Self-compacting concrete is an innovative construction material / its priority to normal vibrated concrete is that there is not any vibration requirement. Bond strength of reinforcement is one of the key factors that ensures the usefulness of any reinforced concrete structure. In this study, 6 full-scale concrete beams spliced at the mid-span were tested under two-point symmetrical loading. Test variables were bottom cover, side cover, free spacing between longitudinal reinforcement, lap-splice length and presence of transverse reinforcements within the lap-splice region. Specimen SC_22_44_88_800 had cover dimensions close to the code limits and had 36db lap splice length. This specimen showed flexural failure. Specimen SC_44_44_44_710 had 32db lap splice and cover dimensions greater than code minimums. This specimen showed yielding primarily. With the increasing loading, however, bond failure occurred with side splitting. ACI 408 descriptive equation for normal vibrated concrete predicted bar stresses of the unconfined specimens produced with self-compacting concrete acceptably well. The predicted values were lower than the measured values to be on the safe side. The error varied between 3.4% and 6.5%. All predictions of the ACI408 descriptive equation was higher than the measured bar stresses of the confined specimens produced with SCC. All the calculated values were unsafe. The error varied between 10.6% and 34.5%. Specimen SC_44_22_22_530_T4 with 24db lap splice length had side cover and spacing between bars 63.3% and 56.7% less than the ACI 318 limits. The calculated bar stress was 21.6% higher than the measured value. The main reason of the deviation was inadequate cover dimensions. In specimen SC_44_22_22_530_T6, number transverse reinforcement was increased to 6 stirrups to overcome the small cover and spacing problem. However, increased number of stirrups inside a small side and face cover caused weak plane and measured bar stress decreased.

Performance of buildings and bridges during past earthquakes has indicated that many of these structures are vulnerable to seismic damage and structural collapse. The deficiencies in pre-1970s design codes have resulted in poor performance of reinforced concrete structures during seismic excitations. The Richter Magnitude 6.6 - 1971 San Fernando Earthquake raised awareness for seismic retrofit needs of existing buildings for the first time. The majority of deficiencies of vulnerable concrete columns can be overcome through seismic retrofits that involve additional transverse reinforcement. This can be done either by providing reinforced concrete, steel, or fibre-reinforced polymer (FRP) jackets around existing columns; or by applying transverse prestressing to columns (RetroBelt System). The research project presented in this thesis involves a seismic retrofit methodology for seismically deficient building and bridge columns, utilizing the use of high-strength packaging straps as external reinforcement for transverse prestressing. The emphasis in the project is placed on experimental research. Three seismically deficient full-size reinforced concrete columns, with a circular, a square and a rectangular cross- section, either critical in shear or flexure, were designed, built and tested under simulated seismic loading. The results indicate that external prestressing of columns in transverse direction with high-strength steel straps improves ductility and energy dissipation capacity of seismically deficient columns. They further indicate that current analytical techniques can be used to predict the force-displacement relationships of columns. A design approach is presented for the retrofit methodology investigated.

