No description available.

No description available.

The thesis investigates the mechanisms and restraints which influence transverse crack propagation through the bituminous surfacings of semi-flexible pavements. These pavements incorporate continuously laid cement bound roadbases which, during curing, crack into slabs of varying length, ranging from 4-25m. Reciprocal crack growth can occur in the surfacing, known as 'reflection cracks', located through stresses concentrated at the discontinuities within the roadbase . Three mechanisms have been identified and are described as contributing to reflection crack propagation. They have been analysed independently although the majority of conclusions drawn are applicable to their combined action. Their relative importance will vary with respect to pavement geometry, material properties, environmental conditions and traffic intensity. The first mechanism, 'tensile fatigue', induces crack propagation vertically upward through the surfacing. Tensile strains are developed during daily and ru1nual fluctuations of temperature, which cause expansion and contraction of the cement bound roadbase. This mechanism is most prominent on pavements with thin surfacings and long slab lengths. The rate of crack growth is dependent on the range of temperature within the roadbase , slab length, thermal characteristics of the roadbase material and resistance of the surfacing to this form of fatigue . A model has been developed based on a combination of results from an extensive testing programme, the use of fracture mechanics theory and computer simulation of the condition. The results quantify the resistance shown by conventional bituminous mixes to reflection cracking in terms of their mix parameters. Also considered are the use of stress relieving membranes, reinforcement material and modified binders to inhibit crack growth. The second mechanism, 'tensile yield' is also thermally induced but associated with cold weather conditions. Temperature gradients through the pavement structure induce warping and contraction within the uppermost layers. Tensile strains developed at the surface can, under U.K. winter temperatures, exceed the ultimate yield strain of the wearing course material. Preliminary. investigations of four pavements constructed in the early 1970's to motorway specifications indicate that reflection cracking will initiate at the surface if the yield strain, as defined through tensile creep tests, is reduced through binder oxidization to a value of 0.5%. This mechanism will operate on pavements with greater structural layer thicknesses and is only partially dependent on slab length. The influence of a further mechanism, 'shear fatigue' induced through trafficking of the pavement, has been shown to be confined to the acceleration of crack growth in the final stages of propagation unless a breakdown of interlock occurs between adjoining roadbase slabs.

Zugl.: Hamburg, Univ., Diss., 2005

Submitted by Felipe Lapenda (felipe.lapenda@ufpe.br) on 2015-03-04T13:11:06Z No. of bitstreams: 2 DISSERTAÇÃO Manoel de Lima Acidi Neto.pdf: 2950896 bytes, checksum: 589030b1b3b7a8b76ad97be79d9c1887 (MD5) license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) / Made available in DSpace on 2015-03-04T13:11:06Z (GMT). No. of bitstreams: 2 DISSERTAÇÃO Manoel de Lima Acidi Neto.pdf: 2950896 bytes, checksum: 589030b1b3b7a8b76ad97be79d9c1887 (MD5) license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Previous issue date: 2014-02-25 / O objetivo desse estudo foi compreender contextos de uso do crack, a partir da análise das práticas e representações sociais relacionadas, buscando o modo como se constroem nas redes interacionais de seus usuários. Para tanto, foram realizadas entrevistas entre 14 usuários de diferentes contextos e padrões de consumo. O consumo de crack é atualmente um tema de ampla repercussão pública, tratado como um problema social grave que necessita de intervenções urgentes. A ênfase dada ao problema é remetida a um determinismo farmacológico, responsável pela produção de efeitos individuais e sociais diversos, tais como a dependência e o envolvimento com a criminalidade, existindo poucas discussões acerca dos aspectos sociais e culturais envolvidos no fenômeno do consumo. As concepções predominantes sobre o assunto situam que o usuário da substância tende a perder o controle do consumo, voltando-se à compulsão de usá-la, desvinculando-se de suas atividades cotidianas, com prejuízos profissionais e pessoais, além de perda do contato com seus grupos sociais de pertença. Esses efeitos têm sido atribuídos à fissura que a droga pode provocar no usuário, considerada um fator crítico para o desenvolvimento do uso compulsivo e da dependência. Os dados foram analisados através de análise temática de conteúdo e sugerem que as representações encontradas nesses contextos situavam o crack como droga da destruição, responsável pelo fracasso de planos de vida, desconstrução familiar, carreira profissional, etc. Esse aspecto destruidor apareceu também no modo de consumir a droga, que implicava em episódios de ingestão intensa, com deterioração orgânica e prejuízos para a saúde. As práticas de cuidado no consumo se tornavam despercebidas, apesar de presentes, somente sendo focalizado o caráter nocivo da experiência. Diante dessa concepção, a dependência foi apresentada como um reforçador desse modo de lidar com a droga. Ao afirmar que se encontrava doente, dependente do crack, o usuário demonstrava não ter mais preocupações em mediar o consumo, tornando-se “escravo” da droga. Contudo, a experiência do consumo era sempre mediada por algum tipo de controle, mesmo quando o usuário considerava que a droga havia tomado conta de seu ser. Assim sendo, perante essa compreensão da dependência, os usuários consideravam que o envolvimento com a criminalidade era uma consequência do consumo do crack. Nesse âmbito, apesar dos usuários não se limitarem a realizar escolhas diante dessa ética do crime, as representações hegemônicas partilhadas em seus contextos de uso remetem essas características como verdades sobre o crack. Dessa forma, mesmo vivenciando outras experiências com a droga, acreditam que seu uso remete ao âmbito do prazer destrutivo, da impossibilidade de ação voluntária, na encarnação de uma figura de repulsa. As normas sociais vigentes na informalidade desses campos convocam os usuários a se tornarem dependentes, criminosos, incapazes de construir planos de vida. Nesse âmbito, os padrões de controle devem ser evidenciados, pois foram unânimes entre os usuários. A tendência a se destacar os padrões de consumo, demarcando limites de uso, que situam usuários como controlados ou compulsivos, termina por não atentar para a existência de controles sociais mediadores da relação com o crack.

