M. K. El Debs

Bending behavior of mortar reinforced with steel meshes and polymeric fibers

Abstract The main purpose of this research was to study the effects of combining reinforcing steel meshes with discontinuous fibers as reinforcement in thin walled Portland cement based mortar beams. The term ‘thin’ implies thicknesses of less than about 25 mm. The underlying idea behind this combination is to satisfy the ultimate strength limit state through the steel mesh reinforcement (main reinforcement) and to control cracking under service loads through fiber reinforcement (secondary reinforcement). An extensive experimental program with bending tests was undertaken. Specimens were 127 × 457 × 12.7 mm. The following variables were investigated: (a) the reference mesh size — 25.4 × 25.4 mm and 50.8 × 50.8 mm; (b) the transverse wire spacing — 25.4 mm, 50.8 mm, and no transverse wires; (c) the type of fibers — polyvinylalcohol (PVA) and polypropylene (PP); and (d) the fiber volume fraction — 1 and 2% for PVA fibers, and 0.5 and 1% for PP fibers. Some of the main conclusions are: (a) for the same fiber volume fraction, the use of PVA fibers led to a better overall performance than that of PP fibers; (b) an increase in cracking moment and a decrease in crack spacing was observed when 1% PVA, 2% PVA, and 1% PP fibers were used; (c) when 0.5% PP fiber was used, no noticeable change in behavior was observed in comparison to specimens without fibers; and (d) for 1% PVA fibers the transverse wire spacing had little effect on the crack spacing and for 2% PVA fibers, the transverse wire had no influence.

Reliability evaluation of reinforced concrete pipes in crack opening limit state

Resumo Structural reliability theory is used in this paper to verify the capability of Brazilian code on design of concrete structures (NBR 6118:2003) concerning the evaluation of crack width in reinforced concrete pipes. Two limit state equations are defined in terms of crack opening. The First Order Second Moment and Monte Carlo simulation methods are used in the reliability analysis. In an initial reliability analysis, problem parameters that have the largest contributions in failure probabilities are identified. A parametric analysis is performed in these variables, in order to study their influence in failure probabilities. The study shows that the formulation of NBR 6118:2003 leads to nonuniform reliability, for a constant safety factor. This means that the unitary safety coefficient specified by the code for the cracking limit state does not reflect the uncertainty in the tubes resistance parameters.

Projeto da base de pilares pré-moldados embutidos em cálices de fundação com interfaces lisas

Esse trabalho apresenta a comparacao de resultados experimentais e teoricos de uma nova formulacao para o projeto da base de pilares pre-moldados embutidos em calice de fundacao com interfaces lisas. Atraves do modelo de bielas e tirantes proposto, sao dimensionadas as armaduras longitudinal e transversal do pilar e verificado o esmagamento do concreto das bielas. O metodo de projeto proposto mostrou-se bastante racional no dimensionamento das armaduras, apresentando resultados bastante proximos dos resultados experimentais, sendo estes ultimos referentes a ensaios de dois prototipos com interfaces lisas submetidos a forca normal com grande excentricidade, nos quais foi variado o comprimento de embutimento. Para o caso de forca normal com pequena excentricidade, e apresentada uma simplificacao do modelo, em que o atrito das interfaces e a excentricidade da reacao normal na base do pilar sao desprezados.

Determinação do módulo de elasticidade do concreto a partir da resposta acústica

A analise dinâmica de estruturas e cada vez mais relevante para estruturas civis. Nesse contexto, ensaios dinâmicos, de carater nao-destrutivo, sao uma ferramenta promissora, pois permitem que sejam obtidas informacoes integradas e globais da estrutura a respeito da rigidez e do amortecimento e, alem disso, podem ser repetidos e comparados ao longo do tempo. A resposta acustica e uma inovadora metodologia de ensaio nao-destrutivo que visa a determinacao do modulo de elasticidade do concreto, parâmetro que associado a resistencia do concreto a compressao, configura-se como diretriz ao dimensionamento de elementos estruturais de concreto simples, armado ou protendido. Na comparacao com metodologias usuais de ensaios nao-destrutivos, os resultados demostraram que a utilizacao de ferramentas baseadas na resposta acustica apresentam diferenca inferior a 1% na determinacao da frequencia de ressonância.