Neste estudo foram analisados experimentalmente o comportamento de 24 pilares curtos de Concreto de Ultra Alta Resistência - CUAR, confinados por armaduras helicoidais, avaliando especificamente os acréscimos de resistência e ductilidade obtidos com diferentes níveis de pressão lateral de confinamento. Na etapa experimental foram realizados ensaios de pilares curtos de CUAR com as seguintes características: - seção circular de 7,2 cm de diâmetro e comprimento de 23 cm, e quatro níveis de resistência à compressão do concreto sendo eles, 165, 175, 200 e 229 MPa, dosados sem e com adição de fibras metálicas; - diferentes espaçamentos das armaduras helicoidais, de modo que fossem obtidas situações com baixo, médio e alto índice de confinamento e taxa de armadura longitudinal fixa. Os ensaios de compressão centrada foram realizados com controle de deslocamento, de modo que foram obtidas as curvas força x deslocamento completas. Constatou-se que a seção resistente dos pilares de CUAR é a formada pelo núcleo de concreto confinado, área delimitada pelo eixo da armadura transversal. Observou-se que o CUAR com fibras metálicas apresenta maior deformação do núcleo de concreto confinado em relação ao núcleo de concreto confinado de CUAR sem adição de fibras metálicas, indicando dessa forma, que os pilares de CUAR com fibras metálicas apresentam comportamento mais dúctil. Para as situações de alto confinamento foram gerados ao concreto do núcleo confinado significativos acréscimos de resistência e deformação axial, aumentando a resistência do concreto confinado em relação a resistência do concreto não confinado em: 82,26%, 75,34%, 90,46% e 70,51%, respectivamente, e as deformações axiais do concreto confinado em relação a deformação axial do concreto não confinado em: 433%, 474%, 647% e 550%. Finalmente, acredita-se que os resultados obtidos poderão trazer subsídios para aplicações futuras desta técnica de confinamento na construção de novos elementos estruturais e no reforço de pilares submetidos a elevados níveis de solicitação axial. / This study evaluated experimentally the behavior of 24 short columns of Ultra High Strength Concrete - UHSC confined by helical transverse reinforcement, specifically evaluating strength increases and ductility obtained with different levels of lateral pressure of confinement. In the experimental phase short UHSC columns with the following characteristics were tested: - circular cross section of 7.2 cm diameter and 23 cm length, four levels of concrete strength (165, 175, 200 and 229 MPa), with and without addition of metallic fibers; - different spacing of transverse reinforcement, so that situations were obtained with low, medium and high level of confinement, while the longitudinal reinforcement ratio was fixed. The centered compression tests were conducted with displacement control, so that complete force x displacement curves were obtained. It was found that the resistant section of UHSC columns is formed by the confined concrete core delimited by the axis of the transverse reinforcement. It was observed that the axial displacement reached in columns with steel fibers was higher than without fibers, indicating that columns with steel fibers exhibit more ductile behavior. For high confinement levels significant axial strength and displacement increases were observed. Increases of axial strength of confined concrete in comparison to unconfined concrete were 82.26%, 75.34%, 90, 46% and 70.51%. Axial displacements were increased by 433%, 474%, 647% and 550%. Finally, it is believed that the results could provide information for future applications of this technique in construction of a new type of columns or in strengthening of columns subjected to high levels of axial force.

[pt] A flambagem das armaduras longitudinais em pilares de concreto armado pode ocorrer na região entre dois estribos consecutivos, ou pode envolver um certo número de estribos. As normas de projeto existentes não fornecem uma metodologia apropriada para o dimensionamento dos estribos em diferentes situações. O presente trabalho tem por objetivo desenvolver uma formulação que permita analisar a flambagem das armaduras longitudinais em pilares de concreto armado submetidos a carregamento axial levando em conta o espaçamento entre os estribos, o diâmetro e arranjo dos estribos na seção transversal e o diâmetro das armaduras longitudinais. Para este propósito um método analítico para a avaliação da flambagem da armadura longitudinal é proposto, considerando-se as barras longitudinais restringidas pela rigidez axial ou à flexão dos estribos. Admite-se que a armadura longitudinal funciona como uma coluna esbelta. Consideram-se duas formas de modelagem da atuação dos estribos: como apoios elásticos discretos e como base elástica contínua. O presente trabalho trata a coluna com um ou mais modos de deformação, incluindo certas não- linearidades. São fornecidos cargas críticas e caminhos pós-críticos para tais casos. Como resultado deste estudo, apresenta-se uma proposta para dimensionamento racional dos estribos que permite estudar diferentes alternativas em um ábaco de utilização simples para projeto. Apresentam-se comparações com resultados experimentais da literatura em pilares de concreto armado. Isto permite uma avaliação crítica dos desenvolvimentos teóricos realizados e da forma proposta de dimensionamento racional dos estribos. / [en] Buckling of longitudinal reinforcement in reinforced concrete columns may occur in the region between two consecutive ties, or may involve a number of ties. The existing design code specifications do not provide an appropriate methodology for the design of the transversal reinforcement in different situations. The main objective of the present work is to develop a formulation to allow to analyze the buckling of longitudinal bars in reinforced concrete columns taking into account the tie spacing, the diameter and arrangement of the ties in the cross section and the longitudinal bar diameter. For this purpose an analytical method for the evaluation of the buckling load of longitudinal bars is described, as a function of the constraint imposed by the axial or flexural stiffness of the stirrups. The longitudinal bar is considered as a column deforming according to thin beam theory. The tie action is described either by a set discrete elastic supports or by a continuous elastic foundation. The theoretical analysis considers the column with one or more deformation modes, with some degree of nonlinearity, including the analysis of post-critical equilibrium paths. As a result of this study, rational criteria for spacing and sizing of transversal reinforcement are derived, allowing to study different alternatives in an abacus of simple use for design. Several comparisons with the results obtained experimentally by other authors in reinforced concrete columns are presented, allowing for an evaluation of the validity of the theoretical developments and the rational design methodology proposed herein.

