and an error back-propagation learning procedure. It predicts a dimensionless discharge using input describing the main channel and floodplain flow depths, vegetation density over the cross section, channel sinuosity, transverse floodplain slope, and floodplain bend tightness. The discharge is dimensionalized by multiplication with the composite discharge calculated assuming frictional resistance only. The model was trained using 45 data sets representing a range of main channel and floodplain characteristics and tested using 15 additional data sets. The discharge prediction error for all the data used in development and testing the model was -0.19% on average and exceeded 15% for one condition only.Key words: compound channels, channel conveyance, flow resistance, neural networks.

ISSN:0315-1468    

DOI:10.1139/l99-056

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The development of a quasi two-dimensional computational model for simulating the transient variations of bed topography profiles in alluvial river channels is reported. The formulation of the model is based on combining the longitudinal flow momentum with the continuity principle of the sediment bed load. The Engelund-Hansen formula is employed in estimating the total sediment bed load along the reach of a river channel. The lateral bed load contribution from the total load is calculated in the same way as in calculating the lateral secondary currents from the main flow velocities. The numerical scheme and the computational procedure used in the study are described in detail. The simulated bed level profiles are verified through comparisons with experimental and field measurements taken from case studies in the literature for different flow conditions, channel characteristics, and sediment properties. The correlation between flow discharge, bed load, boundary friction, and channel slope is discussed. On the basis of the reasonably good comparisons with field data, it may be deduced that the model can be used for predicting the bottom topography variations in river channels.Key words: meandering rivers, bottom topography, sediment transport, bed load, boundary roughness, field measurements, experimental data, computational modelling, finite difference method.

ISSN:0315-1468    

DOI:10.1139/l99-063

出版商:NRC Research Press    

年代:2000

数据来源: NRC