Cement-base bearing pads mortar for connections in the precast concrete: study of surface roughness

Bearing pads are used in precast concrete connections to avoid concentrated stresses in the contact area between the precast elements. In the present research, the bearing pads are Portland cement mortar with styrene-butadiene latex (SB), lightweight aggregate (expanded vermiculite-term) and short fibers (polypropylene, glass and PVA), in order to obtain a material with low modulus of elasticity and high tenacity, compared with normal Portland cement mortar. The objective of this paper is to analyze the influence of surface roughness on the pads and test other types of polypropylene fibers. Tests were carried out to characterize the composite and test on bearing pads. Characterization tests show compressive strength of 41MPa and modulus of elasticity of 12.8GPa. The bearing pads tests present 30% reduction of stiffness in relation to a reference mortar. The bearing pads with roughness on both sides present a reduction up to 30% in stiffness and an increase in accumulated deformation of more than 120%, regarding bearing pads with both sides smooth.

Desenvolvimento de equações para a limitação do peso de veículos de carga em pontes de concreto através da teoria de confiabilidade

Resumo The increase in gross weight limits allowed by Brazilian legislation and the soaring number of new truck configurations on national highways has called for greater attention regarding the structural safety of bridges when submitted to real traffic. This paper verifies the performance of bridges under Sao Paulo Department of Transportation jurisdiction using the reliability index β and obtains truck weight limits in order to guarantee structural integrity. The superstructures of reinforced and prestressed concrete bridges, classes 36 and 45, are considered. The ultimate limit state is verified in cross sections subjected to positive and negative critical bending moments. In case of prestressed bridges, the cracking limit state in concrete is added. The real traffic is represented by a live load model based on weighting data collected from stations located on highways of the state of Sao Paulo and the statistical resistance parameters are determined using the Monte Carlo technique. The gross weight limits are presented in the form of equations known as bridge formulas which are applicable to any group of two or more consecutive axles. The observed results indicate restrictions to the traffic of some vehicles, especially the 740 kN and 19.80 meters length truck. Considering only the serviceability limit state, class 45 bridges are found to exhibit lower weight limits due to the load factors recommended by the code during design.

Análise comparativa de modelos de cálculo para consolos de concreto

O objetivo deste trabalho e realizar uma analise comparativa de alguns modelos de calculo para consolos de concreto pre-moldado. Para isso, sao analisados os modelos de calculo das normas brasileiras (NBR 9062) e europeia (EUROCODE 2) e de um manual de projeto norte-americano (PCI). Alem disso, sao analisados tres modelos de calculo sugeridos na literatura. Busca-se nessa comparacao identificar os modelos que melhor representam a forca de ruina dos consolos, seja pelo escoamento do tirante ou pelo esmagamento do concreto da biela. Alem disso, busca-se identificar a contribuicao da armadura de costura na resistencia do consolo. Para isso, foi montado um banco de dados a partir de resultados de ensaios de consolos de concreto realizados por diversos pesquisadores presentes na literatura, no qual os modelos em questao foram aplicados. Utilizando-se de ferramentas estatisticas, sao definidos coeficientes de ajustes a serem aplicados aos modelos para uso em projeto e que levam em conta a dispersao de resultados encontrada na analise.

Comportamento de almofadas de argamassa modificada na transferência de tensões de compressão

This research aims to analyze the compressive stress transfer between precast concrete elements using cement mortar pads modified with polypropylene fibers, styrene-butadiene latex and heat-expanded vermiculite. The stress transfer analyses are performed interleaving a cementbearing pad between two concrete blocks, subjecting the entire specimen to different compressive load tests. The parameters analyzed in the tests are: surface roughness (using bosses on the bonded phase of different thicknesses), compressive strength with monotonic and cyclic loadings. The main results obtained in this study are: a) the presence of pad increased the strength in 24% for thicknesses of imperfections of 0.5 mm and approximately 12% for smooth faces blocks; b) gain of effectiveness of the bearing pad when the concrete strength was reduced; c) for cyclic loading, the bearing pad increased in 48% the connections strength.

Recomendações para verificação da estabilidade lateral de vigas pré-moldadas em fases transitórias

Resumo This paper presents recommendations for security check of precast beams in transitory phases, compare results of parametric analyzes with national and international code recommendations and confront the formulations used for the calculation of critical load of lateral instability. In transport and lifting phases, precast beams are susceptible to loss lateral stability because the established supports provides little restriction to the element rotate on its principal axis and move laterally. To recommend limits of slenderness, parametric analysis are performed using formulations based on bifurcacional instability, including eigenvalue problems with the finite element method. The results show that the safety limits for I beams and rectangular beams are different. For the analyzed cases and with reference to beam slenderness equation used by fib Model Code [13], the limit determined for rectangular beams would be 85 and for I beams 53, which could be taken as 50, as recommended by the code. Within the analyzed cases of I beams, only the fib Model Code [13] recommendation attend the slenderness limit for transitory phases.