Performance of buildings and bridges during past earthquakes has indicated that many of these structures are vulnerable to seismic damage and structural collapse. The deficiencies in pre-1970s design codes have resulted in poor performance of reinforced concrete structures during seismic excitations. The Richter Magnitude 6.6 - 1971 San Fernando Earthquake raised awareness for seismic retrofit needs of existing buildings for the first time. The majority of deficiencies of vulnerable concrete columns can be overcome through seismic retrofits that involve additional transverse reinforcement. This can be done either by providing reinforced concrete, steel, or fibre-reinforced polymer (FRP) jackets around existing columns; or by applying transverse prestressing to columns (RetroBelt System). The research project presented in this thesis involves a seismic retrofit methodology for seismically deficient building and bridge columns, utilizing the use of high-strength packaging straps as external reinforcement for transverse prestressing. The emphasis in the project is placed on experimental research. Three seismically deficient full-size reinforced concrete columns, with a circular, a square and a rectangular cross- section, either critical in shear or flexure, were designed, built and tested under simulated seismic loading. The results indicate that external prestressing of columns in transverse direction with high-strength steel straps improves ductility and energy dissipation capacity of seismically deficient columns. They further indicate that current analytical techniques can be used to predict the force-displacement relationships of columns. A design approach is presented for the retrofit methodology investigated.

A resistência à força cortante em lajes de pontes sem armadura transversal têm sido um aspecto preocupante nas verificações de estruturas de concreto estrutural construídas décadas passadas e está diretamente relacionado aos modelos de cálculo de resistência à força cortante e de largura colaborante empregados no caso de cargas parcialmente distribuídas próximas do apoio. Entretanto, não existem ainda estudos nacionais relacionados ao nível de acurácia e precisão das abordagens geralmente empregadas na prática de projetos de pontes no Brasil. Por esta razão, propõem-se apresentar uma contribuição às análises de resistência à força cortante em lajes de pontes com ênfase no modelo de cálculo da ABNT NBR 6118:2014. Para isto foram comparados os resultados experimentais e teóricos utilizando diferentes modelos de resistência à força cortante e uma base de dados construída a partir de 642 resultados experimentais. Posteriormente, alguns modelos experimentais foram explorados por meio de simulações numéricas em elementos finitos no intuito de avaliar o nível de aproximações desta abordagem e investigar a influência de parâmetros como mísulas na proximidade dos apoios. Dentre os principais resultados desta pesquisa destaca-se que o valor médio da relação entre a resistência à força cortante teórica e experimental Vexp/Vcal utilizando a ABNT NBR 6118:2014 variou de 2,145 a 1,140 conforme o modelo de largura colaborante utilizado. Enquanto isso, os modelos numéricos calibrados apresentaram relação Vexp/VMEF variando entre 0,95 e 1,01 e com coeficientes de variação menores que 15%. De maneira geral, identificou-se que os modelos de resistência à força cortante apresentam elevados níveis de dispersão entre resultados teóricos e experimentais no caso de lajes e faixas de laje e que os modelos mais usuais de definição da largura colaborante não são precisamente adequados para o caso de cargas parcialmente distribuídas próximas do apoio. / The shear strength in bridge slabs without transverse reinforcement has been a matter of concern in structural concrete structures checks built in the past decades and is directly related to the shear force and effective width calculation models employed in the case of partially distributed loads close to the support. However, there are still no national studies related to the level of accuracy and precision of the approaches commonly used in the practice of bridge projects in Brazil. For this reason, it is proposed to contribute to the shear strength analyzes in bridge slabs with emphasis on the calculation model of ABNT NBR 6118: 2014. For this, we compared the experimental and theoretical results using different models of shear strength and a database constructed from 642 experimental results. Subsequently, some experimental models were explored by means of numerical simulations in finite elements in order to evaluate the level of approximations of this approach and to investigate the influence of parameters such as greater thickness close to the supports. Among the main results of this research, it is worth noting that the average value of the relationship between theoretical and experimental shear strength Vexp/Vcal using ABNT NBR 6118: 2014 varied from 2,145 to 1,140 according to the effective width model used. Meanwhile, the calibrated numerical models showed Vexp/VMEF ratio varying between 0.95 and 1.01 and with coefficients of variation lower than 15%. In general, it was identified that the shear strength models present high levels of dispersion between theoretical and experimental results in the case of slabs and slab strips and that the most usual models of defining the effective width are not precisely adequate for the partially distributed loads close to the support in slabs.