No description available.

No description available.

Many aircraft are designed using the Damage-Tolerance concept. This concept assumes that a structural component is flawed during the manufacturing process. A central feature in this concept is the fatigue-crack-growth-rate properties of the material used in its construction. ASTM E-647 standard describes how fatigue crack growth tests are to be conducted. In the late 1960’s, Wolf Elber discovered the concept of crack closure. He showed that the crack would close prematurely even while under tensile loading. A central feature in this concept is called the crack opening load. The method in ASTM E-647 used to calculate the crack opening load has been found to be inadequate for high load ratios R (Pmin/Pmax>0.4) tests. Herein, an algorithm implementing the least squares method was developed to determine the crack-opening load for low and high R tests on a variety of materials for fatigue crack growth tests conducted on compact C(T) specimens.

This thesis emphasizes the critical importance of crack control in designing and constructing reinforced concrete (RC) structures. Cracks in such structures can significantly reduce strength and durability, pose safety risks, and lead to high repair costs. Existing codes and standards offer varying approaches, resulting in inconsistent results in designing for serviceability limit state (SLS). The evolution of modern reinforced concrete, incorporating additives like superplasticizers and silica fume, requires an update to crack control models based on outdated conceptions. The thesis aims to compare crack width calculations, understand bond stress in contemporary concrete models, and enhance crack control models. The study covers crack development, mathematical aspects of crack design, laboratory testing, and analysis of RC specimens. The findings aim to offer valuable recommendations and improve crack control measures, contributing to a more robust database and aiding the development of effective global model codes and standards for crack control in RC structures.:1. Introduction 2. General knowledge of cracks in RC structures 2.1. Cause of crack formation 2.1.1. Crack during the hardening process 2.1.1.1. Plastic shrinkage cracks 2.1.1.2. Plastic settlement cracks 2.1.2. Crack of hardened concrete 2.1.2.1. Drying shrinkage cracks 2.1.2.2. Thermal cracks 2.1.2.3. Crack due to chemical reaction 2.1.3. Crack due to external loads 2.2. Crack development in an axially loaded member 3. Crack width calculations 3.1. Design formula according to EN:1992 3.1.1. Calculating crack width 3.1.2. Calculating minimum reinforcement 3.1.3. Detailing of reinforcement 3.2. Design formula according to fib Model Code 3.2.1. Crack width calculation per fib Model Code 1990 3.2.2. Crack width calculation per fib Model Code 2010 3.3. Design formula in other codes and standards 3.3.1. Crack width calculation in American standard (ACI) 3.3.2. Crack width calculation in British standard (BS) 3.3.3. Crack width calculation in Vietnamese standard (TCVN) 3.3.4. Summary and example of crack width calculations a. Crack control per EN 1992-1-1 b. Crack control per Model Code 1990 c. Crack control per Model Code 2010 d. Crack control per ACI e. Crack control per BS f. Crack control per TCVN 4. Pull-out experiments 4.1. Experimental basis 4.2. Experiment setup 4.2.1. Test machine 4.2.2. Test cubes 5. Results and Discussion 5.1. Failure modes and bond-slip curves 5.2. The bond-slip functions 6. Conclusion 7. References

Chloride-induced corrosion of reinforcing steel is widely accepted as the primary cause of premature deterioration in concrete bridge decks (Brown, M.C., 2002). Since linear cracking in concrete cover can potentially accelerate chloride ingress to the depth of the reinforcing steel, there is reason to believe that severity of deck cracking can significantly influence the time to first repair and/or rehabilitation of the bridge deck. Surface width, orientation, and length of cracks in 38 Virginia bridge decks were investigated in order to characterize the general distribution of deck cracking in the commonwealth of Virginia. Crack data was correlated to structural/material design parameters and environmental exposure conditions in order to determine significant predictor-response relationships. The majority of surveyed bridge decks were divided into four classifications of deck type based on superstructure type and construction era, either 1968-1971 or 1984-1991. Surveyed bridge decks that did not fit into any of the four classifications were used to form more generalized subsets. These larger subsets were used to determine if significant influence factors could be translated to broader classifications of bridge decks. Transverse beam spacing, annual average daily truck traffic (AADTT), resistivity of the deck concrete, chloride exposure, and the percentage of concrete clear cover depths less than or equal to 38mm (1.5 in) were all determined to have a significant correlation with linear deck cracking. / Master of Science