The confinement of concrete structures has been used for reinforcement and rehabilitation of compressed structural elements, with the objective of increasing their strength capacity and ductility. Various types of reinforcement are used: metal pipes, fiber reinforced polymers, transverse reinforcement, among others. Understanding the stress-strain behavior of confined concrete columns is an important aspect to be considered in the design of these structural elements. This work presents a study of short columns of concrete confined by transverse reinforcement and fiber reinforced polymers subjected to the centered load, employing a nonlinear theoretical model that allows obtaining the stress-strain curves. Concrete columns with various strength levels and different cross-sectional shapes are considered. The employed model is modified according with the type of analyzed problem and a comparative analysis is carried out to verify the performance of the models, considering the experimental results from several examples of short columns confined available in the literature. / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / O confinamento de estruturas de concreto vem sendo utilizado no reforço e reabilitação de peças comprimidas, com o objetivo de aumentar sua capacidade resistente e dutilidade. Vários tipos de reforços são usados: tubos metálicos, polímeros reforçados com fibras, armaduras transversais, entre outros. A compreensão do comportamento tensão-deformação de colunas de concreto confinadas é um aspecto importante a considerar para um projeto confiável desses elementos estruturais. Este trabalho apresenta um estudo de colunas curtas de concreto confinadas por armadura transversal e polímeros reforçados com fibras submetidas à carga centrada, empregando um modelo teórico não linear que permite a obtenção das curvas tensão-deformação. São considerados concretos de variadas resistências e diferentes formas de seção transversal. O modelo empregado é modificado se adequando aos diversos casos analisados, e para verificação do desempenho deste é realizada uma análise comparativa considerando os resultados experimentais provenientes de vários exemplos de colunas curtas confinadas disponíveis na literatura.

The primary reinforcement used for construction of structural concrete members has a yield strength of 60 ksi. This reinforcement grade was incorporated into construction over 50 years ago and remains the standard. Recent advances in material technology have led to the development of commercially available reinforcing steel with yield strengths of 100 ksi. While greater yield strengths can be utilized in design, it is essential that the bars can be properly anchored and spliced to fully develop their strength. Although design expressions are available for this purpose, they were established considering 60 ksi reinforcement. Therefore, the objective of this research program is to evaluate the development of high-strength reinforcing steel and establish a design expression for the development and splicing of this steel. Two phases of experimental tests were conducted. Phase I was performed by Glucksman (2018) and investigated the influence of splice length and transverse reinforcement on bond strength over four series of beam tests. This study (Phase II) was conducted following Phase I and consisted of reinforced concrete slab and beam testing over three series. An investigation was conducted on reinforcement development with a focus on the effect of splice length, concrete compressive strength, stress-strain relationships of the steel (ASTM A615 vs. ASTM A1035), and transverse reinforcement. Based on the results, the influences of test variables were identified, and a new confinement model was developed that estimates the transverse reinforcement contribution to bond strength. Finally, a design expression is provided for calculating the development and splice lengths of high-strength reinforcement.